Food restriction-induced hyperactivity: addiction or adaptation to famine?

Sex-dependent circadian alterations of both central and peripheral clock genes expression and gut-microbiota composition during activity-based anorexia in mice

RATIONALE

Patients with anorexia nervosa (AN) often present sleep disorders and circadian hormonal dysregulation. The role of the microbiota-gut-brain axis in the regulation of feeding behavior has emerged during the last decades but its relationships with the circadian rhythm remains poorly documented. Thus, we aimed to characterize the circadian clock genes expression in peripheral and central tissues in the activity-based anorexia mouse model (ABA), as well as the dynamics of the gut-microbiota composition.

METHODS

From day 1 to day 17, male and female C57Bl/6 mice were submitted or not to the ABA protocol (ABA and control (CT) groups), which combines a progressive limited access to food and a free access to a running wheel. At day 17, fasted CT and ABA mice were euthanized after either resting (EoR) or activity (EoA) phase (n = 10-12 per group). Circadian clock genes expression was assessed by RT-qPCR on peripheral (liver, colon and ileum) and central (hypothalamic suprachiasmatic nucleus or SCN) tissues. Cecal bacterial taxa abundances were evaluated by qPCR. Data were compared by two-way ANOVA followed by post-tests.

RESULTS

ABA mice exhibited a lower food intake, a body weight loss and an increase of diurnal physical activity that differ according with the sex. Interestingly, in the SCN, only ABA female mice exhibited altered circadian clock genes expression (Bmal1, Per1, Per2, Cry1, Cry2). In the intestinal tract, modification of clock genes expression was also more marked in females compared to males. For instance, in the ileum, female mice showed alteration of Bmal1, Clock, Per1, Per2, Cry1, Cry2 and Rev-erbα mRNA levels, while only Per2 and Cry1 mRNAs were affected by ABA model in males. By contrast, in the liver, clock genes expression was more markedly affected in males compared to females in response to ABA. Finally, circadian variations of gut-bacteria abundances were observed in both male and female mice and sex-dependent alteration were observed in response to the ABA model.

CONCLUSIONS

This study shows that alteration of circadian clock genes expression at both peripheral and central levels occurs in response to the ABA model. In addition, our data underline that circadian variations of the gut-microbiota composition are sex-dependent.

High levels of physical activity in female adolescents with anorexia nervosa: medical and psychopathological correlates

PURPOSE

While overexercise is commonly described in patients who experience anorexia nervosa (AN), it represents a condition still underestimated, especially in the paediatric population.

METHOD

The present study aims at assessing the possible associations between levels of physical activity (PA) and clinical features, endocrinological data and psychopathological traits in a sample of 244 female adolescents hospitalised for AN subdivided into two groups according to PA levels (high PA vs. no/low PA). The two groups were compared through multivariate analyses, while multiple regression analysis was conducted to determine whether physical activity predict specific outcomes.

RESULTS

No significant differences were found between the two groups in terms of last Body Mass Index (BMI) before illness, BMI at admission and disease duration, while a difference emerged in delta BMI(rapidity of weight loss), significantly higher in high-PA group (p = 0.021). Significant differences were observed in Free triiodothyronine- (p < 0.001), Free thyroxine (p = 0.046), Follicle-stimulating hormone (p = 0.019), Luteinising hormone (p = 0.002) levels, with values remarkably lower in high-PA group. Concerning psychopathological scales, the high-PA group showed worst Children's Global Assessment Scale (CGAS) scores (p = 0.035). Regression analyses revealed that higher PA predicts higher delta BMI (p = 0.021), presence of amenorrhea (p = 0.003), lower heart rate (p = 0.012), lower thyroid (Free triiodothyronine p < 0.001, Free thyroxine p = 0.029) and gynaecological hormones' levels (Follicle-stimulating hormone p = 0.023, Luteinising hormone p = 0.003, 17-Beta estradiol p = 0.041). Concerning psychiatric measures, HPA predicts worst scores at CGAS (p = 0.019), and at scales for evaluation of alexithymia (p = 0.028) and depression (p = 0.004).

CONCLUSIONS

Results suggest that high levels of physical activity in acute AN associate with worst clinical conditions at admission, especially in terms of endocrinological and medical features.

LEVEL OF EVIDENCE

Level III.

