In vivo binding of putative delta-selective opioid antagonists to central opiate receptors

The in vivo opiate receptor binding of three putative delta-selective opioid antagonists, the irreversible oripavine ligand NIH 10236 and two enkephalin analogs, ICI 154,129 and ICI 174,866, was studied in rat brain. Following i.c.v. injection, potent and apparently irreversible opiate receptor binding was observed for NIH 10236 with similar affinity for mu, delta and kappa sites. However, opiate receptor binding was not evident either after s.c. injection of NIH 10236 or ICI 154,129, or following i.c.v. administration of ICI 154,129 or ICI 174,864. The receptor sites that mediate the pharmacological effects of these drugs should be reevaluated.