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Despite years of research, our understanding of the mechanisms by which inflammation induces depression is still limited. As clinical data points to a strong association between depression and motivational alterations, we sought to (1) characterize the motivational changes that are associated with inflammation in mice, and (2) determine if they depend on inflammation-induced activation of indoleamine 2,3 dioxygenase-1 (IDO1). Lipopolysaccharide (LPS)-treated or spared nerve injured (SNI) wild type (WT) and Ido1(−/−) mice underwent behavioral tests of antidepressant activity (e.g., forced swim test) and motivated behavior, including assessment of (1) reward expectancy using a food-related anticipatory activity task, (2) willingness to work for reward using a progressive ratio schedule of food reinforcement, (3) effort allocation using a concurrent choice task, and (4) ability to associate environmental cues with reward using conditioned place preference. LPS- and SNI-induced deficits in behavioral tests of antidepressant activity in WT but not Ido1(−/−) mice. Further, LPS decreased food related-anticipatory activity, reduced performance in the progressive ratio task, and shifted effort toward the preferred reward in the concurrent choice task. These effects were observed in both WT and Ido1(−/−) mice. Finally, SNI mice developed a conditioned place preference based on relief from pain in an IDO1-independent manner. These findings demonstrate that the motivational effects of inflammation do not require IDO1. Further, they indicate that the motivational component of inflammation-induced depression is mechanistically distinct from that measured by behavioral tests of antidepressant activity.

Fatigue is the most common symptom of cancer at diagnosis, yet causes and effective treatments remain elusive. As tumors can be highly inflammatory, it is generally accepted that inflammation mediates cancer-related fatigue. However, evidence to support this assertion is mostly correlational. In this study, we directly tested the hypothesis that fatigue results from propagation of tumor-induced inflammation to the brain and activation of the central proinflammatory cytokine, IL1. The heterotopic syngeneic murine head and neck cancer model (mEER) caused systemic inflammation and increased expression of Il1b in the brain while inducing fatigue-like behaviors characterized by decreased voluntary wheel running and exploratory activity. Expression of Il1b in the brain was not associated with any alterations in motivation, measured by responding in a progressive ratio schedule of food reinforcement, depression-like behaviors, or energy balance. Decreased wheel running occurred prior to Il1b detection in the brain, when systemic inflammation was minimal. Furthermore, mice null for two components of IL1β signaling, the type 1 IL1 receptor or the receptor adapter protein MyD88, were not protected from tumor-induced decreases in wheel running, despite attenuated cytokine action and expression. Behavioral and inflammatory analysis of four additional syngeneic tumor models revealed that tumors can induce fatigue regardless of their systemic or central nervous system inflammatory potential. Together, our results show that brain IL1 signaling is not necessary for tumor-related fatigue, dissociating this type of cancer sequela from systemic cytokine expression. Significance: These findings challenge the current understanding of fatigue in cancer patients, the most common and debilitating sequela associated with malignancy. Cancer Res; 78(3); 695–705. ©2017 AACR.

