Control of wakefulness by lateral hypothalamic glutamatergic neurons in male mice

A REM-active basal ganglia circuit that regulates anxiety

Rapid eye movement (REM) sleep has been hypothesized to promote emotional resilience, but any neuronal circuits mediating this have not been identified. We find that in mice, somatostatin (Som) neurons in the entopeduncular nucleus (EPSom)/internal globus pallidus are predominantly active during REM sleep. This unique REM activity is both necessary and sufficient for maintaining normal REM sleep. Inhibiting or exciting EPSom neurons reduced or increased REM sleep duration, respectively. Activation of the sole downstream target of EPSom neurons, Vglut2 cells in the lateral habenula (LHb), increased sleep via the ventral tegmental area (VTA). A simple chemogenetic scheme to periodically inhibit the LHb over 4 days selectively removed a significant amount of cumulative REM sleep. Chronic, but not acute, REM reduction correlated with mice becoming anxious and more sensitive to aversive stimuli. Therefore, we suggest that cumulative REM sleep, in part generated by the EP → LHb → VTA circuit identified here, could contribute to stabilizing reactions to habitual aversive stimuli.

Substantia Innominata Glutamatergic Neurons Modulate Sevoflurane Anesthesia in Male Mice

BACKGROUND

Accumulated evidence suggests that brain regions that promote wakefulness also facilitate emergence from general anesthesia (GA). Glutamatergic neurons in the substantia innominata (SI) regulate motivation-related aversive, depressive, and aggressive behaviors relying on heightened arousal. Here, we hypothesize that glutamatergic neurons in the SI are also involved in the regulation of the effects of sevoflurane anesthesia.

METHODS

With a combination of fiber photometry, chemogenetic and optogenetic tools, behavioral tests, and cortical electroencephalogram recordings, we investigated whether and how SI glutamatergic neurons and their projections to the lateral hypothalamus (LH) regulate sevoflurane anesthesia in adult male mice.

RESULTS

Population activity of glutamatergic neurons in the SI gradually decreased upon sevoflurane-induced loss of consciousness (LOC) and slowly returned as soon as inhalation of sevoflurane discontinued before recovery of consciousness (ROC). Chemogenetic activation of SI glutamatergic neurons dampened the animals' sensitivity to sevoflurane exposure, prolonged induction time (mean ± standard deviation [SD]; 389 ± 67 seconds vs 458 ± 53 seconds; P = .047), and shortened emergence time (305 seconds, 95% confidence interval [CI], 242-369 seconds vs 207 seconds, 95% CI, 135-279 seconds; P = .004), whereas chemogenetic inhibition of these neurons facilitated sevoflurane anesthesia. Furthermore, optogenetic activation of SI glutamatergic neurons and their terminals in LH induced cortical activation and behavioral emergence from different depths of sevoflurane anesthesia.

CONCLUSIONS

Our study shows that SI glutamatergic neuronal activity facilitates emergence from sevoflurane anesthesia and provides evidence for the involvement of the SI-LH glutamatergic pathway in the regulation of consciousness during GA.

Elevation of hypothalamic ketone bodies induces a decrease in energy expenditures and an increase risk of metabolic disorder

OBJECTIVE

Ketone bodies (such as β-hydroxybutyrate or BHB) have been recently proposed as signals involved in brain regulation of energy homeostasis and obesity development. However, the precise role of ketone bodies sensing by the brain, and its impact on metabolic disorder development remains unclear. Nevertheless, partial deletion of the ubiquitous ketone bodies transporter MCT1 in mice (HE mice) results in diet-induced obesity resistance, while there is no alteration under normal chow diet. These results suggest that ketone bodies produced during the high fat diet would be important signals involved in obesity onset.

METHODS

In the present study we used a specific BHB infusion of the hypothalamus and analyzed the energy homeostasis of WT or HE mice fed a normal chow diet.

RESULTS

Our results indicate that high BHB levels sensed by the hypothalamus disrupt the brain regulation of energy homeostasis. This brain control dysregulation leads to peripheral alterations of energy expenditure mechanisms.

CONCLUSIONS

Altogether, the changes induced by high ketone bodies levels sensed by the brain increase the risk of obesity onset in mice.

