Binding of ³H-naloxone in the mouse brain: Effect of ions and tolerance development

The binding of ³H-naloxone (spec. act. 5.2 Ci/mmol) in a crude mitochondrial fraction of the whole mouse brain was examined. Binding was reversed by the narcotic agonists levorphanol, morphine and 1-methadone but not by dextrorphan. Levorphanol sensitive (specific) ³H-naloxone binding was blocked by Na+, Li+, Ca++, Mg++ and Mn++ but not by K+. When the crude mitochondrial fraction was separated on a discontinuous sucrose gradient, the highest activity of specific binding was found in the nerve ending particle fraction. Animals made physically dependent by 3 day morphine pellet implantation did not show an increased binding affinity for ³H-nalovxone. The implantation of a 10 mg naloxone pellet increased the apparent total number of binding sites on the second and third day of implantation.