Epidemiology

• Higher incidence (9 – 20/100,000 person years) and prevalence (156 – 291/100,000 people) in populations of North American and Northern European descent (Lancet 2012;380:1606)
• Incidence increased in industrialized countries and urban versus rural locations, suggestive of environmental triggers, such as improved sanitation, reduced exposure to childhood enteric infections and mucosal immune system maturation (Lancet 2012;380:1606)
• Bimodal age distribution with peaks at 15 – 30 years and 50 – 70 years (Lancet 2012;380:1606)
• Family history of inflammatory bowel disease, particularly that of a first degree relative (5.7 – 15.5%) and Ashkenazi Jewish descent (3 – 5x) show higher risk of disease development (Lancet 2012;380:1606)
• Gastrointestinal infections with Salmonella spp, Shigella spp and Campylobacter spp have twice the risk of developing ulcerative colitis postinfection (Lancet 2012;380:1606)
• M = F
• Former cigarette smoking is strong risk factor (Lancet 2017;389:1756)

• Almost always involves the rectum
  • Continuous pattern of involvement proximally to include up to the entire colon (pancolitis)
  • Rectal sparing can be seen, particularly after treatment (Lancet 2012;380:1606, Histopathology 2014;64:317, Am J Clin Pathol 2004;122:94)
• Patch of inflammation in the cecum, often involving the periappendiceal mucosa (cecal patch), can be present (Lancet 2012;380:1606, Histopathology 2014;64:317)
• Approximately 20% of patients will have inflammation in the terminal ileum (backwash ileitis)
  • Typically present in patients with pancolitis (Am J Surg Pathol 2005;29:1472)
• Focally enhanced gastritis can be seen in approximately 20% of pediatric patients (Pathology 2017;49:808)
• Extraintestinal manifestations:
  • Peripheral arthritis, seronegative
  • Ankylosing spondylitis or sacroiliitis
  • Erythema nodosum
  • Pyoderma granulosum
  • Primary sclerosing cholangitis (PSC)

Pathophysiology

• Not fully known but appears to be a complex multifactorial process involving an overwhelming T helper type 2-like immune response, leading to mucosal injury in response to gut microbial dysbiosis in genetically predisposed patients
• Proposed mechanisms include:
  • Damage to the colonic epithelial barrier due to dysregulation of epithelial tight junctions, which provide a physical barrier between the immune cells and the luminal microbes, leads to increased permeability (Lancet 2012;380:1606)
  • Colonic epithelium upregulation of antimicrobial peptides, known as beta defensins (Lancet 2012;380:1606)
  • Disruption in the homeostatic balance of the mucosal immunity and the enteric nonpathogenic bacteria, resulting in the patient’s aberrant immune response to the enteric commensal bacteria (Lancet 2012;380:1606, Front Microbiol 2018;9:2247)
  • Increased number of colonic epithelium activated and mature dendritic cells with increased stimulatory capacity (Lancet 2012;380:1606)
  • Increased expression of TLR4 by lamina propria cells and TLR4 polymorphism, which can alter susceptibility to enteric infections and tolerance to commensal bacteria (Lancet 2012;380:1606)
  • Disruption in the homeostatic balance between regulatory and effector T cells, leading to a nonclassic natural killer T cell production of IL5 and IL13, which have cytotoxic effects on epithelial cells, mediating an atypical Th2 response
    • IL13 can induce a positive feedback system on the natural killer T cells, leading to increased tissue injury (Lancet 2012;380:1606)
  • Increase in proinflammatory cytokines, chemoattractants such as CXCL8 and adhesion molecules such as MadCAM1 recruit increased leukocytes to the colonic mucosa (Lancet 2012;380:1606)
  • Other genetic risk loci include IL23 and IL10, JAK2 kinase pathway genes, hepatocyte nuclear factor 4α, CDH1 and laminin β1 (Lancet 2012;380:1606)

Etiology :

I propose a mechanism that involves the limbic system and that continuously maintains a state of stress equivalent to a situation of major stress (trauma) and therefore causes all the signs and symptoms of the disease. How does that happen?
Imagine the situation of this gazelle:

Its limbic system will use all means to save its life, by starting it will increase blood circulation in the muscles by dilating the arteries of the muscles (vasodilation), it will increase blood pressure to increase the pressure of perfusion, it will increase heart rate to increase blood flow per minute. This system will trigger other reactions at the level of other organs for example, in the liver and fat tissue it will cause an increase in the release of carbohydrates and fatty acids to bring the necessary energy to the muscles. It will give to this gazelle a deeper field of vision as well as many others possibilities with other systems (see physiopathology of stress) but this system will go further it will reduce the circulation in the internal organs to offer the muscles the blood flow necessary for survival (its survival is at stake the intestines and other organs can wait).

It will also stop the movement of the intestine  and the evacuation of the urinary system, because this is not the time to go to the bathroom!So no defecation and no urination. At the same time  for the same reason the appetite is inhibited  But while this gazelle is running it will see a lot of things it will see flowers, plants, streams, trees, the sun, as well as other animals…… in short it will see its usual environment in complete.Probably consciously it will record only the traumatic part of what happens to it, because it is concerned about its survival, but subconsciously all these elements will be recorded. As a result, it will consciously record a small portion of the information and a large portion of the information will be recorded subconsciously. If this gazelle succeeds in saving its life, it cannot help but record those moments of stress in his memories.

After this event this gazelle is condemned to live with the stress that is recorded and this state of stress will have a continuous effect because the environment of this gazelle’s life is filled with elements which unconsciously remind it of the stressful event and which trigger a major stress with all the resulting consequences. Inhibition of bowel motility will cause constipation. Abdominal and digestive artery vasoconstriction will result in functional ischemia whose intensity will determine the symptomatology. If the vasoconstriction is limited it will only cause slight clinical manifestations without a lesion visible macroscopically or microscopically at the level of the intestine it is the case of the irritable bowel syndrome.

If the vasoconstriction is more important, there will be ischemia creating a minor cellular suffering and especially in the segments where the intestine is in the remote areas of the irrigation network and especially without anastomosis ie in the areas vulnerable. 

This explains the frequency with which the rectum gets affected…

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The limbic system is a group of gray matter and white matter structures lodged deep within the cerebrum that are involved in four functions: olfaction, emotional responses, behavioral activities and memories.

Anatomical structures:

–Limbic lobe: has two main components that are cingulate gyrus (involved in memory and emotional processing and autonomic nervous system) and Para hippocampal gyrus (primarily involved in memory processing).

– Hippocampal formation: composed of three parts: dentate gyrus (afferent information, subiculum (efferent information) hippocampus proper (efferent information)

– Amygdala: which is involved in emotions and emotional behaviors and emotional responses to smell. The Amygdala has two components: cortico medial nuclear group (involved in olfaction) and basolateral nuclear group (all other emotions and behaviors not related to olfaction).

– Hypothalamus: the most important nuclei of hypothalamus in relation with limbic system are mammillary bodies, and autonomic nervous system nuclei (posterior sympathetic anterior parasympathetic)

– Thalamus: there are two main nuclei involved in the limbic system (anterior nucleus of the thalamus which is involved in papez circuit and the other one is called the mediodorsal nucleus which is involved in the circuit connecting the amygdala to prefrontal cortex “emotions and behaviors)

Septal area:

Habenula (part of epithalamus)

Habenula and septal area are connected via a structure called stria medullaris and are involved, particularly in reward pathway and emotional responses as well.

Pathways connecting different structures of the limbic system:

Fornix:

Connects hippocampus to hypothalamus (mammillary bodies) and the septal area.

Striae terminalis: connects the amygdala to the hypothalamus and septal area.

Ventral amygdalofugal pathway : connects the amygdala to the hypothalamus and septal area and also to the mediodorsal nucleus of the thalamus

Striae Medullaris thalami: connects septal area to habenula.

Mamillo-thalamic tract: connects mammillary bodies to the anterior nucleus of the thalamus.

Medial forebrain bundle:  two-way connection that connects the prefrontal cortex to the reticular formation in the brain stem running through the hypothalamus.

Mamillo tegmental tract: connects mammillary bodies of hypothalamus to ventral tegmental area

Mamillary peduncles: connects the ventral tegmental area to the hypothalamus

Functions of limbic system:

-Olfaction: Olfactory epithelial cells, olfactory bulb, action potential in the olfactory tracts, the olfactory tracts bifurcate in medial olfactory striate and lateral olfactory striate which goes into the Para hippocampal gyrus (memory) and to the amygdala (emotions of smell).

-Memory (learning): papez circuit:

Subiculum → Fornix → Mamillary bodies → Mamillo thalamic tract → anterior nucleus of the thalamus → cingulate gyrus →Para hippocampal gyrus → entorhinal cortex → dentate gyrus → subiculum.

Hippocampus (dentate gyrus, which is the receiving portion and subiculum and hippocampus proper which are the leaving portion),

Cingulate gyrus sends also the information to the prefrontal cortex and the purpose of communicating with the prefrontal cortex is to have our memory be involved with our thought and decision making.

-Emotions: particularly the emotional responses, and it’s also involved in behaviors and there are three types of behaviors that the limbic system is involved in :

a) Emotional responses: that means (fear, rage, anger, sadness)

b) Behaviors (feeding behaviors, sexual behaviors, motivational behaviors)

The Amygdala is the center (epicenter) for emotions and behaviors in the limbic system. How does it basically know that we are fearful, angry, or sad? In fact, the limbic system is in communication with our cerebral cortex.

Prefrontal cortex: is involved in thought processing (reasoning, judgment, decision making, personality

Temporal lobe: is involved in multiple functions (smell, taste, visceral sensations, and other areas which are involved in auditory association areas and all are in communication with the amygdala)

Posterior association area: receives information from three different areas: somatosensory association cortex, visual association cortex, and auditory association cortex, and then communicates that with the amygdala, so the amygdala receives informations from all of these structures. And from the cerebral cortex communicating with the amygdala, it will be possible to have enough information to send information to two regions via striae terminalis to septal area and hypothalamus.

The feeding is also controlled by amygdala imagine the sadness amygdala will send information to the nuclei of hypothalamus the ventro-medial nucleus is involved in satiety and the other nucleus is called lateral hypothalamus nucleus and is involved in hunger so the amygdala is responsible for feeding aspect of the organism.

The sexual behavior is also controlled by amygdala:

Depending your psychological situation amygdala feel if you are in sexual mood or not and if yes it will intervene by two nucleus the first one is paraventricular nucleus which releases oxytocin which is commonly secreted in females for uterine contraction and milk ejection, in males it is engaged in sexual orgasm and sexual drives itself. And the other one is Medial Preoptic nucleus which releases Gonadotropin releasing Hormone and GNRH releases in male testosterone which is also responsible for increased sex drive.

