Running wheel choice by Syrian hamsters

The present study investigated the preference of male and female Syrian hamsters, Mesocricetus auratus, for different types of running wheels. Hamsters were placed individually in sets of multiple cages linked by tunnels, each cage with a different running wheel. The number of wheel revolutions in each cage was tallied daily over 40 days. The hamsters did not express a preference when offered a choice of a running surface made of metal rods spaced 9 mm apart and a similar running surface covered in plastic mesh to prevent the possible slippage of feet between the rods. The hamsters did express a clear preference for larger wheels (35 versus 23 cm diameter), and for completely circular wheels over truncated ones. They neither favoured nor rejected wheels with small obstacles along the running surface. In all experiments, preferences were more strongly expressed by males than by females. Running wheels for hamsters may be improved by enlarging their diameter (to the standards often used for rats, if practically possible) and by ensuring good footing on the running surface (a space no larger than 9 mm between evenly spaced rods seems sufficient to achieve this, at least in large wheels and for hamsters older than 55 days). Installing obstacles along the running surface does not appear to make the wheel more interesting to hamsters.     

Effects of food deprivation on locomotor activity, plasma glucose, and circadian clock resetting in Syrian hamsters

Circadian rhythms in Syrian hamsters can be phase advanced by activity or arousal stimulated during the daily rest phase ("subjective day"). A widely used method for stimulating activity is confinement to a novel wheel. Some hamsters decline to run, and some procedures may reduce the probability of running. The authors evaluated food deprivation (FD) as a method to promote running. Given evidence that perturbations of cell metabolism or glucose availability may affect circadian clock function in some tissues or species, they also assessed the effects of FD on free-running circadian phase, resetting responses to photic and nonphotic stimuli and plasma glucose. In constant light, a 27-h fast significantly increased running in a novel wheel and marginally increased the average size of resulting phase shifts. FD, without novel wheel confinement, was associated with some very large phase shifts or disruption of rhythmicity in hamsters that spontaneously ran in their home wheels during the subjective day. Hamsters that ran only during the usual active phase (subjective night) or that were prevented from running did not exhibit phase shifts, despite refeeding in the mid-subjective day. Using an Aschoff Type II design for measuring shifts, a 27-h fast significantly increased the number of hamsters that ran continuously when confined to a novel wheel but did not affect the dose-response relation between the amount of running and the size of the resulting shift. A day of fasting also did not affect the size of phase delay or advance shifts to 30-min light pulses in the subjective night. Plasma glucose was markedly reduced by wheel running in combination with fasting but was increased by running in nonfasted hamsters. These results establish FD as a useful tool for stimulating activity in home cage or novel wheels and indicate that in Syrian hamsters, significant alterations in glucose availability, associated with running, fasting, and refeeding, have surprisingly little effect on circadian pacemaker function.     

Novel wheel running blocks the preovulatory luteinizing hormone surge and advances the hamster circadian pacemaker

In rodents, the preovulatory luteinizing hormone (LH) surge is timed by a circadian rhythm. We recently reported that a phenobarbital-induced delay of the estrous cycle in Syrian hamsters is associated with an approximately 2-h phase advance in both the circadian locomotor activity rhythm and the timing of the LH surge. The following study tests the hypothesis that a >2-h nonpharmacological phase advance in the circadian pacemaker that delays the estrous cycle by a day will also phase advance the LH surge by approximately 2 h. Activity rhythms were continuously monitored in regularly cycling hamsters using running wheels or infrared detectors for about 10 days prior to jugular cannulation. The next day, on proestrus, hamsters were transferred to the laboratory for 1 of 3 treatments: transfer to a "new cage" (and wheel) from zeitgeber time (ZT) 4 to 8 (with ZT12 defined as time of lights-off), or exposure to a "novel wheel" at ZT5 or ZT1. All animals were then placed in constant dark (DD). Blood samples were obtained just before onset of DD and hourly for the next 6 h, on that day and the next day for determination of plasma LH concentrations. Running activity was monitored in DD for about 10 more days. Transfer to a novel wheel at either ZT5 or ZT1 delayed the LH surge to day 2 in most hamsters, whereas exposure to a new cage did not. Only the delayed LH surges were phase advanced at least 2.5 h on average in all 3 groups. However, wheel-running activity was similarly phase advanced in all 3 groups regardless of the timing of the LH surge; thus, the phase advances in circadian activity rhythms were not associated with the 1-day delay of the LH surge. Interestingly, the number of wheel revolutions was closely associated with the 1-day delay of LH surges following exposure to a novel wheel at either ZT1 or ZT5. These results suggest that the intensity of wheel running (or an associated stimulus) plays an important role in the circadian timing mechanism for the LH surge.     

