Seasonal neuroendocrine rhythms in the male Siberian hamster persist after monosodium glutamate-induced lesions of the arcuate nucleus in the neonatal period

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".

Photoperiodic Remodeling of Adiposity and Energy Metabolism in Non-Human Mammals

Energy homeostasis and metabolism in mammals are strongly influenced by seasonal changes. Variations in photoperiod patterns drive adaptations in body weight and adiposity, reflecting changes in the regulation of food intake and energy expenditure. Humans also show distinct patterns of energy balance depending on the season, being more susceptible to gaining weight during a specific time of the year. Changes in body weight are mainly reflected by the adipose tissue, which is a key metabolic tissue and is highly affected by circannual rhythms. Mostly, in summer-like (long-active) photoperiod, adipocytes adopt a rather anabolic profile, more predisposed to store energy, while food intake increases and energy expenditure is reduced. These metabolic adaptations involve molecular modifications, some of which have been studied during the last years and are summarized in this review. In addition, there is a bidirectional relation between obesity and the seasonal responses, with obesity disrupting some of the seasonal responses observed in healthy mammals, and altered seasonality being highly associated with increased risk of developing obesity. This suggests that changes in photoperiod produce important metabolic alterations in healthy organisms. Biological rhythms impact the regulation of metabolism to different extents, some of which are already known, but further research is needed to fully understand the relationship between energy balance and seasonality.

Human torpor: translating insights from nature into manned deep space expedition

During a long-duration manned spaceflight mission, such as flying to Mars and beyond, all crew members will spend a long period in an independent spacecraft with closed-loop bioregenerative life-support systems. Saving resources and reducing medical risks, particularly in mental heath, are key technology gaps hampering human expedition into deep space. In the 1960s, several scientists proposed that an induced state of suppressed metabolism in humans, which mimics 'hibernation', could be an ideal solution to cope with many issues during spaceflight. In recent years, with the introduction of specific methods, it is becoming more feasible to induce an artificial hibernation-like state (synthetic torpor) in non-hibernating species. Natural torpor is a fascinating, yet enigmatic, physiological process in which metabolic rate (MR), body core temperature (T_b ) and behavioural activity are reduced to save energy during harsh seasonal conditions. It employs a complex central neural network to orchestrate a homeostatic state of hypometabolism, hypothermia and hypoactivity in response to environmental challenges. The anatomical and functional connections within the central nervous system (CNS) lie at the heart of controlling synthetic torpor. Although progress has been made, the precise mechanisms underlying the active regulation of the torpor-arousal transition, and their profound influence on neural function and behaviour, which are critical concerns for safe and reversible human torpor, remain poorly understood. In this review, we place particular emphasis on elaborating the central nervous mechanism orchestrating the torpor-arousal transition in both non-flying hibernating mammals and non-hibernating species, and aim to provide translational insights into long-duration manned spaceflight. In addition, identifying difficulties and challenges ahead will underscore important concerns in engineering synthetic torpor in humans. We believe that synthetic torpor may not be the only option for manned long-duration spaceflight, but it is the most achievable solution in the foreseeable future. Translating the available knowledge from natural torpor research will not only benefit manned spaceflight, but also many clinical settings attempting to manipulate energy metabolism and neurobehavioural functions.

Role of hypothalamic tanycytes in nutrient sensing and energy balance

Animal models are valuable for the study of complex behaviours and physiology such as the control of appetite because genetic, pharmacological and surgical approaches allow the investigation of underlying mechanisms. However, the majority of such studies are carried out in just two species, laboratory mice and rats. These conventional laboratory species have been intensely selected for high growth rate and fecundity, and have a high metabolic rate and short lifespan. These aspects limit their translational relevance for human appetite control. This review will consider the value of studies carried out in a seasonal species, the Siberian hamster, which shows natural photoperiod-regulated annual cycles in appetite, growth and fattening. Such studies reveal that this long-term control is not simply an adjustment of the known hypothalamic neuronal systems that control hunger and satiety in the short term. Long-term cyclicity is probably driven by hypothalamic tanycytes, glial cells that line the ventricular walls of the hypothalamus. These unique cells sense nutrients and metabolic hormones, integrate seasonal signals and effect plasticity of surrounding neural circuits through their function as a stem cell niche in the adult. Studies of glial cell function in the hypothalamus offer new potential for identifying central targets for appetite and body weight control amenable to dietary or pharmacological manipulation.

Ultradian rhythms in mammalian physiology and behavior

Diverse mammalian ultradian rhythms (URs) with periods in the 1-6h range, are omnipresent at multiple levels of biological organization and of functional and adaptive significance. Specification of neuroendocrine substrates that generate URs remains elusive. The suprachiasmatic (SCN) and arcuate (ARC) nuclei of the rodent hypothalamus subserve several behavioral URs. Recently, in a major advance, dopaminergic signaling in striatal circuitry, likely at D2 receptors, has been implicated in behavioral and thermoregulatory URs of mice. We propose a neural network in which reciprocal communication among the SCN, the ARC and striatal dopaminergic circuitry modulates the period and waveform of behavioral and physiological URs.

Hypothalamic Agouti-Related Peptide mRNA is Elevated During Natural and Stress-Induced Anorexia

As part of their natural lives, animals can undergo periods of voluntarily reduced food intake and body weight (i.e. animal anorexias) that are beneficial for survival or breeding, such as during territorial behaviour, hibernation, migration and incubation of eggs. For incubation, a change in the defended level of body weight or 'sliding set point' appears to be involved, although the neural mechanisms reponsible for this are unknown. We investigated how neuropeptide gene expression in the arcuate nucleus of the domestic chicken responded to a 60-70% voluntary reduction in food intake measured both after incubation and after an environmental stressor involving transfer to unfamiliar housing. We hypothesised that gene expression would not change in these circumstances because the reduced food intake and body weight represented a defended level in birds with free access to food. Unexpectedly, we observed increased gene expression of the orexigenic peptide agouti-related peptide (AgRP) in both incubating and transferred animals compared to controls. Also pro-opiomelanocortin (POMC) mRNA was higher in incubating hens and significantly increased 6 days after exposure to the stressor. Conversely expression of neuropeptide Y and cocaine- and amphetamine-regulated transcript gene was unchanged in both experimental situations. We conclude that AgRP expression remains sensitive to the level of energy stores during natural anorexias, which is of adaptive advantage, although its normal orexigenic effects are over-ridden by inhibitory signals. In the case of stress-induced anorexia, increased POMC may contribute to this inhibitory role, whereas, for incubation, reduced feeding may also be associated with increased expression in the hypothalamus of the anorexigenic peptide vasoactive intestinal peptide.

Seasonal regulation of structural plasticity and neurogenesis in the adult mammalian brain: focus on the sheep hypothalamus

To cope with variations in the environment, most mammalian species exhibit seasonal cycles in physiology and behaviour. Seasonal plasticity during the lifetime contributes to seasonal physiology. Over the years, our ideas regarding adult brain plasticity and, more specifically, hypothalamic plasticity have greatly evolved. Along with the two main neurogenic regions, namely the hippocampal subgranular and lateral ventricle subventricular zones, the hypothalamus, which is the central homeostatic regulator of numerous physiological functions that comprise sexual behaviours, feeding and metabolism, also hosts neurogenic niches. Both endogenous and exogenous factors, including the photoperiod, modulate the hypothalamic neurogenic capacities. The present review describes the effects of season on adult morphological plasticity and neurogenesis in seasonal species, for which the photoperiod is a master environmental cue for the successful programming of seasonal functions. In addition, the potential functional significance of adult neurogenesis in the mediation of the seasonal control of reproduction and feeding is discussed.

