Species differences in the concentrations and distributions of opioid binding sites

Binding at the mu, delta- and kappa-types of opioid binding sites was compared in homogenates from the brains of guinea-pig, rabbit, rat and two mouse strains, under conditions of selective labelling. Species differences were shown by two observations. Firstly, analysis of saturation curves in homogenates of brain from which the cerebellum had been removed showed that in guinea-pig brain the opioid binding sites consist of 24% mu-sites, 32% delta-sites and 44% kappa-sites. In contrast, in rabbit brain the corresponding values are 43% mu-sites, 19% delta-sites and 37% kappa-sites and in rat brain, 46% mu-sites, 42% delta-sites and 12% kappa-sites. In the brains of DBA/2 mice the opioid binding sites are comprised of 51% mu-sites, 29% delta-sites and 20% kappa-sites and in C57BL/10 mice, of 44% mu-sites, 35% delta-sites and 21% kappa-sites; these strain differences are due to significant differences in the concentrations of the mu-sites. Secondly, species differences were found when the binding of single concentrations of tritiated ligands (1 X KD value in whole brain) was determined at mu-, delta- and kappa-sites in six brain regions from guinea-pig, rat or rabbit.