Circadian locomotor activity rhythm under the influences of temperature cycle in the Djungarian hamster, Phodopus sungorus, entrained by 12 hour light-12 hour dark cycle

Effects of temperature cycle (25 degrees C during light and 10 degrees C during dark) on circadian locomotor activity rhythm entrained by 12 hr light-12 hr dark cycle were studied in the dark active Djungarian hamster. The amounts of activity per 24 hr were significantly greater under temperature cycle than under constant temperature of 25 degrees C. Phase angle difference between activity onset and light off was always more positive under temperature cycle than under constant temperature. These findings are discussed in terms of circadian physiology.     

Circadian Changes in Anxiolysis-Related Behavior of Syrian Hamsters. Correlation with Hypothalamic GABA Release

The diurnal variations in anxiolysis and exploratory behavior were examined in a plus-maze paradigm in Syrian hamsters exposed to 14 h light: 10 h dark photoperiods or to constant darkness for 3 days. The percent of time spent in open arms and the percent of entries to open arms (both indexes of anxiety-related behavior) as well as the number of crosses to both arms (an index of locomotor behavior) showed significant daily variations under the two lighting conditions, maxima being found at night (2400-0400 h). Flumazenil (5 mg/kg) injected at the middle of the light (at 1600 h) or dark period (at 0400 h) decreased by 39-54 % anxiolysis-related behavior without affecting locomotor activity significantly. 3 H-?-aminobutyric acid (GABA) release from preoptic-medial basal hypothalamic explants obtained from hamsters exposed to 14 h light: 10 h dark photoperiods attained maximal values at 2400-0400 h. The results further supported the existence of circadian changes in anxiolysis-related behavior in Syrian hamsters that correlated with an increased hypothalamic GABA release.     

Transmission and adaptation of chronic wasting disease to hamsters and transgenic mice: evidence for strains

In vitro screening using the cell-free prion protein conversion system indicated that certain rodents may be susceptible to chronic wasting disease (CWD). Therefore, CWD isolates from mule deer, white-tailed deer, and elk were inoculated intracerebrally into various rodent species to assess the rodents' susceptibility and to develop new rodent models of CWD. The species inoculated were Syrian golden, Djungarian, Chinese, Siberian, and Armenian hamsters, transgenic mice expressing the Syrian golden hamster prion protein, and RML Swiss and C57BL10 wild-type mice. The transgenic mice and the Syrian golden, Chinese, Siberian, and Armenian hamsters had limited susceptibility to certain of the CWD inocula, as evidenced by incomplete attack rates and long incubation periods. For serial passages of CWD isolates in Syrian golden hamsters, incubation periods rapidly stabilized, with isolates having either short (85 to 89 days) or long (408 to 544 days) mean incubation periods and distinct neuropathological patterns. In contrast, wild-type mouse strains and Djungarian hamsters were not susceptible to CWD. These results show that CWD can be transmitted and adapted to some species of rodents and suggest that the cervid-derived CWD inocula may have contained or diverged into at least two distinct transmissible spongiform encephalopathy strains.     

Non-parametric photic entrainment of Djungarian hamsters with different rhythmic phenotypes

