Long-term cold acclimation leads to high Q10 effects on oxygen consumption of loggerhead sea turtles Caretta caretta

We monitored oxygen consumption (VO2), body temperatures (Tb), submersion intervals, and circadian rhythms of VO2 in nine loggerhead turtles during a 6-mo period. The turtles originated from the Tyrhennian Sea, South Italy (40 degrees 51'N, 14 degrees 17'E) and were kept in indoor tanks at constant photoperiod while being subject to the seasonal decline in water temperature (Tw=27.1 degrees to 15.3 degrees C). From summer to winter, all turtles underwent profound reductions in VO2 (Q10=5.4). Simultaneously, their activity was greatly reduced and submergence intervals increased. Over 24-h periods, however, the turtles showed no circadian rhythm in activity or VO2. However, there was a significant positive correlation between the proportion of a day spent actively swimming and VO2. Tb's were not significantly different from Tw and followed the same seasonal decline. A second experiment was conducted to establish the effect of short-term exposure to various temperatures on VO2. Tb equilibrated with the experimental Tw within 3 h. The metabolic responses were again positively correlated with changes in Tw, but this time the corresponding Q10 was only 1.3. On the basis of the range of body masses of the turtles used in this study (2-60 kg), the intraspecific scaling exponent for VO2 was 0.353.     

Cleavage site of a major yolk protein (MYP) determined by cDNA isolation and amino acid sequencing in sea urchin,Hemicentrotus pulcherrimus

The grey mouse lemur (Microcebus murinus) is a small nocturnal primate exhibiting daily torpor. In constant ambient temperature (22-24 degrees C), body temperature (Tb) and locomotor activity were monitored by telemetry in animals exposed to short (SP: 10 h light/day) or long (LP: 14 light/day) photoperiods. They were first fed ad libitum for 8 days and then subjected to 80% restricted feeding for 8 more days. During ad libitum feeding, locomotor activity was significantly lower in SP-exposed animals than in LP-exposed animals. Whatever the photoperiod, animals entered daily hypothermia within the first hours following the light onset. Depth of daily hypothermia increased irregularly under SP exposure, whereas minimal daily Tb was constantly above 35 degrees C under LP exposure. After the transfer from long photoperiod to short photoperiod corresponding to the induction of seasonal fattening, locomotor activity and depth of controlled daily hypothermia did not change significantly. In contrast, food restriction led to a significant increase in locomotor activity and in frequency of daily torpor (Tb<33 degrees C) and body temperature reached minimum values averaging 25 degrees C. However, SP-exposed animals exhibited lower minimal daily Tb and higher torpor duration than LP exposed animals. Therefore, daily torpor appears as a rapid response to food restriction occurring whatever the photoperiod, although enhanced by short photoperiod.     

Thermoregulatory patterns of two sympatric rodents: Otomys unisulcatus and Parotomys brantsii

1. The adaptations to an arid environment in two closely related rodent species were investigated. 2. The rate of oxygen consumption (VO2), body temperature (Tb), evaporative water loss and minimal conductance in Otomys unisulcatus and Parotomys brantsii were determined under controlled conditions at ambient temperatures (Ta), ranging from 11-31 C. 3. Physiological features atypical of desert-adapted rodents include a basal metabolic rate higher than predicted by body mass, the low "lower critical temperature" and symptoms of heat stress at 31 degrees C. 4. The low Tb and wide thermoneutral zone recorded for both species are characteristic of desert rodent species. 5. These species' physiological abilities reflect their mesic phylogeny and we suggest that behaviour must play an important role in their survival in semi-arid areas.     

