NEUROSCIENCE AND BIOBEHAVIORAL REVIEWS

PERGAMON Neuroscience and Biobehavioral Reviews 25 @2001) 101±116 www.elsevier.com/locate/neubiorev Review Hypothalamic mechanisms for regulating energy balance: from rodent models to human obesity

Julian G. Mercera,*, John R. Speakmana,b

aRowett Research Institute, Aberdeen Centre for Energy Regulation and Obesity ACERO), Bucksburn, Aberdeen AB21 9SB, UK bDepartment of Zoology, University of Aberdeen, Aberdeen AB24 2TZ, UK Received 20 October 2000; accepted 1 December 2000

Abstract In small rodents there is compelling evidence of a lipostatic system of body mass regulation in which peripheral signals of energy storage are decoded in the hypothalamus. The ability of small mammals to defend an appropriate mass against imposed energy imbalance has implicated hypothalamic neuroendocrine systems in body mass regulation. The effect of the neuropeptide systems involved in this regulation is primarily compensatory. However, small mammals can also effect changes in the level of body mass that they will defend, as exempli®ed by seasonal species. Regulatory control over fat mass may be relatively loose in humans; the sizes of long-term storage depots may not themselves be regulated, but rather may be a consequence of temporal variations in the matching of supply and demand. Whether food intake is regulated to match energy demand, or to match demand and to regulate storage, it is clear that physiological defects or genetic variation in hypothalamic and peripheral feedbacksystems will have profound implications for fat storage. Study of mechanisms implicated in energy homeostasis in laboratory rodents is likely to continue to identify targets for pharmacological manipulation in the management of human obesity. q 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Body mass; Body weight; Body weight set point; Siberian hamster; Phodopus; Photoperiod; Hypothalamic ; ;

Contents 1. Introduction ...... 101 2. An evolutionary perspective on the need to regulate fat storage in small mammals ...... 102 3. Hypothalamic involvement in body mass regulation ...... 103 4. Hypothalamic neuropeptides in body mass defence ...... 104 5. Programmed body mass change in mammals ...... 106 6. Body massÐthe sliding `set point' ...... 107 7. Dietary manipulation of defended body mass ...... 107 8. Seasonal body mass regulation in the Siberian hamster ...... 108 9. An evolutionary perspective on fat storage regulation in humans ...... 110 10. The human hypothalamus in control of food intake and adiposity ...... 111 11. Conclusions ...... 112 Acknowledgements ...... 113 References ...... 113

1. Introduction needs vary according to what the individual is doing at any particular time, i.e. growing, reproducing, migrating etc. All animals eat food to satisfy their needs for energy and Moreover the problem of meeting these demands is exacer- nutrients. These requirements are not in a ®xed ratio because bated by the fact that food is extremely heterogeneous in its composition. The taskis therefore to pickfrom the available foods a suitable combination of food types and quantities to * Corresponding author. Tel.: 144-1224-716662; fax: 144-1224- 716653. satisfy diverse and temporally variable requirements. Due to E-mail address: [email protected]@J.G. Mercer). alternative behavioural necessities, food intake must be

0149-7634/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0149-7634@00)00053-1 102 J.G. Mercer, J.R. Speakman / Neuroscience and Biobehavioral Reviews 25 2001) 101±116 and demand must have been matched to a precision of around 0.2% [5]. Since we do not accumulate large stores of protein, calcium or other nutrients it can be presumed that all these other systems must also exhibit the same degree of precise regulation. All mammals vary over time in the amount of energy that they maintain in long-term storage. These changes can occur over both protracted @months and years) and relatively short periods @days and weeks). An important question is whether the magnitudes of the long- term storage depots are themselves a regulated reserve, or whether the levels of storage are simply an epiphenomenon of temporal variations in the matching of supply and demand @Fig. 1). In one simple @unregulated) model, indi- viduals match energy expenditure and energy intake without reference to the level of fat storage. When food intake exceeds demand animals get fat, and when expenditure exceeds intake they get thin. In the alternative model, the fat @energy) depots, and also presumably the levels of other nutrient depots, provide signals to the brain about their current status. These may then be compared with the desir- Fig. 1. A feedbacksystem matching multidimensional nutrient supply and demand @here represented by two dimensions±energy and nitrogen). Flows able upper and lower limits of storage, and food intake and of materials @energy and nutrients) are illustrated by heavy solid lines. expenditure modulated to effect adjustments to the level of Peripheral signals to the brain indicating levels of the relevant parameters long term storage. In energy terms this second model is the are indicated by solid lines. Efferent control signals from the brain are familiar lipostatic model originally formulated by Kennedy illustrated by heavy dashed lines. Important factors in¯uencing levels of [6]. It is important to distinguish between these two alter- pertinent phenomena are illustrated by dashed lines. In a more complex regulatory model @model 2), feedbackis also transmitted to the brain about native systems because they have extremely important levels of long term storage of nutrients and energy @dotted lines). consequences for the manner in which we interpret phenom- ena connected with regulation of energy and nutrient discontinuous in nature, yet the requirements for resources balance. A common point, however, is that both systems are continuous. Consequently there is a need for the short- involve regulation of food intake either to match demands term storage of energy and nutrients, to smooth out the @model 1), or to match demands and regulate storage @model supply so that it meets demand. Because animals are feeding 2). Physiological defects or genetic variation in either to satisfy a variety of different requirements, these short- system will have profound implications for the levels of term storage mechanisms must often be overwhelmed by the fat storage. ¯ux of any particular type of nutrient. Faced with this In this review we aim to do several things. We ®rst mismatch of supply and demand animals must do one of consider the question of the need for a lipostatic system in two things: store the excess in a more long-term storage, or small rodents in an evolutionary context of the adaptive dispose of it. value of such a system. We then explore the nature of the For most mammals, the majority of excess energy is signalling within this system and most particularly the deposited into adipose tissue stores as fat. However, some hypothalamic neuropeptides which form the mechanism mammals appear able to cope with excess energy by burn- by which peripheral signals are decoded and efferent signals ing it off. Frugivorous bats, for example, consume a food controlling food intake and energy utilisation are produced. that is high in energy but low in protein [1]. To meet daily We will use as examples several well-studied rodent protein demands, fruit bats must take in far more energy models. We then turn our attention to an evolutionary than they require. Rather than store the excess as fat they context for the adaptive need for a lipostatic system in burn it off by ¯ying [2]. It has been suggested that humans humans, and review the evidence concerning the roles of may also burn off excess energy intake during dietary hypothalamic neuropeptides in this system. induced thermogenesis [3] and that individual differences in this capacity contribute to differences in long-term storage. The recent implication that there may also be 2. An evolutionary perspective on the need to regulate fat other mechanisms for `burning off' excess energy ®ts into storage in small mammals this framework[4]. The matching of supply and demand over relatively long One assumption implicit in the models presented in the time periods is very good, implying the existence of a introduction was that food is always available. The problem sophisticated regulatory system. The level of fat accumu- facing an animal is therefore only to select from the avail- lated over a typical decade by a human suggests that supply able resources what and how much to eat. However, in the J.G. Mercer, J.R. Speakman / Neuroscience and Biobehavioral Reviews 25 2001) 101±116 103 real world the food supply is not guaranteed. Hence to over- centre for afferent signals of energetic status. However, come the uncertainty in food supply, animals need to main- beyond the interaction of hypothalamic neuropeptides tain fat stores that are larger than are immediately required with their receptors, the mechanisms that determine energy to get them from meal to meal. Due to the scaling effects of balance and body mass/composition are largely unde®ned. size on daily energy demands, the importance of fat storage Lesioning and electrical stimulation studies conducted in for survival is greatest in small animals. Studies of daily the 1950s provided the ®rst evidence that the hypothalamus energy requirements of free-living small mammals [7], was a major centre in the control of food intake and body measured using the doubly-labelled water technique [8], mass [18]. `Hunger' and `satiety' centres were proposed in have revealed that on average the energy requirements of the lateral hypothalamic area @LHA) and the ventromedial a 30 g small mammal amount to about 63 kJ/day. Even by hypothalamic nucleus @VMN), respectively, in the light of storing 10% of its body mass as fat such a small mammal the consumatory and body mass consequences of these would have the capacity to survive for less than 2 days in the manipulations. These broad regional designations have complete absence of food supplies @39 kJ/g £ 3gˆ 117 kJ now been replaced by a more detailed knowledge of the of fat stored/63 kJ/day ˆ 1.86 days survival). There must be anabolic and catabolic molecular substrates involved in strong selective pressures therefore for small animals to energy balance, the neuronal populations involved, and store fat, and these pressures are likely to be greater, result- their regulation and integration [19±22]. ing in greater levels of storage, as the stochasticity in food Prior to the landmarkcloning of the ob or leptin gene supplies increases. [23], the existence of a circulating hormone conveying Despite the strong pressures to store greater quantities of information about the size of adipose tissue reserves had fat, animals in the wild are seldom recovered with a body been predicted from parabiosis studies [24], and was a condition equivalent to morbid obesity. This is because central tenet of the lipostatic theory of food intake and there is a counterbalancing selective pressure. Animals stor- body mass regulation [6]. It was also predicted that the ing large amounts of body fat will be slower and less mouse diabetic @db) mutation induced obesity by functional manoeuvrable, increasing the riskof predation. Body fat lesion of a receptor for this adiposity signal [24]. While storage is therefore likely to be a trait on which there are there is strong evidence that the pancreatic hormone, insu- strong selective pressures both at the upper and lower lin, is involved in the CNS control of food intake [19,20], the margins. Under increasing unpredictability in food supply, molecular genetic explanation for the parabiosis data was animals might be predicted to increase their fat storage, and provided by the cloning of the leptin [23] and conversely when the risks of predation are high, animals [25] genes. Leptin is primarily a product of white adipose would be predicted to reduce their levels of fat storage tissue. In laboratory rodents, with the notable exception of [9±11]. Several experimental tests have con®rmed these the ob/ob mice, serum leptin re¯ects the overall level of predictions in wild animals, most notably in small birds adiposity of the animal and its energy balance state, being [12±17]. Small mammals and birds therefore need to regu- elevated in obesity and lowered following acute manipula- late their body fatness within relatively narrow margins that tions such as food deprivation [26]. Serum leptin concentra- are themselves sensitive to changes in the environment in tions in humans also correlate positively with body which the animals ®nd themselves. adiposity indices such as body mass index @BMI) [27]. Recombinant leptin protein reverses the hyperphagia and obese phenotype of the ob/ob mouse and also reduces 3. Hypothalamic involvement in body mass regulation food intake and body mass in normal lean rodents [28±30]. Exogenous leptin is more potent when administered To maintain a relatively stable body mass within such directly into the cerebroventricular system of the brain, narrow margins, accurate regulatory systems must exist to particularly in normal rodents. This route of delivery match energy intake to energy expenditure. Indeed such would allow a higher concentration of injected hormone sensitivity is essential if a reasonable degree of body mass to access hypothalamic structures. Sites of action within control is to be effected. In a variable environment charac- the brain were quickly con®rmed by the mapping of terised by periodic food shortage there will inevitably be neurones activated following leptin injection @early times when animals are forced to draw on their energy response gene activation; [31,32]), and by the description reserves. The behaviour of many mammals upon refeeding of leptin receptor expression in a number of brain regions, following such challenges supports the notion that there is a including hypothalamic nuclei with an established regula- target body mass or body mass range. The restoration of tory function in energy homeostasis [33]. It is now clear that normal levels of body mass, adiposity and food intake is leptin interacts via its receptors with hypothalamic neurones now known to involve the interaction of a number of neural, with a number of different neurochemical phenotypes, endocrine and neurochemical signals. The critical feature of thereby providing feedbackon peripheral energetic status this regulatory system is the feedbackof signals from to regulatory systems in the brain [34,35]. Leptin provides adipose tissue, as well as from other peripheral sites, to two distinct forms of feedback. First, there are profound the hypothalamus, the latter forming the major integratory diurnal changes in leptin gene expression, which re¯ect 104 J.G. Mercer, J.R. Speakman / Neuroscience and Biobehavioral Reviews 25 2001) 101±116

