Discharge profiles of identified GABAergic in comparison to cholinergic and putative glutamatergic basal forebrain neurons across the sleep-wake cycle

Selective activation of cholinergic basal forebrain neurons induces immediate sleep-wake transitions

The basal forebrain (BF) plays a crucial role in cortical activation [1, 2]. However, the exact role of cholinergic BF (ch-BF) neurons in the sleep-wake cycle remains unclear [3, 4]. We demonstrated that photostimulation of ch-BF neurons genetically targeted with channelrhodopsin 2 (ChR2) was sufficient to induce an immediate transition to waking or rapid eye movement (REM) sleep from slow-wave sleep (SWS). Light stimulation was most likely to induce behavioral arousal during SWS, but not during REM sleep, a result in contrast to the previously reported photostimulation of noradrenergic or hypocretin neurons that induces wake transitions from both SWS and REM sleep. Furthermore, the ratio of light-induced transitions from SWS to wakefulness or to REM sleep did not significantly differ from that of natural transitions, suggesting that activation of ch-BF neurons facilitates the transition from SWS but does not change the direction of the transition. Excitation of ch-BF neurons during wakefulness or REM sleep sustained the cortical activation. Stimulation of these neurons for 1 hr induced a delayed increase in the duration of wakefulness in the subsequent inactive period. Our results suggest that activation of ch-BF neurons alone is sufficient to suppress SWS and promote wakefulness and REM sleep.

Analysis of sleep disorders under pain using an optogenetic tool: possible involvement of the activation of dorsal raphe nucleus-serotonergic neurons

Background

Several etiological reports have shown that chronic pain significantly interferes with sleep. Inadequate sleep due to chronic pain may contribute to the stressful negative consequences of living with pain. However, the neurophysiological mechanism by which chronic pain affects sleep-arousal patterns is as yet unknown. Although serotonin (5-HT) was proposed to be responsible for sleep regulation, whether the activity of 5-HTergic neurons in the dorsal raphe nucleus (DRN) is affected by chronic pain has been studied only infrequently. On the other hand, the recent development of optogenetic tools has provided a valuable opportunity to regulate the activity in genetically targeted neural populations with high spatial and temporal precision. In the present study, we investigated whether chronic pain could induce sleep dysregulation while changing the activity of DRN-5-HTergic neurons. Furthermore, we sought to physiologically activate the DRN with channelrhodopsin-2 (ChR2) to identify a causal role for the DRN-5-HT system in promoting and maintaining wakefulness using optogenetics.

Results

We produced a sciatic nerve ligation model by tying a tight ligature around approximately one-third to one-half the diameter of the sciatic nerve. In mice with nerve ligation, we confirmed an increase in wakefulness and a decrease in non-rapid eye movement (NREM) sleep as monitored by electroencephalogram (EEG). Microinjection of the retrograde tracer fluoro-gold (FG) into the prefrontal cortex (PFC) revealed several retrogradely labeled-cells in the DRN. The key finding of the present study was that the levels of 5-HT released in the PFC by the electrical stimulation of DRN neurons were significantly increased in mice with sciatic nerve ligation. Using optogenetic tools in mice, we found a causal relationship among DRN neuron firing, cortical activity and sleep-to-wake transitions. In particular, the activation of DRN-5-HTergic neurons produced a significant increase in wakefulness and a significant decrease in NREM sleep. The duration of NREM sleep episodes was significantly decreased during photostimulation in these mice.

Conclusions

These results suggest that neuropathic pain accelerates the activity of DRN-5-HTergic neurons. Although further loss-of-function experiments are required, we hypothesize that this activation in DRN neurons may, at least in part, correlate with sleep dysregulation under a neuropathic pain-like state.

Kv2.2: a novel molecular target to study the role of basal forebrain GABAergic neurons in the sleep-wake cycle

STUDY OBJECTIVES

The basal forebrain (BF) has been implicated as an important brain region that regulates the sleep-wake cycle of animals. Gamma-aminobutyric acidergic (GABAergic) neurons are the most predominant neuronal population within this region. However, due to the lack of specific molecular tools, the roles of the BF GABAergic neurons have not been fully elucidated. Previously, we have found high expression levels of the Kv2.2 voltage-gated potassium channel on approximately 60% of GABAergic neurons in the magnocellular preoptic area and horizontal limb of the diagonal band of Broca of the BF and therefore proposed it as a potential molecular target to study this neuronal population. In this study, we sought to determine the functional roles of the Kv2.2-expressing neurons in the regulation of the sleep-wake cycle.

DESIGN

Sleep analysis between two genotypes and within each genotype before and after sleep deprivation.

SETTING

Animal sleep research laboratory.

PARTICIPANTS

Adult mice. Wild-type and Kv2.2 knockout mice with C57/BL6 background.

INTERVENTIONS

EEG/EMG recordings from the basal state and after sleep-deprivation which was induced by mild agitation for 6 h.

RESULTS

Immunostaining of a marker of neuronal activity indicates that these Kv2.2-expressing neurons appear to be preferentially active during the wake state. Therefore, we tested whether Kv2.2-expressing neurons in the BF are involved in arousal using Kv2.2-deficient mice. BF GABAergic neurons exhibited augmented expression of c-Fos. These knockout mice exhibited longer consolidated wake bouts than wild-type littermates, and that phenotype was further exacerbated by sleep deprivation. Moreover, in-depth analyses of their cortical electroencephalogram revealed a significant decrease in the delta-frequency activity during the nonrapid eye movement sleep state.

CONCLUSIONS

These results revealed the significance of Kv2.2-expressing neurons in the regulation of the sleep-wake cycle.

