A new experimental design to study inflammation-related versus non-inflammation-related depression in mice.

BACKGROUND: Major depressive disorder (MDD) represents a major public health concern, particularly due to its steadily rising prevalence and the poor responsiveness to standard antidepressants notably in patients afflicted with chronic inflammatory conditions, such as obesity. This highlights the need to improve current therapeutic strategies, including by targeting inflammation based on its role in the pathophysiology and treatment responsiveness of MDD. Nevertheless, dissecting the relative contribution of inflammation in the development and treatment of MDD remains a major issue, further complicated by the lack of preclinical depression models suitable to experimentally dissociate inflammation-related vs. inflammation-unrelated depression. METHODS: While current models usually focus on one particular MDD risk factor, we compared in male C57BL/6J mice the behavioral, inflammatory and neurobiological impact of chronic exposure to high-fat diet (HFD), a procedure known to induce inflammation-related depressive-like behaviors, and unpredictable chronic mild stress (UCMS), a stress-induced depression model notably renowned for its responsivity to antidepressants. RESULTS: While both paradigms induced neurovegetative, depressive-like and anxiety-like behaviors, inflammation and downstream neurobiological pathways contributing to inflammation-driven depression were specifically activated in HFD mice, as revealed by increased circulating levels of inflammatory factors, as well as brain expression of microglial activation markers and enzymes from the kynurenine and tetrahydrobiopterin (BH4) pathways. In addition, serotoninergic and dopaminergic systems were differentially impacted, depending on the experimental condition. CONCLUSIONS: These data validate an experimental design suitable to deeply study the mechanisms underlying inflammation-driven depression comparatively to non-inflammatory depression. This design could help to better understand the pathophysiology of treatment resistant depression.

Rapeseed oil fortified with micronutrients improves cognitive alterations associated with metabolic syndrome.

Metabolic syndrome represents a major risk factor for severe comorbidities such as cardiovascular diseases or diabetes. It is also associated with an increased prevalence of emotional and cognitive alterations that in turn aggravate the disease and related outcomes. Identifying therapeutic strategies able to improve those alterations is therefore a major socioeconomical and public health challenge. We previously reported that both hippocampal inflammatory processes and neuronal plasticity contribute to the development of emotional and cognitive alterations in db/db mice, an experimental model of metabolic syndrome that displays most of the classical features of the syndrome. In that context, nutritional interventions with known impact on those neurobiological processes appear as a promising alternative to limit the development of neurobiological comorbidities of metabolic syndrome. We therefore tested here whether n-3 polyunsaturated fatty acids (n-3 PUFAs) associated with a cocktail of antioxidants can protect against the development of behavioral alterations that accompany the metabolic syndrome. Thus, this study aimed: 1) to evaluate if a diet supplemented with the plant-derived n-3 PUFA α-linolenic acid (ALA) and antioxidants (provided by n-3 PUFAs-rich rapeseed oil fortified with a mix of naturally constituting antioxidant micronutrients, including coenzyme Q10, tocopherol, and the phenolic compound canolol) improved behavioral alterations in db/db mice, and 2) to decipher the biological mechanisms underlying this behavioral effect. Although the supplemented diet did not improve anxiety-like behavior and inflammatory abnormalities, it reversed hippocampus-dependent spatial memory deficits displayed by db/db mice in a water maze task. It concomitantly changed subunit composition of glutamatergic AMPA and NMDA receptors in the hippocampus that has been shown to modulate synaptic function related to spatial memory. These data suggest that changes in local neuronal plasticity may underlie cognitive improvements in db/db mice fed the supplemented diet. The current findings might therefore provide valuable data for introducing new nutritional strategies for the treatment of behavioral complications associated with MetS.

Brain tumor necrosis factor-α mediates anxiety-like behavior in a mouse model of severe obesity.

