Further experiments on the relationship between the period of circadian rhythms and locomotor activity levels in hamsters

Circadian Responses to Light in the BTBR Mouse

Animals with altered freerunning periods are valuable in understanding properties of the circadian clock. Understanding the relationship between endogenous clock properties, entrainment, and influence of light in terms of parametric and non-parametric models can help us better understand how different populations adapt to external light cycles. Many clinical populations often show significant changes in circadian properties that in turn cause sleep and circadian problems, possibly exacerbating their underlying clinical condition. BTBR T⁺Itpr3^tf/J (BTBR) mice are a model commonly used for the study of autism spectrum disorders (ASD). Adults and adolescents with ASD frequently exhibit profound sleep and circadian disruptions, including increased latency to sleep, insomnia, advanced and delayed sleep phase disorders, and sleep fragmentation. Here, we investigated the circadian phenotype of BTBR mice in freerunning and light-entrained conditions and found that this strain of mice showed noticeably short freerunning periods (~22.75 h). In addition, when compared to C57BL/6J controls, BTBR mice also showed higher levels of activity even though this activity was compressed into a shorter active phase. Phase delays and phase advances to light were significantly larger in BTBR mice. Despite the short freerunning period, BTBR mice exhibited normal entrainment in light-dark cycles and accelerated entrainment to both advanced and delayed light cycles. Their ability to entrain to skeleton photoperiods of 1 min suggests that this entrainment cannot be attributed to masking. Period differences were also correlated with differences in the number of vasoactive intestinal polypeptide–expressing cells in the suprachiasmatic nucleus (SCN). Overall, the BTBR model, with their unique freerunning and entrainment properties, makes an interesting model to understand the underlying circadian clock.

Modeling the Influence of Synaptic Plasticity on After-effects

While circadian rhythms in physiology and behavior demonstrate remarkable day-to-day precision, they are also able to exhibit plasticity in a variety of parameters and under a variety of conditions. After-effects are one type of plasticity in which exposure to non-24-h light-dark cycles (T-cycles) will alter the animal's free-running rhythm in subsequent constant conditions. We use a mathematical model to explore whether the concept of synaptic plasticity can explain the observation of after-effects. In this model, the SCN is composed of a set of individual oscillators randomly selected from a normally distributed population. Each cell receives input from a defined set of oscillators, and the overall period of a cell is a weighted average of its own period and that of its inputs. The influence that an input has on its target's period is determined by the proximity of the input cell's period to the imposed T-cycle period, such that cells with periods near T will have greater influence. Such an arrangement is able to duplicate the phenomenon of after-effects, with relatively few inputs per cell (~4-5) being required. When the variability of periods between oscillators is low, the system is quite robust and results in minimal after-effects, while systems with greater between-cell variability exhibit greater magnitude after-effects. T-cycles that produce maximal after-effects have periods within ~2.5 to 3 h of the population period. Overall, this model demonstrates that synaptic plasticity in the SCN network could contribute to plasticity of the circadian period.

Role of Rhodopsins as Circadian Photoreceptors in the Drosophila melanogaster

Light profoundly affects the circadian clock and the activity levels of animals. Along with the systematic changes in intensity and spectral composition, over the 24-h day, light shows considerable irregular fluctuations (noise). Using light as the Zeitgeber for the circadian clock is, therefore, a complex task and this might explain why animals utilize multiple photoreceptors to entrain their circadian clock. The fruit fly Drosophila melanogaster possesses light-sensitive Cryptochrome and seven Rhodopsins that all contribute to light detection. We review the role of Rhodopsins in circadian entrainment, and of direct light-effects on the activity, with a special emphasis on the newly discovered Rhodopsin 7 (Rh7). We present evidence that Rhodopsin 6 in receptor cells 8 of the compound eyes, as well as in the extra retinal Hofbauer-Buchner eyelets, plays a major role in entraining the fly’s circadian clock with an appropriate phase-to-light–dark cycles. We discuss recent contradictory findings regarding Rhodopsin 7 and report original data that support its role in the compound eyes and in the brain. While Rhodopsin 7 in the brain appears to have a minor role in entrainment, in the compound eyes it seems crucial for fine-tuning light sensitivity to prevent overshooting responses to bright light.

Voluntary exercise stabilizes photic entrainment of djungarian hamsters (Phodopus sungorus) with a delayed activity onset

The Djungarian hamsters of our breeding colony show unstable daily activity patterns when kept under standard laboratory conditions. Moreover, part of them develops a delayed activity onset (DAO) or an arrhythmic phenotype. In former studies, we have shown that the system of photic entrainment works at its limits. If the period length (tau) increases, which is the case in DAO hamsters, the light-induced phase advances are too small to compensate the daily delay of the activity rhythm caused by tau being longer than 24 h. Accordingly, under natural conditions, there must be further (environmental) factors to enable a stable entrainment. One of these may be the higher level of motor activity. Animals must cover long distances to search for food, sexual partners and others. In the laboratory, hamsters are kept singly in small cages. This does restrict animals' options for motor activity. Also, there is less need for moving around as the hamsters are fed ad libitum. In the present study, a series of experiments was performed to investigate the putative effect of the activity level. To begin with, wild type (WT) and DAO animals were given access to running wheels. 50% of DAO hamsters developed a WT activity pattern. As the main reason for the DAO phenomenon is their long tau together with a too weak photic phase response, the effect of wheel running on these parameters was investigated in further experiments. With higher activity level, tau decreased in WT hamsters but increased in DAO animals even though the increase for the activity onset was only close to significance. Moreover, the photic phase responses were weaker though significant only for the activity offset of DAO hamsters. Based on the assumptions that running wheel activity will affect the phase response and/or the free running period, the results of the present paper do not provide an explanation for why part of DAO hamsters developed a WT phenotype when they had access to running wheels. Obviously, mechanisms downstream from the suprachiasmatic nuclei must be taken into account when investigating the stabilizing, improving circadian entrainment effect of motor activity.

