Seasonal thermogenic acclimation of diurnally and nocturnally active desert spiny mice

Sex differences in the response to circadian disruption in diurnal sand rats

Most animal model studies on physiological functions and pathologies are conducted in males. However, diseases such as depression, type 2 diabetes (T2DM) and cardiovascular disease, all show different prevalence and characteristics in females and males. Moreover, most mammal studies are conducted in nocturnal mice and rats, while modelling diurnal humans. We therefore used male and female fat sand rats (Psammomys obesus), which are diurnal in the wild, as an animal model for T2DM, to explore the effects of mild circadian disruption on behavior, glucose tolerance, cholesterol and heart weight. We found significant differences between the sexes: on average, in response to short photoperiods (SP) acclimation, males showed higher levels of depression-like behavior, lower glucose tolerance, and increased plasma cholesterol levels compared with females, with no effect on heart/body weight ratio. Females, however did show an increase in heart/body weight ratio in response to SP acclimation. We also found that regardless of sex, arrhythmic animals showed higher blood glucose levels, cholesterol levels, heart/body weight ratio, and depressive-like behavior compared with rhythmic animals. Hence, we suggest that the expression of the Circadian Syndrome could be different between males and females. Additional work with females is required to clearly delineate the specific effects in both sexes, and promote sex-based health care, prevention measures and therapies.

Sex differences in nonshivering thermogenesis in the wild

Nonshivering thermogenesis (NST) is a key mechanism that allows mammals to control their body temperature. Sex can frequently affect thermoregulatory requirements; therefore, males and females can be expected to differ significantly in their NST capacity. Several sex-related differences in NST have been described in laboratory animals and humans; however, these parameters are relatively rarely studied in animals living under natural conditions. Here, I briefly review factors that may be responsible for this disparity and point out two situations that should be particularly promising in searching for sex differences in NST under natural conditions: the lactation period and potential mitonuclear conflicts over NST control in species with genetic polymorphism.

Effects of prolonged night-time light exposure and traffic noise on the behavior and body temperature rhythmicity of the wild desert rodent, Gerbillus tarabuli

The aim of this study was to demonstrate for the first time in Tarabul's gerbils (Gerbillus tarabuli), the effects of simultaneous exposure to two major environmental stressors - light and noise pollutions - on the body temperature rhythm and anxious behavior. Seven groups, each consisting of 6 adult male gerbils, were subjected to a standard LD cycle (12 L:12D) with lights on at 08:00 h and off at 20:00 h, constant conditions (total darkness, DD), prolonged nighttime exposure to light (PEL: 18 L:6D) with lights on at 08:00 h and off at 02:00 h, mimicking prolonged exposure to light pollution in peri-urban areas, exposure to auditory stress (TNS) of 80 dB, and conditions combining PEL&TNS. The body temperature circadian rhythm was recorded, and behavioral tests were performed at the end of experimental phases. The results revealed the existence, for the first time in Gerbilus tarabuli, of an endogenous circadian rhythm of body temperature with a period of 23.8 ± 0.04 h. Prolonged exposure to light at night (PEL) induced a significant phase delay (02 h 09 min ± 0.16 h) of the rhythm, with an acrophase (peak time) occurring at 04:42 ± 0.13 h instead of 02:33 ± 0.21 h. Exposure to TNS for 4 hours per night induced a significant increase of the amplitude of the rhythm and a decrease of the rhythm regularity (robustness of 73.26% in TNS vs. 82.32 in control condition). While combining TNS and PEL significantly delayed the phase of the Tb rhythm by 3 h 10 min (acrophase at 06:39 ± 0.37 h instead of 02:33 ± 0.21 h), increased the amplitude, and significantly reduced the stability of the rhythm (robustness of 67.25% in PEL&TNS vs. 82.32 in control condition). PEL&TNS and TNS environments induce an important stress in gerbils highlighted by a significant decrease of the number of line crossings and time spent in the center area of the open field test. Furthermore, elevated plus maze test revealed gerbils of the PEL&TNS and TNS conditions significantly visited the lowest number of open arms and spent a shorter amount of time in it. In addition, these conditions were responsible for less activity (total number of entries in arms) than in the control and PEL conditions. These results indicate clearly that in the desert area, peri-urban light and noise pollutions disturb the circadian rhythm components and alter the behavior of Tarabul's gerbils inducing an anxious state.

Circadian rhythmicity of body temperature and metabolism

This article reviews the literature on the circadian rhythms of body temperature and whole-organism metabolism. The two rhythms are first described separately, each description preceded by a review of research methods. Both rhythms are generated endogenously but can be affected by exogenous factors. The relationship between the two rhythms is discussed next. In endothermic animals, modulation of metabolic activity can affect body temperature, but the rhythm of body temperature is not a mere side effect of the rhythm of metabolic thermogenesis associated with general activity. The circadian system modulates metabolic heat production to generate the body temperature rhythm, which challenges homeothermy but does not abolish it. Individual cells do not regulate their own temperature, but the relationship between circadian rhythms and metabolism at the cellular level is also discussed. Metabolism is both an output of and an input to the circadian clock, meaning that circadian rhythmicity and metabolism are intertwined in the cell.

