Coupling of multiple opioid receptors to GTPase following selective receptor alkylation in brain membranes

Abstract

Opioid agonists of the mu, kappa and delta types stimulated low-Km guanosine triphosphatase (GTPase) in membranes, from the brain of the rat by up to 34%, with potencies the rank order of which corresponded to the respective binding affinities to opioid receptor. In general, kappa ligands stimulated GTPase to a lesser degree than mu or delta opiates. The coupling of a given type of opioid receptor to GTPase was resolved by direct or protective alkylation of the other receptors. Treatment of the membranes with [beta]-funaltrexamine abolished the stimulation of GTPase by sufentanil and levorphanol (mu), but not by bremazocine (kappa) or DSLET (delta). On the other hand, prior incubation with Superfit, an alkylating agent with selectivity for the delta opioid receptor, specifically eliminated the effect of DSLET. Partial alkylation by increasing concentrations of Superfit gradually reduced the extent of stimulation of GTPase by DSLET. The successive treatment of membranes with Superfit and [beta]-funaltrex-amine blocked the actions of DSLET, sufentanil and levorphanol, but had no effect on the stimulation of the GTPase by bremazocine. Selective coupling of an opioid receptor to GTPase was also obtained after incubation of membranes with [beta]-chlornaltrexamine in the presence of protective concentrations of mu, kappa or delta opioid ligands. Alkylation resolved the coupling of the non-selective opiate etorphine: the sum of stimulation of GTPase in the receptor-selective membranes equalled maximal stimulation of enzyme in untreated membranes. Naloxone blocked the stimulation of GTPase by mu, kappa or delta agonists, but ICI-174,864 specifically inhibited the effect of DSLET. The results describe the use of receptor-selective membranes from brain to characterize the coupling of multiple opioid receptors to high-affinity GTPase, the inhibitory binding protein for GTP of the adenylate cyclase complex.