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Sleep State Switching

2010; Cell Press; Volume: 68; Issue: 6 Linguagem: Inglês

10.1016/j.neuron.2010.11.032

1097-4199

Clifford B. Saper, Patrick M. Fuller, Nigel P. Pedersen, Jun Lu, Thomas E. Scammell,

Sleep and related disorders

We take for granted the ability to fall asleep or to snap out of sleep into wakefulness, but these changes in behavioral state require specific switching mechanisms in the brain that allow well-defined state transitions. In this review, we examine the basic circuitry underlying the regulation of sleep and wakefulness and discuss a theoretical framework wherein the interactions between reciprocal neuronal circuits enable relatively rapid and complete state transitions. We also review how homeostatic, circadian, and allostatic drives help regulate sleep state switching and discuss how breakdown of the switching mechanism may contribute to sleep disorders such as narcolepsy. We take for granted the ability to fall asleep or to snap out of sleep into wakefulness, but these changes in behavioral state require specific switching mechanisms in the brain that allow well-defined state transitions. In this review, we examine the basic circuitry underlying the regulation of sleep and wakefulness and discuss a theoretical framework wherein the interactions between reciprocal neuronal circuits enable relatively rapid and complete state transitions. We also review how homeostatic, circadian, and allostatic drives help regulate sleep state switching and discuss how breakdown of the switching mechanism may contribute to sleep disorders such as narcolepsy. We spend nearly one-third of our lives asleep, and many mammals, including small laboratory rodents, spend half or more of their existence in this state (Savage and West, 2007Savage V.M. West G.B. A quantitative, theoretical framework for understanding mammalian sleep.Proc. Natl. Acad. Sci. USA. 2007; 104: 1051-1056Crossref PubMed Scopus (25) Google Scholar, Siegel, 2009Siegel J.M. Sleep viewed as a state of adaptive inactivity.Nat. Rev. Neurosci. 2009; 10: 747-753Crossref PubMed Scopus (61) Google Scholar). Because sleeping animals are inherently more vulnerable, it is necessary for an animal to be able to awaken quickly so it can flee or defend itself. Conversely, it is a common experience that one can fall asleep over just a few seconds or minutes. These state transitions involve dramatic alterations in easily observed physiological variables, including eye closure, breathing, arousability, and muscle tone. We measure the changes in cortical activity and muscle tone, respectively, by recording the electroencephalogram (EEG) and electromyogram (EMG), and the actual transitions in electrophysiologically monitored state occur over just a few seconds (Takahashi et al., 2010Takahashi K. Kayama Y. Lin J.S. Sakai K. Locus coeruleus neuronal activity during the sleep-waking cycle in mice.Neuroscience. 2010; 169: 1115-1126Crossref PubMed Scopus (42) Google Scholar). Similarly, during the sleep period, animals and people rapidly transition between rapid eye movement (REM) and non-REM (NREM) sleep states. Recent advances in understanding the brain circuitry underlying the waking and sleeping states have given rise to models that may explain these transitions. The principles that govern these models for state transitions may ultimately apply to many other state changes, such as emotional responses, sexual arousal, or cognitive state changes such as reorienting attention. Hence the mechanisms for wake-sleep state transitions potentially have broad implications for a variety of behavioral states. As an individual falls asleep, the EEG initially transitions from a state of high-frequency, low-voltage waves in the waking state to higher voltage, slower waves representing NREM sleep. These changes take place over a few seconds or less in rodents but may take 10 s to a minute in humans (Takahashi et al., 2010Takahashi K. Kayama Y. Lin J.S. Sakai K. Locus coeruleus neuronal activity during the sleep-waking cycle in mice.Neuroscience. 