Leptin inhibits food-deprivation-induced increases in food intake and food hoarding

Blunted food conditioned place preference-like behavior in adolescent-stressed male hamsters

Social stress during adolescence results in long lasting weight gain, obesity, and enhanced food hoarding behavior in hamsters. We wanted to determine whether stress also enhanced conditioned place preference-like behavior (CPP-like) for food reward, as would be expected from studies with substances like cocaine. Our experimental animals were exposed daily to aggressive adults for two weeks in early puberty, while also trained to explore a V-shaped maze containing a food reward at one end. They were tested for CPP-like behavior on the last day of social stress. Our results showed that while stress enhanced weight gain, food intake, food efficiency, and body fat, it caused a reduction of Place Preference as compared to controls. In fact, the correlated relationship between Place Preference and body fat was inverted by stress exposure: while it was positively correlated in controls, it was mildly negatively correlated in stressed hamsters. These unexpected data illustrate the extent of adaptive behavior in foraging animals once a resource has become untrustworthy.

High-altitude environments enhance the ability of Eothenomys miletus to regulate body mass during food limitation, with a focus on gut microorganisms and physiological markers

Animals' digestion, energy metabolism, and immunity are significantly influenced by interactions between the gut microbiota and the intestinal environment of the host. Previous studies have shown that gut microbiota of Eothenomys miletus can respond to environmental changes, high fiber or fat foods. But how E. miletus in high-altitude adapt to their environment through gut microbiota and physiological changes during winter food shortages period was unclear. In the present study, we evaluated the altitude differences in gut microbiota and their interactions with physiology in terms of body mass regulation in order to study the adaptation of the gut microbiota and physiological indicators of the E. miletus under food restriction settings. E. miletus were collected for this study from Jingdong County (JD, low-altitude) and Xianggelila County (XGLL, high-altitude) in Yunnan Province, China, and split into three groups: control group, food-restricted feeding group for 7 days, and re-feeding group was offered a standard diet for 14 days. 16S rRNA gene sequencing and physiological methods were used to analyze the abundance and community structure of gut microbiota, as well as physiological indicators of each group in E. miletus. The results showed that while the RMR changed more during the period of food restriction, the body mass and major organ masses of E. miletus from high-altitude changed less. After food restriction, RMR in XGLL decreased by 25.25%, while that of in JD decreased by 16.54%. E. miletus from the XGLL had gut bacteria that were more abundant in Firmicutes and had fewer OTUs, and the microbiota had a closer interaction with physiological indicators. Moreover, the gut microbiota adapted to the food shortage environment by enhancing the genera of Bacterroides, Ruminococcus, Turicibacter, and Treponema to improve the utilization of nutrient resources. The interactions between microbial species and the equilibrium of energy homeostasis were further impacted by alterations in physiological indicators and microbial community structure. These variations were important for E. miletus to adapt to the fluctuations and changes of food resources in high-altitude region, which also expand our knowledge of organismal adaptations and the mechanisms behind the interactions between gut bacteria and host physiology.

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

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

Loss of transient receptor potential channel 5 causes obesity and postpartum depression

Hypothalamic neural circuits regulate instinctive behaviors such as food seeking, the fight/flight response, socialization, and maternal care. Here, we identified microdeletions on chromosome Xq23 disrupting the brain-expressed transient receptor potential (TRP) channel 5 (TRPC5). This family of channels detects sensory stimuli and converts them into electrical signals interpretable by the brain. Male TRPC5 deletion carriers exhibited food seeking, obesity, anxiety, and autism, which were recapitulated in knockin male mice harboring a human loss-of-function TRPC5 mutation. Women carrying TRPC5 deletions had severe postpartum depression. As mothers, female knockin mice exhibited anhedonia and depression-like behavior with impaired care of offspring. Deletion of Trpc5 from oxytocin neurons in the hypothalamic paraventricular nucleus caused obesity in both sexes and postpartum depressive behavior in females, while Trpc5 overexpression in oxytocin neurons in knock-in mice reversed these phenotypes. We demonstrate that TRPC5 plays a pivotal role in mediating innate human behaviors fundamental to survival, including food seeking and maternal care.