Effort-motivated behavior resolves paradoxes in appetitive conditioning

Motivated behavior has long been studied by psychologists, ethologists, and neuroscientists. To date, many scientists agree with the view that cue and reward attraction is the product of a dopamine-dependent unconscious process called incentive salience or "wanting". This process allows the influence of multiple factors such as hunger and odors on motivational attraction. In some cases, however, the resulting motivated behavior differs from what the incentive salience hypothesis would predict. I argue that seeking behavior under reward uncertainty illustrates this situation: Organisms do not just "want" (appetite-based attraction) cues that are inconsistent or associated with reward occasionally, they "hope" that those cues will consistently predict reward procurement in the ongoing trial. Said otherwise, they become motivated to invest time and energy to find consistent cue-reward associations despite no guarantee of success (effort-based attraction). A multi-test comparison of performance between individuals trained under uncertainty and certainty reveals behavioral paradoxes suggesting that the concept of incentive salience cannot fully account for responding to inconsistent cues. A mathematical model explains how appetite-based and effort-based attractions might combine their effects.

Exploring the Mechanisms of Recovery in Anorexia Nervosa through a Translational Approach: From Original Ecological Measurements in Human to Brain Tissue Analyses in Mice

Anorexia nervosa (AN) is a severe eating disorder where caloric restriction, excessive physical activity and metabolic alterations lead to life-threatening situations. Despite weight restoration after treatment, a significant part of patients experience relapses. In this translational study, we combined clinical and preclinical approaches. We describe preliminary data about the effect of weight gain on the symptomatology of patients suffering from acute AN (n = 225) and partially recovered (n = 41). We measured more precisely physical activity with continuous cardiac monitoring in a sub-group (n = 68). Using a mouse model, we investigated whether a long-term food restriction followed by nutritional recovery associated or not with physical activity may differentially impact peripheral and central homeostatic regulation. We assessed the plasma concentration of acyl ghrelin, desacyl ghrelin and leptin and the mRNA expression of hypothalamic neuropeptides and their receptors. Our data show an effect of undernutrition history on the level of physical activity in AN. The preclinical model supports an important role of physical activity in the recovery process and points out the leptin system as one factor that can drive a reliable restoration of metabolic variables through the hypothalamic regulation of neuropeptides involved in feeding behavior.

Eating and wheel running across the estrous cycle in rat lines selectively bred on a taste phenotype

The Occidental Low- and High-Saccharin-Consuming rats (respectively, LoS and HiS) have been selectively bred for decades to study the relationship between taste and behaviors in and beyond the ingestive domain. Whether the saccharin phenotype is associated with behavioral periodicities tied to reproductive status is not known. Here we describe for the first time variation across the estrous cycle in chow intake and wheel running by LoS and HiS rats. This study also shed light on why rats, humans, and some other mammals eat less and become more active as fertility increases. Wheel running increases when eating is reduced through restricted chow access, more so in LoS rats than in HiS rats (Dess et al., 2000). If the decrease in food intake from diestrus through estrus causes the increase in running (Eat Less → Run More hypothesis, ELRM), then the running peak should follow the eating nadir and be greater in LoS rats. Bayesian cyclic regression showed that estrous cycles were shorter in LoS rats than in HiS rats; implications are discussed. Contrary to ELRM, the running peak did not follow the eating nadir, and cycle amplitude did not distinguish LoS rats from HiS rats. These results indirectly support the No Time To Eat hypothesis (Fessler, 2003), according to which the periovulatory eating nadir and running peak reflect fitness-enhancing consequences of shifts away from eating and toward mating as fertility increases.

From sign-tracking to attentional bias: Implications for gambling and substance use disorders

Sign-tracking behavior in Pavlovian autoshaping is known to be a relevant index of the incentive salience attributed to reward-related cues. Evidence has accumulated to suggest that animals that exhibit a sign-tracker phenotype are especially vulnerable to addiction and relapse due to their proneness to attribute incentive salience to drug cues, and their relatively weak cognitive and attentional control over their behavior. Interestingly, sign-tracking is also influenced by reward uncertainty in a way that may promote gambling disorder. Research indicates that reward uncertainty sensitizes sign-tracking responses and favors the development of a sign-tracker phenotype, compatible with the conditioned attractiveness of lights and sounds in casinos for problem gamblers. The study of attentional biases in humans (an effect akin to sign-tracking in animals) leads to similar observations, notably that the propensity to develop attraction for conditioned stimuli (CSs) is predictive of addictive behavior. Here we review the literature on drug addiction and gambling disorder, highlighting the similarities between studies of sign-tracking and attentional biases.

