Silencing of Cholinergic Basal Forebrain Neurons Using Archaerhodopsin Prolongs Slow-Wave Sleep in Mice

REM sleep is associated with the volume of the cholinergic basal forebrain in aMCI individuals

BACKGROUND

Rapid-eye movement (REM) sleep highly depends on the activity of cholinergic basal forebrain (BF) neurons and is reduced in Alzheimer's disease. Here, we investigated the associations between the volume of BF nuclei and REM sleep characteristics, and the impact of cognitive status on these links, in late middle-aged and older participants.

METHODS

Thirty-one cognitively healthy controls (66.8 ± 7.2 years old, 13 women) and 31 participants with amnestic Mild Cognitive Impairment (aMCI) (68.3 ± 8.8 years old, 7 women) were included in this cross-sectional study. All participants underwent polysomnography, a comprehensive neuropsychological assessment and Magnetic Resonance Imaging examination. REM sleep characteristics (i.e., percentage, latency and efficiency) were derived from polysomnographic recordings. T1-weighted images were preprocessed using CAT12 and the DARTEL algorithm, and we extracted the gray matter volume of BF regions of interest using a probabilistic atlas implemented in the JuBrain Anatomy Toolbox. Multiple linear regressions were performed between the volume of BF nuclei and REM sleep characteristics controlling for age, sex and total intracranial volume, in the whole cohort and in subgroups stratified by cognitive status.

RESULTS

In the whole sample, lower REM sleep percentage was significantly associated to lower nucleus basalis of Meynert (Ch4) volume (β = 0.32, p = 0.009). When stratifying the cohort according to cognitive status, lower REM sleep percentage was significantly associated to both lower Ch4 (β = 0.48, p = 0.012) and total BF volumes (β = 0.44, p = 0.014) in aMCI individuals, but not in cognitively unimpaired participants. No significant associations were observed between the volume of the BF and wake after sleep onset or non-REM sleep variables.

DISCUSSION

These results suggest that REM sleep disturbances may be an early manifestation of the degeneration of the BF cholinergic system before the onset of dementia, especially in participants with mild memory deficits.

Basal Forebrain Modulation of Olfactory Coding In Vivo

Sensory perception is one of the most fundamental brain functions, allowing individuals to properly interact and adapt to a constantly changing environment. This process requires the integration of bottom-up and topdown neuronal activity, which is centrally mediated by the basal forebrain, a brain region that has been linked to a series of cognitive processes such as attention and alertness. Here, we review the latest research using optogenetic approaches in rodents and in vivo electrophysiological recordings that are shedding light on the role of this region, in regulating olfactory processing and decisionmaking. Moreover, we summarize evidence highlighting the anatomical and physiological differences in the basal forebrain of individuals with autism spectrum disorder, which could underpin the sensory perception abnormalities they exhibit, and propose this research line as a potential opportunity to understand the neurobiological basis of this disorder.

Sleep-Wake Neurobiology

Sleep and wakefulness are complex, tightly regulated behaviors that occur in virtually all animals. With recent exciting developments in neuroscience methodologies such as optogenetics, chemogenetics, and cell-specific calcium imaging technology, researchers can advance our understanding of how discrete neuronal groups precisely modulate states of sleep and wakefulness. In this chapter, we provide an overview of key neurotransmitter systems, neurons, and circuits that regulate states of sleep and wakefulness. We also describe long-standing models for the regulation of sleep/wake and non-rapid eye movement/rapid eye movement cycling. We contrast previous knowledge derived from classic approaches such as brain stimulation, lesions, cFos expression, and single-unit recordings, with emerging data using the newest technologies. Our understanding of neural circuits underlying the regulation of sleep and wakefulness is rapidly evolving, and this knowledge is critical for our field to elucidate the enigmatic function(s) of sleep.

Muscarinic Acetylcholine Receptors Chrm1 and Chrm3 Are Essential for REM Sleep

Sleep regulation involves interdependent signaling among specialized neurons in distributed brain regions. Although acetylcholine promotes wakefulness and rapid eye movement (REM) sleep, it is unclear whether the cholinergic pathway is essential (i.e., absolutely required) for REM sleep because of redundancy from neural circuits to molecules. First, we demonstrate that synaptic inhibition of TrkA+ cholinergic neurons causes a severe short-sleep phenotype and that sleep reduction is mostly attributable to a shortened sleep duration in the dark phase. Subsequent comprehensive knockout of acetylcholine receptor genes by the triple-target CRISPR method reveals that a similar short-sleep phenotype appears in the knockout of two Gq-type acetylcholine receptors Chrm1 and Chrm3. Strikingly, Chrm1 and Chrm3 double knockout chronically diminishes REM sleep to an almost undetectable level. These results suggest that muscarinic acetylcholine receptors, Chrm1 and Chrm3, are essential for REM sleep.

