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 antagonis t naltrexone on beta-EP processing in the hypothalamus and on the rele ase of 2 POMC-derived peptides, beta-EP and gamma(3)-MSH, from the per ifused 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 t o 7 control rats. The mean ratio of beta-EP(1-31) compared to beta-EP( 1-27) plus beta-EP(1-26) was 2.34 +/- 0.41 in the naltrexone treated r ats, significantly higher than the ratio of 1.26 +/- 0.09 in the contr ol rats (P < 0.02). Thus in the setting of chronic opioid antagonism a lthough beta-EP content decreases, there is relatively more beta-EP(1_ 31), the biologically active opioid form of the peptide, compared to t he 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 naltrexo ne. Basal release of gamma(3)-MSH was significantly higher from the na ltrexone 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 wa s also significantly higher in the naltrexone group (951 +/- 94 vs. 54 3 +/- 85 pg/60 min) (P < 0.005). Basal release of beta-EP was 136 +/- 45 pg/60 min in the naltrexone treated brains compared to 93 +/- 15 pg in the controls, but this difference was not significant; KCl stimula ted release of beta-EP, however was significantly higher in the naltre xone group (558 +/- 103 vs. 275 +/- 49 pg/60 min) (P < 0.02). To study the acute and chronic effects of naltrexone in vivo on beta-EP and ga mma(3)-MSH release, rats were either injected with naltrexone and sacr ificed 40-60 min later or were infused with naltrexone for 7 days. Bas eline gamma(3)-MSH release was significantly higher in rats treated wi th naltrexone 40-60 min prior to the perifusion (P < 0.01). Baseline b eta-EP release was below the limit of assay detection. No differences were noted in the responses of gamma(3)-MSH or beta-EP to KCl in eithe r group. In contrast after chronic treatment with naltrexone for 1 wee k, baseline peptide release was not different from the control animals despite a more than 50% fall in peptide content. The gamma(3)-MSH and beta-EP responses to KCl stimulation, however, were significantly les s in the naltrexone treated animals. Thus there is an increase in POMC peptide release acutely after treatment with naltrexone in vitro and in vivo. After 1 week of naltrexone, baseline POMC peptide release con tinues unchanged despite the fall in peptide content, however, the res ponse to KCl is blunted possibly reflecting the decrease in peptide co ntent after chronic stimulation with naltrexone. We conclude that nalt rexone has significant effects on POMC peptide release and on beta-EP processing in the hypothalamus. These results further demonstrate that the brain POMC system can respond to opioid blockade at several level s and are consistent with inhibitory feedback mechanisms for the autor egulation of the POMC system by endogenous beta-EP.