Brainstem Circuitry Regulating Phasic Activation of Trigeminal Motoneurons during REM Sleep

Background

Rapid eye movement sleep (REMS) is characterized by activation of the cortical and hippocampal electroencephalogram (EEG) and atonia of non-respiratory muscles with superimposed phasic activity or twitching, particularly of cranial muscles such as those of the eye, tongue, face and jaw. While phasic activity is a characteristic feature of REMS, the neural substrates driving this activity remain unresolved. Here we investigated the neural circuits underlying masseter (jaw) phasic activity during REMS. The trigeminal motor nucleus (Mo5), which controls masseter motor function, receives glutamatergic inputs mainly from the parvocellular reticular formation (PCRt), but also from the adjacent paramedian reticular area (PMnR). On the other hand, the Mo5 and PCRt do not receive direct input from the sublaterodorsal (SLD) nucleus, a brainstem region critical for REMS atonia of postural muscles. We hypothesized that the PCRt-PMnR, but not the SLD, regulates masseter phasic activity during REMS.

Methodology/Principal Findings

To test our hypothesis, we measured masseter electromyogram (EMG), neck muscle EMG, electrooculogram (EOG) and EEG in rats with cell-body specific lesions of the SLD, PMnR, and PCRt. Bilateral lesions of the PMnR and rostral PCRt (rPCRt), but not the caudal PCRt or SLD, reduced and eliminated REMS phasic activity of the masseter, respectively. Lesions of the PMnR and rPCRt did not, however, alter the neck EMG or EOG. To determine if rPCRt neurons use glutamate to control masseter phasic movements, we selectively blocked glutamate release by rPCRt neurons using a Cre-lox mouse system. Genetic disruption of glutamate neurotransmission by rPCRt neurons blocked masseter phasic activity during REMS.

Conclusions/Significance

These results indicate that (1) premotor glutamatergic neurons in the medullary rPCRt and PMnR are involved in generating phasic activity in the masseter muscles, but not phasic eye movements, during REMS; and (2) separate brainstem neural circuits control postural and cranial muscle phasic activity during REMS.     

Brainstem neurons responsible for postural, masseter or pharyngeal muscle atonia during paradoxical sleep in freely-moving cats

In this mini review, we summarize our findings regarding the brainstem neurons responsible for the postural, masseter, or pharyngeal muscle atonia observed during paradoxical sleep (PS) in freely moving cats. Both the pons and medulla contain neurons showing tonic activation selective to PS and atonia, referred to as PS/atonia-on-neurons. The PS/atonia-on neurons, characterized by their most slow conducting property and located in the peri-locus coeruleus alpha (peri-LCa) and adjacent LCa of the mediodorsal pontine tegmentum, play a critical executive role in the somatic and orofacial muscle atonia observed during PS. Slow conducting medullary PS/atonia-on neurons located in the nuclei reticularis magnocellularis (Mc) and parvocellularis (Pc) may play a critical executive role in the generation of, respectively, antigravity or orofacial muscle atonia during PS. In addition, either tonic or phasic cessation of activity of medullary serotonergic neurons may play an important role in the atonia of genioglossus muscles during PS via a mechanism of disfacilitation.     

Muscle tone regulation during REM sleep: neural circuitry and clinical significance

Rapid eye movement (REM) sleep is a distinct behavioral state characterized by an activated cortical and hippocampal electroencephalogram (EEG) and concurrent muscle atonia. Research conducted over the past 50 years has revealed the neuronal circuits responsible for the generation and maintenance of REM sleep, as well as the pathways involved in generating the cardinal signs of REM sleep such as cortical activation and muscle atonia. The generation and maintenance of REM sleep appear to involve a widespread network in the pons and medulla. The caudal laterodorsal tegmental nucleus (cLDT) and sublaterodorsal nucleus (SLD) within the dorsolateral pons contain REM-on neurons, and the ventrolateral periaqueductal grey (vlPAG) contains REM-off neurons. The interaction between these structures is proposed to regulate REM sleep amounts. The cLDT-SLD neurons project to the basal forebrain via the parabrachial-precoeruleus (PB-PC) complex, and this pathway may be critical for the EEG activation seen during REM sleep. Descending SLD glutamatergic projections activate the ventromedial medulla, and spinal cord interneurons mediate muscle atonia and suppress phasic muscle twitches in spinal musculature. In contrast, phasic muscle twitches in the masseter muscles may be driven by glutamatergic neurons in the rostral parvicellular reticular nucleus (PCRt); however, the brain region responsible for generating phasic twitches in the other cranial muscles including facial muscles and tongue are not clear.     

