Circadian temperature rhythms in rabbit pups and in their does.

Circadian rhythms of mammals are generated endogenously, the master oscillator system residing in the suprachiasmatic nuclei (SCN). Previous experiments have indicated that the rabbit has a feeding entrainable circadian oscillator (FEO) which is supposed to be of greatest importance during the early infancy of the rabbit. Here we report the course of telemetrically monitored core body temperature of rabbit pups and of their does. Temperature increased from 37.6+/-0.3 degrees C on day 2 to 39.5+/-0.1 degrees C on day 28 of life. The pups showed a 24 h temperature rhythm even during their first days of life. Temperature increased 2 1/2-3 h prior to nursing for 0.4-0.8 degrees C and rose for an additional 0.4-0.6 degrees C immediately after milk ingestion. The anticipatory, but not the postprandial component persisted when nursing was skipped twice. The persistence of a rhythm in the absence of any entraining agent is crucial for its endogenous generation. In the doe, the core body temperature gradually decreased during the last 2/3 of pregnancy. During parturition it steeply rose for 1.5-1.7 degrees C and attained a plateau of 39.7+/-0.2 degrees C during lactation. The circadian rhythm persisted during the whole course of pregnancy and lactation. Thus, in the rabbit an endogenous, feeding entrainable circadian oscillator appears to operate from the first days of life. It is of functional significance in that it alerts the pup in time so that it is able to utilize the singular short presence of the doe for maximal milk intake.     

Thermoregulation and diurnal rhythms in 1-week-old rat pups

This paper reviews the ontogeny of thermoregulation and diurnal rhythmicity in rats. Additionally, original data are presented that indicate the emergence of an endogenous circadian core temperature rhythm during the first postnatal week. Despite neurological immaturity, newborn rats display autonomic and behavioral thermoregulatory responses within 24 h of birth. Their "biological clock" is already running before birth. The thermal environment of pups changes cyclically owing to diurnal variations in maternal behavior, but the core temperatures of 1-week-old pups huddling in the absence of the dam also show marked diurnal fluctuations. Five- to 8-day-old lean Zucker rat pups artificially reared in the absence of 24-h cycles of ambient temperature and food intake show diurnal changes in core temperature similar to those in huddling mother-reared pups. Diurnal core temperature changes, evident only when regulatory effectors are not overwhelmed, are one of the first self-maintained diurnal rhythms to appear. Because thermoregulation and circadian rhythmicity both appear before maturation of the neural networks believed to be critical for their control in adult animals, studying the immature rat might increase our understanding of the control of these processes in the more complex mature central nervous system.     

Differences in daily rhythms of wrist temperature between obese and normal-weight women: associations with metabolic syndrome features

The circadian rhythm of core body temperature is associated with widespread physiological effects. However, studies with other more practical temperature measures, such as wrist (WT) and proximal temperatures, are still scarce. The aim of this study was to investigate whether obesity is associated with differences in mean WT values or in its daily rhythmicity patterns. Daily patterns of cortisol, melatonin, and different metabolic syndrome (MetS) features were also analyzed in an attempt to clarify the potential association between chronodisruption and MetS. The study was conducted on 20 normal-weight women (age: 38 ± 11 yrs and BMI: 22 ± 2.6 kg/m(2)) and 50 obese women (age: 42 ± 10 yrs and BMI: 33.5 ± 3.2 kg/m(2)) (mean ± SEM). Skin temperature was measured over a 3-day period every 10 min with the "Thermochron iButton." Rhythmic parameters were obtained using an integrated package for time-series analysis, "Circadianware." Obese women displayed significantly lower mean WT (34.1°C ± 0.3°C) with a more flattened 24-h pattern, a lower-quality rhythm, and a higher intraday variability (IV). Particularly interesting were the marked differences between obese and normal-weight women in the secondary WT peak in the postprandial period (second-harmonic power [P2]), considered as a marker of chronodisruption and of metabolic alterations. WT rhythmicity characteristics were related to MetS features, obesity-related proteins, and circadian markers, such as melatonin. In summary, obese women displayed a lower-quality WT daily rhythm with a more flattened pattern (particularly in the postprandial period) and increased IV, which suggests a greater fragmentation of the rest/activity rhythm compared to normal-weight women. These 24-h changes were associated with higher MetS risk.     

