Abstracts

RATIONALE: Early-life seizures permanently decrease seizure threshold and increase the susceptiblity to seizure-induced cell death in adulthood. Even a single seizure in early life can cause long lasting functional alterations in network circuitry despite the absence of acute neuronal injury. To determine anatomical correlates of this functional alteration, we subjected postnatal day (P) 15 rat pups to a single kainate(KA)-induced seizure and evaluated dentate granule cell mossy fiber sprouting by Timm stain at P30.
METHODS: Long-Evans male rats were injected with KA (3mg/kg, i.p.) or saline at P15 and sacrificed by transcardiac perfusion at P30. We prepared 40[mu]m coronal sections, 160[mu]m apart, spanning dorsal hippocampus between 2.8-3.8mm ventral to bregma and processed the sections for Timm staining. Timm scores (0-5) were assigned to 6 hippocampal CA3 fields from 3 sections per brain and averaged for statistical analysis using student t-test.
RESULTS: At P15, KA induced status epilepticus lasting for 30-60 minutes in all rats. Evaluation of Timm staining at P30 showed a continuous dense laminar band of granules in the stratum oriens along the entire CA3 region in 8/10 animals with prior exposure to KA at P15. In P30 control littermate rats, Timm-stainied granules in the stratum oriens were limited to the septal end of dorsal hippocampus (n=8). No supragranular mossy fiber sprouting in the molecular layer of dentate gyrus was seen at P30 after kaiante status at P15. The mean Timm score was significantly higher in the kainate-treated rats (4.1[plusminus]0.7 ) compared to controls (1.1[plusminus]0.9, p[lt]0.0001). [figure1]
CONCLUSIONS: Early-life KA seizures at P15, without causing overt cellular injury, induce mossy fiber sprouting and aberrant innervation of basal dendrites of CA3 pyramidal neurons in the stratum oriens. Recurrent seizures in early life have been shown to cause mossy fiber sprouting in the CA3 pyramidal cell layer (Holmes et al., Ann Neurol 1998;44:845-857) as well as spine loss in apical dendrites of CA3 pyramidal neurons possibly due to pruning and partial deafferentation induced by recurring seizures (Swann et al., Hippocampus 2000;10(5):617-25). Here we show that even a single prolonged seizure is sufficient to cause abnormal afferent targeting during second week of life at the time of active granule cell proliferation and axonal outgrowth. Pathologic connectivity change induced by seizures in the immature brain may provide a structural basis for the priming effect of early-life seizures for later epilepsy and seizure-induced neuronal injury.
[Supported by: KO8NS02068, RO1NS27984, RO1NS31718 and NIH2P30HD18655.]