The effect of cage size and enrichment on core temperature and febrile response of the golden hamster

The aim of this study is to determine the effect of cage size and cage enrichment. Golden hamsters were individually housed in standard cages of four different sizes and in enriched cages of three different sizes since 3 weeks of age. Each of the seven housing groups consisted of 12 hamsters. After 14 weeks of housing in their respective environments the measurements started. The mean baseline rectal temperature was significantly higher in hamsters housed in small cages than in hamsters housed in large cages. After the injection of fever-inducing lipopolysaccharide rectal temperature increased by 1 to 2 degrees C. The increase of rectal temperature and the fever index were the highest in animals housed in large cages and the smallest in animals housed in small cages. Through cage enrichment and increasing cage size the mean febrile response increased while the mean baseline rectal temperature decreased. Cage size and cage enrichment had no effect on the dispersion of the measured values. The differences in microclimate between large and small cages were too small to have an effect on thermoregulation. The results indicate that housing in small cages induce chronic stress which obviously affects thermoregulation. The findings demonstrate that the results of some physiological experiments are significantly influenced by the pre-experimental housing conditions.     

Training Nonhuman Primates Using Positive Reinforcement Techniques

Physical cage enrichment—exercise devices for rodents in the laboratory—often includes running wheels. This study compared responses of mice in enriched physical and social conditions and in standard social conditions to wheel running, individual housing, and open-field test. The study divided into 6 groups, 48 female BALB/c mice group housed in enriched and standard conditions. On alternate days, the study exposed 2 groups to individual running wheel cages. It intermittently separated from their cage mates and housed individually 2 groups with no running wheels; 2 control groups remained in enriched or standard condition cages. There were no significant differences between enriched and standard group housed mice in alternate days' wheel running. Over time, enriched, group housed mice ran less. Both groups responded similarly to individual housing. In open-field test, mice exposed to individual housing without running wheel moved more and faster than wheel running and home cage control mice. They have lower body weights than group housed and wheel running mice. Intermittent withdrawal of individual housing affects the animals more than other commodities. Wheel running normalizes some effects of intermittent separation from the enriched, social home cage.     

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

ABSTRACT

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.     

Nonphotic Phase Shifting in the Old and the Cold

When confined to novel running wheels or when given injections of triazolam in their home cages, old hamsters do not become as active as young hamsters. Therefore, lack of nonphotic phase shifting following such manipulations may stem from insufficient activity or arousal. Phase advances can be obtained in some 10-month-old animals when wheel running during the pulse is increased by the presence of females in estrous condition and in most 18-month-old hamsters by combining confinement to a novel wheel with triazolam injections. These data suggest that there is relatively little if anything wrong in aging hamsters with the nonphotic phase-shifting mechanism itself. The reason why in certain situations old hamsters do not shift appears to be because the nonphotic inputs to these shifting mechanisms are not strong enough. However, when running in novel wheels is increased by carrying out the tests at cold temperatures, most old animals did not show subsequent phase shifts. Evidently it is not running per se that is critical for phase shifts, but probably the motivational context for such running.     

Phase shifting by novelty-induced running: activity dose-response curves at different circadian times

This study compared phase shifting after novelty-induced running at different circadian times (CTs). In Experiment 1, hamsters were confined to novel wheels for 3 h, starting at CTs 2, 4, 6, 8, 10 or 22. The largest shifts were found at CTs 2, 4 and 6. At each CT there was a relationship between the number of revolutions during the pulse and the size of phase shift. Maximum shifts were usually observed at each CT when animals ran 5000–9000 revolutions during the pulse. In Experiment 2, hamsters were confined to novel wheels for 1 h, also starting at CTs 2, 4, 6, 8, 10 or 22. Unlike with 3-h pulses, the largest shifts with 1-h pulses occurred at CT 8. In Experiment 3, hamsters were shut into a small nest box after a 1-h pulse at CT 8; phase shifting was unaffected, showing that movement about the home cage after a 1-h pulse had ended was not required for shifting. At CTs 2, 4 and 22, 3-h pulses produced shifts but 1-h pulses did not. Possibly, there are two different mechanisms of nonphotic phase shifting that can be activated by being placed in a novel wheel, but the results can also be explained in terms of a single mechanism. Accepted: 8 August 1997     