Hypothalamic control of seasonal changes in food intake and body weight

Seasonal cycles of fattening and body weight reflecting changes in both food intake and energy expenditure are a core aspect of the biology of mammals that have evolved in temperate and arctic latitudes. Identifying the neuroendocrine mechanisms that underlie these cycles has provided new insights into the hypothalamic control of appetite and fuel oxidation. Surprisingly, seasonal cycles do not result from changes in the leptin-responsive and homeostatic pathways located in the mediobasal and lateral hypothalamus that regulate meal timing and compensatory responses to starvation or caloric restriction. Rather, they result from changes in tanycyte function, which locally regulates transport and metabolism of thyroid hormone and retinoic acid. These signals are crucial for the initial development of the brain, so it is hypothesized that seasonal neuroendocrine cycles reflect developmental mechanisms in the adult hypothalamus, manifest as changes in neurogenesis and plasticity of connections.

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.

On the value of seasonal mammals for identifying mechanisms underlying the control of food intake and body weight

This article is part of a Special Issue "Energy Balance". Seasonal cycles of adiposity and body weight reflecting changes in both food intake and energy expenditure are the norm in mammals that have evolved in temperate and polar habitats. Innate circannual rhythmicity and direct responses to the annual change in photoperiod combine to ensure that behavior and energy metabolism are regulated in anticipation of altered energetic demands such as the energetically costly processes of hibernation, migration, and lactation. In the last decade, major progress has been made into identifying the central mechanisms that underlie these profound long-term changes in behavior and physiology. Surprisingly they are distinct from the peptidergic and aminergic systems in the hypothalamus that have been identified in studies of the laboratory mouse and rat and implicated in timing meal intervals and in short-term responses to caloric restriction. Comparative studies across rodents, ungulates and birds reveal that tanycytes embedded in the ependymal layer of the third ventricle play a critical role in seasonal changes because they regulate the local availability of thyroid hormone. Understanding how this altered hormonal environment might regulate neurogenesis and plasticity in the hypothalamus should provide new insight into development of strategies to manage appetite and body weight.

The leptin receptor is expressed in the Leydig cells of Merino rams, but its expression is not affected by long-term differences in the plane of nutrition

We studied whether the long-term effects of nutrition on ram reproductive potential are mediated by changes in the expression and distribution of leptin receptor (Ob-Rb) in the testes. Thirty-two young Merino rams were allocated to a 2 × 2 factorial design, with either natural (NP) or improved pastures (IP), with (+S) or without (–S) supplementation. After 14 weeks of treatment, a blood sample was taken to measure leptin concentrations and rams were castrated. The weights of testicles (TW) and epididymides (EW) were recorded before tissue sampling for the detection of Ob-Rb expression by immunohistochemistry. TW was positively affected by the interaction IP × S, but not EW and bodyweight (BW), which were affected either by IP or S (P < 0.05). TW and EW were greatest (g) in IP+S rams (290.6 ± 40.1 and 57.6 ± 2.3), followed by IP–S (284.6 ± 61.4 and 52.5 ± 2.3), NP+S (282.6 ± 37.9 and 55.8 ± 2.3), and NP–S rams (208.5 ± 48.7 and 44.4 ± 2.3; P < 0.01). Expression of Ob-Rb was observed in the cytoplasm of Leydig cells, without any effect of treatments on the percentage of positive Leydig cells or on staining-intensity score. The serum leptin concentrations were similar among groups. The absolute volume of the seminiferous epithelium (mL) was affected by IP and S, as well as by their interaction (IP–S: 179.7 ± 13.1, IP+S; 174.9 ± 9.0; NP+S: 178.0 ± 10.2; NP–S: 120.8 ± 9.8; P < 0.05). We conclude that, under the conditions of the present study, the long-term improvement of nutrition promoted histological changes of increased spermatogenesis of Merino rams, but was not related to changes in the expression of the Ob-Rb receptor in the Leydig cell cytoplasm.

When do we eat? Ingestive behavior, survival, and reproductive success

The neuroendocrinology of ingestive behavior is a topic central to human health, particularly in light of the prevalence of obesity, eating disorders, and diabetes. The study of food intake in laboratory rats and mice has yielded some useful hypotheses, but there are still many gaps in our knowledge. Ingestive behavior is more complex than the consummatory act of eating, and decisions about when and how much to eat usually take place in the context of potential mating partners, competitors, predators, and environmental fluctuations that are not present in the laboratory. We emphasize appetitive behaviors, actions that bring animals in contact with a goal object, precede consummatory behaviors, and provide a window into motivation. Appetitive ingestive behaviors are under the control of neural circuits and neuropeptide systems that control appetitive sex behaviors and differ from those that control consummatory ingestive behaviors. Decreases in the availability of oxidizable metabolic fuels enhance the stimulatory effects of peripheral hormones on appetitive ingestive behavior and the inhibitory effects on appetitive sex behavior, putting a new twist on the notion of leptin, insulin, and ghrelin "resistance." The ratio of hormone concentrations to the availability of oxidizable metabolic fuels may generate a critical signal that schedules conflicting behaviors, e.g., mate searching vs. foraging, food hoarding vs. courtship, and fat accumulation vs. parental care. In species representing every vertebrate taxa and even in some invertebrates, many putative "satiety" or "hunger" hormones function to schedule ingestive behavior in order to optimize reproductive success in environments where energy availability fluctuates.

Neonatal monosodium glutamate treatment counteracts circadian arrhythmicity induced by phase shifts of the light-dark cycle in female and male Siberian hamsters

Studies of rats and voles suggest that distinct pathways emanating from the anterior hypothalamic-retrochiasmatic area and the mediobasal hypothalamic arcuate nucleus independently generate ultradian rhythms (URs) in hormone secretion and behavior. We evaluated the hypothesis that destruction of arcuate nucleus (ARC) neurons, in concert with dampening of suprachiasmatic nucleus (SCN) circadian rhythmicity, would compromize the generation of ultradian rhythms (URs) of locomotor activity. Siberian hamsters retain-->of both sexes treated neonatally with monosodium glutamate (MSG) that destroys ARC neurons were subjected in adulthood to a circadian disrupting phase-shift protocol (DPS) that produces SCN arrhythmia. MSG treatments induced hypogonadism and obesity, retain-->and markedly reduced the size of the optic chiasm and optic nerves. MSG-treated hamsters exhibited normal entrainment to the light-dark cycle, but MSG treatretain-->ment counteracted the circadian arrhythmicity induced by the DPS protocol: only 6% of retain-->MSG-treated hamsters exhibited circadian arrhythmia, whereas 50% of control hamsters were circadian disrupted. In MSG-treated hamsters that retained circadian rhythmicity after DPS treatment, quantitative parameters of URs appeared normal, but in the two MSG-treated hamsters that became circadian arrhythmic after DPS, both dark-phase and light-phase URs were abolished. Although preliminary, these data are consistent with reports in voles suggesting that the combined disruption of SCN and ARC function impairs the expression of behavioral URs. The data also suggest that light thresholds for entrainment of circadian rhythms may be lower than those required to disrupt circadian organization.

Quercetin ameliorates glucose and lipid metabolism and improves antioxidant status in postnatally monosodium glutamate-induced metabolic alterations

We reported the effects of quercetin on metabolic and hormonal profile as well as serum antioxidant activities in a model of MSG (monosodium glutamate)-induced obesity. Rats were divided into 4 groups: MSG group, submitted to neonatal treatment with high doses of MSG, administrated subcutaneously during 10 days, from 2 day-old; control groups, which received the same volume of saline. After completing 30 day-old, these groups were subdivided into 4 groups: control and MSG groups treated and non-treated with quercetin at doses of 75 mg/kg body weight (i.p.) over 42 days. BW gain and food consumption were higher in MSG treated rats and quercetin significantly reduced BW by 25%. While MSG increased triacylglycerol, total cholesterol and fractions, and reduced HDL concentrations, administration of quercetin normalized HDL-cholesterol and reduced others lipids. Insulin, leptin, glucose and creatinine levels were raised in MSG-treated rats and reduced after quercetin treatment. Alanine transaminase, aspartate transaminase, lactate dehydrogenase and alkaline phosphatase activities were lower after MSG-quercetin combination compared to rats given only MSG. MSG-quercetin combination augmented total protein and urea levels as well as glutathione peroxidase and superoxide dismutase activities in contrast to MSG-treated animals. Quercetin normalized serum lipid and glucose profile and minimized the MSG-related toxic effects, which was associated to its antioxidant properties.

