In the present study, we used an animal model of sleep deprivation that mimics chronically insufficient sleep as it often occurs in humans. We found that sleep deprivation leads to behavioral abnormalities, including anxiety and depressive-like behaviors. The consistent loss of sleep activates glial cells, which increase the release of pro-inflammatory cytokines in rats, leading to cognitive impairment and emotional disorders. Sleep deprivation also alters the transcript profile of circadian clock genes, which regulates physical, mental, and behavioral changes in several pathological processes. These observations indicate a new pathway through which sleep deprivation induces mental illness through the impairment the regulation of circadian clock genes and the induction of microglial proinflammatory cytokines and results in the progressive breakdown of the BBB to induce brain injury formation. These novel findings highlight the importance of circadian responses during the period of sleep loss and emphasize the contribution of circadian mechanisms for understanding mental disorders.
Accumulating evidence suggests that loss of sleep is closely related to neuropsychiatric disorders. The connection between sleep loss or deficits and mental disorders is not yet fully understood. SD impairs physiological and behavioral development by dysregulating proinflammatory cytokines, such as TNFα, IL-6 and IL-1β. Meanwhile, the serum and CSF levels of proinflammatory cytokines are also increased. However, the reason for the inflammatory cytokine upregulation in SD mice and its mechanism in SD-induced mental illness remain unknown. Astrocyte/microglial activation, along with the release of inflammatory cytokines, is considered, among other factors, to be the main source of neuroinflammation. In a recent study by Hurtado-Alvarado et al., it was reported that sleep deprivation for 10 consecutive days induces the overexpression of markers of reactive astroglia (GFAP) and microglia (Iba1) and disrupts the blood-brain barrier via adenosine-mediated signaling(He et al., 2014a; Hurtado-Alvarado et al., 2016). Similarly, in our recent study of 3 days of acute sleep deprivation in rats, we observed reactive gliosis in astrocytic and microglial populations along with the secretion of proinflammatory mediators such as TNFα, IL-6 and IL-1β. Glial cell activation was evidenced by increased microglial reactivity and deviation from normal microglial morphology along with changes in the cell body, branching index and cell counting ratio of resting and activated microglial cells during SD. Interestingly, we found a significant increase in astrocyte immunoreactivity and the morphology of astrocytes was significantly altered, indicating astrocyte hypertrophy during SD-induced neuroinflammation. This is in agreement with earlier reports on the efficacy of chronic sleep loss against stress-induced gliosis.
Circadian clock genes are essential for driving and maintaining circadian rhythm. The suprachiasmatic nucleus (SCN) of the anterior hypothalamus is the internal clock that controls various physiological systems on a cycle of 24 hours. There are at least 9 core circadian clock genes (PER1, PER2, CRY1, CRY2, CLOCK, and BMAL1) that regulate central and peripheral circadian oscillators using transcriptional-translational feedback loops(Bass and Takahashi, 2010; Bellesi et al., 2017; Eckel-Mahan et al., 2013; Forde and Kalsi, 2017; Rosenwasser and Turek, 2015). Disruptions in circadian rhythm and alterations in circadian rhythm genes are associated with an increased risk of depression. A previous study also demonstrated the daily pattern of PER1, PER2, CRY1, CRY2, CLOCK and BMAL1 expression levels. In healthy people, circadian genes such as CLOCK and BMAL1 are expressed in a rhythmic fashion over the course of 24 hours. These genes cause hormonal changes that allow people to respond optimally to the demands of their day, especially the demands of wakefulness and sleep. In a study in mice, chronobiology researchers discovered that a mutation in the CLOCK gene leads to unusual behavior, including mania, hyperactivity and sleeplessness in mice(Siegel, 2005; Stickgold, 2005; Vyazovskiy and Faraguna, 2015; Vyazovskiy and Harris, 2013; Wadhwa et al., 2017; Yang et al., 2015; Zhu et al., 2012). This study suggests that mutations in the CLOCK gene may be part of the cause of mental illnesses. In fact, the sleep disturbances associated with depression often disrupt patients’ mood and ability to function as much as the depression itself. However, the link between sleep disruption and depression is not well established. Our study demonstrated that SD-mediated depression is associated with reduced expression of circadian rhythm genes in the hypothalamus in an animal model of acute SD. We demonstrated that circadian rhythm genes contribute to the homeostatic aspect of sleep regulation. Indeed, changes in some circadian rhythm genes modify markers of sleep homeostasis, and an increase in homeostatic sleep drive alters circadian clock gene expression in the forebrain. Here, we established a possible mechanism by which SD can alter circadian rhythm clock genes expression by quantifying DNA binding of the core-clock transcription factors CLOCK and BMAL1 to the target circadian clock genes in animal model(Carreras et al., 2014; He et al., 2014b; Scammell et al., 2017; Takatoshi et al., 2010). Our research indicates that the disruption of the balance of circadian rhythm genes in the brain can induce mental illness caused by insufficient sleep. These data demonstrate that SD lead to abnormal circadian rhythm genes expression is regarded as an important contributing factor to the disruption of circadian rhythm, which is associated with numerous pathological processes in mental disease.