Important GABAergic mechanism within the NTS and the control of sympathetic baroreflex in SHR

Inhibitory neurotransmission has an important role in the processing of sensory afferent signals in the nucleus of the solitary tract (NTS), particularly in spontaneously hypertensive rats (SHR). In the present study, we tested the hypothesis that γ-aminobutyric acid (GABA) mediated neurotransmission within the NTS produces an inhibition of the baroreflex response of splanchnic sympathetic nerve discharge (sSND).In urethane-anesthetized, artificially ventilated and vagotomized male SHR and Wistar Kyoto (WKY) rats we compared baroreflex-response curves evoked after bilateral injections into the NTS of the GABA-A antagonist bicuculline (25 pmol/50 nl) or the GABA-B antagonist CGP 35348 (5 nmol/50 nl). Baseline MAP in SHR was higher than the WKY rats (SHR: 153 ± 5, vs. WKY: 112 ± 6 mm Hg, p < 0.05). Bilateral injection of bicuculline or CGP 35348 into the NTS induced a transient (5 min) reduction in MAP (∆ = −26 ± 4 and −41 ± 6 mm Hg, respectively vs. saline ∆ = + 4 ± 3 mm Hg, p < 0.05) and sSND (∆ = −21 ± 13 and −78 ± 7%, respectively vs. saline: ∆ = + 6 ± 4% p < 0.05). Analysis of the baroreceptor curve revealed a decrease in the lower plateau (43 ± 11 and 15 ± 5%, respectively vs. saline: 78 ± 6%, p < 0.05) and an increase in the sympathetic gain of baroreflex (6.3 ± 0.3, 7.2 ± 0.8% respectively vs. saline: 4.2 ± 0.4%, p < 0.05). Bicuculline or CGP35348 into the NTS in WKY rats did not change MAP, sSND and sympathetic baroreflex gain.These data indicate that GABAergic mechanisms within the NTS act tonically reducing sympathetic baroreflex gain in SHR.