Quantitative differences among EMG activities of muscles innervated by subpopulations of hypoglossal and upper spinal motoneurons during non-REM sleep - REM sleep transitions: a window on neural processes in the sleeping brain

In the rat, a species widely used to study the neural mechanisms of sleep and motor control, lingual electromyographic activity (EMG) is minimal during non-rapid eye movement (non-REM) sleep and then phasic twitches gradually increase after the onset of REM sleep. To better characterize the central neural processes underlying this pattern, we quantified EMG of muscles innervated by distinct subpopulations of hypoglossal motoneurons and nuchal (N) EMG during transitions from non-REM sleep to REM sleep. In 8 chronically instrumented rats, we recorded cortical EEG, EMG at sites near the base of the tongue where genioglossal and intrinsic muscle fibers predominate (GG-I), EMG of the geniohyoid (GH) muscle, and N EMG. Sleep-wake states were identified and EMGs quantified relative to their mean levels in wakefulness in successive 10 s epochs. During non-REM sleep, the average EMG levels differed among the three muscles, with the order being N>GH>GG-I. During REM sleep, due to different magnitudes of phasic twitches, the order was reversed to GG-I>GH>N. GG-I and GH exhibited a gradual increase of twitching that peaked at 70-120 s after the onset of REM sleep and then declined if the REM sleep episode lasted longer. We propose that a common phasic excitatory generator impinges on motoneuron pools that innervate different muscles, but twitching magnitudes are different due to different levels of tonic motoneuronal hyperpolarization. We also propose that REM sleep episodes of average durations are terminated by intense activity of the central generator of phasic events, whereas long REM sleep episodes end as a result of a gradual waning of the tonic disfacilitatory and inhibitory processes.     

Noradrenergic modulation of hypoglossal motoneuron excitability: developmental and putative state-dependent mechanisms

Hypoglossal (XII) motoneurons (MNs) contribute to diverse behaviors. Their innervation of the genioglossus muscle, a tongue protruder, plays a critical role in maintaining upper airway patency during breathing. Indeed, reduced activity in these motoneurons is implicated in sleep related disorders of breathing such as obstructive sleep apnea (OSA). The excitability of these MNs is modulated by multiple neurotransmitter systems. The focus of this review is on the modulation of XII MN excitability by norepinephrine (NE), which increases MN excitability through a variety of mechanisms. The level of noradrenergic drive, however, is very dynamic, varying on developmental, sleep-wake and even millisecond timescales relevant to transitions between behaviours. Here we review and provide new data on the maturation of the noradrenergic modulatory system, focusing on those elements specifically relevant to XII MN excitability including the: i) ontogeny of the noradrenergic cell group that provides the majority of the noradrenergic innervation to the XII nucleus, the Locus subcoeruleus (LsC); ii) time course over which the XII nucleus is innervated by noradrenergic nerve fibres, and; iii) ontogeny of XII MN sensitivity to NE. In the context of state-dependent changes in noradrenergic cell activity, we review mechanisms of NE action most relevant to its role in the muscle atonia of REM sleep. We conclude with a discussion of the hypothesis that the dynamics of MN modulation by NE extend to the spatial domain and recent data suggesting that noradrenergic modulation of the dendritic tree is not uniform but compartmentalized. Implications for information processing are discussed.     

Scoring atonia during normal and pathological rapid eye movement sleep: Visual and automatic quantification methods

One of the essential features of rapid eye movement (REM) sleep behavior disorder is REM sleep without atonia seen during nocturnal polysomnographic recordings. In this paper we provide an overview about the varied scoring criteria proposed for visual analysis of loss of atonia during REM sleep. The automatic quantification of loss of atonia overcomes many of the limitations of visual scoring and these new approaches are reviewed. Finally, the contributions of these automatic methods to the understanding of the complex mechanisms underlying muscle atonia and motor suppression during REM sleep are briefly illustrated.

