Abstracts

Rationale: Epilepsy effects more than 65 million people worldwide. One-third of this population will be refractory to anti-seizure drugs. These patients are at increased risk for sudden unexpected death in epilepsy (SUDEP). A potential mechanism that causes SUDEP is postictal dysfunction of serotonin (5-HT). Our lab has previous demonstrated that dorsal raphe nucleus (DRN) 5-HT neurons are involved in CO₂ arousal, and this chemosensitivity is impaired when these neurons are absent. Additionally, Hypercapnic Ventilatory Response (HCVR) has been found to be diminished postictally. We investigated the effect a seizure has on DRN 5-HT neuronal activity to evaluate whether these neurons are negatively impacted, thus impairing their CO₂ chemosensitivity postictally. We hypothesized that following a seizure, DRN 5-HT neuronal activity is refractory compared to baseline and unresponsive to inspired CO₂.

Methods: To measure DRN 5-HT neuronal activity, we utilized Fiber Photometry to record Ca²⁺ activity as proxy of neuronal activity. Male and female (8-10 wks) Pet1-Cre::GCaMP mice were used in this study. The Pet1::GCaMP mice are a cross between the Pet1-Cre mice and Ai96(RCL-GCaMP6s) mice, which conditionally express fluorescent Ca²⁺ indicator GCaMP6s under control of the Pet1 promoter. Mice were instrumented with EEG and EMG electrodes, photometry fibers in the DRN (AP: -4.60, ML: ±0.0, DV: -2.93), and bipolar electrodes into the amygdala for kindling (AP: -1.34, ML: -2.80, DV: -4.70). Mice were subjected to the amygdala kindling procedure. This involved increase amplitude of electrical stimulation until the mice developed electrically inducible tonic-clonic seizures. Mice were placed in plethysmography chambers and Ca²⁺ signaling was recorded during the trial to compare the change in fluorescence over baseline fluorescence. Room air (21% O₂ / bal N₂) was administered during baseline recording, a seizure was induced, and the mouse was either given a CO2 (7% CO₂ / 21% O₂ / bal N₂) challenge or switched to another room air tank. Upon completion, mice were euthanized, perfused, and stained to verify implant placement and protein expression.

Results: Pet1::GCaMP mice displayed increased DRN 5-HT neuronal activity during the ictal period. However, this activity is impaired postictally. Additionally, there was no response to CO₂ for varying durations upon seizure termination.


Conclusions: Such findings support that seizures negatively impact the neurons that contribute to CO₂ arousal. Recording from the DRN 5-HT neurons demonstrated a refractory period following electrically induced seizures. Future directions include examining the DRN 5-HT projections and identifying subservient CO₂ chemoreceptive populations.


Funding: R01NS129722 (GFB), F31NS125955 (KGJ), T32NS007421 (MJS)