Table 1 Neuropeptides involved in the control of energy balance

Peptide Acronym Relevant site@s) of synthesis

Anabolic/orexigenic NPY Arcuate nucleus @ARC) Agouti-related protein AGRP ARC Melanin-concentrating hormone MCH Lateral hypothalamus @LHA) @hypocretins) ± LHA ± Paraventricular nucleus @PVN) Catabolic/anorexigenic Alpha-melanocyte-stimulating hormone alpha-MSH ARC Cocaine- and amphetamine-regulated transcript CART ARC, other hypothalamic sites? Corticotrophin-releasing factor CRF PVN Glucagon-like -1 GLP-1 Nucleus tractus solitarius @NTS) Glucagon-like peptide-2 GLP-2 NTS -releasing peptide PrRP Dorsomedial nucleus @DMH), NTS the diurnal pattern of food intake. Leptin rises 4±6 h out of or to imposed overfeeding @i.e. gavage or tube feeding), phase with food intake and therefore peaks in humans express a compensatory hyper- or hypophagia upon return during the early night, and in rodents during the later part to ad libitum feeding until they return to a body mass that is of the night [36±38]. Changes in feeding patterns can alter similar to that of freely-feeding controls [46]. the diurnal pattern of leptin expression [39] demonstrating A range of compensatory responses is evoked following that it is food intake rather than an endogenous pattern imposed energetic manipulations that challenge normal triggered by the light:darkcycle that controls the expres- body mass. These compensatory changes may also be sion. This daily leptin surge appears to encode a signal induced in physiological states with similar effects on re¯ecting daily food intake, and its absence is a potent signal energy balance. In either case, regulatory pathways act in of starvation [40±42]. Second, basal levels of leptin during concert to defend body mass at its existing level or to restore the daily nadir encode for the levels of body fat storage. This it to that level once conditions allow. During food depriva- second role may indeed be secondary in importance to the tion, for example, blood concentrations of leptin and , role of leptin as a food intake/starvation signal [41], which re¯ect body adiposity and energy ¯ux, are reduced although commonly it is this second role that is emphasised and are accompanied by appropriate changes in the activity [43±45]. In small mammals, the regulatory loops formed by of a number of neuropeptide systems in the hypothalamus leptin and its downstream signalling systems permits which evoke anabolic and catabolic responses. These `sensing' of both daily food intake and body fat stores by hypothalamic events in turn drive the compensatory adjust- the hypothalamus, and thus expression of the behavioural ments in caloric intake and energy expenditure necessary to and metabolic adjustments required to regulate energy restore body mass and body composition to an appropriate balance. level. In general, the behaviour of endogenous hypothalamic neuropeptide systems during imposed energy imbalance 4. Hypothalamic neuropeptides in body mass defence can be predicted from consideration of the in vivo effects of these upon exogenous administration directly As might be anticipated from the evolutionary scenario into the brain. The energy balance-related peptides can be presented above small mammals appear to possess regula- divided on the basis of in vivo activity into those that tory mechanisms for `defending' their body mass/composi- increase food intake and simultaneously reduce energy tion within certain limits. Much of the available evidence expenditure, and thus have an overall anabolic effect, and suggests that animals attempt to maintain a stable or appro- those with the opposite effects contributing to a net catabolic priate body fat content. While fat mass may be the primary outcome @Table 1). The generic categorisation of anabolic regulated variable, signals from other nutrient stores @such @orexigenic) and catabolic @anorexigenic) effector pathways as lean body tissue) may also be important in long-term has been outlined in a number of recent major reviews mass recovery. In laboratory rodents the recovery response [19,20]. From this perspective, it might be anticipated that following experimentally-induced mass change can be read- in negative energy balance, such as following food depriva- ily demonstrated. Animals habitually return to a body mass tion or food restriction, the activity of anabolic systems that is appropriate for their age, stage of development and/or would be increased, while that of catabolic systems would environment after an imposed perturbation. For example, be reduced. Conversely, in imposed positive energy rats subjected to a period of food deprivation or restriction, balance, the opposite responses would be expected. These J.G. Mercer, J.R. Speakman / Neuroscience and Biobehavioral Reviews 25 2001) 101±116 105 complementary changes would act to restore body mass to tary hypophagia. The likely involvement of the melanocor- an appropriate level. A key attribute for a neuropeptide with tin system in this defence of a lower body mass and/or primary involvement in body mass regulation might be that restoration of energy balance was supported by the ability the activity of such a system should remain appropriately of a melanocortin receptor antagonist to reverse regulatory elevated/lowered until normal body mass is attained, rather hypophagia [58]. NPY gene expression was not affected by than re¯ecting normalisation of energy intake or the period the voluntary hypophagia, suggesting again that the state of of ad libitum feeding. energy balance, rather than the absolute level of food intake, Complete food deprivation @FD) is the best-characterised is the critical parameter in determining the activity of this paradigm for examining the defence of body mass in labora- system [57]. tory rodents. Similarly, neuropeptide Y @NPY) is the best- In addition to analysis of the effects of exogenous studied hypothalamic neuropeptide system [47]. Food depri- peptides in vivo and the responses of endogenous systems vation-induced peripheral feedbackreaches the rat hypotha- to energetic challenge, the study of transgenic and mutant lamus quite rapidly; recent evidence suggests that FD mice has been particularly useful in identifying those beginning 2 h before lights out results in elevated NPY hypothalamic signalling systems that are critical to normal gene expression within 6 h of the withdrawal of food [48]. body mass control [59]. As already discussed, a functional Within the hypothalamic arcuate nucleus @ARC), the leptin signalling pathway is essential for maintenance of expression of anabolic peptide systems such as NPY and normal body mass, with mutations of either the hormone agouti-related protein @AGRP; [49]) is increased by FD, as itself or of its receptor giving rise to profound obesity is VGF mRNA [50]. Conversely, expression of pro-opiome- [23,25]. A competent melanocortin system is also required. lanocortin @POMC; [51]) mRNA, the precursor for alpha- The complex nature of this hypothalamic system is still melanocyte stimulating hormone @alpha-MSH), and of emerging @Fig. 2). Leptin inputs directly into the melano- cocaine- and amphetamine-regulated transcript @CART; cortin system via its receptors on POMC neurones and [52]), both catabolic systems, is reduced. These changes AGRP neurones in the ARC [34,35,60]. Alpha-MSH, one in neuropeptide activity are probably mediated directly by of the cleavage products of the POMC precursor, is the main leptin since these ARC populations of peptidergic neurones agonist of the melanocortin-4 receptor @MC4-R), where it also express leptin receptors [34,35]. The mRNAs for the promotes negative energy balance. Perhaps the ®rst hint of anabolic neuropeptides, melanin-concentrating hormone the importance and complexity of this system came from the @MCH) and the orexins @expressed in the LHA and zona positional cloning of the gene responsible for obesity in the incerta @ZI)), are also elevated during FD [53,54], whereas agouti @Ay/a) mouse [61,62]. The protein product of this corticotrophin-releasing factor @CRF) mRNA in the para- gene, agouti, is an antagonist at the melanocortin-1 receptor ventricular nucleus @PVN) is reduced [55]. The normalisa- and is normally expressed in hair follicles. However, in the tion of neuropeptide gene expression following FD may agouti mouse, ectopic expression of agouti protein in the indeed be primarily determined by the speed of return to a brain results in antagonism at the MC4-R, and obesity. normal @appropriate) body mass; preventing the expression AGRP is the natural homologue of agouti, and is an endo- of hyperphagia during refeeding following FD @food intake genous antagonist at MC4-R, promoting positive energy held at pre-fast levels), such that body mass remains below balance [63]. Transgenic over-expression of AGRP also normal, also maintained elevated NPY gene expression gives rise to obesity [64], presumably through the same [56]. mechanism as ectopic expression of agouti. POMC knock- It would perhaps be surprising if there were not robust out mice, which lackall the peptides that are generated by regulatory systems to minimise the riskof starving to death processing of the POMC precursor including alpha-MSH, and to accelerate recovery from severe negative energy are obese, with impaired adrenal development and altered balance. According to the evolutionary perspective pigmentation [65]. Administration of an alpha-MSH agonist discussed earlier, the response to overfeeding and to atten- causes mass loss in the obese POMC knockout model. Data dant increases in adiposity may be of similar importance, obtained from the MC4-R knockout mouse, which is hyper- although this aspect of the energy balance regulatory phagic and obese [66], is consistent with a critical role for process has received much less attention. This probably this receptor in limiting food intake and the accumulation of re¯ects the relative dif®culty in setting up such studies. fat mass. Involuntary overfeeding via an implanted gastrostomy Another melanocortin receptor subtype, MC3-R, is also tube at a level that produced a 5% mass increase in 9 days expressed in hypothalamic areas known to be involved in @up to 125% of normal caloric intake) reduced voluntary energy balance [67], but until recently the role of this recep- intake of pellet diet to less than 10% of controls. Rats tor was unknown. Alpha-MSH and AGRP also act as remained hypophagic for 3 days after tube feeding was agonist and antagonist, respectively, at the MC3-R. An indi- discontinued [57]. Overfeeding increased CRF gene expres- cation of the function of this receptor came from recent sion in the PVN by approximately 50% [57], and POMC descriptions of the phenotype of the MC3-R knockout mRNA levels in the ARC by 80% [58], increases in negative [68,69]. Inactivation of the MC3-R gives rise to mice with feedbackactivity that could be causally linkedto the volun- a relatively normal body mass, but with increased fat mass 106 J.G. Mercer, J.R. Speakman / Neuroscience and Biobehavioral Reviews 25 2001) 101±116