Distribution and intrinsic membrane properties of basal forebrain GABAergic and parvalbumin neurons in the mouse

The basal forebrain (BF) strongly regulates cortical activation, sleep homeostasis, and attention. Many BF neurons involved in these processes are GABAergic, including a subpopulation of projection neurons containing the calcium-binding protein, parvalbumin (PV). However, technical difficulties in identification have prevented a precise mapping of the distribution of GABAergic and GABA/PV+ neurons in the mouse or a determination of their intrinsic membrane properties. Here we used mice expressing fluorescent proteins in GABAergic (GAD67-GFP knock-in mice) or PV+ neurons (PV-Tomato mice) to study these neurons. Immunohistochemical staining for GABA in GAD67-GFP mice confirmed that GFP selectively labeled BF GABAergic neurons. GFP+ neurons and fibers were distributed throughout the BF, with the highest density in the magnocellular preoptic area (MCPO). Immunohistochemistry for PV indicated that the majority of PV+ neurons in the BF were large (>20 μm) or medium-sized (15-20 μm) GFP+ neurons. Most medium and large-sized BF GFP+ neurons, including those retrogradely labeled from the neocortex, were fast-firing and spontaneously active in vitro. They exhibited prominent hyperpolarization-activated inward currents and subthreshold "spikelets," suggestive of electrical coupling. PV+ neurons recorded in PV-Tomato mice had similar properties but had significantly narrower action potentials and a higher maximal firing frequency. Another population of smaller GFP+ neurons had properties similar to striatal projection neurons. The fast firing and electrical coupling of BF GABA/PV+ neurons, together with their projections to cortical interneurons and the thalamic reticular nucleus, suggest a strong and synchronous control of the neocortical fast rhythms typical of wakefulness and REM sleep.

Short neuropeptide F is a sleep-promoting inhibitory modulator

To advance the understanding of sleep regulation, we screened for sleep-promoting cells and identified neurons expressing neuropeptide Y-like short neuropeptide F (sNPF). Sleep induction by sNPF meets all relevant criteria. Rebound sleep following sleep deprivation is reduced by activation of sNPF neurons, and flies experience negative sleep rebound upon cessation of sNPF neuronal stimulation, indicating that sNPF provides an important signal to the sleep homeostat. Only a subset of sNPF-expressing neurons, which includes the small ventrolateral clock neurons, is sleep promoting. Their release of sNPF increases sleep consolidation in part by suppressing the activity of wake-promoting large ventrolateral clock neurons, and suppression of neuronal firing may be the general response to sNPF receptor activation. sNPF acutely increases sleep without altering feeding behavior, which it affects only on a much longer time scale. The profound effect of sNPF on sleep indicates that it is an important sleep-promoting molecule.

Social and environmental contexts modulate sleep deprivation-induced c-Fos activation in rats

People often sleep deprive themselves voluntarily for social and lifestyle reasons. Animals also appear to stay awake longer as a result of their natural curiosity to explore novel environments and interact socially with conspecifics. Although multiple arousal systems in the brain are known to act jointly to promote and maintain wakefulness, it remains unclear whether these systems are similarly engaged during voluntary vs. forced wakefulness. Using c-Fos immunohistochemistry, we compared neuronal responses in rats deprived of sleep for 2 h by gentle sensory stimulation, exploration under social isolation, or exploration with social interaction, and rats under undisturbed control conditions. In many arousal, limbic, and autonomic nuclei examined (e.g., anterior cingulate cortex and locus coeruleus), the two sleep deprivation procedures involving exploration were similarly effective, and both were more effective than sleep deprivation with sensory stimulation, in increasing the number of c-Fos immunoreactive neurons. However, some nuclei (e.g., paraventricular hypothalamic nucleus and select amygdala nuclei) were more responsive to exploration with social interaction, while others (e.g., histaminergic tuberomammillary nucleus) responded more strongly to exploration in social isolation. In the rostral basal forebrain, cholinergic and GABAergic neurons responded preferentially to exploration with social interaction, whereas resident neurons in general responded most strongly to exploration without social interaction. These results indicate that voluntary exploration with/without social interaction is more effective than forced sleep deprivation with gentle sensory stimulation for inducing c-Fos in arousal and limbic/autonomic brain regions, and suggest that these nuclei participate in different aspects of arousal during sustained voluntary wakefulness.

Neurons in the basal forebrain project to the cortex in a complex topographic organization that reflects corticocortical connectivity patterns: an experimental study based on retrograde tracing and 3D reconstruction

The most prominent feature of the Basal Forebrain (BF) is the collection of large cortically projecting neurons (basal nucleus of Meynert) that serve as the primary source of cholinergic input to the entire cortical mantle. Despite its broad involvement in cortical activation, attention, and memory, the functional details of the BF are not well understood due to the anatomical complexity of the region. This study tested the hypothesis that basalocortical connections reflect cortical connectivity patterns. Distinct retrograde tracers were deposited into various frontal and posterior cortical areas, and retrogradely labeled cholinergic and noncholinergic neurons were mapped in the BF. Concurrently, we mapped retrogradely labeled cells in posterior cortical areas that project to various frontal areas, and all cell populations were combined in the same coordinate system. Our studies suggest that the cholinergic and noncholinergic projections to the neocortex are not diffuse, but instead, are organized into segregated or overlapping pools of projection neurons. The extent of overlap between BF populations projecting to the cortex depends on the degree of connectivity between the cortical targets of these projection populations. We suggest that the organization of projections from the BF may enable parallel modulation of multiple groupings of interconnected yet nonadjacent cortical areas.

Septo-hippocampal GABAergic signaling across multiple modalities in awake mice

Hippocampal interneurons receive GABAergic input from the medial septum. Using two-photon Ca(2+) imaging of axonal boutons in hippocampal CA1 of behaving mice, we found that populations of septo-hippocampal GABAergic boutons were activated during locomotion and salient sensory events; sensory responses scaled with stimulus intensity and were abolished by anesthesia. We found similar activity patterns among boutons with common putative postsynaptic targets, with low-dimensional bouton population dynamics being driven primarily by presynaptic spiking.

