What Chemical Causes Depression? Not Just Serotonin

No single chemical causes depression. The popular idea that depression results from “low serotonin” took hold in the 1990s alongside the rise of SSRI antidepressants, but a major systematic review published in 2022 found no convincing evidence that depression is caused by lower serotonin levels or reduced serotonin activity.¹ Depression involves a web of chemical systems, stress hormones, immune signals, growth factors, and even metabolites produced by gut bacteria, all interacting in ways that vary from person to person. Understanding that complexity changes how you think about treatment, why some medications work for some people and not others, and why the disorder has proven so stubbornly difficult to solve.

How the Serotonin Myth Took Root

The “chemical imbalance” explanation for depression became one of the most widely believed ideas in mental health. It was neat, simple, and easy to market alongside a new generation of antidepressants. But the theory was always more advertising than science. One analysis described it as having “little evidence it has empirical support” despite being widely accepted by the public and used to justify prescribing SSRIs.² The 2022 umbrella review was even more direct: most studies found no evidence of reduced serotonin activity in depressed people compared to non-depressed people, and experimentally lowering serotonin did not consistently make healthy volunteers feel worse.¹

This does not mean SSRIs are useless. They clearly help some people. But the reason they help probably has less to do with correcting a serotonin deficit and more to do with downstream effects on neural plasticity, stress circuits, and other signaling systems that happen to be nudged when you alter serotonin levels. The “which chemical” question is the wrong frame. Depression is better understood as a systems-level problem, and the chemicals involved differ depending on the person, the subtype of depression, and even the life circumstances that triggered it.

Dopamine and the Inability to Feel Pleasure

If serotonin is the molecule everyone blames for depression, dopamine is the one that often better explains one of its cruelest symptoms: anhedonia, the inability to enjoy things you once loved. The brain’s reward circuit runs on dopamine, connecting a deep midbrain region to areas involved in motivation and pleasure. When that circuit is disrupted, activities that used to feel satisfying register as flat or empty.³ Feelings of low motivation, reduced energy, appetite changes, and slowed physical movement may all trace back to altered dopamine signaling in this reward pathway.⁴

This matters practically because SSRIs do not primarily target dopamine. Someone whose depression centers on anhedonia and low drive may respond poorly to a serotonin-focused drug but respond better to a medication that also increases dopamine or norepinephrine activity. The symptom profile can hint at the chemistry underneath, even though clinicians rarely test neurotransmitter levels directly.

Glutamate, the Brain’s Workhorse Signal

Glutamate is the most abundant excitatory chemical messenger in the brain, involved in the vast majority of neural communication. It has gotten far less public attention than serotonin, but the evidence linking it to depression is growing fast, largely because of ketamine. In imaging studies, ketamine increased glutamate release in the prefrontal cortex of human subjects, and that surge is thought to be part of what triggers its remarkably rapid antidepressant effects.⁵

The mechanism is more nuanced than just “more glutamate helps.” Ketamine blocks one type of glutamate receptor while indirectly stimulating another, and lab work suggests that its downstream effects involve a complex feedback loop where the brain’s own adenosine system dials glutamate release back down through a previously unknown mechanism.⁶ That short burst of increased glutamate activity appears to kickstart synaptic strengthening and growth, producing antidepressant effects within hours rather than weeks. A wave of new drugs now targets different parts of the glutamate system, including compounds that work on both NMDA and AMPA receptors.⁷

GABA, the Brain’s Brake Pedal

If glutamate is the accelerator, GABA is the brake. It is the main inhibitory chemical in the brain, calming neural activity and keeping excitation in check. There is a hypothesis that depression involves a deficit in GABA signaling in certain brain circuits. The clearest evidence for this came from the approval of brexanolone for postpartum depression. Brexanolone is a synthetic form of allopregnanolone, a naturally occurring neurosteroid that enhances GABA receptor function. Its antidepressant effect is attributed to restoring inhibitory tone in circuits where GABA signaling has dropped.⁸

Postpartum depression may be a special case, since levels of neurosteroids like allopregnanolone fluctuate dramatically around childbirth. But GABA deficits have been measured in other forms of depression too, and the success of brexanolone opened the door to oral neurosteroid-based antidepressants that work through a fundamentally different mechanism than traditional antidepressants.

