Photoperiod regulates leptin sensitivity in field voles, Microtus agrestis

Pregnancy-induced mechanisms regulating central and peripheral leptin sensitivity: lessons from sheep

This review describes various aspects of the leptin resistance phenomenon and related physiological mechanisms that occur in pregnant sheep. Its main aim is to analyze the mechanisms that determine the occurrence of pregnancy-induced leptin resistance and to investigate the accompanying processes that affect the physiology of pregnancy and lactation in livestock. The main purpose of this analysis was to comprehensively understand the phenomenon of leptin resistance, including the causes of its emergence and its effects on nonrodent organisms.

Seasonal food intake and energy balance: Neuronal and non-neuronal control mechanisms

Animals inhabiting temperate and high latitudes undergo drastic seasonal changes in energy storage, facilitated by changes in food intake and body mass. Those seasonal changes in the animal's biology are not mere consequences of environmental energy availability but are anticipatory responses to the energetic requirements of the upcoming season and are actively timed by tracking the annual progression in photoperiod. In this review, we discuss how photoperiod is used to control energy balance seasonally and how this is distinct from energy homeostasis. Most notably, we suggest that photoperiodic control of food intake and body mass does not originate from the arcuate nucleus, as for homeostatic appetite control, but is rather to be found in hypothalamic tanycytes. Tanycytes are specialized ependymal cells lining the third ventricle, which can sense metabolites from the cerebrospinal fluid (e.g. glucose) and can control access of circulating signals to the brain. They are also essential in conveying time-of-year information by integrating photoperiod and altering hypothalamic thyroid metabolism, a feature that is conserved in seasonal vertebrates and connects to seasonal breeding and metabolism. We also discuss how homeostatic feedback signals are handled during times of rapid energetic transitions. Studies on leptin in seasonal mammals suggest a seasonal shift in central sensitivity and blood-brain transport, which might be facilitated by tanycytes. This article is part of the Special Issue on "Food intake and feeding states".

Models of body weight and fatness regulation

Body weight and fatness appear to be regulated phenomena. Several different theoretical models are available to capture the essence of this idea. These include the set-point, dynamic equilibrium, adiposity force, control theory-settling point, Hall-Guo, operation point and dual intervention point (DIP) models. The set-point model posits a single reference point around which levels of fat are regulated. The dynamic equilibrium model suggests that the apparent regulation of body fat around a reference point is an illusion owing to the necessary impacts of weight change on energy expenditure. Control theory focuses on the importance of feedback gain and suggests set-point and dynamic equilibrium are ends of a continuum of feedback gain. Control theory models have also been called 'settling point' models. The Hall-Guo, operation point and DIP models also bring together the set-point and dynamic equilibrium ideas into a single framework. The DIP proposes a zone of indifference where dynamic equilibrium 'regulation' predominates, bounded by upper and lower intervention points beyond which physiological mechanisms are activated. The drifty gene hypothesis is an idea explaining where this individual variation in the upper intervention point might come from. We conclude that further experiments to test between the models are sorely required. This article is part of a discussion meeting issue 'Causes of obesity: theories, conjectures and evidence (Part II)'.

Chowing down: diet considerations in rodent models of metabolic disease

Diet plays a substantial role in the etiology, progression, and treatment of chronic disease and is best considered as a multifaceted set of modifiable input variables with pleiotropic effects on a variety of biological pathways spanning multiple organ systems. This brief review discusses key issues related to the design and conduct of diet interventions in rodent models of metabolic disease and their implications for interpreting experiments. We also make specific recommendations to improve rodent diet studies to help better understand the role of diet on metabolic physiology and thereby improve our understanding of metabolic disease.