Activity Based Anorexia as an Animal Model for Anorexia Nervosa-A Systematic Review

Anorexia nervosa (AN) is a severe eating disorder affecting around 1 per 100 persons. However, the knowledge about its underlying pathophysiology is limited. To address the need for a better understanding of AN, an animal model was established early on in the late 1960's: the activity-based anorexia (ABA) model in which rats have access to a running wheel combined with restricted food access leading to self-starving/body weight loss and hyperactivity. Both symptoms, separately or combined, can also be found in patients with AN. The aim of this systematic review was to compile the current knowledge about this animal model as well as to address gaps in knowledge. Using the data bases of PubMed, Embase and Web of science 102 publications were identified meeting the search criteria. Here, we show that the ABA model mimics core features of human AN and has been characterized with regards to brain alterations, hormonal changes as well as adaptations of the immune system. Moreover, pharmacological interventions in ABA animals and new developments, such as a chronic adaptation of the ABA model, will be highlighted. The chronic model might be well suited to display AN characteristics but should be further characterized. Lastly, limitations of the model will be discussed.

Effect of Sucrose Ingestion at the End of a Critical Window that Increases Hypertension Susceptibility on Peripheral Mechanisms Regulating Blood Pressure in Rats. Role of Sirtuins 1 and 3

Susceptibility to develop hypertension may be established during early stages of life that include the intrauterine period, infancy and childhood. We recently showed that blood pressure increased when rats reached adulthood when sucrose was ingested for a short-term critical window from postnatal day 12 to 28 in the rat, which corresponds to days around weaning. Here, we studied several factors that might participate in the increased susceptibility to hypertension when adulthood is reached by analyzing the changes produced at the end of the sucrose ingestion during this critical period. Body weight of the rats at the end of the sucrose period was decreased even if there was an increased ingestion in Kcal. We found an increase in blood pressure accompanied by a decrease in endothelial nitric oxide synthase (eNOS) expression in the aorta. When insulin was administered to rats receiving sucrose, glucose in plasma diminished later than in controls and this slight insulin resistance may reduce nitric oxide synthase action. Oleic acid that modulates eNOS expression was increased, lipoperoxidation was elevated and total non-enzymatic anti-oxidant capacity was decreased. There was also a decrease in SOD2 expression. We also studied the expression of Sirt1, which regulates eNOS expression and Sirt3, which regulates SOD2 expression as possible epigenetic targets of enzyme expression involved in the long- term programming of hypertension. Sirt3 was decreased but we did not find an alteration in Sirt1 expression. We conclude that these changes may underpin the epigenetic programming of increased susceptibility to develop hypertension in the adults when there was exposure to high sucrose levels near weaning in rats.

Metabolic and neuroendocrine adaptations to undernutrition in anorexia nervosa: from a clinical to a basic research point of view

The exact mechanisms linking metabolic and neuroendocrine adaptations to undernutrition and the pathophysiology of anorexia nervosa (AN) are not fully understood. AN is a psychiatric disorder of complex etiology characterized by extreme starvation while the disease is progressing into a chronic state. Metabolic and endocrine alterations associated to this disorder are part of a powerful response to maintain whole body energy homeostasis. But these modifications may also contribute to associated neuropsychiatric symptoms (reward abnormalities, anxiety, depression) and thus participate to sustain the disease. The current review presents data with both a clinical and basic research point of view on the role of nutritional and energy sensors with neuroendocrine actions in the pathophysiology of the disease, as they modulate metabolic responses, reproductive functions, stress responses as well as physical activity. While clinical data present a full description of changes occurring in AN, animal models that integrate either spontaneous genetic mutations or experimentally-induced food restriction with hyperactivity and/or social stress recapitulate the main metabolic and endocrine alterations of AN and provide mechanistic information between undernutrition state and symptoms of the disease. Further progress on the central and peripheral mechanism involved in the pathophysiology of eating disorders partly relies on the development and/or refinement of existing animal models to include recently identified genetic traits and better mimic the complex and multifactorial dimensions of the disease.

