The Circadian Activity and Body Temperature Rhythms of Mice During Their Last Days of Life

Motor activity and body temperature rhythms have been investigated telemetrically in old mice until their death. The present paper is based mainly on the data of two animals which could be monitored over a sufficient length of time (more than three weeks), though the other three animals showed similar results. The daily body temperature rhythm of old mice was more stable as compared to the activity rhythm and was detectable until the last day of life. Its magnitude, defined as the difference between maximum and minimum, was similar to that obtained in middle-aged mice. By contrast, the activity rhythm disappeared or started to become erratic earlier. Unlike the position in younger animals, the body temperature rhythm was phase delayed with respect to the activity rhythm. In one mouse, the increased instability of the phase position two weeks before its death led to a free run with a period length of about 23.7 h. Both activity and temperature rhythms were fragmented in old mice. In the case of the body temperature this was obviously caused by masking. After purifying the raw temperatures, the fragmentation disappeared. On the other hand, the free-run condition was not caused by masking. The sensitivity of body temperature to motor activity was different from younger mice, and this probably reflects changes/deteriorations in the physiology of thermoregulation during the last days of life. In one mouse, during the last 4 days of life, a sharp, torpor-like decrease of body temperature corresponding with the time of the daily minimum was observed. This phenomenon was not found in other mice, though all of them died during the falling period of the temperature rhythm. The results confirm our hypothesis that the endogenous clock may work even during the very last days of life. The ability to synchronize with the periodic environment deteriorates earlier. Also they suggest that these phenomena are not only typical for the activity rhythm but apply also to the body temperature rhythm.     

Different Behavior of the Circadian Rhythms of Activity and Body Temperature During Resynchronization Following an Advance of the LD Cycle

Spontaneous activity and the body temperature of laboratory mice were recorded telemetrically using implantable transmitters. Following ten control days (L : D = 12 : 12; light from 07:00 to 19:00), the LD cycle was phase-advanced by shortening the light time by 8 h. Recordings were continued for a further 3 weeks. The raw temperature data were unmasked or ‘purified’ — that is, the temperature changes due to locomotor activity were removed, so revealing the endogenous component of the rhythm — using a regression method previously developed by us. The circadian rhythms of activity and measured body temperature resynchronized on average after 8 days. During resynchronization, both rhythms tended to show two components, one adjusting by a phase advance and the other by a phase delay. However, after purification of the body temperature rhythm, only the advancing component remained. These results indicate that the delaying component of the measured temperature rhythm was caused by masking due to activity, and that the endogenous component of this rhythm did not divide into two components during the resynchronization process. Also, the endogenous component of the circadian rhythm of body temperature and one component of the activity rhythm seemed to be controlled by the same oscillator. It remains uncertain how the other component of the activity rhythm is regulated.     

Different Behavior of the Circadian Rhythms of Activity and Body Temperature During Resynchronization Following an Advance of the LD Cycle

Spontaneous activity and the body temperature of laboratory mice were recorded telemetrically using implantable transmitters. Following ten control days (L : D = 12 : 12; light from 07:00 to 19:00), the LD cycle was phase-advanced by shortening the light time by 8 h. Recordings were continued for a further 3 weeks. The raw temperature data were unmasked or 'purified' — that is, the temperature changes due to locomotor activity were removed, so revealing the endogenous component of the rhythm — using a regression method previously developed by us. The circadian rhythms of activity and measured body temperature resynchronized on average after 8 days. During resynchronization, both rhythms tended to show two components, one adjusting by a phase advance and the other by a phase delay. However, after purification of the body temperature rhythm, only the advancing component remained. These results indicate that the delaying component of the measured temperature rhythm was caused by masking due to activity, and that the endogenous component of this rhythm did not divide into two components during the resynchronization process. Also, the endogenous component of the circadian rhythm of body temperature and one component of the activity rhythm seemed to be controlled by the same oscillator. It remains uncertain how the other component of the activity rhythm is regulated.     

Temporal structuring of delivery in the absence of a photoperiod: preparturient myometrial activity of the rhesus monkey is related to maternal body temperature and depends on the maternal circadian system.

