Pneumotaxic mechanisms in the non-human primate: effect of the N-methyl-D-aspartate (NMDA) antagonist ketamine

We tested the possible involvement of N-methyl-D-aspartate (NMDA) receptors in the central inspiratory-termination mechanism in non-human primates. Inspiratory bursts were recorded from the phrenic nerve in Macaca fascicularis monkeys paralyzed and ventilated by means of a servoventilator driven by the inspiratory discharge of the phrenic nerve. The central inspiratory termination mechanism was tested by withholding lung inflation. This transiently suppressed the vagal feedback from the lungs which produces inspiratory off-switching independent from the central mechanism. Under anaesthesia with ketamine, a potent NMDA antagonist, non inflation increased inspiratory time to 4s (1s with lungs inflated) whereas no such effect was observed during halothane anaesthesia. We conclude that the termination of inspiration in primates is controlled via central mechanisms in which NMDA receptors are involved.