A hamster model for stress-induced weight gain

This review addresses the translational relevance of animal models of stress and their effects on body weight. In humans, stress, whether chronic or acute, has often been associated with increased food intake and weight gain. In view of the current obesity epidemic, this phenomenon is especially relevant. Such observations contrast with reports with commonly used laboratory animals, especially rats and mice. In these species, it is common to find individuals gaining less weight under stress, even with potent social stressors. However, there are laboratory species that present increased appetite and weight gain under stress, such as golden hamsters. Furthermore, these animals also include metabolic and behavioral similarities with humans, including hoarding behavior which is also enhanced under stress. Consequently, we propose that our comparative perspective provides useful insights for future research on the development of obesity in humans as a consequence of chronic stress exposure.

When translational neuroscience fails in the clinic: Dexamethasone prior to virtual reality exposure therapy increases drop-out rates

Posttraumatic stress disorder (PTSD) is characterized by exaggerated expression of fear responses to danger and safety cues. Translational research suggests that dexamethasone facilitates fear extinction in animal and human fear conditioning models. For this randomized, placebo-controlled trial (N = 27), we aimed to translate these findings to the clinic by using virtual reality exposure (VRE) therapy for OEF/OIF/OND veterans with PTSD to determine whether dexamethasone will increase the efficacy of exposure therapy for VRE relative to placebo. VRE sessions involved imaginal exposure to the most traumatic war memories while viewing a computer-generated view of virtual Iraq or Afghanistan with multisensory stimulus options used to match patient's description of the trauma. VRE was effective in reducing PTSD symptoms but there was no interaction with dexamethasone. Drop-out rate was significantly higher in the dexamethasone group, with 10 of 13 (76.9%) participants in this group discontinuing, compared to only 4 of 14 (28.5%) in the placebo group, χ² = 6.31, p = 0.02. Results indicate that the dexamethasone group may have experienced an increase in PTSD symptoms, particularly re-experiencing, at session 2 following first drug administration. Contrary to study hypotheses, dexamethasone did not enhance exposure therapy outcomes and was associated with increased drop-out. This demonstrates potential pitfalls in translating neuroscience models to the clinic; future research carefully examining glucocorticoid mechanisms involved in therapy augmentation is warranted.

Peripherally injected ghrelin and leptin reduce food hoarding and mass gain in the coal tit (Periparus ater)

In birds little is known about the hormonal signals that communicate nutritional state to the brain and regulate appetitive behaviours. In mammals, the peptide hormones ghrelin and leptin elevate and inhibit consumption and food hoarding, respectively. But in birds, administration of both ghrelin and leptin inhibit food consumption. The role of these hormones in the regulation of food hoarding in avian species has not been examined. To investigate this, we injected wild caught coal tits (Periparus ater) with leptin, high-dose ghrelin, low-dose ghrelin and a saline control in the laboratory. We then measured food hoarding and mass gain, as a proxy of food consumption, every 20 min for 2 h post-injection. Both high-dose ghrelin and leptin injections significantly reduced hoarding and mass gain compared with controls. Our results provide the first evidence that hoarding behaviour can be reduced by both leptin and ghrelin in a wild bird. These findings add to evidence that the hormonal control of food consumption and hoarding in avian species differs from that in mammals. Food hoarding and consumptive behaviours consistently show the same response to peripheral signals of nutritional state, suggesting that the hormonal regulation of food hoarding has evolved from the consumption regulatory system.