2010; 169: 1115-1126Crossref PubMed Scopus (42) Google Scholar, Wright et al., 1995Wright Jr., K.P. Badia P. Wauquier A. Topographical and temporal patterns of brain activity during the transition from wakefulness to sleep.Sleep. 1995; 18: 880-889PubMed Google Scholar). The EEG then progressively slows during NREM sleep until it is dominated by high-voltage, slow wave (0.5–4 Hz) activity, after which the slow waves progressively diminish, a typical bout lasting from 40 min to an hour or more in humans. In rodents, this process is much shorter, with slow waves established within seconds of entering NREM sleep, and the entire NREM bout generally lasting three to five minutes, although occasionally it may extend to 20 min or more. Across species, wake bout lengths follow a power law distribution (the log of probability of a bout of a certain length and the log of the bout length forming a linear relationship), whereas the durations of sleep bouts follow an exponential distribution (Lo et al., 2004Lo C.C. Chou T. Penzel T. Scammell T.E. Strecker R.E. Stanley H.E. Ivanov P. Ch Common scale-invariant patterns of sleep-wake transitions across mammalian species.Proc. Natl. Acad. Sci. USA. 2004; 101: 17545-17548Crossref PubMed Scopus (81) Google Scholar, Phillips et al., 2010Phillips A.J. Robinson P.A. Kedziora D.J. Abeysuriya R.G. Mammalian sleep dynamics: how diverse features arise from a common physiological framework.PLoS Comput. Biol. 2010; 6: e1000826Crossref PubMed Scopus (8) Google Scholar). In each case though, the transitions between NREM sleep and wakefulness typically take less than 1% of the duration of an average NREM bout. The EEG then makes another abrupt transition over a few seconds from NREM into REM sleep, with lower voltage, higher frequency activity. In rodents, the EEG recorded from the cortical surface during REM sleep is dominated by 5–8 Hz theta activity generated by the underlying hippocampus; in humans, theta activity is present during REM sleep, particularly in the hippocampus, but the dominant cortical frequencies are faster and lower voltage. During REM sleep, there is almost complete loss of tone in skeletal muscles (except those used for breathing and eye movements), accompanied by rapid eye movements that give the state its name. Humans report active dreams during REM sleep but less lively mentation during NREM sleep. Over the sleep period, an individual may switch back and forth from NREM to REM sleep, with occasional transitions to periods of wakefulness. The duration of the NREM, REM, and wake bouts varies with the species, age, and health of the individual, but the electrographic transitions between these states are relatively rapid in comparison to bout duration. Researchers first began mapping the general circuitry that controls wakefulness and sleep over 50 years ago, and in the last 10–20 years, much has been learned about the specific systems that regulate these states. Progress over the last few years has been especially rapid, leading to an improved understanding of the neurochemicals, pathways, and firing patterns that regulate NREM and REM sleep. Other new work has examined the ways in which behavioral drives, including homeostatic, circadian, and allostatic influences, may affect these switching mechanisms. We will first review these advances and place them into the context of a model we have proposed for sleep/wake state transitions based upon mutually inhibitory circuits, as are seen in electronic flip-flop switches (Saper et al., 2001Saper C.B. Chou T.C. Scammell T.E. The sleep switch: hypothalamic control of sleep and wakefulness.Trends Neurosci. 2001; 24: 726-731Abstract Full Text Full Text PDF PubMed Scopus (699) Google Scholar, Saper et al., 2005Saper C.B. Lu J. Chou T.C. Gooley J. The hypothalamic integrator for circadian rhythms.Trends Neurosci. 