Sildenafil Enhances Quantity of Immature Neurons and Promotes Functional Recovery in the Developing Ischemic Mouse Brain

<b><i>Background:</i></b> Hypoxic-ischemic (HI) injury to the developing brain occurs in 1 out of 1,000 live births and remains a major cause of significant morbidity and mortality. A large number of survivors suffer from long-term sequelae including seizures and neurological deficits. However, the pathophysiological mechanisms of recovery after HI insult are not clearly understood, and preventive measures or clinical treatments are nonexistent or not sufficiently effective in the clinical setting. Sildenafil as a specific phosphodiesterase 5 inhibitor leads to increased levels of the second messenger cyclic guanosine monophosphate (cGMP) and promotes functional recovery and neurogenesis after ischemic injury to the adult brain. <b><i>Objective:</i></b> Here, we investigated the effect of sildenafil treatment on activation of intracellular signaling pathways, histological and neurogenic response including functional recovery after an ischemic insult to the developing brain. <b><i>Design/Methods:</i></b> Nine-day-old C57BL/6 mice were subjected either to sham operation or underwent ligation of the right common carotid artery followed by hypoxia (8%) for 60 min. Animals were either administered sildenafil (10 mg/kg, i.p.) or vehicle 2 h after hypoxia. A subgroup of animals received multiple injections of 10 mg/kg daily on 5 consecutive days. Pups were either perfusion fixed at postnatal days 14 or 47 for immunohistochemical analysis, or brains were dissected 2, 6, 12, and 24 h after the end of hypoxia and analyzed for cGMP, pAkt, pGSK-3β, and β-catenin by means of ELISA or immunoblotting. In addition, behavioral studies using the wire hang test and elevated plus maze were conducted 21 and 38 days after HI injury. <b><i>Results:</i></b> Based on cresyl violet staining, single or multiple sildenafil injections did not reveal any differences in injury scoring compared to sham animals. However, cerebral levels of cGMP were altered after sildenafil therapy. Treatment significantly increased numbers of immature neurons, as indicated by doublecortin immunoreactivity in the ipsilateral subventricular zone and striatum. In addition, animals treated with sildenafil after HI insult demonstrated improved functional recovery. pAkt, pGSK-3β, and β-catenin levels vary after HI injury but additional sildenafil treatment had no impact on protein expression compared to the level of sham controls. <b><i>Conclusions:</i></b> Here, we report that treatment with sildenafil after HI insult did not improve histological brain injury scores. Nevertheless, our results suggest involvement of the cGMP and PI3K/Akt/GSK-3β signaling pathway with promotion of a neurogenic response and reduction of neurological deficits. In summary, sildenafil may have a role in promoting recovery from HI injury in the developing brain.