Serotonergic (5-HT) neurons of the dorsal raphe nucleus (DRN) and cholinergic neurons in the adjacent laterodorsal

Serotonergic (5-HT) neurons of the dorsal raphe nucleus (DRN) and cholinergic neurons in the adjacent laterodorsal (LDT) and pedunculopontine tegmental nuclei (PPT) have together been implicated in the regulation and production of REM sleep (reviewed in McCarley et al., 1995; Jones, 1993). Several lines of evidence indicate that the activity of cholinergic neurons in the PPT is involved in the generation of REM sleep via projections to the pontine reticular formation and thalamus. For example, electrophysiological studies of PPT neurons reveal sub-populations that discharge preferentially just before and during REM sleep (termed REMon neurons), or discharge during both wakefulness (W) and REM sleep (referred to as Wake/REM-on neurons) (El Mansari et al., 1989; Steriade et al., 1990; Kayama et al., 1992; Thakkar et al., 1998). In contrast to mesopontine neurons that preferentially discharge in REM sleep, monoaminergic neurons in the noradrenergic locus coeruleus and serotonergic DRN exhibit a pattern of discharge activity that is nearly opposite to that of the cholinergic PPT neurons: discharge is greatest during waking, declines during slow wave sleep (SWS) and virtually ceases prior to and during REM sleep for both DRN (McGinty and Harper, 1976; Lydic et al., 1987; Jacobs and Fornal, 1991) and locus coeruleus (Hobson et al., 1975; Foote et al., 1983). This inverse correlation with REM sleep led to suggestions that norepinephrine (McCarley and Hobson, 1975) and 5-HT activity (McGinty and Harper, 1976) might suppress REM sleep, and formed the basis of a structural and mathematical model of REM sleep control, the reciprocal interaction model (McCarley and Hobson, 1975), that had as one of its postulates that the REM-off neurons inhibited the REM-promoting, REM-on neurons (McCarley and Massaquoi, 1992; McCarley et al., 1995). For many years this was regarded as a highly controversial postulate. However, in recent years, in vitro work has supported this hypothesis, revealing that 5-HT directly inhibits identified cholinergic neurons of the LDT and PPT (Mühlethaler et al., 1990; Luebke et al., 1992; Leonard and Llinas, 1994). Furthermore, there is anatomical evidence indicating that the DRN sends 5-HT projections to both LDT and PPT (Semba and Fibiger, 1992; Honda and Semba, 1994; Steininger et al., 1997). Other in vivo work has demonstrated that the 5-HT inhibitory control of the LDT/PPT REMgenerating region is sufficiently strong to influence the behavioral expression of REM sleep (Cespuglio et al., 1979; Portas et al., 1996; Sanford et al., 1994; Horner et al., 1997). In summary, previous work supports the following model of 5-HT involvement in REM sleep generation: when the discharge activity of 5-HT DRN neurons slows in drowsiness and SWS, less 5-HT is released from the 5-HT terminals onto inhibitory 5HT1a receptors on cholinergic neurons of the PPT, Neurons of the cholinergic mesopontine tegmentum preferentially discharge during REM sleep and are thought to promote this state. It has been hypothesized they are inhibited during wakefulness by serotonergic input. The present study used the microdialysis sampling procedure coupled to microbore HPLC to measure extracellular serotonin levels in the pedunculopontine tegmental nucleus (PPT) in naturally sleeping cats. Extracellular serotonin levels were found to be highest during periods of wakefulness, lower during slow wave sleep, and lowest during periods of REM sleep. During wakefulness serotonin levels (mean ± SEM) measured in 10 μl samples were 1.14 ± 0.13 fmol/sample, whereas during slow wave sleep levels declined significantly to 72% of the wakefulness baseline (0.85 ± 0.11 fmol/sample), and dropped further to 45% of the wakefulness baseline in REM samples (0.52 ± 0.10 fmol/sample; all p's<0.003). The decrease in PPT serotonin levels during sleep may be an important determinant in the timing of REM sleep cyclicity. The data support the hypothesis that, during slow wave sleep and REM sleep, the declining levels of serotonin release the PPT REM-promoting neurons from serotonergic inhibition, which, in turn, leads to increases in acetylcholine release in terminal areas, facilitating the emergence of REM sleep.