New Quantitative Trait Loci for the Genetic Variance in Circadian Period of Locomotor Activity between Inbred Strains of Mice

Provisional quantitativetrait loci (QTL) for circadian locomotor period and wheel-running period have been identified in recombinant inbred (RI) mouse strains. To confirm thoseQTLand identify newones, the geneticcomponent of variance of the circadian period was partitionedamongan F2 intercross of RI mouse strains (BXD19 and CXB07). First, a genomic survey using 108 SSLP markers with an average spacing of 15 cM was carried out in a population of 259 (BXD19 · CXB07)F2 animals. The genome-wide survey identified two significant QTLfor period of locomotor activity measured by infrared photobeam crossings on mousechromosomes 1 (lod score5.66) and 14 (lod score4.33). TheQTL on distal chromosome 1 confirmed a previous report based on congenic B6.D2-Mtv7a/ Ty mice. Lod scores greater than 2.0 were found on chromosomes 1, 2, 6, 12, 13, and 14. In a targeted extension study, additional genotyping was performed on these chromosomes in the full sample of 341 F2 progeny. The 6 chromosome-wide surveys identified 3 additional QTL on mouse chromosomes 6, 12, and 13. The QTLon chromosome 12 overlaps with circadian period QTLidentified in several prior studies. For wheel-running period, the chromosome-wide surveys identified QTLon chromosomes 2 and 13 and one highly suggestive QTLon proximal chromosome 1. The results are compared to other published studies of QTL of circadian period.

Non-Photic Modulation of Phase Shifts to Long Light Pulses

Circadian rhythms can be reset by both photic and non-photic stimuli. Recent studies have used long light exposure to produce photic phase shifts or to enhance non-photic phase shifts. The presence or absence of light can also influence the expression of locomotor rhythms through masking; light during the night attenuates locomotor activity, while darkness during the day induces locomotor activity in nocturnal animals. Given this dual role of light, the current study was designed to examine the relative contributions of photic and non-photic components present in a long light pulse paradigm. Mice entrained to a light/dark cycle were exposed to light pulses of various durations (0, 3, 6, 9, or 12 h) starting at the time of lights-off. After the light exposure, animals were placed in DD and were either left undisturbed in their home cages or had their wheels locked for the remainder of the subjective night and subsequent subjective day. Light treatments of 6, 9, and 12 h produced large phase delays. These treatments were associated with decreased activity during the nocturnal light and increased activity during the initial hours of darkness following light exposure. When the wheels were locked to prevent high-amplitude activity, the resulting phase delays to the light were significantly attenuated, suggesting that the activity following the light exposure may have contributed to the overall phase shift. In a second experiment, telemetry probes were used to assess what effect permanently locking the wheels had on the phase shift to the long light pulses. These animals had phase shifts fully as large as animals without any form of wheel lock, suggesting that while non-photic events can modulate photic phase shifts, they do not play a role in the full phase-shift response observed in animals exposed to long light pulses. This paradigm will facilitate investigations into non-photic responses of the mouse circadian system.

Effects of lighting condition on circadian behavior in 5-HT1A receptor knockout mice

Serotonin (5-HT) is an important regulator of the mammalian circadian system, and has been implicated in modulating entrained and free-running rhythms, as well as photic and non-photic phase shifting. In general, 5-HT appears to oppose the actions of light on the circadian system of nocturnal rodents. As well, 5-HT mediates, at least in part, some non-photic responses. The 5-HT1A, 1B and 7 receptors regulate these acute responses to zeitgebers. 5-HT also regulates some entrained and free-running properties of the circadian clock. The receptors that contribute to these phenomena have not been fully examined. Here, we use 5-HT1A receptor knockout (KO) mice to examine the response of the mouse circadian system to a variety of lighting conditions, including a normal light-dark cycle (LD), T-cycles, phase advanced LD cycles, constant darkness (DD), constant light (LL) and a 6 hour dark pulse starting at CT5. Relative to wildtype mice, the 5-HT1A receptor KO mice have lower levels of activity during the first 8h of the night/subjective night in LD and LL, later activity onsets on transient days during re-entrainment, shorter free-running periods in LL when housed with wheels, and smaller phase shifts to dark pulses. No differences were noted in activity levels during DD, alpha under any light condition, free-running period in DD, or phase angle of entrainment in LD. While the 5-HT1A receptor plays an important role in regulating photic and non-photic phase shifting, its contribution to entrained and free-running properties of the circadian clock is relatively minor.

Social forces can impact the circadian clocks of cohabiting hamsters

A number of field and laboratory studies have shown that the social environment influences daily rhythms in numerous species. However, underlying mechanisms, including the circadian system's role, are not known. Obstacles to this research have been the inability to track and objectively analyse rhythms of individual animals housed together. Here, we employed temperature dataloggers to track individual body temperature rhythms of pairs of cohabiting male Syrian hamsters (Mesocricetus auratus) in constant darkness and applied a continuous wavelet transform to determine the phase of rhythm onset before, during, and after cohabitation. Cohabitation altered the predicted trajectory of rhythm onsets in 34% of individuals, representing 58% of pairs, compared to 12% of hamsters single-housed as 'virtual pair' controls. Deviation from the predicted trajectory was by a change in circadian period (τ), which tended to be asymmetric-affecting one individual of the pair in nine of 11 affected pairs-with hints that dominance might play a role. These data implicate a change in the speed of the circadian clock as one mechanism whereby social factors can alter daily rhythms. Miniature dataloggers coupled with wavelet analyses should provide powerful tools for future studies investigating the principles and mechanisms mediating social influences on daily timing.

