Abstracts

Rationale: The δ subunit of the GABA_A receptor (GABA_AR) plays an important role in controlling neuronal excitability through tonic inhibition. Our previous studies have demonstrated that the δ subunit is decreased in dentate granule cells but increased in dentate interneurons in a mouse model of temporal lobe epilepsy. However, it is not known whether such changes also exist in other regions of the hippocampal formation that might be involved in seizure initiation and propagation. The goals of this study were to determine if changes in δ subunit labeling occur throughout the hippocampal formation in epileptic animals, and if the regions with altered δ subunit labeling might correspond to regions with increased neuronal activation following spontaneous seizures. Methods: Young adult C57BL/6 male mice were treated with pilocarpine (330mg/kg) to induce status epilepticus (SE), and diazepam was administered after 2 hours to limit seizure activity. All mice subsequently developed spontaneous seizures. Sections from control and pilocarpine-treated mice at 2 weeks to 3 months following SE were processed for immunohistochemistry of either the δ subunit of the GABA_AR, phosphorylated extracellular signal-regulated kinase (pERK), Fos or NeuN. Results: In control mice, high levels of δ subunit labeling in the molecular layer of the dentate gyrus have been described previously. In addition, moderate levels were observed in the lateral entorhinal cortex (LEnt) and parasubiculum (PaS); lower levels were present in the medial entorhinal cortex (MEnt), CA1, presubiculum and subiculum; and extremely low levels were found in CA3. In pilocarpine-treated mice, decreased diffuse δ labeling was generally observed throughout the hippocampal formation with the most prominent changes in the dentate gyrus, CA1, LEnt and a distinct area between the PaS and MEnt. Nissl and NeuN staining showed substantial preservation of neurons in the regions with decreased δ subunit expression. Increased δ subunit labeling in interneurons could be seen in entorhinal cortex as well as dentate gyrus. Labeling for pERK and Fos was examined at short intervals after spontaneous seizures, and strong increases in labeling occurred in principal neurons of the regions in which there were substantial changes of δ subunit labeling. These included the region between PaS and MEnt, as well as LEnt and dentate gyrus. Conclusions: Decreases in δ subunit of the GABA_AR in epileptic animals extended beyond the dentate gyrus and were apparent in additional regions of the hippocampal formation. Increased pERK and Fos expression following spontaneous seizures was first observed in the regions with a striking decrease of diffuse δ subunit labeling. These findings suggest that changes in the δ subunit of GABA_AR could alter the excitability of principal neurons, and extremely low levels of the δ subunit could contribute to the initiation of seizure activity. Grant Support: VA Medical Research Funds and NIH Grant NS051311.