Pyramidal cells accumulate chloride at seizure onset

► Seizures in vitro are preceded by an interneuron-dominated pre‐ictal burst. ► Interneuron-pyramidal cell firing is tightly coupled following the pre‐ictal burst. ► Chloride transiently increases during seizure, raising EGABA above spike threshold. ► Blocking GABAA receptors results in short bursting, small chloride transients.