Inhibitory effects of angiotensin II on barosensitive rostral ventrolateral medulla neurons of the rat.

1. The brain renin-angiotensin system can influence arterial baroreceptor reflex control of blood pressure (BP) through both direct and indirect effects on sympathetic premotor neurons of the rostral ventrolateral medulla (RVLM). The present study examined the direct effect of angiotensin (Ang) II applied by microiontophoresis on the ongoing activity of single RVLM neurons. 2. In 26 urethane-anaesthetized Wistar rats, recordings of single unit activities of barosensitive RVLM neurons were made from one barrel of a six-barrel micropipette assembly. The other five barrels were filled with either L-glutamate, AngII, valsartan (an AT1 receptor antagonist), PD 123177 (an AT2 receptor antagonist) and saline. All drugs were applied by microiontophoresis. 3. Mean BP was 83 +/- 3 mmHg. Application of AngII inhibited the ongoing activity of RVLM neurons, identified as barosensitive because their activity was inhibited by a phenylephrine- induced increase in BP, from 12.6 +/- 1.5 to 5.4 +/- 1.1 Hz (n=24; P < 0.001). Angiotensin II also inhibited the glutamate-evoked excitation of barosensitive RVLM neurons from 15 +/- 3 to 5.8 +/- 2.0 Hz (n=6; P < 0.001). Valsartan significantly increased neuronal activity from 9.5 +/- 2.3 to 13.5 +/- 3.2 Hz (n=7, P < 0.01), whereas PD 123177 significantly decreased neuronal activity from 13.5 +/- 3.5 to 9.9 +/- 2.8 Hz (n=13; P < 0.01). 4. The results suggest that AngII exerts a tonic inhibitory effect on barosensitive RVLM neurons, which is presumably mediated through AT1 receptor stimulation.