Cortical neuronal activity does not regulate sleep homeostasis

Sleep Phylogeny: Have We Jumped the Shark?

Sleep is considered to be ubiquitous in the animal kingdom. Scientists in part believe this because sleep is increasingly defined solely by behavioral measures. This has resulted in an astonishing expansion of species reported to sleep. But have we gone too far? Have scientists jumped the shark? This idiom refers to when an idea has exhausted its core intent and creates ideas that are discordant with its original purpose. In this commentary, I discuss what I see as an emerging problem in how we define sleep and the attendant implications.

Linking Hypothermia and Altered Metabolism with TrkB Activation

Many mechanisms have been proposed to explain acute antidepressant drug-induced activation of TrkB neurotrophin receptors, but several questions remain. In a series of pharmacological experiments, we observed that TrkB activation induced by antidepressants and several other drugs correlated with sedation, and most importantly, coinciding hypothermia. Untargeted metabolomics of pharmacologically dissimilar TrkB activating treatments revealed effects on shared bioenergetic targets involved in adenosine triphosphate (ATP) breakdown and synthesis, demonstrating a common perturbation in metabolic activity. Both activation of TrkB signaling and hypothermia were recapitulated by administration of inhibitors of glucose and lipid metabolism, supporting a close relationship between metabolic inhibition and neurotrophic signaling. Drug-induced TrkB phosphorylation was independent of electroencephalography slow-wave activity and remained unaltered in knock-in mice with the brain-derived neurotrophic factor (BDNF) Val66Met allele, which have impaired activity-dependent BDNF release, alluding to an activation mechanism independent from BDNF and neuronal activity. Instead, we demonstrated that the active maintenance of body temperature prevents activation of TrkB and other targets associated with antidepressants, including p70S6 kinase downstream of the mammalian target of rapamycin (mTOR) and glycogen synthase kinase 3β (GSK3β). Increased TrkB, GSK3β, and p70S6K phosphorylation was also observed during recovery sleep following sleep deprivation, when a physiological temperature drop is known to occur. Our results suggest that the changes in bioenergetics and thermoregulation are causally connected to TrkB activation and may act as physiological regulators of signaling processes involved in neuronal plasticity.

Melatonin supplementation in the subacute phase after ischemia alleviates postischemic sleep disturbances in rats

BACKGROUND

Sleep disorders are highly prevalent among stroke survivors and impede stroke recovery. It is well established that melatonin has neuroprotective effects in animal models of ischemic stroke. However, as a modulator of endogenous physiological circadian rhythms, the effects of melatonin on poststroke sleep disorders remain unclear. In the present study, we investigated how melatonin delivered intraperitoneally once daily in the subacute phase after stroke onset, influencing neuronal survival, motor recovery, and sleep-wake profiles in rats.

METHODS

Transient ischemic stroke in male Sprague-Dawley rats was induced with 30 min occlusion of the middle cerebral artery. Melatonin or vehicle was delivered intraperitoneally once daily in the subacute phase, from 2 to 7 days after stroke. Electroencephalogram and electromyogram recordings were obtained simultaneously.

RESULTS

Compared to the effects observed in the vehicle-treated ischemic group, after 6 daily consecutive treatment of melatonin at 10 mg/kg starting at ischemic/reperfusion day 2, the infarct volume was significantly decreased (from 39.6 to 26.2%), and the degeneration of axons in the ipsilateral striatum and the contralateral corpus callosum were significantly alleviated. Sensorimotor performances were obviously improved as evidenced by significant increases in the latency to falling off the wire and in the use of the impaired forelimb. In addition to those predictable results of reducing brain tissue damage and mitigating behavioral deficits, repeated melatonin treatment during the subacute phase of stroke also alleviated sleep fragmentation through reducing sleep-wake stage transitions and stage bouts, together with increasing stage durations. Furthermore, daily administration of melatonin at 9 a.m. significantly increased the nonrapid eye movement sleep delta power during both the light and dark periods and decreased the degree of reduction of the circadian index.

CONCLUSIONS

Melatonin promptly reversed ischemia-induced sleep disturbances. The neuroprotective effects of melatonin on ischemic injury may be partially associated with its role in sleep modulation.

Consciousness among delta waves: a paradox?

