Long-term effect of chemogenetic suppression of excitatory hippocampal neurons on spontaneous seizures in a rat model for temporal lobe epilepsy

Objective. One third of epilepsy patients cannot be helped using conventional medication. Selective hippocampal suppression using Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) have potential to address this unmet need. We evaluated whether DREADD-mediated seizure suppression could be obtained in the intraperitoneal kainic acid (IPKA) rat model for temporal lobe epilepsy. Methods. The inhibitory DREADD hM4Di was selectively expressed in excitatory neurons in one hippocampus of IPKA rats. Naïve IPKA rats were included as control group. The effect of clozapine-mediated DREADD activation on dentate gyrus evoked potentials (DGEPs) was evaluated. Next, the effect of DREADD activation on spontaneous seizures was examined using continuous video-electroencephalography. Animals were systemically treated with single (0.1 mg/kg/24h) and repeated (0.1 mg/kg/6h) clozapine injections. Finally, long-term continuous release of clozapine and olanzapine (both 2.8 mg/kg/7d) using implantable osmotic minipumps was evaluated. Results. In the DREADD group, inhibition of DGEPs was observed after clozapine treatment, whereas no effect was observed in control animals. Only in DREADD-expressing animals, a single dose of clozapine reduced seizure frequency during the first six hours post injection. When clozapine was administered every six hours, seizures were suppressed the entire day. Long term treatment resulted in a significant seizure-suppressing effect during the first four days, after which tolerance developed. Significance. This study shows that unilateral inhibition of hippocampus using chemogenetics results in potent seizure suppressing effects in the IPKA rat model, even months after vector injection. It indicates that chemogenetic neuromodulation could contribute to a more optimal treatment for temporal lobe epilepsy