Norepinephrine and the Fog of Low Drive

Norepinephrine is closely related to adrenaline and plays a central role in alertness, attention, and motivation. It contributes to executive functioning, the ability to plan, initiate tasks, and sustain focus, all of which deteriorate in depression.⁹ The “brain fog” and physical lethargy that many people with depression describe may involve norepinephrine as much as, or more than, serotonin. This is one reason why SNRIs (which target both serotonin and norepinephrine) or norepinephrine-focused drugs sometimes help people who didn’t improve on an SSRI alone.

Cortisol and the Stress System

Depression and chronic stress are deeply intertwined, and the body’s main stress system, the HPA axis, is one of the most consistently disrupted biological systems in depressed people. Under chronic stress, this system stays activated far too long, flooding the body with cortisol. That prolonged cortisol exposure damages the brain, particularly the hippocampus, and contributes to the decline of both physical and mental health.¹⁰

The dysfunction is not just “too much cortisol.” The feedback loop that is supposed to shut cortisol production off stops working properly. Receptors in the brain that should detect high cortisol and tell the system to stand down become less sensitive, so the alarm keeps sounding. This impaired feedback, sustained high cortisol, and disrupted daily cortisol rhythms all contribute to neural vulnerability and the progression of depressive symptoms.¹¹ Cortisol is not a neurotransmitter in the classical sense, but it powerfully shapes the chemical environment in which all the other signaling molecules operate.

Inflammation and the Immune Connection

One of the more surprising developments in depression research has been the discovery that the immune system plays a significant role. Depressed people often have elevated levels of inflammatory molecules circulating in their blood, and brain-resident immune cells called microglia show abnormal activation in both animal models and clinical imaging studies of depressed patients.¹² Both excessive microglial activation (from infections, stress, or other inflammatory conditions) and microglial decline (from aging or chronic unpredictable stress) can contribute to depression.¹³

Inflammation also reroutes the metabolism of tryptophan, the amino acid the body uses to make serotonin. When inflammatory signals are high, tryptophan gets shunted down an alternative pathway that produces neurotoxic compounds instead of serotonin. This shift is triggered by inflammatory processes and disturbs the balance between neurotoxic and neuroprotective branches of that metabolic pathway.¹⁴ So inflammation does not just cause misery on its own; it also sabotages the brain’s ability to produce the very chemicals it needs to regulate mood.

BDNF and the Shrinking Brain

Depression is not only about which chemical messengers are too high or too low. It also involves physical changes in brain structure. Brain-derived neurotrophic factor, or BDNF, is a protein that supports the survival of existing neurons and encourages the growth of new connections. It plays a critical role in synaptic plasticity, the brain’s ability to rewire itself in response to experience.¹⁵

Chronic stress reduces BDNF production, and the resulting impairment in structural plasticity may be an initial factor driving depression.¹⁶ Imaging studies have confirmed that key brain regions, particularly the hippocampus and prefrontal cortex, tend to be smaller in people with chronic depression. The disruptions in plasticity occur at both regional and circuit levels.¹⁷ This is partly why exercise consistently shows antidepressant effects: it is one of the most reliable ways to boost BDNF levels and promote the growth of new neural connections.

Acetylcholine, Endocannabinoids, and Opioid Peptides

Several other chemical systems have credible links to depression, though they get less public attention. Acetylcholine, best known for its role in memory and muscle control, has been implicated in depression for over fifty years.¹⁸ Too much acetylcholine activity in the hippocampus appears to promote anxiety and depression-like behavior and reduce resilience to social stress. In animal studies, blocking the enzyme that breaks down acetylcholine increased depression-like behavior, and the effect was reversed by drugs that dampened acetylcholine receptor activity.¹⁹

The endocannabinoid system, the body’s own cannabis-like signaling network, has also emerged as a player. Dysfunction in this system has been identified as a contributing mechanism in the core symptoms of depression.²⁰ And the brain’s own opioid system, particularly a peptide called dynorphin that acts on kappa opioid receptors, has been directly linked to the negative feelings associated with stress and may amplify the dysphoric mood that characterizes depression.²¹

Signals From the Gut

The gut-brain axis has gone from fringe idea to mainstream research area in the last decade. Gut bacteria produce short-chain fatty acids through the fermentation of dietary fiber, and these metabolites are vital signaling molecules that maintain intestinal barrier integrity, influence immune function, and affect microglial activity in the brain.²² Research is focused on understanding how these metabolites enter systemic circulation, cross into the brain, and exert effects on brain structure and function.²³

The practical implication is that the health of your digestive tract may influence your mood in ways that have nothing to do with classical neurotransmitters. This is still an area where the research is running ahead of clinical application. Probiotic supplements marketed for mood are already widely available, but the evidence for specific strains treating depression remains preliminary.

