Modulation of Opioid Receptor Binding by Cis and Trans Fatty Acids

Abstract

In synaptosomal brain membranes, the addition of oleic acid (cis), elaidic acid (trans), and the cis and trans isomers of vaccenic acid, at a concentration of 0.87 Μmol of lipid/mg of protein, strongly reduced the B max and, to a lesser degree, the binding affinity of the Μ-selective opioid [ 3 H]Tyr-D-Ala-Gly-(Me)Phe-Gly-ol ([ 3 H]DAMGO). At comparable membrane content, the cis isomers of the fatty acids were more potent than their trans counterparts in inhibiting ligand binding and in decreasing membrane microviscosity, both at the membrane surface and in the core. However, trans- vaccenic acid affected opioid receptor binding in spite of just marginally altering membrane microviscosity. If the receptors were uncoupled from guanine nucleotide regulatory protein, an altered inhibition profile was obtained: the impairment of K D by the fatty acids was enhanced and that of B max reduced. Receptor interaction of the Δ-opioid [ 3 H](D-Pen 2 ,D-Pen 5 )enkephalin was modulated by lipids to a greater extent than that of [ 3 H]DAMGO: saturable binding was abolished by both oleic and elaidic acids. The binding of [ 3 H]naltrexone was less susceptible to inhibition by the fatty acids, particularly in the presence of sodium. In the absence of this cation, however, cis -vaccenic acid abolished the low-affinity binding component of [ 3 H]naltrexone. These findings support the membrane model of opioid receptor sequestration depicting different ionic environments for the Μ- and Δ-binding sites. The results of this work show distinct modulation of different types and molecular states of opioid receptor by fatty acids through mechanisms involving membrane fluidity and specific interactions with membrane constituents.