2005; 28: 152-157Abstract Full Text Full Text PDF PubMed Scopus (231) Google Scholar). We will then explore recently proposed mathematical models based on this circuitry that can explain many of the features of natural sleep and state transitions. Finally, we will examine how this circuitry can explain many of the features of the sleep disorder narcolepsy, an example of state instability in which the circuitry that stabilizes switching is damaged. Current models of the ascending arousal system are still generally based on the observations by Moruzzi and Magoun, 1949Moruzzi G. Magoun H.W. Brain stem reticular formation and activation of the EEG.Electroencephalogr. Clin. Neurophysiol. 1949; 1: 455-473Abstract Full Text PDF PubMed Google Scholar that electrical stimulation of the paramedian reticular formation, particularly within the midbrain, produces EEG desynchronization consistent with arousal. Subsequent studies identified a slab of tissue at the junction of the rostral pons and caudal midbrain as critical for maintaining the waking state (Lindsley et al., 1949Lindsley D.B. Bowden J.W. Magoun H.W. Effect upon the EEG of acute injury to the brain stem activating system.Electroencephalogr. Clin. Neurophysiol. 1949; 1: 475-486Abstract Full Text PDF PubMed Google Scholar). Although the neurons responsible for arousal were initially thought to be part of the undifferentiated reticular formation, subsequent studies showed that the cell groups at the mesopontine junction that project to the forebrain mainly consist of monoaminergic and cholinergic neurons that reside in specific cell groups rather than the reticular core (Figure 1) (see Saper, 1987Saper C.B. Diffuse Cortical Projection Systems: Anatomical Organization and Role.in: Plum F. Cortical Function. In Handbook Of Physiology. The Nervous System V. American Physiological Society, Bethesda1987: 169-210Google Scholar for review). Many wake-promoting projections arise from neurons in the upper brainstem (A). Cholinergic neurons (aqua) provide the major input to the thalamus, whereas monoaminergic and (presumably) glutamatergic neurons (dark green) provide direct innervation of the the hypothalamus, basal forebrain, and cerebral cortex. The orexin neurons in the lateral hypothalamus (blue) reinforce activity in these brainstem arousal pathways and also directly excite the cerebral cortex and BF. The main sleep-promoting pathways (magenta in B) from the ventrolateral (VLPO) and median (MnPO) preoptic nuclei inhibit the components of the ascending arousal pathways in both the hypothalamus and the brainstem (pathways that are inhibited are shown as open circles and dashed lines). However, the ascending arousal systems are also capable of inhibiting the VLPO (C). This mutually inhibitory relationship of the arousal- and sleep-promoting pathways produces the conditions for a flip-flop switch, which can generate rapid and complete transitions between waking and sleeping states. The following abbreviations are used: DR, dorsal raphe nucleus (serotonin); LC, locus coeruleus (norepinephrine); LDT, laterodorsal tegmental nucleus (acetylcholine); PB, parabrachial nucleus (glutamate); PC, precoeruleus area (glutamate); PPT, pedunculopontine tegmental nucleus (acetylcholine); TMN, tuberomammillary nucleus (histamine); vPAG, ventral periaqueductal gray (dopamine). Cholinergic neurons that project to the forebrain are found in the pedunculopontine and laterodorsal tegmental nuclei (PPT and LDT). They provide the main innervation from the mesopontine junction to the thalamic relay nuclei but also innervate the intralaminar and reticular thalamic nuclei, as well as the lateral hypothalamus, basal forebrain, and prefrontal cortex (Hallanger et al., 1987Hallanger A.E. Levey A.I. Lee H.J. Rye D.B. Wainer B.H. The origins of cholinergic and other subcortical afferents to the thalamus in the rat.J. Comp. 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These nuclei are heterogeneous, but extracellular recordings combined with juxtacellular labeling confirm that cholinergic neurons in the LDT fire during cortical activation, usually increasing their firing rates just before the transition from cortical slow waves to faster frequencies (Boucetta and Jones, 2009Boucetta S. Jones B.E. Activity profiles of cholinergic and intermingled GABAergic and putative glutamatergic neurons in the pontomesencephalic tegmentum of urethane-anesthetized rats.J. Neurosci. 