Medullary responses to chemoreceptor activation are inhibited by locus coeruleus and nucleus raphe magnus.

The effect of electrical and L-glutamate stimulation of the nucleus locus coeruleus (LC) and nucleus raphe magnus (NRM) on multiunit activity evoked in the nucleus tractus solitarius (NTS) by activation of arterial chemoreceptors (15-25 micrograms kg-1 of sodium cyanoborohydride, i.v.) was studied in rats anaesthetized with urethane (1.1 g kg-1 i.p.). Multiunit discharge of NTS neurones in response to cyanide injection was composed by spikes higher than 200 microV and about 10 Hz frequency. Electrical and L-glutamate stimulation of the LC and the NRM significantly reduced the frequency of the cyanide-induced multiunit discharge. The results show that neurones of the NTS with input from arterial chemoreceptors can be inhibited by LC and and NRM cells, suggesting that these nuclei play a role in controlling chemosensory input at the NTS.