Relationship between seizure-induced transcription of the DNA damage-inducible gene GADD45, DNA fragmentation, and neuronal death in focally evoked limbic epilepsy

We investigated the temporal and spatial profile of mRNA transcription for the growth arrest and DNA damage-inducible gene GADD45, DNA fragmentation, and neuronal death in rat brain following focally evoked limbic seizures. G ADD45 mRNA was detected by in situ hybridization, whereas fragmented DNA wa s detected using in situ nick end-labeling by the large (Klenow) fragment o f DNA polymerase I, Kainic acid (0.1 mu g) was injected into the right amyg dala of rats to induce seizures for 45 min, after which diazepam (30 mg/kg) was administered. GADD45 mRNA, DNA fragmentation, and cell death were quan tified bilaterally within six limbic brain regions 0-96 h following seizure cessation. All animals underwent seizures of equivalent severity and durat ion as determined electrographically. in situ hybridization detected bilate ral up-regulation of GADD45 mRNA throughout the CA1, CA3, and dentate gyrus of the hippocampus, the piriform and retrosplenial cortices, and the thala mus within 1 h of seizure termination. GADD45 mRNA levels remained elevated for up to 6 h, declining to baseline within all structures by 16 h. Klenow -positive cells were only found within the CA3 pyramidal layer of the ipsil ateral hippocampus and appeared 16-72 h following seizure cessation. Morpho logic cell death was also restricted to the CA3 subfield, These data demons trate that focally evoked limbic seizures trigger early bihemispheric GADD4 5 mRNA transcription within connected limbic structures, whereas subsequent DNA fragmentation and cell death are restricted to selectively vulnerable brain regions.