Abstracts

RATIONALE: It is well known that seizures induce the expression of neurotrophic factors in the hippocampus. The functions of vitamin D (1[alpha],25-dihydroxyvitamin D3) in the brain has not been studied extensively, even though the receptor for vitamin D [VDR] is widely distributed in neurons of the CNS including the hippocampus. Vitamin D has been shown to regulate the synthesis of NGF, NT-3, and NT-4/5 in the nervous system and in neurally derived cells. Furthermore, vitamin D has recently been shown to be neuroprotective. Our hypothesis is that vitamin D through VDR and neurotrophic factor pathway mediates neuroprotective effect and contributes to neuronal plasticity after an insult. To begin to test this hypothesis, we examined the changes in VDR expression after kainate (KA)-induced seizures.
METHODS: Adult mice were administered KA (30 mg/kg) intraperitoneally and observed for 2 hours. Experimental group included mice that had multiple limbic seizures but no status epilepticus. Saline injected mice served as controls. The expression of VDR protein was assessed by immunohistochemistry and measured by Western blot in the hippocampus isolated at various times (2, 4, 6, 12, 24, 48, 72 hours) after KA injection.
RESULTS: VDR expression was relatively weak in control mice in the neurons of dentate gyrus, CA3 and CA1 region of hippocampus. Immunoreactivity appeared to be predominantly cytoplasmic. Following KA-induced seizures, VDR-immunoreactivity increased dramatically in all regions of hippocampus. In each region, a slightly different temporal course was observed. VDR immunoreactivity in CA1 peaked early at 2 hours after KA injection, starting to decrease at 12 hours and returning to basal level by 24 hours. The VDR expression in CA3 peaked at 4 hours through 12 hours and returned to basal level at 24 hours. Expression in dentate gyrus peaked at 4 hours, starting to decrease at 12 hours and returning toward basal level by 24 hours. Western blot revealed that VDR immunoreactivity was increased at 4 hours after KA injection and returned to basal level by 24 hours.
CONCLUSIONS: KA induced limbic seizures result in a transient increase in VDR expression in neurons of dentate, CA1 and CA3 of mouse hippocampus. The rapid time course suggests a role similar to immediate early genes. Indeed the time course observed here is consistent with the idea that the vitamin D-VDR complex acts on the VDR responsive element of NGF gene as a transcription factor and regulates the later induction of NGF observed after seizures. These findings suggest that vitamin D may play a critical role in neuroprotection or neuronal plasticity after a neurotoxic insult such as seizures.
Support: NS37125 and Mayo Foundation.