Identification and characterization of delta opioid binding sites in the bovine pineal

Bovine pineal membranes were shown to possess a single class of high-affinity binding sites for the opioid peptide, [125I]iodiotyrosyl27-beta-endorphin (beta E) (Kd = 47 pM, Bmax = 2.4 fmol/mg of tissue). The rank order of potency at this beta E site was deltorphin greater than [D-Ser2]-Leu-enkephalin-Thr greater than [D-Pen2,D-Pen5]enkephalin much greater than dermorphin greater than [D-Ala2,MePhe4,Gly5-ol]enkephalin much greater than (5 alpha,7 alpha,8 beta)-(-)-N-methyl-N-[7-(1-pyrrolidinyl)-1- oxaspiro(4,5)dec-8-yl]] benzeneacetamide (U69593) greater than [des-Tyr1]beta E greater than beta E(6-31). These results suggest that beta E binds to delta opioid sites and excludes the possibility of significant binding to mu, kappa and epsilon sites. The presence of delta binding sites was confirmed by use of the delta selective ligand [3H][D-Pen2,D-Pen5]enkephalin (Kd = 1.5 nM). The Bmax observed using [D-Pen2,D-Pen5]enkephalin is similar to that obtained with [125I]beta E, confirming that essentially all pineal opioid sites are of the delta type. The virtual absence of mu opioid sites was confirmed using the mu-selective opioid ligand [3H][D-Ala2,MePhe4,Gly5-ol]enkephalin. These results suggest that endogenous or circulating opioid peptides may modulate pineal function by interaction with delta opioid sites.