Select cognitive deficits in Vasoactive Intestinal Peptide deficient mice

Research articl

Rapid Changes in the Light/Dark Cycle Disrupt Memory of Conditioned Fear in Mice

Background: Circadian 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 Findings: We 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/Significance: Acute changes to the LD cycle affect the recall of fear-conditioned behavior. This suggests that

The peak recall of conditioned fear behavior occurred 24 hrs after training with or without a phase shift.

<p><b>A</b>) Schematic illustration of the experimental design. On Day 0, six cohorts (<i>n</i> = 6 per group) were trained at ZT 3. Acquisition of fear conditioned freezing behavior was not significantly different between the six groups (data not shown). After training, three cohorts were subjected to a 6 hr phase advance of the LD cycle. Cohorts, one phase shifted and one control group were tested for recall of contextual fear-conditioned behavior at 18, 24 or 30 hrs intervals after training. <b>B</b>) Recall of the conditioned fear was reduced by the phase advance of the LD cycle at each of the three intervals tested. The peak recall for both groups was found 24 hrs after training. 2RM ANOVA was performed with post-hoc Bonferroni's <i>t</i>-tests, and “*” indicates significant differences between the phase shifted and control groups. Within groups (control and phase advanced), one way ANOVA revealed differences between testing times (18, 24, 30 hrs post training) with the interval of peak recall in the control group indicated with a “†”, and within the phase advanced group as indicated by a “‡”.</p

Phase advance of the LD cycle enhanced the magnitude of the stress-evoked corticosterone response.

<p>Four cohorts (<i>n</i> = 6–8 per group) of adult male mice were entrained to a 12∶12 LD cycle for at least 2 weeks. On Day -1, two cohorts were subjected to a 6 hr phase shift of the LD cycle. On Day 0, serum corticosterone levels were measured at ZT 3. Baseline concentration of serum corticosterone was not different between the control and phase shifted groups. The cohorts that underwent training for fear conditioning exhibited significant increases in corticosterone. The magnitude of this stress-evoked response was significantly increased in the phase-shifted group. 2RM ANOVA was performed with post-hoc Bonferroni's <i>t</i>-tests and “*” indicates significant differences between training evoked corticosterone responses, while “†” indicates significant differences between baseline and trained mice.</p

Phase advances of 6 hrs or more reduced recall of contextual fear-conditioned behavior.

<p>On Day 0, four cohorts (<i>n</i> = 6–8 per group) were trained at ZT 3. Acquisition of fear conditioned freezing behavior was not significantly different between the groups (data not shown). After training, one cohort was subjected to a 12 hr phase shift, a second cohort was subjected to a 6 hr phase advance and the third cohort subjected to a 3 hr phase advance. All cohorts of mice were tested for recall of contextual fear conditioned behavior in 24 hr intervals. Recall of the conditioned fear was significantly reduced by the 12 and 6 hr phase advance of the LD cycle. 2RM ANOVA was performed with post-hoc Bonferroni's <i>t</i>-tests, and “*” indicates significant differences between the 12 hr phase shifted group and control; the “†” symbol indicates significant differences between the 6 hr phase advanced group and control; “‡” indicates significant differences between the 3 hr phase advanced group and control.</p

Percentage of time spent awake and in NREM and REM sleep as determined by EEG/EMG recordings from mice 24 h before and after an acute 6 hr phase advance of an LD cycle.

<p>No significant differences were found between the baseline and post-phase shift amounts of wake and sleep (REM and NREM sleep).</p

Phase advance of the LD cycle alters the distribution but not the total amount of sleep.

<p>A single cohort (<i>n</i> = 8) of adult male mice were entrained to a 12∶12 LD cycle for 2 weeks. The mice were videotaped for 24 hrs to establish a baseline and an additional 48 hr during which the mice were subjected to a 6 hr phase shift of the LD cycle. The video was scored every 5 min to determine if the mice were awake or asleep. In both <b>A</b> and <b>B</b>, the significant differences between baseline and the first day of the phase shift as determined by 2RM ANOVA with post-hoc Bonferroni's <i>t</i>-tests are denoted by “*” and the significant differences between baseline and the second day of the phase shift by “†”. <b>A</b>) A 6 h phase advance of the LD cycle did not change the total amount of sleep over a 24 h cycle compared to the baseline recordings, but resulted in minor changes in the distribution of sleep. <b>B</b>) A 6 h phase delay of the LD cycle resulted in increased sleep during and after the phase shift, and caused an immediate increase in sleep during the extended 6 h of light, as well as a corresponding decrease in sleep during the dark hours of the following day.</p