Functional activation of Gα protein via α -adrenoceptor in rat cerebral cortical membranes.

Although it is recognized that α -adrenoceptors are coupled to diverse intracellular signalling pathways, its primary transduction mechanisms are evoked by activating phospholipase C in the cell membrane through Gα , resulting in production of inositol 1,4,5-trisphosphate and diacylglycerol. However, there have been few studies that indicate directly the involvement of Gα proteins in this signalling pathway in the central nervous system. In the current study, we tried to pharmacologically characterize (-)-adrenaline-stimulated [ S]GTPγS binding to Gα in rat brain membranes. Functional activation of Gα coupled to α -adrenoceptor was investigated by using [ S]GTPγS binding/immunoprecipitation assay in the membranes prepared from rat cerebral cortex, hippocampus, and striatum. The specific [ S]GTPγS binding to Gα was stimulated by (-)-adrenaline in a concentration-dependent and saturable manner in rat cerebral cortical membranes. In hippocampal or striatal membranes, the stimulatory effects of (-)-adrenaline were scarce. The effect of (-)-adrenaline was potently inhibited by prazosin, a potent and selective α -adrenoceptor antagonist, but not by yohimbine, a selective α -adrenoceptor antagonist. The response was mimicked by cirazoline, but not by R(-)-phenylephrine. Although oxymetazoline also stimulated the specific [ S]GTPγS binding to Gα as an apparent "super-agonist", detailed pharmacological characterization revealed that its agonistic properties in this experimental system were derived from off-target effects on 5-HT receptors, but not via α -adrenoceptors. In conclusion, functional coupling of α -adrenoceptors to Gα proteins are detectable in rat brain membranes by means of [ S]GTPγS binding/immunoprecipitation assay. It is necessary to interpret the experimental data with caution when oxymetazoline is included as an agonist at α -adrenoceptors.