Distinct lateral hypothalamic CaMKIIα neuronal populations regulate wakefulness and locomotor activity

For nearly a century, evidence has accumulated indicating that the lateral hypothalamus (LH) contains neurons essential to sustain wakefulness. While lesion or inactivation of LH neurons produces a profound increase in sleep, stimulation of inhibitory LH neurons promotes wakefulness. To date, the primary wake-promoting cells that have been identified in the LH are the hypocretin/orexin (Hcrt) neurons, yet these neurons have little impact on total sleep or wake duration across the 24-h period. Recently, we and others have identified other LH populations that increase wakefulness. In the present study, we conducted microendoscopic calcium imaging in the LH concomitant with EEG and locomotor activity (LMA) recordings and found that a subset of LH neurons that express Ca2+/calmodulin-dependent protein kinase IIα (CaMKIIα) are preferentially active during wakefulness. Chemogenetic activation of these neurons induced sustained wakefulness and greatly increased LMA even in the absence of Hcrt signaling. Few LH CaMKIIα-expressing neurons are hypocretinergic or histaminergic while a small but significant proportion are GABAergic. Ablation of LH inhibitory neurons followed by activation of the remaining LH CaMKIIα neurons induced similar levels of wakefulness but blunted the LMA increase. Ablated animals showed no significant changes in sleep architecture but both spontaneous LMA and high theta (8 to 10 Hz) power during wakefulness were reduced. Together, these findings indicate the existence of two subpopulations of LH CaMKIIα neurons: an inhibitory population that promotes locomotion without affecting sleep architecture and an excitatory population that promotes prolonged wakefulness even in the absence of Hcrt signaling.

Propofol-induced anesthesia involves the direct inhibition of glutamatergic neurons in the lateral hypothalamus

Propofol is the most widely used intravenous general anesthetic; however, the neuronal circuits that mediate its anesthetic effects are still poorly understood. Glutamatergic neurons in the lateral hypothalamus have been reported to be involved in maintenance of arousal and consciousness. Using Vglut2-Cre transgenic mice, we recorded this group of cells specifically and found that propofol can directly inhibit the glutamatergic neurons, and enhance inhibitory synaptic inputs on these cells, thereby reducing neuronal excitability. Through chemogenetic interventions, we found that inhibition of these neurons increased the duration of propofol-induced anesthesia and reduced movement in the animals after the recovery of right reflex. In contrast, activating this group of cells reduced the duration of propofol anesthesia and increased the animals' locomotor activity after the recovery of right reflex. These results suggest that propofol-induced anesthesia involves the inhibition of glutamatergic neurons in the lateral hypothalamus.

The regulation of the pedunculopontine tegmental nucleus in sleep-wake states

The pedunculopontine tegmental nucleus (PPTg) plays a vital role in sleep/wake states. There are three main kinds of heterogeneous neurons involved: cholinergic, glutamatergic, and gamma-aminobutyric acidergic (GABAergic) neurons. However, the precise roles of cholinergic, glutamatergic and GABAergic PPTg cell groups in regulating sleep-wake are unknown. Recent work suggests that the cholinergic, glutamatergic, and GABAergic neurons of the PPTg may activate the main arousal-promoting nucleus, thus exerting their wakefulness effects. We review the related projection pathways and functions of various neurons of the PPTg, especially the mechanisms of the PPTg in sleep-wake, thus providing new perspectives for research of sleep-wake mechanisms.

Hypothalamic supramammillary neurons that project to the medial septum modulate wakefulness in mice

The hypothalamic supramammillary nucleus (SuM) plays a crucial role in controlling wakefulness, but the downstream target regions participating in this control process remain unknown. Here, using circuit-specific fiber photometry and single-neuron electrophysiology together with electroencephalogram, electromyogram and behavioral recordings, we find that approximately half of SuM neurons that project to the medial septum (MS) are wake-active. Optogenetic stimulation of axonal terminals of SuM-MS projection induces a rapid and reliable transition to wakefulness from non-rapid-eye movement or rapid-eye movement sleep, and chemogenetic activation of SuMMS projecting neurons significantly increases wakefulness time and prolongs latency to sleep. Consistently, chemogenetically inhibiting these neurons significantly reduces wakefulness time and latency to sleep. Therefore, these results identify the MS as a functional downstream target of SuM and provide evidence for the modulation of wakefulness by this hypothalamic-septal projection.

