Autoradiographic localization of mu- and delta-type opioid receptors in the gastrointestinal tract of the rat and guinea pig.

The distribution of delta- and mu-type opioid binding sites in the gastrointestinal tract of the rat and guinea pig was studied by autoradiography after in vitro incubation of tissue slices with 3H-D-Ala2,D-Leu5-enkephalin, and 3H-naloxone or 3H-dihydromorphine to locate delta- and mu-type opioid receptors, respectively. In the gastric fundus, both mu- and delta-type binding sites were found to occur associated with the circular muscle, muscularis mucosae, and submucosal plexus, whereas in the corpus and antrum, binding was located primarily in the submucosal plexus, deep muscular plexus, and mucosa. Some mu-type opioid receptor sites were present in the myenteric plexus. A dense distribution of both mu- and delta-type binding sites was observed throughout the mucosa of the duodenum and ileum of the rat. In guinea pig ileal tissue, however, only mu-type binding could be demonstrated, occurring in the submucosal plexus and diffusely over the muscle layers. Endogenous opioid peptides, acting at these receptors sites, might be involved in the control of gastrointestinal motility, endocrine and exocrine secretions, as well as intestinal fluid and electrolyte transport.