     

Respiratory motor activity: influence of neuromodulators and implications for sleep disordered breathing

Sleep, especially rapid-eye-movement sleep, causes fundamental modifications of respiratory muscle activity and control mechanisms, modifications that can predispose individuals to sleep-related breathing disorders. One of the most common of these disorders is obstructive sleep apnea (OSA) that affects approximately 4% of adults. OSA is caused by repeated episodes of pharyngeal airway obstruction that can occur hundreds of times per night, leading to recurrent asphyxia, arousals from sleep, daytime sleepiness, and adverse cardiovascular and cerebrovascular consequences. OSA is caused by the effects of sleep on pharyngeal muscle tone in individuals with already narrow upper airways. Moreover, since OSA occurs only in sleep, this disorder by definition is a state-dependent process ultimately caused by the influence of sleep neural mechanisms on the activity of pharyngeal motoneurons. This review synthesizes recent findings relating to control of pharyngeal muscle activity across sleep-wake states, with special emphasis on the influence of neuromodulators acting at the hypoglossal motor nucleus that inervates the genioglossus muscle of the tongue. The results of such basic physiological studies may be relevant to identifying and developing new pharmacological strategies to augment pharyngeal muscle activity in sleep, especially rapid-eye-movement sleep, as potential treatments for OSA.     

Brainstem structures involved in rapid eye movement sleep behavior disorder

Rapid eye movement (REM) sleep behavior disorder (RBD) is a parasomnia characterized by the loss of muscle atonia during paradoxical (REM) sleep (PS). The neuronal dysfunctions responsible for RBD are not known. In the present review, we propose an updated integrated model of the mechanisms responsible for PS and explore different hypotheses explaining RBD. We propose that RBD appears based on a specific degeneration of PS-on glutamatergic neurons localized in the caudal pontine sublaterodorsal tegmental nucleus or the glycinergic/GABAergic premotoneurons localized in the medullary ventral gigantocellular reticular nucleus.

     

Control of motoneuron function and muscle tone during REM sleep, REM sleep behavior disorder and cataplexy/narcolepsy

REM sleep triggers a potent suppression of postural muscle tone - i.e., REM atonia. However, motor control during REM sleep is paradoxical because overall brain activity is maximal, but motor output is minimal. The skeletal motor system remains quiescent during REM sleep because somatic motoneurons are powerfully inactivated. Determining the mechanisms triggering loss of motoneuron function during REM sleep is important because breakdown in REM sleep motor control underlies sleep disorders such as REM sleep behavior disorder (RBD) and cataplexy/narcolepsy. For example, RBD is characterized by dramatic REM motor activation resulting in dream enactment and subsequent patient injury. In contrast, cataplexy a pathognomonic symptom of narcolepsy - is caused by the involuntary onset of REM-like atonia during wakefulness. This review highlights recent work from my laboratory that examines how motoneuron function is lost during normal REM sleep and it also identifies potential biochemical mechanisms underlying abnormal motor control in both RBD and cataplexy. First, I show that both GABAB and GABAA/glycine mediated inhibition of motoneurons is required for generating REM atonia. Next, I show that impaired GABA and glycine neurotransmission triggers the cardinal features of RBD in a transgenic mouse model. Last, I show that loss of an excitatory noradrenergic drive onto motoneurons is, at least in part, responsible for the loss of postural muscle tone during cataplexy in narcoleptic mice. Together, this research indicates that multiple transmitters systems are responsible for regulating postural muscle tone during REM sleep, RBD and cataplexy.     

Effect of dorsolateral pontine lesions on diaphragmatic activity during REMS

Muscle atonia is a feature of normal rapid-eye-movement sleep (REMS). The suppression of accessory respiratory muscle activity has been investigated and a role for sleep-disordered breathing hypothesized, but the suppression of diaphragmatic activity has rarely been considered. We hypothesized that the activity of the diaphragm was suppressed by an area of the dorsolateral pons during REMS. Lesions in this region have previously been shown to abolish the atonia of REMS. The diaphragmatic electromyogram (EMG) activity was analyzed in five naturally sleeping cats before and after pontine lesions leading to REMS without atonia. Although respiratory timing parameters were not altered by the lesion, the inspiratory rate of rise was significantly increased in all cats, and the brief pauses (40-100 ms) in the diaphragmatic EMG normally seen in REMS were virtually abolished. We conclude that the dorsolateral pons has a role in suppressing diaphragmatic activation during REMS. This suppression affects the average rate of rise of diaphragmatic activity and also leads to brief intermittent complete cessation of ongoing muscle activity. These decrements in diaphragm activity could jeopardize ventilation during REMS.     

