Abstract
Recordings from serotonin (5HT)-containing neurons of the brainstem suggest that their effects on upper airway, including hypoglossal (XII), motoneurons are maximal during wakefulness and minimal during rapid eye movement (REM) sleep. Several distinct 5HT receptors are present in the XII nucleus. In this study, we assessed the effects mediated by type 1 receptors. We microinjected 5HT1 receptor agonists or antagonists into the XII nucleus of anesthetized, paralyzed, vagotomized, and artificially ventilated rats and observed the resulting changes in XII nerve activity. Both CGS-12066 and 8-OH-DPAT (5HT1B and 5HT1A receptor agonists, respectively) suppressed XII nerve activity in a dose-dependent manner by about 20%. MAN-190, a putative 5HT1A receptor antagonist, had no effect, Based on the relative potencies of the two agonists, we conclude that they may both act through 5HT1B receptors and that 5HT1A receptors are absent from the XII nucleus of adult rats. If the effects mediated by 5HT1B receptors are endogenously active, their withdrawal during REM sleep may to some degree counterbalance the disfacilitation of XII motoneurons resulting from the withdrawal of their 5HT-mediated postsynaptic excitation.
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