NTS neurons with carotid chemoreceptor inputs arborize in the rostral ventrolateral medulla

Neurons that were excited by hypoxic stimulation of carotid chemoreceptors were recorded in the caudal portion of the nucleus of the solitary tract (cNTS) of urethan-anesthetized, vagotomized, aortic-deafferented, artificially ventilated rats (n = 23). The focus of the study is on 26 chemosensitive neurons (classified as early- and late-response cells) that were tonically activated by chemoreceptor stimulation and never fired in bursts synchronized with the phrenic nerve discharge (PND) cycle. The discharge of early-response cells (n = 14) started up to 2.5 s before the onset of PND activation, whereas the discharge of late-response cells (n = 14) started 1.5-5 s after onset of PND response. Four early-response cells were antidromically activated from the rostral ventrolateral medulla (RVLM; latencies: 7-13 ms), and two had axonal collaterals in the region of the nucleus ambiguus. Four late-response neurons were antidromically activated from the RVLM (latencies: 6-12 ms), but no collateral was found in this area. The basal discharge of early- and late-response cells ranged from 0 to 10 and 0 to 30 spikes/s, respectively, but most of them had a very low spontaneous firing rate (median: 0.2 and 0.6 spikes/s, respectively). Neither type was excited by baroreceptor stimulation. The cNTS also contained neurons that were firing in bursts synchronized with the PND cycle. These cells were activated by chemoreceptor stimulation and were not antidromically activated from the RVLM. Chemosensitive neurons made up 33% of cNTS neurons antidromically activated from the RVLM (8/24). In conclusion, a population of cNTS chemosensitive neurons devoid of respiratory modulation projects through the RVLM and arborizes in this region.