Sleep and serotonin modulate paracapsular nitric oxide synthase expressing neurons of the amygdala

eNeuro, 2016

Bocchio M, Fisher SP, Unal G, Ellender TJ, Vyazovskiy VV, Capogna M

Unraveling the roles of distinct neuron types is a fundamental challenge to understand brain function in health and disease. In the amygdala, a brain structure regulating emotional behavior, the diversity of GABAergic neurons is only partially explored. We report a novel population of GABAergic amygdala neurons expressing high levels of neuronal nitric oxide synthase (nNOS). Notably, these cells are predominantly localized along basolateral amygdala (BLA) boundaries. Performing ex vivo patch clamp recordings from nNOS+ neurons in Nos1-CreER;Ai9 mice, we observed that nNOS+ neurons located along the external capsule display distinctive electrophysiological properties, axonal and dendritic arborization and connectivity. Examining their c-Fos expression, we found that paracapsular nNOS+ neurons were activated during a period of undisturbed sleep following sleep deprivation, but not during sleep deprivation. Consistently, we found that dorsal raphe serotonin (5-HT) neurons, which are involved in sleep-wake regulation, innervate nNOS+ neurons. Bath application of 5-HT hyperpolarizes nNOS+ neurons via 5-HT1A receptors. This hyperpolarization produces a reduction in firing rate and, occasionally, a switch from tonic to burst firing mode, thereby contrasting with the classic depolarizing effect of 5-HT on BLA GABAergic cells reported so far. Thus, nNOS+ cells are a distinct cell type of the amygdala that controls the activity of downstream neurons in both amygdaloid and extra-amygdaloid regions in a vigilance state-dependent fashion. Given the strong links between mood, sleep deprivation and 5-HT, the recruitment of paracapsular nNOS+ neurons following high sleep pressure may represent a novel mechanism in emotional regulation.

 

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Stereotypic wheel running decreases cortical activity in mice

Nature Communications, 2016

Fisher SP, Cui N, McKillop LM, Gemignani J, Bannerman DM, Oliver PL, Peirson SN, Vyazovskiy VV.

Abstract
Prolonged wakefulness is thought to gradually increase ‘sleep need’ and influence subsequent sleep duration and intensity, but the role of specific waking behaviours remains unclear. Here we report the effect of voluntary wheel running during wakefulness on neuronal activity in the motor and somatosensory cortex in mice. We find that stereotypic wheel running is associated with a substantial reduction in firing rates among a large subpopulation of cortical neurons, especially at high speeds. Wheel running also has longer-term effects on spiking activity across periods of wakefulness. Specifically, cortical firing rates are significantly higher towards the end of a spontaneous prolonged waking period. However, this increase is abolished when wakefulness is dominated by running wheel activity. These findings indicate that wake-related changes in firing rates are determined not only by wake duration, but also by specific waking behaviours.

 

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Lempel-Ziv complexity of cortical activity during sleep and waking in rats

The Journal of Neurophysiology, 2015

D. Abásolo, S. Simons, R. Morgado da Silva, G. Tononi, and V.V. Vyazovskiy

Abstract
Understanding the dynamics of brain activity manifested in the electroencephalogram (EEG), local-field potentials (LFP) and neuronal spiking is essential for explaining their underlying mechanisms and physiological significance. Much has been learned about sleep regulation using conventional EEG power spectrum, coherence and period-amplitude analyses, which focus primarily on frequency and amplitude characteristics of the signals and on their spatio-temporal synchronicity. However, little is known about the effects of ongoing brain state or preceding sleep-wake history on the nonlinear dynamics of brain activity. Recent advances in developing novel mathematical approaches for investigating temporal structure of brain activity based on such measures, as Lempel-Ziv complexity (LZC) can provide insights that go beyond those obtained with conventional techniques of signal analysis. Here we used extensive data sets obtained in spontaneously awake and sleeping adult male laboratory rats, as well as during and after sleep deprivation, to perform a detailed analysis of cortical local field potential (LFP) and neuronal activity with LZC approach. We found that activated brain states - waking and rapid-eye movement (REM) sleep are characterized by higher LZC as compared to non-rapid-eye movement (NREM) sleep. Notably, LZC values derived from the LFP were especially low during early NREM sleep after sleep deprivation, and towards the middle of individual NREM sleep episodes. We conclude that LZC is an important and yet largely unexplored measure with a high potential for investigating neurophysiologic mechanisms of brain activity in health and disease.   