Fig. 2. The hypothalamic melanocortin system and its involvement in the control of body weight through regulation of food intake and feed ef®ciency. The activity of MC3-R and MC4-R is likely to be regulated by melanocortin peptide agonists, such as alpha-MSH, and the antagonist AGRP. Gamma-MSH shows selectivity to MC3-R. AGRP is a high af®nity antagonist of MC3-R and MC4-R, while agouti protein has highest af®nity for MC1-R and MC4-R, but relatively high concentrations are required for antagonism at MC3-R. The following perturbations to the signalling pathways illustrated result in obesity in rodents: loss of function mutation of leptin or leptin receptor, transgenic knockout of POMC or MC4-R, transgenic over-expression of AGRP, ectopic expression of agouti protein. and reduced lean mass. The mice are hypophagic relative to knockout mouse highlights the likely involvement of this wild-type littermates and have a higher feed ef®ciency @ratio gene in the physiological regulation of arousal and sleep of mass gain to food intake). Analysis of mice lacking both processes, rather than in body mass regulation [74]. MC3 and MC4 receptor subtypes reveals an even more rapid rate of mass gain than that observed in MC4-R knockouts. This suggests that these two receptor subtypes have differ- 5. Programmed body mass change in mammals ent, non-redundant, roles in the regulation of energy balance, thereby emphasising the importance of the mela- The ability to defend body mass or composition against nocortin system overall. energetic challenge by induction of compensatory mechan- The MCH-de®cient mouse is lean, with reduced body isms is clear evidence of regulatory capability, and the mass and hypophagia and an increased metabolic rate. pieces of this jigsaw are beginning to fall into place. These observations and others suggest that MCH is a critical However, mammals also provide examples of anticipatory regulator of feeding and energy balance which probably acts or programmed adjustments to body mass and body compo- downstream of leptin and the melanocortin system [70]. sition. The cycles of body mass and composition exhibited Similarly, a signi®cant role for VGF in energy homeostasis by seasonal mammals are a particularly good example of is indicated from the phenotype of VGF knockout mice, this [75]. These cycles, often cued from a single environ- which are small and thin, with elevated energy expenditure mental variable, photoperiod, have evolved as part of a and altered gene expression for NPY, AGRP and POMC pro®le of adaptations that enhance survival at temperate [71]. However, the outcome of such transgenic studies has latitudes. The ability to programme changes in body mass not always been so clear-cut. Mice lacking NPY feed and composition is therefore superimposed on the capacity normally and have a normal body mass phenotype [72], to defend an appropriate body mass against de®cit or excess suggesting that compensatory or redundant systems have using the neuroendocrine systems already described. The been recruited to make up for this de®ciency. However, relationship between these two levels of regulation has when viewed on the ob/ob background, the NPY knockout received little attention at a mechanistic level. It is not reduces the hyperphagic response to leptin de®ciency and clear at present whether the same downstream signalling obesity is attenuated [73]. The phenotype of the systems will be involved both in the defence of appropriate J.G. Mercer, J.R. Speakman / Neuroscience and Biobehavioral Reviews 25 2001) 101±116 107 balance. In such experiments [76], food restriction is super- imposed on short photoperiod-induced mass loss @Fig. 3), accelerating the rate of mass loss. When restriction is lifted and hamsters are again allowed to feed ad libitum, body mass increases, but only to the point where it approximates to the declining mass of control animals fed ad libitum throughout. Thus the seasonal timekeeping mechanism continues to operate even when the animals are prevented from maintaining their desired body mass [77]. The animals are subsequently able to `defend' an appropriate body mass irrespective of their recent imposed body mass manipulation. In the case of the Siberian hamster, seasonal mass changes are driven exclusively by photoperiod, and are therefore likely to involve melatonin feedback onto energy balance Fig. 3. The ªsliding set pointº of body weight regulation in Siberian pathways. Although the precise interaction between the hamsters. Effect of food restriction, followed by refeeding @SD/R), on pineal hormone, melatonin, and the body mass regulatory body weight in hamsters undergoing short day @SD) weight loss. circuitry remains to be established, there is clearly some reprogramming of appropriate body mass by the accumulat- body mass @i.e. shorter-term regulation of energy balance) ing melatonin signal. This interaction is likely to take place and in adjustments to the level of body mass that will be somewhere within the hypothalamus or thalamus, where the defended. A major goal is the description of how an `appro- majority of CNS melatonin receptors are located [78]. priate' body mass is encoded in the brain and how its regu- lation is effected. Seasonal body mass regulation may or may not involve the 7. Dietary manipulation ofdefended body mass hypothalamic systems that are implicated in energetic compensation @Table 1). These `homeostatic' systems may Although effectively illustrated by the seasonal rodent, not exhibit any perturbation from baseline as long as actual central encoding of appropriate body mass is certainly not body mass is able to accurately trackthe seasonally appro- unique to these animals. Similar regulatory systems are priate optimum [75]. Alternatively, discrete changes in activ- evident from studies of non-seasonal rodents. Whereas ity of neuropeptide systems may lead to a gradual compensatory regulation of body mass is probably effected programmed state of mass loss or mass gain, or abrupt activ- through changes in neuroendocrine signals in the energy ity changes may be involved in priming the animal for seaso- homeostasis centres of the hypothalamus, the fact that nal body mass change. In addition to the resetting of these systems act to restore an appropriate body mass appropriate body mass, changes are likely in the way in implies that there is retained information encoding this para- which feedbacksignals such as leptin are integrated into meter. Experimental evidence demonstrates that other hypothalamic regulatory systems. Such change is probably environmental manipulations are capable of inducing necessary in view of the paradoxical nature of the leptin substantial changes in defended body mass in non-seasonal signal in seasonal animals undergoing programmed mass rodents. This is illustrated by experiments in outbred Spra- change where plasma leptin will vary in parallel with seaso- gue±Dawley rats [79±81]. Rats fed on a normal stockdiet nal body adiposity changes. This raises the question of regulate their body mass normally. When transferred to a how the leptin signal is integrated into hypothalamic path- moderately high fat/energy diet, a range of responses is ways without acting to reverse the programmed body mass observed, and from this normal distribution rats have been cycle [75]. arbitrarily divided into groups that develop obesity @DIO) and those that are apparently resistant to excessive mass gain @diet resistant; DR). DR rats can be made comparably 6. Body massÐthe sliding `set point' obese if also supplied with a highly palatable, high carbo- hydrate, liquid diet. When both groups are transferred back The Siberian hamster provides some of the most compel- to a normal diet, the DIO rats defend their elevated @obese) ling evidence available in support of a `target' or optimum body mass through hyperphagia, whereas the DR rats do body mass. These animals are apparently able to continually not. The DR animals that developed obesity by feeding on adjust the body mass that will be defended according to their the liquid supplement expressed a relative hypophagia on photoperiodic history. This is the so-called `sliding set the pellet diet until their body mass falls to the level of point' of body mass regulation. Adjustments are apparently control animals [79,81]. The DIO rats also defend their made to this encoded `appropriate' body mass even when obese body mass against an imposed food restriction. actual body mass is perturbed by imposed negative energy Thus although both groups of rats expressed a similar 108 J.G. Mercer, J.R. Speakman / Neuroscience and Biobehavioral Reviews 25 2001) 101±116 regain truly represents the body attempting to default to its former mass, or whether this outcome merely re¯ects the return to relative hyperphagia and/or hypoactivity.

