Hypoxia-induced astrocytic reaction and increased vascular permeability in the rat cerebellum

Hypoxia is an important factor linked to induction of vascular leakage and formation of brain edema. In this connection, astrocytes associated closely with the blood vessels are deemed to be involved. This study investigated the response of astrocytes to hypoxia in the adult rat cerebellum, and along with this, the integrity of the blood-brain barrier (BBB) was assessed using fluorescent and electron dense tracers. In rats subjected to hypoxia, mRNA and protein expression of hypoxia inducible factor-1alpha (HIF-1alpha), vascular endothelial growth factor (VEGF), glial fibrillary acidic protein (GFAP), and aquaporin-4 (AQ4) was significantly increased. VEGF and AQ4 immunoreactive cells were identified as astrocytes by double immunofluorescence labeling. Increased VEGF tissue concentration and astrocytic swelling as observed in hypoxic rats were reduced after melatonin administration. Following intraperitoneal or intravenous injection of rhodamine isothiocyanate (RhIC) or horseradish peroxidase (HRP), leakage of both tracers was observed in hypoxic rats but not in the controls indicating that functional integrity of BBB is compromised in hypoxia/reoxygenation. Enhanced gene and protein expression of VEGF may contribute to increased permeability of blood vessels. AQ4, a water transporting protein, is upregulated in astrocytes in hypoxia suggesting the cells are involved in edema formation. To this end, melatonin may be beneficial in reducing edema as it reduced VEGF concentration and, hence, vascular permeability.