Inflammation linked to brain reward dysfunction in American Indians with depression

Recent research indicates that bodily inflammation may disrupt the brain’s ability to process rewards and risks in American Indian adults who have experienced depression. The study found that higher levels of specific inflammatory markers in the blood corresponded with reduced activity in brain regions essential for motivation. These findings were published in Biological Psychiatry: Cognitive Neuroscience and Neuroimaging.

Major Depressive Disorder is a complex mental health condition that goes beyond feelings of sadness. One of its hallmark symptoms is anhedonia, which is a reduced ability to experience pleasure or interest in daily activities. This symptom is often linked to dysfunctions in the brain’s reward circuitry. This system governs how the brain anticipates positive outcomes, such as winning a prize, or negative outcomes, like a financial loss.

Scientists are increasingly looking at the immune system to understand these brain changes. Physical inflammation is the body’s natural response to injury or stress. However, chronic stress can lead to persistent, low-grade inflammation that affects the entire body. Over time, the immune system releases signaling proteins called cytokines that can cross into the brain. Once there, these proteins may alter how neural circuits function.

This biological connection is particularly relevant for American Indian populations. Many Indigenous communities face unique and chronic stressors rooted in historical trauma. These stressors include the long-term psychological impacts of colonization and systemic health disparities. Previous research links symptoms of historical loss to higher risks for both depression and physical health issues.

The researchers hypothesized that this unique stress environment might elevate inflammation levels. They proposed that this inflammation could, in turn, impair the brain’s reward system. This pathway might explain why depression prevalence and severity can be higher in these communities. To test this, the study focused on American Indian individuals who had been diagnosed with Major Depressive Disorder at some point in their lives.

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Leading the investigation was Lizbeth Rojas from the Department of Psychology at Oklahoma State University. She collaborated with a team of experts from the Laureate Institute for Brain Research and other academic institutions. The team aimed to move beyond simple surveys by looking at direct biological and neurological evidence. They sought to connect blood markers of inflammation with real-time brain activity.

The study included 73 adult participants who identified as American Indian. All participants had a history of clinical depression. To assess their biological state, the researchers collected blood samples from each individual. They analyzed these samples for specific biomarkers related to the immune system.

The team measured levels of proinflammatory cytokines, which promote inflammation. These included tumor necrosis factor (TNF) and interleukin-6 (IL-6). They also measured C-reactive protein (CRP), a general marker of inflammation produced by the liver. Additionally, they looked at interleukin-10 (IL-10), a cytokine that helps reduce inflammation.

To observe brain function, the researchers utilized two advanced imaging technologies simultaneously. Participants entered a functional magnetic resonance imaging (fMRI) scanner. This machine measures brain activity by tracking changes in blood oxygen levels. At the same time, participants wore caps to record electroencephalography (EEG) data. EEG measures the electrical activity of the brain with high time precision.

While inside the scanner, the participants performed a specific psychological test called the Monetary Incentive Delay task. This task is designed to activate the brain’s reward centers. Participants viewed a screen that displayed different visual cues. Some cues indicated a chance to win money, while others indicated a risk of losing money.

After seeing a cue, the participant had to press a button rapidly. If they were fast enough on a “win” trial, they gained a small amount of cash. If they were fast enough on a “loss” trial, they avoided a financial penalty. The researchers focused on the “anticipation phase” of this task. This is the brief moment after seeing the cue but before pressing the button.

During this anticipation phase, a healthy brain typically shows high activity in the basal ganglia. This is a group of structures deep in the brain that includes the striatum. The striatum is essential for processing incentives and generating the motivation to act. In people with depression, this area often shows “blunted” or reduced activity.

The study’s results revealed a clear link between the immune system and this brain activity. The researchers used statistical models to predict brain response based on inflammation levels. They found that higher concentrations of TNF were associated with reduced activation in the basal ganglia during the anticipation of a potential win.

This relationship was notably influenced by the sex of the participant. The negative association between TNF and brain activity was observed specifically in male participants. This suggests that for men in this sample, high inflammation dampened the brain’s excitement about a potential reward.

The researchers also examined how the brain reacted to the threat of losing money. In this context, they looked at the interaction between TNF and CRP. They found that elevated levels of both markers predicted reduced brain activation. The basal ganglia were less responsive even when the participant was trying to avoid a negative outcome.

Another finding involved the nucleus accumbens, a key part of the brain’s reward circuit. The study showed that medication status played a role here. Among participants taking psychotropic medication, higher TNF levels were linked to lower activity in this region during loss anticipation. This highlights the complexity of how treatments and biology interact.

The study also attempted to use EEG to measure a specific brain wave called the P300. The P300 is a spike in electrical activity that relates to attention and updating working memory. Previous studies have suggested that people with depression have a smaller P300 response. The researchers expected inflammation to predict the size of this brain wave.

However, the analysis did not find a statistical link between the inflammatory markers and the P300 amplitude. The electrical signals did not show the same clear pattern as the blood flow changes measured by the fMRI. This suggests that inflammation might affect the metabolic demand of brain regions more than the specific electrical timing measured by this task.

These findings support the idea that the immune system plays a role in the biology of depression. The presence of high inflammation appears to “turn down” the brain’s sensitivity to incentives. When the brain is less responsive to rewards, a person may feel less motivation. This aligns with the clinical experience of patients who feel a lack of drive or pleasure.

The authors described several limitations that provide context for these results. The study relied on a relatively small sample size of 73 people. A larger group would provide more statistical certainty. Additionally, the data came from parent studies that were not designed exclusively for this specific investigation.

Another limitation was the lack of a healthy control group. The study only looked at people with a history of depression. Without a non-depressed comparison group, it is difficult to determine if these patterns are unique to depression. They might also appear in people with high inflammation who are not depressed.

The study also could not fully account for cultural factors. While the background emphasizes the role of historical trauma, the analysis did not measure cultural connectedness. Previous research suggests that connection to one’s culture can protect against stress. It acts as a buffer that might improve mental health outcomes.

Despite these caveats, the research offers a specific biological target for understanding depression in American Indian populations. It moves away from purely psychological explanations. Instead, it frames mental health within a “biopsychosocial” model. This model considers how biological stress and social history combine to affect the brain.

The authors suggest that future research should focus on resilience. Understanding how some individuals maintain low inflammation despite stress could be key. This could lead to better prevention strategies. Interventions might focus on reducing inflammation as a way to help restore normal brain function.

Treating depression in these communities may require addressing physical health alongside mental health. If inflammation drives brain dysfunction, then reducing stress on the body is vital. This reinforces the need for holistic healthcare approaches. Such approaches would respect the unique history and challenges faced by American Indian communities.

The study, “Major Depressive Disorder and Serum Inflammatory Biomarkers as Predictors of Reward-Processing Dysfunction in an American Indian Sample,” was authored by Lizbeth Rojas, Eric Mann, Xi Ren, Danielle Bethel, Nicole Baughman, Kaiping Burrows, Rayus Kuplicki, Leandra K. Figueroa-Hall, Robin L. Aupperle, Jennifer L. Stewart, Salvador M. Guinjoan, Sahib S. Khalsa, Jonathan Savitz, Martin P. Paulus, Ricardo A. Wilhelm, Neha A. John-Henderson, Hung-Wen Yeh, and Evan J. White.