Red white and blue - bright light effects in a diurnal rodent model for seasonal affective disorder

Despite the common use of bright light exposure for treatment of seasonal affective disorder (SAD), the underlying biology of the therapeutic effect is not clear. Moreover, there is a debate regarding the most efficacious wavelength of light for treatment. Whereas according to the traditional approach full-spectrum light is used, recent studies suggest that the critical wavelengths are within the range of blue light (460 and 484 nm). Our previous work shows that when diurnal rodents are maintained under short photoperiod they develop depression- and anxiety-like behavioral phenotype that is ameliorated by treatment with wide-spectrum bright light exposure (2500 lux at the cage, 5000 K). Our current study compares the effect of bright wide-spectrum (3,000 lux, wavelength 420- 780 nm, 5487 K), blue (1,300 lux, wavelength 420-530 nm) and red light (1,300 lux, wavelength range 600-780 nm) exposure in the fat sand rat (Psammomys Obesus) model of SAD. We report results of experiments with six groups of sand rats that were kept under various photoperiods and light treatments, and subjected to behavioral tests related to emotions: forced swim test, elevated plus maze and social interactions. Exposure to either intense wide-spectrum white light or to blue light equally ameliorated depression-like behavior whereas red light had no effect. Bright wide-spectrum white light treatment had no effect on animals maintained under neutral photoperiod, meaning that light exposure was only effective in the pathological-like state. The resemblance between the effects of bright white light and blue light suggests that intrinsically photosensitive retinal ganglion cells (ipRGCs) are involved in the underlying biology of SAD and light therapy.

Chronobiology of interspecific interactions in a changing world

Animals should time activities, such as foraging, migration and reproduction, as well as seasonal physiological adaptation, in a way that maximizes fitness. The fitness outcome of such activities depends largely on their interspecific interactions; the temporal overlap with other species determines when they should be active in order to maximize their encounters with food and to minimize their encounters with predators, competitors and parasites. To cope with the constantly changing, but predictable structure of the environment, organisms have evolved internal biological clocks, which are synchronized mainly by light, the most predictable and reliable environmental cue (but which can be masked by other variables), which enable them to anticipate and prepare for predicted changes in the timing of the species they interact with, on top of responding to them directly. Here, we review examples where the internal timing system is used to predict interspecific interactions, and how these interactions affect the internal timing system and activity patterns. We then ask how plastic these mechanisms are, how this plasticity differs between and within species and how this variability in plasticity affects interspecific interactions in a changing world, in which light, the major synchronizer of the biological clock, is no longer a reliable cue owing to the rapidly changing climate, the use of artificial light and urbanization.This article is part of the themed issue 'Wild clocks: integrating chronobiology and ecology to understand timekeeping in free-living animals'.

Comparative analyses of basal rate of metabolism in mammals: data selection does matter

Basal rate of metabolism (BMR) is a physiological parameter that should be measured under strictly defined experimental conditions. In comparative analyses among mammals BMR is widely used as an index of the intensity of the metabolic machinery or as a proxy for energy expenditure. Many databases with BMR values for mammals are available, but the criteria used to select metabolic data as BMR estimates have often varied and the potential effect of this variability has rarely been questioned. We provide a new, expanded BMR database reflecting compliance with standard criteria (resting, postabsorptive state; thermal neutrality; adult, non-reproductive status for females) and examine potential effects of differential selectivity on the results of comparative analyses. The database includes 1739 different entries for 817 species of mammals, compiled from the original sources. It provides information permitting assessment of the validity of each estimate and presents the value closest to a proper BMR for each entry. Using different selection criteria, several alternative data sets were extracted and used in comparative analyses of (i) the scaling of BMR to body mass and (ii) the relationship between brain mass and BMR. It was expected that results would be especially dependent on selection criteria with small sample sizes and with relatively weak relationships. Phylogenetically informed regression (phylogenetic generalized least squares, PGLS) was applied to the alternative data sets for several different clades (Mammalia, Eutheria, Metatheria, or individual orders). For Mammalia, a 'subsampling procedure' was also applied, in which random subsamples of different sample sizes were taken from each original data set and successively analysed. In each case, two data sets with identical sample size and species, but comprising BMR data with different degrees of reliability, were compared. Selection criteria had minor effects on scaling equations computed for large clades (Mammalia, Eutheria, Metatheria), although less-reliable estimates of BMR were generally about 12-20% larger than more-reliable ones. Larger effects were found with more-limited clades, such as sciuromorph rodents. For the relationship between BMR and brain mass the results of comparative analyses were found to depend strongly on the data set used, especially with more-limited, order-level clades. In fact, with small sample sizes (e.g. <100) results often appeared erratic. Subsampling revealed that sample size has a non-linear effect on the probability of a zero slope for a given relationship. Depending on the species included, results could differ dramatically, especially with small sample sizes. Overall, our findings indicate a need for due diligence when selecting BMR estimates and caution regarding results (even if seemingly significant) with small sample sizes.

Role of thermal physiology and bioenergetics on adaptation in tree shrew (Tupaia belangeri): the experiment test

Ambient conditions, as temperature and photoperiod, play a key role in animals’ physiology and behaviors. To test the hypothesis that the maximum thermal physiological and bioenergetics tolerances are induced by extreme environments in Tupaia belangeri. We integrated the acclimatized and acclimated data in several physiological, hormonal, and biochemical markers of thermogenic capacity and bioenergetics in T. belangeri. Results showed that T. belangeri increased body mass, thermogenesis capacity, protein contents and cytochrome c oxidase (COX) activity of liver and brown adipose tissue in winter-like environments, which indicated that temperature was the primary signal for T. belangeri to regulate several physiological capacities. The associated photoperiod signal also elevated the physiological capacities. The regulations of critical physiological traits play a primary role in meeting the survival challenges of winter-like condition in T. belangeri. Together, to cope with cold, leptin may play a potential role in thermogenesis and body mass regulation, as this hormonal signal is associated with other hormones. The strategies of thermal physiology and bioenergetics differs between typical Palearctic species and the local species. However, the maximum thermal physiology and bioenergetic tolerance maybe is an important strategy to cope with winter-like condition of T. belangeri.

