Arterial chemoreceptor input to nucleus tractus solitarius

The arterial chemoreceptors play an important role in the reflex regulation of blood pressure and respiration. To investigate the initial integration of chemoreceptor inputs within the central nervous system, intracellular recordings were obtained in pentobarbital-anesthetized, paralyzed, and mechanically ventilated cats, from 58 cells within the nucleus of the tractus solitarius (NTS) that were depolarized by activation of the ipsilateral carotid body chemoreceptors. Close arterial injection of <100 μl CO₂-saturated bicarbonate evoked depolarizations of membrane potential with amplitudes of 2.2-4.6 mV and durations of 1.8-6.7 s in 46 cells. In 12 cells, activation of the carotid body chemoreceptors evoked a depolarization- hyperpolarization sequence. Electrical stimulation of the carotid sinus nerve (500 μA, 0.2 ms) evoked EPSPs [mean latency 6.4 ± 0.5 (SE) ms; range 2.1- 18.4 ms] in 46 cells and EPSP-IPSPs (7.3 ± 0.8 ms; range 4.2-12.4 ms) in 12 cells. The distribution of EPSP latencies exhibited two peaks, one in the 2- to 4-ms range and another in the 7- to 8-ms range. Twenty-nine chemoreceptive cells were tested for the presence of convergent inputs from the ipsilateral carotid sinus baroreceptors. No evidence was found of a convergent postsynaptic inhibitory input from the baroreceptors within the NTS; however, seven cells were found that received an excitatory input from the baroreceptors. The observation that NTS neurons do not integrate chemoreceptor afferent inputs in a homogeneous manner suggests that the multiplicity of NTS unit responses might be related to the specific reflex function of an individual cell (e.g., vagal or sympathetic outflow, respiration). The various patterns may play an important role in determining the signal that is relayed to subsequent sites in the chemoreflex pathways.