Abstracts

Rationale: Optogenetic stimulation of neurons offers advantages over either electrical or pharmacological methods for manipulating the activity of a specific brain regions, in particular, it permits for selective activation of neurons of interest while avoiding issues associated with drug diffusion and activation of fiber tracts. This approach has only recently been applied in epilepsy models, and its efficacy has not been tested in a key seizure-suppressive pathway - the nigrotectal projection. Inhibition of substantia nigra or activation of superior colliculus using pharmacological approaches is effective at restraining seizures in a variety of experimental models. To determine if an optogenetic approach would be efficacious, we selected the deep/intermediate layers of the superior colliculus (DLSC) as our target for stimulation and pentylenetetrazole as our seizure model.Methods: We stereotaxically microinjected AAV5-hSyn-hChR2(H134R)-mCherry bilaterally into DLSC of rats and implanted fiber optic probes into the same sites. After allowing time for gene expression (3-4wk post-op), we titrated the dose of a chemoconvulsant challenge (pentylenetetrazol, PTZ 20-35mg/kg, ip) for each animal to identify a dose that induced a complex partial seizure (a score of between 3 and 5 on a modified Racine scale). To test the effect of optogenetic activation of DLSC against PTZ-evoked seizures, 100Hz (473nm light, 10mW) light stimulation was applied bilaterally to DLSC immediately after the i.p. injection of PTZ. Animals were perfused at the conclusion of testing and virus expression mapped using mCherry immunofluorescence. Only animals with correct virus and cannula placement were included in the analysis. Results: Before testing the optogenetic activation of DLSC in the seizure model, we stimulated unilaterally and observed behavioral responses (less than 60s of stimulation). This stimulation evoked DLSC-related behaviors such as orienting responses and contraversive posturing of the body and head. At least 2d later, we tested for the effect of bilateral stimulation on seizures evoked by PTZ. Stimulation (which remained on for the duration of the observation period) significantly attenuated the severity of PTZ-evoked seizures. Conclusions: These data demonstrate that optogenetic activation of DLSC is capable of attenuating experimental seizures and provide proof-of-concept for the application of this method to seizure control via basal ganglia networks. Experiments are currently underway testing the efficacy of this approach in other seizure models.