SEROTONERGIC EFFECTS ON HYPOGLOSSAL NEURAL ACTIVITY AND REFLEX RESPONSES

We determined the effects of serotonin (5HT; 6 concentrations ranging from 0.005 to 500 mu M) pressure microinjection into the hypoglossal ( XII) motor nucleus (100-500 nl; pH = 7.2-7.4) on XII whole nerve activ ity and reflex response to upper airway negative pressure in 15 decere brated, vagotomized, paralyzed and artificially ventilated cats. Incre asing 5HT concentration resulted in a concentration dependent increase in ipsilateral tonic XII activity, with no change in phasic XII activ ity. Threshold concentrations ranged from 0.005 to 0.5 mu M, with the maximal response reached at 5 mu M. Increasing 5HT concentration also increased the duration of the XIII response. This ranged from 50 s wit h 0.5 mu M, to over 10 min with 500 mu M 5HT. However, 5HT did not sig nificantly change the XII whole nerve reflex response to upper airway negative pressure (-20 cm H2O) at any 5HT concentration (n = 5). All 5 HT 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.