Abstracts

Rationale: Neuronal vulnerability and hippocampal neurogenesis are both age-dependent and inversely related following several episodes of kainic acid (KA)-induced status epilepticus (SE). During the first two weeks of life the hippocampus is highly resistant to damage, but exhibits high proliferation rates that are paradoxically constrained by multiple seizures. In contrast, selective injury of CA1/CA2 regions predominates at prepubescent ages (P20-P30) then switches to preferential CA3 injury in adulthood (P60), while neurogenesis is provoked to different extents within the hippocampal subregions. We hypothesized that age-dependent inhibition or stimulation of hippocampal progenitors may depend upon a number of factors such as the timing and number of insults, location of injury, level of inflammation, and time point examined. Methods: KA-SE was induced once (1 KA) on postnatal (P) days 13, 20, 30 or 60, twice (2 KA) on P25 and P30 or three times (3 KA) on P6, P9 and P20. Immunohistochemistry was carried out with the mitotic Ki67 antibody. Co-labeling, GFAP staining and histological methods were used to monitor cell fate and inflammation at various times (2, 3, 5, and 9 days). Results: In control P13 pups, Ki67-labeled progenitors were plentiful and scattered along the subgranular zone (SGZ) borders and found deep within the hilus. In contrast, control P20, P30 or P60 rats Ki67-labeled cells were few in number (declining with age) and formed tight clusters along the dorsal and ventral hilar borders of the SGZ. After 1 KA at P20 at 3 or 5 days, there was significant CA1/CA2 injury, but only a small increase in the total number of Ki67-labeled cells in the SGZ was observed. There was reduced clustering along the hilar borders and some progenitors migrated into the layer. At P30 and 2 days following 1 KA or 2 KA, the hippocampus rapidly expressed high progenitor numbers in the CA1 and these were associated with marked injury and increased GFAP staining. Moderate Ki67 cell counts of the CA3 and dentate gyrus (DG) were correlated with modest amounts of injury. At P60 and 2 days following 1 KA, high numbers of progenitors were found throughout the hippocampal subfields again in regions of injury (e.g. CA3). After 2 KA and 2 days, Ki67 cell count elevations were attenuated as was the amount of injury which nearly disappeared after 9 days. In contrast, after 3 KA and 3 days, significant decreases in Ki67 cell counts were already observed as well as reduced injury and GFAP staining. Conclusions: Results indicate that elevated proliferation and injury rates in P20-P30 rats is transient after 1 KA and inhibited with increasing number of seizures with early onset suggesting that the timing and number of insults influence both proliferation and injury rates. Co-labeling with neuronal and non-neuronal markers showed progenitor types were of mixed origin. Thus, SE-induced conditioning in the juvenile period may be contributed by replacement of dying pyramidal neurons and successful differentiated progenitors whereas in older animals the injury and cell loss persists due to poor survival and differentiation of progenitors with maturation.