Leptin resistance was involved in susceptibility to overweight in the striped hamster re-fed with high fat diet

Food restriction (FR) is the most commonly used intervention to prevent the overweight. However, the lost weight is usually followed by “compensatory growth” when FR ends, resulting in overweight. The present study was aimed to examining the behavior patterns and hormones mechanisms underpinning the over-weight. Energy budget and body fat content, and several endocrine markers related to leptin signals were examined in the striped hamsters under 20% FR refed by either low-fat diet (LF group) or high-fat diet (HF group). Body mass and fat content significantly regained when FR ended, and the hamsters in HF group showed 49.1% more body fat than in LF group (P < 0.01). Digestive energy intake was higher by 20.1% in HF than LF group, while metabolic thermogenesis and behavior patterns did not differed between the two groups. Gene expression of leptin receptor and anorexigenic peptides of pro-opiomelanocortin and cocaine- and amphetamine-regulated transcript in hypothalamus were significantly up-regulated in LF group, but down-regulated in HF group. It suggests that effective leptin signals to the brain were involved in attenuation of hyperphagia in hamsters refed with LF. However, “leptin resistance” probably occurred in hamsters refed with HF, which impaired the control of hyperphagia, resulting in development of over-weight.

RFamide-related Peptide-3 and the Trade-off between Reproductive and Ingestive Behavior

Ingestive and sex behaviors are important for individual survival and reproductive success, but when environmental energy availability is limited, individuals of many different species make a trade-off, forfeiting sex for ingestive behavior. For example, food-deprived female Syrian hamsters (Mesocricetus auratus) forego vaginal scent marking and lordosis (sex behaviors) in favor of foraging, hoarding, and eating food (ingestive behavior). Reproductive processes tend to be energetically costly, and individual survival requires homeostasis in metabolic energy. Thus, during energetic challenges, the chances of survival are enhanced by decreasing the energy expended on reproductive processes. The entire hypothalamic-pituitary-gonadal (HPG) system is inhibited by severe energetic challenges, but comparatively little is known about the effects of mild energetic challenges. We hypothesized that (1) a trade-off is made between sex and ingestive behavior even when the level of food restriction is insufficient to inhibit the HPG system; (2) mild energetic challenges force a trade-off between appetitive ingestive and sex behaviors, but not consummatory versions of the same behaviors; and (3) the trade-off is orchestrated by ovarian steroid modulation of RFamide-related peptide 3 (RFRP-3). In other species, RFRP-3, an ortholog of avian gonadotropin-inhibitory hormone, is implicated in control of behavior in response to energetic challenges and stressful stimuli. In support of our three hypotheses, there is a "dose-response" effect of food restriction and re-feeding on the activation of RFRP-3-immunoreactive cells in the dorsomedial hypothalamus and on appetitive behaviors (food hoarding and sexual motivation), but not on consummatory behaviors (food intake and lordosis), with no significant effect on circulating levels of estradiol or progesterone. The effect of food restriction on the activation of RFRP-3 cells is modulated at the time of estrus in gonadally-intact females and in ovariectomized females treated with progesterone alone or with estradiol plus progesterone. Intracerebral treatment with RFRP-3 results in significant decreases in sexual motivation and results in significant but small increases in food hoarding in hamsters fed ad libitum. These and other results are consistent with the idea that ovarian steroids and RFRP-3 are part of a system that orchestrates trade-offs in appetitive behaviors in environments where energy availability fluctuates.

Food restriction-induced changes in motivation differ with stages of the estrous cycle and are closely linked to RFamide-related peptide-3 but not kisspeptin in Syrian hamsters