No convincing evidence exists that the shift from myometrial contractures to contractions, which determines the synchronized 24-h rhythm in the dynamics of the primate uterus, may be attributed to an endogenous circadian rhythm. We therefore wished to ascertain whether a 24-h periodic shift would also occur in the myometrial activity of animals kept under constant conditions. We studied five pregnant rhesus monkeys, kept in continuous darkness from 56-77 days gestational age until delivery at 117-167 days gestational age. During the last week before delivery we determined the individual phase, level, and amplitude of circadian changes in maternal body temperature and 24-h myometrial activity patterns in the form of contractions. In all five monkeys, a rhythm with a period of 24-h characterized the temporal incidence of preparturient contraction activity. A consistent phase lag of 6-7 h from the temperature crest was observed in four out of the five animals. The circadian phase of all individual rhythms was idiosyncratic among animals. We conclude that endogenous rhythms in body temperature and preparturient myometrial activity are truly circadian. In addition, these rhythms are either interdependent or subject to the same maternal timekeeping mechanism, supporting the hypothesis that the exact time of the day at which birth occurs in the rhesus monkey depends on the maternal circadian system.     

Plasticity of Circadian Activity and Body Temperature Rhythms in Golden Spiny Mice

Most animals can be categorized as nocturnal, diurnal, or crepuscular. However, rhythms can be quite plastic in some species and vary from one individual to another within a species. In the golden spiny mouse (Acomys russatus), a variety of rhythm patterns have been seen, and these patterns can change considerably as animals are transferred from the field into the laboratory. We previously suggested that these animals may have a circadian time‐keeping system that is fundamentally nocturnal and that diurnal patterns seen in their natural habitat reflect mechanisms operating outside of the basic circadian time‐keeping system (i.e., masking). In the current study, we further characterized plasticity evident in the daily rhythms of golden spiny mice by measuring effects of lighting conditions and access to a running wheel on rhythms in general activity (GA) and body temperature (Tb). Before the wheel was introduced, most animals were active mainly during the night, though there was considerable inter‐individual variability and patterns were quite plastic. The introduction of the wheel caused an increase in the level of nighttime activity and Tb in most individuals. The periods of the rhythms in constant darkness (DD) were very similar, and even slightly longer in this study (24.1±0.2 h) than in an earlier one in which animals had not been provided with running wheels. We found no correlation between the distance animals ran in their wheels and the period of their rhythms in DD. Re‐entrainment after phase delays of the LD cycle occurred more rapidly in the presence than absence of the running wheel. The characteristics of the rhythms of golden spiny mice seen in this study may be the product of natural selection favoring plasticity of the circadian system, perhaps reflecting what can happen during an evolutionary transition as animals move from a nocturnal to a diurnal niche.     

Uncovering Different Masking Factors on Wrist Skin Temperature Rhythm in Free-Living Subjects

Most circadian rhythms are controlled by a major pacemaker located in the hypothalamic suprachiasmatic nucleus. Some of these rhythms, called marker rhythms, serve to characterize the timing of the internal temporal order. However, these variables are susceptible to masking effects as the result of activity, body position, light exposure, environmental temperature and sleep. Recently, wrist skin temperature (WT) has been proposed as a new index for evaluating circadian system status. In light of previous evidence suggesting the important relationship between WT and core body temperature regulation, the aim of this work was to purify the WT pattern in order to obtain its endogenous rhythm with the application of multiple demasking procedures. To this end, 103 subjects (18–24 years old) were recruited and their WT, activity, body position, light exposure, environmental temperature and sleep were recorded under free-living conditions for 1 week. WT demasking by categories or intercepts was applied to simulate a “constant routine” protocol (awakening, dim light, recumbent position, low activity and warm environmental temperature). Although the overall circadian pattern of WT was similar regardless of the masking effects, its amplitude was the rhythmic parameter most affected by environmental conditions. The acrophase and mesor were determined to be the most robust parameters for characterizing this rhythm. In addition, a circadian modulation of the masking effect was found for each masking variable. WT rhythm exhibits a strong endogenous component, despite the existence of multiple external influences. This was evidenced by simultaneously eliminating the influence of activity, body position, light exposure, environmental temperature and sleep. We therefore propose that it could be considered a valuable and minimally-invasive means of recording circadian physiology in ambulatory conditions.     

