Abstracts

Rationale: Generalized absence epilepsy (GAE) involves thalamocortical circuits, yet relative contributions of thalamic vs cortical subcircuits remain controversial, leaving open important questions about therapy. For example, the electrographic signature of GAE, spike wave discharge (SWD) can occur in isolated cortex of GAE models. In addition, low levels of thalamic spiking have been reported in SWD, and studies in animals and patients suggest that early electrographic SWD features appear in cortex before thalamus. This suggests that thalamus may play a passive role in SWD. However, the thalamus is more metabolically active during SWD than cortex, and absences are powerfully modulated by localized thalamic infusion of GABA ligands. To test if thalamus actively participates in absence seizures we adopted an optogenetic approach in two GAE models, stargazer (stg) mouse (monogenic, AMPAR trafficking gene mutation), and WAG/Rij rat (inbred polygenic strain). Methods: Two opsins were used: inhibitory (ENpHR3.0) and excitatory (SSFO), delivered via AAV virus unilaterally into the somatosensory VB complex, and driven by CamK2α promoter. Controls included animals injected with viruses carrying the reporter eYFP, but no opsin, and application of out of spectrum light (e.g. blue light with little activation of eNpHR, compared to yellow light which specifically activates it). Optrodes were implanted days folowing viral injection, allowing delivery of light to viral targeted area along with thalamic micro-electrodes, along with EEG leads. Results: As expected, both stg and WAG/Rij specimens exhibited spontaneous SWDs with associated behavioral absences. We adapted a detection approach that had been used previously for post-stroke seizures in rats. Individual seizures were several seconds long, with detection typically less than one second. Detected events were coupled randomly with either light or sham stimuli. Inhibition of neural activity with eNpHR weakened SWD in both species. By contrast, trains of eNpHR activating pulses entrained and/or initiated SWD. SSFO strongly suppressed SWD and related behavior. SSFO is bistable - it can be rapidly and reversibly activated and inactivated by blue or yellow light, respectively. Typically, ~1 s of activation was followed by deactivation with yellow light. Longer stimuli led to circling behavior, and rarely tonic clonic seizures. Conclusions: Here we provide three lines of evidence supporting an active thalamic role in generalized absence seizures. In both mouse and rat models, simple inhibition of thalamus with halorhodopsin weakened the seizures, while repetitive inhibition, which resulted in rebound burst activation, promoted seizures. By contrast, depolarizing thalamic neurons with SSFO, which reduced their ability to fire robust phasic robust bursts, blocked the seizures. Thus steadily inhibiting thalamic neurons by eNpHR, or mode switching them via SSFO, suppressed the seizures, while promoting activity through repetitive rebound bursting (through trains of eNpHR pulses) triggered seizures. Thus, thalamus is actively involved in GAE and targeting thalamic networks is warranted as a seizure therapy.