Altered Microviscosity at Brain Membrane Surface Induces Distinct and Reversible Inhibition of Opioid Receptor Binding

In synaptosomal membranes from rat and monkey brain cortex, the addition of petroselenic (18:1, cis ‐Δ 6 ) acid, oleic (18:1, m‐Δ 9 ) acid, and vaccenic (18:1, cis ‐Δ 11 ) acid or their corresponding methyl esters at 0.5 μmol/mg of membrane protein caused a similar 7–10% decrease in the microviscosity of the membrane core, whereas at the membrane surface the microviscosity was reduced 5–7% by the fatty acids but only 1% by their methyl esters. Concomi‐tantly, the fatty acids, but not the methyl esters, inhibited the specific binding of the tritiated μ‐, δ‐, and K‐opioids Tyr‐D‐Ala‐Gly‐(Me)Phe‐Gly‐ol (DAMGO), [D‐Pen 2 ,D‐Pen 5 ]‐enkephalin (DPDPE), and U69,593, respectively. As shown with oleic acid, the sensitivity of opioid receptor binding toward inhibition by fatty acids was in the order δ > μκ k , whereby the binding of [ 3 H]DPDPE was abolished, but significant inhibition of [ 3 H]U69,593 binding, determined in membranes from monkey brain, required membrane modification with a twofold higher fatty acid concentration. Except for the unchanged K D of [ 3 H]U69,593, the inhibition by oleic acid involved both the B max and affinity of opioid binding. Cholesteryl hemisuccinate (0.5–3 μmol/mg of protein), added to membranes previously modified by fatty acids, reversed the fluidization caused by the latter compounds and restored inhibited μ‐, δ‐, and k ‐opioid binding toward control values. In particular, the B max of [ 3 H]‐DPDPE binding completely recovered after being undetectable. The results implicate membrane surface fluidity in the modulation of opioid receptor binding, reveal distinct sensitivity of δ, μ, and K receptors toward that modulation, and identify unsaturated fatty acids and cholesterol as possible endogenous regulators of opioid receptor function.