An immunocapture/scintillation proximity analysis of Gαq/11 activation by native serotonin (5-HT)2A receptors in rat cortex: Blockade by clozapine and mirtazapine

Though transduction mechanisms recruited by heterologously expressed 5‐HT2A receptors have been extensively studied, their interaction with specific subtypes of G‐protein remains to be directly evaluated in cerebral tissue. Herein, as shown by an immunocapture/scintillation proximity analysis, 5‐HT, the prototypical 5‐HT2A agonist, DOI, and Ro60,0175 all enhanced [35S]GTPγS binding to Gαq/11 in rat cortex with pEC50 values of 6.22, 7.24 and 6.35, respectively. No activation of Go or Gs/olf was seen at equivalent concentrations of DOI. Stimulation of Gαq/11 by 5‐HT (30 μM) and DOI (30 μM) was abolished by the selective 5‐HT2A vs. 5‐HT2C/5‐HT2B antagonists, ketanserin (pKB values of 9.11 and 8.88, respectively) and MDL100,907 (9.82 and 9.68). By contrast, 5‐HT‐induced [35S]GTPγS binding to Gαq/11 was only weakly inhibited by the preferential 5‐HT2C receptor antagonists, RS102,221 (6.94) and SB242,084 (7.39), and the preferential 5‐HT2B receptor antagonist, LY266,097 (6.66). The antipsychotic, clozapine, which had marked affinity for 5‐HT2A receptors, blocked the recruitment of Gαq/11 by 5‐HT and DOI with pKB values of 8.54 and 8.14, respectively. Its actions were mimicked by the “atypical” antidepressant and 5‐HT2A receptor antagonist, mirtazapine, which likewise blocked 5‐HT and DOI‐induced Gαq/11 protein activation with pKB values of 7.90 and 7.76, respectively. In conclusion, by use of an immunocapture/scintillation proximity strategy, this study shows that native 5‐HT2A receptors in rat frontal cortex specifically recruit Gαq/11 and that this action is blocked by clozapine and mirtazapine. Quantification of 5‐HT2A receptor‐mediated Gαq/11 activation in frontal cortex should prove instructive in characterizing the actions of diverse classes of psychotropic agent. Synapse 63:95–105, 2009. © 2008 Wiley‐Liss, Inc.