Abstract
^3^H‐naloxone specific binding was carried out on synaptosomal membranes isolated from basal ganglia of the cat brain. A high‐ and a low‐affinity site with Kd~1~ = 3.7 n__M__ and Kd~2~ = 35 n__M__ having B~max 1~ = 79 pmole/g protein and B~max 2~ = 224 pmole/g protein were found. The Hill number for the high‐ and low‐affinity sites were, respectively, 1.01 and 0.86. Digitonin and Triton X‐100 had an inhibitory effect on the binding at concentrations between 10^−5^ and 10^−1^% (w/v). Deoxycholate and Nonidet P‐40 also inhibited the binding of ^3^H‐nal‐oxone, but at 10^−4^% produced a 50% enhancement. After the binding to membranes, the ^3^H‐naloxone receptor complex is stable to the action of Triton X‐100 and dissociates slowly. In membranes bound with 10 n__M__ ^3^H‐naloxone and then submitted to 0.1‐0.2% Triton X‐100, in which only the presynaptic membrane disintegrates, the specific radioactivity is decreased. With a more drastic treatment that disintegrates the postsynaptic membrane, the ^3^H‐naloxone binding to synaptosomal membranes is almost completely abolished. These results suggest that opiate receptors may be localized both pre‐ and postsynaptically in central synapses.