Sequel of spontaneous seizures after kainic acid-induced status epilepticus and associated neuropathological changes in the subiculum and entorhinal cortex.

Injection of the seaweed toxin kainic acid (KA) in rats induces a severe status epilepticus initiating complex neuropathological changes in limbic brain areas and subsequently spontaneous recurrent seizures. Although neuropathological changes have been intensively investigated in the hippocampus proper and the dentate gyrus in various seizure models, much less is known about changes in parahippocampal areas. We now established telemetric EEG recordings combined with continuous video monitoring to characterize the development of spontaneous seizures after KA-induced status epilepticus, and investigated associated neurodegenerative changes, astrocyte and microglia proliferation in the subiculum and other parahippocampal brain areas. The onset of spontaneous seizures was heterogeneous, with an average latency of 15 ± 1.4 days (range 3-36 days) to the initial status epilepticus. The frequency of late spontaneous seizures was higher in rats in which the initial status epilepticus was recurrent after its interruption with diazepam compared to rats in which this treatment was more efficient. Seizure-induced neuropathological changes were assessed in the subiculum by losses in NeuN-positive neurons and by Fluoro-Jade C staining of degenerating neurons. Neuronal loss was already prominent 24 h after KA injection and only modestly progressed at the later intervals. It was most severe in the proximal subiculum and in layer III of the medial entorhinal cortex and distinct Fluoro-Jade C labeling was observed there in 75% of rats even after 3 months. Glutamatergic neurons, labeled by in situ hybridization for the vesicular glutamate transporter 1 followed a similar pattern of cell losses, except for the medial entorhinal cortex and the proximal subiculum that appeared more vulnerable. Glutamate decarboxylase65 (GAD65) mRNA expressing neurons were generally less vulnerable than glutamate neurons. Reactive astrocytes and microglia were present after 24 h, however, became prominent only after 8 days and remained high after 30 days. In the proximal subiculum, parasubiculum and entorhinal cortex the number of microglia cells was highest after 30 days. Although numbers of reactive astrocytes and microglia were reduced again after 3 months, they were still present in most rats. The time course of astrocyte and microglia proliferation parallels that of epileptogenesis.

Somatostatin and neuropeptide Y neurons undergo different plasticity in parahippocampal regions in kainic acid-induced epilepsy.

Parahippocampal brain areas including the subiculum, presubiculum and parasubiculum, and entorhinal cortex give rise to major input and output neurons of the hippocampus and exert increased excitability in animal models and human temporal lobe epilepsy. Using immunohistochemistry and in situ hybridization for somatostatin and neuropeptide Y, we investigated plastic morphologic and neurochemical changes in parahippocampal neurons in the kainic acid (KA) model of temporal lobe epilepsy. Although constitutively contained in similar subclasses of γ-aminobutyric acid (GABA)-ergic neurons, both neuropeptide systems undergo distinctly different changes in their expression. Somatostatin messenger RNA (mRNA) is rapidly but transiently expressed de novo in pyramidal neurons of the subiculum and entorhinal cortex 24 hours after KA. Surviving somatostatin interneurons display increased mRNA levels at late intervals (3 months) after KA and increased labeling of their terminals in the outer molecular layer of the subiculum; the labeling correlates with the number of spontaneous seizures, suggesting that the seizures may trigger somatostatin expression. In contrast, neuropeptide Y mRNA is consistently expressed in principal neurons of the proximal subiculum and the lateral entorhinal cortex and labeling for the peptide persistently increased in virtually all major excitatory pathways of the hippocampal formation. The pronounced plastic changes differentially involving both neuropeptide systems indicate marked rearrangement of parahippocampal areas, presumably aiming at endogenous seizure protection. Their receptors may be targets for anticonvulsive drug therapy.