White matter injury in kaolin-induced neonatal rat hydrocephalus

Hydrocephalus is a common neurological condition characterized by obstruction of cerebrospinal fluid (CSF) flow leading to enlargement of CSF-containing ventricular cavities in the brain. The primary target of injury is axons in the periventricular white matter. The stretching of axons leads to dysfunction and ultimately axotomy. Many experiments in mature animals systems indicate that white matter blood flow is decreased. A decrease in oxygen saturation should lead to production of oxygen free radicals and activation of the transcription factor hypoxia inducible factor 1 alpha (HIF). HIF further activates a cascade of cellular responses, notably, vascular endothelial growth factor (VEGF). The main effect of VEGF is to cause production of new blood vessels. Its expression can have an influence on vascular permeability and could play a role in water dysregulation in the hydrocephalic brain. Our first goal was to characterize the structural changes of a neonatal model of kaolin-induced hydrocephalus in rat. Our second goal was to investigate the molecular sequence of events resulting from ischemia. Sprague-Dawley rats underwent kaolin injection into the cisterna magna at postnatal day 1. They had enlarged ventricles by 7 days and severe dilatation by 21 days as assessed by magnetic resonance imaging (MRI) and histology. Periventricular white matter, including corpus callosum and internal capsule, was edematous at 7 days and severely atrophic at 21 days. ELISA and Western blots revealed decreased expression of myelin associated proteins including myelin basic protein (MBP). Pimonidazole hydrochloride (2-nitroimidazole) specifically binds to thiol groups of proteins, peptides and amino acids in a hypoxic environment. Following subcutaneous administration, immunohistochemistry demonstrated hypoxia-associated adducts in periventricular glial cells. Nitrotyrosine, an oxidative stress marker, could be detected by immunohistochemistry extensively in white matter and around some blood vessels. Weak labeling was also seen around large neurons in cerebral cortex. HIF and 4-Hydroxy-2-nonenal (HNE) for endogenous lipid peroxidation are currently under investigation. VEGF immunohistochemistry in normal rats revealed positive cortical neurons in 7-day old rats; this diminished at 21 days. The pattern of expression shifted to white matter glial cells in hydrocephalic rats. VEGF expression determined by ELISA supports those findings. In this model of neonatal kaolin induced hydrocephalus, hypoxia in white matter might contribute to the destructive changes.