Utilization of Diurnal Rodents in the Research of Depression

Preclinical Research Most neuropsychiatric research, including that related to the circadian system, is performed using nocturnal animals, mainly laboratory mice and rats. Mood disorders are known to be associated with circadian rhythm abnormalities, but the mechanisms by which circadian rhythm disruptions interact with depression remain unclear. As the circadian system of diurnal and nocturnal mammals differs, we previously suggested that the utilization of diurnal animal models may be advantageous for understanding these relations. During the last 10 years, we and others established the validity of several diurnal rodent species as a model for the interactions between circadian rhythms and depression. Diurnal rodents respond to photoperiod manipulation in a similar way to humans, the behavioral outcome is directly related to the circadian system, and treatment that is effective in patients is also effective in the model. Moreover, less effective treatments in patients are also less effective in the model. We, therefore, suggest that using diurnal animal models to study circadian rhythms-related affective disorders, such as depression, will provide new insights that will hopefully lead to the development of more effective treatments. Drug Dev Res 77 : 347-356, 2016. © 2016 Wiley Periodicals, Inc.

The increase in fat content in the warm-acclimated striped hamsters is associated with the down-regulated metabolic thermogenesis

It has been well known that metabolic thermogenesis plays an important role in the thermoregulation of small mammals under different temperatures, while its role in fat accumulation is far from clear. In the present study, several physiological, hormonal, and biochemical measures indicative of metabolic thermogenesis were measured in the weaning striped hamsters after acclimated to a warm condition (30°C) for 1, 3 and 4months. The warm-acclimated groups significantly decreased energy intake, and simultaneously decreased nonshivering thermogenesis compared to those housed at 21°C. Body fat content increased by 29.9%, 22.1% and 19.6% in the hamsters acclimated to 1, 3 or 4months, respectively relative to their counterparts maintain at 21°C (P<0.05). The cytochrome c oxidase (COX) activity of brain, liver, heart and skeletal muscle, and the ratio of serum tri-iodothyronine to thyroxine significantly decreased in warm-acclimated groups compared with 21°C group. COX activity and uncoupling protein 1 (UCP1) mRNA expression of brown adipose tissue (BAT) were significantly down-regulated under the warm conditions. COX activity of BAT, liver, heart and muscle were significantly negatively correlated with body fat content, and the correlation between UCP1 expression and body fat content tended to be negative. These findings suggest that the decrease in the energy spent on metabolic thermogenesis plays an important role in the fat accumulation. The attenuation of COX and UCP1-based BAT activity may be involved in body fat accumulation in animals under warm conditions.

Foraging Activity Pattern Is Shaped by Water Loss Rates in a Diurnal Desert Rodent

Although animals fine-tune their activity to avoid excess heat, we still lack a mechanistic understanding of such behaviors. As the global climate changes, such understanding is particularly important for projecting shifts in the activity patterns of populations and communities. We studied how foraging decisions vary with biotic and abiotic pressures. By tracking the foraging behavior of diurnal desert spiny mice in their natural habitat and estimating the energy and water costs and benefits of foraging, we asked how risk management and thermoregulatory requirements affect foraging decisions. We found that water requirements had the strongest effect on the observed foraging decisions. In their arid environment, mice often lose water while foraging for seeds and cease foraging even at high energetic returns when water loss is high. Mice also foraged more often when energy expenditure was high and for longer times under high seed densities and low predation risks. Gaining insight into both energy and water balance will be crucial to understanding the forces exerted by changing climatic conditions on animal energetics, behavior, and ecology.

The role of dietary fiber content on energy metabolism, thermogenesis, and leptin in Chevrier’s field mouse (Apodemus chevrieri)

Food quality and availability are important factors influencing the survival and reproduction of animals. The aim of the present study was to examine the effect of dietary fiber content high-fiber (HF) diet treatment or low-fiber (LF) diet treatment) on energy metabolism, thermogenesis, and leptin concentrations in Chevrier’s field mouse (Apodemus chevrieri (Milne-Edwards, 1868)) (Mammalia: Rodentia: Muridae). Mice on the HF treatment showed a lower body mass compared with LF treatment from day 27 to day 37, and a lower but insignificant body mass to day 71. Dry matter intake (DMI) and gross energy intake (GEI) were greater in HF compared with LF, whereas the digestible energy intake (DEI) was similar for both treatments. Nonshivering thermogenesis (NST) decreased in HF mice, whereas LF mice remained stable; no significant differences were detected in the basal metabolic rate (BMR), uncoupling protein 1 (UCP1) content in brown adipose tissue (BAT), or the levels of serum tri-iodothyronine (T3) and thyroxine (T4) between HF and LF mice. Although there were no differences in body fat content and serum leptin concentrations between HF and LF mice, serum leptin levels were positively correlated with body fat mass. These results support the hypothesis that A. chevrieri can compensate the poor-quality diet physiologically by way of increasing food intake and decreasing thermogenesis.

