EFFECT OF DELTA-OPIOID ANTAGONISTS ON THE FUNCTIONAL COUPLING BETWEENOPIOID RECEPTORS AND G-PROTEINS IN RAT-BRAIN MEMBRANES

It is currently accepted that occupancy of opioid receptors by agonist s, but not antagonists, promotes the association of the receptors to g uanine nucleotide binding proteins (G-proteins) and stimulates a high affinity GTPase as part of the mechanism that links the receptor-ligan d complex to adenylate cyclase inhibition. In this work we report that in rat brain membranes selective delta-opioid antagonists, the peptid es N,N-Diallyl-Tyr-D-Leu-Gly-Tyr-Leu-OH (Diallyl-G) and N-N-Diallyl-Ty r-Aib-Aib-Phe-Leu-OH (ICI174 864), inhibit the low K(m) GTPase activit y in a concentration dependent way. On the other hand the delta-opioid agoniStS D-Ala2-D-Leu5-enkephalin (DADLE) and D-Ser2-Leu5-Thr6-enkeph alin stimulate dose-dependently the low K(m) GTPase activity in rat br ain membranes. This stimulation was blocked in the presence of Diallyl -G, and reciprocally the inhibition induced by Diallyl-G was reversed by DADLE. The inhibitory effect of Diallyl-G as well as the stimulatio n induced by DADLE were abolished when membranes were exposed to low c oncentrations of N-ethylmaleimide or by ADP ribosylation with pertussi s toxin which interferes with the ability of the receptor to couple to G-proteins. These observations indicate that the inhibitory effect of Diallyl-G on GTPase requires a functional G-protein and suggest that certain delta-opioid antagonists exhibit negative intrinsic activity a nd may have the ability to inhibit the receptor-mediated activation of G-proteins.