To investigate the role of non-parametric light effects in entrainment, Djungarian hamsters of two different circadian phenotypes were exposed to skeleton photoperiods, or to light pulses at different circadian times, to compile phase response curves (PRCs). Wild-type (WT) hamsters show daily rhythms of locomotor activity in accord with the ambient light/dark conditions, with activity onset and offset strongly coupled to light-off and light-on, respectively. Hamsters of the delayed activity onset (DAO) phenotype, in contrast, progressively delay their activity onset, whereas activity offset remains coupled to light-on. The present study was performed to better understand the underlying mechanisms of this phenomenon. Hamsters of DAO and WT phenotypes were kept first under standard housing conditions with a 14:10 h light–dark cycle, and then exposed to skeleton photoperiods (one or two 15-min light pulses of 100 lx at the times of the former light–dark and/or dark–light transitions). In a second experiment, hamsters of both phenotypes were transferred to constant darkness and allowed to free-run until the lengths of the active (α) and resting (ρ) periods were equal (α:ρ = 1). At this point, animals were then exposed to light pulses (100 lx, 15 min) at different circadian times (CTs). Phase and period changes were estimated separately for activity onset and offset. When exposed to skeleton-photoperiods with one or two light pulses, the daily activity patterns of DAO and WT hamsters were similar to those obtained under conditions of a complete 14:10 h light–dark cycle. However, in the case of giving only one light pulse at the time of the former light–dark transition, animals temporarily free-ran until activity offset coincided with the light pulse. These results show that photic entrainment of the circadian activity rhythm is attained primarily via non-parametric mechanisms, with the “morning” light pulse being the essential cue. In the second experiment, typical photic PRCs were obtained with phase delays in the first half of the subjective night, phase advances in the second half, and a dead zone during the subjective day. ANOVA indicated no significant differences between WT and DAO animals despite a significantly longer free-running period (tau) in DAO hamsters. Considering the phase shifts induced around CT0 and the different period lengths, it was possible to model the entrainment patterns of both phenotypes. It was shown that light-induced phase shifts of activity offset were sufficient to compensate for the long tau in WT and DAO hamsters, thus enabling a stable entrainment of their activity offsets to be achieved. With respect to activity onsets, phase shifts were sufficient only in WT animals; in DAO hamsters, activity onset showed increasing delays. The results of the present paper clearly demonstrate that, under laboratory conditions, the non-parametric component of light and dark leads to circadian entrainment in Djungarian hamsters. However, a stable entrainment of activity onset can be achieved only if the free-running period does not exceed a certain value. With longer tau values, hamsters reveal a DAO phenotype. Under field conditions, therefore, non-photic cues/zeitgebers must obviously be involved to enable a proper circadian entrainment.     

A short red light pulse during dark phase of LD-cycle perturbs the hamster's circadian clock

In this study we investigated the influence of red light, which naturally occurs during dawn and dusk, on locomotor activity and body temperature rhythms of Djungarian hamsters (Phodopus sungarus). A single weak red light pulse given 2 h before regular lights on had acute as well as long-term effects persisting for several days following exposure. The hamsters immediately stopped their locomotor activity, accompanied by a drop in body temperature. In the following undisturbed nights (LD 168) the nocturnal activity stopped earlier than usual. This lasting effect of the light pulse was more pronounced than the acute effect. The activity phase compressed gradually during 3 to 5 days after the light pulse was administered while time of activity onset was almost unaffected. It took 6 to 11 days for complete recovery of the original activity phase. The maximal activity compression and the recovery period depended on the duration of the single red light pulse and its intensity. Red light pulses of 15 min duration were about twice effective as 1 min pulses; and the effect of a red light pulse of 130 mW/m² was about 1.5 times stronger than a 30 mW/m² red light pulse. The maximal value of activity phase compression reached in this experiment was 2.5+0.2 h with a recovery period of 11.1±0.3 days following a given red light pulse of 90 mW/m² and 15 min. The morning oscillator seems to be persistently affected. This indicates a very high photosensitivity of the Djungarian hamster's circadian system to red light.Abbreviations T _b body temperature - DD constant darkness - LD light:dark cycle - LL constant light - duration of activity phase - CT circadian time - PRC phase response curve - SCN suprachiasmatic nuclei     

Entrainment of 2 subjective nights by daily light:dark:light:dark cycles in 3 rodent species

Recent work with exotic 24-h light:dark:light:dark (LDLD) cycles indicates surprising flexibility in the entrainment patterns of Syrian hamsters. Following exposure to an LDLD cycle, hamsters may adopt a form of rhythm splitting in which markers of subjective night (e.g., activity, melatonin) are expressed in each of the twice daily scotophases. This pattern contrasts markedly with that of conventionally entrained hamsters in which markers of subjective night are expressed once daily in only 1 of the 2 dark periods. The "split" entrainment pattern was examined further here in Syrian and Siberian hamsters and in mice exposed to LDLD 7:5:7:5, a condition that reliably induces split activity rhythms in all 3 species. The phase angle of entrainment and activity duration were generally similar comparing the 2 daily activity bouts in each species. The stability of this split entrainment state was assessed by deletions of photophases on individual days, by exposure to skeleton photoperiods, and by transfer to constant darkness. As in Syrian hamsters, the one-time substitution of darkness for one 7-h photophase did not grossly alter activity patterns of Siberian hamsters but acutely disrupted the split rhythms of mice. Skeleton light pulses of progressively shorter duration did not significantly alter split entrainment patterns of either Syrian or Siberian hamsters. Both species continued to exhibit stable entrainment with activity expressed in alternate scotophases of an LD 1:5 cycle presented 4 times daily. In contrast, the split activity rhythms of mice were not maintained under skeleton pulses. In constant darkness, rhythms of Siberian hamsters remained distinctly split for a minimum of 2 cycles. Split entrainment to these novel LDLD and 4-pulse skeleton lighting regimes demonstrates a marked degree of plasticity common to the circadian systems of several rodent species and identifies novel entrainment patterns that may be reliably elicited with simple environmental manipulations. Inter- and intraspecific differences in the stability of split activity rhythms likely reflect differences in coupling interactions between the component circadian oscillators, which, adopting separate phase relations to these novel LD cycles, yield a split entrainment pattern.     

