Activation of GABA receptors in the NTS of awake rats reduces the gain of baroreflex bradycardia

In the present study we evaluated the effects of bilateral microinjection o f muscimol (a GABA(A) receptor agonist) and baclofen (a GABA(B) receptor ag onist) into the lateral commissural nucleus tractus solitarii (NTS) of awak e rats on the gain of the baroreflex (BG) activated by a short duration (10 -15 s) infusion of phenylephrine (Phe, 2.5 mu g/0.05 ml, i.v.). Microinject ion of muscimol (50 pmol/50 nl, n=8) into the NTS produced a significant in crease in baseline mean arterial pressure ((MAP) 122+/-6 vs. 101+/-2 mmHg), no changes in baseline heart rate (HR) and a reduction in BG (-1.59+/-0.1 vs. -0.69+/-0.1 beats/mmHg). Microinjection of baclofen (6.25 pmol/50 nl, n =6) into the NTS also produced a significant increase in baseline MAP (138/-5 vs. 103+/-2 mmHg), no changes in baseline HR and a reduction in BG (-1. 54+/-0.3 vs. -0.53+/-0.2 beats/mmHg). Considering that the reduction in BG could be secondary to the increase in MAP in response to microinjection of muscimol (n=6) or baclofen (n=7) into the NTS, in these two groups of rats we brought the MAP back to baseline by infusion of sodium nitroprusside (NP 3.0 mu g/0.05 ml, i.v.). Under these conditions, we verified that the BG r emained significantly reduced after muscimol (-1.49+/-0.2 vs. -0.35+/-0.2 b eats/mmHg) and after baclofen (-1.72+/-0.2 vs. -0.33+/-0.2 beats/mmHg) when compared to control. Reflex tachycardia was observed during the normalizat ion of MAP by NP infusion and, in order to prevent the autonomic imbalance from affecting BG, we used another group of rats treated with atenolol (5 m g/kg, i.v.), a beta(1) receptor antagonist. In rats previously treated with atenolol and submitted to NP infusion, we verified that BG remained reduce d after microinjection of muscimol or baclofen into the NTS. The data show that activation of GABA(A) and GABA(B) receptors, independently of the chan ges in the baseline MAP or HR, inhibited the neurons of the NTS involved in the parasympathetic component of the baroreflex. (C) 2000 Elsevier Science B.V. All rights reserved.