Modulation of opioid receptor binding by G protein and membrane lipid.

Abstract

The role of guanine nucleotide binding protein (G protein) and membrane lipid in opioid receptor-ligand interaction was investigated in a synaptosomal membrane preparation from rat cerebral cortex. Both agonist dihydromorphine and antagonist naltrexone bound to two populations of saturable binding sites. Opioid agonist, but not antagonist, binding affinity was attenuated by GPT-$\gamma$-S. In order to assess the role of G protein in opioid agonist binding, it was inactivated by alkaline treatment of the membranes. Following exposure of membranes to alkaline conditions, (pH 11.75), a loss of high affinity agonist binding was observed while antagonist binding affinity was unchanged. High affinity (0.7 nM) guanine nucleotide-sensitive agonist binding and opioid-stimulated GTPase (intrinsic to G protein) was restored by poly(ethyleneglycol)-induced fusion of the alkali-treated membranes with (opioid receptor devoid) C6 glioma cell membranes. Antagonist binding in the fused membranes was unaltered. These results indicate that high and low affinity opioid agonist binding reflects binding of the ligand to G protein-coupled and -uncoupled form of the receptor, respectively. In contrast, as shown by displacement with receptor-selective opioids, the heterogeneity of naltrexone binding in brain membranes reflects its interaction with both $\mu$ (high affinity component) and $\delta$ (low affinity component) opioid receptors. In order to characterize the modulation of opioid receptors by membrane lipids, cis and trans fatty acids, as well as phospholipids, were incorporated into brain membranes. The results demonstrate that the fatty acids inhibited receptor-ligand interaction in proportion to their membrane fluidizing property (cis $>$ trans), but also acted by specific interactions with the phospholipid bilayer. The fatty acids preferentially inhibited the $\delta$ opioid receptor and their effect was influenced by receptor conformation and state of coupling to G protein. Phosphatidylinositol (PI) but not phosphatidylcholine, incorporated into the lipid bilayer by liposome-brain membrane fusion, strongly inhibited opioid ligand binding exhibiting thereby dependence on its fatty acid composition. The results indicate an important role for the membrane microenvironment in modulating opioid receptor function.