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

Abstract

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.