The role of hypothalamic tri-iodothyronine availability in seasonal regulation of energy balance and body weight

Seasonal cycles of body weight provide a natural model system to understand the central control of energy balance. Studies of such cycles in Siberian hamsters suggest that a change in the hypothalamic availability of thyroid hormone is the key determinant of annual weight regulation. Uptake of thyroid hormone into the hypothalamus from the peripheral circulation occurs largely through a specific monocarboxylate transporter expressed by tanycyte cells lining the third ventricle. Tanycytes are the principal brain cell type expressing type II and type III deiodinases, so they control the local concentrations of T4, T3, and inactive metabolites. Type III deiodinase mRNA in tanycytes is photoperiodically upregulated in short photoperiod. This would be expected to reduce the availability of T3 in the hypothalamus by promoting the production of inactive metabolites such as rT3. Experimental microimplantation of T3 directly into the hypothalamus during short-days promotes a long-day phenotype by increasing food intake and body weight without affecting the peripheral thyroid axis. Thus, thyroid hormone exerts anabolic actions within the brain that play a key role in the seasonal regulation of body weight. Understanding the precise actions of thyroid hormone in the brain may identify novel targets for long-term pharmacological manipulation of body weight.

Short-days induce weight loss in Siberian hamsters despite overexpression of the agouti-related peptide gene

Many vertebrates express profound annual cycles of body fattening, although it is not clear whether these represent differential activity of the central pathways known to mediate homeostatic control of food intake and energy expenditure, or whether the recent discovery of a major role for pars tuberalis-ependymal signalling points towards novel mechanisms. We examined this in the Siberian hamster (Phodopus sungorus) by using gene transfection to up-regulate a major orexigenic peptide, agouti-related peptide (AgRP), and then determined whether this increased anabolic drive could prevent the short-day induced winter catabolic state. Infusions of a recombinant adeno-associated virus encoding an AgRP construct into the hypothalamus of hamsters in the long-day obese phase of their seasonal cycle produced a 20% gain in body weight over 6 weeks compared to hamsters receiving a control reporter construct, reflecting a significant increase in food intake and a significant decrease in energy expenditure. However, all hamsters showed a significant, prolonged decrease in body weight when exposed to short photoperiods, despite the hamsters expressing the AgRP construct maintaining a higher food intake and lower energy expenditure relative to the control hamsters. Visualisation of the green fluorescent protein reporter and analysis of AgRP-immunoreactivity confirmed widespread expression of the construct in the hypothalamus, which was maintained for the 21-week duration of the study. In conclusion, the over-expression of AgRP in the hypothalamus produced a profoundly obese state but did not block the seasonal catabolic response, suggesting a separation of rheostatic mechanisms in seasonality from those maintaining homeostasis of energy metabolism.

Effects of photoperiod on daily locomotor activity, energy expenditure, and feeding behavior in a seasonal mammal

The objective of this study was to determine whether the previously observed effects of photoperiod on body weight in Siberian hamsters were due to changes in the daily patterns of locomotor activity, energy expenditure, and/or feeding behavior. Adult males were monitored through a seasonal cycle using an automated comprehensive laboratory animal monitoring system (CLAMS). Exposure to a short-day photoperiod (SD; 8:16-h light-dark cycle) induced a significant decline in body weight, and oxygen consumption (Vo(2)), carbon dioxide production (Vco(2)), and heat production all decreased reaching a nadir by 16 wk of SD. Clear daily rhythms in locomotor activity, Vo(2), and Vco(2) were observed at the start of the study, but these all progressively diminished after prolonged exposure to SD. Rhythms in feeding behavior were also detected initially, reflecting an increase in meal frequency but not duration during the dark phase. This rhythm was lost by 8 wk of SD exposure such that food intake was relatively constant across dark and light phases. After 18 wk in SD, hamsters were transferred to a long-day photoperiod (LD; 16:8-h light-dark cycle), which induced significant weight gain. This was associated with an increase in energy intake within 2 wk, while Vo(2), Vco(2), and heat production all increased back to basal levels. Rhythmicity was reestablished within 4 wk of reexposure to long days. These results demonstrate that photoperiod impacts on body weight via complex changes in locomotor activity, energy expenditure, and feeding behavior, with a striking loss of daily rhythms during SD exposure.

Arcuate nucleus destruction does not block food deprivation-induced increases in food foraging and hoarding

The mechanisms underlying the control of food intake are considerably better understood than those underlying the appetitive ingestive behaviors of foraging and hoarding of food, despite the prevalence of the latter across species including humans. Neuropeptide Y (NPY) and Agouti-related protein (AgRP), two orexigenic neuropeptides known to stimulate food intake in a variety of species, applied centrally to Siberian hamsters increases foraging and especially hoarding with lesser increases in food intake. Both are expressed in the arcuate nucleus (Arc) and their synthesis increases with food deprivation, a naturally-occurring stimulus that markedly increases foraging and hoarding in Siberian hamsters. Therefore, we tested whether destruction of Arc neurons blocks these ingestive behaviors. This was accomplished either by microinjecting NPY conjugated to saporin (NPY-SAP) bilaterally into the Arc to kill NPY receptor-bearing neurons or via neonatal monosodium glutamate (MSG) treatment. For both methods, Arc cresyl violet staining (cell density) and NPY and Y1 receptor-immunoreactivity (ir) were significantly decreased. Although baseline foraging and food hoarding were not affected, food deprivation-induced increased food hoarding was surprisingly exaggerated approximately 100% with both types of Arc destruction. We found a substantial amount of remaining NPY-ir fibers, likely emanating from the brainstem, and a significant up-regulation of Y1 receptors in Arc NPY projections areas (hypothalamic paraventricular nucleus and perifornical area) after Arc denervation and their activation may have accounted for the exaggerated increases. The converging evidence from both Arc destruction methods suggests an intact Arc is not necessary for food deprivation-induced increases in food foraging and hoarding.

Acute brown adipose tissue temperature response to cold in monosodium glutamate-treated Siberian hamsters

Neonatal monosodium glutamate (MSG) administration increases adiposity, decreases energy expenditure and is associated with arcuate nucleus (Arc) destruction. Disrupted brown adipose tissue (BAT) thermogenesis underlies some of these effects, although, interscapular BAT temperature (T(IBAT)) has not been measured. Therefore, we tested the effects of neonatal MSG or vehicle administration in Siberian hamsters and, when they were adults, measured T(IBAT) during acute cold exposure. The Arc and its projection to the hypothalamic paraventricular nucleus (PVH) are both components of the CNS outflow circuits to IBAT, with the latter implicated in BAT thermogenesis that could be compromised by MSG treatment. Using a viral transneuronal tract tracer, pseudorabies virus (PRV), we also tested whether the components of these circuits were intact. As adults, MSG-treated hamsters had significantly increased body mass and some white fat pad masses, markedly reduced Arc Nissl and neuropeptide staining, and PVH neuropeptide fiber staining. Cold-exposed (18 h at 5 degrees C) MSG- and vehicle-treated hamsters initially maintained T(IBAT), but the ability of the former waned after 2 h being significantly decreased by 18 h. PRV immunoreactive fibers/cells were not altered by neonatal MSG treatment despite substantial Arc and PVH destruction. MSG- and vehicle-treated hamsters given an exogenous norepinephrine challenge showed identical increases in the duration and peak of T(IBAT). Thus, the inability of MSG-treated animals to sustain T(IBAT) in the cold is not due to any obvious MSG-induced deletions of central sympathetic outflow circuits to IBAT, but appears to be extrinsic to the tissue nevertheless.

