A neuroimmune circuit mediates cancer cachexia-associated apathy.

Cachexia, a severe wasting syndrome associated with inflammatory conditions, often leads to multiorgan failure and death. Patients with cachexia experience extreme fatigue, apathy, and clinical depression, yet the biological mechanisms underlying these behavioral symptoms and their relationship to the disease remain unclear. In a mouse cancer model, cachexia specifically induced increased effort-sensitivity, apathy-like symptoms through a cytokine-sensing brainstem-to-basal ganglia circuit. This neural circuit detects elevated interleukin-6 (IL-6) at cachexia onset and translates inflammatory signals into decreased mesolimbic dopamine, thereby increasing effort sensitivity. We alleviated these apathy-like symptoms by targeting key circuit nodes: administering an anti–IL-6 antibody treatment, ablating cytokine sensing in the brainstem, and optogenetically or pharmacologically boosting mesolimbic dopamine. Our findings uncovered a central neural circuit that senses systemic inflammation and orchestrates behavioral changes, providing mechanistic insights into the connection between chronic inflammation and depressive symptoms. Editor's summary: Cachexia, or wasting syndrome, arises during advanced stages of many chronic diseases, including cancer, and is often associated with loss of motivation and apathy. It is known that inflammation is a critical factor in cancer cachexia. However, the link between tumor-induced inflammation and the underlying brain mechanisms that produce motivational deficits remains elusive. Using a well-established mouse model of cancer cachexia, Zhu et al. uncovered a brainstem-to-basal ganglia circuit that suppresses dopamine signaling (see the Perspective by Dalley and Lynall). Neurons in the area postrema sense the inflammatory cytokine interleukin-6 and relay this signal to the parabrachial nucleus. The parabrachial nucleus drives inhibitory neurons in the substantia nigra pars reticulata, which then suppress dopamine neurons in the ventral tegmental area. This sequence reduces dopamine levels in the nucleus accumbens, ultimately increasing effort sensitivity and lowering motivation. —Peter Stern INTRODUCTION: Cachexia is a debilitating wasting syndrome that affects most advanced cancer patients, characterized by profound involuntary weight loss, muscle and fat depletion, and disrupted energy balance. Beyond physical decline, patients commonly experience severe fatigue, apathy, and depression that further diminish their quality of life. Despite these pervasive neuropsychiatric symptoms, the biological mechanisms linking peripheral wasting to brain dysfunction and behavioral changes remain poorly understood, hindering the development of effective treatments. RATIONALE: Systemic inflammation, marked by elevated circulating cytokines, is a key driver of cancer cachexia. Although inflammation could broadly compromise neuronal function, we hypothesized that it acts through specialized circuits that conserve energy during acute illness. In cancer, persistent activation of these circuits may drive chronic fatigue and depression, providing a mechanistic framework for how inflammation dynamically regulates motivation. RESULTS: To investigate cachexia's impact on motivation, we used an established preclinical model: mice subcutaneously implanted with colon adenocarcinoma cells (C26). Within weeks, the mice exhibited classical cachexia symptoms—weight loss, muscle wasting, and decreased feeding—accompanied by pronounced motivational deficits. A battery of behavioral assays uncovered specific deficits in effort-sensitive tasks such as patch foraging and progressive ratio tests. By contrast, tests of physical activity (open-field exploration), reward sensitivity (sucrose preference, a measure of anhedonia), and despair (tail suspension, forced swim tests) remained unchanged, although home-cage activity was reduced. This identifies a specific reduction in effort-based motivation, indicative of apathy and distinct from general weakness or anhedonia. A comprehensive cytokine screen showed rising interleukin-6 (IL-6) levels in the blood and brain that paralleled cachexia progression. Whole-brain, cellular-resolution activity mapping and viral tracing uncovered a circuit originating in the area postrema, a circumventricular organ specialized for detecting blood-borne signals. IL-6–sensing neurons in this region project to the parabrachial nucleus, which in turn activates the substantia nigra pars reticulata inhibitory neurons, ultimately suppressing dopamine release in the nucleus accumbens. Optogenetic stimulation of area postrema neurons rapidly suppressed accumbens dopamine release. Longitudinal dopamine monitoring during cachexia progression revealed a progressive decline in accumbens dopamine that tracked with worsening motivational deficits. Targeting this IL-6–sensing pathway reversed cachexia-induced apathy. Blocking IL-6 with systemic antibodies, knocking down (genetic reduction of gene expression) IL-6 receptors in the area postrema, or ablating area postrema–to–parabrachial neurons each alleviated the motivational deficits. Conversely, boosting dopamine signaling in the nucleus accumbens—through optogenetic stimulation of dopamine neurons or local dopamine agonist infusion—restored motivation despite ongoing inflammation, even at late stages of cancer progression. CONCLUSION: We identified an immune-to-neural circuit through which inflammation drives motivational deficits in cancer cachexia. Area postrema neurons detect circulating IL-6 and relay this signal to the basal ganglia to suppress mesolimbic dopamine, thereby increasing effort sensitivity. This specialized interoceptive pathway demonstrates that inflammation engages discrete neural circuits rather than causing broad disruption and cumulative neural damage. Although this circuit is likely adaptive during acute illness—conserving energy by dampening motivation—its persistent engagement in chronic conditions such as cancer cachexia is detrimental. This reveals that psychiatric symptoms such as apathy in cachexia are not secondary effects of physical decline but can arise directly from the same pathological mechanisms driving the illness itself. Targeting this circuit through ablation of IL-6 sensing or boosting dopamine release relieved motivational deficits, opening therapeutic avenues for neuropsychiatric symptoms in cachexia and possibly other inflammatory conditions where IL-6 is elevated. Moreover, by quantifying effort sensitivity as a dimension of motivation that is measurable across species establishes that inflammation specifically drives apathy-like behavior, enabling clinical translation and treatments for inflammation-driven motivational deficits common to conditions ranging from cancer to depression. Immune-to-brain circuit drives apathy-like behavior in cancer cachexia.: The area postrema (ArP) detects elevated IL-6 levels in cancer cachexia, leading to reduced mesolimbic dopamine in the nucleus accumbens (NAc). This reduction heightens effort sensitivity, producing apathy-like behaviors in cachectic mice. Targeting specific circuit nodes (black circles) through gain- or loss-of-function manipulations rescued these behaviors. DA, dopamine; PBN, parabrachial nucleus; SNpr, substantia nigra pars reticulata; VTA, ventral tegmental area. [ABSTRACT FROM AUTHOR]