Regulation of cardiac sympathetic afferent reflex by GABAA and GABAB receptors in paraventricular nucleus in rats

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The aim of the present study was to determine the role of GABAA and GABAB receptors in paraventricular nucleus (PVN) in regulating cardiac sympathetic afferent reflex (CSAR). Under urethane (800 mg/kg) and α-chloralose (40 mg/kg) anesthesia, renal sympathetic nerve activity (RSNA), mean arterial pressure (MAP) and heart rate (HR) were recorded in sinoaortic-denervated and cervical-vagotomized rats. CSAR was evaluated based in the response of RSNA to epicardial application of capsaicin (0.3 nmol) or bradykinin (1 nmol). Bilateral PVN microinjection of the GABAA receptor agonist isoguvacine (10 nmol) attenuated CSAR, while the GABAB receptor agonist baclofen (1 nmol) abolished CSAR. Both isoguvacine and baclofen greatly decreased baseline RSNA and MAP. The GABAA receptor antagonist gabazine (0.1 nmol) had no significant effect on CSAR, but the GABAB receptor antagonist CGP-35348 (10 nmol) enhanced CSAR. Gabazine caused greater increases in baseline RSNA and MAP than CGP-35348. Vigabatrin (10 nmol), a selective GABA-transaminase inhibitor which increases endogenous GABA level, abolished CSAR, and decreased baseline RSNA, MAP and HR. The effects of vigabatrin were antagonized by combined gabazine (0.1 nmol) and CGP-35348 (10 nmol). The results indicate that activation of either GABAA or GABAB receptors in the PVN inhibits CSAR, while blockage of GABAB receptors in the PVN enhances CSAR. Endogenous GABA in the PVN could have an important role in regulating CSAR.

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