Although the high prevalence of anxiety in obesity increasingly emerges as significant risk factor for related severe health complications, the underlying pathophysiological mechanisms remain poorly understood. Considering that chronic inflammation is a key component of obesity and is well known to impact brain function and emotional behavior, we hypothesized that it may similarly contribute to the development of obesity-related anxiety. This hypothesis was experimentally tested by measuring whether chronic food restriction, a procedure known to reduce inflammation, or chronic anti-inflammatory treatment with ibuprofen improved anxiety-like behavior and concomitantly decreased peripheral and/or hippocampal inflammation characterizing a model of severe obesity, the db/db mice. In both experiments, reduced anxiety-like behaviors in the open-field and/or elevated plus-maze were selectively associated with decreased hippocampal tumor necrosis factor-α (TNF-α) mRNA expression. Highlighting the causality of both events, chronic central infusion of the TNF-α blocker etanercept was then shown to be sufficient to improve anxiety-like behavior in db/db mice. Lastly, by measuring the impact of ex-vivo etanercept on hippocampal synaptic processes underlying anxiety-like behaviors, we showed that the anxiolytic effect of central TNF-α blockade likely involved modulation of synaptic transmission within the ventral hippocampus. Altogether, these results uphold the role of brain TNF-α in mediating obesity-related anxiety and provide important clues about how it may modulate brain function and behavior. They may therefore help to introduce novel therapeutic strategies to reduce anxiety associated with inflammatory conditions.

Impact of prebiotics on metabolic and behavioral alterations in a mouse model of metabolic syndrome.

Mounting evidence shows that the gut microbiota, an important player within the gut-brain communication axis, can affect metabolism, inflammation, brain function and behavior. Interestingly, gut microbiota composition is known to be altered in patients with metabolic syndrome (MetS), who also often display neuropsychiatric symptoms. The use of prebiotics, which beneficially alters the microbiota, may therefore be a promising way to potentially improve physical and mental health in MetS patients. This hypothesis was tested in a mouse model of MetS, namely the obese and type-2 diabetic db/db mice, which display emotional and cognitive alterations associated with changes in gut microbiota composition and hippocampal inflammation compared to their lean db/+ littermates. We assessed the impact of chronic administration (8weeks) of prebiotics (oligofructose) on both metabolic (body weight, food intake, glucose homeostasis) and behavioral (increased anxiety-like behavior and impaired spatial memory) alterations characterizing db/db mice, as well as related neurobiological correlates, with particular attention to neuroinflammatory processes. Prebiotic administration improved excessive food intake and glycemic dysregulations (glucose tolerance and insulin resistance) in db/db mice. This was accompanied by an increase of plasma anti-inflammatory cytokine IL-10 levels and hypothalamic mRNA expression of the anorexigenic cytokine IL-1ß, whereas unbalanced mRNA expression of hypothalamic orexigenic (NPY) and anorexigenic (CART, POMC) peptides was unchanged. We also detected signs of improved blood-brain-barrier integrity in the hypothalamus of oligofructose-treated db/db mice (normalized expression of tight junction proteins ZO-1 and occludin). On the contrary, prebiotic administration did not improve behavioral alterations and associated reduction of hippocampal neurogenesis displayed by db/db mice, despite normalization of increased hippocampal IL-6 mRNA expression. Of note, we found a relationship between the effect of treatment on dentate gyrus neurons and spatial memory. These findings may prove valuable for introducing novel approaches to treat some of the comorbidities associated with MetS.

Juvenile obesity enhances emotional memory and amygdala plasticity through glucocorticoids.