We tested the hypothesis that the effects of food restriction on behavioral motivation are mediated by one or both of the RFamide peptides, RFamide-related peptide-3 (RFRP-3) and kisspeptin (Kp) in female Syrian hamsters (Mesocricetus auratus). Female hamsters fed ad libitum and given a choice between food and adult male hamsters are highly motivated to visit males instead of food on all four days of the estrous cycle, but after 8days of mild food restriction (75% of ad libitum intake) they shift their preference toward food every day of the estrous cycle until the day of estrus, when they shift their preference back toward the males. In support of a role for RFRP-3 in these behavioral changes, the preference for food and the activation of RFRP-3-immunoreactive (Ir) cells in the dorsomedial hypothalamus (DMH) showed the same estrous cycle pattern in food-restricted females, but no association was observed between behavior and the activation of Kp cells in the hypothalamic arcuate nucleus or preoptic area. Next, we tested the hypothesis that food-restriction-induced activation of RFRP-3-Ir cells is modulated by high levels of ovarian steroids at the time of estrus. In support of this idea, on nonestrous days, mild food restriction increased activation of RFRP-3-Ir cells, but failed to do so on the day of estrus even though this level of food restriction did not significantly decrease circulating concentrations of estradiol or progesterone. Furthermore, in ovariectomized females, food-restriction-induced increases in activation of RFRP-3-Ir cells were blocked by systemic treatment with progesterone alone, estradiol plus progesterone, but not estradiol alone. Central infusion with RFRP-3 in ad libitum-fed females significantly decreased sexual motivation and produced significant increases in 90-minute food hoarding, in support of the hypothesis that elevated central levels of RFRP-3 are sufficient to create the shift in behavioral motivation in females fed ad libitum. Together, these results are consistent with the hypothesis that high levels of ingestive motivation are promoted during the nonfertile phase of the estrous cycle by elevated activation of RFRP-3-Ir cells, and RFRP-3-Ir cellular activation is modulated by ovarian steroids around the time of estrus, thereby diverting attention away from food and increasing sexual motivation.

Leptin Suppresses the Rewarding Effects of Running via STAT3 Signaling in Dopamine Neurons

The adipose hormone leptin potently influences physical activity. Leptin can decrease locomotion and running, yet the mechanisms involved and the influence of leptin on the rewarding effects of running ("runner's high") are unknown. Leptin receptor (LepR) signaling involves activation of signal transducer and activator of transcription-3 (STAT3), including in dopamine neurons of the ventral tegmental area (VTA) that are essential for reward-relevant behavior. We found that mice lacking STAT3 in dopamine neurons exhibit greater voluntary running, an effect reversed by viral-mediated STAT3 restoration. STAT3 deletion increased the rewarding effects of running whereas intra-VTA leptin blocked it in a STAT3-dependent manner. Finally, STAT3 loss-of-function reduced mesolimbic dopamine overflow and function. Findings suggest that leptin influences the motivational effects of running via LepR-STAT3 modulation of dopamine tone. Falling leptin is hypothesized to increase stamina and the rewarding effects of running as an adaptive means to enhance the pursuit and procurement of food.

An acute method to test leptin responsiveness in rats

Continuous subcutaneous administration of leptin normalizes blood glucose levels in rodent models of Type 1 and Type 2 diabetes independent of changes in food intake, body weight, and plasma insulin. We tested whether an acute intravenous leptin infusion changed blood glucose in normal and diet-induced leptin-resistant rats to determine whether this measure could be used as a marker of leptin sensitivity. Leptin-responsive chow-fed rats and diet-induced leptin-resistant male Sprague-Dawley rats were fitted with thoracic jugular vein catheters. Four days after surgery, conscious rats were infused intravenously with either saline for 32 min, low-dose (LD) leptin (1.9 μg·kg(-1)·min(-1)) followed by high-dose (HD) leptin (3.8 μg·kg(-1)·min(-1)) for 16 min each, or only HD leptin for 16 min. There was no change in blood glucose after an acute intravenous infusion of either LD leptin or HD leptin alone for 16 min. An intravenous infusion of LD followed by HD leptin for 16 min each significantly decreased serum glucose in leptin-responsive rats but not in leptin-resistant rats. Leptin infusions increased serum leptin in all rat groups but had no effect on plasma glucagon or 12-h weight gain and energy intake in any group of rats. These results show that leptin has an acute glucose-lowering effect that reflects the leptin responsiveness of the rat. This effect is consistent across controls and different leptin-resistant rat models, and the acute nonlethal test provides a novel method of testing leptin responsiveness in rats.