A common observation in EEG research is that consciousness vanishes with the appearance of delta (1 - 4 Hz) waves, particularly when those waves are high amplitude. High amplitude delta oscillations are very frequently observed in states of diminished consciousness, including slow wave sleep, anaesthesia, generalised epileptic seizures, and disorders of consciousness such as coma and vegetative state. This strong correlation between loss of consciousness and high amplitude delta oscillations is thought to stem from the widespread cortical deactivation that occurs during the "down states" or troughs of these slow oscillations. Recently, however, many studies have reported the presence of prominent delta activity during conscious states, which casts doubt on the hypothesis that high amplitude delta oscillations are an indicator of unconsciousness. These studies include work in Angelman syndrome, epilepsy, behavioural responsiveness during propofol anaesthesia, postoperative delirium, and states of dissociation from the environment such as dreaming and powerful psychedelic states. The foregoing studies complement an older, yet largely unacknowledged, body of literature that has documented awake, conscious patients with high amplitude delta oscillations in clinical reports from Rett syndrome, Lennox-Gastaut syndrome, schizophrenia, mitochondrial diseases, hepatic encephalopathy, and nonconvulsive status epilepticus. At the same time, a largely parallel body of recent work has reported convincing evidence that the complexity or entropy of EEG and magnetoencephalogram or MEG signals strongly relates to an individual's level of consciousness. Having reviewed this literature, we discuss plausible mechanisms that would resolve the seeming contradiction between high amplitude delta oscillations and consciousness. We also consider implications concerning theories of consciousness, such as integrated information theory and the entropic brain hypothesis. Finally, we conclude that false inferences of unconscious states can be best avoided by examining measures of electrophysiological complexity in addition to spectral power.

Roles of motor and cortical activity in sleep rebound in rat

Sleep pressure that builds up gradually during the extended wakefulness results in sleep rebound. Several lines of evidence, however, suggest that wake per se may not be sufficient to drive sleep rebound and that rapid eye movement (REM) and non-rapid eye movement (NREM) sleep rebound may be differentially regulated. In this study, we investigated the relative contribution of brain versus physical activities in REM and NREM sleep rebound by four sets of experiments. First, we forced locomotion in rats in a rotating wheel for 4 hr and examined subsequent sleep rebound. Second, we exposed the rats lacking homeostatic sleep response after prolonged quiet wakefulness and arousal brain activity induced by chemoactivation of parabrachial nucleus to the same rotating wheel paradigm and tested if physical activity could rescue the sleep homeostasis. Third, we varied motor activity levels while concurrently inhibiting the cortical activity by administering ketamine or xylazine (motor inhibitor), or ketamine + xylazine mixture and investigated if motor activity in the absence of activated cortex can cause NREM sleep rebound. Fourth and finally, we manipulated cortical activity by administering ketamine (that induced active wakefulness and waking brain) alone or in combination with atropine (that selectively inhibits the cortex) and studied if cortical inhibition irrespective of motor activity levels can block REM sleep rebound. Our results demonstrate that motor activity but not cortical activity determines NREM sleep rebound whereas cortical activity but not motor activity determines REM sleep rebound.

Global sleep homeostasis reflects temporally and spatially integrated local cortical neuronal activity

Sleep homeostasis manifests as a relative constancy of its daily amount and intensity. Theoretical descriptions define 'Process S', a variable with dynamics dependent on global sleep-wake history, and reflected in electroencephalogram (EEG) slow wave activity (SWA, 0.5-4 Hz) during sleep. The notion of sleep as a local, activity-dependent process suggests that activity history must be integrated to determine the dynamics of global Process S. Here, we developed novel mathematical models of Process S based on cortical activity recorded in freely behaving mice, describing local Process S as a function of the deviation of neuronal firing rates from a locally defined set-point, independent of global sleep-wake state. Averaging locally derived Processes S and their rate parameters yielded values resembling those obtained from EEG SWA and global vigilance states. We conclude that local Process S dynamics reflects neuronal activity integrated over time, and global Process S reflects local processes integrated over space.