Characterization of circadian function in Djungarian hamsters insensitive to short day photoperiod

Summary Djungarian hamsters (Phodopus sungorus sungorus) depend mainly on day length to cue seasonal adjustments. However, not all individuals respond to short day conditions. A previous study from this laboratory proposed that nonresponsiveness to short day conditions rests with a defect in the circadian organization of these hamsters.In this study we found pronounced differences between responsive and nonresponsive hamsters in the expression of circadian rhythmicity under constant darkness and under constant illumination. While responsive hamsters showed a free-running activity pattern with a period of 23.86+0.04 h and responded to brief light pulses with the expected phase delays and phase advances, nonresponsive hamsters exhibited a period of 24.04+0.05 h and responded to light pulses with phase advances. Furthermore, 9 out of 15 responsive hamsters showed a clear split in the activity pattern within 8 weeks under constant light (80–100 lux), while only 1 of the 7 nonresponsive hamsters exhibited a split activity pattern. As a result of these differences in circadian function, nonresponsive Djungarian hamsters are incapable of proper photoperiod time measurement and photoperiod-induced seasonality.Abbreviations PRC phase response curve - ct circadian time - DD constant dark - LL constant light     

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.     

Light-induced c-Fos expression in the SCN and behavioural phase shifts of Djungarian hamsters with a delayed activity onset

C-Fos expression in the suprachiasmatic nucleus (SCN) and phase shifts of the activity rhythm following photic stimulation were investigated in Djungarian hamsters (Phodopus sungorus) of two different circadian phenotypes. Wild-type (WT) hamsters display robust daily patterns of locomotor activity according to the light/dark conditions. Hamsters of the DAO (delayed activity onset) phenotype, however, progressively delay the activity onset, whereas activity offset remains coupled to “light-on”. Although the exact reason for the delayed activity onset is not yet clarified, it is connected with a disturbed interaction between the light/dark cycle and the circadian clock. The aim was to test the link between photoreception and the behavioral output of the circadian system in hamsters of both phenotypes, to get further insight in the underlying mechanism of the DAO phenomenon. Animals were exposed to short light pulses at different times during the dark period to analyze phase shifts of the activity rhythm and expression of Fos protein in the SCN. The results indicate that the photosensitive phase in DAO hamsters is shifted like the activity onset. Also, phase shifts were significantly smaller in DAO hamsters. At the same time, levels of Fos expression did not differ between phenotypes regarding the circadian phase. The results provide evidence that the shifted photosensitivity of the circadian system in DAO hamsters does not differ from that of WT animals, and lead us to conclude that processes within the SCN that enable light information to reset the circadian pacemaker might offer an explanation for the DAO phenomenon.     