Nonphotic Phase Shifting in the Old and the Cold

When confined to novel running wheels or when given injections of triazolam in their home cages, old hamsters do not become as active as young hamsters. Therefore, lack of nonphotic phase shifting following such manipulations may stem from insufficient activity or arousal. Phase advances can be obtained in some 10-month-old animals when wheel running during the pulse is increased by the presence of females in estrous condition and in most 18-month-old hamsters by combining confinement to a novel wheel with triazolam injections. These data suggest that there is relatively little if anything wrong in aging hamsters with the nonphotic phase-shifting mechanism itself. The reason why in certain situations old hamsters do not shift appears to be because the nonphotic inputs to these shifting mechanisms are not strong enough. However, when running in novel wheels is increased by carrying out the tests at cold temperatures, most old animals did not show subsequent phase shifts. Evidently it is not running per se that is critical for phase shifts, but probably the motivational context for such running.     

Testosterone and photoperiod interact to regulate locomotor activity in male hamsters

Testicular size, plasma testosterone levels, copulatory behavior, and daily locomotor activity are reduced in male hamsters after 10 weeks of exposure to short days. The role of testosterone in the short day-induced decline in locomotor activity was investigated, determining whether or not photoperiod could alter the effect of testosterone on activity. Castrated adult hamsters were allowed to acclimate to running wheels (wired to digital counters) and then were kept on either long (L:D 14:10) or short (L:D 6:18) days for 60 days. On Day 60, half of the animals on each light cycle were implanted with 12-mm-long testosterone-filled Silastic capsules; half received empty capsules. Digital counting of wheel-running activity continued for another 140 days. Blood samples taken on Day 200 confirmed L:D 14:10 and L:D 6:18 testosterone-treated hamsters had equivalent plasma testosterone levels. After an initial decline in activity, L:D 14:10 animals exhibited a progressive rise in mean running activity (from ~2000 to ~5000 wheel revolutions per day) through 100 days after the initiation of testosterone treatment. In contrast, activity levels in testosterone-treated L:D 6:18 animals remained uniform (~2000 wheel revolutions per day) during this time, indicating exposure to short days rendered the hamsters less sensitive to the stimulatory effect of testosterone on activity. Of further interest was a marked increase in activity after 160–200 short days in animals treated with either testosterone-filled or empty capsules. It appears the total amount of daily locomotor activity in the hamster is modulated by circulating testosterone levels in a manner which is dependent upon the environmental photoperiod.     

The precision of circadian clocks: assessment and analysis in Syrian hamsters

Locomotor activity recordings of Syrian hamsters were systematically analyzed to estimate the precision of the overt circadian activity rhythm in constant darkness. Phase variation, i.e., the standard deviation of phase markers around the regression line, varied with the definition of phase. Smallest phase variation was found in the onset of wheel running activity defined by 1h running means of the raw data. Both lower and higher degrees of smoothing lead to decreased precision measured in the overt rhythm. With passive infrared recordings, the midpoint of activity defined by 3h running means was the least variable. This demonstrates that the choice of phase marker should vary between recording methods. Phase variation decreased with increasing activity and was larger in females than in males. By calculating the average cycle variation and serial covariance of consecutive cycles, we estimated the contribution of 'clock' and 'non-clock' related processes to the overt rhythm variability. Variance in precision between phase markers could be shown to be attributable mainly to nonclock processes. Variance in pacemaker cycle length appeared reduced in wheel running activity records compared with passive infrared sensing records, suggesting feedback from running activity onto pacemaker function.     

