Beate C. Finger

High-fat diet selectively protects against the effects of chronic social stress in the mouse

Obesity and stress-related psychiatric disorders are frequently comorbid. However, our understanding of the relationship between diet, everyday life stress and psychiatric disorders is limited. Although the ability of stress to increase the likelihood to develop obesity and its comorbidities in a feed-forward loop has been studied there is a dearth of studies especially at the behavioural level investigating the feedback hypothesis, that is, the consequences of high-fat diet consumption on chronic stress-induced alterations. The effects of unpredictable chronic psychosocial stress on anxiety-like behaviour in the light-dark box, depressive-like behaviour in the forced swim test, hedonic behaviour in the female urine sniffing test and social avoidance in the social interaction test were investigated in a mouse model of diet-induced obesity. Changes in plasma levels of leptin, insulin and corticosterone were also assessed. A clear dissociation in behaviours was observed in mice subjected to diet-induced obesity coupled with chronic stress, with anxiety- and depressive-like behaviour observed in mice on a low- but not on a high-fat diet exposed to chronic social stress. On the other hand, social avoidance and anhedonic behaviour was present following stress independent of diet. Moreover, the effect of chronic stress in lowering leptin levels was most apparent in mice on a high-fat diet. Plasma insulin levels however where only decreased in mice on high- but not low-fat diet. In conclusion, long-term exposure to high-fat diet selectively and robustly protects against some of the behavioural sequelae of chronic unpredictable social stressors. These data show that there is a clear discrimination in the nature of stress-induced behavioural effects sensitive to protection by high-fat diet. Moreover, these results illustrate the strong influence of dietary components on stress-induced psychological factors and thereby emphasize the importance of the brain-gut-axis as a point of future therapeutic intervention.

Increased sensitivity to the effects of chronic social defeat stress in an innately anxious mouse strain.

Stress and genetic predisposition are two of the major risk factors for a variety of psychiatric illnesses. Inbred mouse strains are considered useful tools in dissecting the genetic basis of complex disorders. Indeed, mice of the C57BL/6 and BALB/c strains, differing markedly in anxiety behaviours, are among the most widely used in psychopharmacological research. However, there is a paucity of studies investigating the impact of social stress in these two strains. Moreover, whether these two mouse strains exhibit different sensitivities to chronic social defeat stress remains poorly studied. Thus in this study we compared the impact of repeated (10 days) social defeat stress on a variety of behavioural and endocrine parameters including social interaction, locomotor activity, plasma corticosterone, body weight and stress-related physiological parameters in both mouse strains. Given that the duration of stress exposure may differentially affect such responses we also compared stressors of short (Social Defeat-Short; SD-S) and of long (Social Defeat-Long; SD-L) duration. Our results show that although mice from both strains were defeated in both social defeat paradigms, only BALB/c mice displayed social interaction impairments following SD-S, whereas both strains were behaviourally sensitive to SD-L. Moreover, both strains also differed in some of the physiological alterations induced by social defeat stress. Specifically, SD-S did not induce any change in corticosterone levels in either of the two strains, whereas SD-L was able to induce significant changes in C57BL/6 mice only. SD-S induced differential effects on bodyweight gain in both strains, increasing it in C57BL/6 and decreasing it in BALB/c mice, whereas SD-L had no effect. On the other hand, exposure to SD-S resulted in cardiac hypertrophy in C57BL/6 mice and SD-L induced spleen hypertrophy and thymus atrophy in BALB/c mice in addition to decreasing faecal output. Overall, the innately anxious BALB/c mice were more sensitive to social stress than C57BL/6, with differential behavioural and physiological alterations emerging as a function of stress severity. These data suggest different coping strategies to social interaction stress between the two mouse strains. The genetic basis of this stress-resilience/susceptibility warrants further investigation.

