Ponto-medullary nuclei involved in the generation of sequential pharyngeal swallowing and concomitant protective laryngeal adduction in situ

Both swallowing and respiration involve postinspiratory laryngeal adduction. Swallowing-related postinspiratory neurons are likely to be located in the nucleus of the solitary tract (NTS) and those involved in respiration are found in the Kolliker-Fuse nucleus (KF). The function of KF and NTS in the generation of swallowing and its coordination with respiration was investigated in perfused brainstem preparations of juvenile rats (n=41). Orally injected water evoked sequential pharyngeal swallowing (s-PSW) seen as phasic, spindle-shaped bursting of vagal nerve activity (VNA) against tonic postinspiratory discharge. KF inhibition by microinjecting isoguvacine (GABAA receptor agonist) selectively attenuated tonic postinspiratory VNA (n=10, P<0.001) but had no effect on frequency or timing of s-PSW. KF disinhibition after bicuculline (GABAA receptor antagonist) microinjections caused an increase of the tonic VNA (n=8, P<0.01) resulting in obscured and delayed phasic s-PSW. Occurrence of spontaneous PSW significantly increased after KF inhibition (P<0.0001) but not after KF disinhibition (P=0.14). NTS isoguvacine microinjections attenuated the occurrence of all PSW (n=5, P<0.01). NTS bicuculline microinjections (n=6) resulted in spontaneous activation of a disordered PSW pattern and long-lasting suppression of respiratory activity. Pharmacological manipulation of either KF or NTS also triggered profound changes in respiratory postinspiratory VNA. Our results indicate that the s-PSW comprises two functionally distinct components. While the primary s-PSW is generated within the NTS, a KF-mediated laryngeal adductor reflex safeguards the lower airways from aspiration. Synaptic interaction between KF and NTS is required for s-PSW coordination with respiration as well as for proper gating and timing of s-PSW.