Sildenafil reduced neuroinflammation and improved white matter injury in a rat model of term neonatal hypoxic-ischemic encephalopathy

Neonatal hypoxic-ischemic encephalopathy (HIE) can cause lifelong neurological impairments. In its tertiary phase, ongoing neuroinflammation creates a toxic environment that promotes neuronal and oligodendrocyte loss. Sildenafil has shown neuroprotective effects in adult models by reducing inflammation and supporting oligodendrocyte survival, but its role in HIE remains unexplored. This study investigated the effects of sildenafil on neuroinflammation and white matter injury in a rat model of term neonatal HIE. Hypoxia-ischemia (HI) was induced in postnatal day 10 (P10) male Long-Evans rats via a left carotid ligation followed by 2 h of hypoxia (8% oxygen). Pups were randomized to receive oral sildenafil or vehicle starting 12 h post-HI, twice daily for 7 days. White matter integrity (corpus callosum and external capsule), oligodendrocyte presence, and glial activation were assessed by histology and immunohistochemistry. Inflammatory markers were measured by enzyme-linked immunosorbent assay (ELISA), and signaling pathways were examined by Western blot. Outcomes were compared to sham and untreated HI controls. HI significantly increased number of GFAP + reactive astrocytes and Iba1 + microglia, alongside elevated TNFα and IL-1β levels. Thickness of the corpus callosum and left external capsule was reduced. Sildenafil treatment - particularly at medium and high doses - attenuated astrocytes and microglia activation, restored microglial morphology, and normalized cytokine expression. White matter thickness was significantly improved, with increased numbers of total Olig2 + and mature CC1 + oligodendrocytes. Mechanistically, sildenafil restored p-AKT levels, which suggests involvement of the PI3K/AKT/mTOR pathway. Sildenafil significantly reduced neuroinflammation, improved white matter integrity, and supported oligodendrocyte recovery after neonatal HI. These findings highlight the potential of sildenafil as a neurorestorative therapy during the tertiary phase of injury in neonatal HIE.