Abstract
Nocturnal mice fed in the middle of the light period exhibit food anticipatory rhythms of behavior and physiology under control of food-entrainable circadian clocks in the brain and body. This is presumed to be adaptive by aligning behavior and physiology with predictable mealtimes. This assumption is challenged by a report that daytime feeding schedules impair cognitive processes important for survival, including object memory and contextual fear conditioning assessed at two times of day. To further evaluate these effects, mice were restricted to a 6 h daily meal in the middle of the light or dark period and object memory was tested at four times of day. Object memory was not impaired by daytime feeding, and did not exhibit circadian variation in either group. To determine whether impairment might depend on methodology, experimental procedures used previously to detect impairment were followed. Daytime feeding induced food anticipatory rhythms and shifted hippocampal clock genes, but again did not impair object memory. Spontaneous alternation and contextual fear conditioning were also not impaired. Hippocampal memory function appears more robust to time of day and daytime feeding schedules than previously reported; day-fed mice can remember what they have seen, where they have been, and where it is dangerous.
Highlights
Nocturnal rats and mice with free access to food are active and eat predominantly at night, under control of a light-dark (LD)-entrainable circadian pacemaker in the suprachiasmatic nucleus (SCN)
The study confirmed that circadian rhythms of hippocampal electrophysiology and clock gene expression are markedly shifted in day-fed mice, whereas clock gene rhythms in the SCN pacemaker remain aligned with the LD cycle[14,15,16,17]
The results were interpreted as evidence that shifting of hippocampal rhythms relative to the SCN pacemaker represents a state of circadian misalignment that disrupts hippocampal memory functions
Summary
Nocturnal rats and mice with free access to food are active and eat predominantly at night, under control of a light-dark (LD)-entrainable circadian pacemaker in the suprachiasmatic nucleus (SCN). It is possible that daytime restricted feeding shifts these rhythms, and that memory functions may not be impaired at specific circadian phases, such as prior to mealtime, when day-fed mice are spontaneously awake and actively seeking food This might be missed if only two daily time points are sampled, as in the Loh et al.[12] study. There was no impairment of spontaneous alternation or contextual fear conditioning, two additional tests sensitive to hippocampal disruption[22,23] These results support the view that stable alterations in circadian timing induced by daily feeding schedules represent an adaptive re-alignment of internal temporal order, rather than a ‘misalignment’ that disrupts cognitive functions important for survival
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