Endogenous opioids, the enteric nervous system and gut motility.

Opium alkaloids have been used for centuries as potent antidiarrheals and analgesics, their constipating action in the latter instance taken as an unwanted effect. It was only during the last decade that the physiological role of opioid peptides present in both neurons and endocrine cells of the gastrointestinal (GI) tract has been defined. The recognition of distinct opioid receptor types which may be differentially involved in the control of motility, acid and electrolyte secretion in the GI tract presently focuses the attention of researchers in this field on the identification of receptor-type-selective opioid agonists in order to free these clinically extremely useful drugs from side effects. The present review provides a survey of mostly physiological data on the functional role of intestinal opioids.