Substance P and the neurokinin-1 receptor regulate electroencephalogram non-rapid eye movement sleep slow-wave activity locally

What is sleep exactly? Global and local modulations of sleep oscillations all around the clock

Wakefulness, non-rapid eye-movement (NREM) and rapid eye-movement (REM) sleep differ from each other along three dimensions: behavioral, phenomenological, physiological. Although these dimensions often fluctuate in step, they can also dissociate. The current paradigm that views sleep as made of global NREM and REM states fail to account for these dissociations. This conundrum can be dissolved by stressing the existence and significance of the local regulation of sleep. We will review the evidence in animals and humans, healthy and pathological brains, showing different forms of local sleep and the consequences on behavior, cognition, and subjective experience. Altogether, we argue that the notion of local sleep provides a unified account for a host of phenomena: dreaming in REM and NREM sleep, NREM and REM parasomnias, intrasleep responsiveness, inattention and mind wandering in wakefulness. Yet, the physiological origins of local sleep or its putative functions remain unclear. Exploring further local sleep could provide a unique and novel perspective on how and why we sleep.

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.

Neuroinflammation, Sleep, and Circadian Rhythms

Molecules involved in innate immunity affect sleep and circadian oscillators and vice versa. Sleep-inducing inflammatory molecules are activated by increased waking activity and pathogens. Pathologies that alter inflammatory molecules, such as traumatic brain injury, cancer, cardiovascular disease, and stroke often are associated with disturbed sleep and electroencephalogram power spectra. Moreover, sleep disorders, such as insomnia and sleep disordered breathing, are associated with increased dysregulation of inflammatory processes. Inflammatory molecules in both the central nervous system and periphery can alter sleep. Inflammation can also modulate cerebral vascular hemodynamics which is associated with alterations in electroencephalogram power spectra. However, further research is needed to determine the interactions of sleep regulatory inflammatory molecules and circadian clocks. The purpose of this review is to: 1) describe the role of the inflammatory cytokines interleukin-1 beta and tumor necrosis factor-alpha and nucleotide-binding domain and leucine-rich repeat protein-3 inflammasomes in sleep regulation, 2) to discuss the relationship between the vagus nerve in translating inflammatory signals between the periphery and central nervous system to alter sleep, and 3) to present information about the relationship between cerebral vascular hemodynamics and the electroencephalogram during sleep.

Somatostatin+/nNOS+ neurons are involved in delta electroencephalogram activity and cortical-dependent recognition memory

Slow-wave activity (SWA) is an oscillatory neocortical activity occurring in the electroencephalogram delta (δ) frequency range (~0.5-4 Hz) during nonrapid eye movement sleep. SWA is a reliable indicator of sleep homeostasis after acute sleep loss and is involved in memory processes. Evidence suggests that cortical neuronal nitric oxide synthase (nNOS) expressing neurons that coexpress somatostatin (SST) play a key role in regulating SWA. However, previous studies lacked selectivity in targeting specific types of neurons that coexpress nNOS-cells which are activated in the cortex after sleep loss. We produced a mouse model that knocks out nNOS expression in neurons that coexpress SST throughout the cortex. Mice lacking nNOS expression in SST positive neurons exhibited significant impairments in both homeostatic low-δ frequency range SWA production and a recognition memory task that relies on cortical input. These results highlight that SST+/nNOS+ neurons are involved in the SWA homeostatic response and cortex-dependent recognition memory.

Sleep State Dependence of Optogenetically evoked Responses in Neuronal Nitric Oxide Synthase-positive Cells of the Cerebral Cortex

Slow-wave activity (SWA) in the electroencephalogram during slow-wave sleep (SWS) varies as a function of sleep-wake history. A putative sleep-active population of neuronal nitric oxide synthase (nNOS)-containing interneurons in the cerebral cortex, defined as such by the expression of Fos in animals euthanized after protracted deep sleep, may be a local regulator of SWA. We investigated whether electrophysiological responses to activation of these cells are consistent with their role of a local regulator of SWA. Using a Cre/loxP strategy, we targeted the population of nNOS interneurons to express the light-activated cation channel Channelrhodopsin2 and the histological marker tdTomato in mice. We then performed histochemical and optogenetic studies in these transgenic mice. Our studies provided histochemical evidence of transgene expression and electrophysiological evidence that the cerebral cortex was responsive to optogenetic manipulation of these cells in both anesthetized and behaving mice. Optogenetic stimulation of the cerebral cortex of animals expressing Channelrhodopsin2 in nNOS interneurons triggered an acute positive deflection of the local field potential that was followed by protracted oscillatory events only during quiet wake and slow wave sleep. The response during wake was maximal when the electroencephalogram (EEG) was in a negative polarization state and abolished when the EEG was in a positive polarization state. Since the polarization state of the EEG is a manifestation of slow-wave oscillations in the activity of underlying pyramidal neurons between the depolarized (LFP negative) and hyperpolarized (LFP positive) states, these data indicate that sleep-active cortical neurons expressing nNOS function in sleep slow-wave physiology.

