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

Prolactin Mediates Long-Term, Seasonal Rheostatic Regulation of Body Mass in Female Mammals

A series of well-described anabolic and catabolic neuropeptides are known to provide short-term, homeostatic control of energy balance. The mechanisms that govern long-term, rheostatic control of regulated changes in energy balance are less well characterized. Using the robust and repeatable seasonal changes in body mass observed in Siberian hamsters, this report examined the role of prolactin in providing long-term rheostatic control of body mass and photoinduced changes in organ mass (ie, kidney, brown adipose tissue, uterine, and spleen). Endogenous circannual interval timing was observed after 4 months in a short photoperiod, indicated by a significant increase in body mass and prolactin mRNA expression in the pituitary gland. There was an inverse relationship between body mass and the expression of somatostatin (Sst) and cocaine- and amphetamine-regulated transcript (Cart). Pharmacological inhibition of prolactin release (via bromocriptine injection), reduced body mass of animals maintained in long photoperiods to winter-short photoperiod levels and was associated with a significant increase in hypothalamic Cart expression. Administration of ovine prolactin significantly increased body mass 24 hours after a single injection and the effect persisted after 3 consecutive daily injections. The data indicate that prolactin has pleiotropic effects on homeostatic sensors of energy balance (ie, Cart) and physiological effectors (ie, kidney, BAT). We propose that prolactin release from the pituitary gland acts as an output signal of the hypothalamic rheostat controller to regulate adaptive changes in body mass.

Pleiotropic effects of proopiomelanocortin and VGF nerve growth factor inducible neuropeptides for the long-term regulation of energy balance

Seasonal rhythms in energy balance are well documented across temperate and equatorial zones animals. The long-term regulated changes in seasonal physiology consists of a rheostatic system that is essential to successful time annual cycles in reproduction, hibernation, torpor, and migration. Most animals use the annual change in photoperiod as a reliable and robust environmental cue to entrain endogenous (i.e. circannual) rhythms. Research over the past few decades has predominantly examined the role of first order neuroendocrine peptides for the rheostatic changes in energy balance. These anorexigenic and orexigenic neuropeptides in the arcuate nucleus include neuropeptide y (Npy), agouti-related peptide (Agrp), cocaine and amphetamine related transcript (Cart) and pro-opiomelanocortin (Pomc). Recent studies also indicate that VGF nerve growth factor inducible (Vgf) in the arcuate nucleus is involved in the seasonal regulation of energy balance. In situ hybridization, qPCR and RNA-sequencing studies have identified that Pomc expression across fish, avian and mammalian species, is a neuroendocrine marker that reflects seasonal energetic states. Here we highlight that long-term changes in arcuate Pomc and Vgf expression is conserved across species and may provide rheostatic regulation of seasonal energy balance.

Effect of exercise on photoperiod-regulated hypothalamic gene expression and peripheral hormones in the seasonal Dwarf Hamster Phodopus sungorus

The Siberian hamster (Phodopus sungorus) is a seasonal mammal responding to the annual cycle in photoperiod with anticipatory physiological adaptations. This includes a reduction in food intake and body weight during the autumn in anticipation of seasonally reduced food availability. In the laboratory, short-day induction of body weight loss can be reversed or prevented by voluntary exercise undertaken when a running wheel is introduced into the home cage. The mechanism by which exercise prevents or reverses body weight reduction is unknown, but one hypothesis is a reversal of short-day photoperiod induced gene expression changes in the hypothalamus that underpin body weight regulation. Alternatively, we postulate an exercise-related anabolic effect involving the growth hormone axis. To test these hypotheses we established photoperiod-running wheel experiments of 8 to 16 weeks duration assessing body weight, food intake, organ mass, lean and fat mass by magnetic resonance, circulating hormones FGF21 and insulin and hypothalamic gene expression. In response to running wheel activity, short-day housed hamsters increased body weight. Compared to short-day housed sedentary hamsters the body weight increase was accompanied by higher food intake, maintenance of tissue mass of key organs such as the liver, maintenance of lean and fat mass and hormonal profiles indicative of long day housed hamsters but there was no overall reversal of hypothalamic gene expression regulated by photoperiod. Therefore the mechanism by which activity induces body weight gain is likely to act largely independently of photoperiod regulated gene expression in the hypothalamus.

