Characterization and localization of delta opioid binding sites in the bovine pineal gland.

Opioid binding sites in the bovine pineal were characterized using the highly selective delta opioid agonist 3H-[D-Pen2,pCl-Phe4,D-Pen5] enkephalin (DPDP(C1)E). Pineal membranes possess a single class of high affinity binding sites for this delta ligand (Kd = 0.26 nM; Bmax = 250 fmol/mg protein). The specific opioid antagonist naloxone dose dependently inhibited 3H-DPDP(C1)E binding, confirming that this ligand is indeed binding to opioid receptors. The delta selective ligands deltorphin and [D-Pen2,5]enkephalin (DPDPE) were much more potent than the mu selective compounds dermorphin and [D-Ala2,MePhe4,Gly5- ol]enkephalin (DAMGO) in inhibiting 3H-DPDP(C1)E binding. These results demonstrate that in bovine pineal membranes, DPDP(C1)E binds to delta opioid sites. Autoradiographic studies showed a uniform distribution of 3H-DPDP(C1)E binding over the bovine pineal in the sections we analyzed. This distribution suggests that delta opioid binding sites are associated with pinealocytes which account for the majority of cell types in the pineal. However, it is not possible to rule out that these receptors may also be associated with other cell types which are present in the bovine pineal. The density and widespread distribution of delta opioid receptors supports the hypothesis that endogenous opioid peptides directly modulate pineal function.