Origin of the expiratory inhibition of nucleus tractus solitarius inspiratory neurones.

Respiratory neurones from retrofacial (NRF) and Bötzinger nuclei (BOT) were tested electrophysiologically for projection to the contralateral inspiratory region of the nucleus of the tractus solitarius (NTS). Antidromic activation was demonstrated for 30 expiratory neurons (out of a mixed population of 61). There was evidence of extensive branching within NTS for these axons. Histological localization of projecting BOT neurones was confined to the medial border of the NRF. NTS inspiratory neurones were impaled in the vicinity of BOT axonal arborizations, and averages of post-synaptic noise were made, using the simultaneously recorded BOT spike trains to trigger the averager. Unitary IPSPs were found in 5 out of 9 pairs so analyzed. The waveforms of these IPSPs were similar to previously recorded monosynaptic IPSPs. 25 expiratory neurones, recorded from the caudal part of nucleus retroambigualis (NRA), were tested for projection to NTS; no projections could be demonstrated. 8 NTS inspiratory neurones were tested, using spike-triggered averaging of post-synaptic noise, for functional connections from caudal NRA expiratory neurones; no PSPs were seen. It is concluded that the expiratory inhibition of NTS inspiratory neurones is due, in part at least, to inputs from BOT expiratory neurones. Caudal NRA expiratory neurones do not appear to contribute to this inhibition.