Autonomic nerve activity and cardiovascular changes during discrete seizures in rats

Activity in both divisions of the autonomic nervous system (ANS) can increase during seizures and result in tachy- or bradyarrhythmias. We sought to determine the patterns of ANS activity that led to heart rate (HR) changes and whether the character of ANS and HR changes can impact the seizures themselves. Simultaneous recordings of vagus nerve and cervical sympathetic ganglionic or nerve activity, EEG, ECG, and blood pressure were acquired from 16 urethane-anesthetized rats that received systemic kainic acid to induce seizures. After initial continuous seizure activity, discrete seizures were observed in 11/16 rats. Individual seizures were classified based on HR changes as tachycardic (n = 3), bradycardic (n = 17), or one of two more severe categories in which (a) the seizure appeared to be terminated by severe bradyarrhythmia (n = 5) or (b) the animal died (n = 6). Interestingly, even simple bradycardic seizures had episodes of dramatically increased respiratory effort, which we interpret as evidence of airway occlusion based on muscle artifacts in the recordings with transient blood pressure decreases. In the severe outcomes, ANS activity increased during seizures until it caused a drastic HR reduction (>50%), in which case seizure and ANS activity decreased dramatically. Sympathetic activity during this late vulnerable period was important for survival. We conclude that individual seizures produce (a) stereotypical changes in autonomic activity and HR, (b) persistence of sympathetic tone helps to protect against death, and (c) bradycardic seizures may indicate increased risk for seizure-associated obstructive apnea.