Seasonal changes of thermoregulatory efficiency in Djungarian hamsters

In their natural habitat, Djungarian hamsters are faced with dramatic seasonal changes. This requires various morphological and physiological adaptations allowing cope with harsh climate and food shortage, particularly in winter. These seasonal changes are controlled by the photoperiod and can be observed also in the laboratory at room temperature. The aim of the present study was to investigate if the efficiency of thermoregulation also depends on the photoperiod. For this reason, Djungarian hamsters were transferred to short-day conditions (SDC) with 8 h light and 16 h darkness. Two-thirds of the animals were classified as responders showing the typical seasonal changes – decrease of body mass, fur change, testes regression, vagina closing. The total activity per day did not change but, the nocturnal activity was spread over the longer dark time. The body temperature decreased, and the animals showed regular daily torpor. To investigate the thermoregulatory efficiency, body temperatures were correlated with motor activity. The obtained regression coefficients describe formally the effect of motor activity on body temperature, a measure for the efficiency of thermoregulation. In SDC, the coefficients were elevated, both during rest and activity, i.e. the same amount of activity did produce a larger increase in body temperature. Under field conditions, this might be an additional mechanism to compensate the bigger in winter heat loss. Also, the high coefficients may support the increase in body temperature at the end of a torpor phase by a bout of motor activity. The results show that, seasonal changes of thermoregulatory efficiency are an effective accessory way to cope with different temperatures in hamsters’ natural environment.     

Circadian regulation of pineal melatonin and reproduction in the Djungarian hamster

Gonadal state, pineal melatonin rhythms, and locomotor activity rhythms were examined in juvenile male Djungarian hamsters exposed to non-24-hr light cycles ("T-cycles") or to full photoperiods. At the end of 1 month, hamsters exposed to a 1-hr pulse of light every 24.33 hr (T 24.33) exhibited small testes, whereas those receiving the same amount of light every 24.78 hr (T 24.78) displayed stimulated gonads, ten-fold larger in size. Accompanying the nonstimulatory effect of the T 24.33 cycle were nocturnal peaks in both pineal melatonin content and serum melatonin concentration which were longer by approximately 4 hr than those observed on the photostimulatory T 24.78 cycle. Exposure to an intermediate-length T-cycle (T 24.53) resulted in a mixed gonadal response and in pineal and serum melatonin peaks of intermediate duration. Wheel-running activity was entrained to the T-cycles such that light was present only near the beginning of the subjective night, its phase (relative to activity onset) differing only slightly among T-cycle groups. Hence the durational differences observed in the melatonin peaks were apparently not due to the acute suppressive or phase-advancing effects of morning light on melatonin biosynthesis, but were rather the result of differences in the endogenous control of pineal activity by the circadian pacemaker system. While no strong correlation was detected between gonadal state and the phase of locomotor activity onset relative to the light pulse, a significant correlation was observed between gonadal state and the duration of daily locomotor activity (alpha). These data were compared to similar measures obtained from hamsters exposed to long-versus short-day full photoperiods (LD 16:8 vs. LD 10:14). In summary, the results of this study indicate involvement of the circadian pacemaker system of Djungarian hamsters in the control of pineal melatonin synthesis and secretion, and in photoperiodic time measurement. Furthermore, these data strengthen the hypothesis that it is the duration of nocturnal pineal melatonin secretion that is the critical feature of this neuroendocrine gland's photoperiodic signal.     

Variability of diurnality in laboratory rodents

The locomotor activity rhythms of domestic mice, laboratory rats, Syrian hamsters, Siberian hamsters, Mongolian gerbils, degus, and Nile grass rats were compared. Running-wheel activity was monitored under a light–dark cycle with 12 h of light and 12 h of darkness per day. Nile grass rats were found to be reliably diurnal, whereas laboratory rats, Siberian hamsters, domestic mice, and Syrian hamsters were reliably nocturnal. Both diurnal and nocturnal subgroups were observed in Mongolian gerbils and degus. A downward gradient of diurnality was observed from Mongolian gerbils classified as diurnal, degus classified as diurnal, gerbils classified as nocturnal, and degus classified as nocturnal. Nocturnal degus remained nocturnal when tested with an infrared motion detector without running wheels. Thus, although the diurnal–nocturnal dichotomy could be applied to some of the species, it was not appropriate for others. The dichotomy may reflect researchers’ needs for systematization more than a natural distinction between species. Through mechanisms as yet poorly understood, the balance between entraining and masking processes seems to generate a gradient of temporal niches that runs from predominantly diurnal species to predominantly nocturnal species with many chronotypes in between, including species that exhibit wide intra-species gradients of temporal niche.     

