Physiological Causes of Depression: A Current Evidence Summary

Neural Circuit Dysregulation

Depression involves a measurable shift in how the brain's reward and emotional circuits function. In clinical depression, a mismatch exists between activity in the brain's reward-seeking and misery-avoidance circuits  the balance is shifted toward the misery-avoiding side. The hippocampus and prefrontal cortex are particularly affected, as these are the regions responsible for mood regulation, memory, and rational thought  and they are among the first to show structural change under chronic stress. nih

HPA Axis & Cortisol Dysregulation

Chronic stress drives the brain's stress hormone system into overdrive, with lasting consequences. Overactivation of the HPA axis results in elevated cortisol levels that contribute to neuronal damage, particularly in the hippocampus and prefrontal cortex, both of which are involved in mood regulation. The damage doesn't stop there excess cortisol interferes with BDNF signaling, a protein crucial for neuronal survival and growth, and reduced BDNF has been associated with the severity of depressive symptoms and resistance to antidepressant treatment. nihPubMed Central

Neurotransmitter Imbalance

The monoamine deficiency model — low serotonin, norepinephrine, and dopamine — remains relevant but is now understood as only part of the picture. Depression is characterized by impaired monoaminergic neurotransmission, diminished neurotrophic signaling, and structural alterations in limbic circuits. More recent models also implicate glutamate dysregulation, which is why ketamine,  a glutamate modulator  has shown rapid antidepressant effects where traditional agents fail. nih

The Microbiota-Gut-Brain Axis

The gut's influence on depressive illness is increasingly well-documented. Imbalances in gut microbiota can affect the HPA axis, promoting inflammation and increasing gut permeability, which allows endotoxins to enter the bloodstream and contribute to neuroinflammation, while also altering serotonin production, the majority of which occurs in the gut. Individuals with depression often exhibit lower levels of SCFA-producing bacteria, which appear to play a protective role against depressive symptoms. nihFrontiers

Neuroinflammation

Inflammation has emerged as one of the most consistent biological findings in depression. Pro-inflammatory cytokines such as IL-6 and TNF-α can cross the blood-brain barrier, negatively influencing neuroplasticity and neuronal function, thereby contributing to depressive symptoms. This inflammatory burden impairs the same hippocampal and prefrontal regions already stressed by cortisol excess, compounding the damage from multiple directions simultaneously. Frontiers

Clinical Takeaway

Depression, like anxiety, is a multi-system phenomenon. The current evidence points to converging dysfunction across stress hormone regulation, neurotransmitter signaling, neuroplasticity, gut microbiota, and neuroinflammation. This explains why a single medication often falls short — and why interventions targeting BDNF, the microbiome, inflammation, and glutamate pathways are gaining traction alongside traditional pharmacotherapy.

Sources: Frontiers in Neuroscience (2025), Frontiers in Cellular Neuroscience (2025), International Journal of Molecular Sciences (2025), Frontiers in Microbiology (2025)

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