8. Seasonal body mass regulation in the Siberian hamster

The Siberian hamster @Phodopus sungorus) is the best- studied laboratory model of seasonal body mass regulation, and a number of studies performed in our laboratory and others have sought to describe the involvement of known energy balance-related neuropeptide and receptor systems in these cycles. Laboratory manipulations of these hamsters usually involve square wave transfers between long and short photoperiods, mimicking an abrupt switch between winter and summer daylength. Transfer of mature male hamsters from long days @16 h light/8 h dark; LD) to short days @8 h light/16 h dark; SD) induces a characteristic body mass response in the SD hamsters @Fig. 4@a)). After a 2±3 weekdelay, the body mass of SD hamsters begins to fall and after 16±20 weeks stabilises at approximately 70% of start- ing mass. This change is physiological and reversible. The majority of mass loss in SDs is due to mobilisation of adipose tissue [84], and body mass cycles are accompanied by changes in food intake, coat colour, reproductive status and energy expenditure @shallow daytime torpor). We have examined leptin signalling and hypothalamic Fig. 4. Photoperiodic regulation of body mass in @a) adult male @from [85]), gene expression in adult male Siberian hamsters kept for and @b) juvenile female Siberian hamsters @from [87]). LD, long days @16 h 18 weeks in LDs or SDs, with or without a ®nal 24 h FD. light:8 h dark); SD, short days @8 h light:16 h dark). SD hamsters had reduced body mass and adiposity, and lower adipose tissue leptin gene expression [85]. Hypotha- level of obesity following manipulation of diet, these lamic gene expression was measured by in situ hybridisa- manipulations may or may not alter @or reset) the level of tion. Gene expression in the ARC was lower in SDs for body mass that the animals will defend. leptin receptor @OB-Rb) and POMC, but higher for AGRP Whereas some form of `set-point' determining defended @Table 2). There was no effect of photoperiod on ARC NPY body mass is consistent with the characteristics of body gene expression, orexin or MCH mRNA in the LHA/ZI, or mass regulation in rodents, it is far from clear whether CRF mRNA in the PVN. An independent study of male equivalent control is present in humans. Several aspects of Siberian hamsters exposed to opposite photoperiods for 12 the DIO rat model ®nd parallel in the human population. weeks also reported reduced POMC and unaltered NPY and Consumption of a high fat diet constitutes a behavioural risk orexin mRNA abundance in SDs compared to LDs [86]. The factor for obesity in humans. However, the consumption of a effect of 24 h FD was broadly in line with the rodent data high fat diet does not necessarily result in the development discussed above, with up-regulation of OB-Rb, AGRP and of obesity. This is evidenced by description of young men NPY @caudal ARC) [85]. with a high fat and high-energy diet but with a normal BMI We have also studied changes in leptin signalling and [82,83]. The problems experienced by obese subjects hypothalamic gene expression in growing juvenile female attempting to lose weight by dieting are well documented. Siberian hamsters [87]. Hamsters were kept for up to 12 If we extrapolate from the rodent literature, it is tempting to weeks in LDs or SDs from weaning at 3 weeks of age. SD speculate that the phenomenon of weight rebound after diet- hamsters had retarded growth, and lower asymptotic body ing in human obesity could represent the outcome of mass @Fig. 4@b)), adiposity and leptin gene expression in compensatory neuroendocrine systems attempting to restore adipose tissue than LD hamsters [87]. Gene expression in fat stores to their earlier level once caloric restriction is the ARC for OB-Rb, POMC, and MC3-R was higher in LDs eased. Thus the mechanism that determines the level at than SDs; in contrast, gene expression for CART was higher which body mass is defended, i.e. the functioning of the in SDs than LDs @Table 2). From comparisons with hypothetical comparator system, may be a critical factor hamsters at weaning, the differences in OB-Rb, POMC in determining the success of weight loss strategies for the and MC3-R mRNA levels re¯ected increases in LDs that obese [46,80]. Clearly, a key issue here is whether weight were blocked by SDs, whereas CART gene expression was J.G. Mercer, J.R. Speakman / Neuroscience and Biobehavioral Reviews 25 2001) 101±116 109

Table 2 Summary of the effect of short photoperiod @SD) and food deprivation or restriction @FD/FR) on hypothalamic gene expression in Siberian hamsters. Arrows indicate levels of gene expression relative to ad libitum-fed LD controls @ $ Ðno change) v LD ad libitum-fed controls FD/FR males Adult SD males Juvenile SD females

NPY @ARC) "$ $ AGRP @ARC) "" #? POMC @ARC) ## # CART @ARC) $ a " a " MCH @LHA) $$ ± Orexins @LHA) $$ ± CRF @PVN) $$ ± OB-Rb @ARC) "# # MC3-R @ARC) # a # a # MC4-R @PVN) ± ± $

a Mercer et al., unpublished.

