Allosteric modulation of GABAB receptor function in human frontal cortex

Abstract In the present study, the effects of different allosteric modulators on the functional activity of γ-aminobutyric acid (GABA) B receptors in membranes of post-mortem human frontal cortex were examined. Western blot analysis indicated that the tissue preparations expressed both GABA B1 and GABA B2 subunits of the GABA B receptor heterodimer. In [ 35 S]-GTPγS binding assays, Ca 2+ ion (1 mM) enhanced the potency of the agonists GABA and 3-aminopropylphosphinic acid (3-APA) and that of the antagonist CGP55845, but not that of the GABA B receptor agonist (−)-baclofen. CGP7930 (2,6-di- t -Bu-4-(3-hydroxy-2,2-dimethyl-propyl)-phenol), a positive allosteric modulator of GABA B receptors, potentiated both GABA B receptor-mediated stimulation of [ 35 S]-GTPγS binding and inhibition of forskolin (FSK)-stimulated adenylyl cyclase activity. Chelation of Ca 2+ ion by EGTA reduced the CGP7930 enhancement of GABA potency in stimulating [ 35 S]-GTPγS binding by two-fold. Fendiline, also reported to act as a positive allosteric modulator of GABA B receptors, failed to enhance GABA stimulation of [ 35 S]-GTPγS binding but inhibited the potentiating effect of CGP7930. The inhibitory effect was mimicked by the phenothiazine antipsychotic trifluoperazine (TFP), but not by other compounds, such as verapamil or diphenydramine (DPN). These data demonstrate that the function of GABA B receptors of human frontal cortex is positively modulated by Ca 2+ ion and CGP7930, which interact synergistically. Conversely, fendiline and trifluoperazine negatively affect the allosteric regulation by CGP7930.