NMDA receptor-mediated inspiratory off-switching in pneumotaxic-disconnected cats

The pneumotaxic center is thought to govern inspiratory off-switching (IOS), and blockade of N-methyl-D-aspartate (NMDA) receptors by dizocilpine impairs IOS causing apneusis. The present study is to examine whether the NMDA receptor-mediated IOS mechanism functions in the medullary respiratory network after disconnecting the pneumotaxic center. In decerebrate and vagotomized cats, the nucleus parabrachialis medialis (NPBM) and vagus nerves were stimulated to evoke IOS and a dorsal pontine transection was performed while the central respiratory activity was recorded in phrenic neurogram. The transection eliminated the NPBM-stimulated IOS but not the vagally evoked IOS, and developed two types of respiration; eupnea in 12 and apneusis in six out of 18 cats. Apneustic respiration was not changed into eupneic one by changing the end-tidal CO2 level. In animals displaying eupnea after the transection, dizocilpine (0.3 mg/kg i.v.) produced apneusis, characterized by a prolonged inspiration, a shortened stage 1 expiration and an unchanged stage 2 expiration. Dizocilpine caused no further change in the apneustic pattern induced by the transection. The present results suggest that the medullary respiratory network is able to generate a eupneic respiration after disconnecting the pontine pneumotaxic center, and the NMDA mechanism plays an important role in the medullary respiratory network.