RESUMEN
The mu-opioid receptor (muOR), which mediates many of the opioid effects in the nervous system, is expressed by enteric neurons. The aims of this study were to determine whether 1) different classes of myenteric neurons in the guinea pig ileum contain muOR immunoreactivity by using double- and triple-labeling immunofluorescence and confocal microscopy, 2) muOR immunoreactivity is localized to enteric neurons immunoreactive for the endogenous opioid enkephalin, and 3) muOR immunoreactivity is localized to interstitial cells of Cajal visualized by c-kit. In the myenteric plexus, 50% of muOR-immunoreactive neurons contained choline acetyltransferase (ChAT) immunoreactivity, whereas about 43% of ChAT-immunoreactive neurons were muOR immunoreactive. Approximately 46% of muOR myenteric neurons were immunoreactive for vasoactive intestinal polypeptide (VIP), and about 31% were immunoreactive for nitric oxide synthase (NOS). MuOR immunoreactivity was found in about 68% of VIP-containing neurons and 60% of NOS-immunoreactive neurons. Triple labeling showed that about 32% of muOR neurons contained VIP and ChAT immunoreactivities. The endogenous opioid enkephalin (ENK) was observed in about 30% of muOR neurons; conversely, 48% of ENK neurons contained muOR immunoreactivity. MuOR was not detected in neurons containing calbindin, nor in interstitial cells of Cajal. MuOR-immunoreactive fibers formed a dense network around interstitial cells of Cajal in the deep muscular plexus. This study demonstrates that muOR is expressed by neurochemically distinct classes of myenteric neurons that are likely to differ functionally, is colocalized with the endogenous opioid ENK, and is not expressed by interstitial cells of Cajal.