Shift work, circadian rhythms and the brain : Identifying biological mechanisms underlying the metabolic and cognitive consequences of work timing, using a rat model

Shift work, and night shift work in particular, is associated with negative health effects. In the short term, night shift work is associated with increased risk of errors and accidents. In the long term, night shift work is associated with metabolic disturbances, including increased risk of obesity and type 2 diabetes. Still, individual tolerance to shift and night shift work varies considerably. Night shift work challenges the body’s normal circadian rhythmicity. Virtually every biological process within the body exhibits daily rhythms. Individual rhythms of cells and tissues are synchronized to the outside world by time cues (zeitgebers). The most prominent zeitgeber is light, but timing of food intake is an important zeitgeber for the metabolic system. The suprachiasmatic nucleus (SCN) of the hypothalamus sets and synchronizes rhythms within individual tissues and cells. Within cells, rhythms are regulated by clock genes and clock proteins. One clock protein, BMAL1, has also been shown to regulate protein synthesis by acting as a promoter of translation initiation. The mechanisms that underlie the negative health effects of night shift work are not fully understood. Circadian misalignment resulting from altered timing of food intake is thought to underlie much of the long term negative metabolic effects, but the acute effects of shifted timing of food intake are less clear. When it comes to the cognitive disturbance associated with shift work, disturbed sleep (both quantity and quality) has been shown to play a part, but less is known about the role of the circadian clock. The aims of this thesis are twofold. Firstly, to investigate the acute effects of simulated night shift work on metabolic (paper Ⅰ) and brain (paper Ⅱ) functioning. Secondly, to understand how individual factors may predict brain functioning following simulated night shift work (paper Ⅲ). These aims are addressed using a rat model of shift work. In this model, rats are exposed to forced activity in automatically rotating wheels for 8 hours a day for 3-4 consecutive days, either in the middle of their active phase to simulate human day shift work (“active workers”) or in the middle of their rest phase to simulate human night shift work (“rest workers”). In paper Ⅰ, the effect of, and recovery from, 3-4 consecutive days of simulated night shift work and accompanying shift in the rhythm of food intake on markers of energy balance and liver metabolism are investigated. Food intake, body temperature, and body weight were monitored as markers of energy balance throughout a 4-day shift work protocol and 8-day subsequent recovery and compared to simulated day shift work. After a 5-week washout period, rats were again exposed to simulated shift work for three consecutive days, fasted for two hours, then sacrificed for collection of liver tissue and analysis of liver gene expression, compared to time-matched controls. The results showed dysregulation of markers of energy balance during simulated night shift work, which took more than eight days to recover. Markers of liver energy storage were upregulated, and markers of energy breakdown were downregulated after just three days of simulated night shift work. In paper Ⅱ, the effects of simulated shift work on BMAL1-driven translation initiation and related markers within the hippocampus and prefrontal cortex (PFC), brain areas important for cognitive functioning, were investigated. Rats were exposed to three days of simulated shift work, recovered in their home cage for two hours, then sacrificed for collection of brain tissue. Expression of protein markers regulating translation initiation was analyzed using m7GTP (cap) pulldown and western blot and compared to time-matched controls. Results showed that after simulated night shift work, BMAL1-driven translation initiation was impaired within the PFC, but not the hippocampus, at a time-point when translation initiation is normally promoted. In paper Ⅲ, the effects of simulated shift work on cognitive performance on a spatial memory task, the Morris Water Maze (MWM), were first investigated. Rats were taught to identify a hidden platform location before being exposed to three consecutive days of simulated shift work. Immediately after the third shift, recall on the MWM task was tested. Rest workers took longer to locate the hidden platform compared to active workers. However, there were considerable individual differences in MWM performance, and some rest workers performed on par with active workers. Individual differences were also observed in PFC markers of brain protein synthesis. Therefore, hierarchical regression analysis was utilized to test how individual variation in factors relating to daily rhythmicity, sleep drive, and glucocorticoid levels might predict spatial memory performance and PFC markers of protein synthesis. Results showed that that type of work, as well as individual differences in daily rhythmicity, sleep drive, and serum glucocorticoids, predicted different aspects of spatial memory performance and PFC markers of protein synthesis. The present findings suggest that just 3-4 days of simulated night shift work is sufficient to disturb metabolic regulation and markers of brain functioning, and that individual variation in a range of predictors relating to circadian rhythmicity and sleep can predict different aspects of brain functioning after simulated shift work. Much is still unknown about the mechanisms that underlie the negative health effects of shift work. The present findings may allow further elucidation of how circadian misalignment impacts all aspects of health, both in those who are engaged in shift work, and in other populations.Doktorgradsavhandlin

