Novel Optogenetic Approaches in Epilepsy Research

Abstract

Epilepsy is a major neurological disorder characterized by repeated seizures afflicting 1% of the global population. The emergence of seizures is associated with several comorbidities and severely decreases the quality of life of patients. Unfortunately, around 30% of patients do not respond to first-line treatment using anti-seizure drugs (ASDs). Furthermore, it is still unclear how seizures arise in the healthy brain. Therefore, it is critical to have well developed models where a causal understanding of epilepsy can be investigated. While the development of seizures has been studied in several animal models, using chemical or electrical induction, deciphering the results of such studies has been difficult due to the uncertainty of the cell population being targeted as well as potential confounds such as brain damage from the procedure itself. Here we describe novel approaches using combinations of optical and genetic methods for studying epileptogenesis. These approaches can circumvent some shortcomings associated with the classical animal models and may thus increase the likelihood of developing new treatment options.

Keywords: animal models; epilepsy; kindling; optogenetics; plasticity; seizures.

FIGURE 1

Optogenetic manipulation of seizure propensity. (A) Optogenetics can be used to drive seizures acutely (Khoshkhoo et al., 2017) or to (B) put a brake on already active seizures (Wykes et al., 2012; Krook-Magnuson et al., 2013) as a form of pro- or anti-convulsant, respectively. This latter approach has potential clinical applications for interrupting ongoing seizures (Paz et al., 2013), whereas both are useful for studying epilepsy. (C) Seizures can also be gradually elicited over time via optogenetic kindling, or optokindling (Cela et al., 2019). Optokindling is fundamentally distinct from directly driving seizures by optogenetic stimulation, since optokindling requires long-lasting changes of neuronal circuits (Cela et al., 2019), whereas direct optogenetic drive does not (Khoshkhoo et al., 2017). (D) It may also be possible to gradually decrease seizure propensity via optogenetic dekindling. As far as we know, this remains to be experimentally demonstrated optogenetically, but such findings have been reported with electrical stimulation (Bains et al., 1999; Ozen and Teskey, 2009). The kindling and dekindling modes may both enable the study of epileptogenesis, e.g. to test therapies that slow down or reverse the development of seizures.

FIGURE 2

Optokindling via simultaneous EEG recording and ChR2 stimulation in awake behaving animals. (A) Coronal M1 section immunostained for EYFP indicated ChR2 expression in layer 2/3 (L2/3), 5, and 6, though predominantly in L2/3. Inset shows close-up of L2/3 ChR2-expressing PCs. (B) To simultaneously activate ChR2 and acquire EEG, ferrules and recording screws were implanted bilaterally above M1, without penetrating the cortex. Fiber optic cables were air-coupled to 445-nm lasers. EEG signals were processed by an extracellular amplifier, but not pre-amplified. A computer (not shown) TTL-gated the lasers and digitized amplified EEG signals. (C) Sample EEG trace illustrating that M1 optokindling did not elicit seizures in stimulation session 1 of 25 (Top). Spectrogram shows direct light-driven responses in the 50-Hz band but no seizures (Bottom). (D) M1 optokindling in session 13 elicited a prominent seizure in this sample EEG sweep (Top). Spectrogram reveals both light-driven responses in the 50-Hz band as well as increased power in low-frequency bands (Bottom). Reproduced and modified from Figure 1 in Cela et al. (2019), with permission.