Abstracts

RATIONALE: AMPA receptors may play an important role in epileptogenesis in the developing brain. AMPA receptors peak during the first two weeks of life in the rat at a time of increased seizure susceptibility. Also, the AMPA receptor antagonist, NBQX, is uniquely effective in suppressing both the acute and longterm epileptogenic effect of perinatal hypoxia. AMPA receptor subunit composition affects calcium permeability; the heteromeric receptors containing GluR2 subunit are impermeable to calcium, while those lacking GluR2 are calcium permeable. We have previously shown that following hypoxia-induced seizures, both GluR2 mRNA and protein are selectively downregulated and concurrently, AMPA-mediated cobalt uptake in hippocampal neuron is increased. To determine wheather an increased expression of calcium-permeable AMPA receptors is a more general response of immature brain to seizures, we studied AMPA receptor subunit protein expression following kainate(KA)-induced seizures at P10.
METHODS: Status epilepticus was induced by KA (2mg/kg, i.p) injection to P10 Long Evans rat pups. Hippocampi were dissected 96h after KA seizures, rapidly frozen and membrane fraction prepared. Western blot was performed using polyclonal rabbit anti GluR1 and GluR2 antibodies (Chemicon) and quantified by chemiluminescence signal detection system. Neuronal injury after KA status was assessed using in situ detection of DNA fragmentaion on perfused brains.
RESULTS: KA-induced seizures at P10 caused 43% decrease in GluR2 protein expression (13.9[plusminus]0.8 vs 5.8[plusminus]1.6, p=0.01) in the hippocampus at 96 hours, without inducing significant changes in GluR1 protein levels. No KA seizure-induced hippocampal DNA fragmentation was detected.
CONCLUSIONS: Early-life KA seizure induces selective downregulation of GluR2 protein expression without causing cell death. Downregulation of GluR2 expression after seizures or ischemia has been hypothesized to serve as a [dsquote]molecular switch[dsquote] to form calcium permeable AMPA receptors and enhance glutamate-mediated neuronal death in adult brain. In contrast, seizure-induced GluR2 downregulation is not associated with cell death in the immature brain. Increased AMPA-mediated calcium influx due to decreased GluR2 protein expression may contribute to a long term alteration in neuronal excitability and epileptogenesis in the immature brain.
Support: K08NS02068(SK) and NIH NINDS 31718(FEJ)