To Do or Not to Do: Dopamine, Affordability and the Economics of Opportunity

Five years ago, we introduced the thrift hypothesis of dopamine (DA), suggesting that the primary role of DA in adaptive behavior is regulating behavioral energy expenditure to match the prevailing economic conditions of the environment. Here we elaborate that hypothesis with several new ideas. First, we introduce the concept of affordability, suggesting that costs must necessarily be evaluated with respect to the availability of resources to the organism, which computes a value not only for the potential reward opportunity, but also the value of resources expended. Placing both costs and benefits within the context of the larger economy in which the animal is functioning requires consideration of the different timescales against which to compute resource availability, or average reward rate. Appropriate windows of computation for tracking resources requires corresponding neural substrates that operate on these different timescales. In discussing temporal patterns of DA signaling, we focus on a neglected form of DA plasticity and adaptation, changes in the physical substrate of the DA system itself, such as up- and down-regulation of receptors or release probability. We argue that changes in the DA substrate itself fundamentally alter its computational function, which we propose mediates adaptations to longer temporal horizons and economic conditions. In developing our hypothesis, we focus on DA D2 receptors (D2R), arguing that D2R implements a form of “cost control” in response to the environmental economy, serving as the “brain’s comptroller”. We propose that the balance between the direct and indirect pathway, regulated by relative expression of D1 and D2 DA receptors, implements affordability. Finally, as we review data, we discuss limitations in current approaches that impede fully investigating the proposed hypothesis and highlight alternative, more semi-naturalistic strategies more conducive to neuroeconomic investigations on the role of DA in adaptive behavior.

Ghrelin treatment prevents development of activity based anorexia in mice

Stimulation of feeding is necessary for treatment of pathological conditions of chronic malnutrition due to anorexia. Ghrelin, a hunger hormone, is one of the candidate for pharmacological treatments of anorexia, but because of its instability in plasma has limited efficacy. We previously showed that plasmatic IgG protect ghrelin from degradation and that IgG from obese subjects and mice may increase ghrelin׳s orexigenic effect. In this study we tested if ghrelin alone or combined with IgG may improve feeding in chronically food-restricted mice with or without physical activity-based anorexia (ABA) induced by free access to a running wheel. Mice received a single daily intraperitoneal injection of ghrelin (1nM) together or not with total IgG (1nM) from obese ob/ob or lean mice before access to food during 8 days of 3h/day feeding time. We found that both ghrelin and ghrelin combined with IgG from obese, but not lean mice, prevented ABA, however, they were not able to diminish body weight loss. Physical activity was lower during the feeding period and was increased shortly after feeding in mice receiving ghrelin together with IgG from obese mice. In food-restricted mice without ABA, ghrelin treatments did not have significant effects on food intake. Thus, this study supports pharmacological use of ghrelin or ghrelin combined with IgG from obese animals for treatment of anorexia accompanied by elevated physical activity. The utility of combining ghrelin with protective IgG should be further determined in animal models of anorexia with unrestricted access to food.

The use of animal models to decipher physiological and neurobiological alterations of anorexia nervosa patients

Extensive studies were performed to decipher the mechanisms regulating feeding due to the worldwide obesity pandemy and its complications. The data obtained might be adapted to another disorder related to alteration of food intake, the restrictive anorexia nervosa. This multifactorial disease with a complex and unknown etiology is considered as an awful eating disorder since the chronic refusal to eat leads to severe, and sometimes, irreversible complications for the whole organism, until death. There is an urgent need to better understand the different aspects of the disease to develop novel approaches complementary to the usual psychological therapies. For this purpose, the use of pertinent animal models becomes a necessity. We present here the various rodent models described in the literature that might be used to dissect central and peripheral mechanisms involved in the adaptation to deficient energy supplies and/or the maintenance of physiological alterations on the long term. Data obtained from the spontaneous or engineered genetic models permit to better apprehend the implication of one signaling system (hormone, neuropeptide, neurotransmitter) in the development of several symptoms observed in anorexia nervosa. As example, mutations in the ghrelin, serotonin, dopamine pathways lead to alterations that mimic the phenotype, but compensatory mechanisms often occur rendering necessary the use of more selective gene strategies. Until now, environmental animal models based on one or several inducing factors like diet restriction, stress, or physical activity mimicked more extensively central and peripheral alterations decribed in anorexia nervosa. They bring significant data on feeding behavior, energy expenditure, and central circuit alterations. Animal models are described and criticized on the basis of the criteria of validity for anorexia nervosa.