Low temperature limits photoperiod control of smolting in atlantic salmon through endocrine mechanisms

We have examined the interaction of photoperiod and temperature in regulating the parr-smolt transformation and its endocrine control. Atlantic salmon juveniles were reared at a constant temperature of 10 degrees C or ambient temperature (2 degrees C from January to April followed by seasonal increase) under simulated natural day length. At 10 degrees C, an increase in day length [16 h of light and 8 h of darkness (LD 16:8)] in February accelerated increases in gill Na(+)-K(+)-ATPase activity, whereas fish at ambient temperature did not respond to increased day length. Increases in gill Na(+)-K(+)-ATPase activity under both photoperiods occurred later at ambient temperature than at 10 degrees C. Plasma growth hormone (GH), insulin-like growth factor, and thyroxine increased within 7 days of increased day length at 10 degrees C and remained elevated for 5-9 wk; the same photoperiod treatment at 2 degrees C resulted in much smaller increases of shorter duration. Plasma cortisol increased transiently 3 and 5 wk after LD 16:8 at 10 degrees C and ambient temperature, respectively. Plasma thyroxine was consistently higher at ambient temperature than at 10 degrees C. Plasma triiodothyronine was initially higher at 10 degrees C than at ambient temperature, and there was no response to LD 16:8 under either temperature regimen. There was a strong correlation between gill Na(+)-K(+)-ATPase activity and plasma GH; correlations were weaker with other hormones. The results provide evidence that low temperature limits the physiological response to increased day length and that GH, insulin-like growth factor I, cortisol, and thyroid hormones mediate the environmental control of the parr-smolt transformation.     

Seasonal changes in the thermoregulation of laboratory golden hamsters during acclimation to seminatural outdoor conditions

Proper adjustments of the thermoregulatory mechanisms ensure survival in the natural environment. In the present study, we tested the hypothesis that laboratory golden hamsters (Mesocricetus auratus) housed under seminatural outdoor conditions are able to acclimatize to daily and seasonal changes in the environment despite their long history of breeding in captivity. The animals experienced natural changes in the photoperiod and ambient temperature characteristic for central Poland. During experiments in the thermal gradient system, the daily rhythms of body temperature (measured as the temperature of brown adipose tissue, TBAT), preferred ambient temperature (PTa) and activity were measured in summer, autumn and spring. We found that mean TBAT was highest in autumn and least in summer, reflecting seasonal changes in the capacity for nonshivering thermogenesis (NST). In summer, TBAT followed the robust daily rhythm with the amplitude of 1.1+/-0.1 degrees C. This amplitude was depressed in autumn (0.2+/-0.1 degrees C) and partially restored in spring (0.4+/-0.1 degrees C). Seasonal changes in the daily amplitude of TBAT recorded during both transitional periods, i.e., in autumn and spring, seem to be associated with hamsters' hibernation. In autumn, mean daily PTa was lower than in summer and spring, indicating the lowering of a set point for core body temperature (Tb) regulation. Locomotor activity was much higher in spring than in summer and autumn, and it always predominated at night. We conclude that laboratory golden hamsters housed under seminatural conditions express daily and seasonal changes in the thermoregulatory mechanisms that, despite long history of breeding in captivity, enable proper acclimatization to seasonally changing environment and ensure successful hibernation and winter survival.     

Behavioral thermoregulation by hypoxic rats.

To address whether a shift in hypothalamic thermal setpoint might be a significant factor in induction of hypoxic hypothermia, behavioral thermoregulation was examined in 7 female Sprague-Dawley rats implanted with radiotelethermometers for deep body temperature (Tb) measurement in a thermocline during normoxia (PO2 = 125 torr) and hypoxia (PO2 = 60 torr). Normoxic rats (TNox) selected a mean ambient temperature of 19.7 +/- 1.4 (SE) degrees C and maintained Tb at 37.0 +/- 0.2 degrees C. Hypoxic rats selected a significantly higher ambient temperature (THox = 28.6 +/- 2.2 degrees C) but maintained Tb significantly lower at 35.5 +/- 0.3 degrees C. Without a thermal gradient (ambient temperature = 25 degrees C), Tb during hypoxia was 35.4 +/- 0.4 degrees C. The maintenance of a lower body temperature during hypoxia through behavioral thermoregulation despite having warmer temperatures available supports the hypothesis that the thermoregulatory setpoint of hypoxic rats is shifted to promote thermoregulation at a lower Tb, effectively reducing oxygen demand when oxygen supply is limited.     

