EVIDENCE THAT BETA-ENDORPHIN IS AN AGONIST AT BOVINE PINEAL DELTA-OPIOID RECEPTORS

Since beta-endorphin is the putative endogenous ligand for epsilon-opi oid receptors, the previous demonstration of saturable, high affinity beta-endorphin binding sites on bovine pineal membranes suggests the p ossible presence of epsilon-opioid receptors. To determine the identit y of pineal beta-endorphin binding sites, the inhibition of [I-125]bet a-endorphin binding by ligands with varying affinities for epsilon-, m u-, delta- or kappa-opioid receptors was investigated. A high positive correlation was observed between the K-i values for these drugs to in hibit [I-125]beta-endorphin binding to pineal membranes and for these drugs to bind to delta-opioid receptors but not to mu-, kappa- or epsi lon-opioid receptors, demonstrating that in the pineal beta-endorphin binds to delta-opioid receptors. Both NaCl and a GTP analogue were pot ent inhibitors of [I-125]beta-endorphin binding, providing evidence th at beta-endorphin is an agonist at pineal delta-opioid receptors. Thes e results suggest that endogenous bovine beta-endorphin may modulate p ineal function.