Metabolism and Energy Production

Neurons are extraordinarily energy-hungry cells, and there is growing evidence that depression involves impaired energy production at the cellular level. When mitochondria, the energy factories inside cells, are not functioning properly, neurons cannot meet their energy demands. Animal studies have shown that chronic mild stress damages mitochondrial structure and inhibits energy production in brain regions including the hippocampus, cortex, and hypothalamus, leading researchers to propose that energy impairment in the brain is a core feature of depression.²⁴

Insulin signaling in the brain adds another metabolic layer. Depression and metabolic dysfunction have a bidirectional relationship: people with depression face a substantially increased risk of developing type 2 diabetes, and people with diabetes face an increased risk of developing depression. Even without full-blown diabetes, insulin resistance is more common in depressed people than in non-depressed controls and has been linked to atypical symptoms, greater illness burden, and poor response to standard antidepressants. This has led researchers to propose a distinct metabolic subtype of depression.²⁵

Why Your Life History Changes Your Brain Chemistry

One reason depression’s chemistry differs so much between individuals is that life experience physically alters how genes are expressed, without changing the genes themselves. This field, called epigenetics, has shown that adverse experiences, especially early in life, can change which genes are turned on or off in the brain’s stress circuits. In animal research, rat pups that received less maternal care showed lasting changes in the regulation of stress hormone receptors in the hippocampus, driven by chemical modifications to the DNA packaging around those genes.²⁶

These modifications can persist into adulthood and affect vulnerability to depression decades later. The emerging model suggests that genetic predisposition and environmental exposure interact through epigenetic mechanisms to alter stress response pathways and neural plasticity, the very systems already described in this article.²⁷ This helps explain why two people with similar life stressors can have completely different outcomes: their brains may literally be wired differently at the molecular level due to earlier experiences.

What This Means for Treatment

If depression were truly caused by one chemical, we would have a simple blood test and a single class of drugs that worked for everyone. Instead, we have dozens of medications that each help a fraction of patients, and roughly a third of people with depression do not respond adequately to any first-line antidepressant. That treatment-resistance problem makes much more sense once you appreciate how many different chemical systems can be disrupted.

The new generation of treatments reflects this shift. Ketamine-based therapies target the glutamate system. Brexanolone targets GABA. Anti-inflammatory agents are being tested for people whose depression features elevated immune markers. Researchers are investigating compounds that target trace amine receptors, which modulate dopamine, serotonin, and glutamate signaling simultaneously, offering a multi-system approach rather than a single-target one.²⁸ Clinical trials are also exploring compounds aimed at different glutamate receptor subtypes, from NMDA receptor blockers to AMPA receptor enhancers, all converging on mechanisms that promote rapid synaptic strengthening.⁷

The honest state of the science is this: depression is not one disease with one chemical cause. It is a collection of overlapping syndromes, each with a different mix of disrupted chemical, hormonal, immune, and metabolic pathways. The serotonin story was not completely wrong, just radically incomplete. The more useful question is not “what chemical causes depression?” but “which systems are disrupted in this particular person’s depression?” That question is harder to answer, but it is the one that will eventually lead to treatments matched to the individual rather than prescribed by trial and error.

Neuropeptide Y and Stress Resilience

Not all brain chemistry relevant to depression involves the molecules that make you feel bad. Some involve molecules that protect you from feeling bad. Neuropeptide Y is one of the most studied examples: a peptide that appears to act as an endogenous buffer against stress. Research has identified it as a mediator of resilience, the ability to experience stress without tipping into a depressive episode.²⁹ People with lower neuropeptide Y activity may be more vulnerable not because their “depression chemicals” are abnormal, but because their protective chemicals are insufficient. This flips the usual framing. Instead of asking only what goes wrong in the brains of depressed people, researchers are increasingly studying what goes right in the brains of people who stay well under severe stress, and whether those protective mechanisms can be enhanced pharmacologically.