Identification of opiate receptor binding in intact animals

After intravenous administration of ³H-naloxone to rats, particulate bound radioactivity accumulated in the brain is selectively associated with opiate receptor binding sites, providing a means of labeling the opiate receptor in vivo. The regional distribution of ³H-naloxone bound in vivo closely parallels regional differences in opiate receptor binding in vitro with highest levels in the corpus striatum, negligible receptor-associated binding in the cerebellum and intermediate levels in other regions. ³H-Naloxone binding in vivo is saturable with the same total number of binding sites determined in vivo as by in vitro procedures. Nalorphine is markedly more potent than morphine in inhibiting ³H-naloxone binding in vivo and non-opiates are ineffective. The half-life for dissociation of ³H-naloxone bound to particles in vivo is the same as its dissociation rate after binding occurs in vitro, and sodium stabilizes ³H-naloxone bound in vivo from initial rapid dissociation as predicted from the known properties of the opiate receptor in vitro.