Divergent regulation of hypothalamic neuropeptide Y and agouti-related protein by photoperiod in F344 rats with differential food intake and growth

Hypothalamic genes involved in food intake and growth regulation were studied in F344 rats in response to photoperiod. Two sub-strains were identified: F344/NHsd (F344/N) and F344/NCrHsd (F344/NCr); sensitive and relatively insensitive to photoperiod respectively. In F344/N rats, marked, but opposite, changes in the genes for neuropeptide Y (NPY) (+97.5%) and agouti-related protein (AgRP) (-39.3%) expression in the arcuate nucleus were observed in response to short (8 : 16 h light/dark cycle, SD) relative to long (16 : 8 h light/dark cycle, LD) day photoperiods. Changes were associated with both reduced food intake and growth. Expression of the genes for cocaine and amphetamine-regulated transcript (CART) and pro-opiomelanocortin (POMC) in the arcuate nucleus was unchanged by photoperiod. POMC in the ependymal layer around the third ventricle was markedly inhibited by SD. Parallel decreases in the genes for growth hormone-releasing hormone (GHRH) and somatostatin (Somatostatin) mRNA in the arcuate nucleus and Somatostatin in the periventricular nucleus were observed in SD. Serum levels of insulin-like growth factor (IGF)-1 and insulin were lower in F344/N rats in SD, whereas neither leptin nor corticosterone levels were affected. By contrast, F344/NCr rats that show only minor food intake and growth rate changes showed minimal responses in these genes and hormones. Thus, NPY/AgRP neurones may be pivotal to the photoperiodic regulation of food intake and growth. Potentially, the SD increase in NPY expression may inhibit growth by decreasing GHRH and Somatostatin expression, whereas the decrease in AgRP expression probably leads to reduced food intake. The present study reveals an atypical and divergent regulation of NPY and AgRP, which may relate to their separate roles with respect to growth and food intake, respectively.

Photoperiod and acute energy deficits interact on components of the thyroid hormone system in hypothalamic tanycytes of the Siberian hamster

In the Siberian hamster, seasonal weight loss occurs gradually over many weeks during autumn and winter. This is driven by a regulatory mechanism that is able to integrate duration of exposure to short days (SDs) with the size of body energy reserves. After food restriction in SDs, followed by ad libitum refeeding, body weight of the hamster does not return to its former level; rather, it increases to a level defined by the length of time spent in SDs. In this report, we show that components of the thyroid hormone system that are involved in seasonal weight loss change expression in response to 48 h of starvation. Eight weeks in an SD photoperiod induced weight loss in the Siberian hamster. In the hypothalamus of these hamsters, type II deiodinase expression was decreased and type III deiodinase expression was induced, but there was no change in hypothalamic neuropeptide Y or thyrotropin-releasing hormone gene expression. For the first time, we show that the thyroid hormone transporter monocarboxylate transporter 8 is expressed in tanycytes and is increased in response to an SD photoperiod. Food restriction (48 h of starvation) reversed the direction of gene expression change for type II and III deiodinase and monocarboxylate transporter 8 induced by SD photoperiods. Furthermore, fasting increased neuropeptide Y expression and decreased thyrotropin-releasing hormone expression. VGF, a gene upregulated in SDs in the dorsal region of the medial posterior area of the arcuate nucleus, was not changed by starvation. These data point to a mechanism whereby energy deprivation can interact with an SD photoperiod on hypothalamic tanycytes to regulate components of the thyroid hormone system involved in photoperiodic regulation of seasonal physiology.

Photoperiod regulates genes encoding melanocortin 3 and serotonin receptors and secretogranins in the dorsomedial posterior arcuate of the Siberian hamster

The mechanism(s) involved in the regulation of the seasonal-appropriate body weight of the Siberian hamster are currently unknown. We have identified photoperiodically regulated genes including VGF in a sub-region of the arcuate nucleus termed the dorsomedial posterior arcuate (dmpARC). Gene expression changes in this nucleus so far account for a significant number of those reported as photoperiodically regulated and are therefore likely to contribute to seasonal physiological responses of the hamsters. The present study aimed to identify additional genes expressed in the dmpARC regulated by photoperiod that could be involved in regulating the activity of this nucleus with respect to seasonal physiology of the Siberian hamster. Using laser capture microdissection coupled with a microarray analysis and a candidate gene approach, we have identified several photoperiodically regulated genes in the dmpARC that are known to have roles in secretory and intracellular signalling pathways. These include secretogranin (sg) III and SgVI (secretory pathway), melanocortin 3 receptor (MC3-R) and serotonin (5-HT) receptors 2A and 7 (signalling pathway), all of which increase in expression under a short photoperiod. The spatial relationship between receptor signalling and potential secretory pathways was investigated by dual in situ hybridisation, which revealed that 5-HT2A and 5-HT7 receptors are expressed in neurones expressing VGF mRNA and that a sub-population (approximately 40%) of these neurones express MC3-R. These gene expression changes in dmpARC neurones may reflect the functional requirement of these neurones for seasonal physiological responses of the hamster.

The regulation of seasonal changes in food intake and body weight

Seasonal rhythms of body weight, reflecting altered food intake, energy storage and expenditure, are a common feature of mammals inhabiting temperate and arctic latitudes. They have evolved so that predictable annual changes in the external environment can be anticipated and animals can adjust their physiology and behaviour in preparation for the changing demands of the seasons. These long-term changes in energy balance are not simply effected by the brain centres and peptidergic pathways known to underlie short-term homeostatic regulation. Screens of altered gene expression in Siberian hamsters comparing the anabolic summer state in long photoperiods and the catabolic 'winter' state in short photoperiods have identified differential gene expression in the hypothalamus. The majority of gene expression changes are confined to two restricted areas: the dorsomedial posterior arcuate nucleus, and the ventral ependymal layer of the third ventricle. Functions encoded by these 'seasonal' genes include thyroid hormone metabolism, retinoic acid and histaminergic signalling, and VGF and secretogranin production. The changes in thyroid hormone availability that are brought about by differential activity of deiodinase enzymes are of particular importance because experimental manipulation of central thyroid levels can prevent seasonal cyclicity. Given the importance of thyroid hormone in the initial development of the brain, we hypothesise that thyroid hormone-dependent plasticity of hypothalamic connections and neurogenesis underlie seasonal cycles of food intake and body weight.

Food deprivation-induced changes in body fat mobilization after neonatal monosodium glutamate treatment

The reversal of obesity is a difficult feat at best and is a growing problem as the obesity epidemic increases worldwide. Considerable focus has been made on the arcuate nucleus (Arc) in the control of body and lipid mass and food intake. To test the role of the Arc in body fat mobilization, we compared the effects of food deprivation on white adipose tissue (WAT) mass in adult Siberian hamsters by making exocytotic lesions of the Arc via neonatal subcutaneous injections of monosodium glutamate (MSG). MSG-treated hamsters had significantly increased body mass, total and individual WAT pad masses, and serum leptin concentrations compared with their vehicle-injected counterparts. MSG produced marked reductions in Arc Nissl staining, tyrosine hydroxylase-immunoreactive (ir) neurons, and neuropeptide Y (NPY)- and agouti-related protein (AgRP)-ir fibers compared with controls. MSG significantly decreased hypothalamic paraventricular nucleus (PVN) NPY- and AgRP fiber-ir compared with controls, likely because of Arc projections to this nucleus. MSG treatment also reduced area postrema (AP) tyrosine hydroxylase (TH)-ir fibers compared with controls. MSG treatment did not, however, block food deprivation-induced decreases in WAT pad mass compared with controls. Thus, despite considerable damage to the Arc and some of its projections to the PVN, as well as the AP, body fat was mobilized apparently normally, bringing into question the necessity of these structures for food deprivation-induced lipid mobilization. These data support recent evidence that chronically decerebrate rats, in which the forebrain is surgically isolated from the caudal brainstem, show normal food deprivation responses, including lipid mobilization.