In addition to metabolic and cardiovascular disorders, obesity is associated with adverse cognitive and emotional outcomes. Its growing prevalence during adolescence is particularly alarming since recent evidence indicates that obesity can affect hippocampal function during this developmental period. Adolescence is a decisive period for maturation of the amygdala and the hypothalamic-pituitary-adrenal (HPA) stress axis, both required for lifelong cognitive and emotional processing. However, little data are available on the impact of obesity during adolescence on amygdala function. Herein, we therefore evaluate in rats whether juvenile high-fat diet (HFD)-induced obesity alters amygdala-dependent emotional memory and whether it depends on HPA axis deregulation. Exposure to HFD from weaning to adulthood, i.e., covering adolescence, enhances long-term emotional memories as assessed by odor-malaise and tone-shock associations. Juvenile HFD also enhances emotion-induced neuronal activation of the basolateral complex of the amygdala (BLA), which correlates with protracted plasma corticosterone release. HFD exposure restricted to adulthood does not modify all these parameters, indicating adolescence is a vulnerable period to the effects of HFD-induced obesity. Finally, exaggerated emotional memory and BLA synaptic plasticity after juvenile HFD are alleviated by a glucocorticoid receptor antagonist. Altogether, our results demonstrate that juvenile HFD alters HPA axis reactivity leading to an enhancement of amygdala-dependent synaptic and memory processes. Adolescence represents a period of increased susceptibility to the effects of diet-induced obesity on amygdala function.

Diet-induced obesity progressively alters cognition, anxiety-like behavior and lipopolysaccharide-induced depressive-like behavior: focus on brain indoleamine 2,3-dioxygenase activation.

Obesity is associated with a high prevalence of mood symptoms and cognitive dysfunctions that emerges as significant risk factors for important health complications such as cardiovascular diseases and type 2 diabetes. It is therefore important to identify the dynamic of development and the pathophysiological mechanisms underlying these neuropsychiatric symptoms. Obesity is also associated with peripheral low-grade inflammation and increased susceptibility to immune-mediated diseases. Excessive production of proinflammatory cytokines and the resulting activation of the brain tryptophan catabolizing enzyme indoleamine 2,3-dioxygenase (IDO) have been shown to promote neurobehavioral complications, particularly depression. In that context, questions arise about the impact of diet-induced obesity on the onset of neuropsychiatric alterations and the increased susceptibility to immune-mediated diseases displayed by obese patients, particularly through brain IDO activation. To answer these questions, we used C57Bl/6 mice exposed to standard diet or western diet (WD; consisting of palatable energy-dense food) since weaning and for 20 weeks. We then measured inflammatory and behavioral responses to a systemic immune challenge with lipopolysaccharide (LPS) in experimental conditions known to alter cognitive and emotional behaviors independently of any motor impairment. We first showed that in absence of LPS, 9 weeks of WD is sufficient to impair spatial recognition memory (in the Y-maze). On the other hand, 18 weeks of WD increased anxiety-like behavior (in the elevated plus-maze), but did not affect depressive-like behavior (in the tail-suspension and forced-swim tests). However, 20 weeks of WD altered LPS-induced depressive-like behavior compared to LPS-treated lean mice and exacerbated hippocampal and hypothalamic proinflammatory cytokine expression and brain IDO activation. Taken together, these results show that WD exposure alters cognition and anxiety in unstimulated conditions and enhances activation of neurobiological mechanisms underlying depression after immune stimulation. They suggest therefore that obesity, and possibly obesity-associated inflammatory priming, may represent a vulnerability state to immune-mediated depressive symptoms.

Impairment of hippocampal-dependent memory induced by juvenile high-fat diet intake is associated with enhanced hippocampal inflammation in rats.

In addition to metabolic and cardiovascular disorders, obesity pandemic is associated with chronic low-grade inflammation as well as adverse cognitive outcomes. However, the existence of critical periods of development that differ in terms of sensitivity to the effects of diet-induced obesity remains unexplored. Using short exposure to a high-fat diet (HFD) exerting no effects when given to adult mice, we recently found impairment of hippocampal-dependent memory and plasticity after similar HFD exposure encompassing adolescence (from weaning to adulthood) showing the vulnerability of the juvenile period (Boitard et al., 2012). Given that inflammatory processes modulate hippocampal functions, we evaluated in rats whether the detrimental effect of juvenile HFD (jHFD) on hippocampal-dependent memory is associated with over-expression of hippocampal pro-inflammatory cytokines. jHFD exposure impaired long-term spatial reference memory in the Morris water maze without affecting acquisition or short-term memory. This suggests an effect on consolidation processes. Moreover, jHFD consumption delayed spatial reversal learning. jHFD intake did neither affect basal expression of pro-inflammatory cytokines at the periphery nor in the brain, but potentiated the enhancement of Interleukin-1-beta and Tumor Necrosis Factor-alpha expression specifically in the hippocampus after a peripheral immune challenge with lipopolysaccharide. Interestingly, whereas the same duration of HFD intake at adulthood induced similar weight gain and metabolic alterations as jHFD intake, it did neither affect spatial performance (long-term memory or reversal learning) nor lipopolysaccharide-induced cytokine expression in the hippocampus. Finally, spatial reversal learning enhanced Interleukin-1-beta in the hippocampus, but not in the frontal cortex and the hypothalamus, of jHFD-fed rats. These results indicate that juvenile HFD intake promotes exaggerated pro-inflammatory cytokines expression in the hippocampus which is likely to contribute to spatial memory impairment.