Methylphenidate modifies the motion of the circadian clock

People with attention-deficit/hyperactivity disorder (ADHD) often experience sleep problems, and these are frequently exacerbated by the methylphenidate they take to manage their ADHD symptoms. Many of the changes to sleep are consistent with a change in the underlying circadian clock. The present study was designed to determine if methylphenidate alone could alter properties of the circadian clock. Young male mice were examined in light-dark cycles and in constant darkness and recordings were performed on behavioral activity, sleep, and electrical activity in the suprachiasmatic nucleus (SCN) of freely moving mice. Methylphenidate in the drinking water (0.08%) significantly increased activity in the mid-to-late night, and led to a delay in the onset of activity and sleep relative to the light-dark cycle. While locomotor levels returned to baseline after treatment ended, the phase angle of entrainment required at least a week to return to baseline levels. In constant darkness, the free-running period of both wheel-running and general locomotor rhythms was lengthened by methylphenidate. When the treatment ended, the free-running period either remained stable or only partially reverted to baseline levels. Methylphenidate also altered the electrical firing rate rhythms in the SCN. It induced a delay in the trough of the rhythm, an increment in rhythm amplitude, and a reduction in rhythm variability. These observations suggest that methylphenidate alters the underlying circadian clock. The observed changes are consistent with clock alterations that would promote sleep-onset insomnia.

Circadian rhythms, the molecular clock, and skeletal muscle

Almost all organisms ranging from single cell bacteria to humans exhibit a variety of behavioral, physiological, and biochemical rhythms. In mammals, circadian rhythms control the timing of many physiological processes over a 24-h period, including sleep-wake cycles, body temperature, feeding, and hormone production. This body of research has led to defined characteristics of circadian rhythms based on period length, phase, and amplitude. Underlying circadian behaviors is a molecular clock mechanism found in most, if not all, cell types including skeletal muscle. The mammalian molecular clock is a complex of multiple oscillating networks that are regulated through transcriptional mechanisms, timed protein turnover, and input from small molecules. At this time, very little is known about circadian aspects of skeletal muscle function/metabolism but some progress has been made on understanding the molecular clock in skeletal muscle. The goal of this chapter is to provide the basic terminology and concepts of circadian rhythms with a more detailed review of the current state of knowledge of the molecular clock, with reference to what is known in skeletal muscle. Research has demonstrated that the molecular clock is active in skeletal muscles and that the muscle-specific transcription factor, MyoD, is a direct target of the molecular clock. Skeletal muscle of clock-compromised mice, Bmal1(-/-) and Clock(Δ19) mice, are weak and exhibit significant disruptions in expression of many genes required for adult muscle structure and metabolism. We suggest that the interaction between the molecular clock, MyoD, and metabolic factors, such as PGC-1, provide a potential system of feedback loops that may be critical for both maintenance and adaptation of skeletal muscle.

Effects of Transient and Continuous Wheel Running Activity on the Upper and Lower Limits of Entrainment to Light‐Dark Cycles in Female Hamsters

The entrainment limits to light-dark cycles can be modified by the experimental conditions under which they are tested. Among the factors that may influence entrainment is the amount of wheel running exerted by the animal. In the present work, the effects of transitory and continuous wheel running on entrainment to light-dark cycles were tested using a range of T cycles at the entrainment limits. Four groups of female hamsters were submitted to 1 h stepwise changes in T cycles. Two groups were exposed to T cycles of which the period was shortened at the lower limit from T22 to T18, and the other two groups were exposed to cycles that lengthened at the upper limit from T27 to T32. One of the groups at the lower limit and one at the upper limit had continuous access to a running wheel, while the others had the wheel locked, except at certain T when a lack of period control by T cycle appeared. The study demonstrates that access to running wheel widens the limits of entrainment to LD cycles. Specifically, the following observations were made: the effects of wheel running for entrainment were more evident in the groups with continuous access to wheel, as they did entrain to T19 and T32; continuous access to a wheel produced aftereffects only after T19, but not under T32; and when animals without a wheel showed relative coordination, unlocking the wheel favored entrainment in all the animals at T31, but in only 1 out 6 at T19. All of these indicate a different effect of the wheel running on the upper and lower limits of entrainment.

Lithium and genetic inhibition of GSK3beta enhance the effect of methamphetamine on circadian rhythms in the mouse

Lithium, a drug commonly used to treat mood disorders, and the psychostimulant methamphetamine are both capable of altering circadian rhythmicity. Although the actions of lithium on the circadian system are thought to occur through inhibition of glycogen synthase kinase-3beta (GSK3beta), the mechanism by which methamphetamine alters circadian rhythms is unknown. We tested the effects of concurrent methamphetamine and lithium treatment on the circadian wheel-running behavior of mice. Methamphetamine alone lengthened both the active duration and the free-running period of locomotor activity in animals housed in constant conditions. Administering lithium enhanced the period-lengthening effects of methamphetamine in animals housed in constant darkness. This effect was even more pronounced when animals were housed in constant light. Lithium increased both methamphetamine intake and serum levels of methamphetamine, possibly contributing to the effects on circadian behavior. We also tested the effect of methamphetamine in mutant mice possessing only one allele for Gsk3beta. These animals, when treated with methamphetamine, responded like wild-type mice treated with a combination of methamphetamine and lithium, displaying long, free-running rhythms. These data, together with many others in the literature, point to a complicated interaction between the circadian system and the development and possible treatment of psychopathologies such as bipolar disorder and drug addiction.

