Alpha1-adrenoceptors in the rat cerebral cortex: new insights into the characterization of alpha1L-and alpha1D-adrenoceptors

Abstract

Among the three alpha1-adrenoceptor subtypes (alpha1A, alpha1B and alpha1D) a peculiar intracellular localization and poor coupling to membrane signals of cloned alpha1D-adrenoceptor has been reported. In addition, the alpha1L-adrenoceptor (low affinity for prazosin), a functional phenotype of alpha1A, has been described. The purpose of this work was to analyze the expression, cellular localization and coupling to membrane signalling (inositol phosphate accumulation) of alpha1-adrenoceptor subtypes in a native tissue, the rat cerebral cortex. mRNA for the three subtypes was quantified by real-time RT-PCR (alpha1D>alpha1B>>alpha1A). Alpha1-adrenoceptors were also detected by immunoblotting, revealing alpha1A- and alpha1B-adrenoceptors to be predominantly expressed in the membrane fraction and the alpha1D-adrenoceptor to be localized in the cytosolic fraction. Competitive radioligand binding studies revealed the presence of alpha1D-adrenoceptor in tissue homogenates, whereas only alpha1A- and alpha1B-subtypes were detected in membranes. The proportion of alpha1A-adrenoceptor increased after treatment with noradrenaline, suggesting differences in agonist-mediated trafficking. Saturation experiments detected high- and low (alpha1A/L)-prazosin binding sites, the latter of which disappeared on incubation with GppNHp. The alpha1A/L-adrenoceptor was heavily implicated in the inositol phosphate response, while the alpha1D-subtype did not play a relevant role. These results suggest that the predominant cytosolic localization of alpha1D-adrenoceptor lies behind its poor coupling to membrane signalling such as inositol phosphate pathway. The fact that the alpha1Ladrenoceptor detected in radioligand binding studies disappeared in the presence of GppNHp implies that it represents a conformational state of the alpha1A-adrenoceptor coupled to G protein.