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.     

Sparrow circadian rhythm responses to rotated light-dark schedules

House sparrows, Passer domesticus, were individually subjected to light-dark regimens while their perching activity was continuously monitored. The sparrows resynchronized in 5 days when LD8:16 (8 hr of light alternating with 16 hr of dark) was advanced by 8 hr; however, the sparrows were 1.7 hr from resynchronization after 5 days when the schedule was delayed 8 hr. Sparrows subjected to two simultaneously presented light-dark schedules (rotated LD8:16 provided by one light source imposed together with LD12:12 provided by a second light source) did not ignore either cycle; instead the sparrows responded as to a sequence of photoperiods; they resynchronized the onsets of their activity with the first time of lights-on of the collective photoperiod provided by the two light sources.     

Effect of reproductive state on circadian periodicity in the rat

The circadian activity rhythms of adult female rats maintained under a light-dark cycle of 14 hr light, 10 hr dark (LD 14:10) or constant dim illumination (dim LL) were recorded during their 4 or 5 day estrous activity cycles and when they were pseudopregnant. In LD 14:10 both the phase angle difference (ψ), which defines the temporal relationship between the onsets of activity and darkness, and the period (τ) of locomotor activity differed significantly among the days of the 4 and 5 day estrous cycle. Activity-time (α) varied reliably only over the days of the 5 day estrous cycle. The period of the free-running activity rhythm in dim LL also differed significantly among the days of the estrous cycle. In both LD and dim LL the most positive ψ, shortest τ and longest α were observed on the day of estrus. Pseudopregnancy diminished the amplitude and altered the daily pattern of the estrous activity rhythm. We conclude that the periodicity of circadian activity systematically varies as a function of the stage of the estrous cycle and in a manner that cannot be solely explained by corresponding alterations in endogenous estrogen.     

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.     

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.     

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.     

Effects of fasting on the circadian body temperature rhythm of Japanese quail

The effect of food deprivation on the body temperature and activity rhythms of quail was assessed in birds exposed to both light-dark (LD) cycles and to continuous darkness (DD). Quail normally exhibit a daily rhythm of body temperature in LD that will persist in DD (that is, the rhythm is circadian). In LD, 3 days' food deprivation caused the body temperature to drop below its normal nighttime levels, whereas daytime body temperature was unaffected. In DD, food deprivation caused the body temperature to drop below normal at all phases of the circadian rhythm of body temperature. Accordingly, the lack of hypothermia during the light phase of the LD cycle following food deprivation must represent a direct exogenous or "masking" effect of light, and is not an endogenous property of the circadian system. Blind birds exposed to LD 12:12 exhibited an entrained body temperature rhythm, and food deprivation caused a drop in body temperature below normal levels during both the light and dark phases of the LD cycle. Accordingly, the masking effects of light observed in normal birds on LD cycles is mediated via retinal photoreceptors and not via extraretinal photoreceptors. Measurements of activity levels before and during fasting indicate that fasting-induced hypothermia cannot be explained simply as a consequence of decreases in activity levels. Food deprivation was also observed to cause significant phase shifts in the endogenous rhythm of body temperature.     

Partial isolation of the suprachiasmatic nuclei: Effects on circadian rhythms of rat drinking behavior

The rat's daily pattern of contact with a drinking tube was observed in a light-dark cycle (LD 12:12) and in constant light (LL) after partial isolation of the suprachiasmatic nuclei (SCN) with a wire microknife that transected their lateral, dorsal, and caudal connections or the passage of a wire microknife through the SCN. Animals whose SCN had been partially isolated remained entrained to an LD cycle and phase shifted drinking behavior when the LD cycle was delayed 4 hr. Disintegration of circadian rhytmicity was observed when these rats were exposed to LL. Passage of the knife through the SCN resulted in immediate and persistent disruption of circadian rhythmicity as severe as that seen after electrolytic lesions of the SCN. These data confirm the necessity of the SCN in regulating circadian behavior and suggest that the SCN may mediate entrainment and freerunning rhythms via different neural systems.     

Circadian rhythms of pineal melatonin release in the pigeon measured by in vivo microdialysis.