elevated under SDs. Photoperiod had no effect on NPY gene expression in the ARC or on MC4-R gene expression in the PVN. Amounts of AGRP mRNA in the ARC tended to be higher in LD hamsters. Only the changes in CART mRNA preceded the divergence of growth trajectories in the oppo- site photoperiods @2 weeks post-weaning; Fig. 5). These data provide the ®rst evidence for a primary event in SD suppression of growth/body mass and may indicate a role for CART in this regulation. The accumulated data indicate that chronic low plasma leptin signals that result from SD mass loss/growth restric- tion are perceived differently from those acute changes that are induced by negative energy balance, where low leptin acts orexigenically. The apparent insensitivity of the body mass axis to seasonal variation in leptin feedbackmay be explained by the associated differences in hypothalamic OB-Rb gene expression between the opposite photoperiods. Both juvenile SD females and adult SD males have less hypothalamic OB-Rb mRNA in the ARC than their respec- tive LD controls, and regulated sensitivity to leptin feedback may be critical for the maintenance of seasonal bodymass. Thus, as leptin feedbackincreases in LDs so does OB-Rb gene expression. Alternatively, it is possible that leptin may not be primarily involved in long term body mass regula- tion, but rather may be important in protecting the body from acute and harmful mass loss. It is not clear how the observed changes to the endogenous leptin system, which might be interpreted as reducing overall sensitivity, can be rationalised with experimental observations of increased responsiveness to exogenous leptin in the SD hamster [88,89]. Interpretation of these simultaneous changes in neuropep- tide and receptor gene expression is necessarily speculative, but a number of conclusions can be drawn. Several of the Fig. 5. Dark®eld images of CART gene expression in the hypothalamic arcu- changes in hypothalamic mRNA levels induced by SDs ate nucleus @ARC) of female Siberian hamsters @from [87]). CART mRNA levels at 2 weeks post-weaning were higher in hamsters kept in short @SD) as appear counter-intuitive in the context of the functioning of opposed to long @LD) daylength from weaning. 3V, third ventricle. Scale bar, these systems in energy balance, and their likely regulation 180 mm. 110 J.G. Mercer, J.R. Speakman / Neuroscience and Biobehavioral Reviews 25 2001) 101±116 by peripheral feedbacksignals such as leptin. SDs reduce, or genotype) because there is no regulatory control exerted prevent the normal LD increase in, the circulating leptin over upper limits of fat storage. In the alternative model, signal but this low level of leptin signalling does not consis- humans are simply regarded as having experienced the same tently activate anabolic hypothalamic systems and inhibit @generally unstated) selective pressures that pertain to small catabolic systems. Only POMC mRNA ®tted the predicted mammals, and are assumed therefore to have also evolved a pattern in both of the photoperiod models @Table 2), when it very tight regulatory system over the levels of fat storage might have been anticipated that the lower circulating leptin [19,20,43±45]. Under this scenario the problem of obesity is signal in SDs would also give rise to decreased ARC POMC regarded as a more pathological problem within the regula- and CART mRNA, and increased gene expression for OB- tory frameworkÐobese people have defective regulation, Rb, AGRP and NPY. The consequence of low POMC gene highlighted in an environment where energy supplies are expression in SDs would be likely to be reduced catabolic freely available. Environment is an obvious complicating @alpha-MSH-mediated) drive through the MC4-R in the factor in this discussion. Provision of a cafeteria-type diet PVN. This change would tend to oppose SD mass loss, an can effectively overwhelm the normal body mass regulation effect that would be reinforced in male hamsters by increased of laboratory rodents [92]. If such a challenge can overcome AGRP activity, an antagonist at the MC4-R. With the recent the comparatively `tight' regulatory systems operating in characterization of the MC3-R knockout mouse [68,69], small mammals, what will be its consequences be in differences in POMC, AGRP and MC3-R gene expression humans? Is it necessary to assume that regulation is defec- with photoperiod may have consequences for feed ef®ciency tive in this scenario? at a time when animals are anticipating food shortages. The Do the evolutionary arguments outlined above stand up to modulation of CART gene expression in SDs, which is logical scrutiny? The most abundant size class of mammal is elevated in both juvenile female hamsters [87] and in adult around 100 g [93,94]. Humans are about three orders of males @unpublished), is consistent with a role in the induction magnitude heavier. This difference is likely to have played a of mass loss. The consistent absence of any photoperiodic major role in the pattern of selective pressures acting on early regulation of NPY gene expression, perhaps the archetypal humans when compared with small mammals, a difference negative energy balance signal, lends further support to the that is completely ignored in the evolutionary scenarios played notion that ad lib-fed animals perceive themselves to be in out to date. In particular, because of their larger size, humans energy homeostasis in both LDs and SDs despite the large would be expected to be far less sensitive to the risks of starva- differential in body mass between photoperiods, i.e. they are tion. We can estimate the levels of energy expenditure of early at an appropriate body mass/adiposity. hominids from the scaling relationships established for modern wild mammals [7,95] since estimates for primates do not deviate signi®cantly from the allometric means 9. An evolutionary perspective on fat storage regulation [96,97]. On this basis an early 70 kg human would expend in humans energy at a rate of about 14 MJ each day [95]. If such a human were to store 10% of its body mass as fat Estimates of the extent to which food intake and expen- @7 kg ˆ 273 MJ of fat), this would be suf®cient to allow it to diture must be balanced in humans clearly indicate that survive for around 20 days without feeding @about 7.5 times regulatory systems controlling food intake and utilisation longer than the survival time of a 30 g mammal storing the of energy and nutrients do exist in humans, and that these same relative amount). Consequently there is likely to have systems impact upon body mass and adiposity. It remains to been less intense selection on the lower margin of fat storage be determined at what level these controls operate @Fig. 1). than one would expect to occur in small mammals, simply Are body mass and adiposity directly regulated @model 2), because of the body size difference. as appears to be the case in small mammals, or is there a Two things, unique to the evolutionary history of humans, consequential regulation brought about only through match- however, are likely to have exacerbated the difference ing of energy intake and energy expenditure @model 1)? between small mammals and us. First, early hunter-gath- Relatively few studies have addressed the evolutionary erers were extremely ¯exible in their diet choice exploiting context of the regulation of body fatness in humans. At plants and animals with variable frequency as well as foods present the problem of obesity in modern society is consid- from marine sources [98±100]. This ¯exibility in their diets ered in the context of one of two alternative evolutionary would enable early humans to ride out periods of shortage in scenarios. The ®rst is the `thrifty genotype' model, initially one particular food type, not only by withdrawing stored fat, proposed by Neel in the 1960s [90,91]. Under this hypoth- but also by switching their diets to other items. This option esis the capability to ef®ciently store fat is regarded as would be unavailable to most small mammals that are more having an evolutionary advantage because it allowed early rigid in their diet choice. An insectivorous shrew, for exam- humans to avoid starvation. In modern environments, ple, would starve to death on a mountain of hazelnuts that however, where energy is readily available, obesity is would keep a bank vole alive for weeks, whereas the bank hypothesised to develop in those individuals with the most vole would die rapidly in a cage full of beetles that would ef®cient fat storage mechanisms @those with the thrift provide adequate sustenance for the shrew. The selective J.G. Mercer, J.R. Speakman / Neuroscience and Biobehavioral Reviews 25 2001) 101±116 111 pressures on lower margins for fat storage were likely there- crine and neuroendocrine regulatory system characterised in fore to have been less intense. This pressure would have small mammals will also have consequences for the main- been further relaxed as humans developed agriculture and tenance of normal body mass in humans, although there may the capacity to store energy outside their own bodies, further be implications for the ef®cacy of potential therapies such as diluting the need to store fat as a mechanism for riding out administering leptin to obese humans [105]. periods of food unavailability. It is dif®cult to imagine in Consequently, the above hypothesis still ®ts without dif®- this scenario how a `thrifty genotype' predisposing one to culty into the emerging picture that susceptibility to obesity ef®ciently deposit fat stores would evolve, or be maintained, may be controlled largely by genetic factors, with environ- within the human population. ment determining phenotypic expression, and that there may On the other hand, early humans were still probably under be a large pool of genes that impact upon energy intake, considerable riskof predation. Selection to limit fat storage energy expenditure or nutrient partitioning, and that ulti- would probably therefore still be present, but in the absence mately in¯uence body mass. Although body fatness in of any pressure driving humans to deposit large quantities of humans may not be a regulated parameter itself in the fat it seems unlikely that this selective pressure on the upper same way that it is in small mammals, it will nevertheless margins of fat storage would be intense. Moreover, as with be an epiphenomenon of the regulatory systems that control the lower margin of fat storage, two aspects of human evolu- food intake and energy expenditure. These systems are tionary history were probably signi®cant. First, humans live likely to show parallels in regulatory control to those in social groupings. It is well established that groups derive found in small mammals, and their functional impairment bene®ts in terms of their ability to detect, escape from and is likely to lead to problems in body mass control. Speci®c deter predators [101,102]. Indeed it has been suggested that genes and gene products will be critical to the maintenance primate sociality may have evolved because of the bene®ts of energy balance, and studies seeking the nature of regula- linked to reducing predation risk [103]. Animals in groups tory control over body energy balance in rodents form a also increase the amount of time spent in low cost beha- critical initial step in understanding these processes in viours @i.e. resting) compared with more expensive activ- humans. The insight gained from the characterisation of ities [104]. The evolution of sociality would be expected to obesity mutations in mice bears testament to the value of not only reduce predation riskbut also reduce energy this approach. Understanding the processes that control food demands and therefore contribute to further lowering the intake and expenditure may lead to a greater understanding riskof starvation. Second, humans developed tools, and of the development of obesity without the need to consider harnessed ®re, both of which would be potent weapons the small mammal system as an identical model to the @literally) to defend against predation. By 6±10,000 years human system. ago, humans were probably already effectively immune from both predation and starvation risks as signi®cant forces moulding the evolution of body fatness regulatory systems. 10. The human hypothalamus in control offoodintake Over the 300 generations since this time there has been and adiposity ample opportunity for whatever regulation did exist to become less intense. There is only limited direct evidence that hypothalamic The dominant thread then is that the selective pressures neuropeptide and receptor systems are involved in the control which we know have resulted in a very tight regulatory ofbodymassinhumans,whetheritbethroughaputativeadipo- system within small mammals over levels of body fat stat or via the interaction of energy intake and energy expendi- storage are unlikely to have been as potent within the evolu- ture.Indeed,asidefromtherarecasesofobesitythatareinduced tionary history of early humans. We might then expect that byspeci®cgenemutations,mostevidenceiscircumstantial.The regulation of body fatness in humans is at best going to be a identi®cation of rare human obesity mutations does at least sloppy version of model 2 and may in fact conform to model con®rm the existence of signalling pathways within the human 1. Extrapolating uncritically from small rodents to humans hypothalamus that are essential for normal body mass control, is therefore fraught with as many problems as the `thrifty andthathaveaconservedfunctioninmammalianspeciesstudied genotype' model. This novel evolutionary scenario sheds an todate.Unsurprisingly,researchershavedrawnupontherodent entirely different light on the patterns of obesity that we literature and adopted a candidate geneapproachinidentifying observe in modern western societies. Most critically it these mutations within the obese human population [59]. The suggests that obesity is unlikely to be a consequence of a following monogenic mutations have been identi®ed in the selective ef®ciency to store fat @a thrifty genotype), nor a human population as having important effects on fat storage: malfunctioning lipostatic regulatory system. Rather it might leptin [106,107], leptin receptor [108,109], POMC [110], be a consequence of the absence of strong selective pres- MC4-R[111±113]andprohormoneconvertase1@apolypeptide sures for regulatory control over a morphological parameter processingenzyme)[114].Adetailed description ofthe pheno- that probably was not of key importance in early hominid types of patients harbouring these mutations can be found else- evolution. Nevertheless, it is clear that even if humans where [59]. The fact that these genes are all components the conform to model 1, many of the components of the endo- leptin-melanocortin signalling pathway serves to emphasise 112 J.G. Mercer, J.R. Speakman / Neuroscience and Biobehavioral Reviews 25 2001) 101±116 the importance of this system in the maintenance of energy the former group [124]. This suggests that high leptin balance. Indeed, between 3 and 5% of extremely obese indivi- concentrations in obese patients depress the activity of duals may have MC4-R mutations [115], although this hypothalamic neurones during exposure to food. Binge- frequency is made up of a range of defects in MC4-R function. eating obese subjects have different rCBFs to non-bingeing Thus,asdiscussedearlier,ifcompletelossoffunctionofaparti- obese or normal weight controls [125]. The potential for in culargeneproductresultsinobesity,itislikelythatallelicvaria- vivo study of receptor concentrations and mechanisms of tion may induce partial loss of function and generate a action in the hypothalamus of obese individuals has yet to predispositiontoobesitysubjecttotheremaininggeneticback- be realised. ground @where several hundred genes may contribute) and to environmental factors. By contrast, sequence variants in the MC3-R genewere not associatedwithobesephenotypes[116]. 11. Conclusions Comparative neuroanatomical and neurochemical evidence indicates that the substrates and structures Evolutionary pressures on small mammals have led to involved in the maintenance of energy homeostasis are relatively tight control over the upper and lower limits of well conserved between mammalian species. However, body fat stores. Most of our knowledge of the involvement there are few detailed protein or gene expression studies of hypothalamic systems in body mass regulation is based of the post-mortem human brain. One exception to this is on the ability of small mammals to defend an appropriate the leptin receptor. The leptin receptor is widely distributed body mass from imposed energy imbalance. The neuroen- in human brain, with a broadly similar pattern of expression docrine systems involved in this regulation are primarily to laboratory rodents, and with the strongest expression in compensatory in nature. However, body mass regulation the cerebellum and hypothalamus, including the ARC and functions at different levels. The body mass responses PVN [117,118]. No differences were observed between the exhibited by seasonal animals to photoperiodic and food levels of leptin receptor gene expression of obese, lean and restriction stimuli indicate the existence of a different diabetic subjects. Another approach to assessing the applic- level of body mass regulation, through which animals are ability of in vivo rodent data to the human situation is the able to effect changes in the level of body mass that they will measurement of concentrations of neuropeptides and mono- defend when challenged with energy imbalance. Studies of amines in human CSF. These procedures address the rate of hypothalamic gene expression in the Siberian hamster have release of neurochemicals by regulatory centres within the begun to tease apart the neuroendocrine pathways involved brain, they cannot determine whether effects are primary or in programmed and compensatory regulation of body mass secondary to the accompanying pathological state. Results and adiposity. These studies suggest that the isolated study have been contradictory. Lower than normal CSF concen- of compensatory body mass regulation in non-seasonal trations of CRF, b-endorphin and NPY have been recorded species may only be illuminating part of the picture. The in obese women [119]. However, in a group of massively characteristics of body mass regulation in the Siberian obese patients, CSF concentrations of were hamster are indicative of a comparator system whereby reduced whereas b-endorphin levels were elevated [120], body composition is assessed against encoded target para- and no differences were observed in levels of CRF, NPY meters. However the sites and identity of neurochemicals or growth hormone releasing hormone @GHRH). The neuro- involved in encoding body mass set point, as opposed to biology of eating disorders has been investigated using the those involved in energy homeostasis, remain elusive. same approach. Patients with bulimia nervosa had elevated It seems likely that regulatory control over fat mass is CSF levels of peptide YY, an anabolic peptide in the NPY loose in humans compared with small mammals, a situation family [121]. Underweight anorexia nervosa patients had that can be rationalised from an evolutionary context. elevated CSF levels of NPY, CRF and , and Nevertheless, it is also clear from studies of human obesity reduced b-endorphin and [122,123]. Most of genes that certain regulatory systems, most notably those these alterations were normalised after weight recovery. It focussed on the leptin and melanocortin pathways, are is not possible to draw any ®rm conclusions from these data essential for normal body mass regulation. Even in the about the role of the hypothalamic peptides in these patho- assumed absence of tight direct regulation of body adipos- physiological conditions. ity, this parameter is likely to be controlled as a consequence An approach to the human hypothalamus that could have of regulated energy intake and expenditure. Study of potential in obesity and eating disorder research is photon mechanisms implicated in energy homeostasis in laboratory emission computed tomography @PET scanning) either in rodents is therefore likely to continue to highlight possible the measurement of regional cerebral blood ¯ow @rCBF), targets for pharmacological manipulation in the manage- thereby revealing brain circuits in which activity is changed, ment of human obesity. This will include mechanisms or in areas such as receptor imaging in the living human highlighted in the seasonal model, where the manifestation brain. For example, a recent study of obese and normal of programmed as well as compensatory control over energy weight women during exposure to food observed an inverse balance may signi®cantly advance our understanding of association between leptin and hypothalamic rCBF only in body mass regulation across mammalian species, including J.G. Mercer, J.R. Speakman / Neuroscience and Biobehavioral Reviews 25 2001) 101±116 113 man. It remains to be determined whether the best potential and fat storage in response to variable food supply. Behav Ecology targets for pharmacological intervention in human obesity 1992;3:181±8. will be individual components of predominantly short-term [16] Rogers CM. Predation riskand fasting capacity: do winter birds maintain optimal body mass?. Ecology 1987;68:1051±61. compensatory pathways or as yet unknown components of [17] Witter MS, Cuthill IC, Bonser RHC. Experimental investigations of the `set-point' system. In large part the answer to this ques- mass-dependent predation riskin the European Starling, Sturnus tion will be determined by the nature of the regulatory vulgaris. Animal Behav 1994;48:210±22. process in humans, although `silent' mechanisms could [18] Stellar E. The physiology of motivation. Psychol Rev 1954;61:5±22. still be amenable to manipulation. Depending on the [19] Schwartz MW, Woods SC, Porte Jr D, Seeley RJ, Baskin DG. Central nervous system control of food intake. Nature strength of any direct adipostatic regulation, targeting the 2000;404:661±71. former, compensatory, systems will carry the riskof weight [20] Woods SC, Seeley RJ, Porte D, Schwartz MW. Signals that regulate regain at the end of the intervention period, whereas the food intake and energy homeostasis. Science 1998;280:1378±83. latter has at least the prospect of lowering defended body [21] Kalra SP, Dube MG, Pu S, Xu B, Horvath TL, Kalra PS. Interacting weight [80]. appetite-regulating pathways in the hypothalamic regulation of body weight. Endocrine Rev 1999;20:68±100. [22] Friedman JM, Halaas JL. Leptin and the regulation of body weight in mammals. Nature 1998;395:763±70. Acknowledgements [23] Zhang Y, Proenca R, Maffei M, Barone M, Leopold L, Friedman JM. Positional cloning of the mouse obese gene and its human homo- Workin the authors' laboratory was funded by the Scot- logue. Nature 1994;372:425±32. tish Executive Rural Affairs Department and by the BBSRC [24] Coleman DL. Obese and diabetes: two mutant genes causing @1/S12030). JRS was supported by a Royal Society Lever- diabetes-obesity syndromes in mice. Diabetologia 1978;14:141±8. [25] Tartaglia LA, Dembski M, Weng X, Deng N, Culpepper J, Devos R, hulme Senior Research Fellowship. Richards GJ, Camp®eld LA, ClarkFT, Deeds J, Muir C, SankerS, Moriarty A, Moore KJ, Smutko JS, Mays GG, Woolf EA, Monroe CA, Tepper RI. Identi®cation and expression cloning of a leptin References receptor. OB-R Cell 1995;83:1263±71. [26] Trayhurn P, Thomas MEA, Duncan JS, Rayner DV. Effects of fast- [1] Thomas DW. Fruit intake and energy budgets of frugivorous bats. ing and refeeding on ob gene-expression in white adipose-tissue of Physiol Zool 1984;57:457±67. lean and obese @ob/ob) mice. Febs Letters 1995;368:488±90. [2] Winter Y, von Helversen O. The energy cost of ¯ight: do small bats [27] Considine RV, Sinha MK, Heiman ML, Kriauciunas A, Stephens ¯y more cheaply than birds?. J Comp Physiol 1998;168:105±11. TW, Nyce MR, Ohannesian JP, Marco CC, McKee LJ, Bauer TL. [3] Rothwell NJ, StockMJ. A role for brown adipose tissue in diet- Serum immunoreactive leptin concentrations in normal-weight and induced thermogenesis. Nature 1979;281:31±35. obese humans. New England Journal of Medicine 1996;334:292±5. [4] Clapham JC, Arch JR, Chapman H, Haynes A, Lister C, Moore GB, [28] Pelleymounter MA, Cullen MJ, Baker MB, Hecht R, Winters D, Piercy V, Carter SA, Lehner I, Smith SA, Beeley LJ, Godden RJ, Boone T, Collins F. Effects of the obese gene-product on body- Herrity N, Skehel M, Changani KK, Hockings PD, Reid DG, Squires weight regulation in ob/ob mice. Science 1995;269:540±3. SM, Hatcher J, Trail B, Latcham J, Rastan S, Harper AJ, Cadenas S, [29] Halaas JL, Gajiwala KS, Maffei M, Cohen SL, Chait BT, Rabinowitz Buckingham JA, Brand MD, Abuin A. Mice overexpressing human D, Lallone RL, Burley SK, Friedman JM. Weight-reducing effects of uncoupling protein-3 in skeletal muscle are hyperphagic and lean. the plasma-protein encoded by the obese gene. Science Nature 2000;406:415±8. 1995;269:543±6. [5] Weigle DS. Appetite and the regulation of body composition. [30] Camp®eld LA, Smith FJ, Guisez Y, Devos R, Burn P. Recombinant FASEB J 1994;8:302±10. mouse ob proteinÐevidence for a peripheral signal linking adipos- [6] Kennedy GC. The role of depot fat in the hypothalamic control of ity and central neural networks. Science 1995;269:546±9. food intake in the rat. Proc Royal Soc Lond B 1953;140:578±92. [31] Woods AJ, StockMJ. Leptin activation in hypothalamus. Nature [7] Speakman JR. The cost of living: factors in¯uencing the daily energy 1996;381:745. demands of small mammals. Adv Ecol Res 2000;30:177±297. [32] Van DijkG, Thiele TE, Donahey JC, Camp®eld LA, Smith FJ, Burn [8] Speakman JR. Doubly-labelled water: theory and practice. London: P, Bernstein IL, Woods SC, Seeley RJ. Central infusions of leptin Chapman and Hall, 1997. and GLP-1-@7-36) amide differentially stimulate c-FLI in the rat [9] Lima SL. Predation riskand unpredictable feeding conditions: deter- brain. Amer J Physiol 1996;271:R1096±100. minants of body mass in birds. Ecology 1986;67:377±85. [33] Mercer JG, Hoggard N, Williams LM, Lawrence CB, Hannah LT, [10] McNamaraJM.Thestarvation-predationtrade-offandsomebehavioural Trayhurn P. Localization of leptin receptor messenger-RNA and the and ecological consequences. In: Hughes RN, editor. Behavioural long form splice variant @Ob-Rb) in mouse hypothalamus and adja- mechanismsoffood selection, Berlin: Springer, 1990. p. 39±50. cent brain-regions by in-situ hybridization. Febs Letters [11] McNamara JM, Houston AI. The value of fat reserves and the trade- 1996;387:113±6. off between starvation and predation. Acta Biotheoretica [34] Cheung CC, Clifton DK, Steiner RA. neurons 1990;38:37±61. are direct targets for leptin in the hypothalamus. Endocrinology [12] Bednekoff PA, Krebs JR. Great tit fat reserves: effects of changing 1997;138:4489±92. and unpredictable feeding day length. Functional Ecology [35] Mercer JG, Hoggard N, Williams LM, Lawrence CB, Hannah LT, 1995;9:457±62. Morgan PJ, Trayhurn P. Coexpression of leptin receptor and prepro- [13] Ekman JB, Hake MK. Monitoring starvation risk: adjustments of neuropeptide Y mRNA in arcuate nucleus of mouse hypothalamus. J body reserves in green®nches @Carduelis chloris) during periods of Neuroendocrinology 1996;8:733±5. unpredictable foraging success. Behav Ecology 1990;1:62±67. [36] Wolthers OD, HeuckC, SkjaerbaekC. Diurnal rhythm in serum [14] Gosler AG, Greenwood JJD, Perrins C. Predation riskand the cost of leptin. J Pediatr Endocrinol Metab 1999;12:153±866. being fat. Nature 1995;377:621±3. [37] Xu B, Kalra PS, Farmerie WG, Kalra SP. Daily changes in [15] Hurly AT. Energetic reserves of marsh tits @Parus palustris): food hypothalamic gene expression of neuropeptide Y, galanin, 114 J.G. Mercer, J.R. Speakman / Neuroscience and Biobehavioral Reviews 25 2001) 101±116