Temporal profile of circadian clock gene expression in a transplanted suprachiasmatic nucleus and peripheral tissues

The mammalian hypothalamic suprachiasmatic nucleus (SCN) is the master oscillator that regulates the circadian rhythms of the peripheral oscillators. Previous studies have demonstrated that the transplantation of embryonic SCN tissues into SCN-lesioned arrhythmic mice restores the behavioral circadian rhythms of these animals. In our present study, we examined the clock gene expression profiles in a transplanted SCN and peripheral tissues, and also analysed the circadian rhythm of the locomotor activity in SCN-grafted mice. These experiments were undertaken to elucidate whether the transplanted SCN generates a dynamic circadian oscillation and maintains the phase relationships that can be detected in intact mice. The grafted SCN indeed showed dynamic circadian expression rhythms of clock genes such as mPeriod1 (mPer1) and mPeriod2 (mPer2). Furthermore, the phase differences between the expression rhythms of these genes in the grafted SCN and the locomotor activity rhythms of the transplanted animals were found to be very similar to those in intact animals. Moreover, in the liver, kidney and skeletal muscles of the transplanted animals, the phase angles between the circadian rhythm of the grafted SCN and that of the peripheral tissues were maintained as in intact animals. However, in the SCN-grafted animals, the amplitudes of the mPer1 and mPer2 rhythms were attenuated in the peripheral tissues. Our current findings therefore indicate that a transplanted SCN has the capacity to generate a dynamic intrinsic circadian oscillation, and can also lock the normal phase angles among the SCN, locomotor activity and peripheral oscillators in a similar manner as in intact control animals.

A QTL on mouse chromosome 12 for the genetic variance in free-running circadian period between inbred strains of mice

Background

Many genes control circadian period in mice. Prior studies suggested a quantitative trait locus (QTL) on proximal mouse chromosome 12 for interstrain differences in circadian period. Since the B6.D2NAhr^d/J strain has DBA/2 alleles for a portion of proximal chromosome 12 introgressed onto its C57BL/6J background, we hypothesized that these mice would have a shorter circadian period than C57BL/6J mice.

Methods

We compared circadian phenotypes of B6.D2NAhr^d/J and C57BL/6 mice: period of general locomotor activity in constant dark and rest/activity pattern in alternating light and dark. We genotyped the B6.D2NAhr^d/J mice to characterize the size of the genomic insert. To aid in identifying candidate quantitative trait genes we queried databases about the resident SNPs, whole brain gene expression in C57BL/6J versus DBA/2J mice, and circadian patterns of gene expression.

Results

The B6.D2NAhr^d/J inbred mice have a shorter circadian period of locomotor activity than the C57BL/6J strain. Furthermore, the genomic insert is associated with another phenotype: the mean phase of activity minimum in the dark part of a light-dark lighting cycle. It was one hour later than in the background strain. The B6.D2NAhr^d/J mice have a DBA/2J genomic insert spanning 35.4 to 41.0 megabase pairs on Chromosome 12. The insert contains 15 genes and 12 predicted genes. In this region Ahr (arylhydrocarbon receptor) and Zfp277 (zinc finger protein 277) both contain non-synonymous SNPs. Zfp277 also showed differential expression in whole brain and was cis-regulated. Three genes and one predicted gene showed a circadian pattern of expression in liver, including Zfp277.

Conclusion

We not only fine-mapped the QTL for circadian period on chromosome 12 but found a new QTL there as well: an association with the timing of the nocturnal activity-minimum. Candidate quantitative trait genes in this QTL are zinc finger protein 277 and arylhydrocarbon receptor. Arylhydrocarbon receptor is structurally related to Bmal1, a canonical clock gene.

Chronic ethanol intake modulates photic and non-photic circadian phase responses in the Syrian hamster

Chronic alcohol intake disrupts sleep and other circadian biological rhythms in both human alcoholics and in experimental animals. Recent studies from our laboratory indicate that these effects may be due, in part, to ethanol-induced alterations in fundamental properties of the circadian pacemaker. The present study explored the effects of chronic voluntary ethanol intake (25% v/v) on circadian phase responses to both photic and non-photic stimuli in Syrian hamsters. Hamsters were used in these experiments because they are a popular model organism in behavioral chronobiology research, and are characterized by unusually high levels of voluntary ethanol intake. Relative to controls, ethanol-exposed animals showed attenuation of circadian phase responses and wheel running activity following acute administration of the benzodiazepine, triazolam, a non-photic phase-shifting stimulus. In addition, ethanol-exposed animals displayed reduced phase advances, but normal phase delays, in response to brief light pulses. While the mechanisms underlying these effects remain to be elucidated, we hypothesize that ionotropic GABA and glutamate receptors may be involved, since these proteins serve as important targets for the neurobiological effects of ethanol, and are also known to be critically involved in the modulation of photic and non-photic circadian phase responses.

The enigma of behavioral inputs to the circadian clock: A test of function using restraint

Wheel running stimulated during the daily rest period can acutely shift circadian rhythms in Syrian hamsters. Spontaneous running, defining the active phase of the circadian rest-activity cycle, can shorten the circadian periodicity in constant light or dark in several nocturnal rodent species. The adaptive significance of these behavioral effects on pacemaker phase and period is unclear. Here we consider a hypothesis that behavioral inputs to the circadian pacemaker serve primarily to enhance the precision of light-dark entrainment and maintain daily activity onset close to lights-off (i.e., dusk) by stabilizing entrainment on a steeper portion of the delay zone of the phase-response curve to light. This hypothesis rests on the evidence that spontaneous activity early in the active period feeds back on the pacemaker to advance its motion. If so, then preventing activity at this time should induce a phase delay shift. Such delay shifts have been reported in Syrian hamsters physically restrained early in the active period. We show here that restraint can induce phase delays but that, using the Aschoff Type 2 procedure for measuring shifts, these delays are very small, are inversely related to behavioral sleep during restraint, and are positively correlated with 'rebound' increases in running following restraint, at a circadian time when stimulated running is known to induce phase delay shifts. Repeated bouts of restraint, to promote habituation, were associated with strong attenuation of 'rebound' running and no significant delay shifts. These results suggest that, in Syrian hamsters, spontaneous activity early at night has little effect on pacemaker motion, and argue against the stated hypothesis.

