Patterned glutamatergic input from median preoptic nucleus strengthens local GABAergic inhibition of hypothalamic paraventricular nucleus neurons

The hypothalamic paraventricular nucleus (PVN) is a key regulator of sympathetic activity. Studies show that dysregulation of PVN presympathetic neurons (PSNs) contributes to cardiovascular and metabolic diseases. Although PVN PSNs receive strong glutamatergic excitation from the forebrain median preoptic nucleus (MnPO), their firing is strongly suppressed by GABAergic inhibition from the surrounding peri-nuclear zone (PNZ). It would seem, therefore, that factors regulating the strength of PNZ GABAergic inhibition are likely determinants of pathogenic sympathetic hyperactivity. We previously showed that PNZ GABAergic inhibition is acutely enhanced both by bath application of L-glutamate (L-Glu, 10 min, 100 µM) and by L-Glu release from local synapses. We termed this phenomenon, glutamate-GABA strengthening or GGS. We further showed that PVN GGS is dependent on extracellular L-Glu uptake by excitatory amino acid transporter 3 (EAAT3) on PVN GABA terminals. It is presently unknown whether synaptic glutamate released from known excitatory inputs to the PVN are capable of driving PVN GGS and what impact GGS has on the probability of PSN firing. Here, we prepared brain slices that preserve synaptic connections between MnPO and PVN in vesicular GABA transporter (VGAT)-channelrhodopsin2 (ChR2) transgenic mice and applied the whole-cell patch-clamp technique to PVN PSNs. In voltage-clamp mode, glutamatergic excitatory postsynaptic currents (EPSCs) evoked by electrical stimulation of the MnPO were recorded along with inhibitory postsynaptic current (IPSCs) evoked by photo stimulation of the PNZ. Because single pulse (< 0.5 ms) electrical stimulation of MnPO routinely induced only single action potentials in PVN PSNs, we investigated the capacity of MnPO stimulation to strengthen GABAergic responses when MnPO stimuli were applied with a pattern mimicking the pattern of action potentials recorded during bath application of L-Glu. The pattern of applied MnPO stimuli was determined from a group of 9 PVN neurons exposed to bath L-Glu according to our standard GGS protocol. Patterned MnPO stimulation increased the amplitude of PNZ evoked IPSCs (+26 ± 18%, n=7 cells) by an amount comparable to GGS induced by bath L-Glu (+30 ± 8 %, n=19 cells). Decay time of photo-evoked IPSCs was also increased (129 ± 65 to 191 ± 93 ms), consistent with observed prolongation of spontaneous IPSCs after bath L-Glu (60 ± 30 to 76 ± 40 ms). Effects of patterned MnPO stimulation were suppressed in the presence of the EAAT inhibitor TBOA, indicating mediation by the GGS mechanism. In separate current clamp recording, patterned MnPO stimulation significantly reduced action potential firing probability (-11 ± 8%, P=0.034, n=6) among PVN PSNs during the post-MnPO stimulation period. Results indicate that PVN GGS is a robust form of heterosynaptic autoregulatory plasticity induced by MnPO excitation that restrains PSN firing driven by MnPO input. Unknown is whether the GGS mechanism is intact or compromised in specific disease modalities, and hence whether PVN GGS mitigates or contributes to pathogenic sympathoexcitation. Support: NS115072 (GMT) This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.