Differential opioid gene expression in neurons and neuroendocrine cells of rat and human gastrointestinal tract

There is immunocytochemical evidence that peptides derived from all three opioid precursors are present in intrinsic neurons of the gastrointestinal (GI) tract of human and rodents (1,2,3). During development, opioids show a non-neuronal and non-endocrine expression in many tissues and the expression of opioids in immune cells is under discussion. Therefore, the aim of this study was to evaluate the presence, relative abundance and distribution of the mRNAs encoding the opioid precursors proenkephalin (PENK), prodynorphin (PDYN) and proopiomelanocortin (POMC) in the human and rat GI tract using in situ hybridization histochemistry with 35S labeled cRNA probes. PENK mRNA is expressed at high levels in myenteric neurons while the labelling for PDYN mRNA was weaker in intrinsic neurons. In addition, PDYN mRNA was detected in a subpopulation of epithelial cells in the small and large intestine. POMC gene expression in the intestine of both species was below detection level. There was no evidence for expression of opioids in gut immune cells. The expression of PENK mRNA and PDYN mRNA in intrinsic neurons supports the concept of opioid modulation of gut motility. Epithelial expression of PRO-DYN mRNA suggests a role in autocrine and paracrine function of the intestinal epithelium.