Localization of the Brainstem GABAergic Neurons Controlling Paradoxical (REM) Sleep

Paradoxical sleep (PS) is a state characterized by cortical activation, rapid eye movements and muscle atonia. Fifty years after its discovery, the neuronal network responsible for the genesis of PS has been only partially identified. We recently proposed that GABAergic neurons would have a pivotal role in that network. To localize these GABAergic neurons, we combined immunohistochemical detection of Fos with non-radioactive in situ hybridization of GAD67 mRNA (GABA synthesis enzyme) in control rats, rats deprived of PS for 72 h and rats allowed to recover after such deprivation. Here we show that GABAergic neurons gating PS (PS-off neurons) are principally located in the ventrolateral periaqueductal gray (vlPAG) and the dorsal part of the deep mesencephalic reticular nucleus immediately ventral to it (dDpMe). Furthermore, iontophoretic application of muscimol for 20 min in this area in head-restrained rats induced a strong and significant increase in PS quantities compared to saline. In addition, we found a large number of GABAergic PS-on neurons in the vlPAG/dDPMe region and the medullary reticular nuclei known to generate muscle atonia during PS. Finally, we showed that PS-on neurons triggering PS localized in the SLD are not GABAergic. Altogether, our results indicate that multiple populations of PS-on GABAergic neurons are distributed in the brainstem while only one population of PS-off GABAergic neurons localized in the vlPAG/dDpMe region exist. From these results, we propose a revised model for PS control in which GABAergic PS-on and PS-off neurons localized in the vlPAG/dDPMe region play leading roles.     

State-dependent sensory gating in olfactory cortex

Sensory systems show behavioral state-dependent gating of information flow that largely depends on the thalamus. Here we examined whether the state-dependent gating occurs in the central olfactory pathway that lacks a thalamic relay. In urethane-anesthetized rats, neocortical EEG showed a periodical alternation between two states: a slow-wave state (SWS) characterized by large and slow waves and a fast-wave state (FWS) characterized by faster waves. Single-unit recordings from olfactory cortex neurons showed robust spike responses to adequate odorants during FWS, whereas they showed only weak responses during SWS. The state-dependent change in odorant-evoked responses was observed in a majority of olfactory cortex neurons, but in only a small percentage of olfactory bulb neurons. These findings demonstrate a powerful state-dependent gating of odor information in the olfactory cortex that works in synchrony with the gating of other sensory systems. They suggest a state-dependent switchover of signal processing modes in the olfactory cortex.     

Impact of intrinsic properties and synaptic factors on the activity of neocortical networks in vivo.

To investigate the relative impact of intrinsic and synaptic factors in the maintenance of the membrane potential of cat neocortical neurons in various states of the network, we performed intracellular recordings in vivo. Experiments were done in the intact cortex and in isolated neocortical slabs of anesthetized animals, and in naturally sleeping and awake cats. There are at least four different electrophysiological cell classes in the neocortex. The responses of different neuronal classes to direct depolarization result in significantly different responses in postsynaptic cells. The activity patterns observed in the intact cortex of anesthetized cats depended mostly on the type of anesthesia. The intracellular activity in small neocortical slabs was composed of silent periods, lasting for tens of seconds, during which only small depolarizing potentials (SDPs, presumed miniature synaptic potentials) were present, and relatively short-lasting (a few hundred milliseconds) active periods. Our data suggest that minis might be amplified by intrinsically-bursting neurons and that the persistent Na+ current brings neurons to firing threshold, thus triggering active periods. The active periods in neurons were composed of the summation of synaptic events and intrinsic depolarizing currents. In chronically-implanted cats, slow-wave sleep was characterized by active (depolarizing) and silent (hyperpolarizing) periods. The silent periods were absent in awake cats. We propose that both intrinsic and synaptic factors are responsible for the transition from silent to active states found in naturally sleeping cats and that synaptic depression might be responsible for the termination of active states during sleep. In view of the unexpected high firing rates of neocortical neurons during the depolarizing epochs in slow-wave sleep, we suggest that cortical neurons are implicated in short-term plasticity processes during this state, in which the brain is disconnected from the outside world, and that memory traces acquired during wakefulness may be consolidated during sleep.     

