The use of [3H]‐[d‐Pen2,d‐Pen5]enkephalin as a highly selective ligand for the δ‐binding site

Abstract
The characteristics of the binding of [3H]‐[d‐Pen2,d‐Pen5]enkephalin were determined in homogenates of guinea‐pig and rat brain. In the guinea‐pig, the maximum binding capacity for [3H]‐[d‐Pen2,d‐Pen5]enkephalin was 4.19 pmol g−1 and the KD 1.61 nm. In the rat, the corresponding values were 2.47 pmol g−1 and 5.42 nm. In both species, the maximum binding capacity and the affinity were not altered when μ‐binding was suppressed with [d‐Ala2, MePhe4, Gly‐ol5]enkephalin. The μ‐agonists, [d‐Ala2, MePhe4, Gly‐ol5]enkephalin and morphine, displaced a small portion of the binding of [3H]‐[d‐Pen2,d‐Pen5]enkephalin with high affinities.