UC Irvine

Specific binding properties of the tritium-labeled delta opiate receptor agonist [3H][2-D-penicillamine, 5-D-penicillamine]enkephalin [( 3H][D-Pen2, D-Pen5]enkephalin) were characterized in the rat brain and in a mouse neuroblastoma-rat glioma hybrid cell line (NG 108-15). Saturation isotherms of [3H][D-Pen2, D-Pen5]enkephalin binding to rat brain and NG 108-15 membranes gave apparent Kd values of 1-6 nM. These values are in good agreement with the Kd value obtained from the kinetic studies. The Bmax value in NG 108-15 membranes was 235.3 fmol/mg of protein. An apparent regional distribution of [3H][D-Pen2, D-Pen5]enkephalin binding was observed in the rat brain. A number of enkephalin analogues inhibited [3H][D-Pen2, D-Pen5]enkephalin binding with high affinity (IC50 values of 0.5-5.0 nM) in both NG 108-15 and rat brain membranes. However, putative mu receptor-selective ligands such as morphine, [D-Ala2, MePhe4, Gly5-ol]enkephalin, [MePhe3, D-Pro4]morphiceptin, and naloxone were less effective inhibitors of [3H][D-Pen2, D-Pen5]enkephalin binding in both systems tested. These data suggest that [3H][D-Pen2, D-Pen5]enkephalin is a potent and selective ligand for the delta opioid receptor.