Leptin signaling in the medial nucleus tractus solitarius reduces food seeking and willingness to work for food

The adipose-derived hormone leptin signals in the medial nucleus tractus solitarius (mNTS) to suppress food intake, in part, by amplifying within-meal gastrointestinal (GI) satiation signals. Here we show that mNTS leptin receptor (LepRb) signaling also reduces appetitive and motivational aspects of feeding, and that these effects can depend on energy status. Using the lowest dose that significantly suppressed 3-h cumulative food intake, unilateral leptin (0.3 μg) administration to the mNTS (3 h before testing) reduced operant lever pressing for sucrose under increasing work demands (progressive ratio reinforcement schedule) regardless of whether animals were energy deplete (food restricted) or replete (ad libitum fed). However, in a separate test of food-motivated responding in which there was no opportunity to consume food (conditioned place preference (CPP) for an environment previously associated with a palatable food reward), mNTS leptin administration suppressed food-seeking behavior only in chronically food-restricted rats. On the other hand, mNTS LepRb signaling did not reduce CPP expression for morphine reinforcement regardless of energy status, suggesting that mNTS leptin signaling differentially influences motivated responding for food vs opioid reward. Overall results show that mNTS LepRb signaling reduces food intake and appetitive food-motivated responding independent of energy status in situations involving orosensory and postingestive contact with food, whereas food-seeking behavior independent of food consumption is only reduced by mNTS LepRb activation in a state of energy deficit. These findings reveal a novel appetitive role for LepRb signaling in the mNTS, a brain region traditionally linked with processing of meal-related GI satiation signals.

Neuroendocrine regulation of appetitive ingestive behavior

Food availability in nature is often irregular, and famine is commonplace. Increased motivation to engage in ingestive behaviors increases the chance of survival, providing additional potential opportunities for reproduction. Because of the advantages conferred by entraining ingestive behavior to environmental conditions, neuroendocrine mechanisms regulating the motivation to acquire and ingest food have evolved to be responsive to exogenous (i.e., food stored for future consumption) and endogenous (i.e., body fat stores) fuel availability. Motivated behaviors like eating occur in two phases. The appetitive phase brings animals into contact with food (e.g., foraging, food hoarding), and the more reflexive consummatory phase results in ingestion (e.g., chewing, swallowing). Quantifiable appetitive behaviors are part of the natural ingestive behavioral repertoire of species such as hamsters and humans. This review summarizes current knowledge about neuroendocrine regulators of ingestive behavior, with an emphasis appetitive behavior. We will discuss hormonal regulators of appetitive ingestive behaviors, including the orexigenic hormone ghrelin, which potently stimulates foraging and food hoarding in Siberian hamsters. This section includes a discussion of the hormone leptin, its relation to endogenous fat stores, and its role in food deprivation-induced increases in appetitive ingestive behaviors. Next, we discuss how hormonal regulators interact with neurotransmitters involved in the regulation of ingestive behaviors, such as neuropeptide Y (NPY), agouti-related protein (AgRP) and α-melanocyte stimulating hormone (α-MSH), to regulate ingestive behavior. Finally, we discuss the potential impact that perinatal nutrient availability can have on the neuroendocrine regulation of ingestive behavior. Understanding the hormonal mechanisms that connect metabolic fuel availability to central appetite regulatory circuits should provide a better understanding of the neuroendocrine regulation of the motivation to engage in ingestive behavior.