Motivational behavior:

We know that amygdala connects with septal area and hypothalamus via the striae terminalis or Ventral medio amygdalo fugal pathway and from the septal area or from the hypothalamus they both can communicate with the ventral tegmental area located in the brainstem, in the ventral tegmental area there is a lot of dopamine (many dopaminergic neurons are located there) and ventral tegmental area sends neurons to Nucleus Accumbens and prefrontal cortex so the amygdala in some conditions (drug abuse for example)  will sends the information to the septal area and to hypothalamus and these in turn will send informations to the ventral tegmental area and from the ventral tegmental area the dopaminergic neurons will connect with nucleus Accumbens and prefrontal cortex. The pathway from the ventral tegmental area to the prefrontal cortex is called the meso cortical pathway and the pathway from the ventral tegmental area to the nucleus Accumbens is called the mesolimbic pathway. The areas receiving the dopaminergic neurons are engaged in reward sensation

Fear:

In fear induced (as emotion) the amygdala will connect via the stria terminalis and ventral amygdalofugal pathway with hypothalamus (posterior hypothalamic nucleus) and the posterior hypothalamic nucleus sends its descending axons downwards via what is called the hypothalamospinal tract and innervates the preganglionic neurons(sympathetic) located within the thoraco lumbar region of the spinal cord  and from there the sympathetic motor neurons will go to the target organs and the final result will be:

Sympathetic activation: cf autonomic nervous system

And the Paraventricular nucleus of hypothalamus which will secrete the cortico-tropine releasing hormone (CRH) which will stimulate the secretion of ACTH by pituitary gland, which in turn will activate the cortisol secretion in adrenal gland (cortisol is called the stress hormone)

Corticotropin-releasing hormone (CRH), a 41 amino acid neuropeptide, discovered in 1981 by Wiley Vale is likely involved in all three types of stress-response. Behavioral responses may involve CRH present in the cerebral cortex and amygdala. Autonomic responses are controlled in part by brainstem fibers descending from the locus coeruleus, which receives CRH-containing fibers from the amygdala and paraventricular nucleus. Hormonal responses center on activation of the hypothalamic–pituitary–adrenal (HPA) axis, which is initiated by CRH present in the paraventricular nucleus of the hypothalamus (PVH).

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Irritable Bowel Syndrome (IBS), is classified as a functional gastrointestinal disease. Autonomic nervous system remodeling by limbic rehabilitation is a very sophisticated technique allowing a physiological treatment  of bowel diseases . In cases of “pure irritable bowel syndrome” the efficiency of the treatment is now confirmed (hundreds of patients successfully treated)  it’s also the same in IBS associated with nonspecific colitis. Now we begin to try successfully the IBD patients. To understand the efficacy of this technic we must begin to study the etiology and the exact mechanism leading to these abnormalities. IBS, is a chronic condition of the lower gastrointestinal tract that affects as many as 15% of adults in the world. Not easily characterized by structural abnormalities, infection, or metabolic disturbances, the underlying mechanisms of IBS remained for many years unclear.Recent research, however, has lead to an increased understanding of IBS. As a result, IBS is often referred to as spastic, nervous or irritable colon. Its hallmark is abdominal pain or discomfort associated with a change in the consistency and/or frequency of bowel movements.

The frequency of IBS in any given population depends, in part, on the ethnic and cultural background of the population being studied, and the criteria used to diagnose the disease. Eight to 20% of adults in the Western world report symptoms consistent with IBS (approximately 65% of these are women). Asia and Africa have similar rates to those in the Western world in general. In India  IBS is more common among men, although it is possible that this is a result of differences in symptom reporting and health care use between genders.

The symptoms of IBS may include abdominal pain, distention, bloating, indigestion and various symptoms of defecation. There are three subcategories of IBS, according to the principal symptoms. These are pain associated with diarrhea; pain associated with constipation; and pain and diarrhea alternating with constipation. IBS is not a psychiatric disorder, although it is tied to emotional and social stress, which can affect both the onset and severity of symptoms. IBS patients suffer from a disproportionately higher rate of co-morbidity with other disorders, such as fibromyalgia, chronic fatigue, pelvic pain and psychiatric disorders. The primary features of the syndrome include motility, sensation and central nervous system dysfunction. Motility dysfunction may be manifest in muscle spasms; contractions can be very slow or fast. An increased sensitivity to stimuli causes pain and abdominal discomfort. Researchers also suspect that the regulatory conduit between the central and enteric pathway in patients suffering from IBS may be impaired. Research suggests that many patients with Irritable Bowel Syndrome have disorganized and appreciably more intense colonic contractions than normal controls.

Pathophysiology :

Enteric propulsion and sensation are, in part, mediated by acetylcholine and serotonin (5HT). The physiology of sensation in the gut is multifaceted. Entero-endocrine cells transmit mechanical and chemical messages. The communication between gut and brain results in reflex responses mediated at three levels—prevertebral ganglia, spinal cord and brainstem. 5-HT, substance P, CGRP, norepinephrine, kappa opiate and nitric oxide are all involved in the perception and autonomic response to visceral stimulation. Sensation is conveyed from the viscus to the conscious perception via neurons in vagal and parasympathetic fibers. Afferent nerves in the dorsal root ganglion synapse with neurons in the dorsal horn. These signals result in reflexes that control motor and secretory functions as they synapse with efferent paths in the prevertebral ganglia and spinal cord. Pain is processed through spinal afferents in the dorsal horn. Ultimately, stimulation of the brainstem brings sensation to a conscious level :

Bidirectional signaling between the brainstem and the dorsal horn mediate sensation. The descending pathways are primarily adrenergic and serotonergic and affect incoming stimuli. End organ sensitivity, stimulus intensity changes or receptive field size of the dorsal horn neuron and limbic system modulation are the mechanisms involved in visceral hypersensitivity. Enteric inflammatory cells may also play an important role in the pathophysiology of Irritable Bowel Syndrome. Clinicians have for many years recognized that the onset of IBS often follows an episode of acute gastroenteritis. Inflammation may alter intestinal cytokine milieu and motility, both of which can result in an increase in a patient’s pain sensation. The menstrual cycle may also affect gut sensation and motility. Other factors, such as malabsorption of sugars (lactose, fructose, and sorbitol), probably aggravate underlying IBS, rather than serving as root causes of the disorder. In patients with rapid transit times, short or medium chain fatty acids can reach the right colon and cause diarrhea.

Symptoms:

The hallmark of IBS is abdominal pain or discomfort associated with either a change in bowel habits or disordered defecation. The pain or discomfort associated with IBS is often poorly localized and may be migratory and variable. It may occur after a meal, during stress or at the time of menses. In addition to pain and discomfort, altered bowel habits are common, including diarrhea, constipation, and diarrhea alternating with constipation. Patients also complain of bloating or abdominal distension, mucous in the stool, urgency, and a feeling of incomplete evacuation. Some patients describe frequent episodes, whereas others describe long symptom-free periods. Patients with irritable bowel frequently report symptoms of other functional gastrointestinal disorders as well, including chest pain, heartburn, nausea or dyspepsia, difficulty swallowing, or a sensation of a lump in the throat or closing of the throat. Patients with IBS are generally classified according to the type of bowel habits that accompany pain. Some patients have diarrhea-predominant symptomatology, others constipation-predominant, and still others have a combination of the two. Some patients alternate between different subgroups. Symptoms may vary from barely noticeable to debilitating, at times within the same patient. In some patients, stress or life crises may be associated with the onset of symptoms, which may then disappear when the stress dissipates. Other patients seem to have random IBS episodes with spontaneous remissions. Still others describe long periods of symptoms and long symptom-free periods. In general, the symptoms of IBS wax and wane throughout life, but the majority of patients seen by physicians is 20–50 years old. In approximately 50% of patients, symptoms begin before age 35. The disorder is also recognized in children, generally appearing in early adolescence. Many patients can trace the onset of symptoms back to childhood. The prevalence of IBS is slightly lower in the elderly, and in this patient population organic disorders must be excluded. Extraintestinal symptoms are common in patients with IBS. These may include headache, sleep disturbances, post-traumatic stress disorder, temporomandibular joint disorder, sicca syndrome, back/pelvic pain, myalgias, back pain, and chronic pelvic pain. Fibromyalgia and interstitial cystitis are also frequently encountered in patients with IBS. In fact, Fibromyalgia occurs in up to 33% of patients with IBS and almost half of patients with fibromyalgia also have IBS.

The autonomic nervous systems  and the bowel:

Sympathetic (red dashed) and parasympathetic (blue) nervous system innervate the gastrointestinal tract. Both carry sensory stimuli, though it appears that spinal afferent nerves in the dorsal horn of the spinal cord process pain. 

In normal conditions the sympathetic nervous system is active during the stress ( fight and flight and fright system) and the parasympathetic is more active during the rest ( rest and digest system).

Sympathetic activation produces:

1) Increased heart rate and cardiac muscle contractility and bronchodilation of the lungs.

2) Constriction of blood vessels in many parts of the body, especially in the bowels and vasodilation of the muscle vessels.

3) Inhibition of stomach and intestinal action to the point where digestion slows down or stops, the same is for defecation and urination.

4) General effect on the sphincters of the body.

5) Paling or flushing, or alternating between both.

6) Liberation of nutrients (particularly fat and glucose) for muscular action.

7) Inhibition of the lacrimal gland (responsible for tear production) and salivation.

8) Dilation of pupil (mydriasis).

9) Relaxation of bladder.

10) Inhibition of erection.

11) Auditory exclusion (loss of hearing).

12) Tunnel vision (loss of peripheral vision).

13) Disinhibition of spinal reflexes; and shaking

14) Increased glucose production and mobilization by the liver

15) Increased lipolysis within fat tissue.

Parasympathetic activation produces:

1) In the stomach and intestines, parasympathetic stimulation leads to increased motility and relaxation of sphincters and increased gastric secretions to aid in digestion.

2) Parasympathetic stimulation leads to vasodilation of internal blood vessels,  especially intestinal vasculature.

3) In the gallbladder, parasympathetic stimulation induces contraction to release bile.

4) In the pancreas, parasympathetic stimulation leads to the release of digestive enzymes and insulin

5) In salivary glands, parasympathetic stimulation leads to high volume secretion of potassium ions, water, and amylase

6) In the heart, parasympathetic stimulation, causes decreased heart rate and velocity of conduction through the AV node.

7) In the eye, parasympathetic stimulation, causes contraction of the sphincter muscle of the iris, leading to constriction of the pupil (miosis). Additionally, it causes contraction of the ciliary muscle, improving near vision.

8) In the lungs, parasympathetic stimulation of M3 receptors leads to bronchoconstriction. It also increases bronchial secretions.

