Abstracts

Rationale: The loss of GABAergic interneurons has long been hypothesized to be an important mechanism of epilepsy. Pharmacological studies that non-specifically block one or more mechanisms of GABA-mediated inhibition have been shown to produce profound seizures in normal brain, and abundant evidence from animal models and human tissue suggests loss of interneurons can occur in acquired epilepsy. However, relatively little is known about whether selective interneuron lesions cause seizures, or the quantitative relationship between interneuron loss and seizure susceptibility. Methods: To test the hypothesis that selective lesions of GABAergic interneurons induce seizures, GAD2-IRES-CRE mice were injected unilaterally in the dorsal CA1 area with an adeno-associated virus containing the diphtheria toxin receptor (DTR), under cre-dependent control of expression to target interneurons for DTR-mediated ablation. At the same time, an electrode connected to a miniature-telemetry recording device was positioned at the injection site for continuous chronic recordings of local field potentials from the CA1 area of the hippocampus. Focal ablation of interneurons was achieved via intraperitoneal injection of diphtheria toxin (DT) 2-3 weeks after transfection. In a separate group of animals, the effect of the interneuron ablation procedure was examined with whole-cell recordings from pyramidal cells and immunohistochemical staining of GABAergic interneurons. Results: Immunohistochemistry for GAD67 and GFP confirmed that the transfected cells were ablated by 6 days after DT administration, and whole cell recordings from CA1 pyramidal cells in ex vivo slices from the experimental animals confirmed that ablation did in fact specifically reduce GABA-mediated inhibition (i.e., the frequency of miniature inhibitory post-synaptic currents). Likewise, animals in the experimental group (n = 6) were observed to have seizures during the acute and sub-acute periods after the ablative procedure. Seizure onset in all animals was between 2 and 6 days after DT treatment. Three animals had seizures only during the week after the ablation procedure, but they were seizure-free for the remainder of the recording period (18-68 days). Two of the animals had seizures that stopped in the second week (8-16 days seizure free). Only one animal was observed to develop epilepsy; that is, spontaneous recurring seizures were observed for 5 weeks in only one of the mice subjected to the ablative procedure. Conclusions: Unilateral CA1 interneuron ablation is sufficient to produce acute/sub-acute convulsive seizures. However, in 5 of the 6 interneuron-ablated mice, seizures did not persist. Therefore, the chronic condition of spontaneous recurrent seizures (i.e., epilepsy) may require either a more extensive loss of interneurons, more time since the interneuron lesion, or additional changes to the neural network.