Though transduction mechanisms recruited by heterologously expressed
5-HT(2A) 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-HT(2A)
agonist, DOI, and Ro60,0175 all enhanced [(35)S]
GTPgammaS binding to Galphaq/11 in rat cortex with pEC(50) 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 Galphaq/11 by
5-HT (30 muM) and DOI (30 muM) was abolished by the
selective 5-HT(2A) vs.
5-HT(2C)/
5-HT(2B) antagonists,
ketanserin (pK(B) values of 9.11 and 8.88, respectively) and MDL100,907 (9.82 and 9.68). By contrast, 5-HT-induced [(35)S]
GTPgammaS binding to Galphaq/11 was only weakly inhibited by the preferential
5-HT(2C)
receptor antagonists, RS102,221 (6.94) and SB242,084 (7.39), and the preferential
5-HT(2B)
receptor antagonist, LY266,097 (6.66). The
antipsychotic,
clozapine, which had marked affinity for
5-HT(2A) receptors, blocked the recruitment of Galphaq/11 by
5-HT and DOI with pK(B) values of 8.54 and 8.14, respectively. Its actions were mimicked by the "atypical"
antidepressant and
5-HT(2A)
receptor antagonist,
mirtazapine, which likewise blocked
5-HT and DOI-induced Galphaq/11 protein
activation with pK(B) values of 7.90 and 7.76, respectively. In conclusion, by use of an immunocapture/scintillation proximity strategy, this study shows that native
5-HT(2A) receptors in rat
frontal cortex specifically recruit Galphaq/11 and that this action is blocked by
clozapine and
mirtazapine. Quantification of
5-HT(2A) receptor-mediated Galphaq/11
activation in
frontal cortex should prove instructive in characterizing the actions of diverse classes of
psychotropic agent.
Synapse 63:95-105, 2009. (c) 2008 Wiley-Liss, Inc.