Acute Effect of a Saffron Extract (Safr'InsideTM) and Its Main Volatile Compound on the Stress Response in Healthy Young Men: A Randomized, Double Blind, Placebo-Controlled, Crossover Study.

According to animal studies, saffron and its main volatile compound safranal may reduce biological and behavioral signs of acute stress. However, little is known about its impact in humans. This study investigated the acute effect of a saffron extract and safranal on the biological and psychological stress responses in healthy men experiencing a laboratory stress procedure. In this double-blind, placebo-controlled, randomized, cross-over study, 19 volunteers aged 18-25 received a single dose of 30 mg saffron extract (Safr'InsideTM), 0.06 mg synthetic safranal, or a placebo on three visits separated by a 28-day washout. Thirteen minutes after administration, participants were exposed to the Maastricht acute stress test (MAST). Salivary cortisol and cortisone were collected from 15 min before the MAST (and pre-dose), 3 min before the MAST, and then 15, 30, 45, 60, and 75 min after the MAST, and stress and anxiety were measured using visual analogic scales. Compared to the placebo, stress and anxiety were significantly toned down after Safranal and Safr'InsideTM administration and coupled with a delay in the times to peak salivary cortisol and cortisone concentrations (p < 0.05). Safr'InsideTM and its volatile compound seem to improve psychological stress response in healthy men after exposure to a lab-based stressor and may modulate the biological stress response.

Saffron extract interferes with lipopolysaccharide-induced brain activation of the kynurenine pathway and impairment of monoamine neurotransmission in mice.

Background: Although activation of inflammatory processes is essential to fight infections, its prolonged impact on brain function is well known to contribute to the pathophysiology of many medical conditions, including neuropsychiatric disorders. Therefore, identifying novel strategies to selectively counter the harmful effects of neuroinflammation appears as a major health concern. In that context, this study aimed to test the relevance of a nutritional intervention with saffron, a spice known for centuries for its beneficial effect on health. Methods: For this purpose, the impact of an acute oral administration of a standardized saffron extract, which was previously shown to display neuromodulatory properties and reduce depressive-like behavior, was measured in mice challenged with lipopolysaccharide (LPS, 830 µg/kg, ip). Results: Pretreatment with saffron extract (6.5 mg/kg, per os) did not reduce LPS-induced sickness behavior, preserving therefore this adaptive behavioral response essential for host defense. However, it interfered with delayed changes of expression of cytokines, chemokines and markers of microglial activation measured 24 h post-LPS treatment in key brain areas for behavior and mood control (frontal cortex, hippocampus, striatum). Importantly, this pretreatment also counteracted by that time the impact of LPS on several neurobiological processes contributing to inflammation-induced emotional alterations, in particular the activation of the kynurenine pathway, assessed through the expression of its main enzymes, as well as concomitant impairment of serotonergic and dopaminergic neurotransmission. Conclusion: Altogether, this study provides important clues on how saffron extract interferes with brain function in conditions of immune stimulation and supports the relevance of saffron-based nutritional interventions to improve the management of inflammation-related comorbidities.

Circulating Human Serum Metabolites Derived from the Intake of a Saffron Extract (Safr'InsideTM) Protect Neurons from Oxidative Stress: Consideration for Depressive Disorders.

