INACTIVATION OF NALOXONE BINDING TO RAT-BRAIN OPIOID RECEPTORS

The kinetics and temperature dependence of inactivation of mu-opioid r eceptors has been assessed by H-3naloxone binding at 37 degrees C in of rat brain membrane preparations. The following parameters characte rizing the inactivation were determined: rate constant k(in) = (5.2 +/ - 1.0) 10(-3) min(-1); activation energy, E(a) = 82.5 kJ/mole; change in activation entropy, Delta S-# = -14 kJ/K.mole. Addition to the medi um of polyunsaturated fatty acids (linoleic or arachidonic, 10(-7)-10( -4) M), cardiolipin (10(-5)-10(-4) M), phosphatidylinositol (10(-5)-10 (-4) M), or 2,6-di-tert-bulyl-p-cresol (10(-4) M) stabilized the recep tors. Similar results were obtained when the preparations were incubat ed under argon. The observed decrease in naloxone binding correlated s trongly with changes in the bilayer microviscosity (measured by electr on spin resonance of 2,2,6,6-tetramethyl-4-caprylaylpiperidine-N-oxyl) and degree of lipid peroxidation. We suggest that the activity of the opioid receptors is critically affected by the physicochemical charac teristics of the bilayer.