mu Opioid receptor-mediated G-protein activation by heroin metabolites: evidence for greater efficacy of 6-monoacetylmorphine compared with morphine.

Abstract

The efficacy of heroin metabolites for the stimulation of mu opioid receptor-mediated G-protein activation was investigated using agonist-stimulated [(35)S]guanosine-5'-O-(gamma-thio)-triphosphate binding. In rat thalamic membranes, heroin and its primary metabolite, 6-monoacetylmorphine (6-MAM), were more efficacious than morphine or morphine-6-beta D-glucuronide. This increased efficacy was not due to increased action of heroin and 6-MAM at delta receptors, as determined by competitive antagonism by naloxone, lack of antagonism by naltrindole, and competitive partial antagonism with morphine. In agreement with this interpretation, the same relative efficacy profile of heroin and its metabo..