Circadian thermoregulation in suckling rabbit pups

The rabbit pup is well suited to track the age-dependent development of periodic thermoregulation during the suckling period. Since the litters are regularly nursed once per day for a total of 3 to 4 min, an exogenous, metabolic, nonphotic periodic variable is supposed to have an impact on the 24-h rhythm of body temperature. The authors monitored the course of core body temperature during the suckling period of 20 pups by means of a transmitter implanted intraperitoneally on day 3 postpartum. The 24-h mean rose from an average of 37.8+/-0.3 degrees C on day 4 of life to 39.5+/-0.2 degrees C at weaning on day 27, for 2 out of 20 pups, and day 28, for 18 out of 20 pups. In constant dim illumination, the pups exhibited a 24-h rhythm even on postnatal day 4, which consolidated around days 5 to 7. The rhythm consisted of a significant anticipatory rise of 0.4 to 0.6 degrees C above the respective 24-h mean commencing 2.5 to 3.5 h prior to nursing. Milk intake was followed by a further increase of temperature for an additional 0.3 to 0.6 degrees C. Then the temperature dropped for 1.2 to 1.5 degrees C within 1 to 3 h and returned to average 3 to 5 h later. During a 48-h fast, the rhythm continued to exist, though in a modified shape: the anticipatory component persisted almost unchanged; a further elevation of temperature, however, did not occur. Thus, the anticipatory component apparently is generated endogenously and the second surge represents an exogenous suckling-induced, thermogenic peak. When maternal nursing was advanced for 15 min/day for a total of 5 h, the temperature rhythm of the pups followed the shift of the zeitgeber in parallel. These data confirm the assumption that a circadian rhythm exists during the first postnatal days of the rabbit and that this rhythm is entrained by the 24-h nursing rhythm. The authors suggest that the biological significance of a feeding entrainable oscillator (FEO) in the rabbit might be to activate the pups prior to the periodic nursing visit of the rabbit doe. Thus, the pups are prepared to quantitatively use the one and only short nursing episode per day for maximal milk ingestion.     

Seasonal changes in the daily rhythms of animals

Seasonal changes in the timing of circadian activity rhythms have been recorded in mammals, birds, reptiles and arthropods. There are probably far more wide spread than is generally realised and are especially important in desert regions with large daily temperature fluctuations. Synchronisation of the onset of circadian activity with changes in the times of dusk and dawn is achieved mainly by responses to changes in photoperiod.

---

Zusammenfassung An Beispielen wird gezeigt,dass jahreszeitliche Veränderungen in der Steuerung der circadianen Aktivitäts-Rhythmen bei Säugern, Vögeln,Reptilien und Arthropoden bestehen. Sie sind wahrscheinlich viel häufiger, als im allgemeinen erkannt wird und sind besonders wichtig in Wüstenregionen mit grossen täglichen Temperaturschwankungen. Die Synchronisation des Beginns der circadianen Aktivität mit Veränderungen der Zeiten der Abend- und Morgendämmerung wird hauptsächlich durch Reaktionen auf Veränderungen der Tageslänge erreicht.

---

Resume On démontre au moyen d'exemples qu'il existe des modifications saisonnières dans la dispersion des rythmes circadiens de l'activité des mammifères, des oiseaux, des reptiles et des arthropodes.Ces modifications sont probablement beaucoup plus fréquentes qu'on ne l'admet généralement et sont importantes surtout dans les régions désertiques où l'on rencontre de très grandes variations de température. La synchronisation du début de l'activité circadienne avec les changements de l'heure de l'aube et du crépuscule est due, en général,à des réactions aux modifications du photopériodisme.

---

---

Parts of this paper were presented during the Fourth International Biometeorological Congress, New Brunswick, N.J., 26 August–2 September 1966.     