Patients with cancer often experience a high symptom burden prior to the start of treatment. As disease and treatment-related neurotoxicities appear to be additive, targeting disease related symptoms may attenuate overall symptom burden for cancer patients and improve the tolerability of treatment. It has been hypothesized that disease-related symptoms are a consequence of tumor-induced inflammation. We tested this hypothesis using a syngeneic heterotopic murine model of human papilloma virus (HPV)-related head and neck cancer. This model has the advantage of being mildly aggressive and not causing cachexia and weight loss. We previously showed that this tumor leads to increased IL-6, IL-1β, and TNF-α expression in the liver and increased IL-1β expression in the brain. The current study confirmed these features and demonstrated that the tumor itself exhibits high inflammatory cytokine expression (e.g., IL-6, IL-1β, and TNF-α) compared to healthy tissue. While there is a clear relationship between cytokine levels and behavioral deficits in this model, the behavioral changes are surprisingly mild. Therefore, we sought to confirm the relationship between behavior and inflammation by amplifying the effect using a low dose of lipopolysaccharide (LPS, 0.1 mg/kg). In tumor-bearing mice LPS induced deficits in nest building, tail suspension, and locomotor activity approximately 24h after LPS. However, these mice did not display an exacerbation of LPS-induced weight loss, anorexia, or anhedonia. Further, while heightened serum IL-6 was observed there was minimal priming of liver or brain cytokine expression. Next we sought to inhibit tumor-induced burrowing deficits by reducing inflammation using minocycline. Minocycline (~50 mg/kg/day in drinking water) was able to attenuate tumor-induced inflammation and burrowing deficits. These data provide evidence in favor of an inflammatory-like mechanism for the behavioral alterations associated with tumor growth in a syngeneic murine model of HPV-related head and neck cancer. However, the inflammatory state and behavioral changes induced by this tumor clearly differ from other forms of inflammation-induced sickness behavior.

We previously showed that leucine supplementation abrogates lipopolysaccharide (LPS)-induced depression-like behavior by competing with the LAT1-dependent transport of kynurenine into the brain. Since leucine activates mTOR in its target cells the objective of the present series of experiments was to determine whether the antidepressant activity of leucine is mediated by activation of mTOR. Using Western blot we first confirmed that leucine (500 mg/kg IP) activates mTOR in the prefrontal cortex 1 hr after treatment, as measured by phosphorylation of AKT, MAP kinase, and mTOR. We then administered rapamycin (0.75 mg/kg IP) before leucine (500 mg/kg IP) and LPS (0.83 mg/kg IP) to CD-1 mice for behavioral experiments. Leucine abrogated LPS-induced reduction in sucrose preference and increased immobility in the forced swim test. These effects were reversed by rapamycin pretreatment. Because leucine utilization by its target cells requires mTOR-mediated upregulation of LAT1 we examined the possibility that rapamycin blocks the effects of leucine by interfering with this mechanism. Livers and hippocampi of mice treated with rapamycin, leucine and LPS were collected at the end of the experiment and expression of LAT1 was analyzed by qRT-PCR. Administration of LPS and leucine in association increased LAT1 expression in both tissues and this effect was blocked by pretreatment with rapamycin. These findings demonstrate the importance of blood-brain barrier transport mechanisms in inflammation-induced depression. Supported by NIH-NIMH (MH104694) and MDACC.

Purpose Fatigue is a frequent side effect of cancer and cancer treatment, common to multiple tumor sites and therapeutic strategies. The propagation of peripheral inflammation into the brain is thought to mediate this response. The purpose of this study was to evaluate the role of inflammatory signaling in mediating fatigue induced by multiple syngeneic tumors. Materials/Methods WT, IL-1RIKO, and MyD88 were injected in the flank with HPV+ (mEER), HPV- head and neck cancer, Lewis lung carcinoma (LLC), and two ovarian tumors. Fatigue was measured using wheel running and locomotor activity. Cytokine expression was assessed in the brain, liver, serum, and tumor using qRT-PCR or immunoassay. ANOVA with post hoc Dunnett’s test were used for statistical comparisons with significance set at p Results Wheel running decreased in mEER animals beginning after 10 days and continuing until termination. Inflammatory cytokine expression was seen in the brains, livers, and serum of mEER animals 4 weeks after tumor implantation, but not sooner. IL-1RIKO and MyD88 mice exhibited decreased inflammatory cytokine expression and serum corticosterone, but wheel running and locomotor activity deficits were preserved. Additional tumors induced similar decreases in wheel running, despite varying systemic or brain inflammation. Conclusions Inflammation is sufficient, but not necessary for fatigue-like behavior in tumor bearing mice. Together, these data definitively dissociate fatigue from inflammation and suggest an alternative non-inflammatory mechanism likely underlies most cancer-related fatigue.