Circadian rhythms of pineal melatonin release were measured in freely moving pigeons (Columba livia) by in vivo microdialysis. The birds were placed in light-dark cycles with 12 h of light and 12 h of darkness (LD 12:12) or continuous dim light (LLdim) after LD 12:12. Although the level of melatonin was various, daily changes of melatonin with higher levels during the dark and lower levels during the light were observed in all of the birds examined. The daily changes of melatonin persisted in LLdim, indicating circadian nature of pineal melatonin release. Moreover pineal melatonin release was inhibited by acute exposure of light during the dark. These results indicate that microdialysis is useful for studying circadian pineal melatonin rhythms of birds.     

Circadian feeding and drinking rhythms in the rat under complete and skeleton photoperiods

Feeding and drinking rhythms were studied in rats maintained under 24-hr light-dark (LD) cycles with various photoperiods, under two-pulse (2P) and one-pulse (1P) skeleton photoperiods, and under constant dark (DD). Rhythmic waveforms were similar under complete LD cycles and corresponding skeleton photoperiods, indicating that these rhythms mainly reflect the entrainment of underlying circadian pacemakers. Little or no role was found for masking effects of light on circadian feeding and drinking waveforms. Entrainment was found to depend mainly on the timing of the dawn light signal, whether it was a 15-min light pulse or a dark-to-light transition initiating a complete photoperiod. Furthermore, the use of 1P schedules revealed that a dawn signal was sufficient for entrainment. These results closely match those obtained for motor activity measures in other nocturnal rodent species, and generally conform to the predictions of Pittendrigh's nonparametric theory of entrainment. Furthermore, the close correspondence of the two rhythms during entrainment, phase-jumps, and free-running (DD) conditions indicates that they are controlled by common circadian pacemakers.     

The chronobiology of the Natal mole-rat, Cryptomys hottentotus natalensis

The Natal mole-rat, Cryptomys hottentotus natalensis, rarely, if ever, is exposed to external light cues because it occurs in completely sealed tunnel systems. As a result, their classical visual system is regressed, and therefore, their circadian system is expected proportionally to be expanded. Locomotor activity was investigated under a number of different photic regimes. Nine of the 12 mole-rats exhibited endogenous circadian rhythms of locomotor activity under constant darkness, with a mean free run period of 24.13 h (range 23.93-24.13 h), with these animals entrained to a light-dark cycle (12 L:12 D). Because C. hottentotus natalensis are able to entrain their locomotor activity to an external light source, light must reach the suprachiasmatic nucleus (SCN), suggesting a functional circadian clock. A clear day-night rhythm of melatonin secretion in animals housed under a neutral photoperiod (12 L:12 D) was observed, with higher melatonin concentrations in the dark compared with the light phase. The rhythm was maintained after the animals were transferred to either continuous light (LL) or dark (DD), suggesting that the endogenous rhythm was maintained under acute exposure to light and dark. However, under DD, the rhythm appeared to shift slightly, potentially as a result of the rhythm free running. These results show that C. hottentotus natalensis has endogenous rhythms of both locomotor activity and melatonin secretion, which are modulated by light.     

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.     

Circadian rhythms of small carnivores and the effect of restricted feeding on daily activity

Restricted daily feeding can entrain an endogenous circadian clock of rodents. Carnivores have not been tested even though, unlike rodents, the availability of their foods can naturally vary with the light-dark cycle. In addition, very little is known of the characteristics of carnivore circadian rhythms in constant illumination. The locomotor activities of weasels and minks were measured on running wheels and tilt floors in LD 12:12 and constant illumination. The animals were then subjected to daily restricted feeding to determine their ability to anticipate the arrival of food. Weasels and minks anticipated food delivery but endogenous control was not unequivocally demonstrated. Anticipatory activity was suppressed during days of food excess but exhibited during days of deprivation when these conditions were presented on alternating days. The characteristics of mink activity rhythms in constant light and dark are consistent with Aschoff's rule for nocturnal animals. Free-running rhythms were not measurable for most weasels due to arhythmicity or lack of data.     

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.     

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.     

Prehatch entrainment of circadian rhythms in the domestic chick using different light regimes

The onset of circadian rhythms in many animals occurs during prenatal development. We conducted four experiments, using the domestic chick as a model, to assess when these rhythms can first be entrained and the type of light zeitgeber necessary. In Experiment 1, the presence of circadian rhythms was assessed using tonic immobility, an antipredator behavior, whereas in Experiments 2 to 4 body temperature was studied. We demonstrate that (a) circadian rhythms can be entrained during the late stage of the chick's 21-day incubation period (prehatch Days 13-18), (b) only 1 day of light cues [12:12 hr light:dark (12L:12D)] on prehatch Day 13 is necessary for entrainment, and (c) short bouts of light, which simulate the light cues embryos typically experience during natural incubation, can act as zeitgebers although they are not as effective as 12L:12D. The onset of entrainment is earlier than predicted and suggests that the brain structures mediating circadian rhythms mature sooner than proposed by previous research.     

