Locomotor activity and melatonin rhythms in rats under non-24-h lighting cycles

• Published in: Physiology & behavior 1995-05, Vol.57 (5), p.849-856
• Main Authors: Laakso, Maija-Liisa, Leinonen, Lea, Joutsiniemi, Sirkka-Liisa, Porkka-Heiskanen, Tarja, Alila, Aino
• Format: Article
• Language: English
• Subjects: Animals; Behavioral psychophysiology; Biological and medical sciences; Circadian; Circadian Rhythm - physiology; Entrainment; Fundamental and applied biological sciences. Psychology; Hormones and behavior; Illumination; Light; Male; Melatonin - blood; Motor Activity - physiology; Phase shifting; Photoperiod; Pineal gland; Psychology. Psychoanalysis. Psychiatry; Psychology. Psychophysiology; Rats; Rats, Wistar; Rest-activity rhythm; Suprachiasmatic Nucleus - physiology; Synchronization
• ISSN: 0031-9384; 1873-507X
• DOI: 10.1016/0031-9384(94)00311-R

The adjustment of pineal melatonin and locomotor activity rhythms to 10:10-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 10:10-h LD cycles. Under these conditions, a free-running component (25 h) became dominant. Under the 14:14-h LD cycles, however, an unexpected adaptation occurred within 10 days. The profiles of the pineal melatonin contents measured on days 5 and 30 under the 10:10-h LD and on day 7 under the 14:14-h LD schedule were in line with the estimated free-running oscillations, but the profile on day 21 under the 14:14-h LD schedule was not. This melatonin pattern fitted the LD-adjusted activity rhythm. Thus, the melatonin rhythm did not adapt better than the activity rhythm to the exotic LD cycles. Instead, parallel changes were found.