Diet-induced obesity in the short-day-lean Brandt's vole

Disentangling the effects of obesity and high-fat diet on glucose homeostasis using a photoperiod induced obesity model implicates ectopic fat deposition as a key factor

OBJECTIVE

Obesity in laboratory rodents is generally induced by feeding them a high fat diet (HFD). This model does not permit separation of the impact of the HFD from the resultant obesity on metabolic defects such as impaired glucose homeostasis. In Brandt's voles we have previously shown that exposure to long photoperiod (LD: 16L: 8D) induces obesity even when they are fed a low fat diet. We show here that these voles are largely resistant to HFD. This model therefore permits some separation of the effects of HFD and obesity on glucose homeostasis. The objective was therefore to use this model to assess if glucose homeostasis is more related to diet or obesity METHODS: Male voles, which were 35 days old and born in LD, were exposed to SD and a low fat diet for 70 days. We then randomly separated the animals into 4 groups for another 63 days: SL (short day and low fat diet: n = 19) group; SH (short day and high-fat diet, n = 20) group; LL (long day and low-fat diet, n = 20) group; LH (long day and high-fat diet, n = 18) group. Glucose tolerance tests (GTT) were performed after treatment for 56 days, and body compositions of the voles were quantified at the end by dissection.

RESULTS

Consistent with our previous work LD voles were more obese than SD voles. Although total body weight was independent of dietary fat content, HFD did have an effect on fat storage. Photoperiod induced obesity had no effect on glucose homeostasis, and the fat content in both the liver and muscle. In contrast, HFD induced adiposity was linked with elevated fat deposition in muscle (but not in liver) and led to impaired glucose tolerance.

CONCLUSIONS

The contrasting effects of diet and photoperiod were consistent with the predictions of the 'lipotoxicity hypothesis'. This may contribute to our understanding of why some human individuals are able to be obese yet remain metabolically healthy.

Thermo-TRP channels are involved in BAT thermoregulation in cold-acclimated Brandt's voles

Transient receptor potential (TRP) channels, which can sense temperature, pressure and mechanical stimuli, were involved in many physiological and biochemical reactions. Whether thermosensitive TRP channels (Thermo-TRPs) are involved in thermoregulation in small mammals is still not clear. We measured the changes of thermo-TRPs at 4 °C, 23 °C and 30 °C in Brandt's voles (Lasiopodomys brandtii) to test the hypothesis that Thermo-TRPs are involved in cold-induced thermogenesis of brown adipose tissue (BAT) in small mammals. Results showed that air temperatures had no effect on body mass and rectal temperature, but the food intake and basal metabolic rate (BMR) in the 4 °C group were significantly higher than in the 30 °C group. Compared with 30 °C group, the protein contents of uncoupling protein 1(UCP1), TRP vanilloid 2 (TRPV2), TRP ankyrin 1 (TRPA1), TRP melastatin 2 (TRPM2), silent Information Regulator T1 (SIRT1), AMP-activated protein kinase (AMPK) and Calcium/calmodulin-dependent protein kinase II (CaMKII) in BAT increased significantly in 4 °C group, but there was no significant difference in the protein content of Thermo-TRPs in the hypothalamus among groups. Further, the expression of PRDM16 (PR domain containing 16) in inguinal white adipose tissue (iWAT) at 4 °C was significantly higher than that at 30 °C, but no difference was observed in the expression of other browning-related genes or TRPV2. In conclusion, TRP channels may participate in BAT thermoregulation through the CaMKII, AMPK, SIRT1 and UCP1 pathway in cold-acclimated Brandt's voles.

Diversity of Thermogenic Capacity Predicts Divergent Obesity Susceptibility in a Wild Rodent

OBJECTIVE

The objective of the present study was to examine whether wild rodents exhibit diverse obesity susceptibility and what factors predispose subjects to this divergence in response to a high-fat diet (HFD).

METHODS

Sixty male and female Brandt's voles (Lasiopodomys brandtii) were fed an HFD for 8 weeks, and the upper (obesity prone [OP]) and lower (obesity resistant [OR]) one-third for mass gain were selected. Energy budgets and pathologic changes were measured. Another 30 males were fed a low-fat control diet (LFD) for 10 weeks and then fed an HFD for 12 weeks. The energetic parameters of the rodents on an LFD were analyzed for the correlation with body mass of the rodents on an HFD.

RESULTS

OP voles had higher energy intakes, higher levels of noradrenaline-induced nonshivering thermogenesis, and a greater impairment of insulin tolerance than OR voles. Unlike laboratory rodents, there were no differences in physical activity or resting metabolic rate between these groups of voles. The thermogenic capacity during LFD feeding was the strongest predictor for mass gain during HFD feeding.

CONCLUSIONS

This study suggests that a wild rodent species of Brandt's voles exhibits diverse obesity susceptibility in reaction to an HFD, providing a natural model to give insight into the mechanisms for divergent obesity susceptibility. This study also indicates that maximum thermogenic capacity has a predictive power for the development of obesity when an HFD was available.