Shift in Food Intake and Changes in Metabolic Regulation and Gene Expression during Simulated Night-Shift Work:A Rat Model

Night-shift work is linked to a shift in food intake toward the normal sleeping period, and to metabolic disturbance. We applied a rat model of night-shift work to assess the immediate effects of such a shift in food intake on metabolism. Male Wistar rats were subjected to 8 h of forced activity during their rest (ZT2-10) or active (ZT14-22) phase. Food intake, body weight, and body temperature were monitored across four work days and eight recovery days. Food intake gradually shifted toward rest-work hours, stabilizing on work day three. A subgroup of animals was euthanized after the third work session for analysis of metabolic gene expression in the liver by real-time polymerase chain reaction (PCR). Results show that work in the rest phase shifted food intake to rest-work hours. Moreover, liver genes related to energy storage and insulin metabolism were upregulated, and genes related to energy breakdown were downregulated compared to non-working time-matched controls. Both working groups lost weight during the protocol and regained weight during recovery, but animals that worked in the rest phase did not fully recover, even after eight days of recovery. In conclusion, three to four days of work in the rest phase is sufficient to induce disruption of several metabolic parameters, which requires more than eight days for full recovery.publishedVersio

Shift work, circadian rhythms and the brain : Identifying biological mechanisms underlying the metabolic and cognitive consequences of work timing, using a rat model

Shift work, and night shift work in particular, is associated with negative health effects. In the short term, night shift work is associated with increased risk of errors and accidents. In the long term, night shift work is associated with metabolic disturbances, including increased risk of obesity and type 2 diabetes. Still, individual tolerance to shift and night shift work varies considerably. Night shift work challenges the body’s normal circadian rhythmicity. Virtually every biological process within the body exhibits daily rhythms. Individual rhythms of cells and tissues are synchronized to the outside world by time cues (zeitgebers). The most prominent zeitgeber is light, but timing of food intake is an important zeitgeber for the metabolic system. The suprachiasmatic nucleus (SCN) of the hypothalamus sets and synchronizes rhythms within individual tissues and cells. Within cells, rhythms are regulated by clock genes and clock proteins. One clock protein, BMAL1, has also been shown to regulate protein synthesis by acting as a promoter of translation initiation. The mechanisms that underlie the negative health effects of night shift work are not fully understood. Circadian misalignment resulting from altered timing of food intake is thought to underlie much of the long term negative metabolic effects, but the acute effects of shifted timing of food intake are less clear. When it comes to the cognitive disturbance associated with shift work, disturbed sleep (both quantity and quality) has been shown to play a part, but less is known about the role of the circadian clock. The aims of this thesis are twofold. Firstly, to investigate the acute effects of simulated night shift work on metabolic (paper Ⅰ) and brain (paper Ⅱ) functioning. Secondly, to understand how individual factors may predict brain functioning following simulated night shift work (paper Ⅲ). These aims are addressed using a rat model of shift work. In this model, rats are exposed to forced activity in automatically rotating wheels for 8 hours a day for 3-4 consecutive days, either in the middle of their active phase to simulate human day shift work (“active workers”) or in the middle of their rest phase to simulate human night shift work (“rest workers”). In paper Ⅰ, the effect of, and recovery from, 3-4 consecutive days of simulated night shift work and accompanying shift in the rhythm of food intake on markers of energy balance and liver metabolism are investigated. Food intake, body temperature, and body weight were monitored as markers of energy balance throughout a 4-day shift work protocol and 8-day subsequent recovery and compared to simulated day shift work. After a 5-week washout period, rats were again exposed to simulated shift work for three consecutive days, fasted for two hours, then sacrificed for collection of liver tissue and analysis of liver gene expression, compared to time-matched controls. The results showed dysregulation of markers of energy balance during simulated night shift work, which took more than eight days to recover. Markers of liver energy storage were upregulated, and markers of energy breakdown were downregulated after just three days of simulated night shift work. In paper Ⅱ, the effects of simulated shift work on BMAL1-driven translation initiation and related markers within the hippocampus and prefrontal cortex (PFC), brain areas important for cognitive functioning, were investigated. Rats were exposed to three days of simulated shift work, recovered in their home cage for two hours, then sacrificed for collection of brain tissue. Expression of protein markers regulating translation initiation was analyzed using m7GTP (cap) pulldown and western blot and compared to time-matched controls. Results showed that after simulated night shift work, BMAL1-driven translation initiation was impaired within the PFC, but not the hippocampus, at a time-point when translation initiation is normally promoted. In paper Ⅲ, the effects of simulated shift work on cognitive performance on a spatial memory task, the Morris Water Maze (MWM), were first investigated. Rats were taught to identify a hidden platform location before being exposed to three consecutive days of simulated shift work. Immediately after the third shift, recall on the MWM task was tested. Rest workers took longer to locate the hidden platform compared to active workers. However, there were considerable individual differences in MWM performance, and some rest workers performed on par with active workers. Individual differences were also observed in PFC markers of brain protein synthesis. Therefore, hierarchical regression analysis was utilized to test how individual variation in factors relating to daily rhythmicity, sleep drive, and glucocorticoid levels might predict spatial memory performance and PFC markers of protein synthesis. Results showed that that type of work, as well as individual differences in daily rhythmicity, sleep drive, and serum glucocorticoids, predicted different aspects of spatial memory performance and PFC markers of protein synthesis. The present findings suggest that just 3-4 days of simulated night shift work is sufficient to disturb metabolic regulation and markers of brain functioning, and that individual variation in a range of predictors relating to circadian rhythmicity and sleep can predict different aspects of brain functioning after simulated shift work. Much is still unknown about the mechanisms that underlie the negative health effects of shift work. The present findings may allow further elucidation of how circadian misalignment impacts all aspects of health, both in those who are engaged in shift work, and in other populations

