Comparison of opioid receptor distributions in the rat ileum

The cellular expression patterns of mu-, delta- and kappa-opioid receptors in the rat ileum were examined using fluorescence immunohistochemistry. Double-labelling was used to examine cellular receptor co-localisation as a pre-requisite for intracellular molecular interactions, such as heterodimerisation. Tissues were stained as whole-mount preparations. Strong, broadly distributed immunoreactivity (ir) was observed for each receptor in the myenteric and submucous plexuses. Although intracellular mu- and delta-ir patterns differed in ganglion neurons, mu/delta co-expression was extensive in these cells. mu/delta co-expression was also observed in interstitial cells, which were diffusely distributed in submucous plexus preparations but generally located adjacent to myenteric plexus structures. Punctate kappa-ir was seen broadly in nerve fibres in both plexuses, suggesting localisation in varicosities. Neuronal mu/kappa co-localisation was not apparent, although kappa-ir fibres were often apposed against mu-ir cells. mu/kappa co-localisation was detected in interstitial cells in submucous plexus preparations. Similarities in mu and delta expression patterns might reflect similar functional properties previously detected for these receptors. This study indicates that the rat gastrointestinal tract might provide a useful tool for the future study of molecular interactions between opioid receptor types.