Abstracts

Rationale: The inherently unpredictable nature of seizures in epilepsy poses an obvious challenge to the development of ideal therapeutic strategies that act on an ‘as-needed' basis without disrupting normal behaviors. It has recently become possible to control specific subsets of neurons utilizing light-sensitive channels or pumps (opsins), allowing unprecedented specific and immediate control of cell populations of interest in behaving animals. However, the widespread use of these novel optogenetic technologies has so far been difficult to apply to epilepsy, in part because of the unpredictable nature of the seizures and difficulty in anticipating the location of seizure onset with many of the available epilepsy models in rodents. Methods: Mice genetically expressing cre in specific neuronal populations provided selective opsin expression for these experiments. Specifically, either inhibitory opsins (halorhodopsin or archaerhodopsin) were expressed in glutamatergic cells, or excitatory opsins (channelrhodopsin) were expressed in parvalbumin-expressing GABAergic cells. Both viral vector and genetic expression of cre-dependent opsins were tested. Kainic acid was injected unilaterally into the hippocampus to induce chronic epilepsy with recurrent behavioral seizures, and depth electrodes and optical fibers were implanted to allow both EEG recording and opsin activation. We then developed a closed-loop system for detecting and responding to spontaneous electrographic and behavioral seizures, tuned to the EEG signal from individual animals, providing on-line, real-time, seizure detection and laser triggering. In order to determine the effect of opsin activation on seizures, electrographic seizure detection triggered the delivery of light for only 50% of the events, in random sequence. Analysis of the duration of electrographic seizures was performed off-line, blinded to the presence or absence of light. Duration of electrographic events during light-on and light-off conditions was then compared in each animal using a Mann-Whitney test with a level of significance of p<0.05. Results: Both silencing principal cells with inhibitory opsins (n= 4 of 5 animals; p<0.05 in each animal) and exciting GABAergic PV cell populations with excitatory opsins (n=3 of 3 animals; p<0.05 in each animal) had an effect on the duration of electrographic seizures. For a substantial portion of events (up to 80% in individual animals), light delivery stopped the electrographic seizure within 5 seconds. Conclusions: Diverse strategies utilizing both genetic and viral-based expression of opsins can be effective methods of modulating electrographic seizure activity. By investigating the control of electrographic and behavioral seizures with different specific populations of neurons, the potential for targeting these populations for future pharmacological or even optogenetic interventions in patients can be explored. This work is supported by the US National Institutes of Health grant NS074702 (to IS), the Epilepsy Foundation (to CA), and the George E. Hewitt Foundation for Medical Research (to EKM).