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NREM and REM sleep: Complementary roles in recovery after wakefulness

The Neuroscientist, 2014

Vyazovskiy VV, Delogu A

Abstract
The overall function of sleep is hypothesised to provide “recovery” after preceding waking activities, thereby ensuring optimal functioning during subsequent wakefulness. However, the functional significance of the temporal dynamics of sleep, manifested in the slow homeostatic process and the alternation between non-rapid eye movement (NREM) and REM sleep remains unclear. We propose that NREM and REM sleep have distinct and complementary contributions to the overall function of sleep. Specifically, we suggest that cortical slow oscillations, occurring within specific functionally interconnected neuronal networks during NREM sleep enable information processing, synaptic plasticity and prophylactic cellular maintenance (“recovery process”). In turn, periodic excursions into an activated brain state – REM sleep – appear to be ideally placed to perform “selection” of brain networks, which have benefited from the process of “recovery”, based on their offline performance. Such two-stage modus operandi of the sleep process would ensure that its functions are fulfilled according to the current need and in the shortest time possible. Our hypothesis accounts for the overall architecture of normal sleep and opens up new perspectives for understanding pathological conditions associated with abnormal sleep patterns.

 

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The dynamics of cortical neuronal activity in the first minutes after spontaneous awakening in rats and mice

Sleep, 2014

Vyazovskiy VV, Cui N, Rodriguez AV, Funk C, Cirelli C, Tononi G

Abstract

Study Objective:  Upon awakening from sleep, a fully awake brain state is not re-established immediately, but the origin and physiological properties of the distinct brain state during the first minutes after awakening are unclear. To investigate whether neuronal firing immediately upon arousal is different from the remaining part of the waking episode, we recorded and analysed the dynamics of cortical neuronal activity in the first 15 minutes after spontaneous awakenings in freely moving rats and mice. Design:  Intracortical recordings of the local field potential and neuronal activity in freely-moving mice and rats. Setting: Basic sleep research laboratory. Patients or Participants: WKY adult male rats, C57BL/6 adult male mice. Interventions: N/A. Measurements and Results: In both species the average population spiking activity upon arousal was initially low, though substantial variability in the dynamics of firing activity was apparent between individual neurons. A distinct population of neurons was found that was virtually silent in the first minutes upon awakening. The overall lower population spiking initially after awakening was associated with the occurrence of brief periods of generalized neuronal silence (OFF periods), whose frequency peaked immediately after awakening and then progressively declined. OFF periods incidence upon awakening was independent of ongoing locomotor activity but was sensitive to immediate preceding sleep/wake history. Notably, in both rats and mice if sleep before a waking episode was enriched in REM sleep, the incidence of OFF periods was initially higher as compared to those waking episodes preceded mainly by NREM sleep. Conclusion: We speculate that an intrusion of sleep-like patterns of cortical neuronal activity into the wake state immediately after awakening may account for some of the changes in the behaviour and cognitive function typical for sleep inertia.

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Sleep and synaptic homeostasisDownload this file (Vyazovskiy_Faraguna_Curr Top Behav Neurosci 2014.pdf)

Curr Top Behav Neurosci, 2014

Vyazovskiy VV, Faraguna U

Abstract
In the last decades a substantial knowledge about sleep mechanisms has been accumulated. However, the function of sleep still remains elusive. The difficulty with unraveling sleep’s function may arise from the lack of understanding of how the multitude of processes associated with waking and sleep – from gene expression and single neuron activity to the whole brain dynamics and behaviour – functionally and mechanistically relate to each other. Therefore, novel conceptual frameworks, which integrate and take into account the variety of phenomena occurring during waking and sleep at different levels, will likely lead to advances in our understanding of the function of sleep, above and beyond of what merely descriptive or correlative approaches can provide. One such framework, the synaptic homeostasis hypothesis, focuses on wake- and sleep- dependent changes in synaptic strength. The core claim of this hypothesis is that learning and experience during wakefulness are associated with a net increase in synaptic strength. In turn, the proposed sleep’s function is to provide synaptic renormalization, which has important implications with respect to energy needs, intracranial space, metabolic supplies, and, importantly, enables further plastic changes. In this article we review the empirical evidence for this hypothesis, which was obtained at several levels – from gene expression and cellular excitability to structural synaptic modifications and behavioural outcomes. We conclude that although the mechanisms behind the proposed role of sleep in synaptic homeostasis are undoubtedly complex, this conceptual framework offers a unique opportunity to provide mechanistic and functional explanation for many previously disparate observations, and define future research strategies.  
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Sleep and the single neuron: the role of global slow oscillations in individual cell rest