Physical activity: benefit or weakness in metabolic adaptations in a mouse model of chronic food restriction?

In restrictive-type anorexia nervosa (AN) patients, physical activity is usually associated with food restriction, but its physiological consequences remain poorly characterized. In female mice, we evaluated the impact of voluntary physical activity with/without chronic food restriction on metabolic and endocrine parameters that might contribute to AN. In this protocol, FRW mice (i.e., food restriction with running wheel) reached a crucial point of body weight loss (especially fat mass) faster than FR mice (i.e., food restriction only). However, in contrast to FR mice, their body weight stabilized, demonstrating a protective effect of a moderate, regular physical activity. Exercise delayed meal initiation and duration. FRW mice displayed food anticipatory activity compared with FR mice, which was strongly diminished with the prolongation of the protocol. The long-term nature of the protocol enabled assessment of bone parameters similar to those observed in AN patients. Both restricted groups adapted their energy metabolism differentially in the short and long term, with less fat oxidation in FRW mice and a preferential use of glucose to compensate for the chronic energy imbalance. Finally, like restrictive AN patients, FRW mice exhibited low leptin levels, high plasma concentrations of corticosterone and ghrelin, and a disruption of the estrous cycle. In conclusion, our model suggests that physical activity has beneficial effects on the adaptation to the severe condition of food restriction despite the absence of any protective effect on lean and bone mass.

Adolescent eating disorders: update on definitions, symptomatology, epidemiology, and comorbidity

The prevalence of eating disorders among adolescents continues to increase. The starvation process itself is often associated with severe alterations of central and peripheral metabolism, affecting overall health during this vulnerable period. This article aims to convey basic knowledge on these frequent and disabling disorders, and to review new developments in classification issues resulting from the transition to DSM-5. A detailed description is given of the symptomatology of each eating disorder that typically manifests during adolescence. New data on epidemiology, and expanding knowledge on associated medical and psychiatric comorbidities and their often long-lasting sequelae in later life, are provided.

Sex-related effects of nutritional supplementation of Escherichia coli: relevance to eating disorders

OBJECTIVES

The biological background of sex-related differences in the development of eating disorders (EDs) is unknown. Recent data showed that gut bacteria Escherichia coli induce autoantibodies against anorexigenic α-melanocyte-stimulating hormone (α-MSH) associated with psychopathology in ED. The aim of this study was to compare the effects of E. coli on feeding and autoantibodies against α-MSH and adrenocorticotropic hormone (ACTH), between female and male rats.

METHODS

Commensal E. coli K12 were given in a culture medium daily to adult Wistar rats by intragastric gavage over a 3-wk period; control rats received culture medium only.

RESULTS

Before gavage, E. coli K12 DNA was detected in feces of female but not male rats. E. coli provision was accompanied by an increase in body weight gain in females, but a decrease in body weight gain and food intake in males. Independent of E. coli treatment, plasma levels of anti-α-MSH and ACTH immunoglobulin (Ig)G were higher in female than male rats. Females responded to E. coli by increasing α-MSH IgG levels and affinity, but males by increasing α-MSH IgM levels. Affinity of IgG for ACTH was increased in both E. coli-treated females and males, although with different kinetics. IgG from females stimulated more efficiently α-MSH-induced cyclic adenosine monophosphate production by melanocortin 4 receptor-expressing cells compared with IgG from males.

DISCUSSION

Sex-related response to how E. coli affects feeding and anti-melanocortin hormone antibody production may depend on the presence of these bacteria in the gut before E. coli supplementation. These data suggest that sex-related presence of certain gut bacteria may represent a risk factor for ED development.

Enhanced consumption of salient solutions following pedunculopontine tegmental lesions

