AJP - Regulatory, Integrative and Comparative Physiology, Vol 264, Issue 6 1058-R1064, Copyright © 1993 by American Physiological Society
Hormonal and locomotor activity rhythms in rats under 90-min dark-pulse conditions
M. L. Laakso, T. Porkka-Heiskanen, L. Leinonen, S. L. Joutsiniemi and P. T. Mannisto
Department of Physiology, University of Helsinki, Finland.
The ability of a short dark pulse to entrain the circadian rhythms in rats
was investigated. Pineal melatonin contents and serum levels of
corticosterone and thyrotropin, a thyroid-stimulating hormone (TSH), were
measured and locomotor activity was recorded under 12:12-h light-dark
cycles (LD; darkness from 1800 to 0600 h) and under a 22.5:1.5 h LD
lighting schedule (darkness from 1800 to 1930 h). The 90-min dark pulse was
enough to trigger the rise of melatonin synthesis, but a free-running
component was detected in the locomotor activity. Corticosterone levels
showed diurnal variations under both conditions. The decrease of
corticosterone and the increase of melatonin were phase locked, but the
corticosterone pattern was distorted under the dark-pulse conditions. The
24-h rhythm of TSH was detectable in the control but not in the dark-pulse
schedule. The results suggest that the circadian rhythm of pineal melatonin
and the decrease of serum corticosterone levels were entrainable by the
dark pulses, whereas the increase of corticosterone, the variations of TSH,
and the rhythm of locomotor activity were not.