Plasticity of Hamster Circadian Entrainment Patterns Depends on Light Intensity

The multiple oscillatory basis of the mammalian circadian pacemaker is adduced by, among other phenomena, the occurrence of split locomotor activity rhythms in rodents after prolonged exposure to constant light. More recently, split rhythms entrained to a 24h light:dark:light:dark cycle have been documented following scheduled access of hamsters to a novel running wheel or by photoperiod manipulations alone. Because the incidence of constant light-induced splitting depends on light intensity, the role of this variable was assessed in this new splitting paradigm. Male Syrian hamsters, entrained to a 14h light:10h dark cycle, were transferred to individual running wheel cages 7h after light onset. Transfer coincided with the beginning of the scotophase of a new photocycle alternating between 5h of relative dark and 7h of light. For four weeks bright photophases (～350 lux) were alternated with either dim (<0.1 lux) or completely dark (0 lux) scotophases. An additional group received moderate intensity photophases (～45 lux) paired with dim scotophase illumination. For an additional four weeks, all hamsters were exposed to the same bright:dim light:dark cycle. Dim light in the scotophase significantly increased the incidence of split activity rhythms relative to that observed with completely dark scotophases. Overall wheel-running rates and activity induced by a cage change were also increased in dim light-exposed animals. Group differences largely disappeared four weeks later when hamsters previously maintained in completely dark scotophases were exposed to dim scotophases. Photophase light intensity did not affect the overall incidence of splitting, but influenced the timing of activity in the afternoon scotophase. The effects of dim illumination may be mediated in part via enhanced locomotor responses to transfer to a new cage or by changes in coupling interactions between component oscillators.     

Plasticity of hamster circadian entrainment patterns depends on light intensity

The multiple oscillatory basis of the mammalian circadian pacemaker is adduced by, among other phenomena, the occurrence of split locomotor activity rhythms in rodents after prolonged exposure to constant light. More recently, split rhythms entrained to a 24h light:dark:light:dark cycle have been documented following scheduled access of hamsters to a novel running wheel or by photoperiod manipulations alone. Because the incidence of constant light-induced splitting depends on light intensity, the role of this variable was assessed in this new splitting paradigm. Male Syrian hamsters, entrained to a 14h light:10h dark cycle, were transferred to individual running wheel cages 7h after light onset. Transfer coincided with the beginning of the scotophase of a new photocycle alternating between 5h of relative dark and 7h of light. For four weeks bright photophases (approximately 350 lux) were alternated with either dim (< 0.1 lux) or completely dark (0 lux) scotophases. An additional group received moderate intensity photophases (approximately 45 lux) paired with dim scotophase illumination. For an additional four weeks, all hamsters were exposed to the same bright:dim light:dark cycle. Dim light in the scotophase significantly increased the incidence of split activity rhythms relative to that observed with completely dark scotophases. Overall wheel-running rates and activity induced by a cage change were also increased in dim light-exposed animals. Group differences largely disappeared four weeks later when hamsters previously maintained in completely dark scotophases were exposed to dim scotophases. Photophase light intensity did not affect the overall incidence of splitting, but influenced the timing of activity in the afternoon scotophase. The effects of dim illumination may be mediated in part via enhanced locomotor responses to transfer to a new cage or by changes in coupling interactions between component oscillators.     

The influence of bedding depth on behaviour in golden hamsters (Mesocricetus auratus)

Although digging was found to be important in captive rodents, most golden hamsters are provided with only little material to dig. In this study, the influence of different bedding depths and acute stressors on the behaviour and welfare of golden hamsters was analysed. Forty-five male golden hamsters were assigned singly to three experimental groups with 80, 40 or 10 cm deep wood shavings. Behaviour was evaluated by continuous running wheel activity and video recordings, a series of stressors was applied after 7 and 8 weeks. Burrows, if constructed, were mapped monthly. Additionally, adrenals, testes and body mass as well as hormone levels of corticosteroids and testosterone were measured at euthanasia. Hamsters kept with 10 cm deep bedding showed significantly more wire-gnawing and a higher running wheel activity than the hamsters in the other groups. In 80 cm deep bedding wire-gnawing was never observed. Stressor application showed no significant immediate influence on behaviour. All hamsters in 40 and 80 cm bedding constructed burrows which they occupied. The body condition (body weight/body size³) was significantly higher in the animals kept in deep (80 cm) compared with those housed in low (10 cm) bedding cages. The relative testes weights were significantly heaviest in the medium treatment group. No significant differences could be detected for the adrenal glands and testosterone levels. In this study, we showed that cages with at least 40 cm of bedding seemed to enhance the welfare of golden hamsters, although those in 80 cm bedding had more body fat compared with the other groups. However, deep bedding which is appropriate for burrowing can be recommended for golden hamsters.     