Phenotypic flexibility of energetics in acclimated Siberian hamsters has a narrower scope in winter than in summer

As photoperiod shortens with the approach of winter, small mammals should reduce their energy expenditure to survive periods of food limitation. However, within seasons, animals should balance their energy budgets as abiotic conditions change, sometimes unpredictably; cold spells should increase heat production, while warm spells should do the opposite. Therefore, we addressed specific questions about the possible interactions between seasonal acclimatization and the intra-seasonal phenotypic flexibility of metabolic rate. We hypothesized that phenotypic flexibility in small mammals differs seasonally and is greater in summer than in winter, and predicted that seasonal adjustments in energetics, which are driven by photoperiod, overwhelm the influence of variations in the thermal environment. We measured body mass, basal metabolic rate (BMR), facultative non-shivering thermogenesis (fNST), body temperature, and calculated minimum thermal conductance in Siberian hamsters Phodopus sungorus. Animals were acclimated to winter-like, and then to summer-like conditions and, within each season, were exposed twice, for 3 weeks to 10, 20 or 28 °C. We used differences between values measured after these short acclimation periods as a measure of the scope of phenotypic flexibility. After winter acclimation, hamsters were lighter, had lower whole animal BMR, higher fNST than in summer, and developed heterothermy. After these short acclimations to the above-mentioned temperatures, hamsters showed reversible changes in BMR and fNST; however, these traits were less flexible in winter than in summer. We conclude that seasonal acclimation affects hamster responses to intra-seasonal variations in the thermal environment. We argue that understanding seasonal changes in phenotypic flexibility is crucial for predicting the biological consequences of global climate changes.

Plasticity in the physiological energetics of Apodemus chevrieri: the role of dietary fiber content

Small mammals are usually adapted to cope with changes in food quality and availability. In order to investigate the adaptive strategy of small rodents responding to varying dietary fiber content, in the present study, Apodemus chevrieri individuals were acclimated to a high-fiber diet for four weeks and then a relatively low-fiber diet for another four weeks. The results show that body mass was relatively stable over the course of acclimation, but dry matter intake, gross energy intake and the mass of the digestive tract increased significantly and digestibility decreased significantly in high-fiber diet mice, while the digestible energy intake was similar for both high-fiber and low-fiber diet mice except for the first week. High-fiber/low-fiber diet mice showed only a significant lower basal metabolic rate and nonshivering thermogenesis compared to low-fiber diet mice on day R1. The high-fiber diet induced a decrease in serum leptin levels and brown adipose tissue mass associated with a reduction in the cytochrome c oxidase activity and uncoupling protein 1 content of brown adipose tissue. Body mass, thermogenic capacity, energy intake, serum leptin levels and digestive tract morphology returned to the control levels after 4 weeks of refeeding low-fiber diet. Further, serum leptin levels were positively related to body fat mass and negatively related to food intake. These data indicated that body mass, energy intake, serum leptin levels and organ morphological plasticity were the main strategies by which A. chevrieri copes with variations in dietary fiber content.

Short photoperiod condition increases susceptibility to stress in adolescent male rats

The seasonality of depressive symptoms is prevalent in children and adolescents. However, the mechanisms that underlie such susceptibility to seasonal influences on mood disorders are unclear. We examined the effects of a short photoperiod condition on the susceptibility to subchronic unpredictable mild stress (SCUS) and rhythmic alterations of plasma corticosterone (CORT), melatonin, and neuropeptide Y (NPY) in adolescent male rats. Compared with the 12h/12h light/dark photoperiod control (CON) rats, the 8h/16h photoperiod SCUS rats exhibited significant anhedonia, a core symptom of human depression, together with a blunted diurnal rhythm and elevation of 24h CORT, melatonin, and NPY levels. The 8h/16h photoperiod condition also blunted the rhythmicity of CORT, caused a phase inversion of melatonin, and caused a phase delay of NPY compared with 12h/12h CON rats. Such abnormalities of plasma CORT, NPY, and melatonin might cause adolescent individuals to present higher stress reactivity and greater vulnerability to stress over their lifetimes. The present study provides evidence of the susceptibility to the seasonality of stress-related disorders in adolescence.

Brown adipose tissue: physiological function and evolutionary significance

In modern eutherian (placental) mammals, brown adipose tissue (BAT) evolved as a specialized thermogenic organ that is responsible for adaptive non-shivering thermogenesis (NST). For NST, energy metabolism of BAT mitochondria is increased by activation of uncoupling protein 1 (UCP1), which dissipates the proton motive force as heat. Despite the presence of UCP1 orthologues prior to the divergence of teleost fish and mammalian lineages, UCP1's significance for thermogenic adipose tissue emerged at later evolutionary stages. Recent studies on the presence of BAT in metatherians (marsupials) and eutherians of the afrotherian clade provide novel insights into the evolution of adaptive NST in mammals. In particular studies on the 'protoendothermic' lesser hedgehog tenrec (Afrotheria) suggest an evolutionary scenario linking BAT to the onset of eutherian endothermy. Here, we review the physiological function and distribution of BAT in an evolutionary context by focusing on the latest research on phylogenetically distinct species.