9) In the kidneys and bladder, parasympathetic stimulation, stimulates peristalsis of ureters, contraction of the detrusor muscle, and relaxation of the internal urethral sphincter aiding in the flow and excretion of urine.

10) Smooth muscle relaxation in the helicine arteries of the penis, allowing blood to fill the corpora cavernosa and corpus spongiosum, causing an erection. The PNS also gives excitatory signals to the vas deferens, seminal vesicles, and prostate.

Current research on the topic suggests a biopsychosocial model of the disorder, implicating physiological, emotional, behavioral and cognitive factors. Approximately 40–60% of patients with IBS who seek medical care also report psychiatric symptoms, such as depression, anxiety, or somatization. Interestingly, however, psychiatric symptoms in patients with IBS in the general population are not as prevalent. It is thought that these psychiatric disturbances influence coping skills and illness-associated behaviors. A history of abuse (physical, sexual, or emotional) has been correlated with symptom severity. More than half of patients who are seen by a physician for Irritable Bowel Disease report stressful life events coinciding with or preceding the onset of symptoms. Stress is known to alter gastrointestinal function. Patients who suffer from IBS have amplified colonic motility responses when compared to normal volunteers (those who do not have any symptoms of IBS).

We believe the limbic system (an area of the brain where stress is perceived and experienced and recorded) is critically involved. The majority of these recordings are not available in the conscious mind. In everyday life each event is processed between conscious mind and limbic system and the ultimate response is evaluated between past experiences and actual information received and then the final reaction is decided. If the limbic system is in stress mode or imbalanced mode all consecutive behaviour of autonomic nervous  system  will be abnormal. 

Diagnosis :

In the absence of definitive diagnostic markers, the diagnosis of IBS rests on physician’s recognition of classic clinical symptoms and the exclusion of other diseases. To facilitate comparisons among different populations and assist in epidemiological studies of IBS, two sets of criteria for diagnosis have been developed—the Manning and Rome Criteria. A multinational working team subsequently developed the Rome Criteria. The original criteria, Rome 1, were recently revised and the new Rome 2 diagnostic criteria are included below.

Manning Criteria: 

Abdominal pain that is relieved with a bowel movement.

Pain associated with more frequent stools.

Sensation of incomplete evacuation.

Passage of mucous.

Abdominal distension.

Rome Criteria

Continuous or recurrent symptoms

of:

Abdominal pain, relieved with defecation, or associated with change in frequency or consistency of stool

and/or

Disturbed defecation( two or more of):

Altered stool frequency

Altered stool form (hard or loose/watery)

Altered stool passage (straining or urgency, feeling of incomplete evacuation)

Passage of mucus

usually with bloating or feeling of abdominal distension

Revised Rome Criteria (65)

At least 12 weeks or more, which need not be consecutive, in the preceding 12 months of abdominal disconfort or pain that has two out of three features:

-Relieved with defecation

and/or

-Onset associated with a change in frequency of

and/or

-onset associated with a change in form(appearance) of stool

Symptoms that cumulatively support the diagnosis of irritable bowel syndrome

-Abnormal stool frequency(for reasurches puposes “abnormal” may be defined as greater than 3 bowel movements per day and less than three bowel movements per week

– Abnormal stool form

lumpy/hard or loose/waterystool)

-Abnormal stool passage( straining, urgency, or feeling of incomplete evacuation)

-Passage of mucus

-Bloating or feeling of abdominal distension

Diagnostic Approach

Effective diagnosis of IBS begins with a careful history and physical examination. The initial evaluation should also include: a complete blood count, chemistry panel, and erythrocyte sedimentation rate as well as a careful study of food allergies and a stool test for fecal occult blood . A colonoscopy should be performed in patients to exclude other types of bowel diseases. Once a diagnosis of IBS has been made, attention should be shifted to treatment. Continued investigations due to persistent symptoms are not warranted and may ultimately undermine a patient’s confidence in both the disorder diagnosis and the attending physician.

Therapy Overview

Management of patients with Irritable Bowel Syndrome is based on a positive diagnosis of the syndrome, exclusion of organic disorders, and specific therapies. Treatment for IBS should address the three main pathophysiologically important factors : psychosocial disturbances, visceral hypersensitivity, and dysmotility. Treatment should be patient oriented and geared towards symptom–specific relief. The majority of conventional IBS treatments currently used is empiric and has not been formally reviewed and scientifically approuved. Therapies may include fiber consumption for constipation, anti-diarrheals, smooth muscle relaxants for pain, and psychotropic agents for pain, diarrhea and depression. Patients with mild or infrequent symptoms may benefit from the establishment of a physician-patient relationship, patient education and reassurance, dietary modification, and simple measures such as fiber consumption. Patients with constipation-predominant IBS can generally be treated with osmotic mild laxatives. Stronger laxatives should be reserved for patients who do not respond to fiber consumption and gentle osmotic laxatives.

Physician-Patient Relationship :  

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In addition, renin release is stimulated by the sympathetic nervous system.

High concentrations of angiotensin II suppress renin secretion via a negative feedback loop. Angiotensin II acts on specific angiotensin receptors AT1 and AT2, causing a contraction of the smooth muscles and the release of aldosterone, prostacyclins and catecholamines. The renin – angiotensin – aldosterone system plays an important role in controlling blood pressure, including sodium balance.

ADH ( vasopressin):

Vasopressin is a small hormone, synthesized in the hypothalamus and released into the circulation from the posterior lob of hypophysis. Although historically named as a result of its potent vasopressor actions, these actions only occur when plasma vasopressin is present in the plasma in supraphysiological concentrations. The most important action of vasopressin is its antidiuretic action on the collecting ducts of the kidney. This leads to a decrease in renal free water clearance, concentration of urine, and a reduction in urine volume. The net effect is the reabsorption of water into the blood, which, along with thirst-generated water intake, leads to normalization of plasma osmolality.Regulation of vasopressin secretion and action thus represents a key homeostatic process which protects the osmotic milieu of the body, allowing normal cellular function.

Synthesis :

Vasopressin is most abundantly produced in magnocellular neurosecretory neurons in the supraoptic and paraventricular (PVN) nuclei, transported to terminals in the neurohypophysis, and released into the general circulation. Vasopressin production is also found in parvocellular neurons in the PVN and vasopressinn produced in these neurons is transported to terminals in the external layer of the median eminence, from which it is released into the hypophysial portal system.

Release and feedback controle :

Vasopressin release is regulated by osmoreceptors in the hypothalamus (OVLT, SFO), which are exquisitely sensitive to changes in plasma osmolality of as little as 1% to 2%. Under hyperosmolar conditions, osmoreceptor stimulation leads to vasopressin release and stimulation of thirst. These two mechanisms result in increased water intake and retention. Vasopressin release is also regulated by baroreceptors in the carotid sinus and aortic arch, under conditions of hypovolemia, these receptors stimulate vasopressin release to increase plasma volume. At very high concentrations, vasopressin also causes vascular smooth muscle constriction through the V1 receptor, increasing vascular tone and therefore the blood pressure. Accordingly, vasopressin is often administered parenterally as a vasopressor agent in patients with hypotension that is refractory to volume restriction.

Vasopressin has effects on the immune system independent of its effect in stimulating the HPA axis. When given intraventricularly to rats, vasopressin decreases the T-cell response to mitogen independently of the HPA axis, probably via the sympathetic nervous system. Like CRH, vasopressin stimulates immune responses in peripheral tissues. Circulating or local vasopressin enhances lymphocyte reactions and potentiates primary antibody. Elevated vasopressin levels are found in a mouse model of autoimmune disease, and antibody neutralization ameliorates the inflammatory response in these mice. Vasopressin can potentiate the release of prolactin, a proinflammatory  peptide hormone.

Because vasopressin has immunosuppressive effects when present in the central nervous system and immunosupportive effects when present in peripheral tissues, predicting which effect would predominate during vasopressin infusion in the ICU is difficult.

Vasopressin is a hormone of the posterior pituitary, that is secreted in response to high serum osmolarity. Excitation of atrial stretch receptors inhibits vasopressin secretion. Vasopressin is also released in response to stress, inflammatory signals, and some medications. Hypotension, morphine, nicotine, angiotensin II, glucocorticoids, and IL-6 all stimulate release of vasopressin. Circulating vasopressin levels are usually high in the early phase of septic shock,  but vasopressin deficiency has been described in vasodilatory shock states in both adults and children. The level of vasopressin that is normal in the late phase of sepsis is unclear.

Vasopressin selectively raises free water reabsorption through the upregulation of aquaporin-2  water channels in the collecting duct, resulting in blood pressure elevation (Elliot et al., 1996; Linshaw 2011). Although it appears that the developing kidney is less sensitive to circulating vasopressin, plasma levels of vasopressin are markedly elevated in the neonate, especially after vaginal delivery, and its cardiovascular actions facilitate neonatal adaptation (Pohjavuori et al., 1985; Linshaw, 2011). The high vasopressin levels are in part also responsible for the diminished urine output of the healthy term neonate during the first day after birth. Under certain pathologic conditions, the dysregulated release of, or the end-organ unresponsiveness to, vasopressin significantly affects renal and cardiovascular functions and electrolyte and fluid status in the sick preterm and term neonate (Svenningsen et al., 1974). In the syndrome of inappropriate secretion of antidiuretic hormone (SIADH), an uncontrolled release of vasopressin occurs in sick preterm and term neonates, with resulting water retention, hyponatremia, and oligouria. In the syndrome of diabetes insipidus, the lack of pituitary production of vasopressin or renal unresponsiveness to vasopressin results in polyuria and hypernatremia.

