Hippocampal stimulation produces neuronal death in the immature brain

We re-examined the proposed resistance of the immature brain to seizure-induced damage. In awake, freely moving rat pups, intermittent perforant path stimulation produced selective hippocampal cell loss and reduction in paired-pulse inhibition. During 16 h of stimulation, animals showed frequent wet dog shakes and hind-limb scratching movements but no convulsive motor activity. In situ end-labelling performed 2 h after the end of stimulation showed an intense band of positively-labelled eosinophilic cells with condensed profiles bilaterally in the dentate granule cell layer of stimulated animals. Control animals showed no in situ end-labelling positivity in the dentate gyrus. These cells were not observed 24 h later, suggestive of rapidly scavenged apoptotic cells. One day after the end of stimulation, many necrotic interneurons with eosinophilic cytoplasm and pyknotic nuclei were observed in the hilus of the stimulated dentate gyrus in all rats tested. Hippocampal pyramidal cells in CA1, CA3 and subiculum showed bilateral damage greater on the side of stimulation, and prepiriform cortex sustained bilateral symmetrical lesions. One month after perforant path stimulation, Cresyl Violet staining showed the number of large hilar interneurons (>15 microm) was reduced on the stimulated side (54.1 +/- 12.2) compared to the non-stimulated side (100.5 +/- 10.2 cells, P<0.01). Immunohistochemical analysis showed significant losses in somatostatin (8.5 +/- 1.6 stimulated side, 22.8 +/- 3.8 unstimulated side, P<0.05) and neuropeptide Y (12.8 +/- 3.2 stimulated side, 17.0 +/- 4.1 unstimulated side, P<0.05) immunoreactive cells in the stimulated hilus but no loss of parvalbumin-immunoreactive cells. Significant reductions in paired-pulse inhibition were found after stimulation but there was some return of inhibition by one month. These combined data demonstrate that the immature brain can incur damage as a result of prolonged seizure-like hippocampal activity mimicking status epilepticus in immature rats. The hippocampal damage produced by perforant path stimulation is associated with the immediate loss of physiological inhibition suggesting important modification of excitatory control in an extremely epileptogenic region of the brain.