Effects of hyperoxia on the metabolic response to cold of the newborn rat.

We asked what effects hyperoxia may have on the metabolic response to cold of the newborn rat. Whole body gaseous metabolism (VO2 and VCO2) was measured in 2-day old rats by open flow respirometry at ambient temperatures (Tamb) between 40 and 20 degrees C, changed at a rate of 0.5 degrees C/min during normoxia and hyperoxia (100% O2 breathing). In normoxia, the thermoneutral range was very narrow, at Tamb = 33-35 degrees C. A decrease in Tamb at first stimulated VO2; a further drop in Tamb below 28 degrees C reduced metabolic rate. The metabolic response to cold was not sufficient to maintain body temperature (Tb). In hyperoxia average values of VO2 were above the normoxic values at all Tamb, but the difference was mostly apparent at low Tamb; at 20 degrees C, hyperoxic VO2 averaged 73% more than in normoxia. This metabolic increase determined a significant but small rise of Tb. We conclude that in the 2-days-old rat hyperoxia has a stimulatory effect on metabolism which is Tamb-dependent, being much more apparent in the cold. This supports the concept that the normoxic VO2 of the newborn is limited by the supply of O2. However, the fact that in the cold, even in hyperoxia, VO2 did not reach very high values, and Tb was not maintained, suggests that not only O2 availability, but also the rate of O2 utilization limits the aerobic metabolic response of the newborn.     

The effect of body temperature on the locomotory energetics of lizards

Oxygen consumption (VO2), carbon dioxide production (VCO2), and stamina were measured in the lizard Tupinambis nigropunctatus running at sustainable and non-sustainable velocities (v) on a motor-driven treadmill. Three experimental groups were measured: field-fresh animals at body temperature (Tb) = 35 degrees C and laboratory-maintained animals at Tb = 35 and 25 degrees C. Mean preferred Tb was determined to be 35.2 degrees C. At 35 degrees C, field-fresh animals had a greater maximal oxygen consumption (VO2max corr) (4.22 vs 3.60 ml O2 g-0.76h-1) and a greater endurance. The net cost of transport (slope of VO2 on v) did not differ between the groups (= 2.60 ml O2 g-0.76)km-1). Velocity at which VO2max is attained (MAS) is 0.84 km h-1. The respiratory exchange ratio (R) exceeded 1.0 at v above MAS, indicating supplementary anaerobic metabolism. At 25 degrees C, VO2max corr was lower (2.34 ml O2 g-0.76h-1) as was endurance, MAS occurring at 0.5 km h-1. Net cost of transport was not significantly different than at 35 degrees C. The effect of Tb on locomotory costs was analyzed for this lizard and other species. It was concluded that the net cost of transport is temperature independent in all species examined and the total cost of locomotion (VO2 v-1) is temperature dependent in Tupinambis (Q10 = 1.4-2.0) and all other species examined except one. The energetic cost of locomotion [(VO2active-VO2rest)v-1], previously reported to be temperature independent in lizards, is temperature dependent in Tupinambis (Q10 = 1.3-1.6) and in two other species.2r     