A single copy of carbonic anhydrase 2 restores wild-type circadian period to carbonic anhydrase II-deficient mice

Carbonic anhydrase II (CA-II)-deficient mice have long circadian periods compared to their siblings with normal CA-II levels. The CA-II-deficient mice differ genetically from their siblings at proximal chromosome three, where the mutated carbonic anhydrase 2 gene sits on a small insert of DNA from the DBA/2J strain. The rest of the genome is that of the C57BL/6J strain. The goal of this study was to test the hypothesis that the null mutation in carbonic anhydrase 2 and the long circadian period phenotype were linked. In order to separate the effect of the null mutation in carbonic anhydrase 2 from the effect of DBA/2J alleles of other genes on the insert, two new lines of mice were studied. The first line, Kar, was developed from a CA-II-deficient mouse that had a fortuitous recombination restoring functional CA-II without affecting the rest of the DBA/2J insert. The second line was generated by breeding DBA/2J mice and C57BL/6J mice until they had the genomic composition of CA-II-deficient mice without the null mutation. Both lines of mice had circadian periods not different from C57BL/6J mice and shorter than CA-II-deficient mice. The phenotype of the new lines showed that the long circadian period characteristic of the CA-II-deficient mice arises when functional CA-II is absent, not when DBA/2J alleles are present on proximal chromosome three.

Activity rhythm of golden hamster (Mesocricetus auratus) can be entrained to a 19-h light-dark cycle

Both temporary access to a running wheel and temporary exposure to light systematically influence the phase producing entrainment of the circadian activity rhythm in the golden hamster (Mesocricetus auratus). However, precise determination of entrainment limits remains methodologically difficult, because such calculations may be influenced by varying experimental paradigms. In this study, effects on the entrainment of the activity pattern during successive light-dark (LD) cycles of stepwise decreasing periods, as well as wheel running activity, were investigated. In particular, the hamster activity rhythm under LD cycles with a period (T) shorter than 22 h was studied, i.e., when the LD cycle itself had been shown to be an insufficiently strong zeitgeber to synchronize activity rhythms. Indeed, it was confirmed that animals without a wheel do not entrain under 11:11-h LD cycles (T = 22 h). Subsequently providing hamsters continuous access to a running wheel established entrainment to T = 22 h. Moreover, this paradigm underwent further reductions of the T period to T = 19.6 h without loss of entrainment. Furthermore, restricting access to the wheel did not result in loss of entrainment, while even entrainment to T = 19 h was observed. To explain this observed shift in the lower entrainment limit, our speculation centers on changes in pacemaker response facilitated by stepwise changes of T spaced very far apart, thus allowing time for adaptation.

Dexamethasone induces different wheel running activity than corticosterone through vasopressin release from the suprachiasmatic nucleus

During the analysis of wheel running activity, we found that corticosterone (1 mg/100 g BW) injection decreased wheel activity, while dexamethasone (0.1 mg/100 g) increased the activity. To clarify the functional differences between corticosterone and dexamethasone, we measured Arg-vasopressin (AVP) release from the suprachiasmatic nucleus (SCN) slice culture in vitro and AVP coding mRNA in the SCN in vivo. The corticosterone (0.2 and 2 microg/ml, final concentration in medium) decreased the AVP release, while it increased by dexamethasone (0.2 and 2 microg/ml). An AVP mRNA in the SCN was decreased by both corticosterone (1 mg/100 g) and dexamethasone (0.1 mg/100 g). The differences in wheel activity by corticosterone and dexamethasone are discussed from the changes of AVP in the SCN.

Selective deficits in the circadian light response in mice lacking PACAP

Previous studies indicate that light information reaches the suprachiasmatic nucleus through a subpopulation of retinal ganglion cells that contain both glutamate and pituitary adenylyl cyclase-activating peptide (PACAP). Although the role of glutamate in this pathway has been well studied, the involvement of PACAP and its receptors is only beginning to be understood. To investigate the functions of PACAP in vivo, we developed a mouse model in which the gene coding for PACAP was disrupted by targeted homologous recombination. RIA was used to confirm a lack of detectable PACAP protein in these mice. PACAP-deficient mice exhibited significant impairment in the magnitude of the response to brief light exposures with both light-induced phase delays and advances of the circadian system impacted. This mutation equally impacted phase shifts induced by bright and dim light exposure. Despite these effects on phase shifting, the loss of PACAP had only limited effects on the generation of circadian oscillations, as measured by rhythms in wheel-running activity. Unlike melanopsin-deficient mice, the mice lacking PACAP exhibited no loss of function in the direct light-induced inhibition of locomotor activity, i.e., masking. Finally, the PACAP-deficient mice exhibited normal phase shifts in response to exposure to discrete dark treatments. The results reported here show that the loss of PACAP produced selective deficits in the light response of the circadian system.

