ROLE OF 5HT(1) RECEPTORS IN THE CONTROL OF HYPOGLOSSAL MOTONEURONS IN-VIVO

Recordings from serotonin (5HT)-containing neurons of the brainstem su ggest that their effects on upper airway, including hypoglossal (XII), motoneurons are maximal during wakefulness and minimal during rapid e ye movement (REM) sleep. Several distinct 5HT receptors are present in the XII nucleus. In this study, we assessed the effects mediated by t ype 1 receptors. We microinjected 5HT(1) receptor agonists or antagoni sts 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 (5HT(1B) and 5HT(1A) rec eptor agonists, respectively) suppressed XII nerve activity in a dose- dependent manner by about 20%. NAN-190, a putative 5HT(1A), receptor a ntagonist, had no effect. Based on the relative potencies of the two a gonists, we conclude that they mau both act through 5HT(1B) receptors and that 5HT(1A) receptors are absent from the XII nucleus of adult ra ts. If the effects mediated by 5HT(1B) receptors are endogenously acti ve, their withdrawal during REM sleep may to some degree counterbalanc e the disfacilitation of XII motoneurons resulting from the withdrawal of their 5HT-mediated postsynaptic excitation.