Whole-brain monosynaptic inputs to lateral periaqueductal gray glutamatergic neurons in mice

OBJECTIVE

The lateral periaqueductal gray (LPAG), which mainly contains glutamatergic neurons, plays an important role in social responses, pain, and offensive and defensive behaviors. Currently, the whole-brain monosynaptic inputs to LPAG glutamatergic neurons are unknown. This study aims to explore the structural framework of the underlying neural mechanisms of LPAG glutamatergic neurons.

METHODS

This study used retrograde tracing systems based on the rabies virus, Cre-LoxP technology, and immunofluorescence analysis.

RESULTS

We found that 59 nuclei projected monosynaptic inputs to the LPAG glutamatergic neurons. In addition, seven hypothalamic nuclei, namely the lateral hypothalamic area (LH), lateral preoptic area (LPO), substantia innominata (SI), medial preoptic area, ventral pallidum, posterior hypothalamic area, and lateral globus pallidus, projected most densely to the LPAG glutamatergic neurons. Notably, we discovered through further immunofluorescence analysis that the inputs to the LPAG glutamatergic neurons were colocalized with several markers related to important neurological functions associated with physiological behaviors.

CONCLUSION

The LPAG glutamatergic neurons received dense projections from the hypothalamus, especially nuclei such as LH, LPO, and SI. The input neurons were colocalized with several markers of physiological behaviors, which show the pivotal role of glutamatergic neurons in the physiological behaviors regulation by LPAG.

Neural Control of REM Sleep and Motor Atonia: Current Perspectives

PURPOSE OF REVIEW

Since the formal discovery of rapid eye movement (REM) sleep in 1953, we have gained a vast amount of knowledge regarding the specific populations of neurons, their connections, and synaptic mechanisms regulating this stage of sleep and its accompanying features. This article discusses REM sleep circuits and their dysfunction, specifically emphasizing recent studies using conditional genetic tools.

RECENT FINDINGS

Sublaterodorsal nucleus (SLD) in the dorsolateral pons, especially the glutamatergic subpopulation in this region (SLDGlut), are shown to be indispensable for REM sleep. These neurons appear to be single REM generators in the rodent brain and may initiate and orchestrate all REM sleep events, including cortical and hippocampal activation and muscle atonia through distinct pathways. However, several cell groups in the brainstem and hypothalamus may influence SLDGlut neuron activity, thereby modulating REM sleep timing, amounts, and architecture. Damage to SLDGlut neurons or their projections involved in muscle atonia leads to REM behavior disorder, whereas the abnormal activation of this pathway during wakefulness may underlie cataplexy in narcolepsy. Despite some opposing views, it has become evident that SLDGlut neurons are the sole generators of REM sleep and its associated characteristics. Further research should prioritize a deeper understanding of their cellular, synaptic, and molecular properties, as well as the mechanisms that trigger their activation during cataplexy and make them susceptible in RBD.

Ventral pallidal glutamatergic neurons regulate wakefulness and emotion through separated projections

Insomnia is often comorbid with depression, but the underlying neuronal circuit mechanism remains elusive. Recently, we reported that GABAergic ventral pallidum (VP) neurons control wakefulness associated with motivation. However, whether and how other subtypes of VP neurons regulate arousal and emotion are largely unknown. Here, we report glutamatergic VP (VPVglut2) neurons control wakefulness and depressive-like behaviors. Physiologically, the calcium activity of VPVglut2 neurons was increased during both NREM sleep-to-wake transitions and depressive/anxiety-like behaviors in mice. Functionally, activation of VPVglut2 neurons was sufficient to increase wakefulness and induce anxiety/depressive-like behaviors, whereas inhibition attenuated both. Dissection of the circuit revealed that separated projections of VPVglut2 neurons to the lateral hypothalamus and lateral habenula promote arousal and depressive-like behaviors, respectively. Our results demonstrate a subtype of VP neurons is responsible for wakefulness and emotion through separated projections, and may provide new lines for the intervention of insomnia and depression in patients.

