Effect of age on kainate-induced seizure severity and cell death

Abstract

While the onset and extent of epilepsy increases in the aged population, the reasons for this increased incidence remain unexplored. The present study used two inbred strains of mice (C57BL/6J and FVB/NJ) to address the genetic control of age-dependent neurodegeneration by building upon previous experiments that have identified phenotypic differences in susceptibility to hippocampal seizure-induced cell death. We determined if seizure induction and seizure-induced cell death are affected differentially in young adult, mature, and aged male C57BL/6J and FVB/NJ mice administered the excitotoxin, kainic acid. Dose response testing was performed in three to four groups of male mice from each strain. Following kainate injections, mice were scored for seizure activity and brains from mice in each age group were processed for light microscopic histopathologic evaluation 7 days following kainate administration to evaluate the severity of seizure-induced brain damage. Irrespective of the dose of kainate administered or the age group examined, resistant strains of mice (C57BL/6J) continued to be resistant to seizure-induced cell death. In contrast, aged animals of the FVB/NJ strain were more vulnerable to the induction of behavioral seizures and associated neuropathology after systemic injection of kainic acid than young or middle-aged mice. Results from these studies suggest that the age-related increased susceptibility to the neurotoxic effects of seizure induction and seizure-induced injury is regulated in a strain-dependent manner, similar to previous observations in young adult mice.

Fig. 1

Dose-response curves of kainate-induced seizures in 3 different age groups of mice (C57BL/6J and FVB/NJ). Dose-response curves for convulsant effects of KA were expressed as the percentage of animals (n=12 mice per data point) displaying Racine Stage 5 seizures in response to systemic kainate injection (mg/kg). The dotted horizontal line depicts the ED₅₀ value, which is a dose predicted to induce Racine Stage 5 seizures in 50% of animals.

Fig. 2

Quantification of kainate-induced neuronal damage in hippocampal subfields at 4 different doses of KA in young adult C57BL/6J and FVB/NJ mice. A strain-dependent difference in cell loss in the dentate hilus, area CA3 and area CA1 was observed at 7 days following KA administration in FVB/NJ mice irrespective of the dose of kainate administered. Data represent the mean ± SEM of at least 8 mice/strain. *P<0.05. DG, dentate gyrus; H, hilus; CA3, area CA3; CA1, area CA1.

Fig. 3

Quantitative analysis of neuronal density in hippocampal subfields following administration of 3 different doses of KA to middle-aged C57BL/6 and FVB/NJ mice. While no cell loss as observed at the lowest dose of KA (A), a significant increase in cell loss was observed in the dentate hilus, and areas CA3 and CA1 of FVB/NJ mice seven days following KA administration. Data represent the mean ± SEM of at least 8 mice/strain. *P<0.05.

Fig. 4

Quantitative analysis of neuronal density in hippocampal subfields following systemic administration of KA at 3 different doses to two strains of aged mice. Strain-dependent differences in cell loss in the dentate hilus were observed at all 3 doses of KA. In contrast, strain-dependent differences in cell loss in area CA3 were observed at only the two highest doses (B and C), while cell loss in area CA1 was only observed at a dose of 15 mg/kg (B). No significant differences in the extent of cell loss were observed 7 days following administration of KA, irrespective of dose, in C57BL/6J mice. Data represent the mean ± SEM of eight mice/dose for each strain. *P<0.05.

Fig. 5

Neuronal cell loss following kainate administration in C57BL/6J and FVB/NJ mice is strain-dependent in three different age groups of mice. Low- and high-power photomicrographs of cresyl violet-stained horizontal sections of the hippocampus showing the destruction of neurons in the CA3 and CA1 subfields and within the dentate hilus 7 days after kainate administration in all three age groups of FVB/NJ mice. In contrast, no cell loss was evident in C57BL/6J mice of any age group following kainate administration. CA3, CA3 pyramidal cell layer; CA1, CA1 pyramidal cell layer; H, hilus. Scale bars, 750 μm (low-power photomicrographs); 350 μm (high-power photomicrographs).

Fig. 6

Comparison of susceptibility to kainate-induced cell death in FVB/NJ mice of three different age groups. Photomicrographs of Fluoro-Jade C-stained horizontal hippocampal sections at low and high magnification showing loss of neurons within the hippocampus in three age groups of FVB/NJ mice 7 days following kainate administration. Note the degeneration of neurons in the dentate hilus, and in the CA3 and CA1 subfields of the hippocampus as demonstrated with the fluorescent marker, Fluoro-Jade C. CA1 and CA3 denote the hippocampal subfields; H, dentate hilus. Scale bars: low-power photomicrographs, 750 μm; high-power photomicrographs, 100 μm.