Behavioral thermoregulation in obese and lean Zucker rats in a thermal gradient

Genetically obese Zucker (Z) rats have been reported to display a body core temperature (Tb) that is consistently below that of their lean littermates. We asked the question whether the lower Tb was a result of deficits in thermoregulation or a downward resetting of the set point for Tb. For a period of 45 consecutive hours, lean and obese Z rats were free to move within a thermal gradient with an ambient temperature (T(a)) range of 15-35 degrees C, while subjected to a 12:12-h light-dark cycle. Tb was measured using a miniature radio transmitter implanted within the peritoneal cavity. Oxygen consumption (VO2) was measured using an open flow technique. Movements and most frequently occupied position in the gradient (preferred T(a)) were recorded using a series of infrared phototransmitters. Obese Z rats were compared with lean Z rats matched for either age (A) or body mass (M). Our results show that obese Z rats have a lower Tb [37.1 +/- 0.1 degrees C (SD) vs. 37.3 +/- 0.1 degrees C, P < 0.001] and a lower VO2 (25.3 +/- 1.9 ml x kg(-1) x h(-1)) than lean controls [33.1 +/- 3.7 (A) and 33.9 +/- 3.9 (M) ml x kg(-1) x h(-1), P < 0.001]. Also, the obese Z rats consistently chose to occupy a cooler T(a) [20.9 +/- 0.6 degrees C vs. 22.7 +/- 0.6 degrees C (A) and 22.5 +/- 0.7 degrees C (M), P < 0.001] in the thermal gradient. This suggests a lower set point for Tb in the obese Z rat, as they refused the option to select a warmer T(a) that might allow them to counteract any thermoregulatory deficiency that could lead to a low Tb. Although all rats followed a definite circadian rhythm for both Tb and VO2, there was no discernible circadian pattern for preferred T(a) in either obese or lean rats. Obese Z rats tended to show a far less definite light-dark activity cycle compared with lean rats.     

Thermoregulatory responses to manipulations of photoperiod in wood mice Apodemus sylvaticus from high latitudes (57°N)

1. 1. The thermoregulatory responses to manipulations of photoperiod in wood mice (Apodemus sylvaticus), which were drawn from a population living at a high latitude (57°N) were studied.

2. 2. Mice captured in spring were acclimated to two different photoperiod regimes 16L:8D and 8L:16D at a constant ambient temperature of 24°C, for 3 weeks.

3. 3. Daily rhythms of body temperature, oxygen consumption and body temperature at various ambient temperatures, nonshivering thermogenesis (the response to a noradrenaline injection) and body mass were measured. Minimal overall thermal conductance was calculated for both groups.

4. 4. Acclimation to long photophase increased the thermoregulatory abilities at relatively high ambient temperatures while that of long-scotophase increased thermoregulatory abilities at low ambient temperatures.

5. 5. Changes in photoperiod may therefore be used as cues for seasonal acclimatization of thermoregulatory mechanisms in this population of wood mice.

     

Stress fever magnitude in laboratory-maintained California ground squirrels varies with season

A previous study demonstrated that California ground squirrels (Spermophilus beecheyi) living in the natural environment had, independent of season, a significantly higher mean diurnal body temperature (T(b)) (39.6 degrees C) than either summer (37.5 degrees C) or winter (36.5 degrees C) laboratory maintained animals. Based upon the previous study it has been suggested that California ground squirrels living in the natural environment may have an elevated set-point for body temperature in a manner analogous to a stress fever response. The present study was conducted to determine if season and/or duration of laboratory open-field exposure influenced the magnitude of laboratory open-field stress fever. If stress fever was involved to some extent in the higher body temperature observed in animals from the natural environment, laboratory maintained animals should exhibit a lower magnitude stress fever during the summer months and a higher magnitude stress fever during the winter months. It was hypothesized that laboratory maintained animals would exhibit the same set-point for stress fever T(b) independent of season, and that the duration of open-field exposure would not influence the magnitude of stress fever. Adult California ground squirrels were acclimated to an ambient temperature of 20+/-1.0 degrees C under either LD 14:10 (summer) or LD 10:14 (winter) photoperiod conditions and individuals from both photoperiod conditions were exposed for periods of 2, 4, and 6 h to an open-field arena. An analysis of the data with a two-factor ANOVA demonstrated that season (photoperiod) significantly influenced the magnitude of the stress fever response (1.1+/-0.1 degrees C for summer animals; 2.1+/-0.2 degrees C for winter animals) while there was no significant influence of open-field exposure duration on stress fever magnitude. These results demonstrate that although the set-point for body temperature in unstressed laboratory maintained California ground squirrels varies with season, the set-point for body temperature in open-field stressed animals does not vary with season. These data lend support to the hypothesis that something like stress fever may play some role in the higher body temperature observed in California ground squirrels living in the natural environment.     