Neuropeptide S alters anxiety, but not depression-like behaviour in Flinders Sensitive Line rats: a genetic animal model of depression

Neuropeptide S (NPS) and its receptor (NPSR) have been implicated in the mediation of anxiolytic-like behaviour in rodents. However, little knowledge is available regarding the NPS system in depression-related behaviours, and whether NPS also exerts anxiolytic effects in an animal model of psychopathology. Therefore, the aim of this work was to characterize the effects of NPS on depression- and anxiety-related parameters, using male and female rats in a well-validated animal model of depression: the Flinders Sensitive Line (FSL), their controls, the Flinders Resistant Line (FRL), and Sprague-Dawley (SD) rats. We found that FSL showed greater immobility in the forced swim test (FST) than FRL, confirming their phenotype. However, NPS did not affect depression-related behaviour in any rat line. No significant differences in baseline anxiety levels between the FSL and FRL strains were observed, but FSL and FRL rats displayed less anxiety-like behaviour compared to SD rats. NPS decreased anxiety-like behaviour on the elevated plus-maze in all strains. The expression of the NPSR in the amygdala, periventricular hypothalamic nucleus, and hippocampus was equal in all male strains, although a trend towards reduced expression within the amygdala was observed in FSL rats compared to SD rats. In conclusion, NPS had a marked anxiolytic effect in FSL, FRL and SD rats, but did not modify the depression-related behaviour in any strain, in spite of the significant differences in innate level between the strains. These findings suggest that NPS specifically modifies anxiety behaviour but cannot overcome/reverse a genetically mediated depression phenotype.

The temporal impact of chronic intermittent psychosocial stress on high-fat diet-induced alterations in body weight.

BACKGROUND Chronic stress and diet can independently or in concert influence the bodys homeostasis over time. Thus, it is crucial to investigate the interplay of these parameters to gain insight into the evolution of stress-induced metabolic and eating disorders. METHODS C57BL/6J mice were subjected to chronic psychosocial (mixed model of social defeat and overcrowding) stress in combination with either a high- or low-fat diet for three or six weeks. To determine the evolution of stress and dietary effects, changes in body weight, caloric intake and caloric efficiency were determined as well as circulating leptin, insulin, glucose and corticosterone levels and social avoidance behaviour. RESULTS Exposure to stress for three weeks caused an increase in weight gain, in caloric intake and in caloric efficiency only in mice on a low-fat diet. However, after six weeks, only stressed mice on a high-fat diet displayed a pronounced inhibition of body weight gain, accompanied by reduced caloric intake and caloric efficiency. Stress decreased circulating leptin levels in mice on a low-fat diet after three weeks and in mice on a high-fat diet after three and six weeks of exposure. Plasma levels of insulin and markers of insulin resistance were blunted in mice on high-fat diet following six weeks of stress exposure. Social avoidance following chronic stress was present in all mice after three and six weeks. CONCLUSIONS This study describes the evolution of the chronic effects of social defeat/overcrowding stress in combination with exposure to high- or low-fat diet. Most importantly, we demonstrate that a six week chronic exposure to social defeat stress prevents the metabolic effects of high-fat diet, by inhibiting an increase in weight gain, caloric intake and efficiency and insulin resistance as well as in plasma leptin and insulin levels. This study highlights the importance of considering the chronic aspects of both parameters and their time-dependent interplay.

Leptin-deficient mice retain normal appetitive spatial learning yet exhibit marked increases in anxiety-related behaviours

