Melanocortin peptides are reported to antagonize opiate dependence and tolerance, but the neural substrates underlying these actions are unknown. In this study, we characterize the rat melanocortin-4 receptor (MC4-R) and demonstrate that this receptor is regulated by opiate administration. The rat MC4-R is 95% identical to the human MC4-R, and the potency of melanocortin peptides to stimulate cAMP production is similar in these two species homologs (alpha-melanocyte-stimulating hormone = adrenocorticotropic hormone > gamma-melanocyte-stimulating hormone). Expression of MC4-R mRNA was found to be enriched in the striatum, nucleus accumbens, and periaque-ductal gray, all of which are regions implicated in the behavioral effects of opiates. In contrast, MC1-, MC3-, and MC5-R are expressed at very low or undetectable levels in these brain regions. Chronic administration of morphine (5 days) resulted in a time-dependent down-regulation of MC4-R mRNA expression in the striatum and periaqueductal gray. Expression of MC4-R mRNA was also decreased in the nucleus accumbens/ olfactory tubercle, but this effect was observed after 1 or 3 days of morphine treatment. In the striatum, the reduction of MC4-R mRNA was accompanied by a concomitant decrease in melanocortin receptor levels, shown by quantitative radioligand binding and autoradiography. In contrast, morphine administration did not influence levels of MC4-R mRNA in several other brain regions, including frontal cortex, olfactory bulb, hypothalamus, and ventral tegmentum/substantia nigra. In light of previous findings that melanocortins antagonize opiate self-administration, analgesic tolerance, and physical dependence, we hypothesize that decreased melanocortin function, via down-regulation of MC4-R expression, may contribute to the development of these opiate-induced behaviors.