Roles of leptin on energy balance and thermoregulation in Eothenomys miletus

Leptin is a hormone mainly synthesized and secreted by white adipose tissue (WAT), which regulates various physiological processes. To investigate the role of leptin in energy balance and thermoregulation in Eothenomys miletus, voles were randomly divided into leptin-injected and PBS-injected groups and placed at 25°C ± 1°C with a photoperiod of 12 L:12 D. They were housed under laboratory conditions for 28 days and compared in terms of body mass, food intake, water intake, core body temperature, interscapular skin temperature, resting metabolic rate (RMR), nonshivering thermogenesis (NST), liver and brown adipose tissue (BAT) thermogenic activity, and serum hormone levels. The results showed that leptin injection decreased body mass, body fat, food intake, and water intake. But it had no significant effect on carcass protein. Leptin injection increased core body temperature, interscapular skin temperature, resting metabolic rate, non-shivering thermogenesis, mitochondrial protein content and cytochrome C oxidase (COX) activity in liver and brown adipose tissue, uncoupling protein 1 (UCP1) content and thyroxin 5'-deiodinase (T45'-DII) activity in brown adipose tissue significantly. Serum leptin, triiodothyronine (T3), thyrotropin-releasing hormone (TRH) and corticotropin-releasing hormone (CRH) concentrations were also increased significantly. Correlation analysis showed that serum leptin levels were positively correlated with core body temperature, body mass loss, uncoupling protein 1 content, thyroxin 5'-deiodinase activity, nonshivering thermogenesis, and negatively correlated with food intake; thyroxin 5'-deiodinase and triiodothyronine levels were positively correlated, suggesting that thyroxin 5'-deiodinase may play an important role in leptin-induced thermogenesis in brown adipose tissue. In conclusion, our study shows that exogenous leptin is involved in the regulation of energy metabolism and thermoregulation in E. miletus, and thyroid hormone may play an important role in the process of leptin regulating energy balance in E. miletus.

Circadian control of brown adipose tissue

Disruption of circadian (~24 h) rhythms is associated with an increased risk of cardiometabolic diseases. Therefore, unravelling how circadian rhythms are regulated in different metabolic tissues has become a prominent research focus. Of particular interest is brown adipose tissue (BAT), which combusts triglyceride-derived fatty acids and glucose into heat and displays a circannual and diurnal rhythm in its thermogenic activity. In this review, the genetic, neuronal and endocrine generation of these rhythms in BAT is discussed. In addition, the potential risks of disruption or attenuation of these rhythms in BAT, and possible factors influencing these rhythms, are addressed.

The evolution of body fatness: trading off disease and predation risk

Human obesity has a large genetic component, yet has many serious negative consequences. How this state of affairs has evolved has generated wide debate. The thrifty gene hypothesis was the first attempt to explain obesity as a consequence of adaptive responses to an ancient environment that in modern society become disadvantageous. The idea is that genes (or more precisely, alleles) predisposing to obesity may have been selected for by repeated exposure to famines. However, this idea has many flaws: for instance, selection of the supposed magnitude over the duration of human evolution would fix any thrifty alleles (famines kill the old and young, not the obese) and there is no evidence that hunter-gatherer populations become obese between famines. An alternative idea (called thrifty late) is that selection in famines has only happened since the agricultural revolution. However, this is inconsistent with the absence of strong signatures of selection at single nucleotide polymorphisms linked to obesity. In parallel to discussions about the origin of obesity, there has been much debate regarding the regulation of body weight. There are three basic models: the set-point, settling point and dual-intervention point models. Selection might act against low and high levels of adiposity because food unpredictability and the risk of starvation selects against low adiposity whereas the risk of predation selects against high adiposity. Although evidence for the latter is quite strong, evidence for the former is relatively weak. The release from predation ∼2-million years ago is suggested to have led to the upper intervention point drifting in evolutionary time, leading to the modern distribution of obesity: the drifty gene hypothesis. Recent critiques of the dual-intervention point/drifty gene idea are flawed and inconsistent with known aspects of energy balance physiology. Here, I present a new formulation of the dual-intervention point model. This model includes the novel suggestion that food unpredictability and starvation are insignificant factors driving fat storage, and that the main force driving up fat storage is the risk of disease and the need to survive periods of pathogen-induced anorexia. This model shows why two independent intervention points are more likely to evolve than a single set point. The molecular basis of the lower intervention point is likely based around the leptin pathway signalling. Determining the molecular basis of the upper intervention point is a crucial key target for future obesity research. A potential definitive test to separate the different models is also described.