Effect of food restriction on energy budget in warm-acclimated striped hamsters

The capacity of small mammals to sustain periods of food shortage largely depends on the adaptive regulation of energy budget in response to the decrease in food supply. In addition to food availability, ambient temperature (Ta) is an important factor affecting the rates of both energy intake and expenditure. To examine the effect of Ta on energy strategy and the capacity to sustain food shortage, striped hamsters were exposed to a warm condition (30°C) and were then restricted to 70% of ad libitum food intake. Body mass, energy intake and expenditure and physiological markers indicative of thermogenesis were measured. Warm exposure had no effect on body mass and digestibility, but decreased energy intake, basal metabolic rate and maximum nonshivering thermogenesis. The mitochondria protein content, cytochrome c oxidase activity and uncoupling protein 1 level of brown adipose tissue were significantly lower in hamsters at 30°C than at 21°C. Food restriction induced a significant decrease in body mass, but the decreased body mass was attenuated at 30°C relative to 21°C. This suggests that striped hamsters could not compensate for the limited food supply by decreasing daily energy expenditure at 21°C, whereas they could at 30°C. The significant reductions in the rates of metabolism and thermogenesis in warm-acclimated hamsters increase the capacity to cope with food shortage. Although, it remains uncertain whether this response represents some generalized evolutionary adaptation, the Ta-dependent adjustment in the capacity to survive food restriction may reflect that warm acclimation plays an important role in adaptive regulation of both physiology and behavior in response to the variations of food availability.

An intact dorsomedial posterior arcuate nucleus is not necessary for photoperiodic responses in Siberian hamsters

Seasonal responses of many animal species are triggered by changes in daylength and its transduction into a neuroendocrine signal by the pineal gland through the nocturnal duration of melatonin (MEL) release. The precise central sites necessary to receive, transduce, and relay the short day (SD) fall-winter MEL signals into seasonal responses and changes in physiology and behavior are unclear. In Siberian hamsters, SDs trigger decreases in body and lipid mass, testicular regression and pelage color changes. Several candidate genes and their central sites of expression have been proposed as components of the MEL transduction system with considerable recent focus on the arcuate nucleus (ARC) and its component, the dorsomedial posterior arcuate nucleus (dmpARC). This site has been postulated as a critical relay of SD information through the modulation of a variety of neurochemicals/receptors important for the control of energy balance. Here the necessity of an intact dmpARC for SD responses was tested by making electrolytic lesions of the Siberian hamster dmpARC and then exposing them to either long days (LD) or SDs for 12wks. The SD typical decreases in body and fat mass, food intake, testicular volume, serum testosterone concentrations, pelage color change and increased UCP-1 protein expression (a proxy for brown adipose tissue thermogenesis) all occurred despite the lack of an intact dmpARC. Although the Siberian hamster dmpARC contains photoperiod-modulated constituents, these data demonstrate that an intact dmpARC is not necessary for SD responses and not integral to the seasonal energy- and reproductive-related responses measured here.

Seasonal changes in body mass, serum leptin levels and hypothalamic neuropeptide gene expression in male Eothenomys olitor

The present study examined seasonal changes in body mass and energy metabolism in the Chaotung vole (Eothenomys olitor) and the physiological mechanisms underpinning these changes. Seasonal changes in the following parameters were measured in male E. olitor, body mass, food intake, thermogenesis, enzyme activity, masses of tissues and organs, hormone concentrations and expression of hypothalamic arcuate nucleus energy balance genes including neuropeptide Y (NPY), agouti-related protein (AgRP), pro-opiomelanocortin (POMC), and cocaine- and amphetamine-regulated transcript (CART). Body mass was constant over the year, but the masses of tissues and organs differed significantly between seasons. There were significant changes in body fat mass and serum leptin levels over the four seasons. E. olitor showed significant seasonal changes in food intake and thermogenesis, uncoupling protein 1 (UCP1) content, enzyme activity, and serum tri-iodothyronine (T3) and thyroxine (T4) levels. Moreover, mRNA expression in the hypothalamus showed significant seasonal changes. All of our results suggested that E. olitor had constant body mass over the year, which was inconsistent with the prediction of the 'set-point' hypothesis. However, body fat mass and serum leptin levels were significantly different among the four seasons, providing support for the 'set-point' hypothesis. The changes in leptin, NPY, AgRP, POMC, and CART mRNA levels may play a role in the regulation of energy intake in E. olitor. Furthermore, the role of leptin and hypothalamic neuropeptide gene in the regulation of energy metabolism and body mass may be different in animals that are acclimated to different seasons.

The expression of leptin, hypothalamic neuropeptides and UCP1 before, during and after fattening in the Daurian ground squirrel (Spermophilus dauricus)

The Daurian ground squirrel (Spermophilus dauricus) accumulates large amounts of body fat during pre-hibernation fattening. Leptin, an adipose-derived hormone, plays important roles in energy balance and thermogenesis. We predicted that body fat accumulation would lead to the elevation of leptin concentration while its effect on satiety would be suppressed in hypothalamus during fattening. In addition, the uncoupling protein 1 (UCP1) in brown adipose tissue (BAT) would increase and correlated positively with leptin concentration before hibernation. Here, we measured serum leptin concentration and leptin mRNA in white adipose tissue (WAT), hypothalamic neuropeptides involved in energy regulation and UCP1 in BAT before, during and after fattening in squirrels. The fat mass gradually increased during fattening but serum leptin increased mainly in the late phase of fattening, which was consistent with leptin mRNA expression in WAT. During fattening, the mRNA of hypothalamic leptin receptor was up-regulated and correlated positively with serum leptin. Orexigenic neuropeptide Y mRNA increased by 67%; however agouti-related peptide remained unchanged before hibernation. There was no significant change in anorexigenic neuropeptide mRNA. No change in suppressor of cytokine signaling-3 and protein tyrosine phosphatase-1B was detected. UCP1 mRNA expression and protein content in BAT increased significantly after fattening. These changes were independent of environmental conditions and serum leptin concentration. Our results suggest that the dissociation of leptin production and adiposity during fattening may facilitate fat accumulation. No evidence of suppressed leptin signal was found in fattening squirrels. The UCP1 recruitment in post-fattening squirrels could occur without winter-like acclimation and increased leptin.

