Endogenous ultradian rhythms in rats exposed to prolonged continuous light

Area of Science:

• Chronobiology
• Animal Physiology
• Neuroendocrinology

Background:

• Circadian rhythms regulate physiological processes over ~24 hours.
• Disruption of circadian rhythms can lead to health issues.
• The impact of prolonged constant light on ultradian rhythms is not fully understood.

Purpose of the Study:

• To investigate the effects of prolonged constant light exposure on circadian and ultradian rhythms in rats.
• To determine if ultradian rhythms are fundamental components of circadian function.
• To examine the synchronization between locomotor activity, body temperature, and plasma corticosterone under constant light.

Main Methods:

• Simultaneous monitoring of locomotor activity, body temperature, and plasma corticosterone in individual rats.
• Exposure to 200 lx continuous light for over 3 months.
• Serial blood sampling at 2-hour intervals for corticosterone analysis.
• Least-squares spectrum analysis to identify periodicities.

Main Results:

• Free-running circadian rhythms persisted for ~2 months before gradually decomposing under constant light.
• After 3 months, circadian rhythms disappeared, replaced by ultradian activity bursts (1-2 hours).
• Body temperature and plasma corticosterone fluctuations synchronized with locomotor activity bursts, exhibiting 4-6 hour periods.

Conclusions:

• Prolonged constant light reveals an underlying ultradian rhythmicity in rats.
• This ultradian component appears to be a fundamental unit of circadian rhythm.
• A common oscillator may govern the ultradian rhythms observed in activity, temperature, and corticosterone.