Investigating mechanisms of diminished GABAergic inhibitory effcacy in the hypothalamic paraventricular nucleus (PVN) of AngII-salt hypertensive mice

Abstract

The PVN receives glutamatergic excitation from sodium and AngII sensing regions of the forebrain along with dense GABAergic inhibition from the surrounding peri-nuclear zone (PNZ). Studies in various models of hypertension and heart failure report that GABAergic inhibitory tonus is diminished, contributing to the net increase in PVN-driven sympathetic nerve activity that contributes to the pathogenesis of these chronic and prevalent cardiovascular diseases. Here we compared GABAergic inhibition of the PVN in normotensive mice consuming a normal salt (0.4% NaCl) diet (NSD) and hypertensive mice consuming a high salt (4% NaCl) diet (HSD) with systemic infusion of AngII (600 ng/kg/min, s.c.) (AngII-salt HTN). Using gramicidin perforated patch recordings in brain slices, we determined that the GABA equilibrium potential (EGABA) of PVN neurons was significantly depolarized in the AngII-salt HTN group (Δ+34.3 ± 7.5 mV) compared to the NSD group (n=3 cells) (P=0.003), consistent with diminished inhibitory effcacy of GABA and reflecting increased intracellular [Cl−]. Western blot analysis of PVN punches yielded perplexing results, showing increased expression of the Cl− efflux transporter KCC2 (+12%) and diminished expression of the Cl− influx transporter NKCC1 (-38%) — suggestive of compensation for a separate cause of elevated intracellular [Cl−]. Previously, we determined that EGABA can be rapidly and reversibly depolarized (+9 mV) by perfusing slices with high K+ (+10 mM) aCSF in the presence of ionotropic glutamate receptor blockade with CNQX (10 μM) and APV (50 μM). Hence it seems that heightened synaptic GABA release is capable of increased intracellular [Cl−]. Given that the magnitude of the latter effect is small relative to the measured depolarization of EGABA in AngII-salt HTN neurons, increased synaptic GABA release appears inadequate to explain the measured increase of EGABA in AngII-salt HTN. We posit that glial buffering of extracellular Cl− might contribute to the unexpectedly large EGABA depolarization in AngII-salt HTN. Notably, EGABA among PNZ GABAergic neurons was also significantly depolarized in the AngII-salt HTN group (Δ+22.4 ± 8.8 mV, n=3 cells) compared to NSD controls (n=3 cells) (P=0.037). Mechanism(s) supporting this are presently unknown. Of special interest, preliminary data indicate that most GABAergic PNZ neurons have recurrent GABAergic responses that are predicted (based on having depolarized EGABA) to have diminished feedback inhibition in the AngII-salt group. If true, PNZ neurons would be expected to increase synaptic GABA release in the PVN of mice with AngII-salt HTN. What contribution the latter effect makes to the net depolarization of EGABA in PVN neurons is under investigation. NS115072 (GMT). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.