Angiotensin II stimulates spinally projecting paraventricular neurons through presynaptic disinhibition

Paraventricular nucleus (PVN) neurons that project to the spinal cord are important in the control of sympathetic outflow. Angiotensin II (Ang II) can stimulate PVN neurons, but its cellular mechanisms are not clear. In this study, we determined the effect of Ang II on the excitatory and inhibitory synaptic inputs to spinally projecting PVN neurons. Whole-cell patch-clamp recordings were performed on PVN neurons labeled by a retrograde fluorescence tracer injected into the thoracic spinal cord of rats. Immunocytochemistry labeling revealed that the immunoreactivity of angiotensin type I (AT(1)) receptors was colocalized with a presynaptic marker, synaptophysin, in the PVN. Application of 0.1-5 muM Ang II significantly decreased the amplitude of evoked GABAergic IPSCs in a concentration-dependent manner. Also, Ang II decreased the frequency of miniature IPSCs from 2.56 +/- 0.45 to 1.05 +/- 0.20 Hz (p < 0.05; n = 12), without affecting the amplitude and the decay time constant. The effect of Ang II on miniature IPSCs was blocked by losartan but not PD 123319. However, Ang II had no effect on the evoked glutamatergic EPSCs and did not alter the frequency and amplitude of miniature EPSCs at concentrations that attenuated IPSCs. Furthermore, Ang II increased the firing rate of PVN neurons from 3.75 +/- 0.36 to 7.89 +/- 0.85 Hz (p < 0.05; n = 9), and such an effect was abolished by losartan. In addition, Ang II failed to excite PVN neurons in the presence of bicuculline. Thus, this study provides substantial new evidence that Ang II excites spinally projecting PVN neurons by attenuation of GABAergic synaptic inputs through activation of presynaptic AT(1) receptors.