The role of leptin in striped hamsters subjected to food restriction and refeeding

Food restriction (FR) and refeeding (Re) have been suggested to impair body mass regulation and thereby making it easier to regain the lost weight and develop over-weight when FR ends. However, it is unclear if this is the case in small mammals showing seasonal forging behaviors. In the present study, energy budget, body fat and serum leptin level were measured in striped hamsters that were exposed to FR-Re. The effects of leptin on food intake, body fat and genes expressions of several hypothalamus neuropeptides were determined. Body mass, fat content and serum leptin level decreased during FR and then increased during Re. Leptin supplement significantly attenuated the increase in food intake during Re, decreased genes expressions of neuropepetide Y (NPY) and agouti-related protein (AgRP) of hypothalamus and leptin of white adipose tissue (WAT). Hormone-sensitive lipase (HSL) gene expression of WAT increased in leptin-treated hamsters that were fed ad libitum, but decreased in FR-Re hamsters. This indicates that the adaptive regulation of WAT HSL gene expression may be involved in the mobilization of fat storage during Re, which partly contributes to the resistance to FR-Re-induced overweight. Leptin may be involved in the down regulations of hypothalamus orexigenic peptides gene expression and consequently plays a crucial role in controlling food intake when FR ends.

Circannual changes in stress and feeding hormones and their effect on food-seeking behaviors

Seasonal fluctuations in food availability show a tight association with seasonal variations in body weight and food intake. Seasonal variations in food intake, energy storage, and expenditure appear to be a widespread phenomenon suggesting they may have evolved in anticipation for changing environmental demands. These cycles appear to be driven by changes in external daylength acting on neuroendocrine pathways. A number of neuroendocrine pathways, two of which are the endocrine mechanisms underlying feeding and stress, appear to show seasonal changes in both their circulating levels and reactivity. As such, variation in the level or reactivity to these hormones may be crucial factors in the control of seasonal variations in food-seeking behaviors. The present review examines the relationship between feeding behavior and seasonal changes in circulating hormones. We hypothesize that seasonal changes in circulating levels of glucocorticoids and the feeding-related hormones ghrelin and leptin contribute to seasonal fluctuations in feeding-related behaviors. This review will focus on the seasonal circulating levels of these hormones as well as sensitivity to these hormones in the modulation of food-seeking behaviors.

Limits to sustained energy intake. XVIII. Energy intake and reproductive output during lactation in Swiss mice raising small litters

Limits to sustained energy intake (SusEI) during lactation in Swiss mice have been suggested to reflect the secretory capacity of the mammary glands. However, an alternative explanation is that milk production and food intake are regulated to match the limited growth capacity of the offspring. In the present study, female Swiss mice were experimentally manipulated in two ways – litter sizes were adjusted to be between 1 and 9 pups and mice were exposed to either warm (21°C) or cold (5°C) conditions from day 10 of lactation. Energy intake, number of pups and litter mass, milk energy output (MEO), thermogenesis, mass of the mammary glands and brown adipose tissue cytochrome c oxidase activity of the mothers were measured. At 21 and 5°C, pup mass at weaning was almost independent of litter size. Positive correlations were observed between the number of pups, litter mass, asymptotic food intake and MEO. These data were consistent with the suggestion that in small litters, pup requirements may be the major factor limiting milk production. Pups raised at 5°C had significantly lower body masses than those raised at 21°C. This was despite the fact that milk production and energy intake at the same litter sizes were both substantially higher in females raising pups at 5°C. This suggests that pup growth capacity is lower in the cold, perhaps due to pups allocating ingested energy to fuel thermogenesis. Differences in observed levels of milk production under different conditions may then reflect a complex interplay between factors limiting maternal performance (peripheral limitation and heat dissipation: generally better when it is cooler) and factors influencing maximum pup growth (litter size and temperature: generally better when it is hotter), and may together result in an optimal temperature favouring reproduction.

Uncoupling protein 1 expression and high-fat diets

Uncoupling protein 1 (Ucp1) is the key component of β-adrenergically controlled nonshivering thermogenesis in brown adipocytes. This process combusts stored and nutrient energy as heat. Cold exposure not only activates Ucp1-mediated thermogenesis to maintain normothermia but also results in adaptive thermogenesis, i.e., the recruitment of thermogenic capacity in brown adipose tissue. As a hallmark of adaptive thermogenesis, Ucp1 synthesis is increased proportionally to temperature and duration of exposure. Beyond this classical thermoregulatory function, it has been suggested that Ucp1-mediated thermogenesis can also be employed for metabolic thermogenesis to prevent the development of obesity. Accordingly, in times of excess caloric intake, one may expect a positive regulation of Ucp1. The general impression from an overview of the present literature is, indeed, an increased brown adipose tissue Ucp1 mRNA and protein content after feeding a high-fat diet (HFD) to mice and rats. The reported increases are very variable in magnitude, and the effect size seems to be independent of dietary fat content and duration of the feeding trial. In white adipose tissue depots Ucp1 mRNA is generally downregulated by HFD, indicating a decline in the number of interspersed brown adipocytes.