Adenosine inhibits the excitatory synaptic inputs to Basal forebrain cholinergic, GABAergic, and parvalbumin neurons in mice

Coffee and tea contain the stimulants caffeine and theophylline. These compounds act as antagonists of adenosine receptors. Adenosine promotes sleep and its extracellular concentration rises in association with prolonged wakefulness, particularly in the basal forebrain (BF) region involved in activating the cerebral cortex. However, the effect of adenosine on identified BF neurons, especially non-cholinergic neurons, is incompletely understood. Here we used whole-cell patch-clamp recordings in mouse brain slices prepared from two validated transgenic mouse lines with fluorescent proteins expressed in GABAergic or parvalbumin (PV) neurons to determine the effect of adenosine. Whole-cell recordings were made from BF cholinergic neurons and from BF GABAergic and PV neurons with the size (>20 μm) and intrinsic membrane properties (prominent H-currents) corresponding to cortically projecting neurons. A brief (2 min) bath application of adenosine (100 μM) decreased the frequency but not the amplitude of spontaneous excitatory postsynaptic currents (EPSCs) in all groups of BF cholinergic, GABAergic, and PV neurons we recorded. In addition, adenosine decreased the frequency of miniature EPSCs in BF cholinergic neurons. Adenosine had no effect on the frequency of spontaneous inhibitory postsynaptic currents in cholinergic neurons or GABAergic neurons with large H-currents but reduced them in a group of GABAergic neurons with smaller H-currents. All effects of adenosine were blocked by a selective, adenosine A1 receptor antagonist, cyclopentyltheophylline (CPT, 1 μM). Adenosine had no postsynaptic effects. Taken together, our work suggests that adenosine promotes sleep by an A1 receptor-mediated inhibition of glutamatergic inputs to cortically projecting cholinergic and GABA/PV neurons. Conversely, caffeine and theophylline promote attentive wakefulness by inhibiting these A1 receptors in BF thereby promoting the high-frequency oscillations in the cortex required for attention and cognition.

Control of arousal by the orexin neurons

The orexin-producing neurons in the lateral hypothalamus play an essential role in promoting arousal and maintaining wakefulness. These neurons receive a broad variety of signals related to environmental, physiological and emotional stimuli; they project to almost every brain region involved in the regulation of wakefulness; and they fire most strongly during active wakefulness, high motor activation, and sustained attention. This review focuses on the specific neuronal pathways through which the orexin neurons promote wakefulness and maintain high level of arousal, and how recent studies using optogenetic and pharmacogenetic methods have demonstrated that the locus coeruleus, the tuberomammillary nucleus, and the basal forebrain are some of the key sites mediating the arousing actions of orexins.

From bench to bed: putative animal models of REM sleep behavior disorder (RBD)

REM behavior disorder (RBD) is a parasomnia characterized by REM sleep without atonia, leading to abnormal and potentially injurious behavior during REM sleep. It is considered one of the most specific predictors of neurodegenerative disorders, such as Parkinson's disease. In this paper, we provide an overview of animal models contributing to our current understanding of REM-associated atonia, and, as a consequence, the pathophysiology of RBD. The generator of REM-associated atonia is located in glutamatergic neurons of the pontine sublaterodorsal nucleus (SLD), as shown in cats, rats and mice. These findings are supported by clinical cases of patients with lesions of the homologous structure in humans. Glutamatergic SLD neurons, presumably in conjunction with others, project to (a) the ventromedial medulla, where they either directly target inhibitory interneurons to alpha motor neurons or are relayed, and (b) the spinal cord directly. At the spinal level, alpha motor neurons are inhibited by GABAergic and glycinergic interneurons. Our current understanding is that lesions of the glutamatergic SLD are the key factor for REM sleep behavior disorder. However, open questions remain, e.g. other features of RBD (such as the typically aggressive dream content) or the frequent progression from idiopathic RBD to neurodegenerative disorders, to name only a few. In order to elucidate these questions, a constant interaction between basic and clinical researchers is required, which might, ultimately, create an early therapeutic window for neurodegenerative disorders.

Cell-type-specific modulation of neocortical activity by basal forebrain input

Activation of the cholinergic neurons in the basal forebrain (BF) desynchronizes cortical activity and enhances sensory processing during arousal and attention. How the cholinergic input modulates the activity of different subtypes of cortical neurons remains unclear. Using in vivo two-photon calcium imaging of neurons in layers 1 and 2/3 of mouse visual cortex, we show that electrical stimulation of the BF bi-directionally modulates the activity of excitatory neurons as well as several subtypes of inhibitory interneurons. While glutamatergic activity contributed to the activation of both excitatory and inhibitory neurons, the contribution of acetylcholine (ACh) was more complex. Excitatory and parvalbumin-positive (PV+) neurons were activated through muscarinic ACh receptors (mAChRs) at low levels of cortical desynchronization and suppressed through nicotinic ACh receptors (nAChRs) when cortical desynchronization was strong. In contrast, vasoactive intestinal peptide-positive (VIP+) and layer 1 interneurons were preferentially activated through nAChRs during strong cortical desynchronization. Thus, cholinergic input from the BF causes a significant shift in the relative activity levels of different subtypes of cortical neurons at increasing levels of cortical desynchronization.

