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A Crucial Part of The Brain's Memory Center Seems to Shrink in People With Depression
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New research has identified a specific region of the brain’s memory center that appears to shrink in older adults with depression, offering a fresh clue in the long-standing mystery of how depression and Alzheimer’s disease may be linked. Scientists have known for years that depression raises the risk of developing Alzheimer’s, but the biological mechanisms behind that connection have remained unclear. The new study, published in Translational Psychiatry, narrows the focus to the hippocampus, a structure critical for memory, and zeroes in on a subsection called CA23DG that may be especially vulnerable to depression.
Led by neuroscientist Danielle Luu of the University of Southern California, the research team analyzed MRI and PET scans from more than 2,000 cognitively unimpaired adults aged 50 to 90, about one-third of whom had experienced depression. After adjusting for age, sex, body mass index, and physical activity, the researchers found that CA23DG—a composite region encompassing hippocampal subfields CA2, CA3, and the dentate gyrus—was significantly smaller in depressed participants. Notably, this reduction remained even when known Alzheimer’s risk factors such as amyloid and tau protein levels or the APOE ε4 gene variant were included in the analysis, suggesting that this shrinkage is not driven by the classic biological hallmarks of Alzheimer’s.
The findings hint at two potentially independent mechanisms that can affect memory in aging. Other hippocampal regions, such as CA1 and the subiculum, did not show reduced volume in people with depression in the base analysis, indicating that CA23DG may be uniquely sensitive. However, when the researchers factored in antidepressant use, they found that participants taking these medications exhibited reduced volume in both CA23DG and CA1. This counterintuitive result led the team to consider that people with more severe depression are more likely to receive medication prescriptions, rather than the drugs themselves causing harm.
Duration of depression also appeared to matter. People who had experienced depression for longer periods showed a stronger association with reduced CA23DG volume, even if their current symptoms were mild or well-controlled, compared to those with recently diagnosed, more severe depression. This suggests that chronic exposure to depression may accumulate damage over time, although the authors caution that the data cannot rule out that medication use itself is responsible for the observed brain structure differences.
“It is possible that medications themselves are driving the association with hippocampal structure,” the researchers wrote, acknowledging that the link might stem more from treatment history than from the duration of the illness. The study is observational, so it cannot prove cause and effect, and much more work is needed to untangle the complex relationships between depression, medication, and brain health.
Still, the discovery offers a promising new target for future research. If CA23DG truly is a sensitive hotspot for depression-related brain changes, it could help doctors identify older adults at higher risk for cognitive decline and potentially guide earlier interventions. The team emphasizes that these findings open up many new questions, and they call for additional studies to explore the underlying biological pathways and to clarify how depression duration, severity, and treatment interact to shape the aging brain.