RationaleThe individual’s emotional state influences food intake in both humans and rodents. Moreover, specific cognitive processes regulating the salient aspects of food reward are also critical for ingestive behaviour. However, the molecular mechanisms underlying such influence remain unclear. Genetic mouse models thus are important tools in dissecting the molecular and pathophysiological processes which cause complex human diseases. Leptin, encoded by the ob gene, plays an important part in the energy homeostasis and is critical for the development of obesity.ObjectivesIn these studies, we assess the impact of leptin on behaviours relevant to anxiety and appetitive learning.MethodsAnxiety-related behaviour was assessed in the light dark box and two tests of hyponeophagia. Spatial learning and behavioural flexibility by re-learning was assessed in an appetitive Y-maze task.ResultsLeptin-deficient (ob/ob) mice displayed higher levels of anxiety-related behaviour in both anxiety tests. In the appetitive Y-maze task, leptin deficiency caused no deficit in learning or re-learning and acute restrained stress had no influence on the learning process.ConclusionsThese results emphasise that whilst leptin has previously been shown to modulate aversively motivated learning we found no difference between leptin-deficient mice and their controls in an appetitive learning task. Moreover, both groups showed behavioural flexibility under stressful conditions. On the other hand, leptin deficiency resulted in marked alterations in behaviours relevant to anxiety.

Toll-Like Receptor 4 Regulates Chronic Stress-Induced Visceral Pain in Mice

BACKGROUND Functional gastrointestinal disorders, which have visceral hypersensitivity as a core symptom, are frequently comorbid with stress-related psychiatric disorders. Increasing evidence points to a key role for toll-like receptor 4 (TLR4) in chronic pain states of somatic origin. However, the central contribution of TLR4 in visceral pain sensation remains elusive. METHODS With pharmacological and genetic approaches, we investigated the involvement of TLR4 in the modulation of visceral pain. The TLR4-deficient and wild-type mice were exposed to chronic stress. Visceral pain was evaluated with colorectal distension. Protein expression levels for TLR4, Cd11b, and glial fibrillary acidic protein (glial cells markers) were quantified in the lumbar region of the spinal cord, prefrontal cortex (PFC), and hippocampus. To evaluate the effect of blocking TLR4 on visceral nociception, TAK-242, a selective TLR4 antagonist, was administered peripherally (intravenous) and centrally (intracerebroventricular and intra-PFC) (n = 10-12/experimental group). RESULTS The TLR4 deficiency reduced visceral pain and prevented the development of chronic psychosocial stress-induced visceral hypersensitivity. Increased expression of TLR4 coupled with enhanced glia activation in the PFC and increased levels of proinflammatory cytokines were observed after chronic stress in wild-type mice. Administration of a TLR4 specific antagonist, TAK-242, attenuated visceral pain sensation in animals with functional TLR4 when administrated centrally and peripherally. Moreover, intra-PFC TAK-242 administration also counteracted chronic stress-induced visceral hypersensitivity. CONCLUSIONS Our results reveal a novel role for TLR4 within the PFC in the modulation of visceral nociception and point to TLR4 as a potential therapeutic target for the development of drugs to treat visceral hypersensitivity.

Selective Breeding for High Anxiety Introduces a Synonymous SNP That Increases Neuropeptide S Receptor Activity

Neuropeptide S (NPS) has generated substantial interest due to its anxiolytic and fear-attenuating effects in rodents, while a corresponding receptor polymorphism associated with increased NPS receptor (NPSR1) surface expression and efficacy has been implicated in an increased risk of panic disorder in humans. To gain insight into this paradox, we examined the NPS system in rats and mice bred for high anxiety-related behavior (HAB) versus low anxiety-related behavior, and, thereafter, determined the effect of central NPS administration on anxiety- and fear-related behavior. The HAB phenotype was accompanied by lower basal NPS receptor (Npsr1) expression, which we could confirm via in vitro dual luciferase promoter assays. Assessment of shorter Npsr1 promoter constructs containing a sequence mutation that introduces a glucocorticoid receptor transcription factor binding site, confirmed via oligonucleotide pull-down assays, revealed increased HAB promoter activity—an effect that was prevented by dexamethasone. Analogous to the human NPSR1 risk isoform, functional analysis of a synonymous single nucleotide polymorphism in the coding region of HAB rodents revealed that it caused a higher cAMP response to NPS stimulation. Assessment of the behavioral consequence of these differences revealed that intracerebroventricular NPS reversed the hyperanxiety of HAB rodents as well as the impaired cued-fear extinction in HAB rats and the enhanced fear expression in HAB mice, respectively. These results suggest that alterations in the NPS system, conserved across rodents and humans, contribute to innate anxiety and fear, and that HAB rodents are particularly suited to resolve the apparent discrepancy between the preclinical and clinical findings to date.