NPY Y1 receptor antagonist prevents NPY-induced torporlike hypothermia in cold-acclimated Siberian hamsters

Siberian hamsters ( Phodopus sungorus) undergo bouts of daily torpor during which body temperature decreases by as much as 20°C and provides a significant savings in energy expenditure. Natural torpor in this species is normally triggered by winterlike photoperiods and low ambient temperatures. Intracerebroventricular injection of neuropeptide Y (NPY) reliably induces torporlike hypothermia that resembles natural torpor. NPY-induced torporlike hypothermia is also produced by intracerebroventricular injections of an NPY Y1 receptor agonist but not by injections of an NPY Y5 receptor agonist. In this research, groups of cold-acclimated Siberian hamsters were either coinjected with a Y1 receptor antagonist (1229U91) and NPY or were coinjected with a Y5 receptor antagonist ( CGP71683 ) and NPY in counterbalanced designs. Paired vehicle + NPY induced torporlike hypothermia in 92% of the hamsters, whereas coinjection of Y1 antagonist + NPY induced torporlike hypothermia in 4% of the hamsters. In contrast, paired injections of vehicle + NPY and Y5 antagonist + NPY induced torporlike hypothermia in 100% and 91% of the hamsters, respectively. Although Y5 antagonist treatment alone had no effect on body temperature, Y1 antagonist injections produced hyperthermia compared with controls. Both Y1 antagonist and Y5 antagonist injections significantly reduced food ingestion 24 h after treatment. We conclude that activation of NPY 1 receptors is both sufficient and necessary for NPY-induced torporlike hypothermia.

Monosodium glutamate-induced arcuate nucleus damage affects both natural torpor and 2DG-induced torpor-like hypothermia in Siberian hamsters

Siberian hamsters ( Phodopus sungorus) have the ability to express daily torpor and decrease their body temperature to ∼15°C, providing a significant savings in energy expenditure. Daily torpor in hamsters is cued by winterlike photoperiods and occurs coincident with the annual nadirs in body fat reserves and chronic leptin concentrations. To better understand the neural mechanisms underlying torpor, Siberian hamster pups were postnatally treated with saline or MSG to ablate arcuate nucleus neurons that likely possess leptin receptors. Body temperature was studied telemetrically in cold-acclimated (10°C) male and female hamsters moved to a winterlike photoperiod (10:14-h light-dark cycle) ( experiments 1 and 2) or that remained in a summerlike photoperiod (14:10-h light-dark cycle) ( experiment 3). In experiment 1, even though other photoperiodic responses persisted, MSG-induced arcuate nucleus ablations prevented the photoperiod-dependent torpor observed in saline-treated Siberian hamsters. MSG-treated hamsters tended to possess greater fat reserves. To determine whether reductions in body fat would increase frequency of photoperiod-induced torpor after MSG treatment, hamsters underwent 2 wk of food restriction (70% of ad libitum) in experiment 2. Although food restriction did increase the frequency of torpor in both MSG- and saline-treated hamsters, it failed to normalize the proportion of MSG-treated hamsters undergoing photoperiod-dependent torpor. In experiment 3, postnatal MSG treatments reduced the proportion of hamsters entering 2DG-induced torpor-like hypothermia by ∼50% compared with saline-treated hamsters (38 vs. 72%). In those MSG-treated hamsters that did become hypothermic, their minimum temperature during hypothermia was significantly greater than comparable saline-treated hamsters. We conclude that 1) arcuate nucleus mechanisms mediate photoperiod-induced torpor, 2) food-restriction-induced torpor may also be reduced by MSG treatments, and 3) arcuate nucleus neurons make an important, albeit partial, contribution to 2DG-induced torpor-like hypothermia.

Histamine H3 receptor and orexin A expression during daily torpor in the Djungarian hamster (Phodopus sungorus)

Seasonal animals use different strategies to reduce energy expenditure in the face of reduced seasonal food availability. For example, the ground squirrel enters a hibernation state with reduced metabolism, hypothermia and suppressed central nervous system activity, whereas the Djungarian hamster (Phodopus sungorus) employs daily bouts of torpor associated with reduced body temperature and energy expenditure. Studies in the hibernating ground squirrel implicate an increase in histamine synthesis and histamine H(3) receptor expression in the brain as a central mechanism governing hibernation. In the present study, we demonstrate an up-regulation of H(3) receptors in several brain nuclei in the Djungarian hamster during bouts of daily torpor, a shallow form of hypothermia, suggesting that histaminergic pathways may play a general role in maintaining low body temperature and torpor state in mammals. These regions include the arcuate nucleus, dorsomedial hypothalamus, suprachiasmatic nucleus, dorsal lateral geniculate nucleus and tuberomammillary nucleus. Interestingly, expression of the mRNA for orexins, a group of neuropeptides that increase wakefulness, remains unchanged during the arousal from daily torpor, suggesting that this classic 'arousal' pathway is not involved in the transition from a hypothermic to the euthermic state.

Thyrotrophin-releasing hormone decreases feeding and increases body temperature, activity and oxygen consumption in Siberian hamsters

Thyrotrophin-releasing hormone (TRH) is known to play an important role in the control of food intake and energy metabolism in addition to its actions on the pituitary-thyroid axis. We have previously shown that central administration of TRH decreases food intake in Siberian hamsters. This species is being increasingly used as a physiological rodent model in which to understand hypothalamic control of long-term changes in energy balance because it accumulates fat reserves in long summer photoperiods, and decreases food intake and body weight when exposed to short winter photoperiods. The objectives of our study in Siberian hamsters were: (i) to investigate whether peripheral administration of TRH would mimic the effects of central administration of TRH on food intake and whether these effects would differ dependent upon the ambient photoperiod; (ii) to determine whether TRH would have an effect on energy expenditure; and (iii) to investigate the potential sites of action of TRH. Both peripheral (5-50 mg/kg body weight; i.p.) and central (0.5 microg/ml; i.c.v.) administration of TRH decreased food intake, and increased locomotor activity, body temperature and oxygen consumption in the Siberian hamster, with a rapid onset and short duration of action. Systemic treatment with TRH was equally effective in suppressing feeding regardless of ambient photoperiod. The acute effects of TRH are likely to be centrally mediated and independent of its role in the control of the production of thyroid hormones. We conclude that TRH functions to promote a catabolic energetic state by co-ordinating acute central and chronic peripheral (thyroid-mediated) function.

Role of melanocortin in the long-term regulation of energy balance: lessons from a seasonal model

Siberian hamsters express photoperiod-regulated seasonal cycles of body weight and food intake, providing an opportunity to study the role of melanocortin systems in regulating long-term adaptive changes in energy metabolism. These hamsters accumulate intraperitoneal fat reserves when kept in long summer photoperiods, but show a profound long-term decrease in food intake and body weight when exposed to a short winter photoperiod. Icv administration of a MC3/4-R agonist (MTII) potently suppresses food intake in hamsters in both the obese and lean state, indicating the potential for melanocortin systems to regulate energy metabolism in the hypothalamus of the Siberian hamster. Icv treatment with the melanocortin antagonist SHU9119 increases food intake in both seasonal states. Moreover, hamsters bearing neurotoxic lesions, which destroy the majority of POMC expressing neurons in the arcuate nucleus are still able to show seasonal regulation of body weight. These studies in a seasonal model substantiate the view that endogenous melanocortin systems exert a tonic inhibition of food intake in mammals. The observations that this melanocortin tone occurs to a similar extent in both an anabolic state induced by a long day photoperiod, and in a catabolic state induced by a short day photoperiod, suggests that alterations in endogenous melanocortin tone are not the primary cause of the lipolysis, weight-loss and hypophagia which characterize the establishment of the short day-induced overwintering state.

