Effects of carotid chemoreceptor excitation on medullary expiratory neurons in cats

A previous report (Lipski et al., 1977, J. Physiol. (London) 269, 797-810) demonstrated an inhibition of the medullary dorsal inspiratory neurons when a phasic chemoreceptor stimulus was applied during expiration. The present study tested the response of expiratory neurons which might mediate this inhibition. Recordings were made in cats anaesthetized with chloralose-urethane from the C5 phrenic rootlet and mainly from the rostral (Bötzinger) and caudal (nucleus retroambigualis, NRA) groups of medullary expiratory neurons. Carotid chemoreceptors were excited by close arterial injections of CO2-equilibrated saline. The stimuli were applied automatically with a preset delay within the respiratory cycle. The stimuli applied in inspiration excited both the phrenic activity and ventral inspiratory neurons within less than 0.5 sec. The stimuli applied in expiration excited 17 out of 25 NRA units, none of which projected to the contralateral dorsal (NTS) respiratory group. Four out of 9 units within be responsible for the expiratory inhibition of NTS inspiratory cells produced by chemoreceptor stimulation, while the caudal expiratory neurons are involved in the mediation of the chemoreceptor-induced effects upon the spinal neurons.