Dual projecting neurons for the laryngeal chemoreflex apnoea

The activation of the SLN produces a strong inhibitory response to stop central breathing (apnoea). The neuronal mechanism underlying this apnoeic response, however, remains unknown. A characteristic, non-respiratory burst activity in the expiratory laryngeal motoneurons plays a critical role in producing the apnoeic response. The potential burst-relaying neurons were predicted in the nucleus tractus solitarius (NTS), with simultaneous projections to the Botzinger complex (BotC) to stop central breathing and to the laryngeal motoneurons in the caudal nucleus ambiguus (NA) to close vocal cords. The aim of this project was to provide anatomical evidence to support the existence of such dual-projecting neurons in the NTS. Experiments were performed on 25 anaesthetised Sprague Dawley rats. Four to five days after microinjection of the fluorescent conjugates of cholera toxin B into BotC and unconjugated CTB into the caudal NA, the rats were perfused transcardially with 4% paraformaldehyde. Unconjugated CTB visualized using Goat anti CTB tagged to Alexa-fluor 488 using immunohistochemistry protocol and sections were visualized on Ziess fluorescence microscope. Retrograde labelled neurons in the NTS projecting to BotC and NA were found in the caudal NTS. A small number of double labelled neurons with simultaneous projections to BotC and NA were found ~360um rostral to ~360um caudal to the obex mainly in interstitial and ventrolateral NTS. The study provided anatomical evidences for dual projecting neurons involved in production of apnoeic response and vocal cords adduction.