Functional activation of Gαq/11 protein via α1‐adrenoceptor in rat cerebral cortical membranes

Abstract

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 Gαq/11, resulting in production of inositol 1,4,5‐trisphosphate and diacylglycerol. However, there have been few studies that indicate directly the involvement of Gαq/11 proteins in this signalling pathway in the central nervous system. In the current study, we tried to pharmacologically characterize (−)‐adrenaline‐stimulated [35S]GTPγS binding to Gαq/11 in rat brain membranes. Functional activation of Gαq/11 coupled to α1‐adrenoceptor was investigated by using [35S]GTPγS binding/immunoprecipitation assay in the membranes prepared from rat cerebral cortex, hippocampus, and striatum. The specific [35S]GTPγS binding to Gαq/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 [35S]GTPγS binding to Gαq/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‐HT2A receptors, but not via α1‐adrenoceptors. In conclusion, functional coupling of α1‐adrenoceptors to Gαq/11 proteins are detectable in rat brain membranes by means of [35S]GTPγS binding/immunoprecipitation assay. It is necessary to interpret the experimental data with caution when oxymetazoline is included as an agonist at α1‐adrenoceptors.