What Causes Depression? Beyond the Serotonin Myth

1. Why the serotonin story is too small

2. Inflammation and the immune system

note

Inflammation can change mood and motivation

The immune system does more than fight infection. It also sends chemical signals that affect the brain.

Key terms

• Cytokines: immune messenger proteins
• C-reactive protein, or CRP: a blood marker of inflammation
• Interleukin-6, or IL-6: another inflammation signal often studied in depression

What inflammation can do

• Increase fatigue
• Reduce motivation and pleasure
• Slow thinking
• Increase pain sensitivity
• Make the body feel “sick” even without an infection

Important caution

Inflammation is not the whole explanation. Many people with depression do not have elevated inflammatory markers.

diagram

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Why some symptoms look inflammatory

Inflammation often produces a cluster called sickness behavior.

That includes:

• Low energy
• Social withdrawal
• Loss of appetite
• Slower movement
• Brain fog

These overlap with depression. That overlap is why researchers pay attention to the immune system when symptoms include strong fatigue and slowed thinking.

Real-world examples

• Rheumatoid arthritis and depression often co-occur.
• People with obesity can have higher baseline inflammation.
• Severe infections can trigger temporary depressive symptoms.

Treatment implication

If inflammation is part of the picture, treating only mood symptoms may not be enough. Sleep, exercise, medical care for chronic illness, and psychotherapy can all matter.

illustration

3. Neuroplasticity and the HPA axis

4. The gut-brain connection

5. The biopsychosocial model: depression has many causes

Transcript

Welcome to Slate. Today we're looking at What Causes Depression? Beyond the Serotonin Myth. We'll cover Why the 'chemical imbalance' theory is an oversimplification, The roles of inflammation, neuroplasticity, and HPA axis dysregulation, Gut-brain connection and the microbiome's role in mood, and The biopsychosocial model: why depression is never one thing. Let's get into it.

Depression is not one broken chemical. The old serotonin story sounds neat, but the brain is not a light switch with one dimmer. It is more like a city with power lines, traffic, weather, and public transit all affecting each other. Serotonin matters, but so do stress hormones, immune signals, sleep, learning, and social experience. A major 2022 review by Joanna Moncrieff and colleagues in Molecular Psychiatry found no consistent evidence that low serotonin causes depression. That does not mean serotonin is irrelevant. It means the simple cause-and-effect story was too tidy. Here’s the key idea shown in the diagram: symptoms can come from many routes that all meet in the brain. A person can become depressed after chronic stress, after inflammation, after trauma, after isolation, or after a mix of these. Two people can both meet criteria for major depression and still have very different biology underneath. That is why a single blood test for depression does not exist. The diagnosis is based on symptoms and time course, not one lab number. When you understand that, treatment starts to make more sense too. Different causes may respond to different combinations of therapy, medication, sleep repair, exercise, or social support.

The immune system can talk to the brain. That sounds strange until you remember that the body is one connected system. When inflammation stays high, immune molecules called cytokines can change how the brain processes reward, energy, and threat. In the visual here, notice how a body infection, obesity, autoimmune disease, or chronic stress can all feed the same immune pathway. Research has found that some people with depression have higher inflammatory markers, such as C-reactive protein, or C-R-P, and interleukin-6, or I-L-6. The effect is not universal, which is important. Depression is not simply an inflammation disorder. But for a subgroup of patients, inflammation may help explain fatigue, slowed thinking, pain, and loss of pleasure. A useful analogy is smoke in a building. The smoke alarm is not the fire itself, but it tells you something in the system is wrong. Inflammation may be one of those alarms. It can also affect treatment response. In some studies, people with higher inflammation respond less well to standard antidepressants, which is one reason researchers are studying anti-inflammatory approaches. The biology is still being worked out, but the message is clear: mood is connected to immune state, not separate from it.

Depression also affects how the brain learns and adapts. That is neuroplasticity. The brain is not fixed. It rewires with experience, and stress can push that wiring in the wrong direction. One major pathway is the H-P-A axis, short for hypothalamic-pituitary-adrenal axis. Here is the sequence shown in the diagram: the hypothalamus signals the pituitary, the pituitary signals the adrenal glands, and the adrenals release cortisol. Cortisol is useful in short bursts. It helps you respond to danger. But when stress is chronic, the system can stay stuck on high alert. Over time, that can affect the hippocampus, amygdala, and prefrontal cortex. The hippocampus helps with memory and context. The amygdala helps detect threat. The prefrontal cortex helps regulate emotion and attention. A common pattern in depression is not a single damaged brain region, but a network that has become less flexible. That is why repeated stress can make the world feel narrower, more threatening, and harder to enjoy. The good news is that plasticity works both ways. Therapy, exercise, sleep, and some medications can all help the brain regain flexibility. The goal is not just symptom suppression. It is restoring the brain’s ability to adapt.

The gut and the brain communicate constantly through nerves, hormones, immune signals, and microbial products. The biggest nerve in that conversation is the vagus nerve. The microbiome, meaning the community of bacteria and other microbes in the gut, does not magically control mood. But it can influence inflammation, metabolism, and signaling molecules that reach the brain. Here is the careful part: the science is promising, but not settled. We should not say one probiotic cures depression. What we can say is that diet, fiber intake, sleep, antibiotics, illness, and stress all shape the microbiome, and those changes can affect the gut-brain system. In animal studies, transferring gut microbes can change behavior. In humans, the effects are smaller and harder to prove, but the connection is real enough to study seriously. A good analogy is a neighborhood power grid. The gut is not the brain, but it helps supply the conditions the brain runs on. If the gut environment is disrupted for months or years, mood can be affected indirectly. This is one reason depression care increasingly pays attention to whole-body health, not just thoughts alone.

The best model for depression is biopsychosocial. That means biology, psychology, and social context all matter at once. No single layer explains every case. A person with a family history of depression may be more biologically vulnerable. A person with trauma may carry a stronger stress response. A person facing poverty, discrimination, loneliness, or unsafe housing may live with constant pressure that keeps the body’s stress systems activated. The chart here makes the point visually: depression risk rises when several forces stack together. That is why treatment often works best when it is matched to the person, not the label. One person may need medication plus therapy. Another may need treatment for insomnia and alcohol use. Another may need social support, trauma therapy, and medical treatment for inflammation-related illness. The real lesson is humility. Depression is not laziness. It is not weak character. It is a whole-person disorder with many pathways. When you replace the serotonin myth with the biopsychosocial model, the condition becomes more understandable, and care becomes more precise. That is the shift from a slogan to science.