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.     

The Adjustment of the Circadian Rhythm of Body Temperature to Simulated Time-Zone Transitions: A Comparison of the Effect of Using Raw Versus Unmasked Data

Deep body temperature and sleep/activity diaries data were recorded during control days and for 6 days after simulated time zone transitions of 8 h to the east (six subjects) or west (seven subjects). Circadian rhythms were assessed by cosinor analysis of both raw data (the conventional method) and purified data (corrected for the effects of sleep and activity). Analysis of raw data gives misleading information about the phase and amplitude of the rhythms due to the masking effects of the exogenous component. Use of purified data indicates that during the process of adjustment after an eastward shift (a) phase changes are more erratic than after a shift to the west; (b) no marked decrease in the amplitude of the rhythms is evident; and (c) no clear evidence exists that the circadian rhythm breaks up temporarily. The masking effect was less after the time zone transition if sleep maintenance was poor.     

The Adjustment of the Circadian Rhythm of Body Temperature to Simulated Time-Zone Transitions: A Comparison of the Effect of Using Raw Versus Unmasked Data

Deep body temperature and sleep/activity diaries data were recorded during control days and for 6 days after simulated time zone transitions of 8 h to the east (six subjects) or west (seven subjects). Circadian rhythms were assessed by cosinor analysis of both raw data (the conventional method) and purified data (corrected for the effects of sleep and activity). Analysis of raw data gives misleading information about the phase and amplitude of the rhythms due to the masking effects of the exogenous component. Use of purified data indicates that during the process of adjustment after an eastward shift (a) phase changes are more erratic than after a shift to the west; (b) no marked decrease in the amplitude of the rhythms is evident; and (c) no clear evidence exists that the circadian rhythm breaks up temporarily. The masking effect was less after the time zone transition if sleep maintenance was poor.     

Circadian Locomotor Activity Rhythm in the Freshwater Crab Pseudothelphusa americana (De Saussure, 1857): Effect of Eyestalk Ablation

The circadian rhythm of locomotor activity in the freshwater crab, Pseudothelphusa americana , was studied in aquaria using infrared crossing sensors. Individuals with ablated eyestalks were compared with intact individuals in constant darkness (DD) and in light-dark cycles (LD). Our results showed that intact animals in DD displayed bimodal rhythms. In LD conditions the two peaks were associated with lights on and lights off, respectively. A significant difference in the free running periods before and after LD was observed in all intact animals. After eyestalk ablation (ES-X), the circadian rhythm of locomotor activity disappeared immediately, but reappeared several days later. Diurnal activity was seen in some ES-X animals when exposed to LD. Our results indicate that locomotor activity rhythm in P. americana is driven primarily by oscillators located outside the eyestalks, and that extraretinal photoreceptors mediate either entrainment or masking effects.     

Circadian Locomotor Activity Rhythm in the Freshwater Crab Pseudothelphusa americana (De Saussure, 1857): Effect of Eyestalk Ablation

The circadian rhythm of locomotor activity in the freshwater crab, Pseudothelphusa americana, was studied in aquaria using infrared crossing sensors. Individuals with ablated eyestalks were compared with intact individuals in constant darkness (DD) and in light-dark cycles (LD). Our results showed that intact animals in DD displayed bimodal rhythms. In LD conditions the two peaks were associated with lights on and lights off, respectively. A significant difference in the free running periods before and after LD was observed in all intact animals. After eyestalk ablation (ES-X), the circadian rhythm of locomotor activity disappeared immediately, but reappeared several days later. Diurnal activity was seen in some ES-X animals when exposed to LD. Our results indicate that locomotor activity rhythm in P. americana is driven primarily by oscillators located outside the eyestalks, and that extraretinal photoreceptors mediate either entrainment or masking effects.     

