Opioid Partial Agonist Effects of 3-O-Methylnaltrexone in Rhesus Monkeys

3-O-Methylnaltrexone (3-MNTX), a putative antagonist of morphine-6-β-D-glucuronide (M6G) receptors, has been reported to block the behavioral effects of heroin at doses that do not block those of morphine, suggesting that M6G receptors may play a unique role in the addictive properties of heroin. This study investigated the effects of 3-MNTX in monkeys trained to discriminate i.v. heroin from vehicle or to self-administer i.v. heroin under a progressive-ratio schedule. Additional in vitro studies determined the effects of 3-MNTX and reference drugs on adenylyl cyclase activity in caudate-putamen membranes of monkeys and rats. In drug discrimination experiments, heroin, morphine, and M6G substituted for heroin in all subjects, whereas 3-MNTX substituted for heroin in one-half the monkeys tested. In these latter monkeys, the effects of 3-MNTX were antagonized by naltrexone, and pretreatment with 3-MNTX enhanced the effects of heroin, M6G, and morphine, indicative of μ-agonist activity. In monkeys showing no substitution of 3-MNTX for heroin, 3-MNTX antagonized the effects of heroin, M6G, and morphine. In self-administration experiments, heroin and 3-MNTX maintained injections per session significantly above those maintained by vehicle when the initial response requirement (IRR) was low; only heroin maintained significant self-administration when the IRR was high. In vitro, 3-MNTX inhibited adenylyl cyclase activity in both monkey and rat brain membranes. The degree of inhibition produced by 3-MNTX was less than that produced by the full agonist [D-Ala²,N-Me-Phe ⁴,Gly5-ol]-enkephalin (DAMGO). The results suggest that 3-MNTX functions primarily as a partial agonist at μ-receptors in monkeys and do not support a singular role for M6G receptors in the abuse-related effects of heroin.