Le Peptide natriurétique auriculaire :
Le peptide natriurétique auriculaire (ANP) est libéré des granules auriculaires. Il produit une natriurèse, une diurèse et une baisse modeste de la pression artérielle, tout en diminuant la rénine et l’aldostérone plasmatiques. Les peptides natriurétiques modifient également la transmission synaptique des osmorécepteurs. Le (PNA) est libéré à la suite de la stimulation de l’oreillette par distension et étirement des récepteurs. Les concentrations de (PNA) sont augmentées par des pressions de remplissage élevées et chez les patients souffrant d’hypertension artérielle et d’hypertrophie ventriculaire gauche comme la paroi du ventricule gauche est épaissie participe à la sécrétion d’ANP.
Eicosanoïdes :
Les métabolites de l’acide arachidonique modifient la pression artérielle par des effets directs sur le tonus musculaire lisse vasculaire et les interactions avec d’autres systèmes vasorégulateurs: système nerveux autonome, le système rénine-angiotensine – aldostérone et autres voies humorales. Chez les patients hypertendus, un dysfonctionnement des cellules endothéliales vasculaires pourrait entraîner une réduction des facteurs de relaxation dérivés de l’endothélium tels que l’oxyde nitrique, la prostacycline et le facteur hyperpolarisant dérivé de l’endothélium, ou une augmentation de la production de facteurs de contraction tels que l’endothéline-1 et le thromboxane A2. Systèmes kallikréine-kinine Les kallikréines tissulaires agissent sur le kininogène pour former des peptides vasoactifs. Le plus important est le bradykinin vasodilatateur. Les kinines jouent un rôle dans la régulation du débit sanguin rénal et de l’excrétion d’eau et de sodium. Les inhibiteurs de l’ECA diminuent la dégradation de la bradykinine en peptides inactifs. Mécanismes endothéliaux L’oxyde nitrique (NO) intervient dans la vasodilatation produite par l’acétylcholine, la bradykinine, le nitroprussiate de sodium et les nitrates. Chez les patients hypertendus, la relaxation dérivée de l’endothélium est inhibée. L’endothélium synthétise les endothélines, les vasoconstricteurs les plus puissants. La génération ou la sensibilité à l’endothéline-1 n’est pas plus élevée chez les sujets hypertendus que chez les sujets normotendus. Néanmoins, les effets vasculaires délétères de l’endothéline-1 endogène peuvent être accentués par une génération réduite d’oxyde nitrique causée par une dysfonction endothéliale hypertensive.
Stéroïdes surrénales :
Les minéraux et glucocorticoïdes augmentent la pression artérielle. Cet effet est médié par la rétention de sodium et d’eau (minéralocorticoïdes) ou une réactivité vasculaire accrue (glucocorticoïdes). De plus, les glucocorticoïdes et les minéralocorticoïdes augmentent le tonus vasculaire en régulant les récepteurs des hormones pressives telles que l’angiotensine II. Vasodépression rénomédullaire Les cellules interstitielles rénomédullaires, situées principalement dans la papille rénale, sécrètent une substance inactive la médullipine I. Ce lipide est transformé dans le foie en médullipine II. Cette substance exerce un effet hypotenseur prolongé, éventuellement par vasodilatation directe, inhibition de la pulsion sympathique en réponse à l’hypotension et une action diurétique. On suppose que l’activité du système rénomédullaire est contrôlée par le flux sanguin médullaire rénal. Excrétion de sodium et d’eau La rétention de sodium et d’eau est associée à une augmentation de la pression artérielle. Il est postulé que le sodium, via le mécanisme d’échange sodium-calcium, provoque une augmentation du calcium intracellulaire dans le muscle lisse vasculaire entraînant une augmentation du tonus vasculaire. La principale cause de rétention de sodium et d’eau peut être une relation anormale entre la pression et l’excrétion de sodium résultant d’une diminution du débit sanguin rénal, d’une masse néphronique réduite et d’une augmentation de l’angiotensine ou des minéralocorticoïdes. Physiopathologie L’hypertension est une élévation chronique de la pression artérielle qui, à long terme, cause des dommages aux organes terminaux et entraîne une augmentation de la morbidité et de la mortalité. La pression artérielle est le produit du débit cardiaque et de la résistance vasculaire systémique. Il s’ensuit que les patients souffrant d’hypertension artérielle peuvent avoir une augmentation du débit cardiaque, une augmentation de la résistance vasculaire systémique, ou les deux. Dans le groupe d’âge plus jeune, le débit cardiaque est souvent élevé, tandis que chez les patients plus âgés, une résistance vasculaire systémique accrue et une rigidité accrue du système vasculaire jouent un rôle dominant. Le tonus vasculaire peut être élevé en raison d’une stimulation accrue des récepteurs a-adrénergiques ou d’une libération accrue de peptides tels que l’angiotensine ou les endothélines. La dernière voie est une augmentation du calcium cytosolique dans le muscle lisse vasculaire provoquant une vasoconstriction. Plusieurs facteurs de croissance, dont l’angiotensine et les endothélines, provoquent une augmentation de la masse musculaire vasculaire lisse appelée remodelage vasculaire. A la fois une augmentation de la résistance vasculaire systémique et une augmentation de la résistance vasculaire
la rigidité augmente la charge imposée au ventricule gauche; cela induit une hypertrophie ventriculaire gauche et un dysfonctionnement diastolique ventriculaire gauche. Chez les jeunes, la pression du pouls générée par le ventricule gauche est
relativement faible et les ondes reflétées par le système vasculaire périphérique se produit principalement après la fin de la systole, augmentant ainsi la pression au début partie de la diastole et l’amélioration de la perfusion coronaire. Avec vieillissement, raidissement de l’aorte et des artères élastiques,augmente la pression du pouls. Les ondes réfléchies passent de la diastole précoce à la systole tardive. Il en résulte une augmentation de la postcharge ventriculaire gauche et contribue à l’hypertrophie ventriculaire gauche. L’élargissement de la pression du pouls avec le vieillissement est un puissant prédicteur des maladies coronariennes. Le système nerveux autonome joue un rôle important dans le contrôle de la pression artérielle. Chez les patients hypertendus, une libération accrue et une sensibilité périphérique accrue à la noradrénaline peuvent être trouvées. De plus, il y a une réactivité accrue aux stimuli stressants. Une autre caractéristique de l’hypertension artérielle est la réinitialisation des baroréflexes diminution de la sensibilité des barorécepteurs. Le système rénine-angiotensine est impliqué au moins dans certaines formes d’hypertension (par exemple l’hypertension rénovasculaire) et est supprimé en présence d’hyperaldostéronisme primaire. Les patients âgés ou noirs ont tendance à souffrir d’hypertension à faible rénine. D’autres ont une hypertension à rénine élevée et ceux-ci sont plus susceptibles de développer un infarctus myocardique et d’autres complications cardiovasculaires. Dans l’hypertension essentielle humaine et expérimentale,l’hypertension, la régulation du volume et la relation entre la pression artérielle et l’excrétion de sodium (natriurèse sous pression) sont anormales. Des preuves considérables indiquent que la réinitialisation de la natriurèse sous pression joue un rôle clé dans la cause de l’hypertension. Chez les patients souffrant d’hypertension essentielle, la réinitialisation de la natriurèse sous pression se caractérise soit par un déplacement parallèle vers des pressions sanguines plus élevées et une hypertension insensible au sel, soit par une diminution de la pente de la natriurèse sous pression et de l’hypertension sensible au sel. Conséquences et complications de l’hypertension Les conséquences cardiaques de l’hypertension sont l’hypertrophie ventriculaire gauche et la maladie coronarienne. L’hypertrophie ventriculaire gauche est causée par une surcharge de pression et est concentrique. Il y a une augmentation de la masse musculaire et de l’épaisseur de la paroi mais pas du volume ventriculaire. L’hypertrophie ventriculaire gauche altère la fonction diastolique, ralentit la relaxation ventriculaire et retarde le remplissage. L’hypertrophie ventriculaire gauche est un facteur de risque indépendant de maladie cardiovasculaire, en particulier de mort subite. Les conséquences de l’hypertension sont fonction de sa gravité. Il n’y a pas de seuil de complications, car l’élévation de la pression artérielle est associée à une morbidité accrue dans toute la plage de pression artérielle (tableau 1).
La maladie coronarienne est associée à, et accélérée par, l’hypertension artérielle chronique, entraînant une ischémie myocardique et un infarctus du myocarde. En effet, l’ischémie myocardique est beaucoup plus fréquente chez les patients hypertendus non traités ou mal contrôlés que chez les patients normotendus. Deux principaux facteurs contribuent à l’ischémie myocardique: une augmentation de la demande en oxygène liée à la pression et une diminution de l’apport coronarien en oxygène résultant des lésions athéromateuses associées. L’hypertension est un facteur de risque important de décès par maladie coronarienne. L’insuffisance cardiaque est une conséquence d’une surcharge de pression chronique. Il peut commencer par un dysfonctionnement diastolique et évoluer vers une insuffisance systolique manifeste avec congestion cardiaque. Les AVC sont des complications majeures de l’hypertension; ils résultent d’une thrombose, d’une thrombo-embolie ou d’une hémorragie intracrânienne. La maladie rénale, initialement révélée par une micro albuminaémie, peut évoluer lentement et se manifester au cours des années suivantes. Traitement à long terme de l’hypertension Tous les antihypertenseurs doivent agir en diminuant le débit cardiaque, la résistance vasculaire périphérique ou les deux. Les classes de médicaments les plus couramment utilisées comprennent les diurétiques thiazidiques, les bbloquants, les inhibiteurs de l’ECA, les antagonistes des récepteurs de l’angiotensine II, les bloqueurs des canaux calciques, les bloqueurs des récepteurs adrénergiques, les bloqueurs combinés des a et b, les vasodilatateurs directs et certains médicaments à action centrale tels que les a2- agonistes des récepteurs adrénergiques et agonistes des récepteurs de l’imidazoline I1. La modification du style de vie est la première étape du traitement de l’hypertension; il comprend une restriction modérée en sodium, une réduction de poids chez les obèses, une diminution de la consommation d’alcool et une augmentation de l’exercice. Un traitement médicamenteux est nécessaire lorsque les mesures ci-dessus n’ont pas réussi ou lorsque l’hypertension est déjà à un stade dangereux (stade 3) lors de sa première reconnaissance.
Thérapie médicamenteuse
Diurétiques:
Le traitement diurétique à faible dose est efficace et réduit le risque d’accident vasculaire cérébral, de maladie coronarienne, d’insuffisance cardiaque congestive et de mortalité totale. Alors que les thiazides sont les plus couramment utilisés, les diurétiques de l’anse
sont également utilisés avec succès et l’association avec un diurétique d’épargne potassique réduit le risque d’hypokaliémie et d’hypomagnésémie. Même à petites doses, les diurétiques potentialisent d’autres antihypertenseurs. Le risque de mort subite est réduit lorsque des diurétiques épargneurs de potassium sont utilisés. À long terme, les spironolactones réduisent la morbidité et la mortalité chez les patients souffrant d’insuffisance cardiaque
c’est une complication typique de l’hypertension de longue date.
Bêta-bloquants:
Un tonus sympathique élevé, l’angine de poitrine et un infarctus du myocarde antérieur sont de bonnes raisons d’utiliser des bêtabloquants. Étant donné qu’une faible dose minimise le risque de fatigue (un effet désagréable du blocage b