While it is known that patients with inflammatory diseases are at increased risk for developing depression, the neurobiological mechanisms underlying this relationship remain elusive. This is partly due to the complexity and heterogeneity of symptom expression. While cognitive/affective symptoms of inflammation-induced depression have been linked to indoleamine 2,3-dioxygenase (IDO1), it is unclear whether this applies to other depression symptoms. As motivational changes are often reported in depression, the current investigation explores the role of IDO1 in motivated behavior using wild type (WT) C57BL/6J and ido1-/- mice. Multiple aspects of motivation were evaluated including reward expectation, willingness to work for a reward, and ability to associate reward to environmental cues. Inflammation was induced with 0.33 mg/kg lipopolysaccharide (LPS) or spared nerve injury (SNI). As previously described, ido1-/- mice were protected from inflammation-induced increased immobility in the forced swim test. LPS also suppressed food-related anticipatory activity (reward expectation) and overall effort in a decision making task (willingness to work for reward). These effects were equivalent in both WT and ido1-/- mice, indicating that inflammation-induced motivational deficits are not IDO1 dependent. Conditioned place preference measured in SNI mice treated with the analgesic, retigabine, was identical in WT and ido1-/- mice. This indicates the ability to associate environmental cues with reward is also not IDO1 dependent. These findings clearly demonstrate that IDO1 does note mediate inflammation-induced motivational processes.

Patients with cancer often experience fatigue and other symptoms even prior to the start of treatment. Because of the strong association between cancer and inflammation it has been hypothesized that these symptoms are a result of tumor-induced inflammation. We sought to test this hypothesis using a syngeneic heterotopic murine model of human papilloma virus-related head and neck cancer. Mice were implanted or not with 1x106 epithelial tonsil cells transfected with E6/7 of HPV16 and HRAS into the hind leg. The resulting tumor expressed high levels of inflammatory cytokines (e.g., IL-6, IL-1beta, and TNF-alpha) and caused elevated cytokine expression within the liver (i.e., IL-6, IL-1beta, and TNF-alpha) and brain (IL-1beta). The tumor reduced burrowing and voluntary wheel running, but did not induce depression-like behavior. To confirm the existence of low grade inflammation, tumor bearing mice were injected with lipopolysaccharide (0.1 mg/kg, IP). Compared to controls, they showed evidence of priming in the form of increased immobility in the tail suspension task and reduced nest building. Chronic treatment with minocycline attenuated tumor-induced burrowing deficits as well as inflammatory cytokine expression without altering tumor response to chemoradation. These data suggest that the tumor-induced inflammation propagates from the tumor to the liver and brain to induce behavioral alterations that may be effectively targeted through the use of anti-inflammatory interventions.

While inflammation is implicated in the development of depression, the neurobiological mechanisms remain elusive. One of the reasons for this is the complexity of depression. To move forward we have decided to deconstruct depression into basic neurobiological units and focus on incentive motivation and anhedonia. Using an operant conditioning task in which mice have the choice between a high effort/high reward and a low effort/low reward mode of response we demonstrated inflammation affects willingness to exert an effort for the reward but does not reduce the sensitivity to the reward. To assess the importance of activation of indoleamine 2,3 dioxygenase (IDO1) in these effects, we compared IDO1-deficient to wild type mice. In accordance with our previous results (Vichaya et al, Neuropsychopharmacology 2014) wild type mice responded to lipopolysaccharide (LPS, 0.33 mg/kg intraperitoneal 24 h before) by a reduction in total responses and an increase in percent chocolate pellets earned. The same response to LPS was observed in IDO1 KO mice. To further assess motivation, food-deprived mice were submitted to a restricted schedule of food delivery that resulted in an increase in locomotor activity just before the expected delivery of food. This food-related anticipatory activity was decreased in the same manner by LPS in wild type and IDO1 KO mice. These findings indicate that inflammation-induced deficits in incentive motivation are not mediated by IDO1 activation.