Absence of respiration modulation of carotid sinus nerve inputs to nucleus tractus solitarius neurons receiving arterial chemoreceptor inputs.

The reflex responses to activation of the arterial chemoreceptors are dependent upon when in the respiratory cycle the chemoreceptor stimulus is given. To determine if the respiratory modulation of the chemoreflex occurs within the nucleus tractus solitarius (NTS), intracellular recordings were obtained in pentobarbital-anesthetized, paralyzed and mechanically ventilated cats, from 22 non-respiratory NTS cells which were depolarized following activation of the ipsilateral carotid body chemoreceptors (by close arterial injection of < 100 microliters CO2 saturated bicarbonate). Activation of the ipsilateral carotid body chemoreceptors evoked depolarizations with amplitudes of 2.9-4.6 mV and durations of 2.1-5.9 s. Three of these cells also received a convergent excitatory input from the carotid sinus baroreceptors. Carotid sinus nerve (CSN) stimulation evoked either an excitatory post-synaptic potential (EPSPs) (n = 14, 8 monosynaptic) or an excitatory/inhibitory sequence (EPSP/IPSPs) (n = 8, 1 monosynaptic). CSN evoked PSPs were separately averaged (25-50 sweeps) during periods of phrenic nerve activity and phrenic nerve silence and during periods when the lungs were inflated and when the lungs were deflated. No parameter of the CSN evoked PSPs (latency, peak amplitude, duration) was altered during periods of phrenic nerve activity or lung inflation (all P values > 0.12, Wilcoxon signed-rank test). The results suggest that there is no respiratory modulation of arterial chemoreceptor inputs by either central respiratory drive or lung stretch receptor afferent inputs at this early stage of the reflex arc.