Hormonal and locomotor activity rhythms in rats under 90-min dark-pulse conditions.

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.