Neural respiratory responses to cortically induced seizures in cats

Seizure activity can lead to profound respiratory stimulation in spontaneously breathing animals with intact respiratory feedback mechanisms (Paydarfar et al., Am. J. Physiol. 260, R934, 1991). The present study was designed to test the hypothesis that peripheral respiratory feedback mechanisms are not important for the genesis of seizure-induced hyperpnea. Studies were performed in 16 anesthetized, vagotomized, glomectomized cats whose end-tidal PCO2 (PETCO2) was kept constant. Integrated phrenic nerve activity was used to represent respiration. Seizures were induced by injection of penicillin into the parietal cortex and electrocorticographic (ECoG) and biceps femoris nerve activities, arterial pressure, airway PCO2 and brain temperature were recorded continuously. Progressive seizure activity was associated with progressive increases of respiratory frequency and peak phrenic activity, despite constancy of PETCO2 and brain temperature. Patterns of entrainment were identified among ECoG spikes, biceps femoris nerve and phrenic nerve activities. Phrenic nerve activity became highly irregular during generalized ictal seizures and ceased to respond to changes of PETCO2. Acute intercollicular decerebration in all experiments resulted in normalization of respiratory rhythm even while ictal ECoG activity continued. We conclude that stimulation of breathing during seizures occurs in the absence of respiratory feedback mechanisms. The findings suggest that an important cause of the respiratory response is a feedforward mechanism, whereby activation of subcortical structures above medulla and pons results in stimulation of breathing.