EFFECTS OF 5'-GUANYLYLIMIDODIPHOSPHATE ON THE LIGAND-BINDING OF MEMBRANE-BOUND AND SOLUBILIZED OPIOID RECEPTORS OF FROG (RANA-ESCULENTA) BRAIN

The effect of the nonhydrolysable analogue of GTP, 5'-guanylylimidodiphosphate (GppNHp) was investigated on the binding of mu and kappa opioid agonists and antagonists in frog brain membrane preparations. The greatest inhibition with GppNHp was observed with labelled dihydromorphine, ([H-3]DHM), a mu opioid receptor agonist. The binding of [H-3]U-69593, a specific kappa opioid receptor agonist was inhibited to a smaller extent by GppNHp. Conversely, [H-3]naloxone (opioid antagonist) binding displayed a dose-dependent increase with GppNHp, which effect was larger at 0-degrees-C than at 24-degrees-C. The binding of [H-3]ethylketocyclazocine (EKC), a ligand with agonist antagonist character was not affected by GppNHp either in the absence or in the presence of sodium ions at 0-degrees-C. Slight inhibition of [H-3]EKC binding in the presence of sodium ions was observed with GppNHp at 24-degrees-C. In displacement experiments GppNHp decreased the ability of mu and kappa agonists to compete for [H-3]naloxone binding sites. This effect was maintained after solubilization with 1% digitonin. However, since the binding of [H-3]naloxone was also reduced, there was no agonist specificity of the GppNHp induced inhibition of binding to solubilized receptors. These results suggest that frog brain opioid receptors are coupled to a GTP-regulatory protein similar to mammalian opioid receptors.