Increases in oxidative stress have been reported to play a central role in the vulnerability to depression, and antidepressant drugs may reduce increased oxidative stress in patients. Among the plants exerting anti-inflammatory and anti-oxidant properties, saffron, a spice derived from the flower of Crocus sativus, is also known for its positive effects on depression, potentially through its SSRI-like properties. However, the molecular mechanisms underlying these effects and their health benefits for humans are currently unclear. Using an original ex vivo clinical approach, we demonstrated for the first time that the circulating human metabolites produced following saffron intake (Safr'InsideTM) protect human neurons from oxidative-stress-induced neurotoxicity by preserving cell viability and increasing BNDF production. In particular, the metabolites significantly stimulated both dopamine and serotonin release. In addition, the saffron's metabolites were also able to protect serotonergic tone by inhibiting the expression of the serotonin transporter SERT and down-regulating serotonin metabolism. Altogether, these data provide new biochemical insights into the mechanisms underlying the beneficial impact of saffron on neuronal viability and activity in humans, in the context of oxidative stress related to depression.

Induction of IDO by bacille Calmette-Guérin is responsible for development of murine depressive-like behavior.

Chronic inflammation activates the tryptophan-degrading enzyme IDO, which is well known to impair T cell proliferation. We have previously established that bacille Calmette-Guérin (BCG), an attenuated form of Mycobacterium bovis, is associated with persistent activation of IDO in the brain and chronic depressive-like behavior, but a causative role has not been established. In these experiments we used both pharmacologic and genetic approaches to test the hypothesis that IDO activation is responsible for the development of chronic depression that follows BCG infection. BCG induced TNF-alpha, IFN-gamma, and IDO mRNA steady-state transcripts in the brain as well as the enzyme 3-hydroxyanthranilic acid oxygenase (3-HAO) that lies downstream of IDO and generates the neuroactive metabolite, quinolinic acid. Behaviors characteristic of depression were apparent 1 wk after BCG infection. Pretreatment with the competitive IDO inhibitor 1-methyltryptophan fully blocked BCG-induced depressive-like behaviors. Importantly, IDO-deficient mice were completely resistant to BCG-induced depressive-like behavior but responded normally to BCG induction of proinflammatory cytokines. These results are the first to prove that the BCG-induced persistent activation of IDO is accompanied by the induction of 3-hydroxyanthranilic acid oxygenase and that IDO is required as an initial step for the subsequent development of chronic depressive-like behavior.

Saffron Extract-Induced Improvement of Depressive-Like Behavior in Mice Is Associated with Modulation of Monoaminergic Neurotransmission.

Depressive disorders represent a major public health concern and display a continuously rising prevalence. Importantly, a large proportion of patients develops aversive side effects and/or does not respond properly to conventional antidepressants. These issues highlight the need to identify further therapeutic strategies, including nutritional approaches using natural plant extracts with known beneficial impacts on health. In that context, growing evidence suggests that saffron could be a particularly promising candidate. This preclinical study aimed therefore to test its antidepressant-like properties in mice and to decipher the underlying mechanisms by focusing on monoaminergic neurotransmission, due to its strong implication in mood disorders. For this purpose, the behavioral and neurobiochemical impact of a saffron extract, Safr'Inside™ (6.5 mg/kg per os) was measured in naïve mice. Saffron extract reduced depressive-like behavior in the forced swim test. This behavioral improvement was associated with neurobiological modifications, particularly changes in serotonergic and dopaminergic neurotransmission, suggesting that Safr'Inside™ may share common targets with conventional pharmacological antidepressants. This study provides useful information on the therapeutic relevance of nutritional interventions with saffron extracts to improve management of mood disorders.

Prevention of Stress-Induced Depressive-like Behavior by Saffron Extract Is Associated with Modulation of Kynurenine Pathway and Monoamine Neurotransmission.

