Studies on opioid receptor selectivity of beta-endorphin antagonists.

Opioid receptor selectivity of several beta-endorphin (beta-EP) analogs which antagonize beta-EP-induced analgesia has been assessed using partially selective binding assays. Although the apparent affinity dissociation constant of beta-EP in these assays varies from 0.2 to 360 nm, the potency of beta-EP antagonists relative to beta-EP remains largely unchanged. It is unlikely that differences in receptor affinities can account for the antagonist properties of these analogs in vivo.