Alterations of motor activity circadian rhythm in rats with adjuvant arthritis

Adjuvant arthritis in rats produces alterations in the motor activity circadian rhythm. Specifically arthritic animals show a decrease in the total daily motor activity and an advance in the acrophase of the rhythm. Slight changes are also observed in the power content of the circadian harmonic as well as in the amplitude.     

Effects of T cycles of light/darkness and periodic forced activity on methamphetamine-induced rhythms in intact and SCN-lesioned rats: explanation by an hourglass-clock model

In this study intact and suprachiasmatic nuclei (SCN)-lesioned female rats were treated with chronic methamphetamine (MA) via the drinking water. Body temperature, feeding, drinking and wheel-running activity were continuously and automatically recorded. The rats were subjected to light-dark (LD) cycles with period T = 23 hr for 4 months and subsequently T = 25 hr for 3 months. Daily 3-hr forced activity (FA 3:21) was imposed during a few weeks under both LD regimes. MA induced infradian rhythms (period tau s = 28-54 hr) that were found to run parallel in all functions. In intact rats these infradian rhythms showed relative coordination by the LD regime and tau s shortened when T lengthened. In SCN-lesioned rats, however, the infradian rhythms were independent of the LD regime. Under the FA cycles tau s lengthening as well as synchronization was observed. We hypothesized that the MA-induced rhythms reflect a long-period sleep-wake cycle of the hourglass type. We investigated this hypothesis with a modified version of the hourglass-clock model of sleep regulation. Computer simulations showed that this model might offer an explanation for the experimental observations.     

Circadian desynchronization of core body temperature and sleep stages in the rat

Proper functioning of the human circadian timing system is crucial to physical and mental health. Much of what we know about this system is based on experimental protocols that induce the desynchronization of behavioral and physiological rhythms within individual subjects, but the neural (or extraneural) substrates for such desynchronization are unknown. We have developed an animal model of human internal desynchrony in which rats are exposed to artificially short (22-h) light-dark cycles. Under these conditions, locomotor activity, sleep-wake, and slow-wave sleep (SWS) exhibit two rhythms within individual animals, one entrained to the 22-h light-dark cycle and the other free-running with a period >24 h (tau(>24 h)). Whereas core body temperature showed two rhythms as well, further analysis indicates this variable oscillates more according to the tau(>24 h) rhythm than to the 22-h rhythm, and that this oscillation is due to an activity-independent circadian regulation. Paradoxical sleep (PS), on the other hand, shows only one free-running rhythm. Our results show that, similarly to humans, (i) circadian rhythms can be internally dissociated in a controlled and predictable manner in the rat and (ii) the circadian rhythms of sleep-wake and SWS can be desynchronized from the rhythms of PS and core body temperature within individual animals. This model now allows for a deeper understanding of the human timekeeping mechanism, for testing potential therapies for circadian dysrhythmias, and for studying the biology of PS and SWS states in a neurologically intact model.     

Effect of melatonin and diazepam on the dissociated circadian rhythm in rats

The main structures involved in the circadian system in mammals are the suprachiasmatic nuclei (SCN) of the hypothalamus. The SCN contain multiple autonomous single-cell circadian oscillators that are coupled among themselves, generating a single rhythm. However, under determined circumstances, the oscillators may uncouple and generate several rhythmic patterns. Rats exposed to an artificially established 22-h light-dark cycle (T22) express two stable circadian rhythms in their motor activity that reflect the separate activities of two groups of oscillators in the morphologically well-defined ventrolateral and dorsomedial SCN subdivisions. In the experiments described in this paper, we studied the effect of melatonin and diazepam (DZP) administration in drinking water on the dissociated components of rat motor activity exposed to T22, to deduce the possible mechanism of these drugs on the circadian system. In order to suppress the endogenous circadian rhythm of melatonin, in some of the rats the pineal gland or the superior cervical ganglia were removed. The results show that melatonin or DZP treatment increased the manifestation of the light-dependent component to the detriment of the manifestation of the non-light-dependent component and that melatonin, but not DZP, shortens the period of the non-light-dependent component. These findings suggest that both DZP and melatonin favor entrainment to external light, and that melatonin could also act on the SCN, producing changes in the period of the circadian cycle.     

Circadian rhythms on skin function of hairless rats: light and thermic influences

Circadian rhythms are present in most functions of living beings. We have demonstrated the presence of circadian rhythms in skin variables (transepidermal water loss, TEWL; stratum corneum hydration, SCH; and skin temperature) in hairless rats under different environmental conditions of light and temperature. Circadian rhythms in TEWL and SCH showed mean amplitudes of about 20% and 14% around the mean, respectively, and appeared under light–dark cycles as well as under constant darkness. Environmental temperature was able to override TEWL, but not SCH rhythm, evidencing the dependency of TEWL on the temperature. Mean daily values of TEWL and SCH, and also the amplitude of TEWL rhythm, increased with the age of the animal. Under constant light, situation that induces arrhythmicity in rats, SCH and TEWL were inversely correlated. The results suggest the importance to take into account the functional skin rhythms in research in dermatological sciences.     

