Characterization of the binding of[3H]CGP54626 to GABAB receptors in the male bullfrog (Rana catesbeiana)

Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the vertebrate brain. GABA activates both ionotropic (GABA(A)) and metabotropic (GABA(B)) receptors in mammals. Whether non-mammalian vertebrates possess receptors with similar characteristics is not well understood. We used a mammalian GABA(B)-specific antagonist to determine the pharmacology of putative receptors in the brain of an anuran amphibian, the male bullfrog (Rana catesbeiana). Receptor binding assays with the antagonist [H-3] CGP54626 revealed a single class of high affinity binding sites (with a K-D of 2.97 nM and a B-max of 2619 fmol/mg protein). Binding was time- and temperature-dependent, saturable and specific. Specific binding of [H-3]CGP54626 was inhibited by several mammalian GABAB receptor agonists and antagonists. The rank order potency of agonists was: GABA = SKF97541 > (R)-Baclofen > 3-APPA. The rank order for antagonists was: CGP54626 = CGP55845 > CGP52432 > CGP35348. The GABA(A) receptor ligands muscimol and SR95531 had very low affinity for [H-3]CGP54626 binding sites, while bicuculline compounds had no affinity. Binding of GABA was positively modulated by CGP7930. Taurine did not allosterically modulate GABA binding but did inhibit [H-3]CGP54626 binding in a linear fashion. Bullfrog brain thus possesses binding sites with significant similarity to mammalian GABAB receptors. These receptors differ from mammalian receptors, however, in dissociation kinetics, ligand specificity and allosteric modulation. (c) 2006 Elsevier B.V. All rights reserved.