Effect of superior laryngeal nerve stimulation on swallowing and respiratory coordination in dexmedetomidine-administered adult rats

It is not clear whether swallowing, which is induced by electrical stimulation of the superior laryngeal nerve (SLN), dominantly suppresses respiration. We examined how the swallowing coordinates with respiration in adult rats under continuous infusion of dexmedetomidine, which possesses α₂-adrenoceptor/imidazoline I₁ receptor (α₂AR/I₁R) mediated sedative-analgesic profile with minimal direct GABAergic potentiation, thereby reducing interference with swallowing circuitry. Prior to the experiment, adult rats (203-322 g, n = 12) under isoflurane anesthesia had their tail vein cannulated for dexmedetomidine administration and tail artery cannulated for analysis of arterial pressure, pulse rate, blood gases (PaO₂, PaCO₂, pH), blood glucose, hematocrit and hemoglobin. After recovery, the animals received continuous infusion of dexmedetomidine intravenously. An incision was made in the neck for SLN electrical stimulation and for recording mylohyoid electromyograms to detect swallowing. Another incision was made in the upper abdomen for recording diaphragm electromyograms to detect inspiratory movements. The spontaneously breathing animals showed no response to tactile or noxious stimulations under continuous infusion of dexmedetomidine. During SLN stimulation at different frequencies (20, 50, or 100 Hz for 10 s), the number of swallows increased in a frequency-dependent manner. Increased swallowing primarily occurred with suppressed respiration (Phase I) and thereafter occurred just after inspiration (Phase II), during which the respiratory frequency recovered to the level before stimulation. The results suggest that, in adult rats receiving dexmedetomidine, SLN stimulation not only triggers swallowing and apnea, but also allows respiration to be resumed and control the occurrence of the swallowing in coordination with respiration.