Abstract

Phenoxybenzamine was found to displace stereospecifically bound [³H]naloxone from mouse brain homogenates in a dose-related manner, with an IC₅₀ of 4.6 µM. In the presence of 100 mM Na⁺, the dose-response curve shifted to give an IC₅₀ of 19 µM and an Na⁺ response ratio of 4.3, which is similar to that of a mixed agonist-antagonist. Scatchard plots of the binding data showed that phenoxybenzamine reduced the number of receptor binding sites for naloxone but did not change the affinity of the unreacted receptors for naloxone. Levallorphan protected the receptor from phenoxybenzamine, but the inhibition of binding activity produced by phenoxybenzamine could not be reversed by repeated washing of the receptor preparation. It thus appears that phenoxybenzamine acts as an irreversible, non-equilibrium inhibitor with binding characteristics similar to those of the agonist-antagonist narcotics. Washed brain homogenates from mice treated in vivo with antinociceptive doses of phenoxybenzamine exhibited enhanced binding of [³H]naloxone and [³H]morphine, which may indicate the displacement of an endogenous material from the opiate receptors in vivo by phenoxybenzamine.