Serotonergic integration of circadian clock and ultradian sleep-wake cycles

In mammals, the suprachiasmatic nucleus (SCN) of the hypothalamus generates a 24 h rhythm of sleep and arousal. While neuronal spiking activity in the SCN provides a functional circadian oscillator that propagates throughout the brain, the ultradian sleep-wake state is regulated by the basal forebrain/preoptic area (BF/POA). How this SCN circadian oscillation is integrated into the shorter sleep-wake cycles remains unclear. We examined the temporal patterns of neuronal activity in these key brain regions in freely behaving rats. Neuronal activity in various brain regions presented diurnal rhythmicity and/or sleep-wake state dependence. We identified a diurnal rhythm in the BF/POA that was selectively degraded when diurnal arousal patterns were disrupted by acute brain serotonin depletion despite robust circadian spiking activity in the SCN. Local blockade of serotonergic transmission in the BF/POA was sufficient to disrupt the diurnal sleep-wake rhythm of mice. These results suggest that the serotonergic system enables the BF/POA to couple the SCN circadian signal to ultradian sleep-wake cycles, thereby providing a potential link between circadian rhythms and psychiatric disorders.

Neuronal nicotinic receptors in sleep-related epilepsy: studies in integrative biology

Although Mendelian diseases are rare, when considered one by one, overall they constitute a significant social burden. Besides the medical aspects, they propose us one of the most general biological problems. Given the simplest physiological perturbation of an organism, that is, a single gene mutation, how do its effects percolate through the hierarchical biological levels to determine the pathogenesis? And how robust is the physiological system to this perturbation? To solve these problems, the study of genetic epilepsies caused by mutant ion channels presents special advantages, as it can exploit the full range of modern experimental methods. These allow to extend the functional analysis from single channels to whole brains. An instructive example is autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE), which can be caused by mutations in neuronal nicotinic acetylcholine receptors. In vitro, such mutations often produce hyperfunctional receptors, at least in heterozygous condition. However, understanding how this leads to sleep-related frontal epilepsy is all but straightforward. Several available animal models are helping us to determine the effects of ADNFLE mutations on the mammalian brain. Because of the complexity of the cholinergic regulation in both developing and mature brains, several pathogenic mechanisms are possible, which also present different therapeutic implications.

GABA-to-ACh ratio in basal forebrain and cerebral cortex varies significantly during sleep

STUDY OBJECTIVES

GABAergic and cholinergic transmission within the basal forebrain and cerebral cortex contribute to the regulation of sleep and wakefulness. In contrast to levels of acetylcholine (ACh), levels of endogenous GABA in basal forebrain and cortex during sleep and wakefulness have not previously been quantified. This study (1) tested the hypothesis that there are differential, state-specific changes in GABA levels within the substantia innominata (SI) region of the basal forebrain and somatosensory cortex; and (2) quantified the ratio of GABAergic to cholinergic transmission in the SI, cortex, and pontine reticular formation during rapid eye movement sleep (REM), non-REM sleep (NREM), and wakefulness.

DESIGN

Within/between subjects.

SETTING

University of Michigan.

PATIENTS OR PARTICIPANTS

Adult, male, purpose bred cats (n = 5).

INTERVENTIONS

In vivo microdialysis, high performance liquid chromatography, electrophysiological recordings.

MEASUREMENTS AND RESULTS

In the SI, GABA levels were significantly greater during NREM (17%) than during REM. In the cortex, GABA levels were significantly greater during NREM than during wakefulness (39%) and REM (63%). During prolonged wakefulness, there was a linear increase in cortical GABA levels, and the amount of time spent awake accounted for 87% of the variance in GABA. The GABA-to-ACh ratio was largest during NREM for all brain regions. REM was characterized by a 68% decrease in the GABA-to-ACh ratio across brain regions, always due to a decrease in GABA levels.

CONCLUSION

Three of the brain regions that comprise the anatomically distributed, sleep-generating network have in common a GABA-mediated, sleep-dependent decrease in the GABA-to-ACh ratio.

Discharge properties of presumed cholinergic and noncholinergic laterodorsal tegmental neurons related to cortical activation in non-anesthetized mice

We have recorded, for the first time, in non-anesthetized, head-restrained mice, a total of 339 single units in and around the laterodorsal (LDT) and sublaterodorsal (SubLDT) tegmental nuclei, which are located, respectively, in, or beneath, the periaqueductal gray and contain cholinergic neurons. The recordings were made during the complete wake-sleep cycle including wakefulness (W), slow-wave sleep (SWS), and paradoxical (or rapid eye movement) sleep (PS). The tegmental neurons displayed either a biphasic narrow or triphasic broad action potential. Seventy-six LDT or SubLDT neurons characterized by their triphasic long-duration action potentials were judged to be cholinergic and this was verified in anesthetized mice using neurobiotin juxtacellular labeling combined with choline acetyltransferase immunohistochemistry of the recorded cell. The 76 presumed cholinergic neurons discharged tonically at the highest rate during W and PS (W/PS-active neurons) as either single isolated spikes or clusters of two to five spikes, and 26 of them discharged selectively during W and PS, these W/PS-selective neurons being found mainly in the SubLDT. The clustering discharge was particularly prominent during PS, when it was associated with an obvious phasic change in the cortical electroencephalogram (EEG), and during waking periods, when it was accompanied by abrupt body movements. During the transition from sleep to waking, the cholinergic W/PS-selective neurons and the LDT or SubLDT noncholinergic W-selective neurons showed firing before the onset of W, while, at the transition from waking to sleep, they ceased firing before sleep onset. At the transition from SWS to PS, all the cholinergic neurons exhibited a significant increase in discharge rate before the onset of PS. The present study in mice supports the view that cholinergic and noncholinergic LDT and SubLDT neurons play an important role in tonic and phasic processes of arousal and cortical EEG activation occurring during W or PS, as well as in the sleep/waking switch.

