Altered transition between agonist- and antagonist-favoring states of mu-opioid receptor in brain membranes with modified microviscosity.

In unmodified synaptosomal brain membranes the presence of NaCl inhibited the binding to mu receptors of the tritiated opioid agonists etorphine, Tyr-D-Ala-Gly-(Me)Phe-Gly-ol, and sufentanil by 53, 43, and 37%, respectively, and increased that of the antagonist [3H]naltrexone by 54%. On the other hand, in membranes whose microviscosity was increased by incorporation of cholesteryl hemisuccinate (CHS) the effects of sodium on opioid agonist and antagonist binding were abolished and strongly reduced, respectively. Furthermore, in the modified membranes the ability of sodium to protect the opioid receptor from inactivation by the sulfhydryl-reactive agent N-ethylmaleimide (NEM) was diminished. In CHS-treated membranes whose elevated microviscosity was reduced by the incorporation of oleic acid, the effectiveness of sodium in modulating opioid binding and attenuating receptor inactivation by NEM was restored. The results implicate membrane microviscosity in the mechanism by which sodium modulates the conversion between agonist- and antagonist-favoring states of mu opioid receptor.