l’ajout d’un diurétique ou d’un inhibiteur calcique est souvent bénéfique. Cependant, le traitement par blocage b estassociée à des symptômes de dépression, de fatigue et de dysfonction sexuelle. Ces effets secondaires
doivent être pris en considération dans l’évaluation des avantages du traitement. Au cours des dernières années, les b-bloquants ont été utilisés de plus en plus fréquemment dans la prise en charge de l’insuffisance cardiaque, complication connue de l’hypertension artérielle. Ils sont efficaces mais leur introduction en présence d’insuffisance cardiaque doit être très prudente, à commencer par des doses très faibles pour éviter une aggravation initiale de l’insuffisance cardiaque. Bloqueurs des canaux calciques Les bloqueurs des canaux calciques peuvent être divisés en dihydropyridines (par exemple nifédipine, nimodipine, amlodipine) et non-dihydropyridines (vérapamil, diltiazem). Les deux groupes diminuent la résistance vasculaire périphérique mais le vérapamil et le diltiazem ont des effets inotropes et chronotropes négatifs. Les dihydropyridines à courte durée d’action telles que la nifédipine provoquent une activation sympathique réflexe et une tachycardie, tandis que les médicaments à longue durée d’action tels que l’amlodipine et les préparations à libération lente de nifédipine provoquent moins d’activation sympathique. Les dihydropyridines à courte durée d’action semblent augmenter le risque de mort subite. Cependant, l’essai sur l’hypertension systolique en Europe (SYST-EUR) qui comparait la nitrendipine au placebo a dû être arrêté tôt en raison des avantages significatifs
thérapie. Les inhibiteurs calciques sont efficaces chez les personnes âgées et peuvent être sélectionnés en monothérapie pour les patients atteints du phénomène de Raynaud, de maladie vasculaire périphérique ou d’asthme, car ces patients ne tolèrent pas les b-bloquants. Le diltiazem et le vérapamil sont contre-indiqués dans l’insuffisance cardiaque. La nifédipine est efficace dans l’hypertension sévère et peut être utilisée par voie sublinguale; il faut faire preuve de prudence en raison du risque d’hypotension excessive. Les bloqueurs des canaux calciques sont souvent associés aux b-bloquants, aux diurétiques et / ou aux inhibiteurs de l’ECA.
Inhibiteurs de l’enzyme de conversion de l’angiotensine:
Les inhibiteurs de l’ECA sont de plus en plus utilisés comme traitement de première intention. Ils ont relativement peu d’effets secondaires et de contre-indications, à l’exception des sténoses bilatérales de l’artère rénale. Bien que les inhibiteurs de l’ECA soient efficaces dans l’hypertension rénovasculaire unilatérale, il existe un risque d’atrophie ischémique. Par conséquent, l’angioplastie ou la reconstruction chirurgicale de l’artère rénale sont préférables à une thérapie purement médicale à long terme. Les inhibiteurs de l’ECA sont des agents de premier choix chez les patients hypertendus diabétiques car ils ralentissent la progression de la dysfonction rénale. Dans l’hypertension avec insuffisance cardiaque, les inhibiteurs de l’ECA sont également des médicaments de premier choix. L’essai HOPE a montré que le ramipril réduisait le risque d’événements cardiovasculaires même en l’absence d’hypertension. Ainsi, cet inhibiteur de l’ECA peut exercer un effet protecteur par des mécanismes autres que la réduction de la pression artérielle. Bloqueurs des récepteurs de l’angiotensine II Comme l’angiotensine II stimule les récepteurs AT1 qui provoquent la vasoconstriction, les antagonistes des récepteurs de l’angiotensine AT1 sont des antihypertenseurs efficaces. Le losartan, le valsartan et le candésartan sont efficaces et provoquent moins de toux que les inhibiteurs de l’ECA. L’étude LIFE est le plus récent essai historique sur l’hypertension. Plus de 9000 patients ont été randomisés pour recevoir soit le losartan, un antagoniste des récepteurs de l’angiotensine, soit un b-bloquant
(aténolol). Les patients du bras losartan ont présenté une meilleure réduction de la mortalité et de la morbidité, en raison d’une plus grande réduction des AVC. Le losartan a également été plus efficace pour réduire l’hypertrophie ventriculaire gauche, un puissant facteur de risque indépendant d’effets indésirables. Chez les patients souffrant d’hypertension systolique isolée, la supériorité du losartan sur l’aténolol était encore plus prononcée que chez ceux souffrant d’hypertension systolique et diastolique. Ces résultats favorables ont conduit à un éditorial intitulé: «Blocus de l’angiotensine dans l’hypertension: une promesse tenue». Il convient de noter que le comparateur de l’étude LIFE était un b-bloquant et que, dans le passé, les b-bloquants n’étaient pas meilleurs que le placebo chez les personnes âgées.
Bloqueurs a1-adrénergiques Exempts d’effets secondaires métaboliques, ces médicaments réduisent le cholestérol sanguin et la résistance vasculaire périphérique. La prazosine a une action plus courte que la doxazosine, l’indoramine et la térazosine. Ces médicaments sont hautement sélectifs pour les récepteurs adrénergiques a1. La somnolence, l’hypotension orthostatique et parfois la tachycardie peuvent être gênantes. La rétention d’eau peut nécessiter l’ajout d’un diurétique. La phénoxybenzamine est un agoniste des récepteurs a-adrénergiques non compétitif utilisé (en association avec un b-bloquant) dans la prise en charge des patients atteints de phaéochromocytome, bien que récemment la doxazosine ait été utilisée avec succès. Vasodilatateurs directs:
L’hydralazine et le minoxidil sont des vasodilatateurs à action directe. Leur utilisation a diminué en raison du potentiel d’effets secondaires graves (syndrome du lupus avec l’hydralazine, hirsutisme avec le minoxidil). Inhibiteurs adrénergiques centraux La méthyldopa est à la fois un faux neurotransmetteur et un agoniste des récepteurs adrénergiques a2. La clonidine et la dexmédétomidine sont des agonistes des récepteurs a2-adrénergiques situés au centre. La sélectivité pour les adrénorécepteurs a2 vs a1 est la plus élevée pour la dexmédétomidine (1620: 1), suivie par la clonidine (220: 1), et la moins pour l’a-méthyldopa (10: 1). La clonidine et la dexmédétomidine rendent la circulation plus stable,
réduire la libération de catécholamines en réponse au stress, et
provoquer une sédation telle que la dexmédétomidine est maintenant utilisée pour la sédation dans les unités de soins intensifs.
La moxonidine est représentative d’une nouvelle classe d’agents antihypertenseurs agissant sur les récepteurs de l’imidazoline1 (I1). La moxonidine réduit l’activité sympathique en agissant sur les centres de la rostrale ventrale
médullaire latérale, réduisant ainsi la résistance vasculaire périphérique.
Peptides natriurétiques:
Les peptides natriurétiques jouent un rôle dans le contrôle du tonus vasculaire et interagissent avec le système rénine-angiotensine-aldostérone. En inhibant leur dégradation, les inhibiteurs de la peptidase rendent ces peptides naturels plus efficaces, réduisant ainsi la résistance vasculaire. Cependant, il n’y a que des essais à petite échelle de leur efficacité. Dans l’ensemble, des études récentes n’ont pas réussi à démontrer la supériorité des agents modernes sur les médicaments plus traditionnels, sauf dans des circonstances particulières, comme l’a démontré une méta-analyse basée sur 15 essais et 75 000 patients. Chez de nombreux patients, un traitement efficace est obtenu par l’association de deux agents ou plus, avec un gain d’efficacité et une réduction des effets secondaires.
Gestion des risques
En plus des mesures pharmacologiques pour le contrôle de la pression artérielle, il devrait y avoir un traitement actif des facteurs connus pour augmenter le risque d’hypertension. Il existe deux mesures distinctes. Premièrement, ceux qui abaissent la tension artérielle, par exemple la réduction de poids, la réduction de la consommation de sel, la limitation de la consommation d’alcool, l’exercice physique, l’augmentation de la consommation de fruits et légumes et la réduction de la consommation totale et de graisses saturées. Deuxièmement, ceux qui réduisent le risque cardiovasculaire, par exemple arrêter de fumer; remplacer les graisses saturées par des graisses polyinsaturées et monoinsaturées; augmentation de la consommation de poisson gras; et réduit l’apport total en graisses. Étant donné que les patients hypertendus courent un risque très élevé de maladie coronarienne, d’autres mesures thérapeutiques comprennent les thérapies par l’aspirine et les statines. L’aspirine à faible dose est efficace dans la prévention des événements thrombotiques tels que les accidents vasculaires cérébraux et l’infarctus du myocarde;
cela est également vrai chez les patients hypertendus dont la pression artérielle est bien contrôlée. Le risque de saignement sévère est très faible à condition que la pression artérielle soit réduite à moins de 150 / 90mmHg. Les avantages du traitement médicamenteux hypolipidémiant avec des statines sont bien établis dans les maladies coronariennes et les maladies cérébrovasculaires, deux conditions fréquemment associées à l’hypertension artérielle.

Références clés
Cain AE, Khalil RA. Physiopathologie de l’hypertension essentielle: rôle de la pompe, du vaisseau et du rein. Semin Nephrol 2002; 22: 3-16

Franklin SS, Khan SA, Wong ND, Larson MG, Levy D. La pression du pouls est-elle utile pour prédire le risque de maladie coronarienne? L’étude cardiaque de

Framingham. Circulation 1999; 100: 354–60

Hansson L, Zanchetti A, Carruthers SG, et al. Effets de la pression artérielle intensive diminution et faible dose d’aspirine chez les patients souffrant d’hypertension: principaux résultats de l’essai randomisé sur le traitement optimal de l’hypertension (HOT). Groupe d’étude HOT. Lancet 1998; 351: 1755–62

Haynes WG, Webb DJ. L’endothéline en tant que régulateur de la fonction cardiovasculaire dans la santé et la maladie. J Hypertension 1998; 16: 1081–98

Howell SJ, Hemming AE, Allman KG, Glover L, Sear JW, Foe¨x P. Prédicteurs de l’ischémie myocardique postopératoire. Le rôle de l’hypertension artérielle intercurrente et d’autres facteurs de risque cardiovasculaire. Anesthésie 1997; 52: 107-11
Prys-Roberts C. Phaeochromocytoma — progrès récents dans sa gestion. Br J Anaesth 2000; 85: 44 57

Weinberger MH. Sensibilité au sel de la pression artérielle chez l’homme. Hypertension 1996; 27: 481–90

Williams B, Poulter NR, Brown MJ. Directive de la British Hypertension Society pour la gestion de l’hypertension. Br J Med 2004; 328: 634–40

Yusuf S, Sleight P, Pogue J, Bosch J, Davies R, Dagenais G. Effets d’un inhibiteur de l’enzyme de conversion de l’angiotensine, le ramipril, sur les événements cardiovasculaires chez les patients à haut risque. Les chercheurs de l’étude d’évaluation de la prévention des résultats cardiaques. New Engl J Med 2000; 342: 145–53

