Serotonergic effects on hypoglossal neural activity and reflex responses

Abstract

We determined the effects of serotonin (5HT; 6 concentrations ranging from 0.005 to 500 μM) pressure microinjection into the hypoglossal (XII) motor nucleus (100–500 nl; pH = 7.2–7.4) on XII whole nerve activity and reflex response to upper airway negative pressure in 15 decerebrated, vagotomized, paralyzed and artificially ventilated cats. Increasing 5HT concentration resulted in a concentration dependent increase in ipsilateral tonic XII activity, with no change in phasic XII activity. Threshold concentrations ranged from 0.005 to 0.5 μM, with the maximal response reached at 5 μM. Increasing 5HT concentration also increased the duration of the XII response. This ranged from 50 s with 0.5 μM, to over 10 min with 500 μM 5HT. However, 5HT did not significantly change the XII whole nerve reflex response to upper airway negative pressure (−20 cm H 2O) at any 5HT concentration ( n = 5). All 5HT effects were reversed by microinjection of 1.0 mM methysergide. We conclude that XII responses to 5HT are elicited at low concentrations of 5HT, which have a relatively short duration of effect, but that 5HT at the XII motor nucleus has no effect on the XII reflex response to upper airway negative pressure.