Cortical zeta-inhibitory peptide injection reduces local sleep need

Local sleep need within cortical circuits exhibits extensive interregional variability and appears to increase following learning during preceding waking. Although the biological mechanisms responsible for generating sleep need are unclear, this local variability could arise as a consequence of wake-dependent synaptic plasticity. To test whether cortical synaptic strength is a proximate driver of sleep homeostasis, we developed a novel experimental approach to alter local sleep need. One hour prior to light onset, we injected zeta-inhibitory peptide (ZIP), a pharmacological antagonist of protein kinase Mζ, which can produce pronounced synaptic depotentiation, into the right motor cortex of freely behaving rats. When compared with saline control, ZIP selectively reduced slow-wave activity (SWA; the best electrophysiological marker of sleep need) within the injected motor cortex without affecting SWA in a distal cortical site. This local reduction in SWA was associated with a significant reduction in the slope and amplitude of individual slow waves. Local ZIP injection did not significantly alter the amount of time spent in each behavioral state, locomotor activity, or EEG/LFP power during waking or REM sleep. Thus, local ZIP injection selectively produced a local reduction in sleep need; synaptic strength, therefore, may play a causal role in generating local homeostatic sleep need within the cortex.

Level of Consciousness Is Dissociable from Electroencephalographic Measures of Cortical Connectivity, Slow Oscillations, and Complexity

Leading neuroscientific theories posit a central role for the functional integration of cortical areas in conscious states. Considerable evidence supporting this hypothesis is based on network changes during anesthesia, but it is unclear whether these changes represent state-related (conscious vs unconscious) or drug-related (anesthetic vs no anesthetic) effects. We recently demonstrated that carbachol delivery to prefrontal cortex (PFC) restored wakefulness despite continuous administration of the general anesthetic sevoflurane. By contrast, carbachol delivery to parietal cortex, or noradrenaline delivery to either prefrontal or parietal cortices, failed to restore wakefulness. Thus, carbachol-induced reversal of sevoflurane anesthesia represents a unique state that combines wakefulness with clinically relevant anesthetic concentrations in the brain. To differentiate the state-related and drug-related associations of cortical connectivity and dynamics, we analyzed the electroencephalographic data gathered from adult male Sprague Dawley rats during the aforementioned experiments for changes in functional cortical gamma connectivity (25–155 Hz), slow oscillations (0.5–1 Hz), and complexity (<175 Hz). We show that higher gamma (85–155 Hz) connectivity is decreased (p ≤ 0.02) during sevoflurane anesthesia, an expected finding, but was not restored during wakefulness induced by carbachol delivery to PFC. Conversely, for rats in which wakefulness was not restored, the functional gamma connectivity remained reduced, but there was a significant decrease (p < 0.001) in the power of slow oscillations and increase (p < 0.001) in cortical complexity, which was similar to that observed during wakefulness induced after carbachol delivery to PFC. We conclude that the level of consciousness can be dissociated from cortical connectivity, oscillations, and dynamics.

SIGNIFICANCE STATEMENT Numerous theories of consciousness suggest that functional connectivity across the cortex is characteristic of the conscious state and is reduced during anesthesia. However, it is unknown whether the observed changes are state-related (conscious vs unconscious) or drug-related (drug vs no drug). We used a novel rat model in which cholinergic stimulation of PFC produced wakefulness despite continuous exposure to a general anesthetic. We demonstrate that, as expected, general anesthesia reduces connectivity. Surprisingly, the connectivity remains suppressed despite pharmacologically induced wakefulness in the presence of anesthetic, with restoration occurring only after the anesthetic is discontinued. Thus, whether an animal exhibits wakefulness or not can be dissociated from cortical connectivity, prompting a reevaluation of the role of connectivity in level of consciousness.

GABA Receptors and the Pharmacology of Sleep

Current GABAergic sleep-promoting medications were developed pragmatically, without making use of the immense diversity of GABA_A receptors. Pharmacogenetic experiments are leading to an understanding of the circuit mechanisms in the hypothalamus by which zolpidem and similar compounds induce sleep at α2βγ2-type GABA_A receptors. Drugs acting at more selective receptor types, for example, at receptors containing the α2 and/or α3 subunits expressed in hypothalamic and brain stem areas, could in principle be useful as hypnotics/anxiolytics. A highly promising sleep-promoting drug, gaboxadol, which activates αβδ-type receptors failed in clinical trials. Thus, for the time being, drugs such as zolpidem, which work as positive allosteric modulators at GABA_A receptors, continue to be some of the most effective compounds to treat primary insomnia.