Medial septum glutamatergic neurons modulate nociception in chronic neuropathic pain via projections to lateral hypothalamus

The medial septum (MS) contributes in pain processing and regulation, especially concerning persistent nociception. However, the role of MS glutamatergic neurons in pain and the underlying neural circuit mechanisms in pain remain poorly understood. In this study, chronic constrictive injury of the sciatic nerve (CCI) surgery was performed to induce thermal and mechanical hyperalgesia in mice. The chemogenetic activation of MS glutamatergic neurons decreased pain thresholds in naïve mice. In contrast, inhibition or ablation of these neurons has improved nociception thresholds in naïve mice and relieved thermal and mechanical hyperalgesia in CCI mice. Anterograde viral tracing revealed that MS glutamatergic neurons had projections to the lateral hypothalamus (LH) and supramammillary nucleus (SuM). We further demonstrated that MS glutamatergic neurons regulate pain thresholds by projecting to LH but not SuM, because the inhibition of MS-LH glutamatergic projections suppressed pain thresholds in CCI and naïve mice, yet, optogenetic activation or inhibition of MS-SuM glutamatergic projections had no effect on pain thresholds in naïve mice. In conclusion, our results reveal that MS glutamatergic neurons play a significant role in regulating pain perception and decipher that MS glutamatergic neurons modulate nociception via projections to LH.

Activation of Ventral Pallidum CaMKIIa-Expressing Neurons Promotes Wakefulness

The ventral pallidum (VP) is involved in the regulation of a variety of behaviors such as motor, reward, and behavioral motivation, and the ability to perform these functions properly is dependent on a high degree of wakefulness. It is unknown whether VP CaMKIIa-expression (VP^CaMKIIa) neurons also have a role in sleep-wake regulation and related neuronal circuit mechanisms. In the present experiment, we first used in vivo fiber photometry to find the population activity of VP^CaMKIIa neurons which increased during the transitions from non-rapid-eye movement (NREM) sleep to wakefulness and NREM sleep to rapid-eye-movement (REM) sleep, with decreased during the transitions from wakefulness to NREM sleep. Then chemogenetic activation of VP^CaMKIIa neurons induced an increase in wakefulness that lasted for 2 h. Mice that were exposed to short-term optogenetic stimulation woke up quickly from stable NREM sleep, and long-term optogenetic stimulation maintained wakefulness. In addition, optogenetic activation of the axons of VP^CaMKIIa neurons in the lateral habenula (LHb) also facilitated the initiation and maintenance of wakefulness and mediated anxiety-like behavior. Finally, the method of chemogenetic inhibition was employed to suppress VP^CaMKIIa neurons, and yet, inhibition of VP^CaMKIIa neuronal activity did not result in an increase in NREM sleep and a decrease in wakefulness. Overall, our data illustrate that the activation of VP^CaMKIIa neurons is of great importance for promoting wakefulness.

Parabrachial nucleus astrocytes regulate wakefulness and isoflurane anesthesia in mice

Background: The parabrachial nucleus (PBN) is an important structure regulating the sleep-wake behavior and general anesthesia. Astrocytes in the central nervous system modulate neuronal activity and consequential behavior. However, the specific role of the parabrachial nucleus astrocytes in regulating the sleep-wake behavior and general anesthesia remains unclear. Methods: We used chemogenetic approach to activate or inhibit the activity of PBN astrocytes by injecting AAV-GFAabc1d-hM3Dq-eGFP or AAV-GFAabc1d-hM4Di-eGFP into the PBN. We investigated the effects of intraperitoneal injection of CNO or vehicle on the amount of wakefulness, NREM sleep and REM sleep in sleep-wake behavior, and on the time of loss of righting reflex, time of recovery of righting reflex, sensitivity to isoflurane, electroencephalogram (EEG) power spectrum and burst suppression ratio (BSR) in isoflurane anesthesia. Results: The activation of PBN astrocytes increased wakefulness amount for 4 h, while the inhibition of PBN astrocytes decreased total amount of wakefulness during the 3-hour post-injection period. Chemogenetic activation of PBN astrocytes decreased isoflurane sensitivity and shortened the emergence time from isoflurane-induced general anesthesia. Cortical EEG recordings revealed that PBN astrocyte activation decreased the EEG delta power and BSR during isoflurane anesthesia. Chemogenetic Inhibition of PBN astrocytes increased the EEG delta power and BSR during isoflurane anesthesia. Conclusion: PBN astrocytes are a key neural substrate regulating wakefulness and emergence from isoflurane anesthesia.

