DIFFERENTIAL SENSITIVITY OF LARYNGEAL AND PHARYNGEAL MOTONEURONS TO IONTOPHORETIC APPLICATION OF SEROTONIN

Serotonergic neurons decrease their activity during sleep, especially rapid eye movement sleep, thereby reducing their facilitatory effect o n upper airway motoneurons. The magnitude of the sleep-related loss of tone varies among upper airway muscles (e.g., pharyngeal dilator moto neurons are more suppressed than laryngeal motoneurons). We hypothesiz ed that these differences may be related to the sensitivity of differe nt groups of upper airway motoneurons to serotonin. Experiments were d one on decerebrate, vagotomized, paralysed and artificially-ventilated cats. Hypoglossal and laryngeal motoneurons were recorded extracellul arly using five-barrel pipettes filled with: serotonin, glutamate and methysergide (serotonergic antagonist) for iontophoresis, and NaCl for recording and current balancing. All but two of the 65 hypoglossal mo toneurons (45 inspiratory, 10 expiratory, IO tonic) and 27 out of 32 l aryngeal motoneurons (14 inspiratory, 18 expiratory) were excited by s erotonin, and the excitation was abolished by methysergide. To compare the magnitude of the excitatory effect among distinct motoneuronal gr oups, we applied small ejection currents in a standardized manner (+15 nA for 3 min; 10 mM serotonin in 150 NaCl) onto spontaneously active motoneurons (13 inspiratory hypoglossal, 11 inspiratory laryngeal and ii expiratory laryngeal). Serotonin increased the number of spikes per respiratory burst of inspiratory hypoglossal motoneurons from 19+/-4. 0 (S.E.M.) to 35+/-4.8, of inspiratory laryngeal motoneurons from 44+/ -8.3 to 55+/-8.8, and of expiratory laryngeal motoneurons from 23+/-4. 8 to 33+/-6.2. The relative increases in activity (to 220%+/-24, 147%/-23 and 148%+/-9 of control, respectively) were significantly higher in hypoglossal than in laryngeal motoneurons. In addition, the excitat ory effect developed significantly faster in hypoglossal than in laryn geal motoneurons. Methysergide reduced the spontaneous activity of abo ut half the hypoglossal and laryngeal motoneurons to 66%+/-5 of contro l. Thus, the sensitivity to the excitatory effects of serotonin varies among different pools of upper airway motoneurons. These differences correlate with the pattern of airway muscle hypotonia seen during slee p. (C) 1997 IBRO. Published by Elsevier Science Ltd.