What can we learn from seasonal animals about the regulation of energy balance?

Weight loss in humans requires, except during an illness, some form of imposed restriction on food intake or increase in energy expenditure. This necessitates overcoming powerful peripheral and central signals that serve to protect against negative energy balance. The identification of the systems and pathways involved has come from mouse models with genetic and targeted mutations, e.g., ob/ob and MC4 R(-/-) as well as rat models of obesity. Study of seasonal animals has shown that they undergo annual cycles of body fattening and adipose tissue loss as important adaptations to environmental change, yet these changes appear to involve mechanisms distinct from those known already. One animal model, the Siberian hamster, exhibits marked, but reversible, weight loss in response to shortening day length. The body weight is driven by a decrease in food intake with the magnitude of the loss of body weight being directly related to the length of time of exposure to short photoperiod. The most important facet of this response is that the point of energy balance is continuously re-adjusted during the transition in body weight reflecting an apparent 'sliding set point'. Studies have focused on identifying the neural basis of this mechanism. Initial studies of known genes (e.g., NPY, POMC, and AgRP) both through the measurement of gene expression in the arcuate nucleus as well as following intracerebroventricular (i.c.v.) injection indicated that the systems involved are not those involved in restoring energy balance following energy deficits. Instead, a novel mechanism of regulation is implied. Recent studies have begun to explore the neural basis of the seasonal body weight response. A discrete and novel region of the posterior arcuate nucleus, the dorsal medial posterior arcuate nucleus (dmpARC) has been identified, where a battery of gene expression changes for signalling molecules (vgf and histamine H3 receptor) and transcription factors (RXRgamma and RAR) occur in association with seasonal changes in body weight. This work provides the basis of a potentially novel mechanism of energy balance regulation.

Regulation of body weight and thermogenesis in seasonally acclimatized Brandt's voles (Microtus brandti)

Seasonal changes in an animal's morphology, physiology, and behavior are considered to be an adaptive strategy for survival and reproductive success. In the present study, we examined body weight and several behavioral, physiological, hormonal, and biochemical markers in seasonally acclimatized Brandt's voles (Microtus brandti) to test our hypothesis that Brandt's voles can decrease energy intake associated with decrease in body weight, body fat content, serum leptin level, and increasing thermogenesis in winter conditions. We found that the body weight of Brandt's voles was lowest in winter (December to February) and highest in spring and early summer (May to June). This seasonal variation in body weight was associated with changes in other markers examined. For example, the winter decrease in body weight was accompanied by increased energy intake and enhanced nonshivering thermogenesis (NST) as well as by decreased body fat mass and reduced levels of circulating leptin. Further, circulating levels of leptin were positively correlated with body weight and body fat mass, and negatively correlated with energy intake and uncoupling protein 1 contents. Together, these data do not support our hypothesis and suggest that leptin may be involved in this process and serve as a starvation signal in Brandt's voles.

Involvement of 5-HT receptors in the regulation of food intake in Siberian hamsters

The Siberian hamster provides a physiological model for understanding the hypothalamic control of energy metabolism as it undergoes annual photoperiod-regulated cycles of body weight (i.e. fattening in summer, and catabolism of fat stores in winter). As a first step to investigate whether enhanced serotonergic (5-HT) tone might underlie the catabolic processes in short days, we investigated whether serotonergic stimulation can produce catabolic actions in fat hamsters housed in long days. Acute treatment with the serotonin reuptake inhibitor (+/-) fenfluramine (8 mg/kg, i.p.) produced a prolonged, dose-dependent reduction in food intake in both photoperiods. Behavioural observations and radiotelemetry analyses revealed that this anorectic effect of fenfluramine was associated with short-term increases in locomotor activity and in core body temperature. In a subsequent series of studies, hamsters were pretreated with the 5-HT2C receptor antagonist SB242084 (4 mg/kg, i.p.). This 5-HT2C receptor antagonist completely blocked the anorectic actions of fenfluramine, but did not decrease the hyperthermia or hyperlocomotion induced by fenfluramine; thus, the anorectic actions of fenfluramine probably reflect actions via the 5-HT2C receptor. Consistent with these observations, treatment of hamsters with the 5-HT2C receptor agonist VER 3323 (10 mg/kg, i.p.) or the 5-HT1B/2C receptor agonist mCPP (3 mg/kg, i.p.) reduced food intake. The response to manipulation of serotonergic pathways was not affected by the ambient photoperiod in any of these studies. We conclude that the anorectic actions of fenfluramine are not an indirect consequence of serotonergic actions on arousal pathways, and that its actions on feeding in the Siberian hamster are most likely to be mediated by the 5-HT2C receptor.

Temporal changes in gene expression in the arcuate nucleus precede seasonal responses in adiposity and reproduction

In anticipation of seasonal climate changes, Siberian hamsters display a strategy for survival that entails profound physiological adaptations driven by photoperiod. These include weight loss, reproductive quiescence, and pelage growth with shortening photoperiod and vice versa with lengthening photoperiod. This study reports gene expression changes in the hypothalamus of Siberian hamsters switched from short days (SD) to long days (LD), and also in photorefractory hamsters. Siberian hamsters were maintained in either LD or SD for 14 wk, conditions that generate physiological states of obesity under LD and leanness under SD. After 14 wk, SD lighting was switched to LD and gene expression investigated after 0, 2, 4, and 6 wk by in situ hybridization. Genes encoding nuclear receptors (RXR/RAR), retinoid binding proteins (CRBP1 and CRABP2), and histamine H3 receptor were photoperiodically regulated with significantly lower expression in SD, whereas VGF mRNA expression was significantly higher in SD, in the dorsomedial posterior arcuate nucleus. After a SD-to-LD switch, gene expression changes of CRABP2, RAR, H3R, and VGF occurred relatively rapidly toward LD control levels, ahead of body weight recovery and testicular recrudescence, whereas CRBP1 responded less robustly and rxrgamma did not respond at the mRNA level. In this brain nucleus in photorefractory animals, the CRABP2, RAR, H3R, and VGF mRNA returned toward LD levels, whereas CRBP1 and rxrgamma remained at the reduced SD level. Thus, genes described here are related to photoperiodic programming of the neuroendocrine hypothalamus through expression responses within a subdivision of the arcuate nucleus.

Neuronal distribution of melanin-concentrating hormone, cocaine- and amphetamine-regulated transcript and orexin B in the brain of the Djungarian hamster (Phodopus sungorus)

The distribution of melanin-concentrating hormone-, cocaine- and amphetamine-regulated transcript- and orexin B-immunoreactive elements as well as their morphological relationships in selected brain structures harbouring the neuroendocrine pathways controlling energy balance and circadian rhythmicity in the Djungarian hamster (Phodopus sungorus) were studied. Cocaine- and amphetamine-regulated transcript-(55-102)-immunoreactive perikarya co-expressed melanin-concentrating hormone-immunoreactivity in the lateral hypothalamic area, dorsomedial hypothalamic nucleus, zona incerta and posterior hypothalamic area. In addition, arcuate nucleus, hypothalamic periventricular nucleus, Edinger-Westphal nucleus, and the rostral aspect of the dorsal raphe nucleus contained cocaine- and amphetamine-regulated transcript-immunoreactive cell profiles. Orexin B-immunoreactive perikarya were distributed in the lateral hypothalamic area, dorsomedial hypothalamic nucleus and retrochiasmatic area. Cells immunoreactive for orexin B did not co-express melanin-concentrating hormone-immunoreactivity, but orexin B-immunoreactive fibers had close apposition to many melanin-concentrating hormone-immunoreactive cells. Whereas immunoreactivity for all examined peptides was absent in the suprachiasmatic nucleus, dense and large orexin B-immunoreactive fibers and to a lesser extent melanin-concentrating hormone- and cocaine- and amphetamine-regulated transcript-immunoreactive fibers of smaller size were present in the intergeniculate leaflet and raphe nucleus. These observations in Djungarian hamsters indicate that the neuronal distribution of the examined peptides is strongly conserved between species. In addition, the presence of fibers within the neuronal components of the circadian timing system suggests that they may indirectly influence circadian rhythms.

