Enhancement by Baclofen of the Gs-Coupled Receptor-Mediated cAMP Production inXenopusOocytes Expressing Rat Brain Cortex Poly (A)+RNA: A Role of G-Protein βγ Subunits

Abstract We investigated the mechanism by which GABA-B receptors enhance the Gs-coupled receptor-mediated cAMP production in Xenopus oocytes expressing poly (A) + RNA derived from rat brain cortex. We expressed the cystic fibrosis transmembrane conductance regulator gene (CFTR) as a reporter for cAMP changes in oocytes. The GABA-B agonist (-)baclofen enhanced the adrenergic β 2 agonist isoproterenol- or vasoactive intestinal peptide (VIP)-induced CFTR currents, whereas (-)baclofen alone did not cause any currents. The (-)baclofen-enhanced currents were inhibited by the GABA-B antagonist 2-OH saclofen. The enhancement by (-)baclofen was further augmented by coexpressing adenylyl cyclase (AC) type II, an isotype activated by Gβγ and Gαs, but not by coexpressing AC type III, an isotype insensitive to Gβγ. Moreover, pretreatment of the oocytes with pertussis toxin (PTX) abolished the enhanced effect of (-)baclofen. These results indicate that upon GABA-B activation, the Gβγ released from PTX-sensitive G-proteins activates the AC type II (or IV), and this process requires the Gαs activation by Gs-coupled receptors.