Abstract
The effect of 24-h pretreatment with epinephrine (EPI) or norepinephrine (NE) on α2- and β-adrenoceptor agonist-induced, cAMP responses and Gi3α-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 β2-adrenoceptor antagonist ICI 118,551 blocked this response. Similarly, the preferential α2-adrenoceptor agonist UK14,304 inhibited forskolin-stimulated cAMP accumulation, implying that cultured SCG neurons possess functional α2- and β2-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 α2-adrenoceptor agonist UK14,304 and NE was without effect. Immunoblotting analyses of Gi3α subunits revealed that 24-h EPI but not NE treatment induces a 3- to 4-fold increase in the expression of Gi3α subunits. Furthermore, EPI-induced up-regulation of α-subunit expression can be blocked by the preferential β2-adrenoceptor antagonist ICI 118,551 but not by the preferential β1-adrenoceptor antagonist CGP 20712A. Our results suggest that changes in α2-adrenoceptor responsiveness induced by EPI may involve activation of β2-adrenoceptors that influence the expression of inhibitory G proteins. Thus, primary cultures of sympathetic neurons by possessing functional α2- 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.
Footnotes
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Send reprint requests to: Douglas C. Eikenburg, Department of Pharmacological and Pharmaceutical Sciences, University of Houston, Houston, TX 77204. E-mail: deikenburg{at}uh.edu
- Abbreviations:
- EPI
- epinephrine
- HBSS
- Hanks’ balanced salt solution
- NGEM
- neuronal growth-enhancing medium
- NGM
- neuronal growth medium
- IBMX
- 3-isobutyl 1-methylxanthine
- NE
- norepinephrine
- SCG
- superior cervical ganglion
- UK 14,304
- 5-bromo-6-[2-imidazolin-2-yl-amino]quinoxaline
- Received February 16, 1999.
- Accepted June 22, 1999.
- The American Society for Pharmacology and Experimental Therapeutics
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