GABAergic synaptic inhibition in the hypothalamic paraventricular nucleus (PVN) is strengthened by acute activation of glutamatergic inputs from the median preoptic nucleus

Abstract

Pre‐sympathetic neurons (PSNs) in the hypothalamic PVN are normally maintained in a state of near discharge quiescence reflecting the resting dominance of GABAergic inhibition. Our studies indicate that synaptic inhibition is actively maintained in the PVN, in part, by EAAT3‐mediated uptake of L‐glutamate (Glu) into local GABAergic synaptic terminals arising from the surrounding peri‐nuclear zone (PNZ). Notably, this Glu‐GABA strengthening (GGS) mechanism relies on apparent increases both in GABA synthesis and packaging into synaptic vesicles. Studies have so far demonstrated that PVN GGS can be elicited either by exposure of brain slices to bath Glu (100 μM) or high K+ (+10 mM). Studies performed in vivo indicate that Glu nanoinjection into the PVN increases GABAergic inhibitory control of renal sympathetic nerve activity (SNA). To investigate whether endogenous synaptic Glu release can drive PVN GGS, glutamatergic EPSCs were evoked in PVN PSNs from the median preoptic nucleus (MnPO). To accomplish this, we created an angled horizontal slice preparation containing the MnPO, PVN and PNZ. Single pulse (0.5 ms, 0.5 to 1.0 mA) electrical stimulation of MnPO evoked amongst PVN PSNs (n=2) mostly unitary EPSCs (ampl.: −139 ± 5 pA, tau: 3.27 ± 0.01). In slices prepared from vGAT‐ChR2(H134R) expressing mice, GABAergic IPSCs were evoked by blue light stimulation (20 ms, 0.4 mW, 0.067 Hz) of PNZ GABA neurons before and after glutamatergic EPSCs were electrically evoked from MnPO (1 Hz, 10 min). The amplitude of photostimulated GABAergic IPSCs at baseline (−152 ± 47 pA) was increased (−220 ± 83 pA) within ~3 minutes following MnPO stimulation (n=2). PNZ eIPSC amplitude was not increased (before: −89.5 ± 9.7 pA, after: −86.5 ± 10.3, pA) when MnPO stimulation was performed in the presence of the pan‐specific Glu uptake inhibitor TBOA (100 μM). Findings indicate that excitatory inputs to PVN from the forebrain lamina terminalis can acutely strengthen GABAergic inhibition through an EAAT3 uptake‐dependent mechanism consistent with PVN GGS. Grading of PVN synaptic inhibition in pace with excitation could help prevent excitotoxic injury of PSNs during prolonged physiological challenges (e.g., water deprivation) and in chronic diseases (e.g., neurogenic hypertension) accompanied by persistently elevated SNA.Support or Funding InformationNIH HL088052 (GMT)