Intracerebroventricular administration of leptin increase physical activity but has no effect on thermogenesis in cold-acclimated rats

Most small homotherms display low leptin level in response to chronic cold exposure. Cold-induced hypoleptinemia was proved to induce hyperphagia. However, it is still not clear whether hypoleptinemia regulates energy expenditure in cold condition. We try to answer this question in chronic cold-acclimated rats. Results showed that 5-day intracerebroventricular(ICV) infusion of leptin (5 μg/day) had no effects on basal and adaptive thermogenesis and uncoupling protein 1 expression. Physical activity was increased by leptin treatment. We further determined whether ghrelin could reverse the increasing effect of leptin on physical activity. Coadministration of ghrelin (1.2 μg/day) completely reversed the effect of leptin on physical activity. Collectively, this study indicated the regulation of leptin on energy expenditure during cold acclimation may be mainly mediated by physical activity but not by thermogenesis. Our study outlined behavioral role of leptin during the adaptation to cold, which adds some new knowledge to promote our understanding of cold-induced metabolic adaptation.

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.

Does leptin signal adiposity in the egg-laying mammal, Tachyglossus aculeatus?

Leptin is a peptide hormone best known for its role in feedback regulation of adiposity in eutherian mammals. Normally an increase in adipose tissue mass leads to an increase in circulating leptin which increases energy expenditure and limits food intake, but in hibernating eutherian mammals this relationship may change to allow prehibernatory fattening. The echidna (Tachyglossus aculeatus) is a monotreme mammal which accumulates significant fat reserves before entering hibernation, and mates immediately at the end of hibernation. We hypothesised that echidnas would show a strong relationship between body mass and plasma leptin for most of the year which would change during the pre-hibernatory period. We measured plasma leptin and body mass in free-ranging echidnas over several reproductive and hibernation cycles. There were significant seasonal variations in plasma leptin in both sexes, with the highest levels occurring in hibernation and in mating females. The lowest levels were found in males when they were foraging maximally after the reproductive period. We used mass%, body mass at the time of sampling as a percentage of long term mean mass, as a proxy for adiposity. There was a weak negative relationship between mass% and plasma leptin, from which we infer a weak negative relationship between adiposity and plasma leptin as has been found in reptiles and birds, rather than the strong positive relationship found in other mammals.

Behavioral and physiological effects of photoperiod-induced migratory state and leptin on a migratory bird, Zonotrichia albicollis: I. Anorectic effects of leptin administration

The hormone leptin is involved in the regulation of energy balance in mammals, mainly by reducing food intake and body adiposity and increasing energy expenditure. During energetically demanding periods, leptin's action is often altered to facilitate fat deposition and maintain high rates of food intake. Despite the present controversy over the existence of an avian leptin, there is evidence that a leptin receptor exists in birds and its activation influences energy intake and metabolism. However, it is unknown whether the effects of the activation of leptin receptor on energy balance are modulated during migration. We manipulated photoperiod to induce migratory behavior in captive white-throated sparrows (Zonotrichia albicollis) and injected migratory and wintering sparrows with either murine leptin or PBS for 7 days. We measured food intake, changes in body composition and foraging behavior to test if leptin's effects are altered during migratory state. Leptin decreased foraging behavior, food intake and fat mass in wintering sparrows, but had no effect on foraging behavior or food intake in migratory sparrows. Migratory sparrows injected with leptin maintained fat better than sparrows injected with PBS. Thus, sparrows' responses to leptin changed with migratory state, possibly to aid in the increase and maintenance of rates of food intake and fat deposition. We also found that long-form leptin receptor and SOCS3 were expressed in tissues of sparrows, including the hypothalamus, but their expression did not change with migratory state. Further study of the leptin receptor system and other regulators of energy balance in migratory birds will increase our understanding of the physiological mechanisms that are responsible for their ability to complete energetically demanding journeys.

Hypothalamic neuropeptides, not leptin sensitivity, contributes to the hyperphagia in lactating Brandt's voles, Lasiopodomys brandtii