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«MacLean, says that three brains operate like “three interconnected biological computers, [each] with its own special intelligence, its own subjectivity, its own sense of time and space and its own memory”. He refers to these three brains as the neocortex or neo-mammalian brain, the limbic or paleo-mammalian system, and the reptilian brain, the brainstem and cerebellum. Each of the three brains is connected by nerves to the other two, but each seems to operate as its own brain system with distinct capacities. This hypothesis has become a very influential paradigm, which has forced a rethink of how the brain functions. It had previously been assumed that the highest level of the brain, the neocortex, dominates the other, lower levels. MacLean has shown that this is not the case, and that the physically lower limbic system, which rules emotions, can hijack the higher mental functions when it needs to.
Reptilian brain:
This brain controls all reflexes, which are automatic and purely regulatory: body temperature, hormonal secretion, blood glucose level, blood pressure, spontaneous respiration…
Ondin’s curse ( lesion or abnormality to the midbrain region: Ondine’s curse—more appropriately known as congenital central hypoventilation syndrome or CCHS—is a rare, severe form of sleep apnea in which an individual completely stops breathing when falling asleep. It is usually congenital, meaning that it is present from birth. It may be noted in the neonatal unit after delivery.  Central sleep apnea is characterized by the brainstem failing to prompt normal breathing. This seems to be due to a decreased responsiveness to high levels of carbon dioxide and low oxygen levels within the blood. This becomes especially dangerous during sleep.
Ondine’s curse is named after a mythical tale in which a heartbroken water nymph curses her unfaithful husband to stop breathing should he ever fall asleep. In medical terms, Ondine’s curse represents an extreme form of sleep apnea.
Limbic system (paleo mammalian brain):
Anatomy:

        Fig 1: in this mid sagittal dissection the hypothalamus and thalamus are                                         delineated by the hypo thalamic sulcus
Anteriorly the hypothalamus extends to the anterior commissure and the optic chiasm. Inferiorly it includes the mammillary bodies and extends to the infundibular stuck where it communicates with the pituitary gland. The hypothalamus is structurally part of the diencephalon but it functions as part of the limbic system through the reciprocal connections. It helps to maintain homeostasis in the entire body through influences on the endocrine system and importantly through its primary influence on both the sympathetic and parasympathetic systems.
-The limbic system is extremely old from an evolutionary perspective and in its connections; it is interposed between the hypothalamus and the neocortex. Limbic lob is not a true lobe rather it spans the frontal, parietal and temporal lobes, it comprises a ring of cortex in the in the medial surface in the brain (the cingulate gyrus and the para hippocampal gyrus)

Fig 2 : cingulate gyrus ( left) Parahippocampale gyrus(right)

-The Hippocampus: is primarily involved in memory and it lies in the inferior horn of … And from its posterior ends fibers emerge to form the fornix which swings over the thalamus to rich the mammillary bodies of the hypothalamus by column of the fornix

Fig 3 : Hippocampus 
– The mammillary bodies are primarily responsible for emotional processing.

    Fig 4 : connection between fornix and mammillary bodies

-The mammillo-thalamic tract connects the mammillary bodies with the anterior nucleus and dorso-medial nucleus of the thalamus and from the thalamus the information travels to the limbic lob (this is the classic papez circuit involved in learning, memory and emotions).
We now know that other structures are involved this circuit including (amygdala…
The amygdala is located in the roof of the inferior horn of lateral ventricle directly underneath the uncus, it lies superior and anterior to the hippocampus (delineated by green dashed cycle)

Fig 5 Amygdala (green dashed)
The structure delineated by red is the uncus.The amygdala is a key structure in the expression of emotions, emotional memory and basic drives.

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1) The Hypothalamus: which is regarded as the highest control system over the autonomic motor neurons that affect the activity of our visceral organs so the hypothalamus is the center of homeostasis (continuously and unconsciously adjusting the activity of our visceral effectors to match the person’s physical activity and energy requirements).

For example, the heart rate will be adjusted when we are sleeping and or when we are running and this will be done automatically or the heart rate will increase if we are frightened. We can see that hypothalamus is influenced by our emotions that means the hypothalamus is connected to our limbic system. The hypo thalamus is in the area of the brain right above the pituitary gland and we can see that the hypothalamus is connected to the limbic system which is associated with emotions and there are various descending tracts that effects autonomic motor neurons.

Branching of the brain and spinal cord these autonomic motor neurons innervate various internal organs.

There are two types of autonomic motor neurons that innervate each visceral organ of the body (Dual innervation):

1) the parasympathetic autonomic motor neurons that exerts the predominant influence on the visceral organs during the state of relaxation (low energy requirement): rest and digest

2) the sympathetic autonomic motor neurons that exerts the predominant influence on the visceral organs during states of stress (high energy requirement): fight, fright, flight

                       General patterns of autonomic outflow from the C.N.S

1. A) Parasympathetic motor neurons:

The parasympathetic motor neurons are present only in certain cranial nerves and in the sacral spinal nerves: (cranio-sacral)

Cranial nerves III (oculomotor), VII ( Facial), IX ( Glossopharyngeal), X (Vagus) and sacral S2, S3, S4 contains parasympathetic motor neurons.

Some of the cranial nerves contain parasympathetic motor neurons and some of the spinal nerves coming out of the sacral level of the spinal cord contains parasympathetic motor neurons. ………

1. B) Sympathetic (thoraco-lumbar) division of the Autonomic nervous system:

The only nerves that contain inside them sympathetic motor neurons are those spinal nerves coming of at thoracic and lumbar levels. That’s why the sympathetic nervous system is also known as thoraco-lumbar division. So, there is clear difference anatomically.

Now what’s interesting is that there are many sympathetic motor neurons that innervate all internal organs instead there is only vagal nerve for parasympathetic nervous system. In other words, all internal organs are innervated by two types of autonomic motor neurons: Dual innervation

“somatic muscles have nicotinic acetylcholine receptors and are totally dependent of their innervation: they are called neurogenic”

To study the ANS we will take the heart as a simple model (but it could be any other organ)

We know that sympathetic as well as parasympathetic innervation takes two motor neurons to go from the C.N.S to the internal organ. The first neuron is myelinated and the second one is not myelinated (it is the same for Para and sympathetic)

                  dual innervation of the heart

The acetylcholine is liberated by parasympathetic nerve at the heart (! Acetylcholine receptor at the heart is muscarinic and not nicotinic like the somatic muscles)

Acetylcholine is also released from preganglionic fiber of sympathetic motor neuron as in the preganglionic motor neurons of parasympathetic system but at the level of target organ there is  epinephrine release ( we say that post ganglionic fiber is adrenergic)

So in the heart there are two types of receptors ( adrenergic receptors and acetycholine muscarinic receptors)

Somatic muscles havent nicotinic receptors of acetyl choline and if their motor neurons are cut the muscle will be paralysed that means they are totally dependant of the motor neuron ( they are neurogenic) .

Here the dual innervation of the heart is  discussed (schematized) but this system can be applied to all of the internal organs of the body .

The adrenergic receptors on the heart are B1 and

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Keilin (1925) was studying the mosquitoes responsible of the malaria. With a spectromicroscope he looked at the legs of one mosquito and saw that they change of colour. At rest the muscles are oxidised but when they are excited the muscles consume the oxygen and are thus less oxidised. The colour is given by the cytochrome, a molecule that changes of colour in function of the oxidation.

This molecule belongs to the mitochondrion. The mitochondrion is a kind of bacteria hosted by eukaryotes.

They have their own membrane, composed of an inner membrane and of a porous outer membrane. The matrix is inside the inner membrane and the intermembrane space is also called cristae where the membrane is protruding. Similarly to the haemoglobin, the cytochrome is a protein that possesses a haem the iron of which can change of oxidation state. This property is used in the respiratory chain. Cytochrome belongs to the inner membrane of the mitochondrion along which it can move and it separate the glycolysis from the citric acid cycle. The cycle of the citric acid takes place in the matrix of the mitochondrion while glycolysis takes place outside of the mitochondrion.

Respiratory chain

The respiratory chain can be summarised by the following figure and table:

 In the inner membrane we find 4 complexes – NADH-ubiquinone reductase, succinate-ubiquinone reductase, ubiquinol-cytochrome c reductase and cytochrome oxidase – that make redox reactions and transfer electrons from one side of the membrane to the other side (sometimes protons instead of electrons). One reaction at one side of the membrane (matrix or intermembrane space) is always coupled with one reaction at the other side of the membrane. One reaction of reduction is coupled with a reduction of oxidation and vice-versa. The whole phenomenon involves transfers of electrons inside the membrane and transfers of protons from one side of the membrane to the other side. Note that some complexes belong to the membrane but only face one of its sides.

Q is the ubiquinone, a cofactor that belongs to the membrane. It interacts with the cytochrome c in a loop of oxidoreduction. One can see that the oxygen is involved by the complex IV. To resume the loop, we can write two reactions of oxidoreduction, one with the NAD and one with the FAD.

However O₂ cannot directly interact with FADH₂ and NADH+H⁺ and this loop is thus necessary. For redox, the free energy of Gibbs is

To determine the difference of potential ∆E°, we look at the couples involved in the reaction:

For FAD we have 43.3kcal/mol. There is a transfer of protons involved in the process but there must be an equilibrium between the concentrations of charge inside the cell and outside the cell. The protons can move out of the matrix of the mitochondrion through some protein complexes that generate ATP from the flow of protons (bottom of the figure). The F₁-ATPase protein is composed of two parts, F₀ in the membrane and F₁ in the matrix of the mitochondrion. F₀ pumps the protons from the intermembrane space towards the matrix. F₁ receives the energy from the transport and transforms ADP into ATP. Those species are charged negatively and require a transport through the membrane (top of the figure). One ADP can go in the matrix only if one ATP is moving out of the matrix. The inorganic phosphates P_i are also transported through the membrane.

The energy to form the ATP comes from the gradient of pH. It generates a proton motive force.

Where the indexes mat and ext respectively matrix and its exterior (the intermembrane space). The gradient of pH generates a difference of electric potential ΔV.

Combining the two effects, we have

Dividing this expression by nF, we obtain the expression of a variation of free energy ∆p associated to the transfer of one mole of protons (in volt).

The coefficient 2.303RT/nF of the gradient of pH has a value of 59mV at 25°C.

We can measure the pH and the difference of potential: ∆V=0.14V and ∆pH=-1.4.

It corresponds to a variation of free energy equal to

To form one mole of ATP, we need 7.3kcal. In the respiratory chain, there are 4 complexes that oxidise the NADH and the FAD.  They respectively form the equivalent of 11 and 8 protons by loop.

We can write a global equation of the “combustion” of a pyruvate during the citric acid cycle and the respiratory chain:To summarise, the oxidation of one NADH generates ~3 ATP and the oxidation of FAD generates ~2ATP.

The 15 ATP come from

• 4 NADH (one before the citric acid cycle and 3 during the cycle) à12ATP,
• from 1 FADH₂ à 2ATP
• and from 1 GTP»1 ATP.

