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

Abstract

1 The characteristics of the binding of [³H]-[d-Pen²,d-Pen⁵]enkephalin were determined in homogenates of guinea-pig and rat brain. 2 In the guinea-pig, the maximum binding capacity for [³H]-[d-Pen²,d-Pen⁵]enkephalin was 4.19 pmol g⁻¹ and the K_D 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. 3 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.