Nature Reviews Neuroscience, 2013

Vyazovskiy VV & Harris KD

Abstract
Sleep is universal in animals, but its specific functions remain elusive. We propose that sleep's primary function is to allow individual neurons to perform prophylactic cellular maintenance. Just as muscle cells must rest after strenuous exercise to prevent long-term damage, brain cells must rest after intense synaptic activity. We suggest that periods of reduced synaptic input ('off periods' or 'down states') are necessary for such maintenance. This in turn requires a state of globally synchronized neuronal activity, reduced sensory input and behavioural immobility — the well-known manifestations of sleep.

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The temporal structure of behaviour and sleep homeostasis

PLoS One, 2012

Vyazovskiy VV & Tobler I

Abstract
The amount and architecture of vigilance states are governed by two distinct processes, which occur at different time scales. The first, a slow one, is related to a wake/sleep dependent homeostatic Process S, which occurs on a time scale of hours, and is reflected in the dynamics of NREM sleep EEG slow-wave activity. The second, a fast one, is manifested in a regular alternation of two sleep states – NREM and REM sleep, which occur, in rodents, on a time scale of ~5-10 minutes. Neither the mechanisms underlying the time constants of these two processes – the slow one and the fast one, nor their functional significance are understood. Notably, both processes are primarily apparent during sleep, while their potential manifestation during wakefulness is obscured by ongoing behaviour. Here, we find, in mice provided with running wheels, that the two sleep processes become clearly apparent also during waking at the level of behavior and brain activity. Specifically, the slow process was manifested in the total duration of waking periods starting from dark onset, while the fast process was apparent in a regular occurrence of running bouts during the waking periods. The dynamics of both processes were stable within individual animals, but showed large interindividual variability. Importantly, the two processes were not independent: the periodic structure of waking behaviour (fast process) appeared to be a strong predictor of the capacity to sustain continuous wakefulness (slow process). The data indicate that the temporal organization of vigilance states on both the fast and the slow time scales may arise from a common neurophysiologic mechanism.

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Local sleep in awake rats

Nature, 2011

Vyazovskiy VV, Olcese U, Hanlon EC, Nir Y, Cirelli C, Tononi G.

Abstract
In an awake state, neurons in the cerebral cortex fire irregularly and electroencephalogram (EEG) recordings display low-amplitude, high-frequency fluctuations. During sleep, neurons oscillate between 'on' periods, when they fire as in an awake brain, and 'off' periods, when they stop firing altogether and the EEG displays high-amplitude slow waves. However, what happens to neuronal firing after a long period of being awake is not known. Here we show that in freely behaving rats after a long period in an awake state, cortical neurons can go briefly 'offline' as in sleep, accompanied by slow waves in the local EEG. Neurons often go offline in one cortical area but not in another, and during these periods of 'local sleep', the incidence of which increases with the duration of the awake state, rats are active and display an 'awake' EEG. However, they are progressively impaired in a sugar pellet reaching task. Thus, although both the EEG and behaviour indicate wakefulness, local populations of neurons in the cortex may be falling asleep, with negative consequences for performance.

 

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Prolonged wakefulness alters neuronal responsiveness to local electrical stimulation of the neocortex in awake rats

Journal of Sleep Research, 2012

Vyazovskiy VV, Olcese U, Cirelli C, Tononi G

Prolonged wakefulness or a lack of sleep lead to cognitive deficits, but little is known about the underlying cellular mechanisms. We recently found that sleep deprivation affects spontaneous neuronal activity in the neocortex of sleeping and awake rats. While it is well known that synaptic responses are modulated by ongoing cortical activity, it remains unclear whether prolonged waking affects responsiveness of cortical neurons to incoming stimuli. By applying local electrical microstimulation to the frontal area of the neocortex, we found that after a 4-hour period of waking the initial neuronal response in the contralateral frontal cortex was stronger and more synchronous, and was followed by a more profound inhibition of neuronal spiking as compared to the control condition. These changes in evoked activity suggest increased neuronal excitability and indicate that after staying awake cortical neurons become transiently bistable. We propose that some of the detrimental effects of sleep deprivation may be a result of altered neuronal responsiveness to incoming intrinsic and extrinsic inputs.