Depressive disorders are a major public health concern. Despite currently available treatment options, their prevalence steadily increases, and a high rate of therapeutic failure is often reported, together with important antidepressant-related side effects. This highlights the need to improve existing therapeutic strategies, including by using nutritional interventions. In that context, saffron recently received particular attention for its beneficial effects on mood, although the underlying mechanisms are poorly understood. This study investigated in mice the impact of a saffron extract (Safr'Inside™; 6.25 mg/kg, per os) on acute restraint stress (ARS)-induced depressive-like behavior and related neurobiological alterations, by focusing on hypothalamic-pituitary-adrenal axis, inflammation-related metabolic pathways, and monoaminergic systems, all known to be altered by stress and involved in depressive disorder pathophysiology. When given before stress onset, Safr'Inside administration attenuated ARS-induced depressive-like behavior in the forced swim test. Importantly, it concomitantly reversed several stress-induced monoamine dysregulations and modulated the expression of key enzymes of the kynurenine pathway, likely reducing kynurenine-related neurotoxicity. These results show that saffron pretreatment prevents the development of stress-induced depressive symptoms and improves our understanding about the underlying mechanisms, which is a central issue to validate the therapeutic relevance of nutritional interventions with saffron in depressed patients.

Interferon-gamma and tumor necrosis factor-alpha mediate the upregulation of indoleamine 2,3-dioxygenase and the induction of depressive-like behavior in mice in response to bacillus Calmette-Guerin.

Although the tryptophan-degrading enzyme, indoleamine 2,3-dioxygenase (IDO), is a pivotal mediator of inflammation-induced depression, its mechanism of regulation has not yet been investigated in this context. Here, we demonstrate an essential role for interferon (IFN)gamma and tumor necrosis factor (TNF)alpha in the induction of IDO and depressive-like behaviors in response to chronic immune activation. Wild-type (WT) control mice and IFNgammaR(-/-) mice were inoculated with an attenuated form of Mycobacterium bovis, bacille Calmette-Guérin (BCG). Infection with BCG induced an acute episode of sickness that was similar in WT and IFNgammaR(-/-) mice. Increased immobility during the forced swim and tail suspension tests occurred in WT mice 7 d after BCG inoculation but was entirely absent in IFNgammaR(-/-) mice. In WT mice, these indices of depressive-like behavior were associated with chronic upregulation of IFNgamma, interleukin(IL)-1beta, TNFalpha, and IDO. Proinflammatory cytokine expression was elevated in BCG-infected IFNgammaR(-/-) mice as well, but upregulation of lung and brain IDO mRNA was completely abolished. This was accompanied by an attenuation of BCG-induced TNFalpha mRNA and the lack of an increase in plasma kynurenine/tryptophan ratio in the BCG-inoculated IFNgammaR(-/-) mice compared with WT controls. Pretreatment of mice with the TNFalpha antagonist, etanercept, partially blunted BCG-induced IDO activation and depressive-like behavior. In accordance with these in vivo data, IFNgamma and TNFalpha synergized to induce IDO in primary microglia. Together, these data demonstrate that IFNgamma, with TNFalpha, is necessary for induction of IDO and depressive-like behavior in mice after BCG infection.

The gut microbiota metabolite indole increases emotional responses and adrenal medulla activity in chronically stressed male mice.

