Circadian entrainment of the squirrel monkey by extreme photoperiods: Interactions between the phasic and tonic effects of light

To examine the role that the phasic and tonic aspects of the light-dark (LD) cycle play in entraining the circadian timing system of primates, squirrel monkeys (Saimiri sciureus) were exposed to 24 hr LD cycles in which the light duration (photoperiod) was varied from 1 sec to 23 hr. The monkeys were maintained in isolation and the circadian rhythm of drinking was monitored. The photoperiod was first gradually shortened until constant darkness was reached. Even in extremely short photoperiods of only one second of light per day, the drinking rhythm remained synchronized to the 24 hr period of the LD cycle. In the second set of experiments, the photoperiod was gradually lengthened until constant light was achieved. The drinking rhythm of all monkeys was synchronized by 21 hr photoperiods (LD 21:3), but free-ran in 23 hr photoperiods (LD 23:1) which provided a 1 hr dark pulse each day. The tonic effects of light may contribute to the difference between the ability to entrain to short versus long photoperiods. In constant darkness the free-running period was close to 24 hr, thus reducing the phase-resetting necessary to achieve entrainment to a 24 hr period by short light pulses. However, in constant light or in the long photoperiods which did not entrain (LD 23:1) the free-running period of the drinking rhythm was greater than 25 hr, thus requiring a much larger daily phase shift to achieve entrainment to a 24 hr period.     

Circadian behavioral rhythms during various light-intensity cycles in rats

Sleep, ambulation and drinking were recorded on 5 kinds of light-intensity cycles fluctuating between 300 lux and 0 lux with a 24-hr period in rats. The waveforms of the cycles were rectangular wave (RA), sinusoidal wave (SO), triangular wave (TA), descending saw-tooth wave (ST-d) and ascending saw-tooth wave (ST-a). Each condition was maintained for 28 days. Thereafter, the rats were released into constant darkness (DD) for 20 days. The circadian behavioral rhythms clearly entrained to RA, SO, TA and ST-a, but not to ST-d. The waveforms of the behavioral rhythms varied depending on those of the fluctuating light-intensity cycles, and were quite different from those in RA and DD. Daily amounts of slow wave sleep and paradoxical sleep were hardly affected by the lighting conditions, while those of ambulation and drinking were decreased on the other light-intensity cycles than RA. These results present new aspects of entrainment and masking of circadian rhythm by light.     

Entrainment and masking of circadian drinking rhythms in primates: Influence of light intensity

The entrained drinking rhythms of squirrel monkeys were studied during exposure to 24 hr illumination cycles of three different intensities (60:0; 66:6; 76:16 lux). Increasing the intensity of ambient illumination significantly delayed the offset of drinking but had no effect on either the onset or the total amount of daily drinking behavior. Comparison of drinking behavior under an alternating schedule of LD 66:6 lux and constant light of 6 lux indicated that the twice daily light transitions consistently altered the temporal distribution of drinking behavior. The daily timing of squirrel monkey drinking behavior thus, depends not only on the mechanisms of circadian entrainment to the LD cycle, but also on the ability of the LD cycle to directly influence, or “mask” behavior.     

Scheduled daily exercise or feeding alters the phase of photic entrainment in Syrian hamsters

Single daily bouts of appropriately timed activity can phase-shift or entrain circadian rhythms in rodents maintained in constant dark (DD). Whether this apparent feedback of behavioral activity to the circadian pacemaker has any adaptive significance is unclear; circadian rhythms are normally entrained by light-dark (LD) cycles, and this may override any effects of activity. To address this issue, the phase of entrainment to LD cycles was examined in hamsters exposed to a daily exercise schedule (3 h of induced wheel running). Hamsters exercised late in the dark showed a significant delay of entrained phase in LD (i.e., they became relative “night owls”) and lengthening of free-running periodicity in DD, compared to controls and hamsters exercised in midlight. Hamsters fed in midlight (arousal without wheel running) showed a significant advance of LD entrained phase (i.e., they became “early birds”). These observations provide the necessary rationale for further examination of the functional significance of behavioral feedback for the normal entrainment process. In addition, they raise the possibility that the entrained phase of human circadian rhythms can be similarly manipulated by behavioral procedures such as timed exercise.     

Circadian rhythms of activity and drinking in mice lacking angiotensin II 1A receptors.