Both pregnancy and lactation are associated with hyperphagia, and circulating leptin levels are elevated during pregnancy but decreased during lactation in Brandt's voles, Lasiopodomys brandtii. Previous findings suggest that impaired leptin sensitivity contributes to hyperphagia during pregnancy. The present study aimed to examine whether the decreased circulating leptin level and/or hypothalamic leptin sensitivity contributed to the hyperphagia during lactation in Brandt's voles. The serum leptin level and mRNA expression of the long form of the leptin receptor (Ob-Rb), suppressor-of-cytokine-signalling-3 (SOCS-3), neuropeptide Y (NPY), agouti-related protein (AgRP), pro-opiomelanocortin (POMC) and cocaine- and amphetamine-regulated transcript (CART) in the hypothalamus were examined on dioestrous, day 5, day 17 of lactation and day 27 (1 week after weaning) in Brandt's voles. Compared with controls, hypothalamic Ob-Rb and SOCS-3 mRNA expression was not significantly changed during lactation. The serum leptin level was significantly lower in lactating females than in the non-reproductive group. Hypothalamic NPY and AgRP mRNA expression significantly increased whereas POMC mRNA expression was significantly decreased during lactation compared with controls. However, there were no significant changes in hypothalamic CART mRNA expression. Food intake was positively correlated with NPY and AgRP mRNA expression but negatively correlated with POMC mRNA expression during lactation. These data suggest that hyperphagia during lactation was associated with low leptin levels, but not impaired leptin sensitivity, and that the hypothalamic neuropeptides NPY, AgRP and POMC are involved in mediating the role of leptin in food intake regulation in lactating Brandt's voles.

Physiological and behavioral responses to intermittent starvation in C57BL/6J mice

The dual intervention point model states that body mass is controlled by upper and lower intervention points, above and below which animals (and humans) intervene physiologically to bring their body mass back into the acceptable range. It has been further suggested that the lower intervention point may be defined by the risk of starvation, while the upper intervention point may be defined by the risk of predation. The objective of the present study was to test whether the risk of starvation determines the lower intervention point and to examine the physiological and behavioral mechanisms that underpin the regulation of body mass, when the risk of starvation is increased. Sixty-four mice were exposed to random days of complete fasting or 50% food restriction and their body mass and fat mass responses were measured. Food intake, physical activity and body temperature were measured throughout the experiment. In addition, plasma leptin and insulin, triglyceride and non-esterified fatty acids, along with hypothalamic neuropeptides gene expression in the arcuate nucleus were assessed after 13 and 42 days of treatment. We found that C57BL/6J mice increased body mass and fatness in response to a short-term (13 days) intermittent fasting, which was restored to baseline as the treatment was prolonged. In contrast, intermittently 50% food restricted mice showed no significant changes in body mass or fatness. Over the first 13 days of treatment the data were consistent with the dual intervention point model as the mice showed both increased body mass and adiposity over this period. Over the more protracted period of 42 days the effect waned and was therefore inconsistent with the model. The body mass and fat mass gains in intermittently fasted mice were mainly accounted for by increased food intake. Elevated NPY gene expression after 13 days (three 24 h fasting events) may have driven the increase in food intake. However, no changes were observed in such neuropeptides as POMC, CART, AgRP, Ob-Rb and SOCS 3 or circulating levels of leptin, insulin, NEFA and TG. Hypothermia during fasting days may have also contributed to the increase in body mass. Over 42 days of treatment (nine 24 h fasting events) cumulative food intake was not affected by intermittent starvation. However physical activity, mainly activity during the light phase was lowered suggesting an adaptation to unpredictable starvation. Overall, mice exhibited different behavioral and physiological responses to intermittent starvation depending on the duration of treatment.

Influence of photoperiod on hormones, behavior, and immune function

Photoperiodism is the ability of plants and animals to measure environmental day length to ascertain time of year. Central to the evolution of photoperiodism in animals is the adaptive distribution of energetically challenging activities across the year to optimize reproductive fitness while balancing the energetic tradeoffs necessary for seasonally-appropriate survival strategies. The ability to accurately predict future events requires endogenous mechanisms to permit physiological anticipation of annual conditions. Day length provides a virtually noise free environmental signal to monitor and accurately predict time of the year. In mammals, melatonin provides the hormonal signal transducing day length. Duration of pineal melatonin is inversely related to day length and its secretion drives enduring changes in many physiological systems, including the HPA, HPG, and brain-gut axes, the autonomic nervous system, and the immune system. Thus, melatonin is the fulcrum mediating redistribution of energetic investment among physiological processes to maximize fitness and survival.