Hypocretin-2 Saporin Lesions of the Ventrolateral Periaquaductal Gray (vlPAG) Increase REM Sleep in Hypocretin Knockout Mice

Ten years ago the sleep disorder narcolepsy was linked to the neuropeptide hypocretin (HCRT), also known as orexin. This disorder is characterized by excessive day time sleepiness, inappropriate triggering of rapid-eye movement (REM) sleep and cataplexy, which is a sudden loss of muscle tone during waking. It is still not known how HCRT regulates REM sleep or muscle tone since HCRT neurons are localized only in the lateral hypothalamus while REM sleep and muscle atonia are generated from the brainstem. To identify a potential neuronal circuit, the neurotoxin hypocretin-2-saporin (HCRT2-SAP) was used to lesion neurons in the ventral lateral periaquaductal gray (vlPAG). The first experiment utilized hypocretin knock-out (HCRT-ko) mice with the expectation that deletion of both HCRT and its target neurons would exacerbate narcoleptic symptoms. Indeed, HCRT-ko mice (n = 8) given the neurotoxin HCRT2-SAP (16.5 ng/23nl/sec each side) in the vlPAG had levels of REM sleep and sleep fragmentation that were considerably higher compared to HCRT-ko given saline (+39%; n = 7) or wildtype mice (+177%; n = 9). However, cataplexy attacks did not increase, nor were levels of wake or non-REM sleep changed. Experiment 2 determined the effects in mice where HCRT was present but the downstream target neurons in the vlPAG were deleted by the neurotoxin. This experiment utilized an FVB-transgenic strain of mice where eGFP identifies GABA neurons. We verified this and also determined that eGFP neurons were immunopositive for the HCRT-2 receptor. vlPAG lesions in these mice increased REM sleep (+79% versus saline controls) and it was significantly correlated (r = 0.89) with loss of eGFP neurons. These results identify the vlPAG as one site that loses its inhibitory control over REM sleep, but does not cause cataplexy, as a result of hypocretin deficiency.     

Interaction between sleep mechanisms and orexin neurons

Orexin is a neuropeptide that plays a highly important role in mechanisms that regulate sleep/wake states. Lack of the orexin gene or orexin-producing neurons (orexin neurons) results in narcolepsy in several mammalian species, suggesting that orexin is an important factor for the maintenance of wakefulness. Constitutive, ectopic expression of orexin in transgenic mice resulted in severe fragmentation of non–rapid eye movement sleep, along with abnormal muscle tone regulation during REM sleep, suggesting that activity of orexin neurons should be appropriately decreased during sleep to maintain consolidated sleep states. This review will discuss the mechanisms by which the orexin system is regulated during sleep.

     

Differential morphofunctional characteristics and gene expression in fast and slow muscle of rats with monocrotaline-induced heart failure

Heart failure (HF) is characterized by limited exercise tolerance, skeletal muscle atrophy, a shift toward fast muscle fiber, and myogenic regulatory factor (MRF) changes. Reactive oxygen species (ROS) also contribute to target organ damage in this syndrome. In this study, we investigated and compared morphofunctional characteristics and gene expression in Soleus (SOL--oxidative and slow twitching muscle) and in Extensor Digitorum Longus (EDL--glycolytic and fast twitching muscle) during HF. Two groups of rats were used: control (CT) and heart failure (HF), induced by a single injection of monocrotaline. MyoD and myogenin gene expression were determined by RT-qPCR, and MHC isoforms by SDS-PAGE; muscle fiber type frequency and cross sectional area (CSA) were analyzed by mATPase. A biochemical study was performed to determine lipid hydroperoxide (LH), glutathione peroxidase (GSH-Px), and superoxide dismutase (SOD); myography was used to determine amplitude, rise time, fall time, and fatigue resistance in both muscles. HF showed SOL and EDL muscle atrophy in all muscle fiber types; fiber frequency decreased in type IIC and muscle contraction fall time increased only in SOL muscle. Myogenin mRNA expression was lower in SOL and myoD decreased in HF EDL muscle. LH increased, and SOD and GSH-Px activity decreased only in HF SOL muscle. HF EDL muscle did not present changes in MHC distribution, contractile properties, HL concentration, and antioxidant enzyme activity. In conclusion, our results indicate that monocrotaline induced HF promoted more prominent biochemical, morphological and functional changes in SOL (oxidative and slow twitching muscle). Although further experiments are required to better determine the mechanisms involved in HF pathophysiology, our results contribute to understanding the muscle-specific changes that occur in this syndrome.     

