Entrainment of Circadian Wheel-Running Rhythms of the Northern Brown Bandicoot,Isoodon macrourus, by Daily Restricted Feeding Schedules

Regulation of the rabbit's once-daily pattern of nursing: a circadian or hourglass-dependent process?

The European rabbit Oryctolagus cuniculus has an unusual pattern of nursing behavior. After giving birth in a nursery burrow (or laboratory nest box), the mother immediately leaves the young and only returns to nurse for a few minutes once approximately every 24 h. It has been assumed this schedule, like a variety of other functions in the rabbit, is under circadian control. This assumption has been largely based on findings from mothers only permitted restricted access to their young once every 24 h. However, in nature and in the laboratory, mothers with free access to young show nursing visits with a periodicity shorter than 24 h, that does not correspond to other behavioral and physiological rhythms entrained to the prevailing 24 h light/dark (LD) cycle. To investigate how this unusual, apparently non-circadian pattern might be regulated, we conducted two experiments using female Dutch-belted rabbits housed individually in cages designed to automatically register feeding activity and nest box visits. In Experiment 1 we recorded the behavior of 17 mothers with free access to their young under five different LD cycles with long photo and short scotoperiods, spanning the limits of entrainment of the rabbit's circadian system. Whereas feeding rhythms were entrained by LD cycles within the rabbit's circadian range of entrainment, nursing visits showed a consistently shorter periodicity regardless of the LD regimen, largely independent of the circadian system. In Experiment 2 we tested further 12 mothers under more conventional LD 16:8 cycles but "trained" by having access to the nest box restricted to 1 h at the same time each day for the first 7 d of nursing. Mothers were then allowed free access either when their young were left in the box (n = 6), or when the litter had been permanently removed (n = 6). Mothers with pups still present returned to nurse them on the following days according to a similarly advancing pattern to the mothers of Experiment 1 despite the previous 7 d of "training" to an experimentally enforced 24 h nursing schedule as commonly used in previous studies of rabbit maternal behavior. Mothers whose pups had been removed entered the box repeatedly several times on the first day of unrestricted access, but on subsequent days did so only rarely, and at times of day apparently unrelated to the previously scheduled access. We conclude that the pattern of the rabbit's once-daily nursing visits has a periodicity largely independent of the circadian system, and that this is reset at each nursing. When nursing fails to occur nest box visits cease abruptly, with mothers making few or no subsequent visits. Together, these findings suggest that the rabbit's once-daily pattern of nursing is regulated by an hourglass-type process with a period less than 24 h that is reset at each nursing, rather than by a circadian oscillator. Such a mechanism might be particularly adaptive for rhythms of short duration that should end abruptly with a sudden change in context such as death or weaning of the young.

The effects of lighting conditions and food restriction paradigms on locomotor activity of common spiny mice, Acomys cahirinus

BACKGROUND

An endogenous circadian clock controls locomotor activity in common spiny mice (Acomys cahirinus). However, little is known about the effects of constant light (LL) on this activity or about the existence of an additional food entrainable clock. A series of experiments were performed to investigate the effects of LL and DD on tau and activity levels.

METHODS

Spiny mice were housed individually and their running wheel activity monitored. One group of mice was exposed to LD, DD and several intensities of LL. Another group was exposed to a restricted feeding (RF) paradigm in light: dark (LD) during one hour before the L to D transition. Significance of rhythmicity was assessed using Lomb-Scargle periodograms.

RESULTS

In LD all animals exhibited nocturnal activity rhythms that persisted in DD. When animals were exposed to RF (during L), all of these animals (n = 11) demonstrated significant food anticipatory activity as well as an increase in diurnal activity. This increase in diurnal activity persisted in 4/11 animals during subsequent ad libitum conditions. Under LL conditions, the locomotor rhythms of 2/11 animals appeared to entrain to RF. When animals were exposed to sequentially increasing LL intensities, rhythmicity persisted and, while activity decreased significantly, the free-running period was relatively unaffected. In addition, the period in LL was significantly longer than the period in DD. Exposure to LL also induced long-term changes (after-effects) on period and activity when animals were again exposed to DD.

CONCLUSIONS

Overall these studies demonstrate clear and robust circadian rhythms of wheel-running in A. cahirinus. In addition, LL clearly inhibited activity in this species and induced after-effects. The results also confirm the presence of a food entrainable oscillator in this species.

A daily palatable meal without food deprivation entrains the suprachiasmatic nucleus of rats

Food is considered a potent Zeitgeber for peripheral oscillators but not for the suprachiasmatic nucleus (SCN), which is entrained principally by the light-dark cycle. However, when food attains relevant properties in quantity and quality, it can be a potent Zeitgeber even for the SCN. Here we evaluated the entrainment influence of a daily palatable meal, without regular food deprivation, on the circadian rhythm of locomotor activity and the c-Fos and PER-1 protein expression in the SCN. Rats fed ad libitum, in constant darkness, received a palatable meal for 6 weeks starting in the middle of the subjective day. Locomotor activity showed entrainment when the offset of activity coincided with the palatable meal-time. In the SCN, the peak expression of c-Fos was observed at palatable meal-time and PER-1 showed a peak during the onset of subjective night, as predicted according to the behavioural entrained pattern. In addition, c-Fos and PER-1 expression in the paraventricular thalamic nucleus (PVT) showed increased expression at palatable meal-time, while the intergeniculate leaflet did not, suggesting that the PVT may be involved as an input pathway of palatable food-entrainment to the SCN. These results demonstrate that daily access to a palatable meal can entrain the SCN; several stimuli can be implicated in this process, including motivation and arousal.

Effects of prolonged exposure to darkness on circadian photic responsiveness in the mouse

Circadian rhythms in mammals are generated by an endogenous pacemaker but are modulated by environmental cycles, principally the alternation of light and darkness. Although much is known about nonparametric effects of light on the circadian system, little is known about other effects of photic stimulation. In the present study, which consists of a series of five experiments in mice, various manipulations of photic stimulation were used to dissect the mechanisms responsible for a variation in the magnitude of light-induced phase-shifts that results from prolonged exposure to darkness. The results confirmed previous observations that prolonged exposure to darkness causes an increase in the magnitude of phase shifts (both phase advances and phase delays) evoked by discrete light pulses. The results also indicated that the increase in responsiveness results from the lack of exposure to light per se and not from collateral effects of exposure to constant darkness such as the lack of previous entrainment. The lack of exposure to light causes the circadian system to undergo a process of dark adaptation similar to dark adaptation in the visual system but with a much slower temporal course. The results suggest that circadian dark adaptation may take place at the retinal level, but it is not clear whether it involves a change in the sensitivity or maximal responsiveness of the system.

Daily hypocaloric feeding entrains circadian rhythms of wheel-running and body temperature in rats kept in constant darkness

Since high levels of physical activity are known to interact with the circadian system, we tested whether hyperactivity induced by hypocaloric feeding may facilitate the synchronizing effect of periodic feeding in rats in constant darkness. Wheel-running (WRR) and body temperature (BTR) rhythms were analyzed in rats fed daily 50% of ad libitum food intake. In addition to a feeding-associated component (FAC) expressed for both rhythms, WRR and BTR were phase-shifted towards the feeding time by 12.1 +/- 0.4 and 12.4 +/- 0.4 h, respectively. BTR and FAC of temperature merged into a single cosine-wave food-entrained rhythm (FER) of temperature. Similarly, WRR and FAC of activity merged into a unimodal FER of wheel-running. This indicates that daily hypocaloric feeding may synchronize circadian rhythms in rats.