Angiotensin II (ANG II) type 1 receptors are found in the mouse suprachiasmatic nucleus (SCN), the site of the circadian pacemaker, but their significance for circadian timekeeping is unknown. We examined circadian rhythms of wheel running and drinking in angiotensin AT(1a) receptor knockout (KO) mice. Mean daily running and drinking activity were elevated in KO mice under a light-dark (LD) cycle and in constant dark (DD). These increases were confined to the usual active (dark) period, thus, the 'amplitude' of running and drinking rhythms was higher in KO mice. The phase of entrainment to LD (measured by the onset of the daily active period) did not differ between groups, either in LD or on the first day of DD ('unmasked' phase). KO mice showed a modestly shorter free-running period (tau) in DD. The direction and magnitude of phase shifts to light pulses at two circadian times (CTs) in DD did not differ between groups. Core functions of the circadian system appear intact following AT(1a) receptor KO. The modestly shorter tau and increased rhythm amplitude in KO mice may be secondary to an effect of the mutation on the level of running and drinking activity.     

Daily activity patterns of a nocturnal and a diurnal rodent in a seminatural environment

The entrainment of circadian rhythms by light-dark (LD) cycles has been extensively investigated in laboratory studies. In almost all of these studies, organisms have not been allowed to modulate their exposure to the LD cycle. In the present study, the rhythm of running-wheel activity was investigated in nocturnal (domestic mice) and diurnal (Nile grass rats) rodents provided with light-tight nest boxes and maintained under long and short photoperiods. Photoperiod length had a significant effect on the duration of the daily active phase (alpha), on the phase angle of entrainment (psi), and on diurnality or nocturnality in both species. The availability of a nest box had a modest effect only on the variability of activity onsets. Neither in the nocturnal nor in the diurnal species was there any evidence of entrainment by frequency demultiplication or of entrainment without photic stimulation at either dawn or dusk. These results indicate that at least in the species studied, the ability of rodents to modulate their exposure to the LD cycle does not have a major effect on photic entrainment.     

Circadian rhythms in house sparrows: lighting ad lib

House sparrows, Passer domesticus, have circadian rhythms of locomotor activity that can be entrained by light-dark cycles. Perch-hopping activity was studied in house sparrows that were given control of their own lighting. In one series of experiments, sparrows permitted to select their own lighting most commonly chose circadian freerunning cycles. The period of the selected freerunning cycles was 23.2 hr (0.9 hr shorter than the period length the sparrows exhibited in constant dark). The average self-imposed "photoperiods" in the selected freerunning cycles ranged from 8.2-10.0 hr. In a second series of experiments, sparrows were exposed to LDLD1:6:1:16. This cycle can be interpreted ambiguously as the skeleton of a short photoperiod, LD8:16, or of a long photoperiod, LD18:6. All the birds (13 birds; 23 trials) interpreted the skeleton cycle as a short photoperiod because they entrained to it as they would to LD8:16.     

Nonphotic entrainment of circadian activity rhythms in suprachiasmatic nuclei-ablated hamsters.

This study examined the role of the suprachiasmatic nuclei (SCN) in nonphotic entrainment. The wheel-running activity of SCN-ablated hamsters was recorded in constant dark (DD) and then under prolonged schedules of 2-hr daily cage changes, restricted food availability, and daily light-dark (LD) cycles. Some hamsters with very large lesions subsuming the SCN and surrounding areas exhibited significant, albeit unstable, circadian activity rhythms in DD. Some hamsters with similar ablations also showed entrained rhythms to daily cage change schedules. All hamsters showed robust rhythms entrained to a daily feeding schedule, but no hamsters showed entrainment to LD cycles. Competent circadian oscillators evidently exist outside the SCN, at least 0.5 mm or more away, and at least some are nonphotically entrainable. Weaker entrainment in animals with larger lesions suggests that nonphotically entrainable oscillators also exist within the SCN or its immediate vicinity.     