Overexpression of suppressor of cytokine signaling 3 in the arcuate nucleus of juvenile Phodopus sungorus alters seasonal body weight changes

The profound seasonal cycle in body weight exhibited by the Djungarian hamster (Phodopus sungorus) is associated with the development of hypothalamic leptin resistance during long day photoperiod (LD, 16:8 h light dark cycle), when body weight is elevated relative to short day photoperiod (SD, 8:16 h light dark cycle). We previously have shown that this seasonal change in physiology is associated with higher levels of mRNA for the potent inhibitor of leptin signaling, suppressor of cytokine signaling-3 (SOCS3), in the arcuate nucleus (ARC) of LD hamsters relative to hamsters in SD. The alteration in SOCS3 gene expression preceded the body weight change suggesting that SOCS3 might be the molecular switch of seasonal body weight changes. To functionally characterize the role of SOCS3 in seasonal body weight regulation, we injected SOCS3 expressing recombinant adeno-associated virus type-2 (rAAV2-SOCS3) constructs into the ARC of leptin sensitive SD hamsters immediately after weaning. Hamsters that received rAAV2 expressing enhanced green fluorescent protein (rAAV2-EGFP) served as controls. ARC-directed SOCS3 overexpression led to a significant increase in body weight over a period of 12 weeks without fully restoring the LD phenotype. This increase was partially due to elevated brown and white adipose tissue mass. Gene expression of pro-opiomelanocortin was increased while thyroid hormone converting enzyme DIO3 mRNA levels were reduced in SD hamsters with SOCS3 overexpression. In conclusion, our data suggest that ARC-directed SOCS3 overexpression partially overcomes the profound seasonal body weight cycle exhibited by the hamster which is associated with altered pro-opiomelanocortin and DIO3 gene expression.

Molecular pathways involved in seasonal body weight and reproductive responses governed by melatonin

Seasonal mammals typically of temperate or boreal habitats use the predictable annual cycle of daylength to initiate a suite of physiological and behavioural changes in anticipation of adverse environmental winter conditions, unfavourable for survival and reproduction. Daylength is encoded as the duration of production of the pineal hormone melatonin, but how the melatonin signal is decoded has been elusive. From the studies carried out in birds and mammals together with the advent of technologies such as microarray analysis of gene expression, progress has been achieved to demystify how seasonal physiology is regulated in response to the duration of melatonin signalling. The critical tissue for the action of melatonin is the pars tuberalis (PT) where melatonin receptors are located. At the molecular level, regulation of cyclic adenosine monophosphate (cAMP) signalling in this tissue is likely to be a key event for melatonin action, either an acute inhibitory action or sensitization of this pathway by prolonged stimulation of melatonin receptors reflecting durational melatonin presence. Melatonin action at the PT has been shown to have both positive and negative effects on gene transcription, incorporating components of the circadian clock as part of the mechanism of decoding the melatonin signal and regulating thyrotrophin-stimulating hormone (TSH) expression, a key output hormone of the PT. Microarray analysis of gene expression of PT tissue exposed to long and short photoperiods has identified important new genes that may be regulated by melatonin and contributing to the seasonal regulation of TSH production by this tissue. In the brain, tanycytes lining the third ventricle of the hypothalamus and regulation of thyroid hormone synthesis by PT-derived TSH in these cells are now established as an important component of the pathway leading to seasonal changes in physiology. Beyond the tanycyte, identified changes in gene expression for neuropeptides, receptors and other signalling molecules pinpoint some of the areas of the brain, the hypothalamus in particular, that are likely to be involved in the regulation of seasonal physiology.

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.

Seasonal regulation of cocaine- and amphetamine-regulated transcript in the arcuate nucleus of Djungarian hamster (Phodopus sungorus)

The hypothalamic neuropeptidergic system involved in the photoperiodic control of energy metabolism in seasonal mammals, is poorly understood. In the present study we examined whether distribution and number of the hypothalamic neuronal cell populations containing cocaine- and amphetamine-regulated transcript (CART) are influenced by different photoperiod and ambient temperature, or by food status in the Djungarian hamster (Phodopus sungorus). Hamsters bred and raised in long day photoperiod at room temperature (16 h light/8h dark at 23 degrees C; LD) were transferred to short day photoperiod and moderate cold (8h light/16 h dark at 16 degrees C; SD). After a 4 weeks acclimation period, uterus and body weight were decreased in SD as compared to controls maintained in LD. The number of CART-immunoreactive cells within the arcuate nucleus (ARC) was significantly higher in SD hamsters compared to LD control. This increase was restricted to the rostro to mid portion of the ARC, specifically in the hypothalamic retrochiasmatic area close to the rostral ARC and in the hypothalamic region lateral to the ARC and ventral to the ventromedial hypothalamic nuclei. In similar hypothalamic regions, food deprivation for 48 h significantly decreased the number of CART-immunoreactive cells in SD hamsters. Shortening of photoperiod combined with lowering of ambient temperature and food deprivation had no effect on the number of CART-immunoreactive cells in the lateral hypothalamic area. These findings suggest that photoperiod and ambient temperature influence energy metabolism potentially by alterations of the CART neuronal system in the rostral portion of the ARC in Djungarian hamsters.