2009; 29: 4664-4674Crossref PubMed Scopus (36) Google Scholar). The monoaminergic cell groups at the mesopontine level that project to the forebrain include the noradrenergic locus coeruleus (LC) and the serotoninergic dorsal and median raphe nuclei (Aston-Jones and Bloom, 1981Aston-Jones G. Bloom F.E. Activity of norepinephrine-containing locus coeruleus neurons in behaving rats anticipates fluctuations in the sleep-waking cycle.J. Neurosci. 1981; 1: 876-886PubMed Google Scholar, Dahlström and Fuxe, 1964Dahlström A. Fuxe K. Evidence for the existence of monoamine-containing neurons in the central nervous system. i. demonstration of monoamines in the cell bodies of brain stem neurons.Acta Physiol. Scand. Suppl. 1964; : 1-55Google Scholar, Kocsis et al., 2006Kocsis B. Varga V. Dahan L. Sik A. Serotonergic neuron diversity: identification of raphe neurons with discharges time-locked to the hippocampal theta rhythm.Proc. Natl. Acad. Sci. USA. 2006; 103: 1059-1064Crossref PubMed Scopus (55) Google Scholar), as well as dopaminergic neurons adjacent to the dorsal raphe nucleus (Lu et al., 2006aLu J. Jhou T.C. Saper C.B. Identification of wake-active dopaminergic neurons in the ventral periaqueductal gray matter.J. Neurosci. 2006; 26: 193-202Crossref PubMed Scopus (142) Google Scholar). Histaminergic neurons in the tuberomammillary nucleus (TMN) have similar projection targets and firing patterns (Panula et al., 1989Panula P. Pirvola U. Auvinen S. Airaksinen M.S. Histamine-immunoreactive nerve fibers in the rat brain.Neuroscience. 1989; 28: 585-610Crossref PubMed Google Scholar, Steininger et al., 1999Steininger T.L. Alam M.N. Gong H. Szymusiak R. McGinty D. Sleep-waking discharge of neurons in the posterior lateral hypothalamus of the albino rat.Brain Res. 1999; 840: 138-147Crossref PubMed Scopus (141) Google Scholar). Axons from these cell groups predominantly target the lateral hypothalamus, basal forebrain, and cerebral cortex, where they terminate extensively, particularly in the prefrontal cortex. Each of these monoaminergic systems also sends smaller but important populations of axons to the thalamus where they largely target the intralaminar and reticular nuclei. Generally, neurons in these cell groups fire most actively during wakefulness, decrease activity during non-REM sleep, and fall silent during REM sleep (Aston-Jones and Bloom, 1981Aston-Jones G. Bloom F.E. Activity of norepinephrine-containing locus coeruleus neurons in behaving rats anticipates fluctuations in the sleep-waking cycle.J. Neurosci. 1981; 1: 876-886PubMed Google Scholar, Kocsis et al., 2006Kocsis B. Varga V. Dahan L. Sik A. Serotonergic neuron diversity: identification of raphe neurons with discharges time-locked to the hippocampal theta rhythm.Proc. Natl. Acad. Sci. USA. 2006; 103: 1059-1064Crossref PubMed Scopus (55) Google Scholar, Steininger et al., 1999Steininger T.L. Alam M.N. Gong H. Szymusiak R. McGinty D. Sleep-waking discharge of neurons in the posterior lateral hypothalamus of the albino rat.Brain Res. 1999; 840: 138-147Crossref PubMed Scopus (141) Google Scholar, Takahashi et al., 2006Takahashi K. Lin J.S. Sakai K. Neuronal activity of histaminergic tuberomammillary neurons during wake-sleep states in the mouse.J. Neurosci. 2006; 26: 10292-10298Crossref PubMed Scopus (113) Google Scholar, Takahashi et al., 2010Takahashi K. Kayama Y. Lin J.S. Sakai K. Locus coeruleus neuronal activity during the sleep-waking cycle in mice.Neuroscience. 2010; 169: 1115-1126Crossref PubMed Scopus (42) Google Scholar). Another source of arousal influence from the rostral pons may be glutamatergic neurons in the parabrachial nucleus and the adjacent precoeruleus area (PC, the lateral corner of the rostral pontine periventricular gray matter, just rostral to the main body of the LC), which have been found to send major projections to the lateral hypothalamus, basal forebrain, and cerebral cortex (Hur and Zaborszky, 2005Hur E.E. Zaborszky L. Vglut2 afferents to the medial prefrontal and primary somatosensory cortices: a combined retrograde tracing in situ hybridization study [corrected].J. Comp. Neurol. 