Photoperiod and body weight in female Syrian hamsters: skeleton photoperiods, response magnitude, and development of photorefractoriness

Two experiments examined the effects of various photoperiods on body weight and reproductive function in Syrian hamsters (Mesocricetus auratus). In Experiment 1 female hamsters were exposed to symmetrical skeleton photoperiods in which dawn and dusk were mimicked by 1-hr light pulses. A skeleton long (LD 16:8) photoperiod had no effect on body weight or estrous cyclicity (compared with animals in a complete LD 16:8 photoperiod), but exposure to a skeleton short (LD 10:14) photoperiod increased body weight and interrupted estrous cycles. Thus, body weight appears to respond to the relative timing of the two light pulses (a circadian mechanism) rather than to the absolute amount of light or darkness (an hourglass mechanism), just as does reproduction. In Experiment 2 female hamsters were exposed to a long photoperiod (LD 16:8) or to one of two short photoperiods (LD 10:14 and LD 8:16). Both short photoperiods increased body weight and interrupted estrous cycles, but the LD 8:16 photoperiod was substantially more effective at increasing body weight than was the LD 10:14. Thus, hamster body weight appears to be capable of a graded response to photoperiod, with shorter days producing a greater obesity. With prolonged exposure to the two short photoperiods (greater than 30 weeks), body weights spontaneously returned to the same level as the long-day controls, and estrous cycles resumed. When these hamsters were treated with the pineal gland hormone, melatonin, only those housed in long days exhibited the characteristic body weight gains, growth of brown and white adipose tissues, and decreases in uterine weight. Therefore, with prolonged exposure to short days, energy balance develops a photorefractoriness and an insensitivity to melatonin treatment, just as with reproductive function.     

Mistimed wheel running interferes with re‐entrainment of circadian Per1 rhythms in the mouse skeletal muscle and lung

Previously, we showed the acceleration of re‐entrainment to 8‐h phase‐advanced light/dark cycles (LD) in the circadian Per1 expression rhythms of the mouse lung and skeletal muscle by 3‐h wheel running (WR) at the beginning of shifted dark phase. In the present study, the effects of WR at the end of shifted dark phase were examined on the re‐entrainment in mice. LD was advanced by shortening and was delayed by lengthening the first light period in the phase‐advance and phase‐delay protocol, respectively. Shifted LD was continued for 4 days, which was followed by constant darkness (DD). Per1 expression was measured in the cultured tissues obtained on the first day of DD from mice carrying a bioluminescence reporter of Per1 expression. In the phase‐advance protocol, re‐entrainment was not influenced by WR in any circadian rhythm examined. In the phase‐delay protocol, re‐entrainment of the circadian locomotor rhythm was not affected by WR. However, re‐entrainment of circadian Per1 rhythm was significantly decelerated in the skeletal muscle and lung. These findings indicate that the effects of WR on re‐entrainment depend on the time of day and the peripheral tissues. Mistimed WR interferes with re‐entrainment of circadian rhythms in the lung and skeletal muscle.     

Circadian rhythms in the rat: Constant darkness,entrainment to T cycles and to skeleton photoperiods

Free running activity and drinking rhythms of male Sprague-Dawley rats were observed in constant darkness (DD) for up to 44 days. The average period of the rhythms ($\text{τ}{}_{\mathrm{<mtext>dd</mtext>}}$) was 24.2 hr (±0.12 hr) and the activity time was near one half of the circadian cycle. In the second experiment, rats were entrained to T cycles (T=period) with 2 hr of light per cycle. At T=23 and T=26 about one half of the rats entrained indicating that these periods are near the limits of entrainment. T=23 induced a lasting aftereffect on $\text{τ}{}_{\mathrm{<mtext>dd</mtext>}}$ while T=26 affected $\text{τ}{}_{\mathrm{<mtext>dd</mtext>}}$ only briefly. In contrast to some other nocturnal rodents, activity time was not compressed as T neared the limits of entrainment. In the third experiment, rats and hamsters were entrained to 24 hr skeleton photoperiods (two 1 hr light pulses/cycle). Rats phase jumped to the longer subjective night when the interval between the light pulses was reduced to 6 or 5 hr, while most hamsters phase jumped at 3.5 hr. Furthermore, all rats phase jumped by means of delaying transients while most hamsters showed advancing transients. Finally, while skeleton photoperiods compressed activity time in hamsters to 6 hr or less, activity time remained fairly constant in rats. These results demonstrate considerable differences in the organization of the circadian system among commonly studied nocturnal rodents.     