Behavioral assessment of intermittent wheel running and individual housing in mice in the laboratory

Physical cage enrichment—exercise devices for rodents in the laboratory—often includes running wheels. This study compared responses of mice in enriched physical and social conditions and in standard social conditions to wheel running, individual housing, and open-field test. The study divided into 6 groups, 48 female BALB/c mice group housed in enriched and standard conditions. On alternate days, the study exposed 2 groups to individual running wheel cages. It intermittently separated from their cage mates and housed individually 2 groups with no running wheels; 2 control groups remained in enriched or standard condition cages. There were no significant differences between enriched and standard group housed mice in alternate days' wheel running. Over time, enriched, group housed mice ran less. Both groups responded similarly to individual housing. In open-field test, mice exposed to individual housing without running wheel moved more and faster than wheel running and home cage control mice. They have lower body weights than group housed and wheel running mice. Intermittent withdrawal of individual housing affects the animals more than other commodities. Wheel running normalizes some effects of intermittent separation from the enriched, social home cage.     

Cage enrichment with paper tissue, but not plastic tunnels, increases variability in mouse model of asthma

Environmental enrichment, besides having a great impact on animal welfare, can also be a potential variable in experimental research. Thus, we investigated whether enrichment of cages with paper tissues or plastic tunnels affects scientific outcome in the well-described mouse model of allergic asthma. BALB/cJ mice were introduced to paper tissues as nesting material, transparent plastic tunnels serving as shelters or kept in non-enriched cages. Afterwards, mice were sensitized to chicken egg ovalbumin (OVA) precipitated in aluminium sulphate and then intranasally challenged with OVA to induce allergic lung inflammation. Mice housed in cages enriched with paper tissues, but not with plastic tunnels, had increased total cell number, eosinophil number and IL-13 concentration in bronchoalveolar lavage fluid in comparison with the non-enriched control group. These results indicate that the effect of environmental enrichment on mice asthma models depends on the type of enrichment used. Therefore, it is important to consider the potential effects of any environmental enrichment on animal welfare and more importantly, on research results in order to standardize and obtain more accurate data from rodent studies.     

Nonphotic phase shifting in female Syrian hamsters: interactions with the estrous cycle

Nonphotic phase shifting of circadian rhythms was examined in female Syrian hamsters. Animals were stimulated at zeitgeber time 4.5 by either placing them in a novel running wheel or by transferring them to a clean home cage. Placement in a clean home cage was more effective than novel wheel treatment in stimulating large (> 1.5 h) phase shifts. Peak phase shifts (ca. 3.5 h) and the percentage of females showing large phase shifts were comparable to those found in male hamsters stimulated with novel wheels. The amount of activity induced by nonphotic stimulation and the amount of phase shifting varied slightly with respect to the 4-day estrous cycle. Animals tended to run less and shift less on the day of estrus. Nonphotic stimulation on proestrus often resulted in a 1-day delay of the estrous cycle reflected in animals' postovulatory vaginal discharge and the expression of sexual receptivity (lordosis). This delay of the estrous cycle was associated with large phase advances and high activity. These results extend the generality of nonphotic phase shifting to females for the first time and raise the possibility that resetting of circadian rhythms can induce changes in the estrous cycle.     

Using the Activity-based Anorexia Rodent Model to Study the Neurobiological Basis of Anorexia Nervosa

Anorexia nervosa (AN) is a psychiatric illness characterized by excessively restricted caloric intake and abnormally high levels of physical activity. A challenging illness to treat, due to the lack of understanding of the underlying neurobiology, AN has the highest mortality rate among psychiatric illnesses. To address this need, neuroscientists are using an animal model to study how neural circuits may contribute toward vulnerability to AN and may be affected by AN. Activity-based anorexia (ABA) is a bio-behavioral phenomenon described in rodents that models the key symptoms of anorexia nervosa. When rodents with free access to voluntary exercise on a running wheel experience food restriction, they become hyperactive – running more than animals with free access to food. Here, we describe the procedures by which ABA is induced in adolescent female C57BL/6 mice. On postnatal day 36 (P36), the animal is housed with access to voluntary exercise on a running wheel. After 4 days of acclimation to the running wheel, on P40, all food is removed from the cage. For the next 3 days, food is returned to the cage (allowing animals free food access) for 2 hr daily. After the fourth day of food restriction, free access to food is returned and the running wheel is removed from the cage to allow the animals to recover. Continuous multi-day analysis of running wheel activity shows that mice become hyperactive within 24 hr following the onset of food restriction. The mice run even during the limited time during which they have access to food. Additionally, the circadian pattern of wheel running becomes disrupted by the experience of food restriction. We have been able to correlate neurobiological changes with various aspects of the animals’ wheel running behavior to implicate particular brain regions and neurochemical changes with resilience and vulnerability to food-restriction induced hyperactivity.     