Effects of activity of daily living and gender on circadian rhythms of the elderly in a nursing home

The objective of this study was to confirm the effects of ADL (Activity of Daily Living) and gender on circadian rhythms of the elderly in a nursing home. Twenty-one elderly volunteers, aged over 65 years, were divided in four groups depending on their ADL and gender: subjects with almost no problem in ADL (H males, H females) and those who were almost bedridden (L males, L females). Oral temperature, heart rate, blood pressure, time of sleep and wake, subjective sleepiness, overall feeling and vitality were measured every 4 hours during the day continuously for six days. The circadian rhythm was calculated by using the least squares fit of cosine function. Subjective sleep quality was also surveyed. In the sleep/wake rhythm, the mesor was significantly higher in L males compared to the other groups and the amplitude was significantly lower in L females compared to other groups. The subjective sleepiness was higher in L males compared to the other groups and L females showed a higher sleepiness compared to H females. No significant difference among the group was observed in subjective sleep quality. In conclusion, these results indicate that the subjective sleepiness and sleep/wake rhythm differ depending on ADL and gender, although no significant difference was observed in physiological parameters. ADL and gender based difference in subjective sleepiness and sleep/wake rhythm should be taken into account with regard to the care of the elderly in nursing homes.     

Development of sucrose preference in rabbit pups

Taste preference behavior in developing rabbit pups was examinedat 2–4 days, 6–8 days and 14–16 days postnatally.0.3 M sucrose and distilled water were presented alternatelyat 2 min. intervals, and volumes ingested were recorded. Sweetpreferences were not expressed until late in the second postnatalweek. Since it is known that rabbit pups exhibit distinctivefacial displays to different taste stimuli at birth (Ganchrowet al., 1979) the delay in the expression of sweet preferencewas postulated to be related to the postnatal maturation ofneural processes central to peripheral and brainstem gustatorycoordinating mechanisms.     

Patterns of ultradian rhythms of activity and metabolic rate in relation to average daily energy expenditure in root voles

Microtine species are known to have an ultradian pattern of activity. Earlier study suggested that period of activity oscillations decreased after magnification of the average energy expenditure. Moreover, the lack of daily variation in nonshivering thermogenesis capacity suggests that the level of metabolic rate (MR) depends mainly on animal activity. Thus, (1) one could expect that ultradian oscillations of MR and activity were similarly dependent on the average level of energy expenditure. Moreover, I expected that animals at lower ambient temperature (T_a) should minimize their energy expenditure and the time of exposure to low T_a outside the nest. Thus (2) their mean activity near a feeder does not increase together with average level of energy expenditure. I tested these hypotheses using root voles Microtus oeconomus Pallas, 1776 acclimated to T_a of 20°C (12L∶12D). The MR and activity both inside (A_n) and outside the nest (A_f), were measured continuously at T_a of 10 and 20°C. Average oxygen consumption differed between T_as. The period of MR and A_f rhythm was shorter at lower T_a, but the period of A_n did not differ significantly. The level of energy expenditure did not affect mean A_f. Close correlation between MR and A_f length period suggests that oscillations of MR during the day are affected by activity related to feeding, rather than low cost A_n.     

Twenty-four hour daily rhythms of glycine in the rodent brain

Male Sprague Dawley rats weighing 200–250 grams each were adapted for a minimum period of three weeks to a 12-hour light: 12-hour dark programmed illumination cycle with a temperature of 23 ± 1°C before used in this study. Every four hours for a twenty-four hour period glycine levels were measured in specific brain areas. Glycine levels were determined in the caudate nucleus, cerebellum, cortex, midbrain and pons by the method of Aprison and Shank (16). The highest level of glycine was observed in the pons. Through concentrations appeared in the cerebellum, cortex, midbrain and pons at D-0400 hours and the peak concentrations occurred at L-1600 hours. The glycine levels were significantly lower during the dark period of the light-dark cycle for all brains except the caudate nucleus.     

Diversity and development of circadian rhythms in the European rabbit

Three main concerns underlie this review: 1) The need to draw together the widely dispersed information available on the circadian biology of the rabbit. Although the rabbit is a classic laboratory mammal, this extensive body of information is often overlooked by chronobiologists, and despite several advantages of this species. In terms of its general biology the rabbit is the best studied laboratory mammal in the wild, it demonstrates a wide variety of robust circadian functions, and being a lagomorph, it provides a useful comparison with more commonly studied rodent species. 2) The need to more fully exploit a developmental approach to understanding circadian function, and the particular suitability of the rabbit for this. Female rabbits only visit their altricial young for a few minutes once every 24 h to nurse, and survival of the young depends on the tight circadian-controlled synchronization in behavior and physiology of the two parties. Patterns of circadian rhythmicity in neonatal pups associated with nursing do not form a smooth continuum into weaning and adult life, and may reflect the action of separate mechanisms operating in their own right. 3) Using information from the first two points, to emphasize the diversity and complexity of circadian rhythms underlying behavioral and physiological functions in adult and developing mammals. Information accruing on circadian functions in the rabbit makes it increasingly difficult to account for these in terms of one or two regulatory mechanisms or "oscillators." Thus, it is argued that in addition to the reductionist, molecular approaches currently dominating much of chronobiology, the study of circadian systems as emergent characteristics of whole organisms operating in complex environments merits special attention.     