Effects of hypoxia and cold acclimation on thermoregulation in the rat.

The effects of hypoxia (inspired O2 fraction = 0.12) on thermoregulation and on the different sources of thermogenesis were studied in rats before and after periods of 1-4 wk of cold acclimation. Measurements of metabolic rate (VO2) and body temperature (Tb) were made at 5-min intervals, and shivering activity was recorded continuously in groups of rats subjected to three protocols. In protocol 1, rats were exposed to normoxia to an ambient temperature (Ta) of 5 degrees C for 2 h. In protocol 2, at Ta of 5 degrees C, rats were exposed for 30 min to normoxia, then for 45 min to hypoxia, and finally for 30 min to normoxia. In protocol 3, in the non-cold-acclimated (NCA) rats, Ta was decreased from 30 to 5 degrees C in steps of 5 degrees C and of 30-min duration while in cold-acclimated (CA) rats at 5 degrees C for 4-wk, Ta was increased from 5 to 30 degrees C in steps of 5 degrees C and of 30-min duration. Recordings were made in normoxia and in hypoxia on different days in the same animals. The results showed that 1) in NCA rats, cold exposure in normoxia induced increases in VO2 and shivering that were proportional to the decrease in Ta; 2) in CA rats in normoxia, for a given Ta, VO2 and Tb were higher than in NCA rats, whereas shivering was generally lower; and 3) in both NCA and CA rats, hypoxia induced a transient decrease in shivering and a sustained decrease in nonshivering thermogenesis associated with a marked decrease in Tb that was about the same in NCA and CA rats. We speculate that hypoxia acts on Tb control to produce a general inhibition of thermogenesis. Nonshivering thermogenesis is markedly sensitive to hypoxia, especially demonstrable in CA rats; a recovery or even an increase in shivering can compensate for the decrease in nonshivering thermogenesis.     

Oxygen consumption and thermoregulatory responses in three species of South American marsupials

Oxygen consumption (VO(2)), body temperature (T(b)) and wet thermal conductance (C(wet)), under resting conditions, exposure to low ambient temperature (T(a)) and during sustained exercise (treadmill running) were measured in three phylogenetic related (same family; Didelphidae) South American marsupials possessing similar body masses: Caluromys philander (arboreal/fruit and insect eating), Philander opossum (terrestrial and arboreal/omnivore), and Metachirus nudicaudatus (terrestrial/omnivore). Our measurements of VO(2) and C(wet) under resting conditions agree with those previously reported for other marsupials. We expected that C. philander would show a lower maximal sustained VO(2), compared to the other two species, based on its more reduced skeletal muscle mass. However, the values obtained for C. philander were not statistically different (ANOVA) from those obtained for the other two species. When exposed to low ambient temperature (12 degrees C), differences among the three species were detected, i.e., M. nudicaudatus did not survive, while the other two species were able to reduce their T(b) under such conditions. C. philander gradually decreases its T(b) when cold exposed, and P. opossum shows a more pronounced T(b) drop only when exposure to low ambient temperatures occurs for a more prolonged period of time.     

Rhythms of mammalian body temperature can sustain peripheral circadian clocks

BACKGROUND: Low-amplitude temperature oscillations can entrain the phase of circadian rhythms in several unicellular and multicellular organisms, including Neurospora and Drosophila. Because mammalian body temperature is subject to circadian variations of 1 degrees C-4 degrees C, we wished to determine whether these temperature cycles could serve as a Zeitgeber for circadian gene expression in peripheral cell types. RESULTS: In RAT1 fibroblasts cultured in vitro, circadian gene expression could be established by a square wave temperature rhythm with a (Delta)T of 4 degrees C (12 hr 37 degrees C/12 hr 33 degrees C). To examine whether natural body temperature rhythms can also affect circadian gene expression, we first measured core body temperature cycles in the peritoneal cavities of mice by radiotelemetry. We then reproduced these rhythms with high precision in the liquid medium of cultured fibroblasts for several days by means of a homemade computer-driven incubator. While these "in vivo" temperature rhythms were incapable of establishing circadian gene expression de novo, they could maintain previously induced rhythms for multiple days; by contrast, the rhythms of control cells kept at constant temperature rapidly dampened. Moreover, circadian oscillations of environmental temperature could reentrain circadian clocks in the livers of mice, probably via the changes they imposed upon both body temperature and feeding behavior. Interestingly, these changes in ambient temperature did not affect the phase of the central circadian pacemaker in the suprachiasmatic nucleus (SCN) of the hypothalamus. CONCLUSIONS: We postulate that both endogenous and environmental temperature cycles can participate in the synchronization of peripheral clocks in mammals.     