Differential Role of Prefrontal and Parietal Cortices in Controlling Level of Consciousness

Consciousness is determined both by level (e.g., being awake versus being anesthetized) and content (i.e., the qualitative aspects of experience). Subcortical areas are known to play a causal role in regulating the level of consciousness [1-9], but the role of the cortex is less well understood. Clinical and correlative data have been used both to support and refute a role for prefrontal and posterior cortices in the level of consciousness [10-22]. The prefrontal cortex has extensive reciprocal connections to wake-promoting centers in the brainstem and diencephalon [23, 24], and hence is in a unique position to modulate level of consciousness. Furthermore, a recent study suggested that the prefrontal cortex might be important in regulating level of consciousness [25] but causal evidence, and a comparison with more posterior cortical sites, is lacking. Therefore, to test the hypothesis that prefrontal cortex plays a role in regulating level of consciousness, we attempted to reverse sevoflurane anesthesia by cholinergic or noradrenergic stimulation of the prefrontal prelimbic cortex and two areas of parietal cortex in rat. General anesthesia was defined by loss of the righting reflex, a widely used surrogate measure in rodents. We demonstrate that cholinergic stimulation of prefrontal cortex, but not parietal cortex, restored wake-like behavior, despite continuous exposure to clinically relevant concentrations of sevoflurane anesthesia. Noradrenergic stimulation of the prefrontal and parietal areas resulted in electroencephalographic activation but failed to produce any signs of wake-like behavior. We conclude that cholinergic mechanisms in prefrontal cortex can regulate the level of consciousness.

Unraveling the Evolutionary Determinants of Sleep

Despite decades of intense study, the functions of sleep are still shrouded in mystery. The difficulty in understanding these functions can be at least partly attributed to the varied manifestations of sleep in different animals. Daily sleep duration can range from 4-20 hrs among mammals, and sleep can manifest throughout the brain, or it can alternate over time between cerebral hemispheres, depending on the species. Ecological factors are likely to have shaped these and other sleep behaviors during evolution by altering the properties of conserved arousal circuits in the brain. Nonetheless, core functions of sleep are likely to have arisen early and to have persisted to the present day in diverse organisms. This review will discuss the evolutionary forces that may be responsible for phylogenetic differences in sleep and the potential core functions that sleep fulfills.

Why Does Sleep Slow-Wave Activity Increase After Extended Wake? Assessing the Effects of Increased Cortical Firing During Wake and Sleep

During non-rapid eye movement (NREM) sleep, cortical neurons alternate between ON periods of firing and OFF periods of silence. This bi-stability, which is largely synchronous across neurons, is reflected in the EEG as slow waves. Slow-wave activity (SWA) increases with wake duration and declines homeostatically during sleep, but the underlying mechanisms remain unclear. One possibility is neuronal "fatigue": high, sustained firing in wake would force neurons to recover with more frequent and longer OFF periods during sleep. Another possibility is net synaptic potentiation during wake: stronger coupling among neurons would lead to greater synchrony and therefore higher SWA. Here, we obtained a comparable increase in sustained firing (6 h) in cortex by: (1) keeping mice awake by exposure to novel objects to promote plasticity and (2) optogenetically activating a local population of cortical neurons at wake-like levels during sleep. Sleep after extended wake led to increased SWA, higher synchrony, and more time spent OFF, with a positive correlation between SWA, synchrony, and OFF periods. Moreover, time spent OFF was correlated with cortical firing during prior wake. After local optogenetic stimulation, SWA and cortical synchrony decreased locally, time spent OFF did not change, and local SWA was not correlated with either measure. Moreover, laser-induced cortical firing was not correlated with time spent OFF afterward. Overall, these results suggest that high sustained firing per se may not be the primary determinant of SWA increases observed after extended wake.

SIGNIFICANCE STATEMENT

A long-standing hypothesis is that neurons fire less during slow-wave sleep to recover from the "fatigue" accrued during wake, when overall synaptic activity is higher than in sleep. This idea, however, has rarely been tested and other factors, namely increased cortical synchrony, could explain why sleep slow-wave activity (SWA) is higher after extended wake. We forced neurons in the mouse cortex to fire at high levels for 6 h in 2 different conditions: during active wake with exploration and during sleep. We find that neurons need more time OFF only after sustained firing in wake, suggesting that fatigue due to sustained firing alone is unlikely to account for the increase in SWA that follows sleep deprivation.