Diurnal profiles of hypothalamic energy balance gene expression with photoperiod manipulation in the Siberian hamster, Phodopus sungorus

Hypothalamic energy balance genes have been examined in the context of seasonal body weight regulation in the Siberian hamster. Most of these long photoperiod (LD)/short photoperiod (SD) comparisons have been of tissues collected at a single point in the light-dark cycle. We examined the diurnal expression profile of hypothalamic genes in hamsters killed at 3-h intervals throughout the light-dark cycle after housing in LD or SD for 12 wk. Gene expression of neuropeptide Y, agouti-related peptide, proopiomelanocortin, cocaine- and amphetamine-regulated transcript, long-form leptin receptor, suppressor of cytokine signaling-3, melanocortin-3 receptor, melanocortin-4 receptor, and the clock gene Per1 as control were measured by in situ hybridization in hypothalamic nuclei. Effects of photoperiod on gene expression and leptin levels were generally consistent with previous reports. A clear diurnal variation was observed for Per1 in the suprachiasmatic nucleus in both photoperiods. Temporal effects on expression of energy balance genes were restricted to long-form leptin receptor in the arcuate nucleus and ventromedial nucleus, where similar diurnal expression profiles were observed, and melanocortin-4 receptor in the paraventricular nucleus; these effects were only observed in LD hamsters. There was no variation in serum leptin concentration. The 24-h profiles of hypothalamic energy balance gene expression broadly confirm photoperiodic differences that were observed previously, based on single time point comparisons, support the growing consensus that these genes have a limited role in seasonal body weight regulation, and further suggest limited involvement in daily rhythms of food intake.

Hypothalamic neuropeptide gene expression during recovery from food restriction superimposed on short-day photoperiod-induced weight loss in the Siberian hamster

Previously, 40% food restriction of male Siberian hamsters over 21 days in short-day (SD) photoperiod induced characteristic changes in expression of hypothalamic arcuate nucleus energy balance genes; mRNAs for neuropeptide Y, agouti-related peptide, and leptin receptor were upregulated, and those of proopiomelanocortin and cocaine- and amphetamine-regulated transcript were depressed. The present study examined the effect of refeeding hamsters for 6 days (approximately 50% recovery of weight differential) or 19 days (resumption of appropriate weight trajectory). Hyperphagia continued throughout refeeding, but differences in fat pad weights and leptin levels had disappeared after 19 days. Cocaine- and amphetamine-regulated transcript gene expression was depressed by prior restriction in both refed groups. The depressive effect of prior restriction on proopiomelanocortin gene expression had disappeared after 19 days of refeeding. There was no effect of prior food restriction on neuropeptide Y or agouti-related peptide gene expression. Expression of the anorexigenic brain-derived neurotrophic factor was downregulated in the ventromedial nucleus after SD exposure for 12 wk. In the SD food restriction study, there were effects of photoperiod on brain-derived neurotrophic factor gene expression but not of prior food restriction. Hypothalamic energy balance genes in the hamster respond asynchronously to return to a seasonally appropriate body weight. The achievement of this weight rather than the weight at which caloric restriction was imposed is the critical factor. The differential responses of hypothalamic energy balance genes to food restriction and refeeding are poorly characterized in any species, a critical issue given their potential relevance to human weight loss strategies that involve caloric restriction.

Appetite regulation and seasonality: implications for obesity

High circulating concentrations of leptin in obesity are associated with an apparent loss of its characteristic anorexic action within the hypothalamic region of the brain. Central insensitivity to leptin may therefore contribute to the aetiology of this disease, and an increased understanding of the underlying mechanisms will identify potential means of prevention and/or therapeutic targets. Seasonal animals such as sheep and Siberian hamsters (Phodopus sungorus) exhibit annual photoperiod-driven cycles of appetite and body weight. Increased food intake and weight gain in long days (summer) are associated with high circulating leptin, and decreased intake and weight loss in short days (winter) with low leptin. Critically, these cycles are associated with reversible changes in sensitivity to leptin. High sensitivity is seen in short days and relative insensitivity in long days, demonstrated both in sheep given leptin centrally via intracerebroventricular cannulas and in hamsters given leptin peripherally. In addition, primary hypothalamic appetite-regulating targets for leptin (i.e. neuropeptide Y, melanocortin and cocaine- and amphetamine-regulated transcript pathways) respond differently in these species to changes in circulating leptin and nutritional status induced by photoperiod as opposed to such changes induced by food restriction. Studies of seasonal animals will help to resolve causes of altered sensitivity to leptin and whether these changes reflect altered transport into the brain and/or altered signalling at the receptor or post-receptor level. Increased knowledge of the mechanism(s) and time-course for development and reversal of reduced central leptin sensitivity will provide new insights into the development and control of obesity.