Selective REM sleep deprivation during daytime. II. Muscle atonia in non-REM sleep

One of the hallmarks of rapid eye movement (REM) sleep is muscle atonia. Here we report extended epochs of muscle atonia in non-REM sleep (MAN). Their extent and time course was studied in a protocol that included a baseline night, a daytime sleep episode with or without selective REM sleep deprivation, and a recovery night. The distribution of the latency to the first occurrence of MAN was bimodal with a first mode shortly after sleep onset and a second mode 40 min later. Within a non-REM sleep episode, MAN showed a U-shaped distribution with the highest values before and after REM sleep. Whereas MAN was at a constant level over consecutive 2-h intervals of nighttime sleep, MAN showed high initial values when sleep began in the morning. Selective daytime REM sleep deprivation caused an initial enhancement of MAN during recovery sleep. It is concluded that episodes of MAN may represent an REM sleep equivalent and that it may be a marker of homeostatic and circadian REM sleep regulating processes. MAN episodes may contribute to the compensation of an REM sleep deficit.     

Cholinergic stimulation of the pons depresses respiration in decerebrate cats

The injection of carbachol into the pontine tegmentum of decerebrate cats evokes a postural motor atonia that has many of the characteristics of the atonia of natural rapid-eye-movement (REM) sleep (Morales et al. J. Neurophysiol. 57: 1118-1129, 1987). We have used the carbachol-injected decerebrate cat to study the changes in respiratory neuronal activity that accompany the atonia. The activities of representative respiratory motor nerves--phrenic, intercostal, and hypoglossal--and that of a motor branch of C4 were recorded in decerebrate, vagotomized, paralyzed, and artificially ventilated cats. After the microinjection of carbachol, there was a profound suppression of activity in all the nerves and a decrease in respiratory rate. This was a consistent stereotyped response in which the magnitude of the suppression of respiratory-related activity was phrenic (to approximately 65% of control) less than inspiratory intercostal (approximately 50%) less than hypoglossal (approximately 10%) less than expiratory intercostal (approximately 5%). The decrease in respiratory rate (to approximately 70% of control) was caused by a prolongation of both inspiratory and expiratory durations. Complete reversal of the carbachol effect was elicited by the microinjection of atropine into the same site as the carbachol injection. This allowed us to produce a second episode of atonia by the injection of carbachol into the contralateral pons. Thus we have demonstrated the existence of neural pathways originating in the cholinoceptive cells of the pons that have the potential to powerfully and differentially depress various respiratory motoneuronal pools and to reduce the respiratory rate. These pathways are likely to be activated along with the atonia of REM sleep.     

Paradoxical (REM) sleep genesis: The switch from an aminergic–cholinergic to a GABAergic–glutamatergic hypothesis

In the middle of the last century, Michel Jouvet discovered paradoxical sleep (PS), a sleep phase paradoxically characterized by cortical activation and rapid eye movements and a muscle atonia. Soon after, he showed that it was still present in “pontine cats” in which all structures rostral to the brainstem have been removed. Later on, it was demonstrated that the pontine peri-locus coeruleus α (peri-LCα in cats, corresponding to the sublaterodorsal nucleus, SLD, in rats) is responsible for PS onset. It was then proposed that the onset and maintenance of PS is due to a reciprocal inhibitory interaction between neurons presumably cholinergic specifically active during PS localized in this region and monoaminergic neurons. In the last decade, we have tested this hypothesis with our model of head-restrained rats and functional neuroanatomical studies. Our results confirmed that the SLD in rats contains the neurons responsible for the onset and maintenance of PS. They further indicate that (1) these neurons are non-cholinergic possibly glutamatergic neurons, (2) they directly project to the glycinergic premotoneurons localized in the medullary ventral gigantocellular reticular nucleus (GiV), (3) the main neurotransmitter responsible for their inhibition during waking (W) and slow wave sleep (SWS) is GABA rather than monoamines, (4) they are constantly and tonically excited by glutamate and (5) the GABAergic neurons responsible for their tonic inhibition during W and SWS are localized in the deep mesencephalic reticular nucleus (DPMe). We also showed that the tonic inhibition of locus coeruleus (LC) noradrenergic and dorsal raphe (DRN) serotonergic neurons during sleep is due to a tonic GABAergic inhibition by neurons localized in the dorsal paragigantocellular reticular nucleus (DPGi) and the ventrolateral periaqueductal gray (vlPAG). We propose that these GABAergic neurons also inhibit the GABAergic neurons of the DPMe at the onset and during PS and are therefore responsible for the onset and maintenance of PS.