Torpor and hibernation in a basal placental mammal, the Lesser Hedgehog Tenrec Echinops telfairi

The patterns of heterothermy were measured in Lesser Hedgehog Tenrecs, Echinops telfairi, under semi-natural conditions in an outdoor enclosure during the austral mid-winter in southwestern Madagascar. The animals were implanted with miniaturized body temperature (Tb) loggers (iButtons) that measured body temperature every 42 min for 2 months (May and June). The tenrecs entered daily torpor on all 60 consecutive days of measurement, that is, on 100% of animal days, with body temperature closely tracking ambient temperature (Ta) during the ambient heating phase. The mean minimum daily Tb of the tenrecs was 18.44 +/- 0.50 degrees C (n = 174, N = 3), and never exceeded 25 degrees C whereas, apart from a few hibernation bouts in one animal, the mean maximum daily Tb was 30.73 +/- 0.15 degrees C (n = 167, N = 3). Thus during winter, tenrecs display the lowest normothermic Tb of all placental mammals. E. telfairi showed afternoon and early evening arousals, but entered torpor before midnight and remained in torpor for 12-18 h each day. One animal hibernated on two occasions for periods of 2-4 days. We consider E. telfairi to be a protoendotherm, and discuss the relevance and potential of these data for testing models on the evolution of endothermy.     

Comparison in men of physiological responses to exercise of increasing intensity at low and moderate ambient temperatures

In six male subjects the sweating thresholds, heart rate (fc), as well as the metabolic responses to exercise of different intensities [40%, 60% and 80% maximal oxygen uptake (VO2max)], were compared at ambient temperatures (Ta) of 5 degrees C (LT) and 24 degrees C (MT). Each period of exercise was preceded by a rest period at the same temperature. In LT experiments, the subjects rested until shivering occurred and in MT experiments the rest period was made to be of exactly equivalent length. Oxygen uptake (VO2) at the end of each rest period was higher in LT than MT (P less than 0.05). During 20-min exercise at 40% VO2max performed in the cold no sweating was recorded, while at higher exercise intensities sweating occurred at similar rectal temperatures (Tre) but at lower mean skin (Tsk) and mean body temperatures (Tb) in LT than MT experiments (P less than 0.001). The exercise induced VO2 increase was greater only at the end of the light (40% VO2max) exercise in the cold in comparison with MT (P less than 0.001). Both fc and blood lactate concentration [1a]b were lower at the end of LT than MT for moderate (60% VO2max) and heavy (80% VO2max) exercises. It was concluded that the sweating threshold during exercise in the cold environment had shifted towards lower Tb and Tsk. It was also found that subjects exposed to cold possessed a potentially greater ability to exercise at moderate and high intensities than those at 24 degrees C since the increases in Tre, fc and [1a]b were lower at the lower Ta.     

