Supersensitivity of brain opiate receptor subtypes after chronic naltrexone treatment

Chronic administration of the narcotic antagonist naltrexone resulted in a marked increase in brain opiate receptors. Similar changes were observed for putative Mu, Delta, and Kappa opiate receptor subtypes. In contrast, only a modest increase was observed for the putative Sigma receptor. Withdrawal from chronic naltrexone treatment resulted in a decrease from elevated receptor levels to nearly control receptors levels in a period of about 6 days, as revealed by [ 3H] etorphine binding. These results may shed light on the mechanisms of opiate dependence and withdrawal.