proopiomelanocortin, and adipocyte leptin gene expression and [59] Barsh GS, Farooqi IS, O'Rahilly S. Genetics of body-weight regula- secretion: effects of food restriction. Endocrinology 1999; tion. Nature 2000;404:644±51. 140:2858±75. [60] Hahn TM, Breininger JF, Baskin DG, Schwartz MW. Coexpression [38] Nagatani S, Guthikonda P, Foster DL. Appearance of a nocturnal of Agrp and NPY in fasting-activated hypothalamic neurons. Nature peakof leptin secretion in the pubertal rat. Hormones Behav Neurosci 1998;1:271±2. 2000;37:345±52. [61] Miller MW, Duhl DM, Vrieling H, Cordes SP, Ollmann MM, [39] Schoeller DA, Cella LK, Sinha MK, Caro JF. Entrainment of the Winkes BM, Barsh GS. Cloning of the mouse agouti gene predicts diurnal rhythm of plasma leptin to meal timing. J Clin Invest a secreted protein ubiquitously expressed in mice carrying the lethal 1997;100:1882±7. yellow mutation. Genes Dev 1993;7:454±67. [40] Ahima RS, Prabakaran D, Mantzoros C, Qu D, Lowell B, Maratos- [62] Bultman SJ, Michaud EJ, WoychikRP. Molecular characterization Flier E, Flier JS. Role of leptin in the neuroendocrine response to of the mouse agouti locus. Cell 1992;71:1195±204. fasting. Nature 1996;382:250±2. [63] Ollmann MM, Wilson BD, Yang YK, Kerns JA, Chen Y, Gantz I, [41] Ahima RS, Flier JS. Leptin Ann Rev Physiol 2000;25:413±27. Barsh GS. Antagonism of central melanocortin receptors in vitro and [42] Ahima RS, Saper CB, Flier JS, Elmquist JK. Leptin regulation of in vivo by agouti-related protein. Science 1997;278:135±8. neuroendocrine systems. Front Neuroendocrinol 2000;21:263±307. [64] Graham M, Shutter JR, Sarmiento U, Sarosi I, StarkKL. Overex- [43] Baile CA, Della-Fera MA, Martin RJ. Regulation of and pression of Agrt leads to obesity in transgenic mice. Nat Genet body fat mass by leptin. Ann Rev Nut 2000;20:105±27. 1997;17:273±4. [44] Rohner-Jeanrenaud F. Hormonal regulation of energy partitioning. [65] Yaswen L, Diehl N, Brennan MB, Hochgeschwender U. Obesity in Int J Obes 2000;24:34±37. the mouse model of pro-opiomelanocortin de®ciency responds to [45] Lu H, Li C. ReviewÐleptin: a multifunctional hormone. Cell peripheral melanocortin. Nat Med 1999;5:1066±70. Research 2000;10:81±92. [66] Huszar D, Lynch C, Fairchild-Huntress V, Dunmore J, Fang Q, [46] Keesey RE, Hirvonen MD. Body weight set-points: determination Berkemeier L, Gu W, Kesterson R, Boston B, Cone R, Smith F, and adjustment. J Nutr 1997;127:18755±835. Camp®eld L, Burn P, Lee F. Targeted disruption of the melanocor- [47] Leibowitz SF. Brain Neuropeptide Y: an integrator of endocrine, tin-4 receptor results in obesity in mice. Cell 1997;88:131±41. metabolic and behavioral processes. Brain Res Bull 1991;27:333±7. [67] Roselli-Rehfuss L, Mountjoy KG, Robbins LS, Mortrud MT, Low [48] Dallman MF, Akana SF, Bhatnagar S, Bell ME, Choi S, Chu A, MJ, Tatro JB, Entwistle ML, Simerly RB, Cone RD. Identi®cation of Horsley C, Levin N, Meijer O, Soriano LR, StrackAM, Viau V. a receptor for gamma melanotropin and other proopiomelanocortin Starvation: early signals, sensors, and sequelae. Endocrinology peptides in the hypothalamus and limbic system. Proc Natl Acad Sci 1999;140:4015±23. USA 1993;90:8856±60. [49] Mizuno TM, Mobbs CV. Hypothalamic agouti-related protein [68] Chen AS, Marsh DJ, Trumbauer ME, Frazier EG, Guan XM, Yu H, messenger ribonucleic acid is inhibited by leptin and stimulated by Rosenblum CI, Vongs A, Feng Y, Cao L, Metzger JM, StrackAM, fasting. Endocrinology 1999;140:814±7. Camacho RE, Mellin TN, Nunes CN, Min W, Fisher J, Gopal-Truter [50] Salton SR, Ferri GL, Hahm S, Snyder SE, Wilson AJ, Possenti R, S, MacIntyre DE, Chen HY, Van Der Ploeg LH. Inactivation of the Levi A. VGF: a novel role for this neuronal and neuroendocrine mouse melanocortin-3 receptor results in increased fat mass and polypeptide in the regulation of energy balance. Front Neuroendo- reduced lean body mass. Nat Genet 2000;26:97±102. crinol 2000;21:199±219. [69] Butler AA, Kesterson RA, Khong K, Cullen MJ, Pelleymounter MA, [51] Bergendahl M, Wiemann JN, Clifton DK, Huhtaniemi I, Steiner RA. Dekoning J, Baetscher M, Cone RD. A unique metabolic syndrome Short-term starvation decreases POMC mRNA but does not alter causes obesity in the melanocortin-3 receptor-de®cient mouse. GnRH mRNA in the brain of adult male rats. Neuroendocrinology Endocrinology 2000;141:3518±21. 1992;56:913±20. [70] Shimada M, Tritos NA, Lowell BB, Flier JS, Maratos-Flier E. Mice [52] Kristensen P, Judge ME, Thim L, Ribel U, Christjansen KN, Wulff lacking melanin-concentrating hormone are hypophagic and lean. BS, Clausen JT, Jensen PB, Madsen OD, Vrang N, Larsen PJ, Nature 1998;396:670±4. Hastrup S. Hypothalamic CART is a new anorectic peptide regulated [71] Hahm S, Mizuno TM, Wu TJ, Wisor JP, Priest CA, KozakCA, by leptin. Nature 1998;393:72±76. Boozer CN, Peng B, McEvoy RC, Good P, Kelley KA, Takahashi [53] Qu D, Ludwig DS, Gammeltoft S, Piper M, Pelleymounter MA, JS, Pintar JE, Roberts JL, Mobbs CV, Salton SR. Targeted deletion Cullen MJ, Mathes WF, PrzypekJ, KanarekR, Maratos-Flier E. A of the Vgf gene indicates that the encoded secretory peptide precur- role for melanin-concentrating hormone in the central regulation of sor plays a novel role in the regulation of energy balance. Neuron feeding behaviour. Nature 1996;380:243±7. 1999;23:537±48. [54] Sakurai T, Amemiya A, Ishii M, Matsuzaki I, Chemelli RM, Tanaka [72] Erickson JC, Clegg KE, Palmiter RD. Sensitivity to leptin and H, Williams SC, Richardson JA, Kozlowski GP, Wilson S, Arch susceptibility to seizures of mice lacking neuropeptide Y. Nature JRS, Buckingham RE, Haynes AC, Carr SA, Annan RS, McNulty 1996;381:415±8. DE, Liu W-S, Terrett JA, Elshourbagy NA, Bergsma DJ, Yanagi- [73] Erickson JC, Hollopeter G, Palmiter RD. Attenuation of the obesity sawa M. Orexins and orexin receptors: a family of hypothalamic syndrome of ob/ob mice by the loss of neuropeptide Y. Science neuropeptides and G protein-coupled receptors that regulate feeding 1996;274:1704±7. behavior. Cell 1998;92:575±85. [74] Chemelli RM, Willie JT, Sinton CM, Elmquist JK, Scammell T, Lee [55] Brady LS, Smith MA, Gold PW, Herkenham M. Altered expression C, Richardson JA, Williams SC, Xiong Y, Kisanuki Y, Fitch TE, of hypothalamic neuropeptide mRNAs in food-restricted and food- Nakazato M, Hammer RE, Saper CB, Yanagisawa M. Narcolepsy in deprived rats. Neuroendocrinology 1990;52:441±7. orexin knockout mice: molecular genetics of sleep regulation. Cell [56] Davies L, Marks JL. Role of hypothalamic neuropeptide Y gene 1999;98:437±51. expression in body weight regulation. Amer J Physiol [75] Mercer JG. Regulation of appetite and body weight in seasonal 1994;266:R1687±91. mammals. Comp Biochem Physiol 1998;119C:295±303. [57] Seeley RJ, Matson CA, Chavez M, Woods SC, Dallman MF, [76] Steinlechner S, Heldmaier G, Becker H. The seasonal cycle of body Schwartz MW. Behavioral, endocrine, and hypothalamic responses weight in the Djungarian hamster: photoperiodic control and in¯u- to involuntary overfeeding. Amer J Physiol 1996;271:R819±23. ence of starvation and melatonin. Oecologia 1983;60:401±5. [58] Hagan MM, Rushing PA, Schwartz MW, Yagaloff KA, Burn P, [77] Bartness TJ, Elliot JA, Goldman BD. Control of torpor and body Woods SC, Seeley RJ. Role of the CNS melanocortin system in weight patterns by a seasonal timer in Siberian hamsters. Amer J the response to overfeeding. J Neurosci 1999;19:2362±23627. Physiol 1989;257:R142±9. J.G. Mercer, J.R. Speakman / Neuroscience and Biobehavioral Reviews 25 2001) 101±116 115