2005; 483: 351-373Crossref PubMed Scopus (117) Google Scholar, Lu et al., 2006bLu J. Sherman D. Devor M. Saper C.B. A putative flip-flop switch for control of REM sleep.Nature. 2006; 441: 589-594Crossref PubMed Scopus (388) Google Scholar, Saper, 1987Saper C.B. Diffuse Cortical Projection Systems: Anatomical Organization and Role.in: Plum F. Cortical Function. In Handbook Of Physiology. The Nervous System V. American Physiological Society, Bethesda1987: 169-210Google Scholar, Saper and Loewy, 1980Saper C.B. Loewy A.D. Efferent connections of the parabrachial nucleus in the rat.Brain Res. 1980; 197: 291-317Crossref PubMed Scopus (389) Google Scholar). The activity patterns of these glutamatergic neurons have not yet been studied, but recordings in this area in cats and Fos studies in rats have shown predominantly wake- and REM-sleep-active neurons (Chu and Bloom, 1973Chu N. Bloom F.E. Norepinephrine-containing neurons: changes in spontaneous discharge patterns during sleeping and waking.Science. 1973; 179: 908-910Crossref PubMed Google Scholar, Lu et al., 2006bLu J. Sherman D. Devor M. Saper C.B. A putative flip-flop switch for control of REM sleep.Nature. 2006; 441: 589-594Crossref PubMed Scopus (388) Google Scholar, Saito et al., 1977Saito H. Sakai K. Jouvet M. Discharge patterns of the nucleus parabrachialis lateralis neurons of the cat during sleep and waking.Brain Res. 1977; 134: 59-72Crossref PubMed Scopus (46) Google Scholar). Tests of the role of these neurons in wakefulness would be of great interest. Several forebrain neuronal systems also support wakefulness. The importance of these forebrain regions is underscored by the observation that over a period of weeks to months after acute lesions of the brainstem arousal system, animals and humans eventually recover wake-sleep cycles (Adametz, 1959Adametz J.H. Rate of recovery of functioning in cats with rostral reticular lesions; an experimental study.J. Neurosurg. 1959; 16 (discussion 97–98): 85-97Crossref PubMed Google Scholar, Posner et al., 2008Posner J.B. Saper C.B. Schiff N.D. Plum F. Plum and Posner's Diagnosis Of Stupor And Coma. Oxford University Press, New York2008Crossref Google Scholar). Thus, while these forebrain areas appear to depend upon the brainstem arousal influence in the intact individual, they apparently can reorganize to support cortical arousal even without input from the brainstem. One of these forebrain arousal systems is found in the posterior half of the lateral hypothalamus. Just dorsal and rostral to the histaminergic neurons of the TMN, the lateral hypothalamus contains neurons producing the orexin neuropeptides (orexin-A and -B, also known as hypocretin-1 and -2). Many of the orexin neurons also contain glutamate, and nearly all also contain the neuropeptide dynorphin (Chou et al., 2001Chou T.C. Lee C.E. Lu J. Elmquist J.K. Hara J. Willie J.T. Beuckmann C.T. Chemelli R.M. Sakurai T. Yanagisawa M. et al.Orexin (hypocretin) neurons contain dynorphin.J. Neurosci. 2001; 21: RC168PubMed Google Scholar, Torrealba et al., 2003Torrealba F. Yanagisawa M. Saper C.B. Colocalization of orexin a and glutamate immunoreactivity in axon terminals in the tuberomammillary nucleus in rats.Neuroscience. 2003; 119: 1033-1044Crossref PubMed Scopus (70) Google Scholar). They send axons to the entire cerebral cortex, as well as to the brainstem and basal forebrain, with particularly intense input to the TMN and the LC (Peyron et al., 1998Peyron C. Tighe D.K. van den Pol A.N. de Lecea L. Heller H.C. Sutcliffe J.G. Kilduff T.S. Neurons containing hypocretin (orexin) project to multiple neuronal systems.J. Neurosci. 