Adolescent major depressive disorder (MDD) is a major public health concern associated with significant morbidity and mortality. The identification of neurobiological correlates of adolescent MDD has been hampered by the disorder's heterogeneity. To address this challenge, the proposed project adapts a dimensional investigative approach focusing on anhedonia - the loss of pleasure - a core symptom of MDD. Anhedonia can be easily quantified and is tied to specific neural reward circuitry. Among adolescents with MDD, anhedonia is highly variable, with its full range of severity manifesting in this group. These characteristics make anhedonia an ideal candidate for such an approach. Converging data from our laboratory and others' suggest that peripheral activation of the immune system/inflammation and associated CNS alterations mediate anhedonia. We have studied the kynurenine pathway (KP), a central neuroimmunological pathway, which is activated by cytokines and degrades tryptophan into several neurotoxins (kynurenines). We reported increased peripheral activation of the KP and neurotoxic load in highly anhedonic MDD adolescents compared to non-anhedonic and healthy control (HC) adolescents. Further, to assess KP-associated CNS alterations, we used proton MR spectroscopy (1H MRS), a non-invasive imaging technique that measures brain metabolites reflecting different aspects of neural function. We documented associations between blood KP neurotoxins and striatal choline (membrane turnover marker) levels along with decreased anterior cingulate cortex (ACC) 3- aminobutyric acid (GABA) levels in anhedonic MDD adolescents, which were correlated with anhedonia scores in the whole MDD group. Based on these preliminary data, we propose to test the overall hypothesis that peripheral activation of the immune system and accompanying neurometabolic alterations are specifically linked to anhedonia severity. To test our hypothesis, 60 depressed adolescents (psychotropic-free) and 40 HC, ages 12-18, Tanner e 4, will be studied. Anhedonia will be measured quantitatively. Blood samples for cytokines, KP metabolites (including quinolinic acid and 3-hydroxykynurenine), kynurenine 3-monooxygenase enzyme activity (initiates the KP neurotoxic branch), and saliva samples for cortisol measures will be collected at AM after an overnight fast. Within an hour, concentrations of neurometabolites reflecting neuronal viability, GABA, and membrane turnover will be measured in the ACC and striatum, regions within the neural reward circuitry, via 1H MRS.