Social influences on mammalian circadian rhythms: animal and human studies

While light is considered the dominant stimulus for entraining (synchronizing) mammalian circadian rhythms to local environmental time, social stimuli are also widely cited as 'zeitgebers' (time-cues). This review critically assesses the evidence for social influences on mammalian circadian rhythms, and possible mechanisms of action. Social stimuli may affect circadian behavioural programmes by regulating the phase and period of circadian clocks (i.e. a zeitgeber action, either direct or by conditioning to photic zeitgebers), by influencing daily patterns of light exposure or modulating light input to the clock, or by associative learning processes that utilize circadian time as a discriminative or conditioned stimulus. There is good evidence that social stimuli can act as zeitgebers. In several species maternal signals are the primary zeitgeber in utero and prior to weaning. Adults of some species can also be phase shifted or entrained by single or periodic social interactions, but these effects are often weak, and appear to be mediated by social stimulation of arousal. There is no strong evidence yet for sensory-specific nonphotic inputs to the clock. The circadian phase-dependence of clock resetting to social stimuli or arousal (the 'nonphotic' phase response curve, PRC), where known, is distinct from that to light and similar in diurnal and nocturnal animals. There is some evidence that induction of arousal can modulate light input to the clock, but no studies yet of whether social stimuli can shift the clock by conditioning to photic cues, or be incorporated into the circadian programme by associative learning. In humans, social zeitgebers appear weak by comparison with light. In temporal isolation or under weak light-dark cycles, humans may ignore social cues and free-run independently, although cases of mutual synchrony among two or more group-housed individuals have been reported. Social cues may affect circadian timing by controlling sleep-wake states, but the phase of entrainment observed to fixed sleep-wake schedules in dim light is consistent with photic mediation (scheduled variations in behavioural state necessarily create daily light-dark cycles unless subjects are housed in constant dark or have no eyes). By contrast, discrete exercise sessions can induce phase shifts consistent with the nonphotic PRC observed in animal studies. The best evidence for social entrainment in humans is from a few totally blind subjects who synchronize to the 24 h day, or to near-24 h sleep-wake schedules under laboratory conditions. However, the critical entraining stimuli have not yet been identified, and there are no reported cases yet of social entrainment in bilaterally enucleated blind subjects. The role of social zeitgebers in mammalian behavioural ecology, their mechanisms of action, and their utility for manipulating circadian rhythms in humans, remains to be more fully elaborated.

Different effects of intensity and duration of locomotor activity on circadian period

An outstanding unresolved issue in chronobiology is how the level of locomotor activity influences length of the free-running, endogenous circadian period (tau). To address this issue, the authors studied a novel model, 4 replicate lines of laboratory house mice (Mus domesticus) that had been selectively bred for high wheel-running activity (S) and their 4 unselected control (C) lines. Previous work indicates that S mice run approximately twice as many revolutions/day and exhibit an altered dopaminergic function as compared with C mice. The authors report that S mice have a tau shorter by about 0.5 h as compared with C mice. The difference in tau was significant both under constant light (control lines: tau = 25.5 h; selected: tau = 24.9 h) and under constant dark (control lines: 23.7 h; selected: 23.4 h). Moreover, the difference remained statistically significant even when the effects of running speed and time spent running were controlled in ANCOVA. Thus, something more fundamental than just intensity or duration of wheel-running activity per se must underlie the difference in tau between the S and C lines. However, despite significant difference in total wheel-running activity between females and males, tau did not differ between the sexes. Similarly, among individuals within lines, tau was not correlated with wheel-running activity measured as total revolutions per day. Instead, tau tended to decrease with average running speed but increase with time spent running. Finally, within individuals, an increase in time spent running resulted in decreased tau in the next few days, but changes in running speed had no statistically significant effect. The distinctions between effects of duration versus intensity of an activity, as well as between the among- versus within-individual correlations, are critical to understanding the relation between locomotor activity and pace of the circadian clock.

Daily exercise facilitates phase delays of circadian melatonin rhythm in very dim light

Shift workers and transmeridian travelers are exposed to abnormal work-rest cycles, inducing a change in the phase relationship between the sleep-wake cycle and the endogenous circadian timing system. Misalignment of circadian phase is associated with sleep disruption and deterioration of alertness and cognitive performance. Exercise has been investigated as a behavioral countermeasure to facilitate circadian adaptation. In contrast to previous studies where results might have been confounded by ambient light exposure, this investigation was conducted under strictly controlled very dim light (standing ∼0.65 lux; angle of gaze) conditions to minimize the phase-resetting effects of light. Eighteen young, fit males completed a 15-day randomized clinical trial in which circadian phase was measured in a constant routine before and after exposure to a week of nightly bouts of exercise or a nonexercise control condition after a 9-h delay in the sleep-wake schedule. Plasma samples collected every 30–60 min were analyzed for melatonin to determine circadian phase. Subjects who completed three 45-min bouts of cycle ergometry each night showed a significantly greater shift in the dim light melatonin onset (DLMO25%), dim light melatonin offset, and midpoint of the melatonin profile compared with nonexercising controls (Student t-test; P < 0.05). The magnitude of phase delay induced by the exercise intervention was significantly dependent on the relative timing of the exercise after the preintervention DLMO25% ( r = −0.73, P < 0.05) such that the closer to the DLMO25%, the greater the phase shift. These data suggest that exercise may help to facilitate circadian adaptation to schedules requiring a delay in the sleep-wake cycle.

Age-Related Changes in the Spontaneous Motor Rhythms of theSenescence-Accelerated Mouse (SAMP8)

The present study examined the effect of age on the spontaneous motor rhythms of mice during wheel running. The spontaneous motor tempo (SMT) of wheel running was measured for the P8 strain of the senescence-accelerated mouse (SAMP8) by recording the sequence of time intervals (measured in milliseconds) for successive revolutions ofa run-wheel over the course of 16 days. Analyses of the distribution of interrevolution intervals of 2-, 7-, and 12-month-old SAMP8 revealed an age-related slowing of wheel running and a corresponding increase in variability consistent with Weber's law. All three age groups also demonstrated a practice effect over the course of testing best described by a power law. These findings provide evidence of age-related changes in the spontaneous motor rhythms of the SAMP8 that occur as early as 7 months of age. The results are consistent with age-related changes in human subjects and suggest that spontaneous wheel-running behavior in rodents may be a good model for studying SMT.