Effects of leptin infusion during peak lactation on food intake, body composition, litter growth, and maternal neuroendocrine status in female Brandt's voles (Lasiopodomys brandtii)

During lactation, female small mammals frequently reduce their fat reserves to very low levels. The function of this reduction is unclear, as calculations suggest that the contribution of the withdrawn energy from fat to the total energy balance of lactation is trivial. An alternative hypothesis is that reducing fat leads to a reduction in circulating adipokines, such as leptin, that play a role in stimulating the hyperphagia of lactation. We investigated the role of circulating leptin in lactation by repleting leptin levels using miniosmotic pumps during the last 7 days of lactation in Brandt's voles (Lasiopodomys brandtii), a model small wild mammal we have extensively studied in the context of lactation energy demands. Repletion of leptin resulted in a dose-dependent reduction of body mass and food intake in lactating voles. Comparisons to nonreproducing individuals suggests that the reduced leptin in lactation, due to reduced fat stores, may account for ∼16% of the lactational hyperphagia. Reduced leptin in lactation may, in part, cause lactational hyperphagia via stimulatory effects on hypothalamic orexigenic neuropeptides (neuropeptide Y and agouti-related peptide) and inhibition of the anorexigenic neuropeptide (proopiomelanocortin). These effects were reversed by the experimental repletion of leptin. There was no significant effect of leptin treatment on daily energy expenditure, milk production or pup growth, but leptin repletion did result in a reversal of the suppression of uncoupling protein-1 levels in brown adipose tissue, indicating an additional role for reducing body fat and leptin during peak lacation.

Role of hypoleptinemia during cold adaptation in Brandt's voles (Lasiopodomys brandtii)

Brandt's voles Lasiopodomys brandtii exhibit large increases in nonshivering thermogenesis to cope with chronic cold exposure, resulting in compensatory hyperphagia and fat mobilization. These physiological events are accompanied by a remarkable reduction in serum leptin levels. However, the role of hypoleptinemia in cold adaptation in this species is still unknown. In the present study, we tested the hypothesis that hypoleptinemia contributes to increases in food intake and brown adipose tissue (BAT) thermogenesis by modifying hypothalamic neuropeptides in cold-exposed Brandt's voles. Adult male voles were transferred to 5°C for 28 days. Accompanied by a decrease in serum leptin levels, hypothalamic agouti-related protein (AgRP) mRNA levels were significantly increased, but there were no changes in the long form of leptin receptor (Ob-Rb), suppressor of cytokine signaling 3 (SOCS3), neuropeptide Y (NPY) mRNA, proopiomelanocortin (POMC), and cocaine- and amphetamine-regulated peptide (CART) mRNA levels in the hypothalamus. When cold-exposed voles were returned to warm (23°C) for 28 days, body mass, food intake, serum leptin, and AgRP mRNA were restored to control levels. Leptin administration in cold-exposed voles decreased food intake as well as hypothalamic AgRP mRNA levels. There were no significant effects of leptin administration on hypothalamic Ob-Rb, SOCS3, NPY, POMC, CART mRNA, and uncoupling protein 1 levels under cold conditions. These results suggest that hypoleptinemia partially contributes to cold-induced hyperphagia, which might involve the elevation of hypothalamic AgRP gene expression.

Physiological models of leptin resistance

In common forms of obesity, animals and humans become leptin resistant associated with impaired regulation of energy homeostasis. Over the last decade, significant advances in delineating the underlying mechanisms have been achieved. As well as the obvious scientific progress obtained by novel transgenic animals, natural and physiological models of leptin resistance such as the Siberian hamster (Phodoups sungorus), the field vole (Microtus agrestis) or the rat during pregnancy have also provided invaluable insight into the dynamic long-term control of energy homeostasis. Seasonal (in the hamster) and pregnancy-induced leptin resistance are characterised by a modulation of the leptin signalling cascade downstream of its receptor in the hypothalamus. In this state, leptin-induced phosphorylation of the important transcription factor, signal transducer and activator of transcription 3 (STAT3), is impaired in the arcuate nucleus and the ventromedial hypothalamus (only during pregnancy). This is accompanied by elevated levels of leptin signalling inhibitors such as the suppressor of cytokine signalling (SOCS3) and the protein tyrosine phosphatase 1B (PTP1B). The janus kinase 2 (JAK2)-STAT3 signalling pathway might be modulated by a dual function of the tyrosine residue Tyr(985) in the intracellular domain of the leptin receptor. In seasonal animals, SOCS3, most importantly seems to act as a 'molecular switch' enabling a photoperiod-induced alteration in leptin signalling and subsequent adjustments in energy homeostasis to allow attainment of a new body weight set-point. These physiological models show that animals can exhibit leptin resistance as an adaptive response to meet new physiological or environmental challenges, promoting the survival of the species during times of increased metabolic demand. The molecular mechanisms mediating physiological and/or pathological leptin resistance, like during diet induced obesity, might be very similar involving hypothalamic SOCS3. Investigation of these models might further provide new insight into the dynamic complexity of energy homeostasis.