Ambient temperature cycles entrain the free-running circadian rhythms of the stripe-faced dunnart,Sminthopsis macroura

Summary We examined the effect of cycles of 12 h warm (35 ± 2 °C) and 12 h (21 ± 2 °C) ambient temperature (Ta) upon the circadian activity rhythms of stripe-faced dunnarts, Sminthopsis macroura, free-running in conditions of constant dark (DD) or constant light (LL). It was hypothesized that dunnarts would entrain to the temperature cycles (TaHLs) or show perturbations of period, and that LL would act synergistically with the TaHLs in these effects. Under DD, 2 of 6 animals showed clear entrainment to the TaHLs. Other animals exhibited changes of period () and heavy negative masking of activity during the warm fraction of the TaHLs. Under LL, 3 of 12 animals entrained to the TaHLs. It was concluded that Ta is a significant though weak Zeitgeber for S. macroura compared to light. It is possible that TaHLs entrain homeotherm activity rhythms by altering the rhythm of body temperature, which is usually tightly coupled to activity.Abbreviations TaHL a cycle of Higher and Lower ambient temperature - TaC Constant Ta - Tb body temperature     

Effect of Chronic Caloric Restriction on the Circadian Regulation of Physiological and Behavioral Variables in Old Male B6C3F1 Mice

The circadian rhythms of food and water consumption, the number of feeding and drinking episodes, oxygen consumption, carbon dioxide production, respiratory quotient, gross motor activity, and body temperature were measured in male B6C3F, mice that were fed ad libitum (AL) or fed a caloric-restricted diet (CR). The CR regimen (60% of the normal AL consumption) was fed to mice during the daytime (5 hr after lights on). CR animals exhibited fewer feeding episodes but consumed more food per feeding bout and spent more total time feeding than AL mice. It appears that CR caused mice to change from their normal “nibbling behavior” to meal feeding. Compared to AL animals, the mean body temperature was reduced in CR animals, while the amplitude of the body temperature rhythm was increased. Spans of reduced activity, metabolism, and body temperature (torpor) occurred in CR mice for several hours immediately before feeding, during times of high fatty acid metabolism (low RQ). The acute availability of exogenous substrates (energy supplies) seemed to modulate metabolism shifting metabolic pathways to promote energy efficiency. CR was also associated with lower DNA damage, higher DNA repair, and decreased proto-oncogene expression. Most of the circadian rhythms studied seemed to be synchronized primarily to the feeding rather than the photoperiod cycle. Night-time CR feeding was found to be better than daytime feeding because the circadian rhythms for AL and CR animals were highly synchronized when this regimen was used.     

Effect of Chronic Caloric Restriction on the Circadian Regulation of Physiological and Behavioral Variables in Old Male B6C3F1 Mice

The circadian rhythms of food and water consumption, the number of feeding and drinking episodes, oxygen consumption, carbon dioxide production, respiratory quotient, gross motor activity, and body temperature were measured in male B6C3F, mice that were fed ad libitum (AL) or fed a caloric-restricted diet (CR). The CR regimen (60% of the normal AL consumption) was fed to mice during the daytime (5 hr after lights on). CR animals exhibited fewer feeding episodes but consumed more food per feeding bout and spent more total time feeding than AL mice. It appears that CR caused mice to change from their normal “nibbling behavior” to meal feeding. Compared to AL animals, the mean body temperature was reduced in CR animals, while the amplitude of the body temperature rhythm was increased. Spans of reduced activity, metabolism, and body temperature (torpor) occurred in CR mice for several hours immediately before feeding, during times of high fatty acid metabolism (low RQ). The acute availability of exogenous substrates (energy supplies) seemed to modulate metabolism shifting metabolic pathways to promote energy efficiency. CR was also associated with lower DNA damage, higher DNA repair, and decreased proto-oncogene expression. Most of the circadian rhythms studied seemed to be synchronized primarily to the feeding rather than the photoperiod cycle. Night-time CR feeding was found to be better than daytime feeding because the circadian rhythms for AL and CR animals were highly synchronized when this regimen was used.     

