Properties of the putative epsilon opioid receptor: Identification in rat, guinea pig, cow, pig and chicken brain

[³H]Ethylketocyclazocine, in the presence of mu, delta and kappa-1 blocking agents, labels a high-affinity, saturable binding site in rat brain which we previously concluded might be the β-endorphin-selective epsilon opioid receptor. However, studies failing to establish clearly the existence of the site in species other than that raised the possibility that it might be an artifact or a unique constituent of rat tissues. Neither appears to be the case. We report here that, like other types of receptors, the site is proteinaceous with a sulfhydryl group within the binding site itself. Furthermore, the site appears to exist in two interchangeable, GTP-γ-S-sensitive states which are readily distinguished by β-endorphin, but not (-)- [³H]ethylketocyclazocine, and which are likely to be the site coupled to and uncoupled from a G protein. The putative epsilon receptor is not a peculiarity of rat brain but is readily measurable in forebrain of guinea pig, cow, chicken and pig brain as well. In fact, in all of the species examined, it is more abundant than mu, delta or kappa-1 receptors, representing 38 to 55% of the total opioid receptor population. The binding selectivity profile for drugs and the structure-activity relationship for β-endorphin analogs indicated that the pharmacological properties of the putative epsilon receptor in brain are remarkably correlated with those of the epsilon receptor that was first hypothesized to exist in rat vas deferens and of the epsilon receptor that has been hypothesized to mediate the supraspinal analgesic effects of β-endorphin.