The effects of temperature and artificial rain on the metabolism of American kestrels (Falco sparverius)

The effect of rainfall on the metabolism of birds is poorly understood. We measured the metabolism as rate of oxygen consumption (VO2) of four male and four female American kestrels (Falco sparverius) using open-circuit respirometry. We measured VO2 during the spring at ambient temperatures (Ta) of 5, 10, 15, and 25 degrees C in air without rainfall and with simulated rainfall of 2.5 (low rainfall) and 6.1 cm h(-1) (high rainfall). Kestrel metabolism was significantly higher when exposed to the two rainfall levels compared to no rainfall. However, kestrel metabolism was not significantly different at the two rainfall levels. Body temperature (Tb) was significantly lower under high rainfall compared to low rainfall. In addition, under both rainfall levels Tb decreased with decreasing Ta. Calculated thermal conductance was significantly higher in kestrels exposed to rain compared to no rainfall. Kestrels may use sleeking behavior at high rainfall levels to decrease water penetration of the plumage. Daily energy expenditure (DEE) of kestrels exposed to rain may increase markedly, and kestrel energetics may be further exacerbated by wind that often accompanies natural rainstorms.     

Temperature-independence of circannual variations in circadian rhythms of golden-mantled ground squirrels

In golden-mantled ground squirrels, phase angles of entrainment of circadian locomotor activity to a fixed light-dark cycle differ markedly between subjective summer and winter. A change in ambient temperature affects entrainment only during subjective winter when it also produces pronounced effects on body temperature (Tb). It was previously proposed that variations in Tb are causally related to the circannual rhythm in circadian entrainment. To test this hypothesis, wheel-running activity and Tb were monitored for 12 to 14 months in castrated male ground squirrels housed in a 14:10 LD photocycle at 21 degrees C. Animals were treated with testosterone implants that eliminated hibernation and prevented the marked winter decline in Tb; these squirrels manifested circannual changes in circadian entrainment indistinguishable from those of untreated animals. Both groups exhibited pronounced changes in phase angle and alpha of circadian wheel-running and Tb rhythms. Seasonal variation in Tb is not necessary for circannual changes in circadian organization of golden-mantled ground squirrels.     

Bombesin, somatostatin, and related peptides: actions on thermoregulation

Bombesin acts within the anterior hypothalamic preoptic area to interfere with thermoregulation in the rat. The body temperature (Tb) of animals receiving bombesin varies in parallel with ambient temperature (Ta). Bombesin-induced reduction of Tb in animals at low Ta is associated with a marked reduction of oxygen consumption (VO2). Some somatostatin-related peptides, e.g., desAA 1,2,4,5,12,13 [D-Trp8]-somatostatin (ODT8-SS), act within the brain to prevent bombesin-induced reduction of VO2 and Tb. ODT8-SS also produces hyperthermia not associated with an increase in VO2.     

The bear circadian clock doesn’t ‘sleep’ during winter dormancy

Background

Most biological functions are synchronized to the environmental light:dark cycle via a circadian timekeeping system. Bears exhibit shallow torpor combined with metabolic suppression during winter dormancy. We sought to confirm that free-running circadian rhythms of body temperature (Tb) and activity were expressed in torpid grizzly (brown) bears and that they were functionally responsive to environmental light. We also measured activity and ambient light exposures in denning wild bears to determine if rhythms were evident and what the photic conditions of their natural dens were. Lastly, we used cultured skin fibroblasts obtained from captive torpid bears to assess molecular clock operation in peripheral tissues. Circadian parameters were estimated using robust wavelet transforms and maximum entropy spectral analyses.

Results

Captive grizzly bears housed in constant darkness during winter dormancy expressed circadian rhythms of activity and Tb. The rhythm period of juvenile bears was significantly shorter than that of adult bears. However, the period of activity rhythms in adult captive bears was virtually identical to that of adult wild denning bears as was the strength of the activity rhythms. Similar to what has been found in other mammals, a single light exposure during the bear’s active period delayed subsequent activity onsets whereas these were advanced when light was applied during the bear’s inactive period. Lastly, in vitro studies confirmed the expression of molecular circadian rhythms with a period comparable to the bear’s own behavioral rhythms.

Conclusions

Based on these findings we conclude that the circadian system is functional in torpid bears and their peripheral tissues even when housed in constant darkness, is responsive to phase-shifting effects of light, and therefore, is a normal facet of torpid bear physiology.