Photoperiodic regulation of insulin receptor mRNA and intracellular insulin signaling in the arcuate nucleus of the Siberian hamster,Phodopus sungorus

During the last 5 years it has been well established that photoperiod-induced changes in body weight in the seasonal hamster, Phodopus sungorus, are accompanied by a marked seasonal cycle in leptin sensitivity. In the present study, we investigated the possible involvement of insulin signaling in seasonal body weight regulation. We analyzed the expression pattern and relative intensity of insulin receptor (IR), phosphatidylinositol 3-kinase (PI3-kinase), and protein tyrosine phosphatase 1B (PTP1B) mRNAs by in situ hybridization in the brains of juvenile female hamsters acclimated to either long- (LD) or short-day length (SD) for 8 wk, with or without superimposed food deprivation for 48 h. Furthermore, the hypothalamic concentration and distribution of phospho-AKT, a marker of PI3-kinase activity was determined by immunoblotting and immunohistochemistry. Eight weeks of acclimation to SD led to a substantial downregulation of IR, PTP1B gene expression, and phospho-AKT concentration in this brain region, whereas PI3-kinase mRNA was unchanged. Food deprivation induced a decrease in PTP1B and a trend toward lowered IR gene expression in LD but not in SD. Additionally, a striking increase in PTP1B gene expression in the thalamus was observed after food deprivation in both photoperiods. The direction of change in neuronal insulin signaling contrasts to the central catabolic nature of this pathway described in other species. SD-induced reduction in insulin signaling may be due to decline in body fat stores mediated by enhanced central leptin sensitivity. Increased anorexigenic tone of leptin may overwrite central insulin signaling to prevent catabolic overdrive.

The suppressor of cytokine signalling 3, SOCS3, may be one critical modulator of seasonal body weight changes in the Siberian hamster, Phodopus sungorus

The Siberian hamster, Phodopus sungorus, exhibits a remarkable cycle of body weight, reproduction and leptin sensitivity in response to a seasonal change in photoperiod. In the present study, we investigated the hypothesis that the suppressor of cytokine signalling 3 (SOCS3) plays a critical role in the regulation of the seasonal body weight cycle. We analysed arcuate nucleus SOCS3 gene expression in short day length (SD; 8 : 16 h light/dark) acclimated Siberian hamsters that were transferred back to long day length (LD; 16 : 8 h light/dark) and in hamsters that spontaneously became photorefractory to SD induced by prolonged exposure. SD acclimated hamsters that were transferred back to LD for 1, 2, 3, 4 or 6 weeks, increased arcuate nucleus SOCS3 gene expression to the LD level within 2 weeks, and maintained this higher level thereafter. The early increase of SOCS3 gene expression preceded the LD-induced rise in body weight by approximately 3 weeks. Hamsters kept in SD for an extended period (25 weeks), began to become refractory to SD and to increase body weight. By this time, there was no difference in level of SOCS3 gene expression between LD and SD photoperiods, although body weight was still suppressed in SD hamsters. Finally, we addressed whether SOCS3 gene expression is related to SD-induced gonadal regression or to body weight decrease by comparing Siberian hamsters with Syrian hamsters. The latter exhibited substantial SD-induced gonadal regression but only limited seasonal changes in body weight. Acclimation to either LD or SD for 14 weeks had no effect on SOCS3 gene expression. This implies that arcuate nucleus SOCS3 gene expression is unlikely to be related to seasonal cycles in reproductive activity. Taken together, the findings further strengthen our hypothesis that SOCS3 may be one molecular trigger of seasonal cycles in body weight.

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.

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.

