EFFECT OF MORPHINE ON PROOPIOMELANOCORTIN GENE-EXPRESSION AND PEPTIDELEVELS IN THE HYPOTHALAMUS

Opiates have been reported to suppress POMC in the medial basal hypoth alamus (MBH) but studies have been complicated by the fact that acutel y, in the rat, opiates stimulate corticosterone and inhibit gonadal st eroid release, which could both affect POMC in brain. We have therefor e examined POMC gene expression and peptide levels in the MBH of castr ated rats after 10 days of treatment with subcutaneous morphine or pla cebo pellets and after pellet removal. POMC mRNA was measured by solut ion hybridization assay and beta-endorphin (beta-EP) and alpha-MSH wer e measured by RIA. In castrated male rats, the mean POMC mRNA concentr ation in the MBH was 1.67 +/- 0.11 pg/mu g RNA in the control animals and decreased to 1.17 +/- 0.11 pg/mu g RNA in the morphine-treated ani mals (P < 0.01). Similarly in castrated, estradiol replaced female rat s, the mean POMC mRNA level in the MBH was 1.36 +/- 0.19 pg/mu g RNA a nd decreased to 0.82 +/- 0.08 pg/mu g RNA after morphine treatment (P < 0.05). beta-EP levels were not significantly different in either stu dy. When castrated male rats were similarly morphine pelleted and kill ed either on day 10 or 2 days later after pellet removal, the mean POM C mRNA level again fell from 1.83 +/- 0.21 in the controls to 1.28 +/- 0.20 pg/mu g RNA after 10 days of morphine; 2 days after pellet remov al levels remained suppressed at 0.80 +/- 0.08 pg/mu g RNA (P < 0.01). In this study the concentrations of beta-EP and alpha-MSH were both n oted to decline in the MBH after morphine treatment (P < 0.05). When t he forms of beta-EP in the MBH were characterized by HPLC, a decrease in the concentration of beta-EP was again seen after morphine but no s ignificant differences in the pattern of beta-EP processing or in the relative amounts of beta-EP(1-31) compared to beta-EP(1-27) and beta-E P(1-26) were noted in morphine-treated animals. There was also no sign ificant effect of 10(-6)-10(-4) M morphine on basal or KCl-stimulated release of beta-EP or gamma(3)-MSH release from the perifused rat hypo thalamus in vitro. We conclude that morphine suppresses POMC gene expr ession in the hypothalamus of chronically treated male and female rats . Persistent changes were also noted during morphine withdrawal. In so me cases this was accompanied by a fall in beta-EP peptide content. Th ese effects were seen in castrated animals with and without sex steroi d replacement and are thus independent of the effects of morphine on t he pituitary-gonadal axis. These results show that opiate drugs modify endogenous opioid systems in the brain and provide further support fo r the hypothesis that such changes may contribute to mechanisms of opi ate dependence and withdrawal.