Control of sleep and wakefulness

This review summarizes the brain mechanisms controlling sleep and wakefulness. Wakefulness promoting systems cause low-voltage, fast activity in the electroencephalogram (EEG). Multiple interacting neurotransmitter systems in the brain stem, hypothalamus, and basal forebrain converge onto common effector systems in the thalamus and cortex. Sleep results from the inhibition of wake-promoting systems by homeostatic sleep factors such as adenosine and nitric oxide and GABAergic neurons in the preoptic area of the hypothalamus, resulting in large-amplitude, slow EEG oscillations. Local, activity-dependent factors modulate the amplitude and frequency of cortical slow oscillations. Non-rapid-eye-movement (NREM) sleep results in conservation of brain energy and facilitates memory consolidation through the modulation of synaptic weights. Rapid-eye-movement (REM) sleep results from the interaction of brain stem cholinergic, aminergic, and GABAergic neurons which control the activity of glutamatergic reticular formation neurons leading to REM sleep phenomena such as muscle atonia, REMs, dreaming, and cortical activation. Strong activation of limbic regions during REM sleep suggests a role in regulation of emotion. Genetic studies suggest that brain mechanisms controlling waking and NREM sleep are strongly conserved throughout evolution, underscoring their enormous importance for brain function. Sleep disruption interferes with the normal restorative functions of NREM and REM sleep, resulting in disruptions of breathing and cardiovascular function, changes in emotional reactivity, and cognitive impairments in attention, memory, and decision making.

Sleep locally, act globally

In most animals, sleep is considered a global brain and behavioral state. However, recent intracortical recordings have shown that aspects of non-rapid eye movement (NREM) sleep and wakefulness can occur simultaneously in different parts of the cortex in mammals, including humans. Paradoxically, however, NREM sleep still manifests as a global behavioral shutdown. In this review, the authors examine this paradox from an evolutionary perspective. On the basis of strategic modeling, they suggest that in animals with brains composed of heavily interconnected and functionally interdependent units, a global regulator of sleep maintains the behavioral shutdown that defines sleep and thereby ensures that local use-dependent functions are performed in a safe and efficient manner. This novel perspective has implications for understanding deficits in human cognitive performance resulting from sleep deprivation, sleep disorders such as sleepwalking, changes in consciousness that occur during sleep, and the function of sleep itself.

Adult mouse basal forebrain harbors two distinct cholinergic populations defined by their electrophysiology

We performed whole-cell recordings from basal forebrain (BF) cholinergic neurons in transgenic mice expressing enhanced green fluorescent protein (eGFP) under the control of the choline acetyltransferase promoter. BF cholinergic neurons can be differentiated into two electrophysiologically identifiable subtypes: early and late firing neurons. Early firing neurons (∼70%) are more excitable, show prominent spike frequency adaptation and are more susceptible to depolarization blockade, a phenomenon characterized by complete silencing of the neuron following initial action potentials. Late firing neurons (∼30%), albeit being less excitable, could maintain a tonic discharge at low frequencies. In voltage clamp analysis, we have shown that early firing neurons have a higher density of low voltage activated (LVA) calcium currents. These two cholinergic cell populations might be involved in distinct functions: the early firing group being more suitable for phasic changes in cortical acetylcholine release associated with attention while the late firing neurons could support general arousal by maintaining tonic acetylcholine levels.

Adenosine inhibits glutamatergic input to basal forebrain cholinergic neurons

Adenosine has been proposed as an endogenous homeostatic sleep factor that accumulates during waking and inhibits wake-active neurons to promote sleep. It has been specifically hypothesized that adenosine decreases wakefulness and promotes sleep recovery by directly inhibiting wake-active neurons of the basal forebrain (BF), particularly BF cholinergic neurons. We previously showed that adenosine directly inhibits BF cholinergic neurons. Here, we investigated 1) how adenosine modulates glutamatergic input to BF cholinergic neurons and 2) how adenosine uptake and adenosine metabolism are involved in regulating extracellular levels of adenosine. Our experiments were conducted using whole cell patch-clamp recordings in mouse brain slices. We found that in BF cholinergic neurons, adenosine reduced the amplitude of AMPA-mediated evoked glutamatergic excitatory postsynaptic currents (EPSCs) and decreased the frequency of spontaneous and miniature EPSCs through presynaptic A(1) receptors. Thus we have demonstrated that in addition to directly inhibiting BF cholinergic neurons, adenosine depresses excitatory inputs to these neurons. It is therefore possible that both direct and indirect inhibition may synergistically contribute to the sleep-promoting effects of adenosine in the BF. We also found that blocking the influx of adenosine through the equilibrative nucleoside transporters or inhibiting adenosine kinase and adenosine deaminase increased endogenous adenosine inhibitory tone, suggesting a possible mechanism through which adenosine extracellular levels in the basal forebrain are regulated.

Sleep neurobiology from a clinical perspective

Many neurochemical systems interact to generate wakefulness and sleep. Wakefulness is promoted by neurons in the pons, midbrain, and posterior hypothalamus that produce acetylcholine, norepinephrine, dopamine, serotonin, histamine, and orexin/hypocretin. Most of these ascending arousal systems diffusely activate the cortex and other forebrain targets. NREM sleep is mainly driven by neurons in the preoptic area that inhibit the ascending arousal systems, while REM sleep is regulated primarily by neurons in the pons, with additional influence arising in the hypothalamus. Mutual inhibition between these wake- and sleep-regulating regions likely helps generate full wakefulness and sleep with rapid transitions between states. This up-to-date review of these systems should allow clinicians and researchers to better understand the effects of drugs, lesions, and neurologic disease on sleep and wakefulness.

