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Abstract
BackgroundCircadian rhythms govern many aspects of physiology and behavior including cognitive processes. Components of neural circuits involved in learning and memory, e.g., the amygdala and the hippocampus, exhibit circadian rhythms in gene expression and signaling pathways. The functional significance of these rhythms is still not understood. In the present study, we sought to determine the impact of transiently disrupting the circadian system by shifting the light/dark (LD) cycle. Such “jet lag” treatments alter daily rhythms of gene expression that underlie circadian oscillations as well as disrupt the synchrony between the multiple oscillators found within the body.Methodology/Principal FindingsWe subjected adult male C57Bl/6 mice to a contextual fear conditioning protocol either before or after acute phase shifts of the LD cycle. As part of this study, we examined the impact of phase advances and phase delays, and the effects of different magnitudes of phase shifts. Under all conditions tested, we found that recall of fear conditioned behavior was specifically affected by the jet lag. We found that phase shifts potentiated the stress-evoked corticosterone response without altering baseline levels of this hormone. The jet lag treatment did not result in overall sleep deprivation, but altered the temporal distribution of sleep. Finally, we found that prior experience of jet lag helps to compensate for the reduced recall due to acute phase shifts.Conclusions/SignificanceAcute changes to the LD cycle affect the recall of fear-conditioned behavior. This suggests that a synchronized circadian system may be broadly important for normal cognition and that the consolidation of memories may be particularly sensitive to disruptions of circadian timing.
Highlights
Rhythms in behavior and physiology are found in almost all organisms
We first tested if an acute phase shift prior to training would alter acquisition of fear conditioned behavior (Fig. 1A)
The phase-shifted group of mice (n = 6) housed in 12:12 light/ dark (LD) was subjected to a 12 hr phase advance by extending the dark phase on the day prior to training (Day -1) resulting in an inversion of the lighting cycle on the day of training
Summary
Rhythms in behavior and physiology are found in almost all organisms. The ability to synchronize ones physiology to anticipate environmental changes is thought to be the driving force behind the evolution of a network of circadian oscillators that adapt and respond, and yet have the ability to ‘‘keep time’’ in absence of any external cues. Components of the circuits involved in learning and memory demonstrate rhythms in gene expression, including the amygdala [5] and the hippocampus [6,7]. These rhythms are autonomous as they continue in hippocampal slices in culture [8]. We sought to determine the impact of transiently disrupting the circadian system by shifting the light/ dark (LD) cycle Such ‘‘jet lag’’ treatments alter daily rhythms of gene expression that underlie circadian oscillations as well as disrupt the synchrony between the multiple oscillators found within the body
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