Selection for high activity-related aerobic metabolism does not alter the capacity of non-shivering thermogenesis in bank voles

An intriguing question is how the capacity of non-shivering thermogenesis (NST)-a special mechanism supporting endothermic thermoregulation in mammals-is affected by selection for high exercise metabolism. It has been proposed that high NST could be a mechanism to compensate for a low basal production of heat. On the other hand, high basal or activity metabolism is associated with physiological characteristics such as high performance of the circulatory system, which are also required for achieving a high NST. Here we tested whether selection for high aerobic exercise performance, which correlates with an increased basal metabolic rate, led to a correlated evolution of maximum and facultative NST. Therefore, we measured the NST of bank voles, Myodes (= Clethrionomys) glareolus, from lines selected for 13-14 generations (n=46) for high aerobic metabolism achieved during swimming and from unselected, control lines (n=46). Open-flow respirometry was used to measure the rate of oxygen consumption (V(·)O2) in anesthetized bank voles injected with noradrenaline (NA). After adjusting for body mass, maximum NST (maximum V(·)O2 recorded after injection of NA) did not differ between the selected (2.38±0.08 mLO2min(-1)) and control lines (2.36±0.08 mLO2min(-1); P=0.891). Facultative NST (= maximum NST minus resting metabolic rate of anesthetized animals) did not differ between the selected (1.49±0.07 mLO2min(-1)) and control lines (1.50±0.07 mLO2min(-1); P=0.985), either. Therefore, our results suggest that NST capacity is not strongly linked to maximum activity-related aerobic metabolic rate.

The shift of thermoneutral zone in striped hamster acclimated to different temperatures

Temperature affects all biological functions and will therefore modulate ecologically significant interactions between animals and their environment. Here, we examined the effect of ambient temperature (T_a) on the thermal biology and energy budget in striped hamsters acclimated to cold (5°C), warm (21°C) and hot temperatures (31°C). Thermoneutral zone (TNZ) was 22.5–32.5°C, 25–32.5°C and 30–32.5°C in the cold-, warm- and hot-acclimated hamsters, respectively. The cold acclimation decreased the lower critical temperature and made the TNZ wider, and hot exposure elevated the lower critical temperature, resulting in a narrow TNZ. Within the TNZ, cold-acclimated hamsters showed a significantly higher rate of metabolism and thermogenesis than those acclimated to hot temperature. Digestive enzymes activities, including intestinal sucrase, maltase, L-alanine aminopeptidase-N and leucine aminopeptidase were higher in the cold than in the hot. The changes in metabolic rate and thermogenesis at different temperatures were in parallel with cytochrome c oxidase activity and uncoupling protein 1 gene expression of brown adipose tissue. This suggests that the shift of the lower critical temperature of TNZ is possibly associated with the rate of metabolism and thermogenesis, as well as with the digestive capacity of the gastrointestinal tract at different T_a. The upper critical temperature of TNZ may be independent of the changes in T_a. The changes of lower critical temperature of TNZ are an important strategy in adaption to variations of T_a.

Diurnal rodents as an advantageous model for affective disorders: novel data from diurnal degu (Octodon degus)

Circadian rhythms are strongly associated with affective disorders and recent studies have suggested utilization of diurnal rodents as model animal for circadian rhythms-related domains of these disorders. Previous work with the diurnal fat sand rat and Nile grass rat demonstrated that short photoperiod conditions result in behavioral changes including anxiety- and depression-like behavior. The present study examined the effect of manipulating day length on activity rhythms and behavior of the diurnal degu. Animals were housed for 3 weeks under either a short photoperiod (5-h:19-h LD) or a neutral photoperiod (12-h:12-h LD) and then evaluated by sweet solution test and the forced swim test for depression-like behavior, and in the light/dark box and open field for anxiety-like behavior. Results indicate that short photoperiod induced depression-like behavior in the forced swim test and the sweet solution preference test and anxiety-like behavior in the open field compared with animals maintained in a neutral photoperiod. No effects were shown in the light/dark box. Short photoperiod-acclimated degu showed reduced total activity duration and activity was not restricted to the light phase. The present study further supports the utilization of diurnal rodents to model circadian rhythms-related affective change. Beyond the possible diversity in the mechanisms underlying diurnality in different animals, there are now evidences that in three different diurnal species, the fat sand rat, the grass Nile rat and the degu, shortening of photoperiod results in the appearance of anxiety- and depression-like behaviors.