Leptin signaling in brain: A link between nutrition and cognition?

Leptin is a protein hormone that acts within the hypothalamus to suppress food intake and decrease body adiposity, but it is increasingly clear that the hypothalamus is not the only site of leptin action, nor food intake the only biological effect of leptin. Instead, leptin is a pleiotropic hormone that impinges on many brain areas, and in doing so alters food intake, motivation, learning, memory, cognitive function, neuroprotection, reproduction, growth, metabolism, energy expenditure, and more. This diversity of function also means that a dysregulation of leptin secretion and signaling can have far reaching effects. To date research on leptin signaling has focused primarily on the hypothalamus, and the result is a relative lack of information regarding the impact of leptin signaling and leptin resistance in non-hypothalamic areas, despite a growing literature implicating leptin in the regulation of neuronal structure and function in the hippocampus, cortex and other brain areas associated with cognition.

The contribution of animal models to the study of obesity

Obesity results from prolonged imbalance of energy intake and energy expenditure. Animal models have provided a fundamental contribution to the historical development of understanding the basic parameters that regulate the components of our energy balance. Five different types of animal model have been employed in the study of the physiological and genetic basis of obesity. The first models reflect single gene mutations that have arisen spontaneously in rodent colonies and have subsequently been characterized. The second approach is to speed up the random mutation rate artificially by treating rodents with mutagens or exposing them to radiation. The third type of models are mice and rats where a specific gene has been disrupted or over-expressed as a deliberate act. Such genetically-engineered disruptions may be generated through the entire body for the entire life (global transgenic manipulations) or restricted in both time and to certain tissue or cell types. In all these genetically-engineered scenarios, there are two types of situation that lead to insights: where a specific gene hypothesized to play a role in the regulation of energy balance is targeted, and where a gene is disrupted for a different purpose, but the consequence is an unexpected obese or lean phenotype. A fourth group of animal models concern experiments where selective breeding has been utilized to derive strains of rodents that differ in their degree of fatness. Finally, studies have been made of other species including non-human primates and dogs. In addition to studies of the physiological and genetic basis of obesity, studies of animal models have also informed us about the environmental aspects of the condition. Studies in this context include exploring the responses of animals to high fat or high fat/high sugar (Cafeteria) diets, investigations of the effects of dietary restriction on body mass and fat loss, and studies of the impact of candidate pharmaceuticals on components of energy balance. Despite all this work, there are many gaps in our understanding of how body composition and energy storage are regulated, and a continuing need for the development of pharmaceuticals to treat obesity. Accordingly, reductions in the use of animal models, while ethically desirable, will not be feasible in the short to medium term, and indeed an expansion in activity using animal models is anticipated as the epidemic continues and spreads geographically.

The brain, appetite, and obesity

Food intake and energy expenditure are controlled by complex, redundant, and distributed neural systems that reflect the fundamental biological importance of adequate nutrient supply and energy balance. Much progress has been made in identifying the various hormonal and neural mechanisms by which the brain informs itself about availability of ingested and stored nutrients and, in turn, generates behavioral, autonomic, and endocrine output. While hypothalamus and caudal brainstem play crucial roles in this homeostatic function, areas in the cortex and limbic system are important for processing information regarding prior experience with food, reward, and emotion, as well as social and environmental context. Most vertebrates can store a considerable amount of energy as fat for later use, and this ability has now become one of the major health risks for many human populations. The predisposition to develop obesity can theoretically result from any pathological malfunction or lack of adaptation to changing environments of this highly complex system.

