Abstracts

Rationale: Temporal lobe epilepsy (TLE) is one of the most difficult to treat forms of epilepsy. More than one third of the patients continue to have seizures despite optimal treatment with anti-epileptic drugs. Only a subset of the patients can be treated with epilepsy surgery where the epileptogenic mesial temporal lobe structures are resected. This invasive procedure does not give seizure freedom in all cases and has a high risk for cognitive side effects. In this study we evaluate whether selective silencing of excitatory hippocampal neurons using the inhibitory Designer Receptor Exclusively Activated by Designer Drugs (DREADD) hM4Di could lead to suppression of spontaneous hippocampal seizures in the intrahippocampal kainic acid (IHKA) mouse model for TLE. Methods: Thirty-two mice were injected with kainic acid (KA, 200 ng/50 nl) in the hippocampus. Three weeks after KA injection, an adeno-associated viral (AAV 2/7) vector carrying the gene encoding for the inhibitory DREADD hM4Di and the mCherry reporter under control of the CamKIIa promotor was injected (500 nl) in the KA lesioned hippocampus of 20 mice. In the other 12 mice a control AAV 2/7 - CamKIIa-mCherry vector was injected. At the same time a bipolar recording electrode was implanted in the hippocampus. Three weeks after implantation surgery four hM4Di mice and six mCherry control mice were selected for treatment with CNO based on the presence of a stable and high number of hippocampal seizures. Results: Systemic administration of CNO resulted in a dose-dependent inhibition of epileptic seizures in mice expressing the inhibitory DREADD hM4Di in CamKIIa excitatory neurons in the sclerotic hippocampus, but had no effect in non-DREADD control mice (fig 1a). During 24 hours before treatment the four hM4Di mice had an average of 13, 20, 22, 21 seizures per hour respectively. Treatment with 1 mg/kg and 10 kg/mg CNO completely suppressed hippocampal seizures for 9, 7, 15, 0 hours and 14, 12, 12 and 8 hours respectively. Repetitive injection of the mice with 10 mg/kg CNO during five consecutive days reproducibly suppressed hippocampal seizures for multiple hours (fig 1b). Besides suppression of hippocampal seizures, CNO treatment also significantly reduced total spectral EEG power (1-100 Hz) and the occurrence of pathological high frequency oscillations (pHFOs) in the sclerotic hippocampus. Conclusions: These results support a crucial role for excitatory hippocampal neurons in the generation of spontaneous hippocampal seizures and pHFOs in the intrahippocampal KA mouse model and provide an approach for the development of an epilepsy therapy where a systemically administered drug very selectively modulates specific cell-types inside the seizure focus without affecting other cells in the brain or body, resulting in very potent suppression of epileptic seizures without any side effects. Funding: Robrecht Raedt received funding from the Special Research Funds of Ghent University to perform this research