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

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

PYY(3-36) into the arcuate nucleus inhibits food deprivation-induced increases in food hoarding and intake

Central administration of neuropeptide Y (NPY) increases food intake in laboratory rats and mice, as well as food foraging and hoarding in Siberian hamsters. The NPY-Y1 and Y5 receptors (Rs) within the hypothalamus appear sufficient to account for these increases in ingestive behaviors. Stimulation of NPY-Y2Rs in the Arcuate nucleus (Arc) has an anorexigenic effect as shown by central or peripheral administration of its natural ligand peptide YY (3-36) and pharmacological NPY-Y2R antagonism by BIIE0246 increases food intake. Both effects on food intake by NPY-Y2R agonism and antagonism are relatively short-lived lasting ∼4h. The role of NPY-Y2Rs in appetitive ingestive behaviors (food foraging/hoarding) is untested, however. Therefore, Siberians hamsters, a natural food hoarder, were housed in a semi-natural burrow/foraging system that had (a) foraging requirement (10 revolutions/pellet), no free food (true foraging group), (b) no running wheel access, free food (general malaise control) or (c) running wheel access, free food (exercise control). We microinjected BIIE0246 (antagonist) and PYY(3-36) (agonist) into the Arc to test the role of NPY-Y2Rs there on ingestive behaviors. Food foraging, hoarding, and intake were not affected by Arc BIIE0246 microinjection in fed hamsters 1, 2, 4, and 24h post injection. Stimulation of NPY-Y2Rs by PYY(3-36) inhibited food intake at 0-1 and 1-2h and food hoarding at 1-2h without causing general malaise or affecting foraging. Collectively, these results implicate a sufficiency, but not necessity, of the Arc NPY-Y2R in the inhibition of food intake and food hoarding by Siberian hamsters.

Energy budget, behavior and leptin in striped hamsters subjected to food restriction and refeeding

Food restriction induces a loss of body mass that is often followed by rapid regaining of the lost weight when the restriction ends, consequently increasing a risk of development of obesity. To determine the physiological and behavioral mechanisms underlining the regaining, striped hamsters were restricted to 85% of initial food intake for 4 weeks and refed ad libitum for another 4 weeks. Changes in body mass, energy budget, activity, body composition and serum leptin level were measured. Body mass, body fat mass and serum leptin level significantly decreased in food-restricted hamsters, and increased when the restriction ended, showing a short “compensatory growth” rather than over-weight or obesity compared with ad libitum controls. During restriction, the time spent on activity increased significantly, which was opposite to the changes in serum leptin level. Food intake increased shortly during refeeding, which perhaps contributed to the rapid regaining of body mass. No correlation was observed between serum leptin and energy intake, while negative correlations were found in hamsters that were refed for 7 and 28 days. Exogenous leptin significantly decreased the time spent on activity during food restriction and attenuated the increase in food intake during refeeding. This suggests that low leptin in restricted animals may function as a starvation signal to induce an increase in activity behavior, and high leptin likely serves as a satiety signal to prevent activity during refeeding. Leptin may play a crucial role in controlling food intake when the restriction ends, and consequently preventing overweight.

Effect of central and peripheral leucine on energy metabolism in the Djungarian hamster (Phodopus sungorus)

Branched-chain amino acids, particularly leucine, are thought to activate nutrient sensing pathways in the hypothalamus that regulate food intake and energy homeostasis. In the light of recent controversial findings of leucine's effect on energy homeostasis further clarification of the metabolic impact of dietary leucine supplementation is required. We examined the pharmacological and dietary effects of leucine on energy metabolism in the Djungarian hamster (Phodopus sungorus), a well-established model for studies of alterations in leptin sensitivity and energy metabolism. We acutely administered leucine into the lateral ventricle (1.1 μg) of hamsters to characterize whether leucine exhibits anorexigenic properties in this species as has been described in other rodents. Next the catabolic effect of dietary administered leucine via supplemented rodent diet (15 % leucine), drinking water (17 g/L leucine) and oral gavages (10 mg/day); as well as the effect of subcutaneously (0.1 and 3 mg/day) and intraperitoneally (0.1, 3 and 6 mg/day) injected leucine which avoids the gastrointestinal-track was analyzed. Centrally administered leucine reduced 24 h food intake (by 32 %) and body weight. Both parameters were also reduced in hamsters with leucine supplemented diet, but this catabolic response was based on a pronounced taste aversion to the leucine-diet. In all other experiments, dietary leucine and peripheral injections of leucine had no effect on food intake, body weight and basal blood glucose levels. Our data suggest that in the Djungarian hamster dietary leucine fails to exhibit catabolic effects that would override the evolutionary conserved adaptations of the species which is critical for its survival.