Leptin resistance and the response to positive energy balance

Animals readily reduce food intake and normalize body weight following a period of involuntary overfeeding, suggesting that regulatory systems are engaged to defend against excess weight gain. However, these data exist in the background of an ongoing obesity epidemic, where the ready availability of palatable, energy dense foods often leads to obesity. Currently we know very little about the mechanisms underlying the normalization of body weight following involuntary overfeeding, nor do we fully understand why select individuals successfully remain lean despite living in an obesigenic environment. Recent progress in the study of leptin signaling indicates that manipulations which enhance leptin sensitivity reduce food intake and attenuate diet-induced obesity, while reductions in leptin signaling predispose to obesity. While it remains unclear whether a failure or insufficiency in the weight regulatory system contributes to obesity, this work highlights the importance of this system for the regulation of body weight and its potential value for the treatment of obesity. Nonetheless, it is necessary to more clearly identify those mechanisms that protect lean individuals from weight gain and mediate the normalization of body weight that follows involuntary overfeeding, because it is only with this knowledge that we can clearly determine whether obesity is dependent on, or independent of, a failure in the weight regulatory system.

Seasonal changes in thermogenesis and body mass in wild Mongolian gerbils (Meriones unguiculatus)

Seasonal adjustments in body mass (BM), nonshivering thermogenesis (NST) and several physiological, hormonal, and biochemical markers were measured in wild-trapped Mongolian gerbils (Meriones unguiculatus) from Inner Mongolia, China. Sexual differences were detected in BM, NST, brown adipose tissue (BAT) mass, and mitochondrial protein content. BM and NST in males were higher in winter (January) and spring (May) than in summer (August), and BM of females was also the highest in winter, but NST remained relatively constant throughout the year. Cytochrome c oxidase activity and mitochondrial uncoupling protein 1 (UCP1) content in BAT were enhanced in winter in males or females, respectively. Serum leptin concentration was the lowest in winter and positively correlated with BM and body fat mass but was negatively correlated with BAT UCP1 content. These data suggest that wild Mongolian gerbils do not depend on a decrease in BM, but instead increase their thermogenic capacity to cope with cold stress. Leptin may be involved in the seasonal regulation in energy balance and thermogenesis in field Mongolian gerbils.

Wheel running affects seasonal acclimatization of physiological and morphological traits in the Djungarian hamster (Phodopus sungorus)

Wheel running was previously shown to influence body mass and torpor in short-day-acclimatized Djungarian hamsters ( Phodopus sungorus). To determine whether the exercise-induced effect on body mass depends on the annual phase, hamsters were exposed to the natural change in photoperiod and given access to a running wheel (RW), either before, in the middle of, or at the end of the descending body mass trajectory during seasonal acclimatization. Due to wheel running, the seasonal weight cycle was prevented or aborted by abruptly rising body mass, resulting in a weight appropriate for summer, despite exposure to short days. Torpor was inhibited, and testicular recrudescence was advanced, compared with controls. In contrast, the change into winter fur remained unaltered. Analysis of body composition and plasma leptin revealed a low body fat mass in RW hamsters, not only in winter but also in summer, suggesting a lack of seasonal adiposity. Chronic leptin infusion in winter only decreased body mass in RW individuals, although their relative body fat mass probably was even lower than in sedentary hamsters. A constantly low body fat mass is conceivably reflecting an exercise-dependent change in metabolism, consistent with increased bone mineral content and density in RW hamsters. Additionally, bone area was increased, again supported by elongated vertebral columns. Together, the results show a striking effect of wheel running on body composition and the seasonal pattern of body mass, and they suggest that the photoperiodic regulation of body mass is regulated differently than the reproductive and pelage responses.

A nonadaptive scenario explaining the genetic predisposition to obesity: the "predation release" hypothesis

The "thrifty gene hypothesis" suggests we evolved genes for efficient food collection and fat deposition to survive periods of famine and that now that food is continuously available, these genes are disadvantageous because they make us obese in preparation for a famine that never comes. However, famines are relatively infrequent modern phenomena that involve insufficient mortality for thrifty genes to propagate. I suggest here that early hominids would have been subjected to stabilizing selection for body fatness, with obesity selected against by the risk of predation. Around two million years ago predation was removed as a significant factor by the development of social behavior, weapons, and fire. The absence of predation led to a change in the population distribution of body fatness due to random mutations and drift. Because this novel hypothesis involves random drift, rather than directed selection, it explains why, even in Western society, most people are not obese.

