Benzomorphan sites are ligand recognition sites of putative epsilon-receptors

The binding characteristics of benzomorphan sites of rat brain membranes are compared with those of kappa-sites of human placenta and guinea pig brain membranes. Enkephalins and the stable analog [D-Ala2,D-Leu5]enkephalin, which are virtually inactive at kappa-sites, possess moderate binding affinity at benzomorphan sites. In contrast, a dynorphin analog, PL017-dynorphin A(6-17), binds well to kappa-sites but poorly to benzomorphan sites. Among all opioid peptides tested, beta h-endorphin, which is essentially inactive at the kappa-receptor sites, is the most potent ligand at benzomorphan sites. The potencies of beta h-endorphin and its fragments at epsilon-receptors of the rat vas deferens correlate well with their binding affinities of benzomorphan sites but not of mu- and delta-sites. These data, as well as the data which show the distinct distribution of benzomorphan sites in rat brain as compared with the distribution of mu- and delta-sites of rat brain and of kappa-sites of guinea pig brain, suggest that benzomorphan sites of rat brain are the ligand-binding sites of epsilon-receptors.