Effect of morphine on proopiomelanocortin gene expression and peptide levels in the hypothalamus.

Opiates have been reported to suppress POMC in the medial basal hypothalamus (MBH) but studies have been complicated by the fact that acutely, in the rat, opiates stimulate corticosterone and inhibit gonadal steroid release, which could both affect POMC in brain. We have therefore examined POMC gene expression and peptide levels in the MBH of castrated rats after 10 days of treatment with subcutaneous morphine or placebo pellets and after pellet removal. POMC mRNA was measured by solution hybridization assay and beta-endorphin (beta-EP) and alpha-MSH were measured by RIA. In castrated male rats, the mean POMC mRNA concentration in the MBH was 1.67 +/- 0.11 pg/microgram RNA in the control animals and decreased to 1.17 +/- 0.11 pg/microgram RNA in the morphine-treated animals (P < 0.01). Similarly in castrated, estradiol replaced female rats, the mean POMC mRNA level in the MBH was 1.36 +/- 0.19 pg/microgram RNA and decreased to 0.82 +/- 0.08 pg/microgram RNA after morphine treatment (P < 0.05). beta-EP levels were not significantly different in either study. When castrated male rats were similarly morphine pelleted and killed either on day 10 or 2 days later after pellet removal, the mean POMC mRNA level again fell from 1.83 +/- 0.21 in the controls to 1.28 +/- 0.20 pg/microgram RNA after 10 days of morphine; 2 days after pellet removal levels remained suppressed at 0.80 +/- 0.08 pg/microgram RNA (P < 0.01). In this study the concentrations of beta-EP and alpha-MSH were both noted to decline in the MBH after morphine treatment (P < 0.05). When the 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 significant differences in the pattern of beta-EP processing or in the relative amounts of beta-EP1-31 compared to beta-EP1-27 and beta-EP1-26 were noted in morphine-treated animals. There was also no significant 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 hypothalamus in vitro. We conclude that morphine suppresses POMC gene expression in the hypothalamus of chronically treated male and female rats. Persistent changes were also noted during morphine withdrawal. In some cases this was accompanied by a fall in beta-EP peptide content. These effects were seen in castrated animals with and without sex steroid replacement and are thus independent of the effects of morphine on the pituitary-gonadal axis. These results show that opiate drugs modify endogenous opioid systems in the brain and provide further support for the hypothesis that such changes may contribute to mechanisms of opiate dependence and withdrawal.

Effect of opioid antagonism on beta-endorphin processing and proopiomelanocortin-peptide release in the hypothalamus.

Previous studies have shown that chronic opioid receptor blockade has significant effects on POMC gene expression and peptide levels in the hypothalamus. We have now examined the effects of the opioid antagonist naltrexone on beta-EP processing in the hypothalamus and on the release of 2 POMC-derived peptides, beta-EP and gamma 3-MSH, from the perifused hypothalamus in vitro. The beta-EP immunoactivity in the medial basal hypothalamus (MBH) of 7 rats infused for 1 week with naltrexone by osmotic minipump, was individually analyzed by HPLC and compared to 7 control rats. The mean ratio of beta-EP1-31 compared to beta-EP1-27 plus beta-EP1-26 was 2.34 +/- 0.41 in the naltrexone treated rats, significantly higher than the ratio of 1.26 +/- 0.09 in the control rats (P < 0.02). Thus in the setting of chronic opioid antagonism although beta-EP content decreases, there is relatively more beta-EP1-31, the biologically active opioid form of the peptide, compared to the C-terminally cleaved forms of beta-EP which have reduced biological activity. To study the effects of naltrexone on beta-EP and gamma 3-MSH release, hypothalami were perifused in vitro with 10(-6) M naltrexone. Basal release of gamma 3-MSH was significantly higher from the naltrexone treated brains compared to the controls (221 +/- 20 pg/60 min vs. 161 +/- 6.7 pg/60 min) (P < 0.01); KCl stimulated gamma 3-MSH was also significantly higher in the naltrexone group (951 +/- 94 vs. 543 +/- 85 pg/60 min) (P < 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)