[78] Morgan PJ, Barrett P, Howell HE, Helliwell R. Melatonin receptors: estimations in worldwide hunter-gatherer diets. Amer J Clin Nut localization, molecular pharmacology and physiological signi®- 2000;71:682±92. cance. Neurochem Int 1994;24:101±46. [101] Slotow R, Coumi N. Vigilance in bronze mannikin groups: the [79] Levin BE. Acurate NPY neurons and energy homeostasis in diet- contributions of predation riskand intra-group competition. Beha- induced obese and resistant rats. Amer J Physiol 1999;276:R382±7. viour 2000;14:565±78. [80] Levin BE, Dunn-Meynell AA. Defense of body weight against [102] Hilton GM, Cresswell W, Ruxton GD. Intra¯ockvariation in the chronic caloric restriction in obesity-prone and -resistant rats. speed of escape-¯ight response on attackby an avian predator. Amer J Physiol 2000;278:R231±7. Behav Ecol 1999;5:391±5. [81] Levin BE, Keesey RE. Defense of differing body weight set points in [103] Hill RA, Dunbar RIM. An evaluation of the roles of predation rate diet-induced obese and resistant rats. Amer J Physiol and predation riskas selective pressures on primate grouping beha- 1998;274:R412±9. viour. Behaviour 1998;135:411±30. [82] Macdiarmid JI, Cade JE, Blundell JE. High and low fat consumers, [104] Blumstein DT, Evans CS, Daniel JC. An experimental study of their macronutrient intake and body mass index: further analysis of behavioural group size effects in tammar wallabies, Macropus euge- the National Diet and Nutrition Survey of British Adults. Eur J Clin nii. Ann Behav 1999;58:351±60. Nutr 1996;50:505±12. [105] Heyms®eld SB, Greenberg AS, Fujioka K, Dixon RM, Kushner R, [83] Cooling J, Blundell J. Differences in energy expenditure and Hunt T, Lubina JA, Patane J, Self B, Hunt P, McCamish M. Recom- substrate oxidation between habitual high fat and low fat consumers binant leptin for weight loss in obese and lean adultsÐa rando- @phenotypes). Int J Obesity 1998;22:612±8. mized, controlled, dose-escalation trial. JAMA 1999;282:1568±75. [84] Wade GN, Bartness TJ. Effects of photoperiod and gonadectomy on [106] Montague CT, Farooqi IS, Whitehead JP, Soos MA, Rau H, Ware- food intake, body weight, and body composition in Siberian ham NJ, Sewter CP, Digby JE, Mohammed SN, Hurst JA, Cheetham hamsters. Amer J Physiol 1984;246:R26±30. CH, Earley AR, Barnett AH, Prins JB, O'Rahilly S. Congenital [85] Mercer JG, Moar KM, Ross AW, Hoggard N, Morgan PJ. Photoper- leptin de®ciency is associated with severe early-onset obesity in iod regulates arcuate nucleus POMC, AGRP, and leptin receptor humans. Nature 1997;387:903±8. mRNA in Siberian hamster hypothalamus. Amer J Physiol [107] Strobel A, Issad T, Camoin L, Ozata M, Strosberg AD. A leptin 2000;278:R271±81. missense mutation associated with hypogonadism and morbid [86] Reddy AB, Cronin AS, Ford H, Ebling FJP. Seasonal regulation of obesity. Nature Genetics 1998;18:213±5. food intake and body weight in the male Siberian hamster: studies of [108] Clement K, Vaisse C, Lahlou N, Cabrol S, Pelloux V, Cassuto D, hypothalamic orexin @hypocretin), neuropeptide Y @NPY) and pro- Gourmelen M, Dina C, Chambaz J, Lacorte JM, Basdevant A, Bougneres P, Lebouc Y, Froguel P, Guy-Grand B. A mutation in opiomelanocortin @POMC). Eur J Neuroscience 1999;11:3255±64. the human leptin receptor gene causes obesity and pituitary dysfunc- [87] Adam CL, Moar KM, Logie TJ, Ross AW, Barrett P, Morgan PJ, tion. Nature 1998;392:398±401. Mercer JG. Photoperiod regulates growth, puberty and hypothalamic [109] Strosberg AD, Issad T. The involvement of leptin in humans neuropeptide and receptor gene expression in female Siberian revealed by mutations in leptin and leptin receptor genes. Trends hamsters. Endocrinology 2000;141:4349±4356. in Pharmacological Sciences 1999;20:227±30. [88] Klingenspor M, Niggemann H, Heldmaier G. Modulation of leptin [110] Krude H, Biebermann H, LuckW, Horn R, Brabant G, Gruters A. sensitivity by short photoperiod acclimation in the Djungarian Severe early-onset obesity, adrenal insuf®ciency and red hair hamster Phodopus sungorus. J Comp Physiol B 2000;170:37±43. pigmentation caused by POMC mutations in humans. Nat Genet [89] Mercer JG, Adam CL, Morgan PJ. Towards an understanding of 1998;19:155±7. physiological body weight regulation: seasonal animal models. [111] Yeo GS, Farooqi IS, Aminian S, Halsall DJ, Stanhope RG, O'Ra- Nutr Neurosci 2000;3:307±320. hilly S. A frameshift mutation in MC4R associated with dominantly [90] Neel JV. The ªthrifty genotypeº in 1998. Nut Rev 1999;57:32±39. inherited human obesity. Nat Genet 1998;20:111±2. [91] Lev-Ran A. Thrifty genotype: how applicable is it to obesity and [112] Hinney A, Schmidt A, Nottebom K, Heibult O, Becker I, Ziegler A, type 2 diabetes?. Diabetes Reviews 1999;7:1±22. Gerber G, Sina M, Gorg T, Mayer H, Siegfried W, Fichter M, [92] Sclafani A, Springer D. Dietary obesity in adult rats: similarities to Remschmidt H, Hebebrand J. Several mutations in the melanocor- hypothalamic and human obesity syndromes. Physiol Behav tin-4 receptor gene including a nonsense and a frameshift mutation 1976;17:461±71. associated with dominantly inherited obesity in humans. J Clin [93] Brown JH, Marquet PA, Taper ML. Evolution of body size: conse- Endocrinol Metab 1999;84:1483±6. quences of an energetic de®nition of ®tness. Amer Naturalist [113] Vaisse C, Clement K, Guy-Grand B, Froguel P. A frameshift muta- 1993;142:573±84. tion in human MC4R is associated with a dominant form of obesity. [94] Lovegrove BG. The zoogeography of mammalian basal metabolic Nat Genet 1998;20:113±4. rate. Amer Naturalist 2000;156:201±19. [114] Jackson RS, Creemers JW, Ohagi S, Raf®n-Sanson ML, Sanders L, [95] Nagy KA, Giraud IA, Brown TK. Energetics of free-ranging Montague CT, Hutton JC, O'Rahilly S. Obesity and impaired mammals, reptiles and birds. Ann Rev Nutr 1999;19:247±77. prohormone processing associated with mutations in the human [96] Schmid J, Speakman JR. Daily energy expenditure of the grey prohormone convertase 1 gene. Nat Genet 1997;16:303±6. mouse lemur @Microcebus murinus): a small primate that uses [115] Vaisse C, Clement K, Durand E, Hercberg S, Guy-Grand B, Froguel torpor. J Comp Physiol 2000 @in press). P. Melanocortin-4 receptor mutations are a frequent and heteroge- [97] DrackS, Ortmann S, BuÈhrmann N, Schmid J, Warren RD, Ganzhorn neous cause of morbid obesity. J Clin Invest 2000;106:253±62. JU. Field metabolic rate and the cost of ranging of the red-tailed [116] Li WD, Joo EJ, Furlong EB, Galvin M, Abel K, Bell CJ, Price RA. sportive lemur. In: Rakotosamimanana B, Rasaminanana H, Ganz- Melanocortin 3 receptor @MC3R) gene variants in extremely obese horn JU, Goodman SM, editors. New directions in lemur studies, women. Int J Obes 2000;24:206±10. New Yorkand London: Plenum Press, 1999. [117] Burguera B, Couce ME, Long J, Lamsam J, Laakso K, Jensen MD, [98] Stewart KM. Early hominid utilization of ®sh resources and implica- Parisi JE, Lloyd RV. The long form of the leptin receptor @OB-Rb) is tions for seasonality and behavior. J Human Evolution widely expressed in the human brain. Neuroendocrinology 1994;27:229±45. 2000;71:187±95. [99] Mann N. Dietary lean red meat and human evolution. Eur J Nut [118] Savioz A, Charnay Y, Huguenin C, Graviou C, Greggio B, Bouras C. 2000;39:71±79. Expression of leptin receptor mRNA @long form splice variant) in the [100] Speth JD. Plant-animal subsistence ratios and macronutrient energy human cerebellum. Neuroreport 1997@29):3123±6. 116 J.G. Mercer, J.R. Speakman / Neuroscience and Biobehavioral Reviews 25 2001) 101±116

