NTS NEURONS WITH CAROTID CHEMORECEPTOR INPUTS ARBORIZE IN THE ROSTRALVENTROLATERAL MEDULLA

Neurons that were excited by hypoxic stimulation of carotid chemorecep tors were recorded in the caudal portion of the nucleus of the solitar y tract (cNTS) of urethan-anesthetized, vagotomized, aortic-deafferent ed, artificially ventilated rats (n = 23). The focus of the study is o n 26 chemosensitive neurons (classified as early- and late-response ce lls) that were tonically activated by chemoreceptor stimulation and ne ver 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 lat e-response cells (n = 14) started 1.5-5 s after onset of PND response. Four early-response cells were antidromically activated from the rost ral ventrolateral medulla (RVLM; latencies: 7-13 ms), and two had axon al collaterals in the region of the nucleus ambiguus. Four late-respon se neurons were antidromically activated from the RVLM (latencies: 6-1 2 ms), but no collateral was found in this area. The basal discharge o f early- and late-response cells ranged from 0 to 10 and 0 to 30 spike s/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 ex cited by baroreceptor stimulation. The cNTS also contained neurons tha t were firing in bursts synchronized with the PND cycle. These cells w ere activated by chemoreceptor stimulation and were not antidromically activated from the RVLM. Chemosensitive neurons made up 33% of cNTS n eurons antidromically activated from the RVLM (8/24). In conclusion, a population of cNTS chemosensitive neurons devoid of respiratory modul ation projects through the RVLM and arborizes in this region.