Multiplicity of opiate receptors in different species.

Three populations of opiate binding sites (mu, delta and kappa) could be distinguished by displacement studies with the mu-agonist (D)-Ala2-MePhe4-Glyol5-enkephalin and the kappa-agonist 3H-bremazocine in brain membranes of different species. Whereas brain membranes from pig or guinea-pig and human cortical membranes had a proportion of approximately 35-50% kappa-, 29-40% mu-, and 21-30% delta-binding sites, rat membranes contained much less kappa-sites (12% kappa, 62% mu and 25% delta).