, 2002 and Lu et al., 2006b). While these studies have found relatively few Fos immunoreactive neurons in the PPT or LDT, Fos expression was elevated in three slightly more caudal cell groups: the sublaterodorsal nucleus (SLD), which is ventral and caudal to the LDT; the precoeruleus region (PC), which lies just dorsal to the SLD and caudal to the LDT; and the medial parabrachial nucleus (MPB), which is just dorsolateral to the SLD (Figure 2). The role of the SLD

in producing REM sleep has been studied by injecting it with bicuculline, A-1210477 clinical trial a GABA antagonist, which disinhibits the SLD neurons and elicits REM sleep-like behavior (Boissard et al., 2002). Lesions in the SLD region of cats, also called the subcoeruleus area, have been known since

the 1970s to disrupt atonia during REM sleep such that animals appear to act out their dreams (Hendricks et al., 1982, Sastre and Jouvet, 1979 and Shouse and Siegel, 1992). However, lesions of the SLD in rats have more profound effects, fragmenting and reducing the amount of REM sleep (Lu et al., SB203580 research buy 2006b). Injections of retrograde tracers into the SLD identified major GABAergic inputs from the vlPAG and adjacent lateral pontine tegmentum (LPT) (Boissard et al., 2003 and Lu et al., 2006b). This same region receives convergence of inputs from the extended VLPO and the orexin neurons in the lateral hypothalamus (Lu et al., 2006b). Because the extended VLPO neurons promote REM sleep but are inhibitory, and the orexin neurons prevent REM sleep and are excitatory, the vlPAG-LPT region would be expected to prevent REM sleep. As neurons in the vlPAG-LPT that project to the SLD are GABAergic, they would be expected to fire when REM sleep is inhibited (i.e., to show a REM-off firing pattern). Indeed, inhibition of the vlPAG and LPT with GABA agonists increases REM sleep (Crochet et al., 2006, Sapin et al., 2009 and Sastre et al., 1996), either and lesions increase REM sleep, particularly during the dark phase (Lu et al., 2006b). Injections

of retrograde tracers into the vlPAG and LPT demonstrate retrogradely labeled GABAergic neurons in the SLD and anterogradely labeled axons from the SLD are found in close apposition to GABAergic neurons in the vlPAG and LPT (Lu et al., 2006b). These findings suggest that the vlPAG-LPT and the SLD have a mutually inhibitory relationship that may govern switching in and out of REM sleep, much like the relationship between the VLPO and the ascending arousal systems, which we hypothesize is the basis for switching between sleep and wake states. Glutamatergic neurons that are mixed in with the REM-on GABAergic neurons give rise to long projections that activate the principle components of the REM state (Lu et al., 2006b, Luppi et al., 2004, Luppi et al., 2006, Shouse and Siegel, 1992 and Webster and Jones, 1988).