The expression of Fos following kainic acid-induced seizures is age-dependent

The expression of limbic seizures following kainic acid (KA) administration starts at approximately postnatal day (P) 19 in rats. In this study we investigated whether the expression of Fos-like immunoreactivity (Fos-IR) in limbic regions occurs concomitantly with the behavioural expression of limbic seizures. Immunohistochemistry for c-Fos protein was examined 1, 2, 4, 12 and 24 h following seizure onset (KA-treated rats) or saline injections (controls) in immature and adult rats at P7, P13, P20 and P60. The expression of Fos-IR in limbic structures following KA-induced seizures is age-dependent. There is a strong and selective induction of Fos-IR in the CA3 region of the hippocampus following KA-induced seizures in rats at P7. However, the expression of Fos-IR in KA-treated rats at P13, P20 and P60 involved other hippocampal structures in addition to CA3. Abundant induction of Fos-IR was found in the CA1, CA3 and dentate gyrus (DG) in KA-treated rats at P13, P20 and P60. While immature rats at P7 and P13 showed very few or no Fos-IR neurons in most amygdala nuclei, rat pups at P20 showed strong induction of Fos-IR in the amygdala. Our results demonstrated that the induction of Fos-IR in most amygdala nuclei and the full expression of behavioural limbic seizures occur at the same developmental age, which is consistent with the idea that the amygdala may play a role in the modulation of limbic seizures.