BACKGROUND AND AIMS: The gut microbiota produces metabolites that are an integral part of the metabolome and, as such, of the host physiology. Changes in gut microbiota metabolism could therefore contribute to pathophysiological processes. We showed previously that a chronic and moderate overproduction of indole from tryptophan in male individuals of the highly stress-sensitive F344 rat strain induced anxiety-like and helplessness behaviors. The aim of the present study was to extend the scope of these findings by investigating whether emotional behaviors of male mice that are moderately stress-sensitive but chronically exposed to environmental stressors would also be affected by indole. METHODS: We colonized germ-free male C3H/HeN mice with a wild-type indole-producing Escherichia coli strain, or with the non-indole producing mutant. Gnotobiotic mice were subjected to an unpredictable chronic mild stress procedure, then to a set of tests aimed at assessing anxiety-like (novelty and elevated plus maze tests) and depression-like behaviors (coat state, splash, nesting, tail suspension and sucrose tests). Results of the individual tests were aggregated into a common z-score to estimate the overall emotional response to chronic mild stress and chronic indole production. We also carried out biochemical and molecular analyses in gut mucosa, plasma, brain hippocampus and striatum, and adrenal glands, to examine biological correlates that are usually associated with stress, anxiety and depression. RESULTS: Chronic mild stress caused coat state degradation and anhedonia in both indole-producing and non-indole producing mice, but it did not influence behaviors in the other tests. Chronic indole production did not influence mice behavior when tests were considered individually, but it increased the overall emotionality z-score, specifically in mice under chronic mild stress. Interestingly, in the same mice, indole induced a dramatic increase of the expression of the adrenomedullary Pnmt gene, which is involved in catecholamine biosynthesis. By contrast, systemic tryptophan bioavailability, brain serotonin and dopamine levels and turnover, as well as expression of gut and brain genes involved in cytokine production and tryptophan metabolism along the serotonin and kynurenine pathways, remained similar in all mice. CONCLUSIONS: Chronic indole production by the gut microbiota increased the vulnerability of male mice to the adverse effects of chronic mild stress on emotional behaviors. It also targeted catecholamine biosynthetic pathway of the adrenal medulla, which plays a pivotal role in body's physiological adaptation to stressful events. Future studies will aim to investigate the action mechanisms responsible for these effects.

Spatio-temporal differences in the profile of murine brain expression of proinflammatory cytokines and indoleamine 2,3-dioxygenase in response to peripheral lipopolysaccharide administration.

The mechanisms underlying in vivo activation of indoleamine 2,3-dioxygenase (IDO), a tryptophan-catabolizing enzyme that mediates in the brain the induction of depressive-like behavior by peripheral innate immune system stimulation are still poorly understood. By monitoring how cytokines parallel IDO mRNA expression in the brain in response to intraperitoneal lipopolysaccharide injection in mice, we report a time-dependent induction of IDO expression in both the hippocampus and hypothalamus that was associated with a specific structure-dependent expression of proinflammatory cytokines, particularly interferon-gamma. This study suggests that different mechanisms regulate the activation of IDO by lipopolysaccharide in various brain structures.

Inoculation of Bacillus Calmette-Guerin to mice induces an acute episode of sickness behavior followed by chronic depressive-like behavior.

Although cytokine-induced sickness behavior is now well-established, the mechanisms by which chronic inflammation and depression are linked still remain elusive. Therefore this study aimed to develop a suitable model to identify the neurobiological basis of depressive-like behavior induced by chronic inflammation, independently of sickness behavior. We chose to measure the behavioral consequences of chronic inoculation of mice with Bacillus Calmette-Guerin (BCG), which has been shown to chronically activate both lung and brain indoleamine 2,3-dioxygenase (IDO), a tryptophan-catabolizing enzyme that mediates the occurrence of depressive-like behavior following acute innate immune system activation. BCG inoculation induced an acute episode of sickness (approximately 5 days) that was followed by development of delayed depressive-like behaviors lasting over several weeks. Transient body weight loss, reduction of motor activity and the febrile response to BCG were dissociated temporarily from a sustained increase in the duration of immobility in both forced swim and tail suspension tests, reduced voluntary wheel running and decreased preference for sucrose (a test of anhedonia). Moreover, we show that a distinct pattern of cytokine production and IDO activation parallels the transition from sickness to depression. Protracted depressive-like behavior, but not sickness behavior, was associated with sustained increase in plasma interferon-gamma and TNF-alpha concentrations and peripheral IDO activation. Together, these promising new data establish BCG inoculation of mice as a reliable rodent model of chronic inflammation-induced depressive-like behaviors that recapitulate many clinical observations and provide important clues about the neurobiological basis through which cytokines may have an impact on affective behaviors.

Brain Kynurenine and BH4 Pathways: Relevance to the Pathophysiology and Treatment of Inflammation-Driven Depressive Symptoms.