Indicators for post-surgery recovery in Mongolian gerbils (Meriones unguiculatus)

Telemetric investigations of various parameters are widely used to estimate an animal's state. However, the implantation of the transmitters includes anaesthesia and surgery and has short and longer lasting impacts on the studied object. The aim of the present paper was to evaluate these effects in Mongolian gerbils, namely the hypothermia caused by the anaesthetic and the enduring disturbance of daily rhythmicity until complete recovery. The surgery associated with the implantation of the transmitters differed both in severity and type of anaesthesia. Whereas normal values of body temperature were restored within hours, restoration of daily rhythm required several days, depending on the severity of the surgical procedure. Also, the sensitivity of the body temperature to activity changes was different until the rhythms were re-established. A method based on the rhythm magnitude and shape was proposed to estimate the time until complete recovery of the animals.     

Circadian rhythms of locomotor activity in cockroach nymphs: free running and entrainment

The temporal organization of locomotor activity was investigated in nymphs of the cockroach Leucophaea maderae. Approximately 40% of the animals examined between 1 and 50 days of age exhibited a circadian activity rhythm in constant darkness (n = 172) with an average free-running period of 23.7 +/- 0.68 hr. Twelve of 17 animals in which activity was recorded for most or all of the final instar also exhibited periods of rhythmic activity. The rhythms of the nymphs could be entrained by light-dark (LD) cycles with periods of 22, 24, or 26 hr. In contrast, neither maternal influences during embryogenesis nor hatching from the egg was effective in synchronizing the rhythms. Although adult cockroaches can be readily entrained by temperature cycles, in nymphs temperature appeared at best to be a weak zeitgeber. Embryonic exposure to an LD cycle until 6 days prior to egg hatch was effective in synchronizing the activity rhythms of the nymphs, indicating that differentiation of an entrainable pacemaking system occurs prior to hatching.     

Circadian Activity Rhythms of Laboratory Mice during the Last Weeks of their Life

The aim of the present paper was a detailed analysis of changes of circadian activity rhythms immediately before natural death. Investigations were carried out on individually housed female laboratory mice. Locomotor activity was measured by passive infrared detectors starting with an age of about 75 weeks up to death. At the beginning all animals had pronounced circadian activity rhythms with a main maximum during the dark time and a secondary one just after light-on. As compared to adult mice the amount of activity and the circadian range of oscillation were lower. The main maximum was phase advanced in most of the animals, so that the percentage of activity during dark time accounted for less than 50% of the total 24-h activity. Towards death the amount of activity and the amplitude decreased even more. A circadian rhythm was preserved however as long as the animals were active, although its synchronization with LD-Zeitgeber deteriorated markedly. The phase position of the main maximum became more unstable, leading in some cases to complete uncoupling (free-run with t < 24 h). The secondary maximum in contrast was more stable in its phase and remained synchronized longer. The results show that in old age the mechanisms of synchronization break down earlier than the circadian rhythms. Therefore it seems possible to stabilize the circadian rhythms, e.g. by strengthening of Zeitgebers, which in turn may improve wellbeing and performance.     

The development of new purification methods to assess the circadian rhythm of body temperature in Mongolian gerbils

Six Mongolian gerbils were studied for 8-10d while housed in separate cages in a 12:12h light-dark (L-D) cycle (lights on at 07:00h). Recordings of body temperature, heart rate, and spontaneous activity were made throughout. The temperature and heart rate rhythms were “purified” to take into account the effects of activity, and then the rhythm of temperature was further purified to take into account other masking influences (“non-activity masking effects” or NAME,). The methods employed in the purification processes involved linear regression analysis or analysis of covariance, the latter using functions of activity and NAME as covariates. From these methods, it was possible to obtain not only an estimate of the endogenous component of the temperature rhythm but also a measure of circadian changes in the sensitivity of temperature to masking effects.