Circulating ghrelin levels and central ghrelin receptor expression are elevated in response to food deprivation in a seasonal mammal (Phodopus sungorus)

Ghrelin is an endogenous ligand for the growth hormone secretagogue receptor (GHSR). However, the functional interaction of ligand and receptor is not very well understood. We demonstrate that GHSR mRNA is up-regulated after food deprivation (48 h) in the hypothalamic arcuate nucleus and ventromedial nucleus of the seasonal Siberian hamster, Phodopus sungorus. This increase is accompanied by a two-fold elevation of circulating ghrelin concentration. Chronic changes in feeding state imposed by food restriction over a period of 12 weeks during long day-length induced increased GHSR gene expression, whereas food restriction for 6 weeks had no effect. Phodopus sungorus reveals remarkable seasonal changes in body weight, fat mass and circulating leptin levels. Ghrelin is generally regarded as having opposing effects on appetite and body weight with respect to those exhibited by leptin. However, our study revealed that seasonal adaptations were not accompanied by changes in either GHSR gene expression or circulating ghrelin concentration. Therefore, we suggest that ghrelin only plays a minor role in modulating long-term seasonal body weight cycles. Our findings imply that ghrelin predominantly acts as a short-term regulator of feeding.

Obesity: the integrated roles of environment and genetics

Obesity represents one of the most serious global health issues with approximately 310 million people presently affected. It develops because of a mismatch between energy intake and expenditure that results from behavior (feeding behavior and time spent active) and physiology (resting metabolism and expenditure when active). Both of these traits are affected by environmental and genetic factors. The dramatic increase in the numbers of obese people in Western societies reflects mostly changing environmental factors and is linked to reduced activity and perhaps also increased food intake. However, in all societies and subpopulations, there are both obese and nonobese subjects. These differences are primarily a consequence of genetic factors as is revealed by the high heritability for body mass index. Most researchers agree that energy balance and, hence, body weight are regulated phenomena. There is some disagreement about exactly how this regulation occurs. However, a common model is the "lipostatic" regulation system, whereby our energy stores generate signals that are compared with targets encoded in the brain, and differences between these drive our food intake levels, activity patterns, and resting and active metabolisms. Considerable advances were made in the last decade in understanding the molecular basis of this lipostatic system. Some obese people have high body weight because they have broken lipostats, but these are a rare minority. This suggests that for the majority of obese people, the lipostat is set at an inappropriately high level. When combined with exposure to an environment where there is ready availability of food at low energy costs to obtain it, obesity develops. The evolutionary background to how such a system might have evolved involves the evolution of social behavior, the harnessing of fire, and the development of weapons that effectively freed humans from the risks of predation. The lipostatic model not only explains why some people become obese whereas others do not, but also allows us to understand why energy-controlled diets do not work. Drug-based solutions to the obesity problem that work with the lipostat, rather than against it, are presently under development and will probably be in regular use within 5-10 y. However, several lines of evidence including genetic mapping studies of quantitative trait loci associated with obesity suggest that our present understanding of the regulatory system is still rudimentary. In particular, we know nothing about how the target body weight in the brain is encoded. As our understanding in this field advances, new drug targets are likely to emerge and allow us to treat this crippling disorder.

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.

Photoperiodic regulation of leptin sensitivity in the Siberian hamster, Phodopus sungorus, is reflected in arcuate nucleus SOCS-3 (suppressor of cytokine signaling) gene expression

We present the first evidence that suppressor of cytokine signaling-3 (SOCS3), a protein inhibiting Janus kinase/signal transducer and activator of transcription (STAT) signaling distal of the leptin receptor, conveys seasonal changes in leptin sensitivity in the Siberian hamster. Food deprivation (48 h) reduced SOCS3 gene expression in hamsters acclimated to either long (LD) or short (SD) photoperiods, suggesting that leptin signals acute starvation regardless of photoperiod. However, SOCS3 mRNA levels were substantially lower in the hypothalamic arcuate nucleus of hamsters acclimated to SD than in those raised in LD. In juveniles raised in LD, a rapid increase in SOCS3 mRNA was observed within 4 d of weaning, which was completely prevented by transfer to SD on the day of weaning. The early increase in SOCS3 gene expression in juvenile hamsters in LD clearly preceded the establishment of different body weight trajectories in LD and SD. In adult LD hamsters, SOCS3 mRNA was maintained at an elevated level despite the chronic food restriction imposed to lower body weight and serum leptin to or even below SD levels. A single injection of leptin in SD hamsters elevated SOCS3 mRNA to LD levels, whereas leptin treatment had no effect on SOCS3 gene expression in LD hamsters. Our results suggest that the development of leptin resistance in LD-acclimated hamsters involves SOCS3-mediated suppression of leptin signaling in the arcuate nucleus. Increased SOCS3 expression in LD hamsters is independent of body fat and serum leptin levels, suggesting that the photoperiod is able to trigger the biannual reversible switch in leptin sensitivity.