Molecular Mechanisms of REM Sleep

Rapid-eye movement (REM) sleep is a paradoxical sleep state characterized by brain activity similar to wakefulness, rapid-eye-movement, and lack of muscle tone. REM sleep is a fundamental brain function, evolutionary conserved across species, including human, mouse, bird, and even reptiles. The physiological importance of REM sleep is highlighted by severe sleep disorders incurred by a failure in REM sleep regulation. Despite the intense interest in the mechanism of REM sleep regulation, the molecular machinery is largely left to be investigated. In models of REM sleep regulation, acetylcholine has been a pivotal component. However, even newly emerged techniques such as pharmacogenetics and optogenetics have not fully clarified the function of acetylcholine either at the cellular level or neural-circuit level. Recently, we discovered that the G_q type muscarinic acetylcholine receptor genes, Chrm1 and Chrm3, are essential for REM sleep. In this review, we develop the perspective of current knowledge on REM sleep from a molecular viewpoint. This should be a starting point to clarify the molecular and cellular machinery underlying REM sleep regulation and will provide insights to explore physiological functions of REM sleep and its pathological roles in REM-sleep-related disorders such as depression, PTSD, and neurodegenerative diseases.

3D Reconstruction of the Neurovascular Unit Reveals Differential Loss of Cholinergic Innervation in the Cortex and Hippocampus of the Adult Mouse Brain

Increasing evidence supports a role for cerebrovasculature dysfunction in the etiology of Alzheimer’s disease (AD). Blood vessels in the brain are composed of a collection of cells and acellular material that comprise the neurovascular unit (NVU). The NVU in the hippocampus and cortex receives innervation from cholinergic neurons that originate in the basal forebrain. Death of these neurons and their nerve fibers is an early feature of AD. However, the effect of the loss of cholinergic innervation on the NVU is not well characterized. The purpose of this study was to evaluate the effect of the loss of cholinergic innervation of components of the NVU at capillaries, arteries and veins in the hippocampus and cortex. Adult male C57BL/6 mice received an intracerebroventricular injection of the immunotoxin p75NTR mu-saporin to induce the loss of cholinergic neurons. Quadruple labeling immunohistochemistry and 3D reconstruction were carried out to characterize specific points of contact between cholinergic fibers and collagen IV, smooth muscle cells and astrocyte endfeet. Innate differences were observed between vessels of the hippocampus and cortex of control mice, including a greater amount of cholinergic contact with perivascular astrocytes in hippocampal capillaries and a thicker basement membrane in hippocampal veins. Saporin treatment induced a loss of cholinergic innervation at the arterial basement membrane and smooth muscle cells of both the hippocampus and the cortex. In the cortex, there was an additional loss of innervation at the astrocytic endfeet. The current results suggest that cortical arteries are more strongly affected by cholinergic denervation than arteries in the hippocampus. This regional variation may have implications for the etiology of the vascular pathology that develops in AD.

Basal forebrain subcortical projections

The basal forebrain (BF) contains at least three distinct populations of neurons (cholinergic, glutamatergic, and GABA-ergic) across its different regions (medial septum, diagonal band, magnocellular preoptic area, and substantia innominata). Much attention has focused on the BF's ascending projections to cortex, but less is known about descending projections to subcortical regions. Given the neurochemical and anatomical heterogeneity of the BF, we used conditional anterograde tracing to map the patterns of subcortical projections from multiple BF regions and neurochemical cell types using mice that express Cre recombinase only in cholinergic, glutamatergic, or GABAergic neurons. We confirmed that different BF regions innervate distinct subcortical targets, with more subcortical projections arising from neurons in the caudal and lateral BF (substantia innominata and magnocellular preoptic area). Additionally, glutamatergic and GABAergic BF neurons have distinct patterns of descending projections, while cholinergic descending projections are sparse. Considering the intensity of glutamatergic and GABAergic descending projections, the BF likely acts through subcortical targets to promote arousal, motivation, and other behaviors.

