LOCOMOTOR-ACTIVITY AND MELATONIN RHYTHMS IN RATS UNDER NON-24-H LIGHTING CYCLES

The adjustment of pineal melatonin and locomotor activity rhythms to 1 0:10-h light:dark (LD) or 14:14-h LD cycles was studied in male Wistar rats. Both lighting conditions were thought to be outside the limits of entrainment of the rest-activity rhythm in this species. We assumed that the rhythm of pineal melatonin synthesis might be more adaptable . As expected, the locomotor activity rhythm was not adjusted to the 1 0:10-h LD cycles. Under these conditions, a free-running component (25 h) became dominant. Under the 14:14-h LD cycles, however, an unexpect ed adaptation occurred within 10 days. The profiles of the pineal mela tonin contents measured on days 5 and 30 under the 10:10-h LD and on d ay 7 under the 14:14-h LD schedule were in line with the estimated fre e-running oscillations, but the profile on day 21 under the 14:14-h LD schedule was not. This melatonin pattern fitted the LD-adjusted activ ity rhythm. Thus, the melatonin rhythm did not adapt better than the a ctivity rhythm to the exotic LD cycles. Instead, parallel changes were found.