Entrainment of the circadian activity rhythm to the light cycle: Effective light intensity for a Zeitgeber in the retinal degenerate C3H mouse and the normal C57BL mouse

Effective light intensities for entrainment of activity rhythms were obtained in the retinal degenerated C3Hf/He mice and the normal C57BL/6J mice. The circadian activity rhythms of the mice were examined under LD 12:12 cycles with one of five different levels of light (L phase) and complete darkness (D phase). The thresholds of a Zeitgeber, defined as the light intensity effective for 50% entrainment, were estimated between 1 lux and 5 lux in the C3H and lower than 0.01 lux in the C57BL mice. These results suggest that at least two different kinds of photoreceptor including rods and cones participate in the photic entrainment of the circadian activity rhythms.     

Melatonin secretion in the Mashona mole-rat, Cryptomys darlingi--influence of light on rhythmicity

The hormone melatonin is synthesised and secreted from the pineal gland in darkness and triggers the daily and seasonal timing of various physiological and behavioural processes. The Mashona mole-rat, Cryptomys darlingi, lives in subterranean burrows that are completely sealed and is therefore rarely, if ever, exposed to light under natural conditions. Hence, this species is of particular interest for studies on rhythms of melatonin secretion. We investigated how plasma melatonin concentrations of the Mashona mole-rat responded to exposure to a long-term standard photoperiod of 12 h light, 12 h dark (12:12 LD), constant light (LL) and constant dark (DD). In addition, we examined whether plasma melatonin concentration was coupled to locomotor activity. Mashona mole-rats displayed rhythms of plasma melatonin concentration that appeared entrained to the standard LD photoperiod, suggesting that the mole-rat is capable of perceiving and entraining to this photic zeitgeber. Furthermore, under chronic constant lighting conditions (DD, LL), circadian rhythms in plasma melatonin concentration were observed, suggesting the possible existence of an endogenous rhythm. Light suppressed melatonin secretion, but constant light did not abolish the rhythm of plasma melatonin concentration. Between active and non-active animals, no difference in plasma melatonin concentration was found for any of the sequential photoperiods (LD1 DD, LD2, LL), tentatively suggesting that the rhythm of melatonin secretion is uncoupled from that of locomotor activity.     

Circadian, sleep and brain temperature rhythms in cats under sustained daily light-dark cycles and constant darkness

In adult cats, sleep-wake states and brain temperatures were determined under experimental LD cycles and prolonged DD. Convincing rhythms had not previously been well-established for this animal. The present results demonstrated daily fluctuation in total sleep time (TST) and in brain temperature (Tb). These comprise predominantly nocturnal wakefulness and high temperatures but include a bi-modal component marking dawn and dusk peaks. In addition there are ultradian variations with a period of 2-4 hour superimposed on the circadian rhythm. The latter continued at the beginning of prolonged DD, but gradually diminished and disappeared after about ten days. The TST circadian rhythm diminished more quickly than the Tb rhythm. This result suggests that the former is coupled more strongly to the circadian oscillator than the latter, and that the brain temperature rhythm could not depend on the circadian rhythm of sleep-wakefulness. These results encourage use of cats in further chronobiological studies of sleep and other physiological rhythms.     

Lability and diversity of circadian rhythms of cotton rats Sigmodon hispidus

Cotton rats (Sigmodon hispidus) were maintained from birth in constant LD 14:10 photoperiods and temperatures. Wheel running was diurnal for 6 of 13 juvenile rats and nocturnal for most others. Most diurnal rats eventually added nocturnal activity components. In constant darkness the activity rhythms of adult rats free-ran with a period of 23.2 +/- 0.3 h; in constant illumination the period was 24.7 +/- 0.1 h, in conformation to Aschoff's rule for nocturnal rodents. Some previously nocturnal adult rats eventually adopted stable diurnal activity cycles and other were successively nocturnal, diurnal for 6 mo, and then nocturnal again while maintained in the LD 14:10 photoperiod. The existence of multiple activity types, as well as the spontaneous inversions from nocturnal to diurnal status substantiate and extend field observations of this species. Seasonal inversions from nocturnal to diurnal activity, previously attributed to fluctuating environmental conditions, may also be subject to regulation by endogenous processes. It is suggested that spontaneous phase reversals in activity reflect changes in entrainment of circadian pacemakers by the light-dark cycle or altered relations between such pacemakers and the overt activity rhythm.     