Circadian activity rhythms in high-alcohol-preferring and low-alcohol-preferring mice

The circadian periods of high-alcohol-preferring (HAP) and low-alcohol-preferring (LAP) selected lines of mice were compared. The mice were ethanol-naive. Circadian periods were calculated from records of running-wheel activity in constant dark. The number of daily wheel revolutions and body weights of the two lines of mice were also compared. The HAP line had a shorter period of wheel running than that of the LAP lines. The HAP mice also had a tendency to run more on wheels than did LAP mice. These findings support the suggestion that genes affecting ethanol consumption in mice have pleiotropic effects on circadian period.

Contribution of classic photoreceptors to entrainment

The ability to phase shift and entrain in response to light is spared in retinally degenerate mice (rd/rd). In the present work, fewer retinally degenerate C57BL/6 mice than wildtypes entrained in dim lights, suggesting that rods and/or cones contribute toward entrainment even though they are not necessary. Thresholds for entrainment appear to be a more sensitive test of deficits in entrainment than phase shifts in response to light pulses.

Aschoff's rule in retinally degenerate mice

Both retinally degenerate and wildtype mice lengthened the period of their free-running circadian rhythms and reduced the amount of wheel running when exposed to increasing levels of constant illumination, in accordance with Aschoff's rule. Decreased locomotor activity may contribute toward lengthening of period in bright light. However, the known effects of activity on free-running period are small compared to those obtained by changing illumination. This suggests that Aschoff's rule in mice is not dependent on changes in nonphotic input, but results from a direct effect of light on the circadian system. The sparing of Aschoff's rule in retinally degenerate mice is further evidence that circadian photoreception depends on mechanisms other than rods and cones.

Effect of cage enrichment on the daily use of running wheels by Syrian hamsters

Institutional animal care committees may one day require for the welfare of captive hamsters more floor space and the introduction of tunnels and toys. As hamsters are popular animal subjects in chronobiological research, and as clock phase is usually measured through running wheel activity, it is important to determine what effect cage enrichment might have on daily wheel use. Here the daily number of wheel revolutions, the daily duration of the running activity phase, the phase relationship between lights-off and onset of running activity, and the free-running period of circadian activity rhythms were measured in Syrian hamsters, Mesocricetus auratus, housed in single cages or in multiple cages linked by tunnels and supplied with commercial wooden toys. Free-running periodicity was not affected by cage enrichment. In multiple-cage systems, there were fewer daily revolutions, shorter wheel-running activity phases, and delayed running activity onsets. These effects, however, were small as compared to interindividual and week-to-week variation. They were statistically significant only under a light:dark cycle, not in constant darkness, and only when interindividual variation was eliminated through a paired design or when the number of cages was increased to five (the maximum tested). Daily wheel use is thus affected by cage enrichment, but only slightly.

Three days of novel wheel access diminishes light-induced phase delays in vivo with no effect on per1 induction by light

The mammalian circadian clock, located in the hypothalamic suprachiasmatic nuclei, synchronizes endogenous behavioral and physiological rhythms to a 24 h period through responses to two types of stimuli: photic (light) and nonphotic (behaviorally induced arousal and/or increases in activity). Photic stimuli can block nonphotic effects and vice versa, although the mechanisms and levels of interactions between these two stimuli types are unknown. Here, we investigated whether 3 d of access to a novel running wheel alters the phase shift to light in vivo, and whether this effect could be seen on induction by light of the circadian gene per1. Through measurement of running wheel activity of golden hamsters, access to a new wheel for 3 d was shown to diminish photic phase delays with no effect on phase advances. As seen using in situ hybridization, however, there was no effect on levels of light-induced per1 mRNA. This study indicates a possible role for this paradigm as a model of interactions between photic and nonphotic stimuli.

Circadian clock resetting by sleep deprivation without exercise in Syrian hamsters: dark pulses revisited

Circadian rhythms in Syrian hamsters can be phase shifted by procedures that stimulate wheel running ("exercise") in the mid-subjective day (the hamster's usual sleep period). The authors recently demonstrated that keeping hamsters awake by gentle handling, without continuous running, is sufficient to mimic this effect. Here, the authors assessed whether wakefulness, independent of wheel running, also mediates phase shifts to dark pulses during the midsubjective day in hamsters free-running in constant light (LL). With running wheels locked during a 3 h dark pulse on day 3 of LL, hamsters (N = 16) averaged only 43+/-15 min of spontaneous wake time and phase shifted only 24+/-43 min. When wheels were open during a dark pulse, two hamsters remained awake, ran continuously, and showed phase advance shifts of 7.3 h and 8.7 h, respectively, whereas the other hamsters were awake <60 min and shifted only 45+/-38 min. No animals stayed awake for 3 h without running. Additional time in LL (10 and 20 days) did not potentiate the waking or phase shift response to dark pulses. When all hamsters were sleep deprived with wheels locked during a dark pulse, phase advance shifts averaged 261+/-110 min and ranged up to 7.3 h. These shifts are large compared to those previously observed in response to the 3 h sleep deprivation procedure. Additional tests revealed that this potentiated shift response is dependent on LL prior to sleep deprivation but not LL after sleep deprivation. A final sleep deprivation test showed that a small part of the potentiation may be due to suppression of spontaneous wheel running by LL. These results indicate that some correlate of waking, other than continuous running, mediates the phase-shifting effect of dark pulses in the mid-subjective day. The mechanism by which LL potentiates shifting remains to be determined. The lack of effect of subsequent LL on the magnitude of shifts to sleep deprivation in the dark suggests that LL reduces responsivity to light by processes that take >3 h of dark to reverse.