The wake-promoting transmitter histamine preferentially enhances α-4 subunit-containing GABAA receptors

Histamine is an important wake-promoting neurotransmitter that activates seven-transmembrane G-protein coupled histamine receptors. However, histamine demonstrates target promiscuity, including direct interaction with the structurally unrelated glutamate (NMDA) and GABA(A) receptor channels. Previous work showed that histamine enhances the activity of recombinant GABA(A) receptor isoforms typically found in synaptic locations, although co-release of histamine and GABA is not known to occur in vivo. Here we used patch clamp recordings of various recombinant GABA(A) receptor isoforms (α1-6, β1-3, γ1-3, δ) to test the hypothesis that histamine might show subunit preference under low GABA concentration (extrasynaptic) conditions. We found that histamine potentiated the whole-cell responses to GABA for all tested subunit combinations. However, the magnitude of enhancement was largest (∼400% of EC(10) GABA-evoked currents) with α4β3 and α4β3X isoforms, where X could be γ or δ. In contrast, histamine (1 mM) had small effects on prolonging deactivation of α4β3γ2 receptors following brief (5 ms) pulses of 1 mM GABA. These findings suggest GABA-histamine cross-talk may occur preferentially at low GABA concentrations, which could theoretically be inhibitory (via enhancing tonic inhibition), directly excitatory (via enhancing presynaptic GABAergic signaling), or indirectly excitatory (via inhibiting GABAergic interneurons).

Sleep-waking discharge profiles of median preoptic and surrounding neurons in mice

The median preoptic nucleus (MnPO), part of the anteroventral third ventricular region, plays a key role in body fluid homeostasis and cardiovascular regulation. Recently, a cluster of neurons showing sleep-related c-fos immunoreactivity was found in the rat MnPO, and a subsequent electrophysiological study found that nearly 76% of rat MnPO neurons exhibit increased discharge during sleep. In a recent single unit recording study in mice, we found that sleep-active neurons are not localized in any specific region of the preoptic/basal forebrain (POA/BFB). However, the discharge profiles of mouse MnPO neurons across wake-sleep states remained to be determined. In this study, we therefore examined whether the mouse MnPO contains a high proportion of sleep-active neurons and constitutes a distinct cluster of sleep-promoting neurons in the median preoptic region. We recorded a total of 234 single units in the MnPO, the laterally adjacent peri-MnPO, the dorsally adjacent medial septum (MS), and the ventrally adjacent periventricular (Pe)/medial preoptic (MPO) area (Pe/MPO). We found that the MnPO contained similar proportions of sleep-active (31.9%) and waking (W)-active (33.0%) neurons, together with many waking/paradoxical sleep (W/PS)-active neurons (23.4%), whereas the Pe/MPO and MS contained a high proportion of sleep-active neurons (66.0 and 62.9%, respectively), while the peri-MnPO contained a high proportion of W-active neurons (57.1%). In the MnPO, both W-active and W/PS-active neurons were distributed throughout the nucleus, whereas sleep-active neurons were mostly located on its border. Only slowly discharging (<5 Hz) slow-wave sleep (SWS)/PS-selective neurons were found in the MnPO. During the transition from W to SWS, all of these SWS/PS-selective neurons fired not before, but after, sleep onset, with a gradual increase in discharge rate. In addition to its well-known homeostatic and cardiovascular functions, the MnPO might modulate the sleep-waking cycle by playing different roles in sleep/wake state regulation.

Immunolocalization of the voltage-gated potassium channel Kv2.2 in GABAergic neurons in the basal forebrain of rats and mice

The Kv2 voltage-gated potassium channels, Kv2.1 and Kv2.2, are important regulators of neuronal excitability in mammalian brain. It has been shown that Kv2.1 channels are expressed in virtually all neurons in the brain. However, the cellular localization of Kv2.2 has not been fully elucidated. In this article we report that Kv2.2 is highly expressed in a subset of neurons in the magnocellular preoptic nucleus (MCPO) and the horizontal limb of the diagonal band of Broca (HDB) of the basal forebrain complex, which are areas highly implicated in the regulation of cortical activity and the sleep/wake cycle. It has been shown that MCPO and HDB contain distinct populations of neurons that differ in their neurochemicals, cholinergic, glutamatergic, and gamma-aminobutyric acid (GABA)ergic neurons. Using specific immunolabeling and knockin mice in which green fluorescent protein (GFP) is expressed in GABAergic neurons, we found that Kv2.2 is abundantly expressed in a large subpopulation of the GABAergic neurons in the MCPO and HDB. These data offer Kv2.2 as a molecular target to study the role of the specific subpopulation of basal forebrain GABAergic neurons.

Role of adenosine and wake-promoting basal forebrain in insomnia and associated sleep disruptions caused by ethanol dependence

Insomnia is a severe symptom of alcohol withdrawal; however, the underlying neuronal mechanism is yet unknown. We hypothesized that chronic ethanol exposure will impair basal forebrain (BF) adenosinergic mechanism resulting in insomnia-like symptoms. We performed a series of experiments in Sprague-Dawley rats to test our hypothesis. We used Majchrowicz's chronic binge ethanol protocol to induce ethanol dependency. Our first experiment verified the effects of ethanol withdrawal on sleep-wakefulness. Significant increase in wakefulness was observed during ethanol withdrawal. Next, we examined c-Fos expression (marker of neuronal activation) in BF wake-promoting neurons during ethanol withdrawal. There was a significant increase in the number of BF wake-promoting neurons with c-Fos immunoreactivity. Our third experiment examined the effects of ethanol withdrawal on sleep deprivation induced increase in BF adenosine levels. Sleep deprivation did not increase BF adenosine levels in ethanol dependent rats. Our last experiment examined the effects of ethanol withdrawal on equilibrative nucleoside transporter 1 and A1 receptor expression in the BF. There was a significant reduction in A1 receptor and equilibrative nucleoside transporter 1 expression in the BF of ethanol dependent rats. Based on these results, we suggest that insomnia observed during ethanol withdrawal is caused because of impaired adenosinergic mechanism in the BF.

