Selective loss of dentate hilar interneurons contributes to reduced synaptic inhibition of granule cells in an electrical stimulation‐based animal model of temporal lobe epilepsy

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Abstract
Neuropeptide‐containing hippocampal interneurons and dentate granule cell inhibition were investigated at different periods following electrical stimulation‐induced, self‐sustaining status epilepticus (SE) in rats. Immunohistochemistry for somatostatin (SOM), neuropeptide Y (NPY), parvalbumin (PV), cholecystokinin (CCK), and Fluoro‐Jade B was performed on sections from hippocampus contralateral to the stimulated side and studied by confocal laser scanning microscopy. Compared to paired age‐matched control animals, there were fewer SOM and NPY‐immunoreactive (IR) interneurons in the hilus of the dentate gyrus in animals with epilepsy (40–60 days after SE), and 1, 3, and 7 days following SE. In the hilus of animals that had recently undergone SE, some SOM‐IR and NPY‐IR interneurons also stained for Fluoro‐Jade B. Furthermore, there was electron microscopic evidence of the degeneration of SOM‐IR interneurons following SE. In contrast, the number of CCK and PV‐IR basket cells in epileptic animals was similar to that in controls, although it was transiently diminished following SE; there was no evidence of degeneration of CCK or PV‐IR interneurons. Patch‐clamp recordings revealed a diminished frequency of inhibitory postsynaptic currents in dentate granule cells (DGCs) recorded from epileptic animals and animals that had recently undergone SE compared with controls. These results confirm the selective vulnerability of a particular subset of dentate hilar interneurons after prolonged SE. This loss may contribute to the reduced GABAergic synaptic inhibition of granule cells in epileptic animals. J. Comp. Neurol. 500:876–893, 2007.