Sleep-inducing effect of substance P-cholera toxin A subunit in mice

Evidence indicates that the neuropeptide substance P (SP) can act through neurokinin receptors to alter sleep and/or non-rapid eye movement (NREM) sleep slow-wave activity. Consequently, drugs acting on SP receptors could potentially be used as a novel treatment for sleep-related disorders. In the present study, we used SP conjugated with cholera toxin A subunit (SP-CTA), which enhances its duration of activity on SP receptor-expressing cells, to determine the effects of selectively activating SP receptor-expressing brain cells on sleep regulation in mice. Herein, we found that intracerebroventricular administration of SP-CTA enhanced amounts of NREM sleep which was highly fragmented. This result suggests that the activation of SP receptor-expressing cells in the brain can produce not only arousal effects as shown in previous studies but also sleep-inducing effects.

Postprandial sleep mechanics in Drosophila

Food consumption is thought to induce sleepiness. However, little is known about how postprandial sleep is regulated. Here, we simultaneously measured sleep and food intake of individual flies and found a transient rise in sleep following meals. Depending on the amount consumed, the effect ranged from slightly arousing to strongly sleep inducing. Postprandial sleep was positively correlated with ingested volume, protein, and salt—but not sucrose—revealing meal property-specific regulation. Silencing of leucokinin receptor (Lkr) neurons specifically reduced sleep induced by protein consumption. Thermogenetic stimulation of leucokinin (Lk) neurons decreased whereas Lk downregulation by RNAi increased postprandial sleep, suggestive of an inhibitory connection in the Lk-Lkr circuit. We further identified a subset of non-leucokininergic cells proximal to Lkr neurons that rhythmically increased postprandial sleep when silenced, suggesting that these cells are cyclically gated inhibitory inputs to Lkr neurons. Together, these findings reveal the dynamic nature of postprandial sleep.

DOI:http://dx.doi.org/10.7554/eLife.19334.001

Many of us have experienced feelings of sleepiness after a large meal. However, there is little scientific evidence that this “food coma” effect is real. If it is, it may vary between individuals, or depend on the type of food consumed. This variability makes it difficult to study the causes of post-meal sleepiness.

Murphy et al. have now developed a system that can measure fruit fly sleep and feeding behavior at the same time. Recordings using this system reveal that after a meal, flies sleep more for a short period before returning to a normal state of wakefulness. The sleep period lasts around 20-40 minutes, with flies that ate more generally sleeping more.

Further investigation revealed that salty or protein-rich foods promote sleep, whereas sugary foods do not. By using genetic tools to turn on and off neurons in the fly brain, Murphy et al. identified a number of brain circuits that play a role in controlling post-meal sleepiness. Some of these respond specifically to the consumption of protein. Others are sensitive to the fruit fly’s internal clock, reducing post-meal sleepiness only around dusk. Thus, post-meal sleepiness can be regulated in a number of different ways.

Future experiments are now needed to explore the genes and circuits that enable meal size and the protein or salt content of food to drive sleep. In nature, sleep is likely a vulnerable state for animals. Thus, another challenge will be to uncover why post-meal sleep is important. Does sleeping after a meal boost digestion? Or might it help animals to form memories about a food source, making it easier to find similar food in the future?

DOI:http://dx.doi.org/10.7554/eLife.19334.002

Functions and Mechanisms of Sleep

Sleep is a complex physiological process that is regulated globally, regionally, and locally by both cellular and molecular mechanisms. It occurs to some extent in all animals, although sleep expression in lower animals may be co-extensive with rest. Sleep regulation plays an intrinsic part in many behavioral and physiological functions. Currently, all researchers agree there is no single physiological role sleep serves. Nevertheless, it is quite evident that sleep is essential for many vital functions including development, energy conservation, brain waste clearance, modulation of immune responses, cognition, performance, vigilance, disease, and psychological state. This review details the physiological processes involved in sleep regulation and the possible functions that sleep may serve. This description of the brain circuitry, cell types, and molecules involved in sleep regulation is intended to further the reader’s understanding of the functions of sleep.