1998; 18: 9996-10015Crossref PubMed Google Scholar). There is also less intense orexin innervation of the intralaminar nuclei of the thalamus as well as the anteroventral thalamic nucleus. There are two known orexin receptors, both of which are G protein coupled receptors with excitatory membrane effects (Sakurai et al., 1998Sakurai T. Amemiya A. Ishii M. Matsuzaki I. Chemelli R.M. Tanaka H. Williams S.C. Richardson J.A. Kozlowski G.P. Wilson S. et al.Orexins and orexin receptors: a family of hypothalamic neuropeptides and G protein-coupled receptors that regulate feeding behavior.Cell. 1998; 92: 573-585Abstract Full Text Full Text PDF PubMed Scopus (1223) Google Scholar). Orexin neurons receive afferents from many components of the ascending arousal system, including the LC, dorsal raphe (DR), and parabrachial nucleus, as well as from cortical (medial prefrontal) and amygdaloid (central nucleus) sources associated with arousal and ventral tegmental sites associated with reward (Yoshida et al., 2006Yoshida K. McCormack S. España R.A. Crocker A. Scammell T.E. Afferents to the orexin neurons of the rat brain.J. Comp. Neurol. 2006; 494: 845-861Crossref PubMed Scopus (230) Google Scholar). They fire predominantly during wakefulness, and fire particularly briskly during active exploration of the environment or during motivated behaviors (Lee et al., 2005Lee M.G. Hassani O.K. Jones B.E. Discharge of identified orexin/hypocretin neurons across the sleep-waking cycle.J. Neurosci. 2005; 25: 6716-6720Crossref PubMed Scopus (327) Google Scholar, Mileykovskiy et al., 2005Mileykovskiy B.Y. Kiyashchenko L.I. Siegel J.M. Behavioral correlates of activity in identified hypocretin/orexin neurons.Neuron. 2005; 46: 787-798Abstract Full Text Full Text PDF PubMed Scopus (350) Google Scholar). Orexin neurons are also driven by low glucose (Moriguchi et al., 1999Moriguchi T. Sakurai T. Nambu T. Yanagisawa M. Goto K. Neurons containing orexin in the lateral hypothalamic area of the adult rat brain are activated by insulin-induced acute hypoglycemia.Neurosci. 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Neural substrates of awakening probed with optogenetic control of hypocretin neurons.Nature. 2007; 450: 420-424Crossref PubMed Scopus (337) Google Scholar, Carter et al., 2009Carter M.E. Adamantidis A. Ohtsu H. Deisseroth K. de Lecea L. Sleep homeostasis modulates hypocretin-mediated sleep-to-wake transitions.J. Neurosci. 2009; 29: 10939-10949Crossref PubMed Scopus (77) Google Scholar). Most importantly, selective destruction of the orexin neurons with a genetically targeted toxin results in the symptoms of narcolepsy (Hara et al., 2001Hara J. Beuckmann C.T. Nambu T. Willie J.T. Chemelli R.M. Sinton C.M. Sugiyama F. Yagami K. Goto K. Yanagisawa M. Sakurai T. Genetic ablation of orexin neurons in mice results in narcolepsy, hypophagia, and obesity.Neuron. 2001; 30: 345-354Abstract Full Text Full Text PDF PubMed Scopus (655) Google Scholar), which will be discussed in a separate section below. Overall, the orexin neurons are thought to sustain wakefulness and suppress REM sleep. On the other hand, large lesions of the posterior lateral hypothalamus (Gerashchenko et al., 2003Gerashchenko D. Blanco-Centurion C. Greco M.A. Shiromani P.J. Effects of lateral hypothalamic lesion with the neurotoxin hypocretin-2-saporin on sleep in Long-Evans rats.Neuroscience. 2003; 116: 223-235Crossref PubMed Scopus (62) Google Scholar, Nauta, 1946Nauta W.J.H. Hypothalamic regulation of sleep in rats; an experimental study.J. Neurophysiol. 1946; 9: 285-316PubMed Google Scholar, Ranson, 1939Ranson S.W. Somnolence caused by hypothalamic lesions in monkeys.Arch. Neurol. Psychiatry. 1939; 41: 1-23Crossref Google Scholar, Swett and Hobson, 1968Swett C.P. Hobson J.A. The effects of posterior hypothalamic lesions on behavioral and electrographic manifestations of sleep and waking in cat.Arch. Ital. Biol. 1968; 106: 270-282Google Scholar) produce much more extensive sleepiness than can be explained by elimination of just orexin and histamine transmission. This suggests the presence of other important arousal-producing neurons in the posterior lateral hypothalamus. There is an additional population of neurons in the supramammillary region and extending laterally to the subthalamic nucleus, which is a known source of projections to the cerebral cortex and basal forebrain (Grove, 1988Grove E.A. Neural associations of the substantia innominata in the rat: afferent connections.J. Comp. Neurol. 