Neuropeptides and anticipatory changes in behaviour and physiology: seasonal body weight regulation in the Siberian hamster

The Siberian hamster, Phodopus sungorus, is a powerful model of physiological body weight regulation. This seasonal model offers the potential to distinguish between the compensatory neuroendocrine systems that defend body weight against imposed negative energy balance, and those that are involved in the programming of the level of body weight that will be defended-a seasonally appropriate body weight. Of the known, studied, components of the hypothalamic energy balance system, the anorexogenic peptide, cocaine- and amphetamine-regulated transcript (CART), is the only candidate where gene expression changes in a manner consistent with a role in initiating or sustaining photoperiod-induced differences in body weight trajectory. Siberian hamsters effect a reversible biannual switch in leptin sensitivity in which only short day (SD)-acclimated hamsters that have undergone a reduction in body weight, adiposity and plasma leptin are sensitive to peripheral exogenous leptin. The suppressor of cytokine signalling protein, SOCS3, appears to be the molecular correlate of this seasonal sensitivity.

Reduced leptin concentrations are permissive for display of torpor in Siberian hamsters

A photoperiod with a short photophase induces a winterlike phenotype in Siberian hamsters that includes a progressive decrease in food intake and body mass and reproductive organ regression, as well as reversible hypothermia in the form of short-duration torpor. Torpor substantially reduces energy utilization and is not initiated until body mass, fat stores, and serum leptin concentrations are at their nadir. Because photoperiod-dependent torpor is delayed until fat reserves are lowest, leptin concentrations may be a permissive factor for torpor onset. This conjecture was tested by implanting osmotic minipumps into Siberian hamsters manifesting spontaneous torpor; the animals received a constant release of leptin or vehicle for 14 days. Exogenous leptin treatment eliminated torpor in a significant proportion of treated hamsters, whereas treatment with the vehicle did not. Similarly, endogenous serum leptin concentrations were markedly reduced in all animals undergoing daily torpor. Although simply reducing leptin concentrations below a threshold value is not sufficient for torpor initiation, reduced leptin concentrations nevertheless appear necessary for its occurrence. It is proposed that drastically reduced leptin concentrations provide a “starvation signal” to an as yet unidentified central mechanism mediating torpor initiation.

Feeding and behavioural effects of central administration of the melanocortin 3/4-R antagonist SHU9119 in obese and lean Siberian hamsters

Siberian hamsters accumulate fat reserves in long photoperiods, but show a long-term decrease in food intake and body weight when exposed to a short winter photoperiod. The aim of this study was to determine the role of central melanocortin 3/4 receptors (MC3/4-R) in generating this chronic catabolic state by investigating the effects of SHU9119, a MC3/4-R antagonist, on food intake and associated behaviours. In adult male hamsters, intra-cerebroventricular infusions of SHU9119 significantly increased food intake in a dose-dependent manner. The time course of action was slow, food intake being increased between 4 and 24 h after intra-cerebroventricular administration. A similar degree of increase in food intake occurred in fat hamsters in long days and in lean hamsters chronically exposed to short days. Intra-cerebroventricular treatment with MTII (a MC3/4-R agonist) significantly decreased food intake for up to 24 h after treatment, and SHU9119 reversed these suppressive effects between 4 and 24 h after treatment, a similar time course to that observed when SHU9119 was administered alone. We conclude that endogenous melanocortin peptides acting via MC3/4-R are involved in the regulation of food intake in hamsters in both anabolic and catabolic states, but these acute studies do not provide evidence that increased activity of this hypothalamic system underlies the seasonal decrease in food intake that contributes to the long-term catabolic state in short days.

Leptin and seasonal mammals

Seasonal mammals commonly exhibit robust annual cycles of adiposity, food intake and energy metabolism. These cycles are driven by changes in the external daylength signal, which generates a diurnal melatonin profile and acts on neuroendocrine pathways. The white adipose tissue hormone leptin reflects overall adiposity in seasonal mammals, and consequently undergoes significant seasonal fluctuations in secretion. The seasonally breeding Siberian (Djungarian) hamster is a convenient laboratory model to study the effect of a seasonal time-keeping clock on energy metabolism, appetite regulation and the control of adiposity. We have shown that administration of exogenous leptin at physiological doses induces significant loss of adipose tissue for short-day housed winter-like hamsters in which endogenous adipose tissue and leptin concentrations are already low. By contrast, long-day housed hamsters with high adipose tissue reserves are refractory to the effects of leptin. This phenomenon of seasonal leptin resistance appears to be a general feature of other seasonally breeding mammals, and may reflect the operation of an annual timer controlling leptin uptake and/or action on central nervous system signal transduction pathways. The mobilization of fat by leptin in short-day housed hamsters is not associated with changes in expression in either anorexic or anabolic peptides expressed in leptin-receptor rich structures in the arcuate region of the hypothalamus, and suggests that leptin may target other structures. These data contrast with studies, which show that homeostatic mechanisms in response to feed-restriction induce changes in hypothalamic peptides in a similar manner to nonphotoperiodic species. Thus, the long-term seasonal regulation of body weight set point and leptin feedback may operate through separate pathways to those responsible for acute responses to food restriction.

Early regulation of hypothalamic arcuate nucleus CART gene expression by short photoperiod in the Siberian hamster

Cocaine- and amphetamine-regulated transcript (CART) mRNA is expressed in a number of hypothalamic nuclei including the arcuate nucleus (ARC). An increase in CART gene expression in the ARC of juvenile female Siberian hamsters (Phodopus sungorus) 14 days after transfer to short photoperiod at weaning and prior to major divergence of body weight trajectory in this seasonal mammal implicates CART in the induction of programmed weight change. In the current series of experiments, elevated CART mRNA in short photoperiod juvenile female animals relative to long photoperiod controls was apparent throughout the caudal-rostral extent of the ARC after 14 days, but was not observed when short photoperiod exposure was limited to 4-7 days. Elevated CART gene expression was also observed in juvenile males 14 days after transfer to short photoperiod at weaning, in adult female hamsters 14 days after transfer to short photoperiod and in adult male hamsters 21 days after transfer to short photoperiod. There were no consistent trends in expression levels of other energy balance-related genes with these relatively short duration photoperiod manipulations, suggesting that CART may be involved in short photoperiod-programmed body weight regulation.

Decrease of food intake by MC4-R agonist MTII in Siberian hamsters in long and short photoperiods

We investigated the role of the hypothalamic melanocortin system in the regulation of food intake in the Siberian hamster, which shows a profound seasonal decrease in food intake and body weight in short photoperiod (SP). In male hamsters maintained in long photoperiod (LP), intracerebroventricular injection of melanotan II (MTII) just before lights off significantly decreased food intake relative to vehicle treatment over the 6-h observation period. Similar effects were observed in age-matched hamsters after exposure to a short daylength for 9 wk, when body weight had significantly decreased. There was no clear difference in either the magnitude of response or the dose required for half-maximal inhibition of food intake in hamsters in SP compared with those in LP. MTII significantly increased grooming in both LP and SP. Our results indicate that the melanocortin system is a potent short-term regulator of food intake. However, the lack of differential response or sensitivity to MTII treatment in the obese (LP) vs. lean (SP) states does not support the hypothesis that changes in this melanocortin pathway underlie the long-term decrease in food intake that occurs in this seasonal model.