[119] Strombom U, Krotkiewski M, Blennow K, Mansson JE, Ekman R, [123] DemitrackMA, Putnam FW, Rubinow DR, Pigott TA, Altemus M, Bjorntorp P. The concentrations of monoamine metabolites and Krahn DD, Gold PW. Relation of dissociative phenomena to levels neuropeptides in the cerebrospinal ¯uid of obese women with differ- of cerebrospinal ¯uid monoamine metabolites and beta-endorphin in ent body fat distribution. Int J Obes 1996;20:361±8. patients with eating disorders: a pilot study. Psychiatry Res [120] Brunani A, Invitti C, Dubini A, Piccoletti R, Bendinelli P, Maroni P, 1993;49:1±10. Pezzoli G, Ramella G, Calogero A, Cavagniini F. Cerebrospinal [124] Karhunen LJ, Lappalainen RI, Vanninen EJ, Kuikka JT, Uusitupa ¯uid and plasma concentrations of SRIH, beta-endorphin, CRH, MI. Serum leptin and regional cerebral blood ¯ow during exposure NPY and GHRH in obese and normal weight subjects. Int J Obes to food in obese and normal-weight women. Neuroendocrinology. 1995;19:17±21. 1999;69:154±9. [121] Kaye WH, Weltzin TE. Neurochemistry of bulimia nervosa. J Clin [125] Karhunen LJ, Lappalainen RI, Vanninen EJ, Kuikka JT, Uusitapa Psychiatry 1991;52:21±28. MI. Serum leptin and regional cerebral blood ¯ow during exposure [122] Krahn DD, Gosnell BA. Corticotropin-releasing hormone: possible to food in obese and normal-weight women. Neuroendocrinology role in eating disorders. Psychiatr Med 1989;7:235±45. 2000;69:154±9.