Thermal Ecology, Environments, Communities, and Global Change: Energy Intake and Expenditure in Endotherms

To survive, animals must maintain a balance between energy acquisition (foraging) and energy expenditure. This challenge is particularly great for endotherm vertebrates that require high amounts of energy to maintain homeothermy. Many of these endotherms use hibernation or daily torpor as a mechanism to reduce energy expenditure during anticipated or stochastic periods of stress. Although ecological researchers have focused extensively on energy acquisition, physiologists have largely studied thermal ecology and the mechanisms allowing endotherms to regulate energy expenditure, with little research explicitly linking ecology and thermal biology. Nevertheless, theoretical considerations and research conducted so far point to a significant ecological role for torpor in endotherms. Moreover, global-change challenges facing vertebrate endotherms are also considered in view of their ability to regulate their energy expenditure. We review the thermal ecology of endothermic vertebrates and some of its ecological and evolutionary implications.

Effects of circadian phase and melatonin injection on anxiety-like behavior in nocturnal and diurnal rodents

Animals show daily rhythms in most bodily functions, resulting from the integration of information from an endogenous circadian clock and external stimuli. These rhythms are adaptive and are expected to be related to activity patterns, i.e., to be opposite in diurnal and nocturnal species. Melatonin is secreted during the night in all mammalian species, regardless of their activity patterns. Consequently, in diurnal species the nocturnal secretion of melatonin is concurrent with the resting phase, whereas in nocturnal species it is related to an increase in activity. In this research, we examined in three diurnal and three nocturnal rodent species whether a daily rhythm in anxiety-like behavior exists; whether it differs between nocturnal and diurnal species; and how melatonin affects anxiety-like behavior in species with different activity patterns. Anxiety-like behavior levels were analyzed using the elevated plus-maze. We found a daily rhythm in anxiety-like behavior and a significant response to daytime melatonin administration in all three nocturnal species, which showed significantly lower levels of anxiety during the dark phase, and after melatonin administration. The diurnal species showed either an inverse pattern to that of the nocturnal species in anxiety-like behavior rhythm and in response to daytime melatonin injection, or no rhythm and, accordingly, no response to melatonin.

Role of photoperiod on hormone concentrations and adaptive capacity in tree shrews, Tupaia belangeri

Environmental factors, such as photoperiod and temperature, play an important role in the regulation of an animal's physiology and behavior. In the present study, we examined the effects of short photoperiod (SD, 8L:16D) on body mass as well as on several physiological, hormonal, and biochemical measures indicative of thermogenic capacity, to test our hypothesis that short photoperiod stimulates increases thermogenic capacity and energy intake in tree shrews. At the end, these tree shrews (SD) had a significant higher body mass, energy intake, cytochrome C oxidase (COX) activity and uncoupling protein-1 (UCP1) content, serum tri-iodothyronine (T(3)) and thyroxine (T(4)) compared to LD (16L:8D) tree shrews. However, there were no significant differences in serum leptin and melatonin between the two groups. Together, these data suggest tree shrews employ a strategy of maximizing body growth and increasing energy intake in response to cues associated with short photoperiod.

Foraging sequence, energy intake and torpor: an individual-based field study of energy balancing in desert golden spiny mice

We studied the relationship between sequence of foraging, energy acquired and use of torpor as an energy-balancing strategy in diurnally active desert golden spiny mice. We hypothesised that individuals that arrive earlier to forage will get higher returns and consequently spend less time torpid. If that is the case, then early foragers can be viewed as more successful; if the same individuals arrive repeatedly early, they are likely to have higher fitness under conditions of resource limitation. For the first time, we show a relationship between foraging sequence and amount of resources removed, with individuals that arrive later to a foraging patch tending to receive lower energetic returns and to spend more time torpid. Torpor bears not only benefits but also significant costs, so these individuals pay a price both in lower energy intake and in extended periods of torpor, in what may well be a positive feedback loop.

Maximal thermogenic capacity and non-shivering thermogenesis in the South American subterranean rodent Ctenomys talarum

Subterranean rodents inhabit closed tunnel systems that are hypoxic and hypercapnic and buffer aboveground ambient temperature. In contrast to other strictly subterranean rodents, Ctenomys talarum exhibits activity on the surface during foraging and dispersion and hence, is exposed also to the aboveground environment. In this context, this species is a valuable model to explore how the interplay between underground and aboveground use affects the relationship among basal metabolic rate (BMR), cold-induced maximum metabolic rate (MMR), shivering (ST), and non-shivering thermogenesis (NST). In this work, we provide the first evidence of the presence of NST, including the expression of uncoupling proteins in brown adipose tissue (BAT), and shivering thermogenesis in Ctenomys talarum, a species belonging to the most numerous subterranean genus, endemic to South America. Our results show no differences in BMR, cold-induced MMR, and NST between cold- (15 °C) and warm- (25 °C) acclimated individuals. Furthermore, thermal acclimation had no effect on the expression of mitochondrial uncoupling protein 1 (UCP1) in BAT. Only cytochrome c oxidase (COX) content and activity increased during cold acclimation. When interscapular BAT was removed, NST decreased more than 30%, whereas cold-induced MMR remained unchanged. All together, these data suggest that cold-induced MMR reaches a maximum in warm-acclimated individuals and so a probable ceiling in NST and UCP1 expression in BAT. Possible thermogenic mechanisms explaining the increase in the oxidative capacity, mediated by COX in BAT of cold-acclimated individuals and the role of ST in subterranean life habits are proposed.