Disruption of masking by hypothalamic lesions in Syrian hamsters

Negative masking of locomotor activity by light in nocturnal rodents is mediated by a non-image-forming irradiance-detection system in the retina. Structures receiving input from this system potentially contribute to the masking response. The suprachiasmatic nucleus (SCN) regulates locomotor activity and receives dense innervation from the irradiance-detection system via the retinohypothalamic tract, but its role in masking is unclear. We studied masking in adult Syrian hamsters (Mesocricetus auratus) with electrolytic lesions directed at the SCN. Hamsters were exposed to a 3.5:3.5 ultradian light/dark cycle and their wheel-running activity was monitored. Intact hamsters showed robust masking, expressing less than 20% of their activity in the light even though light and dark occurred equally during their active times. In contrast, hamsters with lesions showed, on average, as much activity in the light as in the dark. Tracing of retinal projections using cholera toxin subunit showed that the lesions damaged retinal projections to the SCN and to the adjacent subparaventricular zone. Retinal innervation outside the hypothalamus was not obviously affected by the lesions. Our results indicate that retinohypothalamic projections, and the targets of these projections, to the SCN and/or adjacent hypothalamic areas play an important role in masking.     

Comparative study on isolation calls emitted from hamster pups.

Waveforms of isolation calls emitted from hamster pups, which were Syrian hamsters, Djungarian hamsters, and Chinese hamsters, were compared in a basic study on improving reproduction by decrease of cannibalism, because it was reported that maternal behavior was induced by isolation calls in rodents. Isolation calls of hamster pups, isolated from their mother and receiving cold stress, were collected by Real-Time Spectrogram (RTS), and calculated to spectrograms and power spectra by SIGNAL. Isolation calls consisted of ultrasonic vocalizations (USVs) and audible vocalizations (ADVs) in each species. Waveforms of isolation calls emitted by the hamster pups, were shown to have several characteristic features. In this study, the species specificity of isolation calls was shown in hamster pups. It would seem that the species specificity originates in the differences of sensitivity to cold stress via the autonomic nerve in hamsters.     

Maternal influence on activity rhythms and reproductive development in Djungarian hamster pups

Photoperiodism and entrainment of the circadian rhythm of locomotor activity were investigated in juvenile Djungarian hamsters. Animals were housed in simulated burrows. Activity was measured as the animal's emergence from a dark nest chamber into an outer box exposed to the room illumination. This burrow emergence activity exhibited marked circadian rhythmicity. Interactions between mother hamsters and their offspring were examined in the simulated burrow system. Male reproductive responses were determined by measuring testicular weights at the time of weaning. It was shown that photoperiodic information received between Days 1 and 15 of life failed to alter the rate of testicular development, but that after Day 15 testicular growth was photoperiod-dependent. The mother, when entrained to a long photoperiod, did not influence the photoperiodic responses of her pups when they were confined to a dark nest box. In contrast, the mother did influence the circadian entrainment patterns of her pups. Pups exhibited a well-developed circadian activity rhythm at weaning with a phase angle roughly similar to that of the mother's activity rhythm. When the maternal rhythms were discrepant with photoperiod information received by the pups directly from the environment, the pups' activity rhythms were synchronized with the light/dark cycle rather than with the rhythm of their mother. Thus, it appears that although pups may first become entrained by maternal cues, they rapidly adjust to the environmental light cycle after leaving the nest.     

Daily Rhythm of the Response to Noradrenaline in Djungarian Hamsters Acclimated to Cold and Short Photoperiod

We investigated the daily rhythm of the response to noradrenaline injections in Djungarian hamsters (Phodopus sungorus sungorus) at neutral ambient temperature, under long photoperiod (L:D 12:12) and after four weeks of acclimation to cold (10ºC) and short photoperiod (L:D 8:16). Animals were injected with noradrenaline (0.6 mg/kg) every four hours. Body temperature and gross motor activity were measured with MiniMitter transmitters implanted into abdominal cavity. Additionally, we measured body weight and food intake prior to, and after acclimation. After four weeks of acclimation, the experiment was performed under LD cycle and then repeated during one-day of constant light (LL) and constant darkness (DD). In animals acclimated to L:D 12:12 and ambient temperature of 25ºC, noradrenaline injections caused short-lasting increase in body temperature followed by marked decrease. There was no significant difference in the magnitude of the reaction between light and dark phase of the day. After acclimation to cold and L:D 8:16, under LD conditions, we recorded significant differences between the responses to the noradrenaline injections during light and dark phase of the day. Post-injection increase was higher during the day than during the night while following noradrenaline-induced hypothermia was much more pronounced in darkness. In experiments performed after acclimation to cold and short photoperiod but during one day of LL and DD regimes, these differences were attenuated. Data presented here indicate that in cold acclimated hamsters, the response to exogenous noradrenaline depends on the time of injection and it exhibits clear daily rhythm. The rhythmicity is altered under LL and DD regimes. It seems that post-injection increase in body temperature elicits following hypothermia. This hypothermia might be of a great ecological importance. Reasonable lowering of body temperature would be a protective mechanism, allowing for energy charge restoration.     