Loss of circadian organization of sleep and wakefulness during hibernation

We investigated circadian and homeostatic regulation of nonrapid eye movement (NREM) sleep in golden-mantled ground squirrels during euthermic intervals between torpor bouts. Slow-wave activity (SWA; 1-4 Hz) and sigma activity (10-15 Hz) represent the two dominant electroencephalographic (EEG) frequency components of NREM sleep. EEG sigma activity has a strong circadian component in addition to a sleep homeostatic component, whereas SWA mainly reflects sleep homeostasis [Dijk DJ and Czeisler CA. J Neurosci 15: 3526-3538, 1995; Dijk DJ, Shanahan TL, Duffy JF, Ronda JM, and Czeisler CA. J Physiol (Lond) 505: 851-858, 1997]. Animals maintained under constant conditions continued to display circadian rhythms in both sigma activity and brain temperature throughout euthermic intervals, whereas sleep and wakefulness showed no circadian organization. Instead, sleep and wakefulness were distributed according to a 6-h ultradian rhythm. SWA, NREM sleep bout length, and sigma activity responded homeostatically to the ultradian sleep-wake pattern. We suggest that the loss of sleep-wake consolidation in ground squirrels during the hibernation season may be related to the greatly decreased locomotor activity during the hibernation season and may be necessary for maintenance of multiday torpor bouts characteristic of hibernating species.

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.

Activity rhythms of wild and laboratory golden hamsters (Mesocricetus auratus) under entrained and free-running conditions

The golden hamster (Mesocricetus auratus) is one of the most frequently used laboratory animals, particularly in chronobiological studies. One reason is its very robust and predictable rhythms, although the question arises whether this is an inbreeding effect or rather is typical for the species. We compared the daily (circadian) activity rhythms of wild and laboratory golden hamsters. The laboratory hamsters were derived from our own outbred stock (Zoh:GOHA). The wild hamsters included animals captured in Syria and their descendants (F1). Experiments were performed under entrained (light: dark [LD] 14h:0h) and under free-running (constant darkness, DD) conditions. Locomotor activity was recorded using passive infrared detectors. Under entrained conditions, the animals had access to a running wheel for a certain time to induce additional activity. After 3 weeks in constant darkness, a light pulse (15 min, 100 lux) was applied at circadian time 14 (CT14). Both laboratory and wild hamsters showed well-pronounced and very similar activity rhythms. Under entrained conditions, all hamsters manifested about 80% of their total 24h activity during the dark portion of the LD cycle. The robustness of the daily rhythms was also similar. However, interindividual variability was higher in wild hamsters for both measures. All animals used the running wheels almost exclusively during the dark portion of the LD cycle, although the wild hamsters were three times more active. The period length, measured in constant darkness, was significantly shorter in wild (23.93h +/- 0.10h) than in laboratory hamsters (24.06 +/- 0.07h). The light-induced phase changes were not different (about 1.5h). In summary, these results indicate that the laboratory hamster is not much different from the wild type.

Circadian rhythm of drinking and running-wheel activity in rats with 6-hydroxydopamine lesions of the ventral tegmental area

Circadian rhythms in drinking and running-wheel (locomotor) activity of rats with 6-hydroxydopamine (6-OHDA, 4 microg/2 microl per rat)-induced lesions in the ventral tegmental area (VTA) were examined under a light-dark (LD) cycle and constant dim light (5 lux). Under the LD cycle, the length of the locomotor activity period was decreased during the dark, and increased during the light period in the lesioned rats. Under the constant dim light conditions, the free-running circadian period (tau) of drinking and activity rhythm was longer in lesioned rats than in sham-operated controls. The elongation of the circadian period was accompanied by decrements in activity. These observations suggest that the mesolimbic dopaminergic system modulates rhythms in circadian drinking and locomotor activity.

Circadian clock resetting by sleep deprivation without exercise in the Syrian hamster

Circadian rhythms in several species can be phase-shifted by procedures that stimulate locomotor activity ("exercise") during the usual sleep period. The role of arousal or sleep loss, independent of activity, in this effect has not been adequately resolved. We show here, using the sleep deprivation procedure of gentle handling, that comparably large phase shifts (up to 240 min advances) of the rest-activity cycle can be induced in Syrian hamsters by 3 hr of behavioral arousal, with minimal locomotion, beginning 6 hr before the usual active period. Horizontal distance traveled during the deprivation procedure averaged approximately 0.08 km, compared to 2. 5 km typical in exercise studies. Hamsters requiring fewer interventions exhibited larger shifts, suggesting that the level or continuity of spontaneous arousal determines shift size. The circadian rhythm of light-induced c-fos expression in the suprachiasmatic nucleus (SCN) was used as a phase marker to further demonstrate that the clock is reset within 1 hr after a 3 hr deprivation. Sleep deprivation mimicked the effects of exercise on basal c-fos expression in two components of the circadian system, suppressing basal Fos immunoreactivity in the SCN, and increasing Fos in the intergeniculate leaflet. Sleep deprivation without exercise in hamsters can rapidly reset the circadian clock and alter gene expression within the circadian system.

Behavioral feedback regulation of circadian rhythm phase angle in light-dark entrained mice

Induced and spontaneous wheel running can alter the phase and period (tau) of circadian rhythms in rodents. The relationship between spontaneous running and the phase angle (psi) of entrainment to 24-h light-dark (LD) cycles was evaluated in C57BL/6j mice. With a wheel freely available, psi was significantly correlated with the absolute (r = 0.32) and relative (r = 0.44) amount of activity during the first 2 h of the activity period. When wheels were locked during the first half of the night in LD and then unlocked in constant dark (DD), mice exhibited a delayed psi and lengthened tau compared with mice that had wheels locked during the second half of the night. In DD, tau correlated negatively with total daily activity. To evaluate if wheel running modulates the phase-resetting actions of LD, phase shifts to light pulses were measured at two time points in DD, when daily activity levels differed by 40%. Phase delays to light were 56% greater when activity levels were lower. However, in a counterbalanced follow-up experiment, phase advances and delays to light pulses were not affected by the availability of wheels, although an effect of time in DD was replicated. Spontaneous activity can regulate psi and tau without altering the response of the pacemaker to light.