Preparation of [3H]-oxymorphazone and its binding to rat brain membranes

Oxymorphazone is a 14-hydroxydihydromorphinone derivative which contains C-6 hydrazone group and hence could serve as an irreversible label for opioid receptors. 3H-oxymorphazone was synthesized by the reaction of 3H-oxymorphone with excess hydrazine, with 640 GBq/mmol specific radioactivity. Both the unlabelled compound and the tritiated ligand show high affinity to mu and kappa opiate receptor subtypes in rat brain membranes. Two binding sites were detected by equilibrium binding studies. About 60% of the 3H-oxymorphazone specific binding is irreversible at 10nM ligand concentration. Preincubation of the membranes with unlabelled oxymorphazone resulted in an irreversible blockade of the high affinity 3H-naloxone binding sites.