Angiotensin II excites paraventricular nucleus neurons that innervate the rostral ventrolateral medulla: An in vitro patch-clamp study in brain slices

Neurons of the hypothalamic paraventricular nucleus (PVN) are key controllers of sympathetic nerve activity and receive input from angiotensin II (ANG II) - containing neurons in the forebrain. This study determined the effect of ANG II on PVN neurons that innervate in the rostral ventrolateral medulla (RVLM) - a brain stem site critical for maintaining sympathetic outflow and arterial pressure. Using an in vitro brain slice preparation, whole cell patchclamp recordings were made from PVN neurons retrogradely labeled from the ipsilateral RVLM of rats. Of 71 neurons tested, 62 (87%) responded to ANG II. In current-clamp mode, bath-applied ANG II ( 2 muM) significantly ( P < 0.05) depolarized membrane potential from - 58.5 +/- 2.5 to - 54.5 +/- 2.0 mV and increased the frequency of action potential discharge from 0.7 +/- 0.3 to 2.8 +/- 0.8 Hz ( n = 4). Local application of ANG II by low-pressure ejection from a glass pipette ( 2 pmol, 0.4 nl, 5 s) also elicited rapid and reproducible excitation in 17 of 20 cells. In this group, membrane potential depolarization averaged 21.5 +/- 4.1 mV, and spike activity increased from 0.7 +/- 0.4 to 21.3 +/- 3.3 Hz. In voltage- clamp mode, 41 of 47 neurons responded to pressure-ejected ANG II with a dose-dependent inward current that averaged - 54.7 +/- 3.9 pA at a maximally effective dose of 2.0 pmol. Blockade of ANG II AT1 receptors significantly reduced discharge ( P < 0.001, n = 5), depolarization ( P < 0.05, n = 3), and inward current ( P < 0.01, n = 11) responses to locally applied ANG II. In six of six cells tested, membrane input conductance increased ( P < 0.001) during local application of ANG II ( 2 pmol), suggesting influx of cations. The ANG II current reversed polarity at + 2.2 +/- 2.2 mV ( n = 9) and was blocked ( P < 0.01) by bath perfusion with gadolinium (Gd3+, 100 muM, n = 8), suggesting that ANG II activates membrane channels that are nonselectively permeable to cations. These findings indicate that ANG II excites PVN neurons that innervate the ipsilateral RVLM by a mechanism that depends on activation of AT1 receptors and gating of one or more classes of ion channels that result in a mixed cation current.