Jason C. O'Connor

From inflammation to sickness and depression: when the immune system subjugates the brain

In response to a peripheral infection, innate immune cells produce pro-inflammatory cytokines that act on the brain to cause sickness behaviour. When activation of the peripheral immune system continues unabated, such as during systemic infections, cancer or autoimmune diseases, the ensuing immune signalling to the brain can lead to an exacerbation of sickness and the development of symptoms of depression in vulnerable individuals. These phenomena might account for the increased prevalence of clinical depression in physically ill people. Inflammation is therefore an important biological event that might increase the risk of major depressive episodes, much like the more traditional psychosocial factors.

Lipopolysaccharide-induced depressive-like behavior is mediated by indoleamine 2,3-dioxygenase activation in mice

Although elevated activity of the tryptophan-degrading enzyme indoleamine 2,3-dioxygenase (IDO) has been proposed to mediate comorbid depression in inflammatory disorders, its causative role has never been tested. We report that peripheral administration of lipopolysaccharide (LPS) activates IDO and culminates in a distinct depressive-like behavioral syndrome, measured by increased duration of immobility in both the forced-swim and tail suspension tests. Blockade of IDO activation either indirectly with the anti-inflammatory tetracycline derivative minocycline, that attenuates LPS-induced expression of proinflammatory cytokines, or directly with the IDO antagonist 1-methyltryptophan (1-MT), prevents development of depressive-like behavior. Both minocycline and 1-MT normalize the kynurenine/tryptophan ratio in the plasma and brain of LPS-treated mice without changing the LPS-induced increase in turnover of brain serotonin. Administration of L-kynurenine, a metabolite of tryptophan that is generated by IDO, to naive mice dose dependently induces depressive-like behavior. These results implicate IDO as a critical molecular mediator of inflammation-induced depressive-like behavior, probably through the catabolism of tryptophan along the kynurenine pathway.

Inflammation-associated depression: From serotonin to kynurenine

In the field of depression, inflammation-associated depression stands up as an exception since its causal factors are obvious and it is easy to mimic in an animal model. In addition, quasi-experimental studies can be carried out in patients who are treated chronically with recombinant cytokines for a medical condition since these patients can be studied longitudinally before, during and after stimulation of the immune system. These clinical studies have revealed that depression is a late phenomenon that develops over a background of early appearing sickness. Incorporation of this feature in animal models of inflammation-associated depression has allowed the demonstration that alterations of brain serotoninergic neurotransmission do not play a major role in the pathogenesis. This is in contrast to the activation of the tryptophan degrading enzyme indoleamine 2,3-dioxygenase that generates potentially neurotoxic kynurenine metabolites such as 3-hydroxy kynurenine and quinolinic acid. Although the relative importance of peripherally versus centrally produced kynurenine and the cellular source of production of this compound remain to be determined, these findings provide new targets for the treatment of inflammation-associated depression that could be extended to other psychiatric conditions mediated by activation of neuroimmune mechanisms.

Interferon-γ and Tumor Necrosis Factor-α Mediate the Upregulation of Indoleamine 2,3-Dioxygenase and the Induction of Depressive-Like Behavior in Mice in Response to Bacillus Calmette-Guérin

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)γ and tumor necrosis factor (TNF)α in the induction of IDO and depressive-like behaviors in response to chronic immune activation. Wild-type (WT) control mice and IFNγR−− 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 IFNγR−− 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 IFNγR−− mice. In WT mice, these indices of depressive-like behavior were associated with chronic upregulation of IFNγ, interleukin(IL)-1β, TNFα, and IDO. Proinflammatory cytokine expression was elevated in BCG-infected IFNγR−− mice as well, but upregulation of lung and brain IDO mRNA was completely abolished. This was accompanied by an attenuation of BCG-induced TNFα mRNA and the lack of an increase in plasma kynurenine/tryptophan ratio in the BCG-inoculated IFNγR−− mice compared with WT controls. Pretreatment of mice with the TNFα antagonist, etanercept, partially blunted BCG-induced IDO activation and depressive-like behavior. In accordance with these in vivo data, IFNγ and TNFα synergized to induce IDO in primary microglia. Together, these data demonstrate that IFNγ, with TNFα, is necessary for induction of IDO and depressive-like behavior in mice after BCG infection.

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.

Aging exacerbates depressive-like behavior in mice in response to activation of the peripheral innate immune system.

