Differential Effects of Epinephrine and Norepinephrine on cAMP Response and Gi3α Protein Expression in Cultured Sympathetic Neurons
Abstract
Abstract The effect of 24-h pretreatment with epinephrine (EPI) or norepinephrine (NE) on α 2 - and β-adrenoceptor agonist-induced, cAMP responses and G i3 α-protein expression was studied in primary cultures of rat superior cervical ganglionic (SCG) neurons. SCG neurons, 10 to 12 days in culture, accumulated cAMP when stimulated with the β-adrenoceptor agonist isoproterenol and the preferential β 2 -adrenoceptor antagonist ICI 118,551 blocked this response. Similarly, the preferential α 2 -adrenoceptor agonist UK14,304 inhibited forskolin-stimulated cAMP accumulation, implying that cultured SCG neurons possess functional α 2 - and β 2 -adrenoceptors. A 24-h treatment of SCG neurons with EPI or NE induced desensitization of the cAMP response to the β-adrenoceptor agonist isoproterenol. Simultaneously, EPI treatment increased the maximal inhibitory cAMP response to the α 2 -adrenoceptor agonist UK14,304 and NE was without effect. Immunoblotting analyses of G i3 α subunits revealed that 24-h EPI but not NE treatment induces a 3- to 4-fold increase in the expression of G i3 α subunits. Furthermore, EPI-induced up-regulation of α-subunit expression can be blocked by the preferential β 2 -adrenoceptor antagonist ICI 118,551 but not by the preferential β 1 -adrenoceptor antagonist CGP 20712A. Our results suggest that changes in α 2 -adrenoceptor responsiveness induced by EPI may involve activation of β 2 -adrenoceptors that influence the expression of inhibitory G proteins. Thus, primary cultures of sympathetic neurons by possessing functional α 2 - and β-adrenoceptors may be a suitable model system to study the signaling mechanisms of “cross talk” between these adrenoceptor subtypes, which are known to play a central role in cardiovascular function.