Angiotensin II Attenuates Synaptic GABA Release and Excites Paraventricular-Rostral Ventrolateral Medulla Output Neurons

Abstract

The hypothalamic paraventricular nucleus (PVN) neurons regulate sympathetic outflow through projections to the spinal cord and rostral ventrolateral medulla (RVLM). Although the PVN-RVLM pathway is important for the action of brain angiotensin II (Ang II) on autonomic control, the cellular mechanisms involved are not fully known. In this study, we examined the effect of Ang II on the excitability and synaptic inputs to RVLMprojecting PVN neurons. PVN neurons were retrogradely labeled by FluoSpheres injected into the RVLM of rats. Wholecell patch-clamp recordings were performed on labeled PVN neurons in brain slices. Ang II significantly increased the firing rate of PVN neurons from 3.63 Ϯ 0.65 to 6.10 Ϯ 0.75 Hz (P Ͻ 0.05, n ϭ 9), and such an effect was eliminated by an AT 1 receptor antagonist, losartan. Furthermore, inclusion of a G protein inhibitor, guanosine 5Ј-O-(2-thiodiphosphate, in the pipette internal solution did not alter the excitatory effect of Ang II on labeled PVN neurons. Application of 0.5 to 5 M Ang II significantly decreased the amplitude of evoked GABAergic inhibitory postsynaptic currents (IPSCs) in a dose-dependent manner. Also, 2 M Ang II significantly decreased the frequency of miniature IPSCs (mIPSCs) from 3.89 Ϯ 0.84 to 2.06 Ϯ 0.45 Hz (P Ͻ 0.05, n ϭ 11), but did not change the amplitude and decay time constant of mIPSCs. By contrast, Ang II had no significant effect on glutamatergic excitatory postsynaptic currents at the concentrations that inhibited IPSCs. In addition, Ang II failed to excite PVN neurons in the presence of bicuculline. Collectively, this study provides important new information that Ang II excites RVLM-projecting PVN neurons through attenuation of GABAergic synaptic inputs.

AT1 and glutamatergic receptors in paraventricular nucleus support blood pressure during water deprivation

“…The slow response is consistent with electrophysiological studies showing that the reversal of ANG II-induced excitation of PVN sympathoexcitatory neurons following washout of the peptide requires several minutes (9,30). Alternatively, the slow time course could be explained by PVN AT1R blockade decreasing vasopressin secretion and its subsequent clearance from plasma, due to reversal of the known stimulatory effect of ANG II in PVN on vasopressin release (39,48).…”

“…Interestingly, the sum of responses to PVN AT1R and glutamatergic blockade was equivalent to that of complete blockade with muscimol. If ANG II acts distinctly from glutamatergic inputs (29,30), then this result would suggest that excitation of AT 1 and glutamatergic receptors can completely explain the sustained activation of PVN observed during water deprivation.…”

“…Moreover, immunocytochemical visualization of AT1R in PVN failed to detect receptors on neurons that project to RVLM or spinal cord (33). Therefore, the action of ANG II in PVN to stimulate parvocellular sympathoexcitatory (9,30) or magnocellular vasopressinergic neurons may be, in part, indirect. This idea is supported by the recent studies of Latchford and Ferguson (27), which suggested that ANG II stimulates PVN magnocellular neurons via a glutamatergic interneuron.…”

“…This idea is supported by the recent studies of Latchford and Ferguson (27), which suggested that ANG II stimulates PVN magnocellular neurons via a glutamatergic interneuron. Similarly, Li and coworkers (29,30) propose that ANG II-induced excitation of PVN sympathoexcitatory neurons is via inhibition of local GABAergic synaptic activity but is independent of glutamatergic input. On the other hand, Cato and Toney (9) provided support for a parallel direct or indirect excitatory effect of ANG II through AT1R-mediated activation of a mixed cation current.…”

“…Therefore, we speculate that the increase in osmolality underlies at least in part the activation of AT 1 and glutamate receptors observed during water deprivation. If so, and if ANG II acts independently of glutamate as suggested by in vitro electrophysiological studies (29,30), then osmoreceptor-mediated activation of PVN sympathoexcitatory neurons may be conveyed via two separate parallel pathways, which include AT 1 and glutamate receptors, respectively (Fig. 6).…”