The orthosteric GABAA receptor ligand Thio-4-PIOL displays distinctly different functional properties at synaptic and extrasynaptic receptors

Explorations into the heterogeneous population of native GABA type A receptors (GABAARs) and the physiological functions governed by the multiple GABAAR subtypes have for decades been hampered by the lack of subtype‐selective ligands.

Progressive ratio responding in an obese mouse model: Effects of fenfluramine.

The progressive ratio schedule of operant responding is a well utilised task for assessing the rewarding aspects of abused drugs and natural rewards including food. Interestingly, progressive ratio paradigms have mainly been neglected in the field of animal research in obesity. Among the most widely studied mouse models of obesity is the leptin-deficient ob/ob mouse, characterised by hyperphagia and obesity. To date there are no studies on the behaviour of these mice in progressive ratio responding, thus we sought to validate the utility of the progressive ratio paradigm in obese mice and demonstrate its sensitivity to an anorectic drug challenge. Ob/ob mice and their lean controls were tested in fixed ratio paradigms of different demand, extinction learning, and progressive ratio schedules with linear and exponential increments, followed by an anorectic drug challenge with fenfluramine (5 and 10 mg/kg). Obese animals showed equal fixed ratio-acquisition and -responding for ratios 1 and 3, but displayed lower responding in ratios 6 and 9. Interestingly, obese animals showed equal motivation to respond in progressive ratio schedules. Fenfluramine dose-dependently induced anorectic effects in both genotypes and reduced progressive ratio responding significantly. This study, for the first time, describes motivational food intake in an operant progressive ratio paradigm in ob/ob mice. Leptin deficiency did not alter appetitive learning or motivation in the progressive ratio. The utility and sensitivity of the progressive ratio task for studies on motivational food intake was demonstrated by a challenge with the anorectic agent fenfluramine.

Obesity takes its toll on visceral pain: High-fat diet induces toll-like receptor 4-dependent visceral hypersensitivity

Exposure to high-fat diet induces both, peripheral and central alterations in TLR4 expression. Moreover, functional TLR4 is required for the development of high-fat diet-induced obesity. Recently, central alterations in TLR4 expression have been associated with the modulation of visceral pain. However, it remains unknown whether there is a functional interaction between the role of TLR4 in diet-induced obesity and in visceral pain. In the present study we investigated the impact of long-term exposure to high-fat diet on visceral pain perception and on the levels of TLR4 and Cd11b (a microglial cell marker) protein expression in the prefrontal cortex (PFC) and hippocampus. Peripheral alterations in TLR4 were assessed following the stimulation of spleenocytes with the TLR4-agonist LPS. Finally, we evaluated the effect of blocking TLR4 on visceral nociception, by administering TAK-242, a selective TLR4-antagonist. Our results demonstrated that exposure to high-fat diet induced visceral hypersensitivity. In parallel, enhanced TLR4 expression and microglia activation were found in brain areas related to visceral pain, the PFC and the hippocampus. Likewise, peripheral TLR4 activity was increased following long-term exposure to high-fat diet, resulting in an increased level of pro-inflammatory cytokines. Finally, TLR4 blockage counteracted the hyperalgesic phenotype present in mice fed on high-fat diet. Our data reveal a role for TLR4 in visceral pain modulation in a model of diet-induced obesity, and point to TLR4 as a potential therapeutic target for the development of drugs to treat visceral hypersensitivity present in pathologies associated to fat diet consumption.