Entrainment of the rat motor activity rhythm: effects of the light-dark cycle and physical exercise

The circadian system is believed to be composed of a population of oscillators that couple together and generate a single rhythm. If this coupling is not strong enough, the circadian system can be dissociated into two or more groups of oscillators, and this is manifested in a dissociation of the overt rhythm into at least two circadian components. This study aims to examine the influence of factors, such as the difference in impact between T and tau, light intensity, and access to a running wheel, on the distribution of motor activity throughout the light-dark (LD) cycle and the dissociation of the rhythm. Rats were submitted to LD cycles of 23 h (T23) or 25 h. For each such cycle, half the rats were submitted to high light intensity and the other half to low light intensity. For each of these conditions, half the rats were kept in small cages, and the other half were in cages with a running wheel. Rats were maintained first under LD cycles and afterwards under constant darkness (DD). Motor activity was recorded throughout the whole experiment by means of activity meters with infrared beams. Results show that the distribution of motor activity throughout the cycle and the after effects observed in the rhythm under DD depended on light intensity and access to the wheel. Moreover, under T23, some rats showed two simultaneous circadian components whose manifestation also depended on the experimental conditions. The results indicate that the strength of circadian entrainment to LD cycles in the rat depends on three factors: the period length of the LD cycle, light intensity used during the light phase, and access to a running wheel.     

Sex differences in the development of the motor activity circadian rhythm in rats under constant light

Differences between sexes in the maturation of the motor activity rhythm have been studied in rats (10 females and 7 males) born and raised under LL and isolated from the day of weaning. Motor activity data were registered every 30 minutes for 45 days after weaning. Periodogram analysis was applied to each animal data and the daily power spectra were calculated on the basis of the period, tau. The process of rhythm evolution was studied by means of an analysis of variance of a multivariant linear model, and by defining two mathematical functions that fit to the evolution of the total daily motor activity (TDMA) and the power content of the circadian harmonic (PCCH) through time: a sigmoidal function, which presents an inflexion was used for the study of the evolution of the PCCH values through time: and a gaussian function, which presents a minimum was used for the evolution of the TDMA values. Results indicate that males show more ultradian components than females just after weaning, and take a longer time to acquire the circadian harmonic as the main harmonic in the daily power spectrum. The acquisition of the circadian character is preceded by a decrease in the TDMA, in the case of the males and is followed by a decrease in TDMA in the case of the females. These results suggest sexual differences in the organization and maturation of the circadian system in rats.     

Period length of the light-dark cycle influences the growth rate and food intake in mice

The adaptation of the endogenous rhythm of an organism to external cycles may be critical for the development of physiological processes in which energy is expended. We sought to determine whether growth rate depends on the degree of tuning between the external cycle and the manifestation of the circadian rhythms. To do so, we studied the growth rate and the food intake of mice (seven groups of 20 animals each) kept under symmetric LD cycles with different periods (T) of 21, 22, 23, 24, 25, 26, and 27 h, respectively, for 80 days. The mice were then kept in constant darkness for a further 80 days. Throughout the experiment, motor activity was recorded every 15 min for each mouse by means of an actimeter that used crossed infrared beams. Several variables related to the circadian motor activity rhythm were calculated, and correlated with body weight, food intake, and the efficiency of food for growth (food efficiency) calculated as: 100 x body weight increase/ amount of food intake. Results show that these three variables seem to be influenced by the number of circadian cycles that the animal has experienced, but also, and more significantly, by the degree to which the alpha phase of the individual rhythm and the dark phase of the external cycle coincide. Therefore, circadian rhythms would seem to affect the physiological processes that regulate growth and energy expenditure.     

Effect of Robertsonian Translocations on the Motor Activity Rhythm in the House Mouse

Here we studied the circadian rhythm of motor activity in two groups of wild house mice from the chromosomal polymorphic zone of Barcelona, which differed in diploid number (2n): standard (2n = 40), with all acrocentric chromosomes, and Robertsonian (2n = 29–32), with several Robertsonian translocations. Motor activity under three lighting conditions, light-dark cycle, constant darkness, and constant light, was recorded for each mouse. The motor activity rhythm was examined by Fourier analysis and the daily power spectra were obtained. On the basis of the mean power spectrum of each animal and under each lighting condition, stepwise discriminant analyses were performed to classify the two chromosomal groups. This method allowed the correct classification of a large number of animals, the rhythms of about 2–2.6 hour periods being the most significant, with higher values in Robertsonian than in standard mice. Our results indicate that the daily motor activity pattern differs between the two chromosomal groups and its analysis may have a valuable interest for behavioral investigations on Robertsonian polymorphic zones of this species.     

Social interaction and sex differences influence rat temperature circadian rhythm under LD cycles and constant light

Circadian rhythms produce an efficient organization of animal behaviour over the 24h day. In some species, social cues have been found to have a role as synchronizers of these rhythms. Here, the influence of social interaction on rat circadian behaviour was investigated, addressing the question of whether cohabitation would produce a delay in the appearance of arrhythmicity under constant light conditions. To this end, the circadian rhythms of male and female rat body temperature were studied for 10days under light-dark conditions, followed by 33days under constant bright light. Half of the animals were maintained in individual cages, whilst the others were maintained in larger cages in groups of three rats of the same sex. Results showed that individual circadian rhythms under 24hour light-dark (LD) cycles were more stable and with higher amplitude in grouped than in isolated animals, and higher in males than in females. During the first days under constant light (LL), the stability of the rhythm was also higher in males than in females, but there were no differences according to the group. Moreover, we did not find significant differences in the time of circadian rhythm loss under LL, since high individual variability was found for this variable. On the other hand, female rats living in isolation showed a delayed acrophase in the circadian rhythm under LD conditions compared with those living in groups. These results suggest that cohabitation increases the internal coherence of circadian behaviour, and could be interpreted as indicating that living in isolation may induce a level of stress that disturbs manifestation of the circadian rhythm, especially in females, which are also more reactive than males to external signals.