Reentrainment of the spontaneous locomotor activity rhythm to a daylight reversal in C57BL/6 and C3H/He mice: implication of melatonin

Influence of the Corticosterone Rhythm on Photic Entrainment of Locomotor Activity in Rats

The question of involvement of glucocorticoid hormones as temporal signals for the synchronization of the timekeeping system was addressed in rats with different corticosterone status. The authors showed that adrenalectomy had no effects on the synchronization of wheel-running activity rhythms to a steady-state LD 12:12 cycle, regardless of whether it was compensated for by a corticosterone replacement therapy that either reinstated constant plasma concentrations of the hormone or mimicked its natural rhythm. However, after a 12-h phase shift (daylight reversal), the lack of circulating corticosterone induced a significant shortening of the resynchronization rate (less than 3 days vs. 7 days). Normalization required restoration of a rhythmic corticosterone secretion that was synchronized to the new photoperiod. Under constant darkness, the corticosterone rhythm did not show any synchronizing effect, providing evidence that it participates in entrainment of the locomotor activity rhythm through modulation of light effects. It is proposed that, under stable lighting conditions, circulating glucocorticoids contribute to stabilizing activity rhythms by reinforcing resistance of the circadian timing system to variations of the photoperiod. Experimental evidence that serotonergic neurons are involved in relaying their modulatory effects to the clock is also presented.

Influence of estrus cycle and ageing on activity patterns in two inbred mouse strains

Despite the widespread use of inbred mice in research, little is known about aging of the circadian system in female mice, although interactions between female gonadal hormones and circadian rhythms have been established. We investigated the influence of the estrus cycle on circadian aspects of running-wheel activity and changes in the course of aging in female C57BL/6 and C3H/He mice recorded continuously between the ages of 3 and 19 months. In the young, cycling mice the second part of the proestrus night was often, but not consistently, characterized by increased motor activity compared to the remaining estrus cycle nights. After estrus cycling had ceased in the course of ageing, the estrus-dependent day-to-day variability in activity was reduced. The amplitude of the daily rest-activity rhythm decreased progressively after the age of 8 months in C3H/He and 10 months in C57BL/6 mice. The capacity for resynchronisation of activity onset to the LD-cycle was compared in young and old mice after an 8-h phase advance of the LD-cycle. Resynchronisation was significantly slower in old C3H/He mice and unaffected by age in C57BL/6 mice. The circadian period in constant darkness did not change with age in either strain. However, the period was shorter in 17-month old C57BL/6 mice compared to an additional group, which was recorded at the same age, after at least 1-month adaptation to the recording conditions. The results show that the reproductive state as well as ageing influence motor activity patterns of female mice in a strain- and cohort-dependent manner.

Circadian rhythm entrainment with melatonin, melatonin receptor antagonist S22153 or their combination in mice exposed to constant light

The ability of daily melatonin and the melatonin receptor antagonist, S22153, to entrain circadian system function was investigated in mice with atypical melatonin rhythm. B6D2F(1) mice were first synchronized to a LD 12:12 for approximately 2 wk, then exposed to continuous light (LL) until study completion. After 10-18 days of LL exposure, mice received daily subcutaneous (s.c.) melatonin at a dose of 0.1, 1 or 10 mg/kg/day (exp. 1) or daily intraperitoneal (i.p.) S22153 (20 mg/kg/day) with or without melatonin (1 mg/kg/day, exp. 2) at subjective zeitgeber time (ZT) 10 for 19 days. Then all the mice were exposed to LL for another 10 days. Spectral analysis showed that initial LL lengthened the period of both rhythms by approximately 1.5 hr as compared with LD 12:12. No entrainment of either rhythm was found in controls. Conversely, daily melatonin-only, S22153-only or their combination set the temperature and activity periods to approximately 24 hr and produced a significant increase of the circadian amplitude of both rhythms as compared with controls. However, after treatment withdrawal, the dominant period lengthened to approximately 25.5 hr in mice receiving either melatonin or S22153. On the contrary, the period remained close to 24 hr for the 10 days following withdrawal of combined S22153 and melatonin. Such sustained pharmacological resetting of circadian function could display therapeutic potential against external resynchronization resulting from defective photoperiodic entrainment.

Locomotor activity rhythm in inbred strains of mice: implications for behavioural studies

Due to the well-known influence of arousal on behavioural responsiveness, this paper focused on the differences in the daily locomotor activity rhythm between inbred strains. The lighting conditions of the environment can provide a regular, defined LD cycle or be constant, e.g. constant dim light or constant darkness. Under a light:dark cycle (daily rhythm) and under free-running conditions, i.e. constant darkness (circadian rhythm), such differences between mouse strains emphasise the relevance of a defined time of day for performing tests in order to obtain reliable behavioural results, and consequently the importance of a well-controlled LD cycle, which constitutes a necessary condition for the stability of daily rhythms. Both the LD cycle in the animal room (during breeding, as well as during the experiments) and the experiment schedule within the LD cycle appear as important factors for a better understanding of behavioural results. This information may be relevant to explain part of the apparently contradictory behavioural results reported in the literature.