To form one pyruvate, glycolysis forms 2 ATP and uses 2 NAD that are reduced into NADH+H⁺.

So considering 3 ATP by NADH, it corresponds to the production of 8 ATP. Added to the 15 ATP of each pyruvate (x2), we obtain 38ATP by glucose. In comparison to the combustion of the glucose (∆G°’=-686kcal/mol), it represents about 40% of the potential energy (38×7.3kcal/mol=277kcal/mol).

The NAD involved in the glycolysis and that produces a NADH is not at the same place (cytosol) that the NADH required in the cellular respiration (matrix of the mitochondrion). The NADH cannot enter freely into the mitochondrion because it is charged. There is a system of shuttle that may differ for different cells and that will modify the NADH to allow its passage through the membrane, then modify it again so it can be used there. In the brain and in muscles, it is a shuttle of glycophosphate that we will next explain. The shuttle is basically composed of two reactions: one in the cytosol and one in the matrix of the mitochondrion:

The shuttle can be represented like this:

If you remember, dihydroxyacetone-3P was part of the glycolysis (red rectangle in the following figure): it is one of the two trioses phosphate produced from fructose -1,6-diphosphate. The fact that this triose is involved in the shuttle does not really decrease the yield of the glycolysis significantly: the cell needs a given quantity of dihydroxyacetone for the shuttle but the molecules are regenerated in the matrix of mitochondria and can then return to the cytosol. So they just have to be produced once. The rest of the production is turned into glyceraldehyde 3P for the glycolysis.

The FADH₂ is oxidised during the respiration and the dihydroxyacetone can move back to the cytosol. The shuttle has a small cost: it oxidises one NADH into NAD in the cytosol while the reaction in the matrix involves FAD/FADH₂. In term of energy, it represents one ATP used because all the processes don’t take place at the same place.

The malate-aspartate shuttle

The same cofactors (NADH+H⁺/NAD⁺) are used so the yield does not change with this shuttle (38 ATP). The mechanism is based on the reducing power of the oxaloacetate.

Reduced into malate (1), it is transported in the matrix of the mitochondrion (2) through the malate-α-ketoglutarate transporter if one α-ketoglutarate is available to make the displacement in the opposite direction. The reverse reaction, the oxidation of the malate into the oxaloacetate is made in the matrix (3). Yet, the oxaloacetate cannot goes back out of the membrane. To do so, it is transformed into an amino acid, the aspartate, by a reaction of transamination (4):

It is done here by the aspartate transaminase with the use of the glutamate as amino acid. As a result, the oxaloacetate is transformed into aspartate that can move through the membrane through a transporter requiring the passage in the opposite direction of a glutamate (5). In the cytosol, it is turned back into the oxaloacetate with the exact same reaction (6) than at the other side of the membrane.

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The cycle of Krebs: cycle of the citric acid

Szent Fuorgue studied the cellular respiration from an extract of muscle. He analysed the absorption of O₂ in presence of various molecules. He observed that some of them considerably increase the absorption of oxygen by the tissues. It is the case for the succinate and the fumarate.

These molecule are stable to variations of temperature. The combustion of one succinate normally consumes 3 O₂ but during the experiments they observed that the absorption of oxygen was above this value. They also showed that the absorption can be decreased if we add malate to the system. The malate is an inhibitor that affects the oxaloacetate, one catalyst of the cycle of Kerbs that is regenerated at each cycle except in presence of malate.

The cycle can be resumed by the following figure.

Citrate is the product of the first reaction of the cycle. Before explanations on the cycle, we will explain the formation of the acetyl CoA. It is made from pyruvate. The first step is the substitution of one CO₂ by the CoASH through a thioester liaison.

The reaction is irreversible (release of CO₂) and is catalysed by a big enzyme: the pyruvate dehydrogenase. Inside the enzyme we find the B₁ vitamin. Simultaneously, there is an oxidation made by the NAD⁺. The product is the acetyl coenzyme A, or acetyl-coA.

The first step forms the citrate by the condensation between the acetyl-coA and the oxaloacetate. The reaction is helped by the citrate synthase that take a proton from the methyl group of the acetylCoA. It releases the coenzyme that can be used again.

The citrate is symmetrical but it is considered as prochiral by the enzyme of the next reaction. To pursue to cycle, the aconitase changes the conformation of the citrate to obtain the isocitrate.

The equilibrium is heavily in favour of the left but the right species is consumed by the next reactions so the reaction is displaced towards the right. Step 3 is subdivided into one reaction of oxidation and one decarboxylation.

The step 4 is similar to the formation of the acetyl-CoA and leads to the formation of the succinyl-coA.

The cleavage of the thioester liaison gives enough energy to form, not an ATP but a GTP.

The succinate is next oxidised to for the fumarate. The cofactor is the FAD: Flavin adenine dinucleotide, a derivate of the riboflavin (vitamin B2).

The reduction of the FAD is done at the orange area.

Step 7 needs one molecule of water to form the malate. Malate is a chiral molecule, the chirality given by the structure of the active site of the enzyme. As a result, only the l-malate is obtained.

Finally, the oxaloacetate is regenerated from the malate.

Now, you may say that there is none of the 38 ATP promised at the beginning of the section. Only one GTP was formed during the cycle but several CO₂ were rejected and 3 NAD⁺ and one FAD were reduced. These molecules represent the energy of the ATP. Moreover, it is the process that occurs in aerobic cells but no O₂ was ever involved in the cycle. We will see the source of the ATP in the next section: the respiratory chain.

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To facilitate the study of genes, they must be isolated and amplified. One method of isolation and amplification of a gene of interest is to clone the gene by inserting it into another DNA molecule that serves as a vehicle or vector that can be replicated in living cells. When these two DNAs of different origin are combined, the result is a recombinant DNA molecule. The recombinant DNA molecule is placed in a host cell, either prokaryotic or eukaryotic. The host cell then replicates (producing a clone), and the vector with its foreign piece of DNA also replicates. The foreign DNA thus becomes amplified in number, and following its amplification can be purified for further analysis.

In 1962, Allan Campbell noted that the linear genome of bacteriophage λ forms a circle upon entering the host bacterial cell, and a recombination (breaking and rejoining) event inserts the phage DNA into the host chromosome. Reversal of the recombination event leads to normal excision of the phage DNA. Further analysis revealed that phage λ had short regions of single-stranded DNA whose base sequences were complementary to each other at each end of its linear genome. These single-stranded regions were called “cohesive” (cos) sites. Complementary base pairing of the cos sites allowed the linear genome to become a circle within the host bacterium. The idea of joining DNA segments by “cohesive sites” became the guiding principle for the development of genetic engineering. With the molecular characterization of restriction and modification systems in bacteria, it soon became apparent that the ideal engineering tools for making cohesive sites on specific DNA pieces were already available in the form of restriction endonucleases.

Early on, Salvador Luria and other phage workers were intrigued by a phenomenon termed “restriction and modification.” Phages grown in one bacterial host often failed to grow in different bacterial strains (“restriction”). However, some rare progeny phages were able to escape this restriction. Once produced in the restrictive host they had become “modified” in some way so that they now grew normally in this host. The entire cycle could be repeated, indicating that the modification was not an irreversible change. For example, phage λ grown on the C strain of Escherichia coli (λ·C) were restricted in the K-12 strain (the standard strain for most molecular work). However, the rare phage λ that managed to grow in the K-12 strain now had “K” modification (λ·K). These phages grew normally on both C and K-12; however, after growth on C, the phage λ with “C” modification (λ·C) was again restricted in K-12. Thus, the K-12 strain was able to mark its own resident DNA for preservation, but could eliminate invading DNA from another distantly related strain. In 1962, the molecular basis of restriction and modification was defined by Werner Arber and co-workers.

Restriction system :

After demonstrating that phage λ DNA was degraded in a restricting host bacterium, Arber and co-workers hypothesized that the restrictive agent was a nuclease with the ability to distinguish whether DNA was resident or foreign. Six years later, such a nuclease was biochemically characterized in E. coli K-12 by Matt.

Meselson and Bob Yuan. The purified enzyme cleaved λ·C-modified DNA into about five pieces but did not attack λ·K-modified DNA. Restriction endonucleases (also referred to simply as restriction enzymes) thus received their name because they restrict or prevent viral infection by degrading the invading nucleic acid.

Modification system

At the time, it was known that methyl groups were added to bacterial DNA at a limited number of sites. Most importantly, the location of methyl groups varied among bacterial species. Arber and colleagues were able to demonstrate that modification consisted of the addition of methyl groups to protect those sites in DNA sensitive to attack by a restriction endonuclease. In E. coli, adenine methylation (6-methyl adenine) is more common than cytosine methylation (5-methyl cytosine)

Methyl-modified target sites are no longer recognized by restriction endonucleases and the DNA is no longer degraded. Once established, methylation patterns are maintained during replication. When resident DNA replicates, the old strand remains methylated and the new strand is unmethylated. In this hemimethylated state, the new strand is quickly methylated by specific methylases. In contrast, foreign DNA that is unmethylated or has a different pattern of methylation than the host cell DNA is degraded by restriction endonucleases.

The first cloning experiments

Hamilton Smith and co-workers demonstrated unequivocally that restriction endonucleases cleave a specific DNA sequence. Later, Daniel Nathans used restriction endonucleases to map the simian virus 40 (SV40) genome and to locate the origin of replication. These major breakthroughs underscored the great potential of restriction endonucleases for DNA work. Building on their discoveries, the cloning experiments of Herbert Boyer, Stanley Cohen, Paul Berg, and their colleagues in the early 1970s ushered in the era of recombinant DNA technology. One of the first recombinant DNA molecules to be engineered was a hybrid of phage λ and the SV40 mammalian DNA virus genome. In 1974 the first eukaryotic gene was cloned. Amplified ribosomal RNA (rRNA) genes or “ribosomal DNA” (rDNA) from the South African clawed frog Xenopus laevis were digested with a restriction endonuclease and linked to a bacterial plasmid. Amplified rDNA was used as the source of eukaryotic DNA since it was well characterized at the time and could be isolated in quantity by CsCl-gradient centrifugation. Within oocytes of the frog, rDNA is selectively amplified by a rolling circle mechanism from an extrachromosomal nucleolar circle . The number of rRNA genes in the oocyte is about 100- to 1000-fold greater than within somatic cells of the same organism. To the great excitement of the scientific community, the cloned frog genes were actively transcribed into rRNA in E. coli. This showed that recombinant plasmids containing both eukaryotic and prokaryotic DNA replicate stably in E. coli. Thus, genetic engineering could produce new combinations of genes that had never appeared in the natural environment, a feat which led to widespread concern about the safety of recombinant DNA work

Cutting and joining DNA

Two major categories of enzymes are important tools in the isolation of DNA and the preparation of recombinant DNA: restriction endonucleases and DNA ligases. Restriction endonucleases recognize a specific, short, nucleotide sequence on a double-stranded DNA molecule, called a restriction site, and cleave the DNA at this recognition site, depending on the type of enzyme. DNA ligase joins two pieces of DNA by forming phosphodiester bonds.