Cocaine and amphetamine-regulated transcript mRNA regulation in the hypothalamus in lean and obese rodents

Cocaine and amphetamine-regulated transcript (CART) mRNA and immunoreactivity are expressed abundantly in the hypothalamus. Central administration of various fragments of this neuropeptide decreases food intake in rodents. To find out whether CART might play a role in the physiological regulation of energy balance, we used in situ hybridization to investigate whether CART mRNA abundance changed in two chronic obese/fat versus lean states and after acute dietary restriction. In the first study, mice were treated with goldthioglucose to destroy glucose-responsive neurones in the ventromedial hypothalamus. This produced hyperphagia and obesity: 7 weeks after treatment, those receiving goldthioglucose weighed 70% more than the controls. CART mRNA abundance in the arcuate nucleus of goldthioglucose-treated mice was decreased by 71% compared to levels in the control mice, but CART expression was unaffected in the dorsolateral hypothalamus. In the second study, male Siberian hamsters were exposed to short days to induce a physiological winter response in which body weight decreases as fat reserves are catabolized, and food intake correspondingly declines. After 8 weeks in short days, body weight had declined by 18% relative to controls maintained in long days in a summer fat state. CART mRNA levels did not differ significantly between the two groups in any hypothalamic areas. In the third study, male Siberian hamsters, either in long days or after 12 weeks exposure to short days to induce weight loss, were subject to a 48-h period of fasting. Although photoperiod per se did not affect CART expression, fasting produced a significant decrease in CART mRNA in the arcuate nucleus of hamsters in both the long- and short-day state. We conclude that CART-producing cells are involved in energy homeostasis: the marked decrease in CART expression in the arcuate nucleus in goldthioglucose-lesioned mice may contribute to the development of obesity, and the decrease following acute dietary restriction in hamsters may reflect a compensatory mechanism to reduce caloric expenditure, but our results do not indicate that CART is involved in long-term seasonal regulation of body weight.

Hypothalamic neuropeptide systems and anticipatory weight change in Siberian hamsters

Seasonal animals are able both to programme changes in body weight in response to annual changes in photoperiod (anticipatory regulation) and to correct changes in body weight caused by imposed energetic demand (compensatory regulation). Experimental evidence from the Siberian hamster suggests that seasonally appropriate body weight is continually reset according to photoperiodic history, even when actual body weight is driven away from this target weight by manipulation of energy intake. These characteristics constitute the "sliding set point" of seasonal body weight regulation. To define the mechanisms and molecules underlying anticipatory body weight regulation, we are investigating the involvement of hypothalamic systems with an established role in the compensatory defence of body weight. Weight loss or restricted growth induced by short days (SD) results in low circulating leptin compared with long day (LD) controls. However, this chronic low leptin signal is read differently from acute low leptin resulting from food deprivation; leptin receptor gene expression in the hypothalamic arcuate nucleus (ARC) is lower in SD, whereas food deprivation increases expression levels, suggesting changes in sensitivity to leptin feedback. SD alterations in mRNA levels for a number of hypothalamic neuropeptide and receptor genes appear counter-intuitive for a SD body weight trajectory. However, early increases in ARC cocaine-and amphetamine-regulated transcript (CART) gene expression in SDs could be involved in driving body weight loss or growth restriction. The sites of photoperiod interaction with energy balance neuronal circuitry and the neurochemical encoding of body weight set point require full characterisation. Study of anticipatory regulation in seasonal animals offers new insight into body weight regulation across mammalian species, including man.

The regulation of body weight: lessons from the seasonal animal

The hypothalamus is a major regulatory centre involved in the control of many important physiological axes. One of these axes is the regulation of ingestive behaviour. Recent work using a combination of genetic-mutant mouse models together with targeted gene deletions has contributed much to our understanding of how neural pathways of the hypothalamus are involved in the regulation of energy balance in animals. These pathways are also relevant to human energy homeostasis. as mutations in key genes are correlated with obesity. Many of the genes identified mediate the effects of leptin, and are therefore primarily involved in sensing and responding to peripheral signals. In seasonal animals, such as the Siberian hamster (Phodopus sungorus), there is evidence for a higher level of regulation. The systems involved regulate body weight around an apparent 'set-point' through the action of photoperiod via the neurohormone, melatonin. The ability to manipulate energy balance through photoperiod (and melatonin) in the seasonal-animal model offers novel opportunities to identify further fundamental aspects of the control mechanisms involved in the central control of energy homeostasis and body weight.

Leptin acts on metabolism in a photoperiod-dependent manner, but has no effect on reproductive function in the seasonally breeding Siberian hamster (Phodopus sungorus)

Leptin may play a role in appetite regulation and metabolism, but its reproductive role is less clear. In photoperiodic Siberian hamsters, seasonal changes in fatness, leptin gene expression, and metabolism occur synchronously with activation or suppression of reproduction, analogous to puberty. Here, we test the hypothesis that seasonal changes in leptin secretion mediate the photoperiodic regulation of reproduction. Mature male and ovariectomized estrogen-treated female Siberian hamsters were kept in long (LD; 16 h of light, 8 h of darkness) or short days (SD; 8 h of light, 16 h of darkness) for 8 weeks, and recombinant murine leptin (15 microg/day) was infused for 2 weeks via osmotic minipumps. SD hamsters exhibited significant weight and fat losses, reduced serum leptin and food intake, and suppressed pituitary LH concentration. Leptin did not suppress food intake over the 2-week treatment on either photoperiod, but significantly reduced fat reserves in SD hamsters. Leptin had no significant effect on pituitary LH concentrations in either sex or photoperiod or on testicular size and testosterone concentrations in males. These results suggest hamsters are more responsive to leptin on SD than on LD and that effects on food intake and fat loss can be dissociated in this species. Our data suggest that leptin does not mediate photoperiodic reproductive changes.

Seasonal regulation of food intake and body weight in the male Siberian hamster: studies of hypothalamic orexin (hypocretin), neuropeptide Y (NPY) and pro-opiomelanocortin (POMC)

The aim of these experiments was to investigate the relationship between hypothalamic expression of orexin (also called hypocretin), neuropeptide Y (NPY) and pro-opiomelanocortin (POMC) mRNA and seasonal cycles of body weight and food intake in the Siberian hamster. Adult males were transferred from long days of 16 h light and 8 h dark to short days of 8 h light and 16 h dark, a procedure known to induce major reductions in food intake and body weight in this species. After 8 weeks of exposure to short days, while body weight was declining, hypothalamic NPY mRNA levels as assessed by in situ hybridization were slightly lower (P < 0.05) than in age-matched controls exposed to long days. After 12 weeks with short days, when body weight would be expected to have reached its seasonal nadir, POMC mRNA levels were lower (P < 0.05) than in hamsters under long days. At no stage did orexin mRNA levels in hamsters under short days differ significantly from levels in those under long days. To investigate further the role of these peptide systems in seasonal changes in body weight and food intake, two provocative tests were carried out. Firstly, a 48-h fast induced a significant increase (P < 0.025) in hypothalamic NPY mRNA levels in both long- and short-day conditions, but did not change hypothalamic POMC or orexin mRNA levels. Secondly, systemic (intraperitoneal) treatment with recombinant murine leptin (5 mg/kg body weight) significantly decreased (P < 0.01) food intake over a 6-h post-treatment period in both long- and short-day conditions. However, this acute leptin treatment did not induce significant changes in hypothalamic orexin, NPY or POMC mRNA abundance. The increase in NPY expression in both long- and short-day conditions following food restriction and the suppression of food intake by leptin in both conditions suggests that acute homeostatic mechanisms operate in both long-day (obese) and short-day (nonobese) conditions. The lack of major changes in orexin, NPY and POMC in such different metabolic states suggest that other central systems must play a greater role in generating these states. Such findings are consistent with the 'sliding set-point' hypothesis, that is, seasonal cycles in food intake and fat metabolism are brought about by as yet unknown central mechanisms that chronically alter the level ('set point') around which homeostasis occurs, rather than resulting from changes in the potency of the acute feedback mechanisms themselves.