Onset of Work-Life Conflict Increases Risk of Subsequent Psychological Distress in the Norwegian Working Population

We aimed to assess whether the onset of work-life conflict is associated with a risk of subsequent onset of psychological distress. Respondents from a randomly drawn cohort of the general Norwegian working population were interviewed in 2009 (T1), 2013 (T2), and 2016 (T3) (gross sample n = 13,803). Participants reporting frequent work-life conflict at T1 and/or psychological distress (five-item Hopkins Symptom Checklist mean score ≥ 2) at T2 were excluded to establish a design that allowed us to study the effect of the onset of work-life conflict at T2 on psychological distress at T3. Logistic regression analysis showed that the onset of frequent work-life conflict more than doubled the risk of the onset of psychological distress at T3 (OR = 2.55; 95% CI 1.44–4.51). The analysis of the association between occasional work-life conflict and psychological distress was not conclusive (OR = 1.21; 95% CI 0.77–1.90). No differential effects of sex were observed (log likelihood ratio = 483.7, p = 0.92). The calculated population attributable risk (PAR) suggests that 12.3% (95% CI 2.84–22.9%) of psychological distress onset could be attributed to frequent work-life conflict. In conclusion, our results suggest that the onset of frequent work-life conflict has a direct effect on the future risk of developing symptoms of psychological distress in both male and female workers

Cognitive function and brain plasticity in a rat model of shift work: role of daily rhythms, sleep and glucocorticoids

Many occupations require operations during the night-time when the internal circadian clock promotes sleep, in many cases resulting in impairments in cognitive performance and brain functioning. Here, we use a rat model to attempt to identify the biological mechanisms underlying such impaired performance. Rats were exposed to forced activity, either in their rest-phase (simulating night-shift work; rest work) or in their active-phase (simulating day-shift work; active work). Sleep, wakefulness and body temperature rhythm were monitored throughout. Following three work shifts, spatial memory performance was tested on the Morris Water Maze task. After 4 weeks washout, the work protocol was repeated, and blood and brain tissue collected. Simulated night-shift work impaired spatial memory and altered biochemical markers of cerebral cortical protein synthesis. Measures of daily rhythm strength were blunted, and sleep drive increased. Individual variation in the data suggested differences in shift work tolerance. Hierarchical regression analyses revealed that type of work, changes in daily rhythmicity and changes in sleep drive predict spatial memory performance and expression of brain protein synthesis regulators. Moreover, serum corticosterone levels predicted expression of brain protein synthesis regulators. These findings open new research avenues into the biological mechanisms that underlie individual variation in shift work tolerance