A reduction of the number and a decrease in the dissociation constant of low-affinity opiate receptor binding sites for metenkephalin in brain membranes was noted in rats after chronic ethanol treatment. These changes were reversed with time, and at 48 hr after withdrawal, tissue from ethanol and pair-fed groups demonstrated similar binding characteristics. Preincubation of the control rat brain membrane fraction with the ultrafiltrate of the supernatant obtained from the brain membrane fraction of rats chronically treated with ethanol resulted in changes of affinity and number of opiate receptor binding sites which resembled the changes observed in rats after long-term treatment with ethanol. Acute ethanol administration (4 g/kg) was shown to eliminate the biphasic character of Scatchard plots usually seen in control animals. Addition of ethanol to control brain membranes was also found to after significantly the binding of‘opiate’peptides to such membrane fracti
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