HYPOXIA-INDUCED LONG-TERM FACILITATION OF RESPIRATORY ACTIVITY IS SEROTONIN DEPENDENT

Repeated isocapnic hypoxia evokes long-term facilitation (LTF) of phre nic nerve activity in rats. We wished to determine: (1) whether hypoxi a-induced LTF is serotonin dependent; and (2) whether hypoxia-induced LTF is a property of upper airway motoneurons. Phrenic and hypoglossal nerve activities were recorded in urethane anesthetized, vagotomized, paralyzed and artificially ventilated rats (n = 7). Rats were exposed to three, 5-min hypoxic episodes (FIo2 = 0.10) separated by 5 min of hyperoxia (FIo2 = 0.50). One hour after the final hypoxic episode, int egrated phrenic and hypoglossal amplitudes and burst frequency were in creased above control values (63 +/- 17%, 78 +/- 26% and 9.6 +/- 2.1 b ursts/min, respectively; p < 0.05). In rats pretreated with methysergi de (n = 7; 4 mg/kg), no changes in phrenic or hypoglossal activity fro m pre-stimulus control values were observed at any time post-stimulati on. The results indicate that hypoxia-induced LTF requires 5-HT recept ors and is characteristic of both hypoglossal and phrenic motor output .