Cholinergic system in sleep regulation of emotion and motivation

Sleep profoundly regulates our emotional and motivational state of mind. Human brain imaging and animal model studies are providing initial insights on the underlying neural mechanisms. Here, we focus on the brain cholinergic system, including cholinergic neurons in the basal forebrain, ventral striatum, habenula, and brain stem. Although much is learned about cholinergic regulations of emotion and motivation, less is known on their interactions with sleep. Specifically, we present an anatomical framework that highlights cholinergic signaling in the integrated reward-arousal/sleep circuitry, and identify the knowledge gaps on the potential roles of cholinergic system in sleep-mediated regulation of emotion and motivation. Sleep impacts every aspect of brain functions. It not only restores cognitive control, but also retunes emotional and motivational regulation [1]. Sleep disturbance is a comorbidity and sometimes a predicting factor for various psychiatric diseases including major depressive disorder, anxiety, post-traumatic stress disorder, and drug addiction [2-9]. Although it is well recognized that sleep prominently shapes emotional and motivational regulation, the underlying neural mechanisms remain elusive. The brain cholinergic system is essential for a diverse variety of functions including cognition, learning and memory, sensory and motor processing, sleep and arousal, reward processing, and emotion regulation [10-14]. Although cholinergic functions in cognition, learning and memory, motor control, and sleep and arousal have been well established, its interaction with sleep in regulating emotion and motivation has not been extensively studied. Here we review current evidence on sleep-mediated regulation of emotion and motivation, and reveal knowledge gaps on potential contributions from the cholinergic system.

Optogenetic Dissection of Sleep-Wake States In Vitro and In Vivo

Optogenetic tools have revolutionized insights into the fundamentals of brain function. This is particularly true for our current understanding of sleep-wake regulation and sleep rhythms. This is illustrated here through a comprehensive and step-by-step review over the major brain areas involved in transitions between sleep and wake states and in sleep rhythmogenesis.

The Effect of Donepezil on Arousal Threshold and Apnea-Hypopnea Index. A Randomized, Double-Blind, Cross-Over Study

RATIONALE

Obstructive sleep apnea (OSA) has multiple pathophysiological causes. A low respiratory arousal threshold (ArTh) and a high loop gain (unstable ventilatory control) can contribute to recurrent respiratory events in patients with OSA. Prior studies have shown that donepezil, an acetylcholinesterase inhibitor, might improve OSA, but the mechanism is unknown.

OBJECTIVES

To determine whether a single dose of donepezil lowers the apnea-hypopnea index by modulating the ArTh or loop gain.

METHODS

In this randomized, double-blind, crossover trial, 41 subjects with OSA underwent two polysomnograms with ArTh and loop gain evaluated, during which 10 mg of donepezil or placebo was administered.

MEASUREMENTS AND MAIN RESULTS

Compared with placebo, sleep efficiency (77.2 vs. 71.9%; P = 0.015) and total sleep time decreased with donepezil (372 vs. 351 min; P = 0.004). No differences were found in apnea-hypopnea index (51.8 vs. 50.0 events/h; P = 0.576) or nadir oxygen saturation as determined by pulse oximetry (80.3 vs. 81.1%; P = 0.241) between placebo and donepezil, respectively. ArTh was not significantly changed (-18.9 vs. -18.0 cm H₂O; P = 0.394) with donepezil. As a whole group, loop gain (ventilatory response to a 1-cycle/min disturbance) did not change significantly (P = 0.089).

CONCLUSIONS

A single dose of donepezil did not appear to affect the overall severity of OSA in this patient group, and no consistent effects on ArTh or loop gain were observed. Donepezil may have minor effects on sleep architecture. Clinical trial registered with www.clinicaltrials.gov (NCT02264353).

A Genetically Defined Circuit for Arousal from Sleep during Hypercapnia

The precise neural circuitry that mediates arousal during sleep apnea is not known. We previously found that glutamatergic neurons in the external lateral parabrachial nucleus (PBel) play a critical role in arousal to elevated CO2 or hypoxia. Because many of the PBel neurons that respond to CO2 express calcitonin gene-related peptide (CGRP), we hypothesized that CGRP may provide a molecular identifier of the CO2 arousal circuit. Here, we report that selective chemogenetic and optogenetic activation of PBel^CGRP neurons caused wakefulness, whereas optogenetic inhibition of PBel^CGRP neurons prevented arousal to CO2, but not to an acoustic tone or shaking. Optogenetic inhibition of PBel^CGRP terminals identified a network of forebrain sites under the control of a PBel^CGRP switch that is necessary to arouse animals from hypercapnia. Our findings define a novel cellular target for interventions that may prevent sleep fragmentation and the attendant cardiovascular and cognitive consequences seen in obstructive sleep apnea. VIDEO ABSTRACT.