Rats with lesions of the pedunculopontine tegmental nucleus (PPTg) reliably overconsume high concentration sucrose solution. This effect is thought to be indicative of response-perseveration or loss of behavioral control in conditions of high excitement. While these theories have anatomical and behavioral support, they have never been explicitly tested. Here, we used a contact lickometer to examine the microstructure of drinking behavior to gain insight into the behavioral changes during overconsumption. Rats received either excitotoxic (ibotenic acid) damage to all PPTg neuronal subpopulations or selective depletion of the cholinergic neuronal sub-population (diphtheria toxin-urotensin II (Dtx-UII) lesions). We offered rats a variety of pleasant, neutral and aversive tastants to assess the generalizability and specificity of the overconsumption effect. Ibotenic-lesioned rats consumed significantly more 20% sucrose than sham controls, and did so through licking significantly more times. However, the behavioral microstructure during overconsumption was unaffected by the lesion and showed no indications of response-perseveration. Furthermore, the overconsumption effect did not generalize to highly consumed saccharin. In contrast, while only consuming small amounts of quinine solution, ibotenic-lesioned rats had significantly more licks and bursts for this tastant. Selective depletion of cholinergic PPTg neurons had no effect on consumption of any tastant. We then assessed whether it is the salience of the solution which determines overconsumption by ibotenic-lesioned rats. While maintained on free-food, ibotenic-lesioned rats had normal consumption of sucrose and hypertonic saline. After mild food deprivation ibotenic PPTg-lesioned rats overconsumed 20% sucrose. Subsequently, after dietary-induced sodium deficiency, lesioned rats consumed significantly more saline than controls. These results establish that it is the salience of the solution which is the determining factor leading to overconsumption following excitotoxic PPTg lesion. They also find no support for response-perseveration contributing to this effect. Results are discussed in terms of altered dopamine (DA) and salience signaling.

Ingestive and locomotor behaviours induced by pharmacological manipulation of <Alpha>-adrenoceptors into the median raphe nucleus

The present study evaluated the involvement of α-adrenoceptors of the median raphe nucleus (MRN) in satiated rats, in food and water intake and motor behaviour. Control groups were treated with saline (SAL) or adrenaline (ADR), injected into the MRN seven minutes after injection of the vehicle used to solubilize the antagonists, propylene glycol (PLG) or SAL. Experimental groups were treated with an α-adrenoceptor antagonist, prazosin (α1, 20 or 40 nmol) or yohimbine (α2, 20 or 40 nmol) or phentolamine (non-selective α, 20 or 40 nmol), followed (later) by injection of ADR or SAL. Behaviour was recorded for 30 min. The injection of ADR and the blockade of α1 receptors resulted in hyperphagia whereas blocking α2 or α1 and α2 simultaneously did not change feeding behaviour. Pre-treatment with prazosin, followed by injection of ADR was not able to cause an increase in the amount of food ingested, while the higher dose of the α1 antagonist reduced the latency to start feeding. Pre-treatment with prazosin also caused hyperactivity. However, pre-treatment with phentolamine or yohimbine was able to block ADR-induced feeding. The present study supports the hypothesis that there is a tonic activation of α1-adrenoceptors in the MRN in satiated rats, which activates an inhibitory influence in areas that control food intake. Injection of ADR seems to activate α2 receptors, resulting in a decrease in the availability of endogenous catecholamines, which reduces the release of the signal that inhibits food intake, leading to hyperphagia.

Additive antidepressant-like effects of fasting with imipramine via modulation of 5-HT2 receptors in the mice

Recently, studies show that intermittent fasting and caloric restriction may improve symptoms of depression. However, there is little scientific evidence regarding the literature on the antidepressant-like effects of acute fasting. The present study aims to investigate the antidepressant-like effects and its influence on brain levels of the transcription factor cAMP response element-binding protein (CREB) and its phosphorylated form (p-CREB) in different time periods of fasting mice. Furthermore, the additive antidepressant-like effects of fasting with imipramine and the possible involvement of the 5-HT2 receptors were examined. In the present study 9h, but not 3h and 18h of fasting significantly reduced immobility time in the forced swimming test (FST) without alteration in locomotor activity in the open field test. 9h fasting also enhanced the ratio of p-CREB/CREB in the frontal cortex and hippocampus. Co-administration of 9h of fasting and imipramine (30mg/kg, i.p) produced the additive antidepressant-like effects in the FST and increased the ratio of p-CREB/CREB. Meanwhile, the additive effects were partially reversed by treatment with a 5-HT2A/2C receptor agonist, (±)-1-(2, 5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI) (5mg/kg, s.c). Furthermore, the antidepressant-like effects of 9h fasting was also blocked by DOI compared to the non-fasting control group. Serum corticosterone level, but not 5-HT and noradrenaline, was significantly increased in a time-dependent manner following different time periods of fasting. Taken together, these results suggest that acute fasting produces antidepressant-like effects via enhancement of the p-CREB/CREB ratio, and additive antidepressant-like effects of fasting with imipramine may be related to modulating 5-HT2 receptors.