Daily torpor in the Djungarian hamster (Phodopus sungorus): interactions with food intake,activity, and social behaviour

Summary In Djungarian hamsters,Phodopus sungorus, daily torpor occurs spontaneously in winter in the presence of abundant food, but individuals show different tendencies to enter torpor. The results show that in hamsters fed rodent chow ad libitum individual torpor frequencies were negatively correlated with both food consumption and the amount of nocturnal locomotor activity. Provision of cafeteria diet at ambient temperatures below thermoneutrality significantly lowered torpor frequencies and induced body weight gains. However, in hamsters fed seeds with a high fat or carbohydrate content (i.e., sunflower seeds or wheat, respectively) neither a decrease of torpor frequencies nor an increase of body weights was observed. The results suggest that in Djungarian hamsters, daily torpor is an intrinsic component of energy balance control and is functionally linked to individual physiological adjustments of food consumption and foraging activity. In addition, the employment of daily torpor can be affected by social interactions, since the long-term pattern of alternations between torpor and normothermia was found to be synchronized in breeding pairs caged together.Abbreviations T _a ambient temperature - DIT diet-induced thermogenesis     

Evidence for independence of circadian characters and extent of photoresponsiveness in the Djungarian hamster, Phodopus sungorus.

Djungarian hamsters (Phodopus sungorus) exposed to a short-day photoperiod generally respond with a syndrome of physiological and behavioral changes, such as body weight loss and molt to a white pelage. The extent of the short-day-induced responses differs among individuals. Furthermore, some hamsters show no photoresponse. In this study, we sought to determine whether variation in the photoresponse would be associated with circadian function: whether phase angle or free-running period (tau) would differ between responsive and nonresponsive hamsters; and whether changes in these circadian characters would correlate with the extent of weight loss and molt (and the timing of molt onset) in photoresponsive hamsters. Adult hamsters were kept in a short-day photoperiod (9 hr light, 15 hr dark) for 14 weeks, during which time body weight and molt were measured biweekly. Hamsters were then transferred to cages equipped with running wheels; we measured the phase angle of activity onset under a short-day photoperiod and tau in constant dark. Hamsters exhibiting a short-day-induced molt had a significantly shorter tau and a less negative phase angle than nonmolting animals. Hamsters that exhibited weight loss also had a significantly less negative phase angle, but no difference in tau. No significant Pearson's or Spearman's correlation coefficients were found between extent (or timing) of the photoresponse and the circadian characters in responsive hamsters. Although these results indicate that threshold for photoresponsiveness is related to circadian function, the extent (and timing) of the photoresponse may not be.     

Expression of circadian rhythmicity in Djungarian hamsters under constant light: Effects of light intensity and the circadian system's state

Summary Djungarian hamsters (Phodopus sungorus), were exposed to constant light with increasing intensities (20, 60, 350 lux), and wheel running activity was recorded. With increasing light intensity the percentage of hamsters showing a split in their daily activity pattern increased and the free running period was lengthened for both the unsplit and the split state. The fact that the free running period of both states depended on the light intensity together with the observation that the highest incidence of acircadian activity occurred under 350 lux, provoked the idea that the emergence of splitting or acircadian rhythmicity is a direct consequence of the light induced lengthening of the free running period. However, analysis of the data failed to support the idea that emergence of a split or acircadian activity is a threshold phenomenon with respect to the free running period.Due to differences in circadian function some Djungarian hamsters do not exhibit photoinduction following short day exposure. In these individuals splitting also occurred but required exposure to a higher light intensity than in photo-responsive hamsters. This observation is in accordance with the idea that the two phenotypes differ in the interaction of the two component oscillators underlying circadian rhythmicity.Abbreviations LD long day photoperiod - LL constant light - SD short day photoperiod - free running period