Evolving concepts in functional gastrointestinal disorders: promising directions for novel pharmaceutical treatments
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In recent years there has been an increasing appreciation of the complexity of functional gastrointestinal disorders. These represent a spectrum of conditions which may affect any part of the gastrointestinal tract in which there appears to be dysregulation of visceral function and afferent sensation and a strong association with emotional factors and stress. There is a clear psychological dimension, with up to 60% of irritable bowel syndrome (IBS) patients reported to have psychological co-morbidities and altered pain perception is also common in comparison with control populations. The role of the enteric nervous system, the sensory pathways and the brain as well as the influence of the latter on sympathetic and parasympathetic outflow have likewise attracted increasing interest and have led to exciting new methods to study their complex interactions. The concept of low-grade inflammation, such as might occur after infection, acting as a trigger for neuromuscular dysfunction has also led to the broad integrative hypotheses that help to explain the biopsychosocial dimensions seen in functional gastrointestinal disease. The multi-component model places a major emphasis on neurogastroenterology and enteric and neuro-immune interactions where new approaches to pharmacotherapy lie. Drugs may affect motility, visceral sensation and other aspects of gut function such as secretion or absorption. More particularly, however, has been the search for and attempts to influence important mediators of these primary gut functions. Such targets include serotonin and selected 5-HT receptors, which are involved in gut motility, visceral sensation and other aspects of gut function, CCK receptors which are involved in the mediation of pain in the gut and nociception in the CNS, opioid receptors involved in pain in the brain, spinal cord and periphery, muscarinic M3-receptors, substance P and neurokinin A and B receptors which are involved in motor adaptation and pain transmission in association with inflammation, gabba receptors involved in nociception and cannabinoid receptors which are involved in the control of acetyl choline release in the gut. With a better understanding of the structures and pathways involved in visceral perception and hyperalgesia, in the CNS, spinal cord and the gut and new pharmacological tools we will be better able to elucidate the neuropharmacology of visceral perception and its relationship to gut dysfunction. It is likely that there will be multiple therapeutic options based on the spectrum of abnormalities capable of causing the spectrum of symptoms of functional gastrointestinal disorders in any individual patient.
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