Contribution of a2-adrenoceptors in caudal ventrolateral medulla to cardiovascular regulation in rat
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چکیده
Sesoko, Shogo, Hiromi Muratani, Masanobu Yamazato, Hiroshi Teruya, Shuichi Takishita, and Koshiro Fukiyama. Contribution of a2-adrenoceptors in caudal ventrolateral medulla to cardiovascular regulation in rat. Am. J. Physiol. 274 (Regulatory Integrative Comp. Physiol. 43): R1119–R1124, 1998.—The inhibitory action of a2agonists on the cardiovascular neurons has been elucidated in the rostral ventrolateral medulla (RVLM) but not in the caudal ventrolateral medulla (CVLM). Our study aimed to clarify whether microinjection of clonidine into the CVLM elicits any cardiovascular effect and whether endogenous a2-adrenoceptor-mediated mechanisms contribute to the tonic activity of the CVLM neurons. In male Sprague-Dawley rats (7–9 wk old, 270–320 g) anesthetized with urethan, unilateral microinjection of 8 nmol of clonidine into the CVLM (n 5 10) increased mean arterial pressure (MAP) and renal sympathetic nerve activity (RSNA) by 12.1 6 1.8 mmHg (mean 6 SE, P , 0.01) and 25.8 6 4.8% (P , 0.01), while heart rate (HR) remained unaltered. Unilateral microinjection of 2 nmol of SKF-86466, a selective blocker of the a2-adrenoceptors, into the CVLM (n 5 10) decreased MAP, HR, and RSNA (211.6 6 2.6 mmHg, 226 6 7 beats/min, and 215.3 6 1.7%, respectively, P , 0.01 for each). Artificial cerebrospinal fluid caused neither a cardiovascular effect nor a sympathetic response. Prior injection of SKF-86466 into the ipsilateral CVLM attenuated the effects of clonidine. Bilateral microinjection of muscimol into the RVLM abolished the effects of both clonidine and SKF-86466 injected into the CVLM. The pressor and sympathoexcitatory effects of clonidine injected into the CVLM suggest a neuroinhibitory action of the drug on the CVLM neurons. In addition,the depressor and sympathoinhibitory effects of SKF-86466 injected into the CVLM indicated that activation of a2-adrenoceptors by endogenous ligand inhibits CVLM neurons. The effects of clonidine and the a2-adrenoceptor antagonist in the CVLM require the integrity of the RVLM.
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