Mu-opioid receptor mRNA expression in proopiomelanocortin neurons of the rat arcuate nucleus

It has been previously demonstrated that the activity of proopiomelanocortin (POMC)-containing neurons in the rat arcuate nucleus is regulated by opiates, but the expression of opioid receptors in POMC neurons has never been reported. In the present study, we have applied a double-labeling in situ hybridization technique to investigate the occurrence of μ-opioid receptor mRNA on POMC neurons. We have found that 20±3% of arcuate POMC neurons express μ-opioid receptor mRNA and that the proportion of POMC neurons expressing μ-opioid receptor is higher in the caudal than in the rostral portion of the arcuate nucleus. Our data suggest that POMC neurons might be both auto-regulated by β-endorphin, and regulated by enkephalins.