The internal synchronization of five circadian functions of the rabbit.

Five different physiological functions of the rabbit (hard faeces and urine excretion, food and water intake and locomotor activity) were registered during LD 12:12 and during continuous light conditions (LL). (1) In LD 12:12 a strong synchronization of the five parameters existed. The minima of all functions consistently occurred during the hours of light. The nocturnal percentage of overall 24-hr events was increased significantly in 'hard faeces excretion' (66 +/- 8(S.D.)%), 'water intake' (64 +/- 15(S.D.)%) and 'urine excretion' (58 +/- 10(S.D.)%). The nocturnal percentage of locomotor activity was significantly increased during the dark-hours in 9 out of 14 animals. In the other five individuals prominent peaks were present even during the photoperiod. On the average of all 14 animals 55 +/- 13(S.D.)% of the 24 hr events of locomotor activity occurred during the night. Despite a trough during the cessation of hard faeces excretion the events of food intake were not elevated significantly during the dark hours. (2) During LL the synchronization of the five functions within each animal persisted during the complete 90-day LL period. A free-running circadian rhythm with tau = 24.8 +/- 0.5(S.D.) hr was present in the four rabbits kept in LL conditions within 5-16 days after the withdrawal of the zeitgeber. (3) In addition to the circadian period the power spectrum analysis of data obtained during LD 12:12 revealed significant ultradian periods of an average period length of 11,6 hr (hard faeces and urine excretion), 8 hr (food and water intake, locomotor activity) and 4 hr (food intake, locomotor activity). During the free-run ultradian periods of 8 and 3.2-4.2 hr were significant in almost all parameters. (4) During LL the level of locomotor activity was reduced for 13 +/- 16(S.D.)%, the events of food intake were increased for 17 +/- 12(S.D.)%. (5) The reinserted LD 12:12 zeitgeber re-entrained the circadian rhythms within 25-45 days. (6) These results provided evidence of a predominant nocturnality of the rabbits under investigation.