Impact of sleep deprivation and sleep enhancement on brain functions

Sleep loss is a common condition that occurs frequently in today’s modern society. The detrimental effects of prolonged lack of sleep on cognitive behavior and performance include reduced attention, impaired reaction times, and an increased likelihood of errors. These effects are particularly pronounced during cognitive tasks such as driving or performing complex mechanical activities at work. Several studies have tried to identify neurobiological mechanisms underlying the behavioral effects of sleep deprivation. While much of the research has focused on neurons, the role of glia cells has been overlooked. Here we hypothesized that sleep loss could negatively affect oligodendrocyte function, leading to altered myelin integrity and impaired neuronal signal propagation along the myelinated axons. These oligodendrocyte-driven changes may slow down the neuronal communications and contribute to behavioral deficits associated with sleep loss. By assessing cortico-cortical evoked responses in rats, we showed that sleep loss increased conduction delays in nerve signal propagation. This effect was associated with impaired interhemispheric synchronization of brain activity and deficits in motor performance and learning. To gain insight into the neurobiological mechanisms, we performed a genome wide analysis of the oligodendrocyte transcriptome and a lipidome assessment of myelin enriched fractions, revealing that sleep loss was associated with endoplasmic reticulum stress and disrupted lipid metabolism, particularly cholesterol transport to myelin sheaths. Mass-spectrometric analysis of myelin membranes confirmed low cholesterol levels following sleep loss. Pharmacologically boosting cholesterol to myelin membranes mitigated the effects of sleep loss on nerve signal propagation, cognitive function, motor performance, and learning. In summary, we established an important link between loss of sleep and myelin alterations, highlighting the critical role of oligodendrocyte cholesterol regulation in cognitive and behavior deficits commonly associated with sleep loss. Second project_Abstract The benefits of a good night’s sleep for motor learning are well established in humans, yet its impact in animals remains less understood. Recent evidence indicates that sleep deprivation significantly impairs motor performance in mice, particularly during complex motor sequence tasks such as the complex wheel task, which requires adaptation to irregularly spaced rungs. These findings prompted us to hypothesize that enhancing sleep could, in contrast, augment motor performance in this paradigm. Emerging research suggests that sleep duration and intensity can be non-invasively enhanced through sensory stimulation, with modalities like auditory and vestibular stimulation showing promise in boosting sleep. However, the cognitive benefits and underlying cellular mechanisms of such sleep enhancement remain poorly defined. In the current study, we investigated how enhancing sleep via gentle vestibular stimulation influences motor learning and cortical gene expression in young adult mice. Building upon recent discovery showing that NREM sleep can be extended in mice via gently rocking, we assigned young adult male C57BL/6 mice (Postnatal day 45) into a sleep enhancement (SE, n=15) group, which received gentle rocking at 1 Hz for 12 hours per day during the light period over 11 days, and a control (S, n=12) group, which remained undisturbed. Both groups had unrestricted access to a complex running wheel during the 12-hour dark period, with food and water available ad libitum. Mouse motion activity was continuously monitored using an infrared camera, serving as a proxy for sleep-wake behavior. Motor learning was quantified as the ratio of maximum wheel-running speeds on day 11 relative to day 1. After completion, the motor cortex was quickly dissected and processed for standard RNA sequencing. Our results revealed that SE mice exhibited significantly longer sleep durations than S mice, with daily increases ranging from 2.05% to 9.04% (p=0.02), alongside fewer sleep-to-wake transitions (p<0.001), indicative of more consolidated sleep. The SE group also outperformed the S group in motor learning, as evidenced by higher maximum (p=0.016) wheel-running speed. Notably, motor learning strongly correlated with total sleep amount across both groups (maximum speed r=0.522, p=0.045). To ensure that these improvements were sleep-specific rather than a result of vestibular stimulation, a control experiment demonstrated that four hours of rocking during the dark phase did not affect motor performance. To investigate underlying molecular changes, we performed RNA sequencing of the motor cortex, which identified 139 differentially expressed genes between S and SE mice (73 upregulated, 66 downregulated), predominantly associated with glutamatergic synapse regulation and synaptic plasticity indicating that improved learning was associated with upregulation of genes involved in synaptic plasticity. Analysis of synapses of layer V of primary motor cortex confirmed increased number of glutamatergic synapses, while GABAergic synapses remained unchanged. Additional control experiments confirmed that rocking alone, without concurrent motor learning, neither improved motor performance nor altered synaptic marker expression, suggesting that the observed synaptic changes were shaped by the dynamic interplay between sleep enhancement and motor learning. Taken together, our findings suggest that sleep enhancement via rocking increases mouse motor learning performance at the complex wheel task by promoting synaptic plasticity of excitatory synapses. Moreover, clarifying the distinct roles of sleep and learning in shaping synaptic dynamics and highlighting molecular mechanisms linking sleep, learning, and brain-cortical plasticity.