Neural and hormonal control of food hoarding

Many animals hoard food, including humans, but despite its pervasiveness, little is known about the physiological mechanisms underlying this appetitive behavior. We summarize studies of food hoarding in humans and rodents with an emphasis on mechanistic laboratory studies of species where this behavior importantly impacts their energy balance (hamsters), but include laboratory rat studies although their wild counterparts do not hoard food. The photoperiod and cold can affect food hoarding, but food availability is the most significant environmental factor affecting food hoarding. Food-deprived/restricted hamsters and humans exhibit large increases in food hoarding compared with their fed counterparts, both doing so without overeating. Some of the peripheral and central peptides involved in food intake also affect food hoarding, although many have not been tested. Ad libitum-fed hamsters given systemic injections of ghrelin, the peripheral orexigenic hormone that increases with fasting, mimics food deprivation-induced increases in food hoarding. Neuropeptide Y or agouti-related protein, brain peptides stimulated by ghrelin, given centrally to ad libitum-fed hamsters, duplicates the early and prolonged postfood deprivation increases in food hoarding, whereas central melanocortin receptor agonism tends to inhibit food deprivation and ghrelin stimulation of hoarding. Central or peripheral leptin injection or peripheral cholecystokinin-33, known satiety peptides, inhibit food hoarding. Food hoarding markedly increases with pregnancy and lactation. Because fasted and/or obese humans hoard more food in general, and more high-density/high-fat foods specifically, than nonfasted and/or nonobese humans, understanding the mechanisms underlying food hoarding could provide another target for behavioral/pharmacological approaches to curb obesity.

Food hoarding and associated neuronal activation in brain reward circuitry in Mongolian gerbils

Mongolian gerbils (Meriones unguiculatus) display food hoarding and thus provide an opportunity to study the neuromechanisms underlying this behavior. In the present study, male gerbils exhibited a bimodal expression of food hoarding behavior-some displayed high levels of food hoarding whereas others virtually lacked this behavior under normal laboratory conditions with free access to food. Food hoarding was found to be associated with an increase in neuronal activation, indicated by Fos immunoreactive (ir) staining, in several brain areas including the nucleus accumbens, ventral tegmental area (VTA), and lateral hypothalamus. Food hoarding was also associated with increases in the number of cells labeled for tyrosine hydroxylase (TH-ir), the rate limiting enzyme for dopamine conversion, and the number of cells co-labeled for TH-ir/Fos-ir in the VTA, suggesting that dopamine in the brain reward circuitry may be involved in food hoarding. Further, we found that 22 h of food deprivation induced food hoarding in some, but not all, males that naturally did not display food hoarding. In these males, however, food hoarding did not increase TH-ir or TH-ir/Fos-ir expression in the VTA. Together, these data indicate that male Mongolian gerbils display diverse phenotypes of food hoarding behavior and that dopamine in the brain reward circuitry may be involved in the control of naturally occurring, but not food deprivation-induced, food hoarding.

