Functional activation of Gα_q/11 protein via α₁ -adrenoceptor in rat cerebral cortical membranes

Although it is recognized that α₁ -adrenoceptors are coupled to diverse intracellular signalling pathways, its primary transduction mechanisms are evoked by activating phospholipase C in the cell membrane through Gα_q/11 , resulting in production of inositol 1,4,5-trisphosphate and diacylglycerol. However, there have been few studies that indicate directly the involvement of Gα_q/11 proteins in this signalling pathway in the central nervous system. In the current study, we tried to pharmacologically characterize (-)-adrenaline-stimulated [³⁵ S]GTPγS binding to Gα_q/11 in rat brain membranes. Functional activation of Gα_q/11 coupled to α₁ -adrenoceptor was investigated by using [³⁵ S]GTPγS binding/immunoprecipitation assay in the membranes prepared from rat cerebral cortex, hippocampus, and striatum. The specific [³⁵ S]GTPγS binding to Gα_q/11 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 α₁ -adrenoceptor antagonist, but not by yohimbine, a selective α₂ -adrenoceptor antagonist. The response was mimicked by cirazoline, but not by R(-)-phenylephrine. Although oxymetazoline also stimulated the specific [³⁵ S]GTPγS binding to Gα_q/11 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_2A receptors, but not via α₁ -adrenoceptors. In conclusion, functional coupling of α₁ -adrenoceptors to Gα_q/11 proteins are detectable in rat brain membranes by means of [³⁵ S]GTPγS binding/immunoprecipitation assay. It is necessary to interpret the experimental data with caution when oxymetazoline is included as an agonist at α₁ -adrenoceptors.