Opiates and the gastrointestinal tract.

Opiate peptides, encephalins and endorphins, originally isolated from the brain, have been detected by the immunocytochemistry and radioimmunoassay in the digestive system and localized in the distinctive endocrine-paracrine cells of APUD series of the gastrointestinal mucosa and the nerve fibers of the myenteric plexus. The physiological role of endogenous opiates is unknown but the spectrum of biological action on the digestive system resembles that of morphine and related drugs. They strongly affect gastrointestinal motility such as gastric and intestinal contractions, gastric emptying, intestinal transit, biliary pressure and defecation. They also increase gastric secretory activity and raise gastric mucosal microcirculation without affecting gastrin release. They inhibit pancreatic bicarbonate and enzyme secretion probably via suppressing the release of intestinal hormones, secretin and cholecystokinin. The actions of opiates on the motility and secretion can be reversed by specific opiate receptor antagonists, e.g. naloxone, indicating that opiate receptors may be involved in these actions.