Abstracts

Rationale: Vascular pathologies and the abnormal leakage of plasma proteins (and/or other solutes) across the blood-brain barrier (BBB) have been implicated in the development and maintenance of epileptic foci in the spontaneously epileptic rat brain. We previously reported that vascular endothelial growth factor (VEGF) expression remains chronically upregulated among reactive astrocytes in the kainic acid model of spontaneous epilepsy, coinciding with pathological microvessels within limbic and paralimbic structures. The persistence of pathological microvessels in other contexts has been shown to be selectively dependent upon elevated VEGF expression. We hypothesized that pathological microvessels in the spontaneously epileptic rat brain might be similarly VEGF-dependent, and that depriving them of VEGF might result in their selective deterioration. To test this hypothesis, we administered a selective small molecule inhibitor of VEGF tyrosine kinases, PTK787 (Novartis), and measured the effects of this compound on seizure expression, vascular pathology, and vascular permeability. Methods: Status epilepticus (SE) was induced in male wistar rats (250g - 350 g) by kainic acid injection (10 mg/kg, i.v.), and was terminated four hours post-onset using pentobarbital (25 mg/kg, i.p.). Spontaneous behavioral seizures were monitored for 6 months (8 hours/day, 5 days/week) prior to initiating PTK787 therapy, and subsequently during two months of PTK787 administration. PTK787 (20 mg/ml, PEG) was administered 7 days/week via oral injection at a dose of 100 mg/kg/day. Vehicle control epileptic rats received PEG injections according to the same schedule. Rats were sacrificed upon completion of PTK787 therapy. Brain tissues were harvested and immunohistochemically stained for vonWillebrand factor (vWF), albumin, glial fibrillary acidic protein (GFAP), and VEGF. Results: PTK787 significantly increased the frequency of spontaneous seizures relative to established baselines, and relative to vehicle control epileptic rats. Seizure severity was not affected by PTK787 therapy. Immunohistochemical staining revealed the persistence of pathological microvessels in PTK787 treated rats, accompanied by an increase in albumin staining in the parenchyma relative to vehicle control epileptic rats. Reactive astrocytes expressing GFAP and/or VEGF were not affected by PTK787 therapy. Conclusions: Our results suggest that persistent abnormal leakage of plasma proteins (and/or other solutes) across the BBB promotes the expression of spontaneous seizures in the chronically epileptic rat brain. The failure of our PTK787 dose regimen to eliminate the pathological microvessels suggests that a more aggressive approach to VEGF inhibition may be required to produce this effect or that other growth factors might also contribute to the persistence of these microvessels in this context. We hypothesize that our PTK787 dose regimen produced an incomplete VEGF blockade, which further compromised BBB function among these pathological microvessels but was insufficient to produce their complete destruction.