Early Microglial Pruning of Inhibitory Synapses in the Paraventricular Nucleus of the Hypothalamus Underlies Sympathetic Overactivation in Pulmonary Hypertension

Pulmonary hypertension (PH) is a progressive cardiopulmonary disorder characterized by elevated pulmonary arterial pressure, right heart dysfunction, and high mortality. Despite advances in prognosis, current therapies remain limited by significant side effects, and PH continues to carry unacceptably high morbidity and mortality. Beyond vascular pathology, PH is marked by sustained sympathetic overactivation that worsens clinical outcomes, yet the central origin of this autonomic imbalance remains unclear. Our group previously identified the paraventricular nucleus (PVN) of the hypothalamus—a major regulator of sympathetic tone that integrates excitatory and inhibitory inputs to control cardiovascular output—and microglia, the brain’s resident immune cells, as potential contributors to this dysregulation. In our prior work, chronic microglial activation alone was sufficient to induce PH and was accompanied by increased microglial density and activation in the PVN. Separately, in chronic PH, we observed altered PVN synaptic architecture: inhibitory presynaptic inputs (GAD + ) were preserved, but inhibitory postsynaptic terminals (NLGN2 + ) were greatly reduced, and excitatory presynaptic inputs (vGLUT2 + ) were more abundant. These synaptic changes aligned with a shift toward greater excitatory drive within the PVN; however, microglial engagement with the synaptic architecture at this chronic stage did not explain the observed changes, as microglial pruning of inhibitory postsynaptic sites was reduced despite their loss, while pruning of excitatory presynaptic terminals was elevated. Because the chronic loss of inhibitory postsynaptic sites did not align with the pattern of microglial pruning at this stage, we hypothesized that microglia may have contributed to these synaptic changes earlier in disease progression rather than during established pathology. To test this possibility, we exposed mice to just 7 days of chronic hypoxia (10% oxygen) rather than the typical 28-day duration used to induce chronic PH (n = 4-5 mice per group; normoxia (Nx) vs. hypoxia (Hx)). We then performed immunohistochemistry on 30-μm brain sections containing the PVN to label microglia and synaptic terminals, followed by high-resolution confocal imaging and 3D reconstruction in Imaris to quantify PVN synaptic density and microglial pruning (puncta engulfed normalized to microglial volume and total PVN puncta). After just 7 days, inhibitory presynaptic inputs (GAD + ) were unchanged, but inhibitory postsynaptic terminals (NLGN2 + ) were already reduced (Nx: 4.54 ± 0.44 vs. Hx: 2.21 ± 0.42 × 10 4 puncta, p = 0.0140). In parallel, excitatory presynaptic inputs (vGLUT2 + ) were elevated (Nx: 0.74 ± 0.03 vs. Hx: 1.28 ± 0.06 × 10 4 puncta, p = 0.0027), indicating that excitatory–inhibitory imbalance emerges rapidly in PH. Mechanistically, microglial pruning reflected a strikingly selective pattern: pruning was upregulated at inhibitory postsynaptic sites (Nx: 0.96 ± 0.16 vs. Hx: 1.68 ± 0.16 × 10 -6 normalized puncta engulfed, p = 0.0246) but unchanged at inhibitory and excitatory presynaptic terminals, identifying a transient period during which microglia preferentially remove inhibitory postsynaptic structures while sparing other synaptic elements. These findings provide strong evidence that early microglial pruning of inhibitory postsynaptic terminals contributes to the loss of inhibitory control in the PVN, suggesting a neuroimmune mechanism that sustains sympathetic activation in PH. Future studies will probe the molecular cues directing microglia to these postsynaptic sites and assess whether preventing this early pruning phase can attenuate the sympathetic overactivation underlying PH. 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.