Characterization of [3H][2-D-$\overline{\text{penicillamine},5\text{ -D-p}}\text{enicillamine}$]-enkephalin Binding to δ Opiate Receptors in the Rat Brain and Neuroblastoma--Glioma Hybrid Cell Line (NG 108-15)

Specific binding properties of the tritium-labeled δ opiate receptor agonist [3H][2-D-$\overline{\text{penicillamine},5\text{ -D-p}}\text{enicillamine}$]-enkephalin ([3H][D-$\text{P}\overline{\text{en}^{2},\text{D -Pe}}\text{n}^{5}$]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-$\text{P}\overline{\text{en}^{2},\text{D- Pe}}\text{n}^{5}$]enkephalin binding to rat brain and NG 108-15 membranes gave apparent Kdvalues of 1-6 nM. These values are in good agreement with the Kdvalue obtained from the kinetic studies. The Bmaxvalue in NG 108-15 membranes was 235.3 fmol/mg of protein. An apparent regional distribution of [3H][D-$\text{P}\overline{\text{en}^{2},\text{D- Pe}}\text{n}^{5}$]enkephalin binding was observed in the rat brain. A number of enkephalin analogues inhibited [3H][D-$\text{P}\overline{\text{en}^{2},\text{D- Pe}}\text{n}^{5}$]enkephalin binding with high affinity (IC50values of 0.5-5.0 nM) in both NG 108-15 and rat brain membranes. However, putative μ 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-$\text{P}\overline{\text{en}^{2},\text{D- Pe}}\text{n}^{5}$]enkephalin binding in both systems tested. These data suggest that [3H][D-$\text{P}\overline{\text{en}^{2},\text{D- Pe}}\text{n}^{5}$]enkephalin is a potent and selective ligand for the δ opioid receptor.