Effect of δ-opioid antagonists on the functional coupling between opioid receptors and G-proteins in rat brain membranes

Abstract

It is currently accepted that occupancy of opioid receptors by agonists, but not antagonists, promotes the association of the receptors to guanine nucleotide binding proteins (G-proteins) and stimulates a high affinity GTPase as part of the mechanism that links the receptor-ligand complex to adenylate cyclase inhibition. In this work we report that if rat brain membranes selective δ-opioid antagonists, the peptides N,N-Diallyl-Tyr- d-Leu-Gly-Tyr-Leu-OH (Diallyl-G) and N-N-Diallyl-Tyr-Aib-Aib-Phe-Leu-OH (ICI174 864), inhibit the low K m GTPase activity in a concentration dependent way. On the other hand the δ-opioid agonists d-Ala 2- d-Leu 5-enkephalin (DADLE) and D-Ser 2Leu 5-Thi 6-enkephalin stimulate dose-dependently the low K m GTPase activity in rat brain 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 stimulation induced by DADLE were abolished when membranes were exposed to low concentrations of N-ethylmaleimide or by ADP ribosylation with pertussis 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 δ-opioid antagonists exhibit negative intrinsic activity and may have the ability to inhibit the receptor-mediated activation of G-proteins.