Neuronal excitation by angiotensin II in the rostral ventrolateral medulla of the rat in vitro

We examined the effects of angiotensin II (ANG II) on spontaneous unit activity in slices of the rat rostral ventrolateral medulla (RVLM), ANG II (1-3 microM) excited 61% of a population of slowly and irregularly firing RVLM neurons (predrug, 1.2 +/- 0.1 spikes/s; postdrug, 4.6 +/- 0.3 spikes/s; n = 52). ANG II had no effect on pacemaker-like rapidly firing neurons (predrug, 8.6 +/- 0.4 spikes/s; n = 33). The effect of ANG II on slowly firing cells was repeatable and was reduced 75% by 3 microM losartan (baseline, 1.7 +/- 0.4 spikes/s; ANG II, 5.3 +/- 0.7 spikes/s; ANG II+losartan, 2.4 +/- 0.6 spikes/s; n = 12). The ongoing activity of slowly firing neurons was unaffected by 0.5-1 mM kynurenic acid (an ionotropic excitatory amino acid receptor antagonist). Most ANG II-responsive neurons (10 of 11) were inhibited by the alpha 2-adrenergic receptor agonist UK-14,304, but pacemaker-like neurons were not. In conclusion, the RVLM contains neurons excited by AT1 receptor agonists. These neurons are distinct from the previously described pacemaker nonadrenergic presympathetic cells. They may be responsible for the pressor effects produced by injecting ANG II into the RVLM in vivo.