In vivo characterization of four types of opioid binding sites in rat brain.

Opioid receptor multiplicity was studied in the brain of intact rats by two experimental approaches. The first involved in vivo labeling by administration of [3H] tracers s.c. to intact rats. The second method was based on ex vivo labeling by incubating fresh brain homogenates immediately after sacrifice with suitable receptor type-specific tracers. The ex vivo labeling approach thus can measure the in vivo receptor occupancy of unlabeled ligands, provided that these ligands do not equilibrate between bound and free form over the assay period. The results suggest the presence of four separate types of opioid binding sites that resemble the mu, delta, kappa, and the newly identified lambda sites.