Compared effects of serotonin on the inspiratory activity of glossopharyngeal, vagal, hypoglossal and cervical motoneurons in neonatal rat brain stem-spinal cord preparations.

Experiments were performed on isolated brain stem-spinal cord preparations from neonatal rats, with a view to comparing the effects of serotonin (5-HT) on the rhythmic inspiratory activity recorded in the glossopharyngeal (IX), vagal (X), hypoglossal (XII) and cervical motor roots. Under normal medium, the inspiratory bursts recorded on the different roots presented slight changes in amplitude from one cycle to the other which were significantly correlated only between IX and X nerves. When the normal medium was replaced by a medium containing 5-HT (30 microM), the frequency of occurrence of the inspiratory bursts increased. After 6 min 5-HT superfusion, the inspiratory bursts observed in the IX, X and cervical recordings decreased slightly but significantly, whereas the XII inspiratory discharges decreased drastically. Upon returning to the control conditions, a total recovery was observed in all the recordings apart from the XII discharge, which remained depressed. Although 5-HT qualitatively depressed all the inspiratory motor outputs, the quantitatively drastic and long-lasting depression of the XII inspiratory activity was particularly noteworthy. 5-HT therefore seems to have specific effects on XII inspiratory activity which may be responsible for obstructive apnea in cases involving abnormal 5-HT biosynthesis mechanisms.