We previously reported (Fenik et al., SFN Abstr. 2002) that simultaneous antagonism of serotonergic (5-HT), noradrenergic (NE) and GABAA receptors in the hypoglossal (XII) nucleus region abolishes the depression of XII nerve activity that occurs during the REM sleep-like episodes elicited by pontine carbachol (cREM). The episodes comprise XII depression, cortical activation, hippocampal theta and silencing of NE neurons (J.Appl.Physiol 93:1448, 2002). We now tested whether the antagonism of only 5-HT and NE receptors is sufficient to achieve this effect. In 6 urethane-anesthetized, paralyzed and artificially ventilated rats, cREM was elicited before, and at different times after, 3 injections (40 nl each) of the 5-HT receptor antagonist methysergide (1 mM) and α1 adrenergic receptor antagonist prazosin (0.2 mM) into the XII nucleus. Integrated XII nerve activity was expressed as a percentage of its level at the beginning of the experiment. Prior to injecting the antagonists, XII nerve activity was depresse...