Neural Correlates of Wakefulness, Sleep, and General Anesthesia: An Experimental Study in Rat

BACKGROUND

Significant advances have been made in our understanding of subcortical processes related to anesthetic- and sleep-induced unconsciousness, but the associated changes in cortical connectivity and cortical neurochemistry have yet to be fully clarified.

METHODS

Male Sprague-Dawley rats were instrumented for simultaneous measurement of cortical acetylcholine and electroencephalographic indices of corticocortical connectivity-coherence and symbolic transfer entropy-before, during, and after general anesthesia (propofol, n = 11; sevoflurane, n = 13). In another group of rats (n = 7), these electroencephalographic indices were analyzed during wakefulness, slow wave sleep (SWS), and rapid eye movement (REM) sleep.

RESULTS

Compared to wakefulness, anesthetic-induced unconsciousness was characterized by a significant decrease in cortical acetylcholine that recovered to preanesthesia levels during recovery wakefulness. Corticocortical coherence and frontal-parietal symbolic transfer entropy in high γ band (85 to 155 Hz) were decreased during anesthetic-induced unconsciousness and returned to preanesthesia levels during recovery wakefulness. Sleep-wake states showed a state-dependent change in coherence and transfer entropy in high γ bandwidth, which correlated with behavioral arousal: high during wakefulness, low during SWS, and lowest during REM sleep. By contrast, frontal-parietal θ connectivity during sleep-wake states was not correlated with behavioral arousal but showed an association with well-established changes in cortical acetylcholine: high during wakefulness and REM sleep and low during SWS.

CONCLUSIONS

Corticocortical coherence and frontal-parietal connectivity in high γ bandwidth correlates with behavioral arousal and is not mediated by cholinergic mechanisms, while θ connectivity correlates with cortical acetylcholine levels.

Sleep slow oscillation and plasticity

It is well documented that sleep contributes to memory consolidation and it is also accepted that long-term synaptic plasticity plays a critical role in memory formation. The mechanisms of this sleep-dependent memory formation are unclear. Two main hypotheses are proposed. According to the first one, synapses are potentiated during wake; and during sleep they are scaled back to become available for the learning tasks in the next day. The other hypothesis is that sleep slow oscillations potentiate synapses that were depressed due to persistent activities during the previous day and that potentiation provides physiological basis for memory consolidation. The objective of this review is to group information on whether cortical synapses are up-scaled or down-scaled during sleep. We conclude that the majority of cortical synapses are up-regulated by sleep slow oscillation.

Sleep, synaptic homeostasis and neuronal firing rates

The synaptic homeostasis hypothesis (SHY) states that wake brings about a net overall increase in synaptic strength in many brain circuits that needs to be renormalized by sleep. I will review recent studies that were either specifically designed to test SHY or were interpreted accordingly, including several experiments that focused on changes in neuronal firing rates. I will emphasize that central to SHY is the idea that what is being regulated across the sleep/wake cycle is synaptic strength, not firing rate, and firing rate taken in isolation is not necessarily an adequate proxy for synaptic strength.

Different Simultaneous Sleep States in the Hippocampus and Neocortex

STUDY OBJECTIVES

Investigators assign sleep-waking states using brain activity collected from a single site, with the assumption that states occur at the same time throughout the brain. We sought to determine if sleep-waking states differ between two separate structures: the hippocampus and neocortex.

METHODS

We measured electrical signals (electroencephalograms and electromyograms) during sleep from the hippocampus and neocortex of five freely behaving adult male rats. We assigned sleep-waking states in 10-sec epochs based on standard scoring criteria across a 4-h recording, then analyzed and compared states and signals from simultaneous epochs between sites.

RESULTS

We found that the total amount of each state, assigned independently using the hippocampal and neocortical signals, was similar between the hippocampus and neocortex. However, states at simultaneous epochs were different as often as they were the same (P = 0.82). Furthermore, we found that the progression of states often flowed through asynchronous state-pairs led by the hippocampus. For example, the hippocampus progressed from transition-to-rapid eye movement sleep to rapid eye movement sleep before the neocortex more often than in synchrony with the neocortex (38.7 ± 16.2% versus 15.8 ± 5.6% mean ± standard error of the mean).