Hepatic and renal cadmium accumulation is associated with mass-specific daily metabolic rate in the bank vole (Clethrionomys glareolus)

Previous study has shown that photoperiod and age affect tissue accumulation of cadmium (Cd) in a small rodent, the bank vole. Since the body mass is also influenced by these factors, the present study was designed to determine whether mass-specific daily metabolic rate might be responsible for differential accumulation of Cd in the liver and kidneys of the short- and long-photoperiod bank voles as well as of the young and old animals. One- and five-month old male bank voles were held under short (8 h light/16 h dark) or long (16 h light/8 h dark) photoperiods and exposed to dietary Cd (100 microg/g) for 6 weeks. The bank voles raised under the short photoperiod and those injected subcutaneously with melatonin (7 micromol/kg/day) under the long photoperiod showed significantly higher concentrations of Cd in the liver (43-60%) and kidneys (40-47%) than the age-matched long-photoperiod animals. The old bank voles accumulated significantly less Cd in both organs than the young animals. These differences in Cd accumulation appeared not to be associated with the relative Cd intake. However, the hepatic and renal Cd levels followed a pattern similar to that of the mass-specific daily metabolic rate (or energy expenditure) and energy assimilation efficiency. These data indicate that mass-specific daily metabolic rate and energy assimilation efficiency (an indicative of digestive and absorptive processes) may be responsible for differential tissue Cd accumulation in the bank vole.     

Non-specific neural stimuli and metabolic rhythms in rats.

Ten rats housed in individual metabolism cages in a room next to a teaching laboratory showed rhythmic fluctuations in urination and defaecation. Sectral analysis of the data indicates a weekly rhythm which is attributed to the lack of neural stimuli at the weekend. Regression analysis using a weekly cycle illustrates this effect.     

Nipple-search performance by rabbit pups: Changes with age and time of day

Developmental changes in the nipple-search behaviour of rabbit pups (Oryctolagus cuniculus) were investigated from day 1 to day 13. The performance of four litters was tested 1 h before and 6 h after the once-daily nursing, and of two litters 6 h before and 1 h after nursing on lactating does restrained on their back. During the first 5 days, the median times taken by pups to attach to nipples decreased from 11.8 s to 3.2 s. This improvement was mainly due to a decrease in the latency to search and suggested an improvement in the ability to react to the nipple-search pheromone. Except for the first day, pups located nipples faster before than after nursing, and were best 1 h before. After the first week, latencies to search remained minimal at this time, but increased again at the other times, and were longest on day 13 at 1 h after nursing. Performance changes with time of day corresponded to daily changes in activity, and, after the first week, suggested a further development of underlying motivational mechanisms.     

Nonentrained circadian rhythms of melatonin in submariners scheduled to an 18-hour day.

The human circadian timing system has previously been shown to free run with a period slightly longer than 24 h in subjects living in the laboratory under conditions of forced desynchrony. In forced desynchrony, subjects are shielded from bright light and periodic time cues and are required to live on a day length outside the range of circadian entrainment. The work schedule used for most personnel aboard American submarines is 6 h on duty alternating with 12 h off duty. This imposed 18-h cycle is too short for human circadian synchronization, especially given that there is no bright-light exposure aboard submarines. However, crew members are exposed to 24-h stimuli that could mediate synchronization, such as clocks and social contacts with personnel who are living on a 24-h schedule. The authors investigated circadian rhythms of salivary melatonin in 20 crew members during a prolonged voyage on a Trident nuclear submarine. The authors found that in crew members living on the 18-h duty cycle, the endogenous rhythm of melatonin showed an average period of 24.35 h (n = 12, SD = 0.18 h). These data indicate that social contacts and knowledge of clock time are insufficient for entrainment to a 24-h period in personnel living by an 18-h rest-activity cycle aboard a submarine.