Seasonal changes in thermogenesis of a free-ranging afrotherian small mammal, the Western rock elephant shrew (Elephantulus rupestris)

We report on the seasonal metabolic adjustments of a small-sized member of the phylogenetically ancient Afrotheria, the Western rock elephant shrew (Elephantulus rupestris). We recorded body temperature (T (b)) patterns and compared the capacity for adrenergically induced nonshivering thermogenesis (NST) in E. rupestris captured in the wild in summer and winter. Noradrenaline (NA) treatment (0.4-0.5 mg/kg, s.c.) induced a pronounced elevation in oxygen consumption compared to controls (saline), and the increase in oxygen consumption following injection of NA was 1.8-fold higher in winter compared to summer. This suggests that the smaller members of Afrotheria possess functional brown adipose tissue, which changes in thermogenic capacity depending on the season. Torpor was recorded in both seasons, but in winter the incidence of torpor was higher (n = 205 out of 448 observations) and minimal T (b) during torpor was lower (T (b)min: 11.9°C) than in summer (n = 24 out of 674 observations; T (b)min: 26°C). In addition to cold, high air humidity emerged as a likely predictor for torpor entry. Overall, E. rupestris showed a high degree of thermoregulatory plasticity, which was mainly reflected in a variable timing of torpor entry and arousal. We conclude that E. rupestris exhibits seasonal metabolic adjustments comparable to what has been long known for many Holarctic rodents.

Circadian rhythms and depression: human psychopathology and animal models

Most organisms (including humans) developed daily rhythms in almost every aspect of their body. It is not surprising that rhythms are also related to affect in health and disease. In the present review we present data that demonstrate the evidence for significant interactions between circadian rhythms and affect from both human studies and animal models research. A number of lines of evidence obtained from human and from animal models research clearly demonstrate relationships between depression and circadian rhythms including (1) daily patterns of depression; (2) seasonal affective disorder; (3) connections between circadian clock genes and depression; (4) relationship between sleep disorders and depression; (5) the antidepressant effect of sleep deprivation; (6) the antidepressant effect of bright light exposure; and (7) the effects of antidepressant drugs on sleep and circadian rhythms. The integration of data suggests that the relationships between the circadian system and depression are well established but the underlying biology of the interactions is far from being understood. We suggest that an important factor hindering research into the underlying mechanisms is the lack of good animal models and we propose that additional efforts in that area should be made. One step in that direction could be the attempt to develop models utilizing diurnal animals which might have a better homology to humans with regard to their circadian rhythms. This article is part of a Special Issue entitled 'Anxiety and Depression'.

Antidepressants reverse short-photoperiod-induced, forced swim test depression-like behavior in the diurnal fat sand rat: further support for the utilization of diurnal rodents for modeling affective disorders

Recent findings demonstrate strong links between abnormalities in circadian rhythms and sleep and the etiology, pathophysiology and treatment of major affective disorders. Further exploration of these interactions requires the development, identification and utilization of good and predictive animal models. The biology and behavior related to circadian rhythms are significantly different in diurnal and nocturnal rodents. Accordingly, it is possible that exploring the interactions between these mechanisms and affective change in diurnal animals may be advantageous. Recent studies demonstrate that diurnal fat sand rats and Nile grass rats show depression-like behavior when maintained under short-photoperiod (SP) conditions compared with animals maintained under neutral photoperiod (NP) conditions. Moreover, these behaviors were ameliorated after treatment with bright light. The present study further explores the possible utility of sand rats as animal models by testing the effects of antidepressants on the SP-induced depression-like behaviors of sand rats. Sand rats maintained in SP or NP conditions for 3 weeks were treated subchronically (5 injections) with the clinically effective antidepressant bupropion, and their behavior was tested in a number of depression-related tests. Results show that antidepressant treatment reverses the effects of SP conditions in the forced swim test, but that neither SP conditions nor antidepressants influenced sweet solution preference. These results partly support the validity of the sand rat model, but suggest that not all tests that were validated in nocturnal laboratory rodents can be applied to other rodent species and that additional tests should be applied to further explore the validity of the model.

The substructure of the suprachiasmatic nucleus: Similarities between nocturnal and diurnal spiny mice

Evolutionary transitions between nocturnal and diurnal patterns of adaptation to the day-night cycle must have involved fundamental changes in the neural mechanisms that coordinate the daily patterning of activity, but little is known about how these mechanisms differ. One reason is that information on these systems in very closely related diurnal and nocturnal species is lacking. In this study, we characterize the suprachiasmatic nucleus (SCN), the primary brain structure involved in the generation and coordination of circadian rhythms, in two members of the genus Acomys with very different activity patterns, Acomys russatus (the golden spiny mouse, diurnal) and Acomys cahirinus (the common spiny mouse, nocturnal). Immunohistochemical techniques were used to label cell bodies containing vasoactive intestinal polypeptide (VIP), vasopressin (VP), gastrin-releasing peptide (GRP) and calbindin (CalB) in the SCN, as well as two sets of inputs to it, those containing serotonin (5-HT) and neuropeptide Y (NPY), respectively. All were present in the SCN of both species and no differences between them were seen. On the basis of neuronal phenotype, the SCN was organized into three basic regions that contained VIP-immunoreactive (-ir), CalB-ir and VP-ir cells, in the ventral, middle and dorsal SCN, respectively. In the rostral SCN, GRP-ir cells were in both the VIP and the CalB cell regions, and in the caudal area they were distributed across a portion of each of the other three regions. Fibers containing NPY-ir and serotonin (5-HT)-ir were most concentrated in the areas containing VIP-ir and CalB-ir cells, respectively. The details of the spatial relationships among the labeled cells and fibers seen here are discussed in relation to different approaches investigators have taken to characterize the SCN more generally.