CONCLUSIONS

We demonstrate that hippocampal and neocortical sleep-waking states often differ in the same epoch. Consequently, electrode location affects estimates of sleep architecture, state transition timing, and perhaps even percentage of time in sleep states. Therefore, under normal conditions, models assuming brain state homogeneity should not be applied to the sleeping or waking brain.

Genetic Dissociation of Daily Sleep and Sleep Following Thermogenetic Sleep Deprivation in Drosophila

STUDY OBJECTIVES

Sleep rebound-the increase in sleep that follows sleep deprivation-is a hallmark of homeostatic sleep regulation that is conserved across the animal kingdom. However, both the mechanisms that underlie sleep rebound and its relationship to habitual daily sleep remain unclear. To address this, we developed an efficient thermogenetic method of inducing sleep deprivation in Drosophila that produces a substantial rebound, and applied the newly developed method to assess sleep rebound in a screen of 1,741 mutated lines. We used data generated by this screen to identify lines with reduced sleep rebound following thermogenetic sleep deprivation, and to probe the relationship between habitual sleep amount and sleep following thermogenetic sleep deprivation in Drosophila.

METHODS

To develop a thermogenetic method of sleep deprivation suitable for screening, we thermogenetically stimulated different populations of wake-promoting neurons labeled by Gal4 drivers. Sleep rebound following thermogenetically-induced wakefulness varies across the different sets of wake-promoting neurons that were stimulated, from very little to quite substantial. Thermogenetic activation of neurons marked by the c584-Gal4 driver produces both strong sleep loss and a substantial rebound that is more consistent within genotypes than rebound following mechanical or caffeine-induced sleep deprivation. We therefore used this driver to induce sleep deprivation in a screen of 1,741 mutagenized lines generated by the Drosophila Gene Disruption Project. Flies were subjected to 9 h of sleep deprivation during the dark period and released from sleep deprivation 3 h before lights-on. Recovery was measured over the 15 h following sleep deprivation. Following identification of lines with reduced sleep rebound, we characterized baseline sleep and sleep depth before and after sleep deprivation for these hits.

RESULTS

We identified two lines that consistently exhibit a blunted increase in the duration and depth of sleep after thermogenetic sleep deprivation. Neither of the two genotypes has reduced total baseline sleep. Statistical analysis across all screened lines shows that genotype is a strong predictor of recovery sleep, independent from effects of genotype on baseline sleep.

CONCLUSIONS

Our data show that rebound sleep following thermogenetic sleep deprivation can be genetically separated from sleep at baseline. This suggests that genetically controlled mechanisms of sleep regulation not manifest under undisturbed conditions contribute to sleep rebound following thermogenetic sleep deprivation.

Sleep, recovery, and metaregulation: explaining the benefits of sleep

A commonly held view is that extended wakefulness is causal for a broad spectrum of deleterious effects at molecular, cellular, network, physiological, psychological, and behavioral levels. Consequently, it is often presumed that sleep plays an active role in providing renormalization of the changes incurred during preceding waking. Not surprisingly, unequivocal empirical evidence supporting such a simple bi-directional interaction between waking and sleep is often limited or controversial. One difficulty is that, invariably, a constellation of many intricately interrelated factors, including the time of day, specific activities or behaviors during preceding waking, metabolic status and stress are present at the time of measurement, shaping the overall effect observed. In addition to this, although insufficient or disrupted sleep is thought to prevent efficient recovery of specific physiological variables, it is also often difficult to attribute specific changes to the lack of sleep proper. Furthermore, sleep is a complex phenomenon characterized by a multitude of processes, whose unique and distinct contributions to the purported functions of sleep are difficult to determine, because they are interrelated. Intensive research effort over the last decades has greatly progressed current understanding of the cellular and physiological processes underlying the regulation of vigilance states. Notably, it also highlighted the infinite complexity within both waking and sleep, and revealed a number of fundamental conceptual and technical obstacles that need to be overcome in order to fully understand these processes. A promising approach could be to view sleep not as an entity, which has specific function(s) and is subject to direct regulation, but as a manifestation of the process of metaregulation, which enables efficient moment-to-moment integration between internal and external factors, preceding history and current homeostatic needs.