Amazing stuff! When will be able to optimize our sleep for best performance and health? Don't hold your breath! 😊
"... Now, researchers ... identified a subgroup of neurons in the thalamus of the mouse brain are crucial for the homeostatic regulation of sleep, providing insights into how animals recover from lost sleep. ...
Curious to learn more about the neurons that regulate sleep, the researchers first mapped neural circuits upstream of known sleep-promoting brain regions in mice. After identifying 11 candidate regions, they injected mice with clozapine N-oxide, a synthetic molecule, to see if chemical activation of the excitatory neurons in those areas promoted sleep. Of these, the activation of a subset of excitatory neurons in the medial thalamic nucleus reuniens (mRE) led to the greatest increase in [non] REM sleep. ...
To further characterize these mRE neurons, the researchers employed chemogenetic and optogenetic tools. Using either method, they found that stimulated neurons led to mice that exhibited deeper and more prolonged NREM sleep several hours later. This suggested that these neurons do not directly induce sleep but instead regulate sleep homeostasis. The researchers also saw that when they activated mRE neurons with light, the mice engaged in enhanced typical pre-sleep behaviors, such as preparing a spot to sleep (nesting) or self-grooming. ..."
From the abstract of the perspective:
"Most animals regularly undergo an astonishing transformation by cycling between wakefulness and sleep. However, if sleep is disrupted or delayed, sleep “debt” accrues, resulting in longer and deeper sleep. Since the discovery of wake-promoting neurons in the brainstem in the 1940s, a hunt for the control mechanisms of sleep-wake behavior has led to a model of reciprocally connected wake-promoting and sleep promoting cell groups [or nuclei (1, 2)] that achieve discrete brain vigilance states by mutually inhibiting one another (3).
But how sleep loss results in rebound sleep to reestablish homeostasis has remained a mystery. On page 1291 of this issue, Lee et al. (4) report a group of neurons in the thalamus (the brain region that relays incoming sensory information to the cerebral cortex) that increases its activity during sleep deprivation and promotes sleep recovery and depth. The findings suggest that these neurons are responsible for promoting sleep homeostasis."
From the editor's summary and abstract:
"Editor’s summary
Sleep is tightly regulated by homeostatic forces, and sleep deficit leads to persistent and consolidated recovery sleep. However, the pathways underlying the homeostatic control of sleep are still unknown. Lee et al. now describe the first homeostatic sleep circuit in mammals (see the Perspective by Gilette and Lipton). This circuit comprises a cluster of excitatory neurons in the nucleus reuniens that is activated by sleep need and is necessary for recovery sleep. Stimulation of these neurons first triggers presleep behavior, followed by deep and persistent sleep that can last hours. This sleep pattern resembles recovery sleep and suggests that activation of reuniens neurons generates sleep pressure even in animals without sleep debt. Silencing this circuit during sleep deprivation disrupts the amount and quality of recovery sleep, suggesting that its activity signals the accrual of sleep need. ...
Structured Abstract
INTRODUCTION
Sleep is under homeostatic control: After prolonged wakefulness, animals engage in persistent, consolidated, and deep sleep. Although the homeostatic regulation of sleep has been intensely studied over the past century, the biological underpinnings of this process remain enigmatic. Progress is being made delineating molecular pathways that mediate sleep homeostasis. By contrast, the identity of neural circuits that sense and/or transmit homeostatic sleep signals is unclear.
RATIONALE
Sleep can be divided into rapid eye movement (REM) and non-REM (NREM) sleep, which is considered the deeper, more restorative form of sleep. To date, many NREM-promoting neural circuits have been identified. However, the identity of specific neuronal clusters required for the accrual of sleep need remains unclear, and the goal of this study was to identify such a neural circuit.
RESULTS
From a circuit screen in mice, a group of excitatory neurons in the thalamic nucleus reuniens (RE) was identified that projected to multiple downstream NREM-promoting clusters.
Brief optogenetic activation of RE neurons led to an unusual phenotype—persistent, consolidated, and deep NREM sleep after a delay. Notably, during this delay period before falling asleep, the animals engaged in sleep-preparatory behaviors, which included grooming and nesting.
Because the persistent, consolidated, and deep sleep phenotype resembled the homeostatic recovery sleep seen after sleep deprivation, we sought to investigate whether RE neurons participate in the homeostatic regulation of sleep.
Most NREM-promoting neurons exhibit increased activity during NREM sleep. To measure the in vivo activity of RE neurons, chronic Neuropixels recordings were performed during sleep deprivation and recovery sleep.
These recordings revealed that RE activity was greater during sleep deprivation and/or wakefulness and reduced during recovery sleep.
Next, we examined whether this elevated RE activity during sleep deprivation was necessary for the accrual of sleep need.
Chemogenetic inhibition of RE neurons during sleep deprivation decreased subsequent homeostatic recovery sleep amount, consolidation, and depth.
RE neurons promote NREM sleep by signaling to a previously identified NREM-promoting cluster in the zona incerta (ZI).
Unexpectedly, sleep deprivation induced neural plastic changes of the RE-ZI connection. The degree of this RE-ZI plasticity correlated with the amount of subsequent homeostatic recovery sleep.
Moreover, this synaptic plasticity enhanced the morphological and functional connectivity between the RE and ZI neuronal clusters.
Calcium- and calmodulin-dependent protein kinase II (CaMKII) is well described to regulate synaptic plasticity and has been implicated in the homeostatic regulation of sleep.
Inhibition of CaMKII activity in RE neurons reduced the RE-ZI plasticity and subsequent homeostatic recovery sleep triggered by sleep deprivation.
CONCLUSION
Our findings suggest that RE neurons are required for the accrual of sleep need and are able to generate persistent, deep sleep, similar to homeostatic recovery sleep.
Sleep deprivation induces plasticity of the RE-ZI circuit, strengthening the connectivity of this sleep-promoting module. The degree of this plasticity correlates with the amount of homeostatic sleep rebound, which suggests that RE-ZI plasticity serves as a molecular readout for sleep need.
These findings reveal a mechanism by which sleep loss transforms the functional coupling of a sleep circuit to promote persistent, deep sleep."
New Brain Circuit Helps Recover Lost Sleep | The Scientist "How do animals bounce back from sleep deprivation? Scientists found a subset of neurons in the mouse thalamus that repay this “sleep debt.”"
Sleep need–dependent plasticity of an RE-ZI circuit promotes homeostatic recovery sleep.