Exposure to peripheral infections may be permissive to cognitive and behavioral complications in the elderly. We have reported that peripheral stimulation of the innate immune system with lipopolysaccharide (LPS) causes an exaggerated neuroinflammatory response and prolonged sickness behavior in aged BALB/c mice. Because LPS also causes depressive behavior, the purpose of this study was to determine whether aging is associated with an exacerbated depressive-like response. We confirmed that LPS (0.33 mg/kg intraperitoneal) induced a protracted sickness response in aged mice with reductions in locomotor and feeding activities 24 and 48 h postinjection, when young adults had fully recovered. When submitted to the forced swim test 24 h post-LPS, both young adult and aged mice exhibited an increased duration of immobility. However, when submitted to either the forced swim test or the tail suspension test 72 h post-LPS, an increased duration of immobility was evident only in aged mice. This prolonged depressive-like behavior in aged LPS-treated mice was associated with a more pronounced induction of peripheral and brain indoleamine 2,3-dioxygenase and a markedly higher turnover rate of brain serotonin (as measured by the ratio of 5-hydroxy-indoleacetic acid over 5-hydroxyt-tryptamine) compared to young adult mice at 24 post-LPS injection. These results provide the first evidence that age-associated reactivity of the brain cytokine system could play a pathophysiological role in the increased prevalence of depression observed in the elderly.

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-α, IFN-γ, 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.

Indoleamine 2,3-dioxygenase mediates anhedonia and anxiety-like behaviors caused by peripheral lipopolysaccharide immune challenge

Upregulation of indoleamine 2,3-dioxygenase (IDO) by proinflammatory cytokines has been implicated as a biological mediator of inflammation-related mood disorders. Clinical reports on this neuro-immune interaction remain correlative, while mechanism-centered preclinical experiments have focused on a relatively narrow, and somewhat controversial, survey of depression-like behaviors that include the forced swim and tail suspension tests. Here, we sought to determine whether peripheral immune challenge with Escherichia coli, lipopolysaccharides (LPS) precipitates the development of translationally relevant depression-like behaviors and to investigate the role of IDO in mediating these LPS-induced behaviors. Intraperitoneal injection of C57BL/6J mice with LPS resulted in a robust, but transient, reduction in exploratory locomotor activity (eLMA) that returned to near baseline levels by 24h. Sucrose preference, a preclinical correlate of anhedonia, was diminished by more than 20% in LPS-treated compared to saline-treated control mice, and LPS induced a significant increase in anxiety-like behavior at 24h that was independent eLMA. Pretreatment of mice with an IDO inhibitor, 1-methyltryptophan (1MT), ablated the anxiogenic effects of LPS, while having no impact on sickness associated changes in body weight or eLMA. Additionally, 1MT pretreatment attenuated the LPS-induced reduction in sucrose preference, which was also confirmed in IDO-1 null mice. Interestingly, acute systemic administration of l-kynurenine, the enzymatic product of IDO, precipitated an anhedonic and anxiogenic effect in naïve mice without effect on eLMA. In a preclinical model, these data implicate IDO as a pivotal mediator of LPS-induced depression- and anxiety-like behavior.

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.

Fractalkine receptor (CX3CR1) deficiency sensitizes mice to the behavioral changes induced by lipopolysaccharide.

BackgroundInteractions between fractalkine (CX3CL1) and fractalkine receptor (CX3CR1) regulate microglial activation in the CNS. Recent findings indicate that age-associated impairments in CX3CL1 and CX3CR1 are directly associated with exaggerated microglial activation and an impaired recovery from sickness behavior after peripheral injection of lipopolysaccharide (LPS). Therefore, the purpose of this study was to determine the extent to which an acute LPS injection causes amplified and prolonged microglial activation and behavioral deficits in CX3CR1-deficient mice (CX3CR1-/-).MethodsCX3CR1-/- mice or control heterozygote mice (CX3CR1+/-) were injected with LPS (0.5 mg/kg i.p.) or saline and behavior (i.e., sickness and depression-like behavior), microglial activation, and markers of tryptophan metabolism were determined. All data were analyzed using Statistical Analysis Systems General Linear Model procedures and were subjected to one-, two-, or three-way ANOVA to determine significant main effects and interactions.ResultsLPS injection caused a prolonged duration of social withdrawal in CX3CR1-/- mice compared to control mice. This extended social withdrawal was associated with enhanced mRNA expression of IL-1β, indolamine 2,3-dioxygenase (IDO) and kynurenine monooxygenase (KMO) in microglia 4 h after LPS. Moreover, elevated expression of IL-1β and CD14 was still detected in microglia of CX3CR1-/- mice 24 h after LPS. There was also increased turnover of tryptophan, serotonin, and dopamine in the brain 24 h after LPS, but these increases were independent of CX3CR1 expression. When submitted to the tail suspension test 48 and 72 h after LPS, an increased duration of immobility was evident only in CX3CR1-/- mice. This depression-like behavior in CX3CR1-/- mice was associated with a persistent activated microglial phenotype in the hippocampus and prefrontal cortex.ConclusionsTaken together, these data indicate that a deficiency of CX3CR1 is permissive to protracted microglial activation and prolonged behavioral alterations in response to transient activation of the innate immune system.