Neural Circuitry of Wakefulness and Sleep

Sleep remains one of the most mysterious yet ubiquitous animal behaviors. We review current perspectives on the neural systems that regulate sleep/wake states in mammals and the circadian mechanisms that control their timing. We also outline key models for the regulation of rapid eye movement (REM) sleep and non-REM sleep, how mutual inhibition between specific pathways gives rise to these distinct states, and how dysfunction in these circuits can give rise to sleep disorders.

The menagerie of the basal forebrain: how many (neural) species are there, what do they look like, how do they behave and who talks to whom?

The diverse cell-types of the basal forebrain control sleep-wake states, cortical activity and reward processing. Large, slow-firing, cholinergic neurons suppress cortical delta activity and promote cortical plasticity in response to reinforcers. Large, fast-firing, cortically-projecting GABAergic neurons promote wakefulness and fast cortical activity. In particular, parvalbumin/GABAergic neurons promote neocortical gamma band activity. Conversely, excitation of slower-firing somatostatin/GABAergic neurons promotes sleep through inhibition of cortically-projecting neurons. Activation of glutamatergic neurons promotes wakefulness, likely by exciting other cortically-projecting neurons. Similarly, cholinergic neurons indirectly promote wakefulness by excitation of wake-promoting, cortically-projecting GABAergic neurons and/or inhibition of sleep-promoting somatostatin/GABAergic neurons. Both glia and neurons increase the levels of adenosine during prolonged wakefulness. Adenosine presynaptically inhibits glutamatergic inputs to wake-promoting cholinergic and GABAergic/parvalbumin neurons, promoting sleep.

Descending projections from the basal forebrain to the orexin neurons in mice

The orexin (hypocretin) neurons play an essential role in promoting arousal, and loss of the orexin neurons results in narcolepsy, a condition characterized by chronic sleepiness and cataplexy. The orexin neurons excite wake-promoting neurons in the basal forebrain (BF), and a reciprocal projection from the BF back to the orexin neurons may help promote arousal and motivation. The BF contains at least three different cell types (cholinergic, glutamatergic, and γ-aminobutyric acid (GABA)ergic neurons) across its different regions (medial septum, diagonal band, magnocellular preoptic area, and substantia innominata). Given the neurochemical and anatomical heterogeneity of the BF, we mapped the pattern of BF projections to the orexin neurons across multiple BF regions and neuronal types. We performed conditional anterograde tracing using mice that express Cre recombinase only in neurons producing acetylcholine, glutamate, or GABA. We found that the orexin neurons are heavily apposed by axon terminals of glutamatergic and GABAergic neurons of the substantia innominata (SI) and magnocellular preoptic area, but there was no innervation by the cholinergic neurons. Channelrhodopsin-assisted circuit mapping (CRACM) demonstrated that glutamatergic SI neurons frequently form functional synapses with the orexin neurons, but, surprisingly, functional synapses from SI GABAergic neurons were rare. Considering their strong reciprocal connections, BF and orexin neurons likely work in concert to promote arousal, motivation, and other behaviors. J. Comp. Neurol. 525:1668-1684, 2017. © 2016 Wiley Periodicals, Inc.

Selective activation of a few limbic structures during paradoxical (REM) sleep by the claustrum and the supramammillary nucleus: evidence and function

We review here classical and recent knowledge on the state of the cortex during paradoxical (REM) sleep (PS). Recent data indicate that only a few limbic cortical structures including the anterior cingulate, retrosplenial and medial entorhinal cortices and the dentate gyrus are strongly activated during PS. In contrast, most of the other cortices including the somatosensory ones are rather deactivated during PS. Further, recent results suggest that tonic activation of limbic cortical neurons during PS is due to projections from glutamate neurons of the claustrum and GABA/glutamate neurons of the supramammillary nucleus while their pacing with theta is induced by projections from GABAergic neurons of the medial septum. The limbic structures activated during PS have all been implicated in spatial memory and it is therefore likely that such activation is crucial for memory consolidation.

