Respiration and Generalized Epilepsies

Project Summary Spike-Wave Discharges (SWDs) are a common type of seizure in the Genetic Generalized Epilepsies (GGEs). Hyperventilation triggers SWDs in the overwhelming majority of patients with absence epilepsy, the most common form of pediatric GGE. We have recently developed a rodent epilepsy model wherein we can evoke a burst of SWDs with hyperventilation. Within 6 minutes of hyperventilation, SWD count increases by over 500%. We now leverage this model to gain unprecedented access to core seizure-generating mechanisms associated with SWDs. By combining plethysmography, EEG and blood measurements in single animals, we show that SWD circuits appear critically sensitive to blood pH. First, we show that hypoxia, a condition that activates hyperventilation, robustly evokes rodent SWDs. Hypoxia-induced hyperventilation results in increased exhalation of CO2 and concomitant blood alkalization (i.e. respiratory alkalosis). We also show that hypoxia-evoked SWDs are abolished when atmospheric CO2 is elevated, thereby supporting the hypothesis that blood alkalization drives hyperventilation-evoked SWDs. Finally, we also show that optogenetic activation of hyperventilation during normal atmospheric conditions – an experimental procedure that reduces blood CO2 but increases O2 – also evokes SWDs. Thus, collectively our data show that SWDs appear to primarily covary with blood CO2. We complement our plethysmography-EEG data with brain slice electrophysiology and calcium imaging. We focus our attention on the intralaminar nuclei of the thalamus because activity-dependent cell tagging approaches (i.e. cFos) consistently label cells within this region after hypoxia-induced hyperventilation. By using whole-cell patch clamp recording techniques we demonstrate that intralaminar thalamic cells produce depolarizing ionic currents during alkalized conditions. A significant portion of this current appears to be mediated by enhanced excitatory synaptic drive. With our preliminary data, we now present a project that aims to test the overarching hypothesis that activation of pH-sensitive intralaminar thalamic neurons by acute respiratory alkalosis precipitates absence seizures. Identifying these neurons and their mechanisms of activation will inform new strategies to treat the most common pediatric GGEs, disorders for which only decades-old, sub-optimal treatments exist. We specifically test the following main hypotheses:  Aim 1 Respiratory alkalosis triggers absence seizures.  Aim 2 Spontaneous and hyperventilation-triggered seizures utilize the same neural ciruitry. When complete, we expect that the results of our project will provide significant, new insights into fundamental cellular- and circuit-level mechanisms that drive generalized spike-wave discharges, and therefore pave new avenues for generalized epilepsy treatments.