Circadian rhythms in blinded rats: correlation between pineal and activity cycles.

Area of Science:

• Chronobiology
• Neuroendocrinology
• Physiology

Background:

• The rat pineal gland shows daily fluctuations in N-acetyltransferase activity and serotonin levels.
• Diurnal rhythms are crucial for physiological regulation in mammals.

Purpose of the Study:

• To investigate the endogenous nature of circadian rhythms in the rat pineal gland.
• To assess the persistence of these rhythms in rats deprived of light cues for extended periods.

Main Methods:

• Rats were blinded for 37 and 60 days to eliminate light-entrainment.
• Continuous monitoring of running activity rhythms identified midrest and midrun phases.
• Pineal gland N-acetyltransferase activity and serotonin levels were measured at these specific activity phases.

Main Results:

• Circadian variations in pineal N-acetyltransferase and serotonin were observed in blinded rats.
• Enzyme levels peaked during the midrun phase, while substrate (serotonin) levels were lowest.
• These rhythms persisted with undamped amplitudes, synchronized with the rats' activity cycle.

Conclusions:

• The study demonstrates the endogenous generation of circadian rhythms in the rat pineal gland.
• These internal rhythms persist robustly for at least 60 days in constant darkness.
• The findings highlight the intrinsic self-sustaining nature of biological clocks.