1988; 277: 315-346Crossref PubMed Google Scholar, Saper, 1985Saper C.B. Organization of cerebral cortical afferent systems in the rat. II. Hypothalamocortical projections.J. Comp. Neurol. 1985; 237: 21-46Crossref PubMed Google Scholar). Many neurons in this region express the vesicular glutamate transporter 2 (Hur and Zaborszky, 2005Hur E.E. Zaborszky L. Vglut2 afferents to the medial prefrontal and primary somatosensory cortices: a combined retrograde tracing in situ hybridization study [corrected].J. Comp. Neurol. 2005; 483: 351-373Crossref PubMed Scopus (117) Google Scholar, Ziegler et al., 2002Ziegler D.R. Cullinan W.E. Herman J.P. Distribution of vesicular glutamate transporter mRNA in rat hypothalamus.J. Comp. Neurol. 2002; 448: 217-229Crossref PubMed Scopus (124) Google Scholar) but whether these glutamatergic neurons promote arousal remains to be determined. The most rostral population of arousal-promoting subcortical neurons is located in the basal forebrain. Many of these neurons contain either acetylcholine or gamma-amino-butyric acid (GABA), and a small number contain glutamate (Manns et al., 2001Manns I.D. Mainville L. Jones B.E. Evidence for glutamate, in addition to acetylcholine and GABA, neurotransmitter synthesis in basal forebrain neurons projecting to the entorhinal cortex.Neuroscience. 2001; 107: 249-263Crossref PubMed Scopus (102) Google Scholar, Hur and Zaborszky, 2005Hur E.E. Zaborszky L. Vglut2 afferents to the medial prefrontal and primary somatosensory cortices: a combined retrograde tracing in situ hybridization study [corrected].J. Comp. Neurol. 2005; 483: 351-373Crossref PubMed Scopus (117) Google Scholar). Basal forebrain cholinergic neurons innervate, both directly and indirectly activate cortical pyramidal cells, and probably augment cortical activation and EEG desynchronization (Jones, 2004Jones B.E. Activity, modulation and role of basal forebrain cholinergic neurons innervating the cerebral cortex.Prog. Brain Res. 2004; 145: 157-169Crossref PubMed Scopus (131) Google Scholar). GABAergic basal forebrain neurons innervate and presumably inhibit cortical GABAergic interneurons and deep layer pyramidal cells (Freund and Meskenaite, 1992Freund T.F. Meskenaite V. gamma-Aminobutyric acid-containing basal forebrain neurons innervate inhibitory interneurons in the neocortex.Proc. Natl. Acad. Sci. USA. 1992; 89: 738-742Crossref PubMed Google Scholar, Henny and Jones, 2008Henny P. Jones B.E. Projections from basal forebrain to prefrontal cortex comprise cholinergic, GABAergic and glutamatergic inputs to pyramidal cells or interneurons.Eur. J. Neurosci. 2008; 27: 654-670Crossref PubMed Scopus (70) Google Scholar), both of which most likely result in disinhibition of cortical circuits. Many of these basal forebrain neurons are wake-active and fire in bursts correlated with specific EEG rhythms. Small ibotenic acid lesions of the basal forebrain result in modest slowing of the EEG without changing the amount of wake or sleep, while specific lesions of basal forebrain cholinergic neurons reduce wakefulness transiently, without affecting the EEG frequency spectrum (Kaur et al., 2008Kaur S. Junek A. Black M.A. Semba K. Effects of ibotenate and 192IgG-saporin lesions of the nucleus basalis magnocellularis/substantia innominata on spontaneous sleep and wake states and on recovery sleep after sleep deprivation in rats.J. Neurosci. 2008; 28: 491-504Crossref PubMed Scopus (44) Google Scholar). On the other hand, acute inactivation of the basal forebrain with the anesthetic procaine produces deep NREM sleep, whereas activation with glutamatergic agonists causes wakefulness (Cape and Jones, 2000Cape E.G. Jones B.E. Effects of glutamate agonist versus procaine microinjec