Food anticipatory activity and photic entrainment in food-restricted BALB/c mice

The BALB/c mouse was evaluated as a model for the study of entrainment of circadian rhythms by feeding schedules. Mice were housed in a 12:12-h light-dark (LD) environment with food available for 3-5 h/day (5 h before dark onset). Food anticipatory activity (FAA) rhythms were evident in all mice, ranging from robust in some to weak and variable in others. Advancing transients of the end of nocturnal activity were evident in many cases, culminating in a significant shortening of the main bout of nocturnal activity. Transients and contraction of nocturnal activity were not dependent on the expression of FAA. Following restricted feeding, nocturnal activity expanded by a series of delaying transients. On the first day of constant dark (DD) with ad libitum food access following restricted feeding in LD, the phase from which activity free-ran was advanced by comparison with control tests. Transients, compressed nocturnal activity, and advanced phase of free-run suggest that feeding schedules cause phase advancement of light-entrained rhythms in BALB/c mice. When restricted feeding was imposed in DD, several mice expressed robust FAA concurrent with a free-running activity component. In some cases, free-running rhythms entrained to feeding time, and in other cases, the period of the free run lengthened toward 24 h. These data show that restricted feeding in BALB/c mice can engage a circadian mechanism driving FAA rhythms and can also modulate the phase of photic entrainment, possibly by a direct entraining effect on the light-entrained rhythm. The BALB/c mouse strain, in several respects, appears to be a useful model for the study of scheduled feeding and circadian rhythms.     

Control of the rat's circadian self-stimulation rhythm by light-dark cycles.

Rats with hypothalamic and septal electrodes were maintained in continuous test environments where bar-press responses produced brief reinforcing electrical stimulations. Long-term trends in response emission were measured under continuous exposure to light, dark and 12 hr light-dark alternations. In addition, transient behavioral adjustment to sudden 180 degrees phase shifts in the light-dark schedule was studied. The ambient light condition was found to control the period and phase of the circadian rhythm of brain self-stimulation behavior, as quantified by Fourier analysis. The circadian period was greatest under constant light (up to 24.90 hr under dim illumination), and approximated 24.00 hr under constant dark. Successful nocturnal entrainment to 12 hr light-dark alternations was obtained, with the peak of the 24 hr Fourier fundamental occurring in the middle-to-late dark segments. Three to 11 days were required for re-entrainment to 180 degrees light-dark phase shifts, during which the behavioral oscillation period increased to values comparable to periods under constant light. The rate of re-entrainment appeared to be proportional to illumination intensity during light segments.     

Effect of a short photoperiod on circadian rhythms of body temperature and motor activity in old rats

Circadian rhythms of body temperature and motor activity were documented in young and old rats (four 8-week-old and five 22-month-old male Wistars, implanted with telemetric probes and housed in a chronobiological facility) under two different photoperiod conditions. The animals were maintained in a light:dark (LD) cycle of 12 h each (LD 12:12) for 4 weeks and then exposed to a LD 6:18 cycle for 7 weeks to assess the effect of age on the desynchronization of the temporal structure of the rhythms. In old rats under LD 12:12, the power of the 24-h component and the circadian amplitude of body temperature and motor activity were markedly lower than in the young and both rhythms were phase-advanced. After the shift to LD 6:18, the circadian rhythmicity was maintained for both variables and the same phase delay (+5+/-1 h) was observed in both age groups, as was a gradual expansion of the patterns of both functions with the longer night. The photoperiod reduction (6 weeks under LD 6:18) did not modify the power of the 24-h component of body temperature and motor activity in old rats. In young rats, however, the power and amplitude of the 24-h component of motor activity rhythm fell to the levels of those in old rats, while the power of the 24-h component of body temperature rhythm and the amplitude did not change. Our data show that the circadian rhythm of motor activity, but not of body temperature, responds age dependently to a photoperiod reduction.     

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.