Tuning arousal with optogenetic modulation of locus coeruleus neurons

Neural activity in the noradrenergic locus coeruleus correlates with periods of wakefulness and arousal. However, whether tonic or phasic activity in these neurons is necessary or sufficient to induce behavioral state transitions and promote long-term arousal is unresolved. We used optogenetic tools in mice to demonstrate a frequency-dependent, causal relationship between locus coeruleus firing, cortical activity, sleep-to-wake transitions, and general locomotor arousal. Surprisingly, we also found that sustained, high-frequency stimulation of the locus coeruleus at frequencies 5 Hz and above caused reversible behavioral arrests. These results suggest that the locus coeruleus is finely tuned to regulate organismal arousal and that bursts of noradrenergic over-excitation cause behavioral attacks similar to those observed in neuropsychiatric disorders.

Activation of cortical interneurons during sleep: an anatomical link to homeostatic sleep regulation?

Although slow wave activity in the EEG has been linked to homeostatic sleep regulation, the neurobiological substrate of sleep homeostasis is not well understood. Whereas cortical neurons typically exhibit reduced discharge rates during slow wave sleep (SWS), a subpopulation of GABAergic interneurons, which express the enzyme neuronal nitric oxide synthase (nNOS), has recently been found to be activated during SWS. The extent of activation of these nNOS neurons is proportional to homeostatic sleep 'drive'. These cells are an exception among cortical interneurons in that they are projection neurons. We propose that cortical nNOS neurons are positioned to influence neuronal activity across widespread brain areas. They could thus provide a long-sought anatomical link for understanding homeostatic sleep regulation.

Slow wave activity during sleep: functional and therapeutic implications

Electroencephalographic slow-wave activity (EEG SWA) is an electrophysiological signature of slow (0.5 to 4.0 Hz), synchronized, oscillatory neocortical activity. In healthy individuals, EEG SWA is maximally expressed during non-rapid-eye-movement (non-REM) sleep, and intensifies as a function of prior wake duration. Many of the cellular and network mechanisms generating EEG SWA have been identified, but a number of questions remain unanswered. For example, although EEG SWA is a marker of sleep need, its precise relationship with sleep homeostasis and its roles in the brain are unknown. In this review, the authors discuss their current understanding of the neural mechanisms and possible functions of EEG SWA.

Sleep and brain energy levels: ATP changes during sleep

Sleep is one of the most pervasive biological phenomena, but one whose function remains elusive. Although many theories of function, indirect evidence, and even common sense suggest sleep is needed for an increase in brain energy, brain energy levels have not been directly measured with modern technology. We here report that ATP levels, the energy currency of brain cells, show a surge in the initial hours of spontaneous sleep in wake-active but not in sleep-active brain regions of rat. The surge is dependent on sleep but not time of day, since preventing sleep by gentle handling of rats for 3 or 6 h also prevents the surge in ATP. A significant positive correlation was observed between the surge in ATP and EEG non-rapid eye movement delta activity (0.5-4.5 Hz) during spontaneous sleep. Inducing sleep and delta activity by adenosine infusion into basal forebrain during the normally active dark period also increases ATP. Together, these observations suggest that the surge in ATP occurs when the neuronal activity is reduced, as occurs during sleep. The levels of phosphorylated AMP-activated protein kinase (P-AMPK), well known for its role in cellular energy sensing and regulation, and ATP show reciprocal changes. P-AMPK levels are lower during the sleep-induced ATP surge than during wake or sleep deprivation. Together, these results suggest that sleep-induced surge in ATP and the decrease in P-AMPK levels set the stage for increased anabolic processes during sleep and provide insight into the molecular events leading to the restorative biosynthetic processes occurring during sleep.

GABAergic neurons intermingled with orexin and MCH neurons in the lateral hypothalamus discharge maximally during sleep

The lateral hypothalamus (LH), where wake-active orexin (Orx)-containing neurons are located, has been considered a waking center. Yet, melanin-concentrating hormone (MCH)-containing neurons are codistributed therein with Orx neurons and, in contrast to them, are active during sleep, not waking. In the present study employing juxtacellular recording and labeling of neurons with Neurobiotin (Nb) in naturally sleeping-waking head-fixed rats, we identified another population of intermingled sleep-active cells, which do not contain MCH (or Orx), but utilize gamma-aminobutyric acid (GABA) as a neurotransmitter. The 'sleep-max' active neurons represented 53% of Nb-labeled MCH-(and Orx) immunonegative (-) cells recorded in the LH. For identification of their neurotransmitter, Nb-labeled varicosities of the Nb-labeled/MCH- neurons were sought within sections adjacent to the Nb-labeled soma and immunostained for the vesicular transporter for GABA (VGAT) or for glutamate. A small proportion of sleep-max Nb+/MCH- neurons (19%) discharged maximally during slow-wave sleep (called 'S-max') in positive correlation with delta electroencephalogram activity, and from VGAT staining of Nb-labeled varicosities appeared to be GABAergic. The vast proportion of sleep-max Nb+/MCH- neurons (81%) discharged maximally during paradoxical sleep (PS, called 'P-max') in negative correlation with electromyogram amplitude, and from Nb-labeled varicosities also appeared to be predominantly GABAergic. Given their discharge profiles across the sleep-wake cycle, P-max together with S-max GABAergic neurons could thus serve to inhibit other neurons of the arousal systems, including local Orx neurons in the LH. They could accordingly dampen arousal with muscle tone and promote sleep, including PS with muscle atonia.

Physiological properties of cholinergic and non-cholinergic magnocellular neurons in acute slices from adult mouse nucleus basalis

BACKGROUND

The basal forebrain is a series of nuclei that provides cholinergic input to much of the forebrain. The most posterior of these nuclei, nucleus basalis, provides cholinergic drive to neocortex and is involved in arousal and attention. The physiological properties of neurons in anterior basal forebrain nuclei, including medial septum, the diagonal band of Broca and substantia innominata, have been described previously. In contrast the physiological properties of neurons in nucleus basalis, the most posterior nucleus of the basal forebrain, are unknown.