Major classes of restriction endonucleases

There are three major classes of restriction endonucleases. Their grouping is based on the types of sequences recognized, the nature of the cut made in the DNA, and the enzyme structure:

– Type I and III restriction endonucleases are not generally  used for gene cloning because they cleave DNA at sites other than the recognition sites and thus cause random cleavage patterns.

– type II endonucleases are widely used for mapping and reconstructing DNA in vitro because they recognize specific sites and cleave just at these sites . In addition, the type II endonuclease and methylase activities are usually separate, single subunit enzymes. Although the two enzymes recognize the same target sequence, they can be purified separately from each other

Nomenclature :

Restriction endonucleases are named for the organism in which they were discovered, using a system of letters and numbers. For example, HindIII  was discovered in Haemophilus influenza strain d and III is for the third enzayme of that type. Sma I is from serratia marcescens ,  Eco RI is Escherichia coli( strain R) and BamHI is from Bacillus amyloliquefaciens ( strain H ). Over 3000 type II esrtriction endonucleases have been isolated and characterised to date.

Recognition sequences for type II restriction endonucleases :

Each orthodox type II restriction endonuclease is composed of two identical polypeptide subunits that join together to form a homodimer. These homodimers recognize short symmetric DNA sequences of 4–8 bp. Six base pair cutters are the most commonly used in molecular biology research. Usually, the sequence read in the 5′ → 3′ direction on one strand is the same as the sequence read in the 5′ → 3′ direction on the complementary strand. Sequences that read the same in both directions are called palindromes . Figure 8.3 shows some common restriction endonucleases and their recognition sequences. Some enzymes, such as EcoR1, generate a staggered cut, in which the single-stranded complementary tails are called “sticky” or cohesive ends because they can hydrogen bond to the singlestranded complementary tails of other DNA fragments. If DNA molecules from different sources share the same palindromic recognition sites, both will contain complementary sticky ends (single-stranded tails) when digested with the same restriction endonuclease. Other type II enzymes, such as SmaI, cut both strands of the DNA at the same position and generate blunt ends with no unpaired nucleotides when they cleave the DNA.

Restriction endonucleases exhibit a much greater degree of sequence specificity in the enzymatic reaction than is exhibited in the binding of regulatory proteins, such as the Lac repressor to DNA . For example, a single base pair change in a critical operator sequence usually reduces the affinity of the Lac repressor by 10- to 100-fold, whereas a single base pair change in the recognition site of a restriction endonuclease essentially eliminates all enzymatic activity.

Like other DNA-binding proteins, the first contact of a restriction endonuclease with DNA is nonspecific. Nonspecific binding usually does not involve interactions with the bases but only with the DNA sugar–phosphate backbone. The restriction endonuclease is loosely bound and its catalytic center is kept at a safe distance from the phosphodiester backbone. Nonspecific binding is a prerequisite for efficient target site location. For example, BamHI moves along the DNA in a linear fashion by a process called “sliding.” Sliding involves helical movement due to tracking along a groove of the DNA over short distances (< 30–50 bp). This reduces the volume of space through which the protein needs to search to one dimension. However, the “random walk” nature of linear diffusion gives equal probabilities for forward and reverse steps, so if the distances between the nonspecific binding site and the recognition site are large (> 30–50 bp), the protein

would return repeatedly to its start point. The main mode of translocation over long distances is thus by “hopping” or “jumping.” In this process, the protein moves between binding sites through three-dimensional space, by dissociating from its initial site before reassociating elsewhere in the same DNA chain. Because of relatively small diffusion constants of proteins, most rebinding events will be short range “hops” back to or near the initial binding site. In the example of BamHI, once the target restriction site is located, the recognition process triggers large conformational changes of the enzyme and the DNA (called coupling), which leads to the activation of the catalytic center. In addition to indirect interaction with the DNA backbone, specific binding is characterized by direct interaction of the enzyme with the nitrogenous bases.

All structures of orthodox type II restriction endonucleases characterized by X-ray crystallography so far show a common structural core composed of four conserved β-strands and one α-helix . In the presence of the essential cofactor Mg²⁺, the enzyme cleaves the DNA on both strands at the same time within or in close proximity to the recognition sequence (restriction site). The enzyme cuts the duplex by breaking the covalent, phosphodiester bond between the phosphate of one nucleotide and the sugar of an adjacent nucleotide, to give free 5′-phosphate and 3′-OH ends. Type II restriction endonucleases do not require ATP hydrolysis for their nucleolytic activity. Although there are a number of models for how this nucleophilic attack on the phosphodiester bond occurs, the exact mechanism by which restriction endonucleases achieve DNA cleavage has not yet been proven experimentally for any type II restriction endonuclease.

DNA ligase :

The study of DNA replication and repair processes led to the discovery of the DNA-joining enzyme called DNA ligase. DNA ligases catalyze formation of a phosphodiester bond between the 5′-phosphate of a nucleotide on one fragment of DNA and the 3′-hydroxyl of another . This joining of linear DNA fragments together with covalent bonds is called ligation. Unlike the type II restriction endonucleases, DNA ligase requires ATP as a cofactor.

Because it can join two pieces of DNA, DNA ligase became a key enzyme in genetic engineering. If restriction-digested fragments of DNA are placed together under appropriate conditions, the DNA fragments from two sources can anneal to form recombinant molecules by hydrogen bonding between the complementary base pairs of the sticky ends. However, the two strands are not covalently bonded by phosphodiester bonds. DNA ligase is required to seal the gaps, covalently bonding the two strands and regenerating a circular molecule. The DNA ligase most widely used in the lab is derived from the bacteriophage T4. T4 DNA ligase will also ligate fragments with blunt ends, but the reaction is less efficient and higher concentrations of the enzyme are usually required in vitro. To increase the efficiency of the reaction, researchers often use the enyzme terminal deoxynucleotidyl transferase to modify the blunt ends. For example, if a single-stranded poly(dA) tail is added to DNA fragments from one source, and a singlestranded poly(dT) tail is added to DNA from another source, the complementary tails can hydrogen bond. Recombinant DNA molecules can then be created by ligation.

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The first step is the phosphorylation of the glucose. The phosphate is added on 6’. For this reaction, as for the others of the glycolysis, an enzyme (the hexokinase/glucokinase) to form the ester and to cleave the anhydride of the ATP.

The second reaction involves the transformation of a pyranose into a furanose.

The third step is dependant of the needs of the cell in energy.

When the cell absolutely needs energy, its consume ADP and forms AMP. If the concentration of AMP in the cell increases then a signal is sent to the phosphofructokinase to increase the rate of the reaction. On the contrary, if the cell is full of ATP, then there is no need to make more of them and the reaction is slowed.

In the next step, the fructose is cleaved into two chains of 3 carbons, an aldose and a ketone that can be transformed into each other by the triose phosphate isomerase.

Because the reaction 5 is reversible, if the C₁ of the glyceraldehyde was marked, we would find the fructose marked on 3’ or in 4’.

The next steps involve chains of 3 carbons. From one glucose, they are thus occurring twice.

The enzyme of the step 6 is the GAPDH (glyceraldehyde 3-phosphate dehydrogenase) and the NAD (nicotinamide adenine dinucleotide) is its cofactor and has to be in the active site of the enzyme for it to work. NAD⁺ is its oxidised form and NADH+H⁺ its reduced form.

The presence of the reduced form can be confirmed by a new peak of absorbance at 340nm. The reduction generates some energy to allow the formation of a phosphoanhydride, what is a liaison high in energy. It will thus generate a lot of energy when it is cleaved.

The seventh step uses the energy of this liaison to regenerate one ATP.

The eighth reaction displaces one phosphate. It is done by an enzyme called mutase.

The ninth step is an intramolecular reaction of oxidation. C₂ is oxidised while C₃ is reduced. We have now a phosphoenol the cleavage of which restores one ATP in the following reaction of tautomerization.

In total, two ATP were used during the glycolysis to form the trioses. These steps are thus using the reserve of energy of the cell. However two reactions form one ATP by triose. As there are two trioses by glucose and the isomerase can turn the ketone into the glyceraldehyde, so we form 4 ATP from the trioses. During the complete glycolysis, 2 ATP are thus formed that can be used by the cell. For anaerobic cells, it is the only way to produce ATP.

The NAD⁺ has to be regenerated by fermentation.

Lactic acid fermentation

The pyruvate, the final product of the glycolysis, is used to oxidise the NADH+H⁺ and form lactate that is next released in the middle.

Ethanol fermentation

Also called alcoholic fermentation, this process also uses the pyruvate but is made in two steps. One first that involves the rejection of CO₂ and the second that oxidises the NADH+H⁺ and frees some ethanol.

Comments on the glycolysis

• In the case of a lack of NAD⁺, or a too large quantity of glucose, more NAD⁺ can be formed from the “unused” triose, the dihydroxyacetone phosphate

The reaction is in equilibrium and is only made in case of need. If the triose was essentially used to do this, the whole process of the glycolysis would be pointless: it would consume 2 ATP to form 2 ATP.

• Coupling of reactions

Some steps of the glycolysis have a positive ∆G⁰’. It is the case of the steps that form rich liaisons. In this way, the step 6 is coupled to the step 7 that cleaves the rich liaison to form an ATP.

If we consider the two reactions at once, i.e. a reaction of oxidation of the aldehyde into a carboxylic acid, we find a ∆G⁰’=-10kcal (7.3kcal are equivalent to one ATP). The two-steps reaction is favoured because we can store the energy as ATP.

The same is true for the steps 8, 9, 10.

The two first steps cost energy but the third is very favourable. In total, we have a ∆G⁰’=-6kcal/mol and the formation of an ATP.

• Uncoupling agent

There are some molecules that can block the formation of the ATP. It is for instance the case of the arsenate ion which is a poison for us. In the step 6 and 7, the ion takes the place of an inorganic phosphate.

We still obtain the product of the step 7 but without the formation of ATP.

• energetic balance

The combustion of one glucose is an exothermic reaction:

The combustion of the products of the glycolysis (two pyruvates) gives

It is about 7% of the energy of one glucose. To that we can add the energy of the phosphoester bond of the 2ATP: 2×7.3kcal/mol.

It is still a very small quantity of energy that was recovered from the glucose. This method was used when there was almost no oxygen available and is used in anaerobic environments.

• Other monosaccharides can make the glycolysis after being transformed by enzymes into glucose or one intermediate of the glycolysis.

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