Daily novel wheel running reorganizes and splits hamster circadian activity rhythms.

The phenomenon of splitting of locomotor activity rhythms in constant light has implied that the mammalian circadian pacemaker is composed of multiple interacting circadian oscillators. Exposure of male Syrian hamsters to novel running wheels also induces splitting in some reports, although novel wheel running (NWR) is better known for its effects on altering circadian phase and the length of the free-running period. In three experiments, the authors confirm and extend earlier reports of split rhythms induced by NWR. Male Syrian hamsters, entrained to LD 14:10, were transferred for 6 to 11 consecutive days to darkened novel Wahmann wheels at ZT 4 and were returned to their home cages at ZT 9. All hamsters ran robustly in the novel wheels. NWR caused a marked reorganization of home cage wheel-running behavior: Activity onsets delayed progressively with each additional day of NWR. After 11 days, activity onset in the nighttime scotophase was delayed by 7 h and disappeared completely in 2 hamsters (Experiment 1). After 6 to 7 days of NWR (Experiment 2), activity onset delayed by 5 h. Transfer of hamsters to constant darkness (DD) after 7 days of NWR revealed clearly split activity rhythms: The delayed nighttime activity bout was clearly identifiable and characterized by a short duration. A second bout associated with the former time of NWR was equally distinct and exhibited a similarly short duration. These components rejoined after 3 to 5 days in DD accomplished via delays and advances of the nighttime and afternoon components, respectively. The final experiment established that rejoining of activity components could be prevented by perpetuating the light-dark:light-dark cycle used to induce split rhythms. The data suggest that NWR causes selective phase shifting of some circadian oscillators and that component oscillators interact strongly in constant darkness.     

Intensive voluntary wheel running may restore circadian activity rhythms and improves the impaired cognitive performance of arrhythmic Djungarian hamsters

Circadian rhythms are highly important not only for the synchronization of animals and humans with their periodic environment but also for their fitness. Accordingly, the disruption of the circadian system may have adverse consequences. A certain number of animals in our breeding stock of Djungarian hamsters are episodically active throughout the day. Also body temperature and melatonin lack 24-h rhythms. Obviously in these animals, the suprachiasmatic nuclei (SCN) as the central pacemaker do not generate a circadian signal. Moreover, these so-called arrhythmic (AR) hamsters have cognitive deficits. Since motor activity is believed to stabilize circadian rhythms, we investigated the effect of voluntary wheel running. Hamsters were bred and kept under standardized housing conditions with food and water ad libitum and a 14 L/10 D lighting regimen. AR animals were selected according to their activity pattern obtained by means of passive infrared motion detectors. In a first step, the daily activity behavior was investigated for 3 weeks each without and with running wheels. To estimate putative photic masking effects, hamsters were exposed to light (LPs) and DPs and also released into constant darkness for a minimum of 3 weeks. A novel object recognition (NOR) test was performed to evaluate cognitive abilities both before and after 3 weeks of wheel availability. The activity patterns of hamsters with low wheel activity were still AR. With more intense running, daily patterns with higher values in the dark time were obtained. Obviously, this was due to masking as LPs did suppress and DPs induced motor activity. When transferred to constant darkness, in some animals the daily rhythm disappeared. In other hamsters, namely those which used the wheels most actively, the rhythm was preserved and free-ran, what can be taken as indication of a reconstitution of circadian rhythmicity. Also, animals showing a 24-h activity pattern after 3 weeks of extensive wheel running were able to recognize the novel object in the NOR test but not so before. The results show that voluntary exercise may reestablish circadian rhythmicity and improve cognitive performance.