Bright light affects human circadian rhythms

The relative effectiveness of external zeitgebers synchronizing circadian rhythms can be evaluated by mesuring the size of the range of entrainment. The experimental approach to measure entrainment limits is the application of an artificial zeitgeber with slowly and steadily changing period. In human circadian rhythms, an absolute light-dark (LD) cycle with a light intensity during L of 100 lux or less, results in an upper entrainment limit of 26.91±0.24 hours. The same limit is reached in constant illumination when only informations are given to the subjects. Consequently, the LD cycle is effective mainly with its behavioral component characterized by the request of the light-dark alternation to go to rest. In experiments with the same experimental protocol but higher intensity of illumination during L (400 lux, i.e., exceeding the threshold beyond which melatonin excretion is suppressed in humans), human circadian rhythms can be synchronized within a much larger range; the upper entrainment limit is, with all overt rhythms measured, beyond 29 hours. This means that bright light has an effect on the human circadian system which is qualitatively different from that of dim light, and which is similar to the effect of light in most animal experiments. This finding has theoretical and practical implications.     

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.     

Rat circadian rhythms entrain to a descending saw-tooth light intensity cycle.

We reexamined whether rat circadian rhythms entrained to the light intensity cycle of a descending saw-tooth (ST-d) form, in which illuminance decreased rectilinearly from 300 lx to 0 lx in 24 h, and abruptly returned to 300 lx (lights-on time). Ambulation, drinking and subcutaneous body temperature were simultaneously monitored in 5 intact, 5 pinealectomized and 5 orchiectomized rats. Additionally, sleep was monitored in the intact rats. In all the rats, entrainment was confirmed during 65 days' exposure to the ST-d cycle. The waveforms of the entrained rhythms were much modified compared with those during LD 12:12. The estimated activity periods of the entrained rhythms straddled the lights-on time of the ST-d cycle. In all the groups of rats, administration of a single ST-d cycle in constant dim red light produced only delay shifts irrespective of its circadian phase, and there was no significant circadian variation in the magnitude of phase shifts. The results indicate that rat circadian rhythms entrain to the ST-d cycle with an unexpected phase position, which cannot be explained by the phase-response curve.     

Disorganization of the rat activity rhythm by chronic treatment with methamphetamine

Remarkable changes in the circadian activity rhythm of rats were observed when they were chronically treated with methamphetamine dissolved in drinking water. The circadian rhythm was phase-delayed with respect to the light-dark (LD) cycle, and showed signs of relative coordination. In some rats, the circadian organization was disturbed and two activity components appeared, with one component free-running and the other entrained by the LD cycle. After methamphetamine withdrawal, these changes disappeared rapidly but there were transient periods of 2-3 days before establishment of a stable entrainment to the LD cycle. The changes in the circadian rhythm persisted even in constant darkness (DD). The period around 24 hr in DD was significantly shorter during methamphetamine treatment than after the drug withdrawal. These results indicate that neither alteration of the sensitivity to light nor lengthening of the intrinsic period is involved in the methamphetamine induced disorganization of the circadian rhythm. Possible mechanisms are discussed in terms of a multi-oscillatory system.     

Comparison of synchronization of primate circadian rhythms by light and food.

Several circadian rhythms in squirrel monkeys (Saimiri sciureus) entrained by two different agents were studied to compare their mode of coupling with the environmental zeitgebers. Synchronization was accomplished either by light-dark cycles consisting of 12 h of 600 lx followed by 12 h of less than 1 lx (LD 12:12), or by eat-fast cycles in which the animals could eat for 3 h and then had to fast for the remaining 21 h each day (EF 3:21). The rhythms of drinking, colonic temperature, and urinary potassium and water excretion were measured in chair-acclimatized monkeys. The drinking and urinary rhythms were more reproducible (smaller mean variance) and more stable (smaller standard deviation of the timing of a phase reference point) in EF than in LD cycles, whereas the temperature rhythm was more tightly controlled by LD cycles than by EF cycles. In constant light an 8-h phase delay in the EF cycle caused the drinking and urinary rhythms to resynchronize to the EF cycle within one day, while the temperature rhythm required about 6 days to resynchronize. In contrast, previously published data for a similar phase delay in the LD cycle with food available ad libitum show that the drinking and temperature rhythms resynchronized more rapidly than the urinary rhythms. These results indicate that separate mechanisms are involved in transducing temporal cues from LD and EF cycles in the circadian timekeeping system of these nonhuman primates.