GABAergic neurons in the rostromedial tegmental nucleus are essential for rapid eye movement sleep suppression

Rapid eye movement (REM) sleep disturbances are prevalent in various psychiatric disorders. However, the neural circuits that regulate REM sleep remain poorly understood. Here, we found that in male mice, optogenetic activation of rostromedial tegmental nucleus (RMTg) GABAergic neurons immediately converted REM sleep to arousal and then initiated non-REM (NREM) sleep. Conversely, laser-mediated inactivation completely converted NREM to REM sleep and prolonged REM sleep duration. The activity of RMTg GABAergic neurons increased to a high discharge level at the termination of REM sleep. RMTg GABAergic neurons directly converted REM sleep to wakefulness and NREM sleep via inhibitory projections to the laterodorsal tegmentum (LDT) and lateral hypothalamus (LH), respectively. Furthermore, LDT glutamatergic neurons were responsible for the REM sleep-wake transitions following photostimulation of the RMTg^GABA-LDT circuit. Thus, RMTg GABAergic neurons are essential for suppressing the induction and maintenance of REM sleep.

Regulation of wakefulness by astrocytes in the lateral hypothalamus

The lateral hypothalamus (LH) is an important brain region mediating sleep-wake behavior. Recent evidence has shown that central nervous system astrocytes modulate the activity of adjacent neurons and participate in several physiological functions. However, the role of LH astrocytes in sleep-wake regulation remains unclear. Here, using synchronous recording of electroencephalogram/electromyogram in mice and calcium signals in LH astrocytes, we show that the activity of LH astrocytes is significantly increased during non-rapid eye movement (NREM) sleep-to-wake transitions and decreased during wake-to-NREM sleep transitions. Chemogenetic activation of LH astrocytes potently promotes wakefulness and maintains long-term arousal, while chemogenetic inhibition of LH astrocytes decreases the total amount of wakefulness in mice. Moreover, by combining chemogenetics with fiber photometry, we show that activation of LH astrocytes significantly increases the calcium signals of adjacent neurons, especially among GABAergic neurons. Taken together, our results clearly illustrate that LH astrocytes are a key neural substrate regulating wakefulness and encode this behavior through surrounding GABAergic neurons. Our findings raise the possibility that overactivity of LH astrocytes may be an underlying mechanism of clinical sleep disorders.

Role of the paraventricular nucleus of the hypothalamus in sleep–wake regulation

The paraventricular nucleus of the hypothalamus (PVH) acts as a cohesive functional unit that regulates neuroendocrine and autonomic function, complex behavior, and negative emotions after stress. However, how the PVH integrates arousal with these biological functions has only recently been explored. Clinical reports, combined with neurotoxic lesioning, immunochemistry, neuronal activity recordings, and the polysomnographic analyses of genetically modified animals, have revealed that the PVH is important for the control of wakefulness. Here, we review emerging anatomical and neural mechanisms for sleep–wake regulation in the PVH to support its essential role in the promotion and maintenance of wakefulness.

Roles of Neuropeptides in Sleep-Wake Regulation

Sleep and wakefulness are basic behavioral states that require coordination between several brain regions, and they involve multiple neurochemical systems, including neuropeptides. Neuropeptides are a group of peptides produced by neurons and neuroendocrine cells of the central nervous system. Like traditional neurotransmitters, neuropeptides can bind to specific surface receptors and subsequently regulate neuronal activities. For example, orexin is a crucial component for the maintenance of wakefulness and the suppression of rapid eye movement (REM) sleep. In addition to orexin, melanin-concentrating hormone, and galanin may promote REM sleep. These results suggest that neuropeptides play an important role in sleep–wake regulation. These neuropeptides can be divided into three categories according to their effects on sleep–wake behaviors in rodents and humans. (i) Galanin, melanin-concentrating hormone, and vasoactive intestinal polypeptide are sleep-promoting peptides. It is also noticeable that vasoactive intestinal polypeptide particularly increases REM sleep. (ii) Orexin and neuropeptide S have been shown to induce wakefulness. (iii) Neuropeptide Y and substance P may have a bidirectional function as they can produce both arousal and sleep-inducing effects. This review will introduce the distribution of various neuropeptides in the brain and summarize the roles of different neuropeptides in sleep–wake regulation. We aim to lay the foundation for future studies to uncover the mechanisms that underlie the initiation, maintenance, and end of sleep–wake states.