Comparison of opioid receptor distributions in the rat ileum

The cellular expression patterns of μ-, δ- and κ-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 μ- and δ-ir patterns differed in ganglion neurons, μ/δ co-expression was extensive in these cells. μ/δ 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 κ-ir was seen broadly in nerve fibres in both plexuses, suggesting localisation in varicosities. Neuronal μ/κ co-localisation was not apparent, although κ-ir fibres were often apposed against μ-ir cells. μ/κ co-localisation was detected in interstitial cells in submucous plexus preparations. Similarities in μ and δ 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.