Abstracts

Rationale: Optogenetics has shown great promise as a direct neuromodulatory therapy for halting seizure activity in various animal models of epilepsy. However, light delivery into the brain is still a major practical challenge that needs to be addressed for future clinical translation. Not only does light delivery into the brain require surgically implanted hardware that can be invasive, but it is also difficult to illuminate large or complicated structures due to light scatter and attenuation. We have bypassed the challenges of external light delivery by directly coupling a bioluminescent light source (a genetically encoded Renilla luciferase) to an inhibitory opsin (Natronomonas halorhodopsin) as a single fusion protein, which we term an inhibitory luminopsin (iLMO).Methods: iLMO was developed, tested, and characterized in vitro using intracellular recordings and multielectrode arrays. iLMO was expressed in various structures of the rat brain by stereotaxic injection of viral vector and evaluated with chronically implanted cannula-electrodes. Epileptic activity was acutely induced with intracerebral injections of bicuculline.Results: iLMO was shown to suppress action potential firing and synchronous bursting activity in vitro in response to both physical light and luciferase substrate. iLMO was further shown to suppress single-unit firing rate and local field potentials in the hippocampus of anesthetized and awake animals. Finally, iLMO was utilized to suppress epileptic activity induced by intracerebral injection of bicuculline. iLMO is currently being tested in the context of several other seizure models.Conclusions: We have developed and tested a novel optogenetic probe that is capable of non-invasive, hardware-independent inhibition of neural activity that will add to the versatility, scalability, and practicality of utilizing optogenetic approaches for halting seizure activity in vivo.