Altered expression of SOCS3 in the hypothalamic arcuate nucleus during seasonal body mass changes in the field vole, Microtus agrestis

We have previously shown that cold-acclimated (8 degrees C) male field voles (Microtus agrestis) transferred from short day (SD, 8 h light) to long day (LD, 16 h light) photoperiod exhibit an increase in body mass lasting 4 weeks, after which they stabilise at a new plateau approximately 7.5 g (24.8%) higher than animals maintained in SD. By infusing voles with exogenous leptin, we have also demonstrated that SD voles respond to the hormone by reducing body mass and food intake, whereas LD animals increasing body mass are resistant to leptin treatment. In the present study, we investigated whether seasonal changes in body mass could be linked to modulation of the leptin signal by suppressor of cytokine signalling-3 (SOCS3). We used in situ hybridisation to examine hypothalamic arcuate nucleus (ARC) expression of SOCS3, neuropeptide Y (NPY), agouti-related peptide (AgRP), pro-opiomelanocortin (POMC) and cocaine- and amphetamine-regulated transcript (CART) genes in 90 voles exposed to either SD or LD for up to 11 weeks. LD voles increasing body mass had significantly higher levels of SOCS3 mRNA than SD or LD voles with a stable body mass. There were no associated changes in expression of NPY, AgRP, POMC and CART genes. These results suggest that voles that regulate body mass at either the lower (SD) or upper (LD) plateau remain sensitive to leptin action, whereas SOCS3-mediated leptin resistance is a short-term mechanism that enables animals to move between the stable body mass plateaus. Our data provide evidence that expression of SOCS3 in the ARC is involved in the modulation of the strength of the leptin signal to facilitate seasonal cycles in body mass and adiposity.

Interactions between the "cognitive" and "metabolic" brain in the control of food intake

If the new environment and modern lifestyle cause obesity in individuals with thrifty genes by increasing energy intake, it is important to know by what mechanisms hyperphagia occurs and why energy balance is not kept in check by the homeostatic regulator. The argument is developed that procuring and ingesting food is an evolutionarily conserved survival mechanism that occupies large parts of the brain's computing capacity including not only the hypothalamus but also a number of cortico-limbic structures. These forebrain systems evolved to engage powerful emotions for guaranteed supply and ingestion of beneficial foods from a sparse and often hostile environment. They are now simply overwhelmed with an abundance of food and food cues that is no longer interrupted by frequent famines. After briefly reviewing structure and functions of the relevant cortico-limbic structures and the better-known hypothalamic homeostatic regulator, the review focuses mainly on interactions between the two systems. Although several cortico-limbic processes are sensitive to metabolic depletion and repletion signals, it appears that they are underlying the same reversible leptin resistance that renders hypothalamic circuits insensible to continuously high leptin levels during periods of feast. It is hypothesized that this naturally occurring leptin resistance allowed temporary neutralization of satiety mechanisms and evolved as a response to survive subsequent periods of famine. With today's continuous and abundant food availability for a segment of the population, the powerful cognitive processes to eat and the resulting overweight can partially escape negative feedback control in prone individuals most strongly expressing such thrifty genes.

Photoperiodic regulation in energy intake, thermogenesis and body mass in root voles (Microtus oeconomus)

The present study was designed to examine whether photoperiod alone was effective to induce seasonal regulations in physiology in root voles (Microtus oeconomus) from the Qinghai-Tibetan plateau noted for its extreme cold environment. Root voles were randomly assigned into either long photoperiod (LD; 16L:8D) or short photoperiod (SD; 8L:16D) for 4 weeks at constant temperature (20 degrees C). At the end of acclimation, SD voles showed lower body mass and body fat coupled with higher energy intake than LD voles. SD greatly enhanced thermogenic capacities in root voles, as indicated by elevated basal metabolic rate (BMR), nonshivering thermogenesis (NST), mitochondrial protein content and uncoupling protein-1 (UCP1) content in brown adipose tissue (BAT). Although no variations in serum leptin levels were found between SD and LD voles, serum leptin levels were positively correlated with body mass and body fat mass, and negatively correlated with energy intake and UCP1 content in BAT, respectively. To summarize, SD alone is effective in inducing higher thermogenic capacities and energy intake coupled with lower body mass and body fat mass in root voles. Leptin is potentially involved in the photoperiod induced body mass regulation and thermogenesis in root voles.