Project Abstract One in 26 Americans will develop epilepsy during their lifetime. One third of these patients have poorly controlled epilepsy that does not respond to medication, placing them at greater risk for sudden unexpected death in epilepsy (SUDEP). While the exact etiology of SUDEP is unknown, dysregulation of the serotonin (5-HT) system has been linked to these untimely deaths. 5-HT plays a critical role in sleep-wake regulation and arousal. Impaired postictal arousal occurs after seizures and is considered a risk factor for SUDEP. Most cases of SUDEP occur at night with the victim found prone in bed. This implies that the victim was unable to arouse, despite the increasing carbon dioxide (CO2) levels which would normally trigger a protective arousal reflex. This reflex is mediated by 5-HT neurons in the dorsal raphe nucleus (DRN). The DRN is a major component of the ascending activating system that produces arousal. The DRN is also a key source of 5-HT projections to the rest of the brain. Neuronal activity in the DRN is dysregulated by seizures. Structural changes in the DRN with chronic epilepsy have been reported. We hypothesize that dysregulation of DRN 5-HT activity in epilepsy patients contributes to loss of postictal arousal and SUDEP risk. In Aim 1 we will use a custom optical fiber-coupled micro- wire array to measure interictal DRN 5-HT activity in epileptic mice during different sleep states and arousal from sleep. We will also assess whether DRN 5-HT activity is suppressed following seizures induced during different sleep-wake states. Aim 2 we will utilize calcium imaging with fiber photometry to determine whether these epileptic animals have blunted DRN 5-HT neuronal activation in response to CO2 exposure. We will assess whether epileptic mice exhibit an increased latency to arouse to CO2 compared to healthy mice. Participation in the proposed training plan and completion of the proposed experiments will advance the applicant's neuroscience training towards a career as an independent neuroscientist. It will also determine the neuronal circuitry involved in causing some seizures to be fatal and provide clinicians a potential biomarker to identify patients at the highest risk for SUDEP.