Cholecystokinin-33 acutely attenuates food foraging, hoarding and intake in Siberian hamsters

Neurochemicals that stimulate food foraging and hoarding in Siberian hamsters are becoming more apparent, but we do not know if cessation of these behaviors is due to waning of excitatory stimuli and/or the advent of inhibitory factors. Cholecystokinin (CCK) may be such an inhibitory factor as it is the prototypic gastrointestinal satiety peptide and is physiologically important in decreasing food intake in several species including Siberian hamsters. Systemic injection of CCK-33 in laboratory rats decreases food intake, doing so to a greater extent than CCK-8. We found minimal effects of CCK-8 on food foraging and hoarding previously in Siberian hamsters, but have not tested CCK-33. Therefore, we asked: Does CCK-33 decrease normal levels or food deprivation-induced increases in food foraging, hoarding and intake? Hamsters were housed in a wheel running-based foraging system with simulated burrows to test the effects of peripheral injections of CCK-33 (13.2, 26.4, or 52.8 microg/kg body mass), with or without a preceding 56 h food deprivation. The highest dose of CCK-33 caused large baseline reductions in all three behaviors for the 1st hour post-injection compared with saline; in addition, the intermediate CCK-33 dose was sufficient to curtail food intake and foraging during the 1st hour. In food-deprived hamsters, we used a 52.8 microg/kg body mass dose of CCK-33 which decreased food intake, hoarding, and foraging almost completely compared with saline controls for 1h. Therefore, CCK-33 appears to be a potent inhibitor of food intake, hoarding, and foraging in Siberian hamsters.

Physiological mechanisms for food-hoarding motivation in animals

The study of ingestive behaviour has an extensive history, starting as early as 1918 when Wallace Craig, an animal behaviourist, coined the terms 'appetitive' and 'consummatory' for the two-part sequence of eating, drinking and sexual behaviours. Since then, most ingestive behaviour research has focused on the neuroendocrine control of food ingestion (consummatory behaviour). The quantity of food eaten, however, is also influenced by the drive both to acquire and to store food (appetitive behaviour). For example, hamster species have a natural proclivity to hoard food and preferentially alter appetitive ingestive behaviours in response to environmental changes and/or metabolic hormones and neuropeptides, whereas other species would instead primarily increase their food intake. Therefore, with the strong appetitive component to their ingestive behaviour that is relatively separate from their consummatory behaviour, they seem an ideal model for elucidating the neuroendocrine mechanisms underlying the control of food hoarding and foraging. This review focuses on the appetitive side of ingestive behaviour, in particular food hoarding, attempting to integrate what is known about the neuroendocrine mechanisms regulating this relatively poorly studied behaviour. An hypothesis is formed stating that the direction of 'energy flux' is a unifying factor for the control of food hoarding.

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

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

Body mass loss during adaptation to short winter-like days increases food foraging, but not food hoarding

Siberian hamsters markedly reduce their body/lipid mass ( approximately 20-45%) in short 'winter-like' days (SD). Decreases in body/lipid mass associated with food deprivation or lipectomy result in increases in foraging and food hoarding. When at their SD-induced body/lipid mass nadir, food hoarding is not increased despite their decreases in body/lipid mass, but hoarding was not tested during the dynamic period of body/lipid mass loss (first 5-6 weeks of SDs). Therefore, we tested for changes in foraging/hoarding during this initial period in Siberian hamsters housed in a simulated burrow with a wheel running-based foraging system and exposed to either long 'summer-like' days (LD) or SDs. Two foraging effort conditions were used: 10 Revolutions/Pellet (pellet delivered after running 10 revolutions) and a Free Wheel/Free Food condition (wheel available, food pellets non-contingently available). Regardless of the foraging condition, body mass was significantly reduced across 8 weeks of SDs ( approximately 15%). Foraging increased after 7 weeks in SDs, but food hoarding did not increase compared to LDs. Instead food hoarding significantly decreased in SDs at Weeks 2-5 compared with Week 0 values, with the 10 Revolutions/Pellet foraging group returning to LD levels thereafter and the Free Wheel/Free Food group remaining reduced from Weeks 2-7. Collectively, we found that SDs decreased body mass, increased foraging after 7 weeks, and increased food hoarding, but only after an initial decrease and not above that seen in LDs. These data suggest that SD-induced body/lipid mass losses do not engender similar behavioral responses as seen with food deprivation or lipectomy.