Vol. 284, Issue 5, R1255-R1259, May 2003
Interruption of the rat circadian clock by short light-dark
cycles
Setsuo
Usui,
Terue
Okazaki, and
Yoshiko
Honda
Department of Psychology, Tokyo Metropolitan Institute for
Neuroscience, Fuchu, Tokyo 183-8526, Japan
Ninety male Sprague-Dawley rats
were exposed to 1:1-h light-dark (LD1:1) cycles for 50-90 days,
and then they were released into constant darkness (DD). During LD1:1
cycles, behavioral rhythms were gradually disintegrated, and circadian
rhythms of locomotor activity, drinking, and urine 6-sulfatoxymelatonin
excretion were eventually abolished. After release into DD, 44 (49%)
rats showed arrhythmic behavior for >10 days. Seven (8%) animals that
remained arrhythmic for >50 days in DD were exposed to brief light
pulses or 12:12-h light-dark cycles, and then they restored their
circadian rhythms. These results indicate that the circadian clock was
stopped, at least functionally, by LD1:1 cycles and was restarted by
subsequent light stimulation.
circadian rhythm; arrhythmicity; constant darkness; melatonin
