SPECTRUM OF THE μ‐, δ‐ AND κ‐BINDING SITES IN HOMOGENATES OF RAT BRAIN

Open Access
Publisher Website
Google Scholar
Abstract
1 In homogenates of rat brain, the binding characteristics of tritiated opiates and opioid peptides were examined and the relative capacities of μ-, δ- and κ-binding sites of the opiate receptor determined by saturation analysis. 2 In competition experiments, binding of the selective μ-ligand [3H]-[d-Ala2,MePhe4,Gly-ol5]enkephalin at the μ-site was displaced by [d-Ala2,d-Leu5]enkephalin with rather low affinity (KI = 12.6 nm) and more readily by the ketazocine-like compounds (−)-ethylketazocine (KI = 3.1 nm) and (−)-bremazocine (KI = 0.32 nm), which also displaced the binding of [3H]-[d-Ala2,d-Leu5]enkephalin from the δ-site. In contrast, the binding to the κ-site was easily displaced by ethylketazocine (1.0 nm) and bremazocine (0.37 nm) but not by the μ-ligand [d-Ala2,MePhe4,Gly-ol5]enkephalin (KI = 2000–3000 nm) or the δ-ligand [d-Ala2,d-Leu5]enkephalin (KI > 20,000 nm). 3 The dissociation equilibrium constant (KD) and the binding capacity (pmol/g) of the μ-binding site were determined with the selective μ-ligand [3H]-[d-Ala2,MePhe4,Gly-ol5]enkephalin. For the δ-site, [3H]-[d-Ala2,d-Leu5]enkephalin was used in the presence of unlabelled [d-Ala2,MePhe4,Gly-ol5]enkephalin in order to suppress cross-reactivity to the μ-binding site. For the estimation of κ-binding, [3H]-(±)-ethylketazocine or [3H]-(−)-bremazocine were used in the presence of unlabelled μ- and δ-ligands for the suppression of cross-reactivities to the μ- and δ-binding sites. 4 In rat brain the capacity of the μ-binding site was 7.3 pmol/g brain, that of the δ-binding site 6.7 pmol/g brain and that of the κ-binding site 2.0 pmol/g brain. Thus, the κ-binding site had the lowest value whereas in the guinea-pig brain the capacity of the μ-binding site was lower than that of the δ- or κ-binding site.

This publication has 12 references indexed in Scilit: