OPIOID BINDING IN GIANT TOAD AND GOLDFISH BRAIN

Opiate receptor expression in phylogenetically different species has p layed an important role in the study of opioid receptor pharmacology. Total opioid binding measured with the nonselective ligand H-3-dipreno rphine reveals a B-max Of 21.7 +/- 1.37 fmol/mg tissue wet wt and a K- D of 0.17 +/- 0.03 nM in Bufo marinus (giant toad),as well as a B-max of 18.17 +/- 0.41 fmol/mg tissue wet wt and a K-D, of 0.47 +/- 0.18nM in Carassius auratus (goldfish). Despite the similar levels of H-3-dip renorphine binding, the composition of binding subtypes in the two spe cies differs. Approximately 30% of total binding corresponds to mu rec eptors in both species, whereas neither kappa(1) nor delta binding can be detected. However, the remaining 70% of binding differs between th e toad and goldfish. In the toad, the non-mu binding corresponds to ka ppa(2) sites, whereas in the goldfish, the non-mu binding corresponds to kappa(3) sites. The sites can be distinguished biochemically, as we ll as pharmacologically. After affinity labeling the sites with H-3-Na lBzoH, the retention times on an ion-exchange column differ for the pe aks of kappa binding in the two species. Although Bufo marinus (giant toad) and Carassius auratus (goldfish) brains express kappa and mu opi oid binding, the kappa subtypes in these two species differ.