Neurotransmitters in neuronal reflexes regulating intestinal secretion.

The intestinal crypt cell secretes chloride into the lumen, resulting in accumulation of fluid that normally thins out mucus or, at higher secretory rates, flushes out the contents. The regulation of chloride secretion occurs by neural reflex pathways within the enteric nervous system. Mechanical stimulation releases 5-hydroxytryptamine (5-HT) from enterochromaffin cells with subsequent activation of intrinsic primary afferents that carry electrical signals to submucosal ganglia. After processing, interneurons activate cholinergic and vasoactive intestinal peptide (VIP) secretomotor neurons. Acetylcholine and VIP bind to epithelial receptors and stimulate sodium chloride and fluid secretion. Reflex-evoked secretory rates can be modulated by a variety of mediators at the level of the enterochromaffin cells, neurons within the reflex pathway, or epithelial cells. Understanding the complex regulatory mechanisms for chloride secretion is likely to provide mechanistic insights into constipation and diarrhea.