The prevalence of depressive disorders is growing worldwide, notably due to stagnation in the development of drugs with greater antidepressant efficacy, the continuous large proportion of patients who do not respond to conventional antidepressants, and the increasing rate of chronic medical conditions associated with an increased vulnerability to depressive comorbidities. Accordingly, better knowledge on the pathophysiology of depression and mechanisms underlying depressive comorbidities in chronic medical conditions appears urgently needed, in order to help in the development of targeted therapeutic strategies. In this review, we present evidence pointing to inflammatory processes as key players in the pathophysiology and treatment of depressive symptoms. In particular, we report preclinical and clinical findings showing that inflammation-driven alterations in specific metabolic pathways, namely kynurenine and tetrahydrobiopterin (BH4) pathways, leads to substantial alterations in the metabolism of serotonin, glutamate and dopamine that are likely to contribute to the development of key depressive symptom dimensions. Accordingly, anti-inflammatory interventions targeting kynurenine and BH4 pathways may be effective as novel treatment or as adjuvants of conventional medications rather directed to monoamines, notably when depressive symptomatology and inflammation are comorbid in treated patients. This notion is discussed in the light of recent findings illustrating the tight interactions between known antidepressant drugs and inflammatory processes, as well as their therapeutic implications. Altogether, this review provides valuable findings for moving toward more adapted and personalized therapeutic strategies to treat inflammation-related depressive symptoms.

Lipopolysaccharide induces delayed FosB/DeltaFosB immunostaining within the mouse extended amygdala, hippocampus and hypothalamus, that parallel the expression of depressive-like behavior.

Proinflammatory cytokines induce both sickness behavior and depression, but their respective neurobiological correlates are still poorly understood. The aim of the present study was therefore to identify in mice the neural substrates of sickness and depressive-like behavior induced by lipopolysaccharide (LPS, 830 microg/kg, intraperitoneal). LPS-induced depressive-like behavior was dissociated from LPS-induced sickness by testing mice either at 6 h (at which time sickness was expected to be maximal) or at 24 h post-LPS (at which time sickness was expected to be minimal and not to bias the measurement of depressive-like behavior). Concurrently, the expression of acute and chronic cellular reactivity markers (c-Fos and FosB/DeltaFosB, respectively) was mapped by immunohistochemistry at these two time points. In comparison to saline, LPS decreased motor activity in a new cage at 6 h but not at 24 h. In contrast, the duration of immobility in the tail suspension test was increased at both 6 and 24 h. This dissociation between decreased motor activity and depressive-like behavior was confirmed at 24 h post-LPS in the forced swim test. LPS also decreased sucrose consumption at 24 and 48 h, despite normal food and water consumption by that time. At 24 h post-LPS, LPS-induced depressive-like behavior was associated with a delayed cellular activity (as assessed by FosB/DeltaFosB immunostaining) in specific brain structures, particularly within the extended amygdala, hippocampus and hypothalamus, whereas c-Fos labeling was markedly decreased by that time in all the brain areas at 6 h post-LPS. These results provide the first evidence in favor of a functional dissociation between the brain structures that underlie cytokine-induced sickness behavior and cytokine-induced depressive-like behavior, and provide important cues about the neuroanatomical brain circuits through which cytokines could have an impact on affect.

Role of Inflammation in the Development of Neuropsychiatric Symptom Domains: Evidence and Mechanisms.

The finding that inflammatory markers are elevated in various neuropsychiatric disorders raises the need of identifying the precise research domain criteria driven by inflammation. Based on the model of inflammation-induced depression it has been possible to identify distinct pathophysiological pathways leading to alterations in neurotransmitter metabolism with specific relevance for the development of symptom constellations that are common to various neuropsychiatric and neurodegenerative conditions. Moreover, converging data indicate that these pathways interact with relevant vulnerability factors and modulatory systems to ultimately impact the presentation of inflammation-driven neuropsychiatric symptoms. Altogether, these findings make inflammation a key pivotal factor in psychopathology. Developing treatments that target inflammation and modulate the pathways and systems by which inflammatory processes selectively affect brain function will be of particular relevance for the treatment of specific neurobehavioral symptom domains.