Even though all purification methods removed many of the effects of spontaneous activity from the temperature record, there remained temperature fluctuations at the L-D and D-L transitions that appeared to be independent of activity. The NAME was of only very marginal value in the purification process. Comparison of the purification methods indicated that the linear methods were inferior (both from a biological viewpoint and when the results were compared mathematically) to those that allowed the rate of rise of temperature due to increasing amounts of activity to become progressively less. The sensitivity of temperature and heart rate to the masking effects of activity showed a circadian rhythm, with sensitivities in the resting phase being greater than those in the active phase. These findings are compatible with the view that thermoregulatory reflexes are induced by spontaneous activity of sufficient amount, and that there is a circadian rhythm in the body temperature at which these reflexes are initiated and in their effectiveness.     

Impact of Chronodisruption during Primate Pregnancy on the Maternal and Newborn Temperature Rhythms

Disruption of the maternal environment during pregnancy is a key contributor to offspring diseases that develop in adult life. To explore the impact of chronodisruption during pregnancy in primates, we exposed pregnant capuchin monkeys to constant light (eliminating the maternal melatonin rhythm) from the last third of gestation to term. Maternal temperature and activity circadian rhythms were assessed as well as the newborn temperature rhythm. Additionally we studied the effect of daily maternal melatonin replacement during pregnancy on these rhythms. Ten pregnant capuchin monkeys were exposed to constant light from 60% of gestation to term. Five received a daily oral dose of melatonin (250 µg kg/body weight) at 1800 h (LL+Mel) and the other five a placebo (LL). Six additional pregnant females were maintained in a 14∶10 light:dark cycles and their newborns were used as controls (LD). Rhythms were recorded 96 h before delivery in the mother and at 4–6 days of age in the newborn. Exposure to constant light had no effect on the maternal body temperature rhythm however it delayed the acrophase of the activity rhythm. Neither rhythm was affected by melatonin replacement. In contrast, maternal exposure to constant light affected the newborn body temperature rhythm. This rhythm was entrained in control newborns whereas LL newborns showed a random distribution of the acrophases over 24-h. In addition, mean temperature was decreased (34.0±0.6 vs 36.1±0.2°C, in LL and control, respectively P<0.05). Maternal melatonin replacement during pregnancy re-synchronized the acrophases and restored mean temperature to the values in control newborns. Our findings demonstrate that prenatal melatonin is a Zeitgeber for the newborn temperature rhythm and supports normal body temperature maintenance. Altogether these prenatal melatonin effects highlight the physiological importance of the maternal melatonin rhythm during pregnancy for the newborn primate.     

Temporal phasing of locomotor activity, heart rate rhythmicity, and core body temperature is disrupted in VIP receptor 2-deficient mice

Neurons of the brain's biological clock located in the hypothalamic suprachiasmatic nucleus (SCN) generate circadian rhythms of physiology (core body temperature, hormone secretion, locomotor activity, sleep/wake, and heart rate) with distinct temporal phasing when entrained by the light/dark (LD) cycle. The neuropeptide vasoactive intestinal polypetide (VIP) and its receptor (VPAC2) are highly expressed in the SCN. Recent studies indicate that VIPergic signaling plays an essential role in the maintenance of ongoing circadian rhythmicity by synchronizing SCN cells and by maintaining rhythmicity within individual neurons. To further increase the understanding of the role of VPAC2 signaling in circadian regulation, we implanted telemetric devices and simultaneously measured core body temperature, spontaneous activity, and heart rate in a strain of VPAC2-deficient mice and compared these observations with observations made from mice examined by wheel-running activity. The study demonstrates that VPAC2 signaling is necessary for a functional circadian clock driving locomotor activity, core body temperature, and heart rate rhythmicity, since VPAC2-deficient mice lose the rhythms in all three parameters when placed under constant conditions (of either light or darkness). Furthermore, although 24-h rhythms for three parameters are retained in VPAC2-deficient mice during the LD cycle, the temperature rhythm displays markedly altered time course and profile, rising earlier and peaking ～4-6 h prior to that of wild-type mice. The use of telemetric devices to measure circadian locomotor activity, temperature, and heart rate, together with the classical determination of circadian rhythms of wheel-running activity, raises questions about how representative wheel-running activity may be of other behavioral parameters, especially when animals have altered circadian phenotype.