Neurons that Project to both the Nucleus Ambiguus and the Bӧtzinger Complex are Ideally Situated to control Laryngeal Adduction and Apnoea During Swallowing

Abstract

Laryngeal stimulation that generates swallow activity is accompanied by laryngeal adduction along with central shutdown of breathing (apnoea) to execute safe swallowing without aspiration. The neural control of this is poorly understood. We hypothesized that the temporal precision of these separate phenomenon i.e., laryngeal adduction and apnoea during swallow is controlled by swallow burst generating neurons in the nucleus tractus solitarius (NTS) that project simultaneously to the caudal nucleus ambiguus (NA) for laryngeal adductions and to the Bӧtzinger complex (BötC) for apnoea. The present study aimed to provide anatomical support for the existence of such dual projecting neurons in the NTS. Retrograde tracing experiments were performed on adult male anaesthetised Sprague Dawley rats (n=2). Each rat received two microinjections of a retrograde tracer: cholera toxin B-fluorescent conjugate (CTB-555) into the BötC and unconjugated CTB into the NA. Following a recovery period of 5-6 days the rats were euthanased and perfused transcardially with 4% paraformaldehyde with immunohistochemistry performed to visualize those neurons traced by the unconjugated CTB. The rostral to caudal distribution of labelled neurons in the NTS was determined by analysing multichannel tile images with fluorescent microscopy. Among the single labelled neurons in the caudal NTS a small number of double-labelled neurons were identified. The double-labelled neurons were found caudal to obex with ≤ 25 per section (n=2). This study supports the existence of neurons in the caudal NTS with simultaneous projection to NA and BötC and which could be potential candidates for the control of laryngeal adduction and apnoea during swallowing.