Cardiovascular effects produced by activation of GABA receptors in the rostral ventrolateral medulla of conscious rats.

The rostral ventrolateral medulla (RVLM) has been proposed as a region playing a major role in the tonic and reflex control of sympathetic vasomotor activity and blood pressure. Pharmacological activation of GABA(A) receptors with muscimol in the RVLM of anesthetized rats results in a large fall in mean arterial pressure (MAP), heart rate (HR) and sympathetic activity. In this study we evaluated the effects of activation of GABA receptors in the RVLM of conscious, freely moving rats. Bilateral microinjections of muscimol into the RVLM of conscious rats produced a large fall in MAP (-38+/-4 mm Hg, n=7) when compared with saline injections (NaCl 0.9%, 7+/-1 mm Hg, n=4). The decrease in MAP evoked by muscimol was accompanied by a significant increase in HR (muscimol 69+/-13 bpm vs. vehicle -33+/-12 bpm, P<0.05), an effect that was completely abolished by beta1 adrenergic receptor blockade. Conversely, bilateral microinjections of GABA(B) agonist, baclofen, evoked a pressor response, but in this case, the increase was not significantly different from that evoked by vehicle injections. These results 1) indicate that GABA(A) receptors have a powerful influence on the resting activity of RVLM neurons in conscious rats; 2) indicate that a compensatory sympathetic-mediated tachycardia is present after inhibition of RVLM neurons in conscious rats; 3) confirm and extend previous findings showing that RVLM neurons are critical for blood pressure maintenance even in normal non-anesthetized conditions.