Neuronal substrates for initiation, maintenance, and structural organization of sleep/wake states

Animals continuously alternate between sleep and wake states throughout their life. The daily organization of sleep and wakefulness is orchestrated by circadian, homeostatic, and motivational processes. Over the last decades, much progress has been made toward determining the neuronal populations involved in sleep/wake regulation. Here, we will discuss how the application of advanced in vivo tools for cell type–specific manipulations now permits the functional interrogation of different features of sleep/wake state regulation: initiation, maintenance, and structural organization. We will specifically focus on recent studies examining the roles of wake-promoting neuronal populations.

Whole-Brain Monosynaptic Afferent Inputs to Basal Forebrain Cholinergic System

The basal forebrain cholinergic system (BFCS) robustly modulates many important behaviors, such as arousal, attention, learning and memory, through heavy projections to cortex and hippocampus. However, the presynaptic partners governing BFCS activity still remain poorly understood. Here, we utilized a recently developed rabies virus-based cell-type-specific retrograde tracing system to map the whole-brain afferent inputs of the BFCS. We found that the BFCS receives inputs from multiple cortical areas, such as orbital frontal cortex, motor cortex, and insular cortex, and that the BFCS also receives dense inputs from several subcortical nuclei related to motivation and stress, including lateral septum, central amygdala, paraventricular nucleus of hypothalamus, dorsal raphe, and parabrachial nucleus. Interestingly, we found that the BFCS receives inputs from the olfactory areas and the entorhinal–hippocampal system. These results greatly expand our knowledge about the connectivity of the mouse BFCS and provided important preliminary indications for future exploration of circuit function.

Cholinergic Neurons in the Basal Forebrain Promote Wakefulness by Actions on Neighboring Non-Cholinergic Neurons: An Opto-Dialysis Study

Understanding the control of sleep-wake states by the basal forebrain (BF) poses a challenge due to the intermingled presence of cholinergic, GABAergic, and glutamatergic neurons. All three BF neuronal subtypes project to the cortex and are implicated in cortical arousal and sleep-wake control. Thus, nonspecific stimulation or inhibition studies do not reveal the roles of these different neuronal types. Recent studies using optogenetics have shown that "selective" stimulation of BF cholinergic neurons increases transitions between NREM sleep and wakefulness, implicating cholinergic projections to cortex in wake promotion. However, the interpretation of these optogenetic experiments is complicated by interactions that may occur within the BF. For instance, a recent in vitro study from our group found that cholinergic neurons strongly excite neighboring GABAergic neurons, including the subset of cortically projecting neurons, which contain the calcium-binding protein, parvalbumin (PV) (Yang et al., 2014). Thus, the wake-promoting effect of "selective" optogenetic stimulation of BF cholinergic neurons could be mediated by local excitation of GABA/PV or other non-cholinergic BF neurons. In this study, using a newly designed opto-dialysis probe to couple selective optical stimulation with simultaneous in vivo microdialysis, we demonstrated that optical stimulation of cholinergic neurons locally increased acetylcholine levels and increased wakefulness in mice. Surprisingly, the enhanced wakefulness caused by cholinergic stimulation was abolished by simultaneous reverse microdialysis of cholinergic receptor antagonists into BF. Thus, our data suggest that the wake-promoting effect of cholinergic stimulation requires local release of acetylcholine in the basal forebrain and activation of cortically projecting, non-cholinergic neurons, including the GABAergic/PV neurons.

SIGNIFICANCE STATEMENT

Optogenetics is a revolutionary tool to assess the roles of particular groups of neurons in behavioral functions, such as control of sleep and wakefulness. However, the interpretation of optogenetic experiments requires knowledge of the effects of stimulation on local neurotransmitter levels and effects on neighboring neurons. Here, using a novel "opto-dialysis" probe to couple optogenetics and in vivo microdialysis, we report that optical stimulation of basal forebrain (BF) cholinergic neurons in mice increases local acetylcholine levels and wakefulness. Reverse microdialysis of cholinergic antagonists within BF prevents the wake-promoting effect. This important result challenges the prevailing dictum that BF cholinergic projections to cortex directly control wakefulness and illustrates the utility of "opto-dialysis" for dissecting the complex brain circuitry underlying behavior.