METHODOLOGY/PRINCIPAL FINDINGS

Here we investigate the physiological properties of neurons in adult mouse nucleus basalis. We obtained cell-attached and whole-cell recordings from magnocellular neurons in slices from P42-54 mice and compared cholinergic and non-cholinergic neurons, distinguished retrospectively by anti-choline acetyltransferase immunocytochemistry. The majority (70-80%) of cholinergic and non-cholinergic neurons were silent at rest. Spontaneously active cholinergic and non-cholinergic neurons exhibited irregular spiking at 3 Hz and at 0.3 to 13.4 Hz, respectively. Cholinergic neurons had smaller, broader action potentials than non-cholinergic neurons (amplitudes 64+/-3.4 and 75+/-2 mV; half widths 0.52+/-0.04 and 0.33+/-0.02 ms). Cholinergic neurons displayed a more pronounced slow after-hyperpolarization than non-cholinergic neurons (13.3+/-2.2 and 3.6+/-0.5 mV) and were unable to spike at high frequencies during tonic current injection (maximum frequencies of approximately 20 Hz and >120 Hz).

CONCLUSIONS/SIGNIFICANCE

Our results indicate that neurons in nucleus basalis share similar physiological properties with neurons in anterior regions of the basal forebrain. Furthermore, cholinergic and non-cholinergic neurons in nucleus basalis can be distinguished by their responses to injected current. To our knowledge, this is the first description of the physiological properties of cholinergic and non-cholinergic neurons in the posterior aspects of the basal forebrain complex and the first study of basal forebrain neurons from the mouse.

Endogenous histamine facilitates long-term potentiation in the hippocampus during walking

Long-term potentiation (LTP) in hippocampal CA1 depends on the behavioral state of LTP induction. We hypothesize that histaminergic activity in the septohippocampal system, which is active during walking compared with other behavioral states, is responsible for the behavioral dependence of LTP. Field basal-dendritic EPSPs of CA1 pyramidal cells were recorded in freely behaving rats, and LTP was induced by a single 200 Hz stimulation train (0.5 s duration). Basal-dendritic LTP was facilitated when induced during walking compared with awake immobility (IMM) or rapid-eye-movement sleep. The facilitation of basal-dendritic LTP during walking was abolished by lesion of tuberomammillary nucleus (TMN) neurons with orexin-saporin or by intramedial-septal infusion of the H(1) histaminergic blocker triprolidine but not the H(2) histaminergic blocker cimetidine. Conversely, histamine infusion in the medial septum enhanced the basal-dendritic LTP induced during IMM to a magnitude similar to that induced during walking. Basal-dendritic LTP induced during walking was not further enhanced by intraseptal histamine infusion. Combined with the previous result that behavior-dependent LTP is mediated by cholinergic septohippocampal neurons, we conclude that the facilitation of basal-dendritic LTP in CA1 during walking was mediated by TMN histaminergic afferents acting on H(1) receptors in the medial septum, which may then enhance cholinergic and noncholinergic inputs to the hippocampus.

Role of wake-promoting basal forebrain and adenosinergic mechanisms in sleep-promoting effects of ethanol

BACKGROUND

Ethanol intake has significant impact on sleep. However, the cellular substrates responsible for sleep promotion following ethanol intake are unknown. The purine nucleoside, adenosine, is responsible for mediating many neuronal and behavioral responses to ethanol. Studies performed in cell cultures suggest that ethanol inhibits equilibrative nucleoside transporter 1 to block the reuptake of adenosine resulting in increased extracellular adenosine. Adenosine also has a pivotal role in sleep regulation. Adenosine acts via A1 receptor to inhibit the wake-promoting neurons of the basal forebrain (BF) resulting in the promotion of sleep. Is ethanol-induced sleep associated with the inhibition of the BF wake-promoting neurons? Do adenosinergic mechanisms in the BF have a role in sleep-promoting effects of ethanol?

METHODS

To address these questions, we performed 3 experiments in Sprague-Dawley rats. First, we verified the effect of ethanol on sleep promotion. Second, we evaluated the effect of ethanol on c-Fos expression (a marker of neuronal activation) in the BF wake-promoting neurons and third we monitored the effects of A1 receptor blockade in the BF on ethanol-induced sleep.

RESULTS

Significant increase in non-rapid eye movement (NREM) sleep with a concomitant decrease in wakefulness was observed during the first 12 hours postethanol. REM sleep remained unaffected. Ethanol administration caused a significant decrease in the number of BF wake-promoting neurons with c-Fos immunoreactivity. Bilateral microinjections of a selective A1R receptor antagonist 8-cyclopentyl-1, 3-dipropylxanthine into the BF significantly attenuated sleep-promoting effects of ethanol.

CONCLUSION

These results suggest that the inhibition of BF wake-promoting neurons by adenosinergic mechanism may be responsible for the sleep promoting effects of ethanol. We believe our study is the first to investigate the cellular mechanisms responsible for the somnogenic effects of ethanol.

Optogenetic deconstruction of sleep-wake circuitry in the brain

How does the brain regulate the sleep–wake cycle? What are the temporal codes of sleep and wake-promoting neural circuits? How do these circuits interact with each other across the light/dark cycle? Over the past few decades, many studies from a variety of disciplines have made substantial progress in answering these fundamental questions. For example, neurobiologists have identified multiple, redundant wake-promoting circuits in the brainstem, hypothalamus, and basal forebrain. Sleep-promoting circuits have been found in the preoptic area and hypothalamus. One of the greatest challenges in recent years has been to selectively record and manipulate these sleep–wake centers in vivo with high spatial and temporal resolution. Recent developments in microbial opsin-based neuromodulation tools, collectively referred to as “optogenetics,” have provided a novel method to demonstrate causal links between neural activity and specific behaviors. Here, we propose to use optogenetics as a fundamental tool to probe the necessity, sufficiency, and connectivity of defined neural circuits in the regulation of sleep and wakefulness.