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.1Molecular Psychiatry. The serotonin theory of depression: a systematic umbrella review of the evidence 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.2SSM – Mental Health. Is the chemical imbalance an ‘urban legend’? An exploration of the status of the serotonin theory of depression in the scientific literature 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.1Molecular Psychiatry. The serotonin theory of depression: a systematic umbrella review of the evidence
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.3PubMed Central. Anhedonia and the brain reward circuitry in depression Feelings of low motivation, reduced energy, appetite changes, and slowed physical movement may all trace back to altered dopamine signaling in this reward pathway.4PubMed. The mesolimbic dopamine reward circuit in depression
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.5PubMed Central. The effects of ketamine on prefrontal glutamate neurotransmission in healthy and depressed subjects
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.6Molecular Psychiatry. Ketamine decreases neuronally released glutamate via retrograde stimulation of presynaptic adenosine A1 receptors 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.7PubMed. All roads lead to glutamate: NMDA and AMPA receptors as targets for rapid-acting antidepressants
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.8PubMed Central. Brexanolone, a neurosteroid antidepressant, vindicates the GABAergic deficit hypothesis of depression and may foster resilience
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.9PubMed Central. The importance of norepinephrine in depression 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.10PubMed Central. Chronic Stress-Associated Depressive Disorders: The Impact of HPA Axis Dysregulation and Neuroinflammation on the Hippocampus-A Mini Review
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.11CNS & Neurological Disorders – Drug Targets. Implication of HPA-axis Dysfunction in Depression: From Neurobiological Mechanisms to Future Treatment Strategies 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.12PubMed Central. Microglia in depression: an overview of microglia in the pathogenesis and treatment of depression Both excessive microglial activation (from infections, stress, or other inflammatory conditions) and microglial decline (from aging or chronic unpredictable stress) can contribute to depression.13Trends in Neurosciences. Microglial hypothesis of depression
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.14PubMed Central. Kynurenine Pathway of Tryptophan Metabolism in Neuropsychiatric Disorders: Pathophysiologic and Therapeutic Considerations 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.15PubMed Central. The role of BDNF in depression on the basis of its location in the neural circuitry
Chronic stress reduces BDNF production, and the resulting impairment in structural plasticity may be an initial factor driving depression.16Frontiers in Cellular Neuroscience. The Role of BDNF on Neural Plasticity in Depression 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.17PubMed Central. Brain-Derived Neurotrophic Factor, Depression, and Physical Activity: Making the Neuroplastic Connection 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.18PubMed Central. Cholinergic regulation of mood: from basic and clinical studies to emerging therapeutics 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.19PubMed Central. Cholinergic signaling in the hippocampus regulates social stress resilience and anxiety- and depression-like behavior
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.20PubMed Central. Bridging reward and resilience: the endocannabinoid system as a unifying mechanism in exercise-induced protection against major depressive disorder 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.21PubMed Central. The dynorphin/kappa opioid system as a modulator of stress-induced and pro-addictive behaviors
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.22PubMed Central. Circulating Short-Chain Fatty Acid (SCFA) profiles as a biomarker of gut-brain axis dysfunction: A meta-analysis for the SCFA signature in major depression Research is focused on understanding how these metabolites enter systemic circulation, cross into the brain, and exert effects on brain structure and function.23PubMed. Short chain fatty acids: Microbial metabolites for gut-brain axis signalling
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.24PubMed Central. Mitochondrial Dysfunction in Depression
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.25PubMed Central. Neuroinflammation and insulin resistance in major depression and bipolar disorder: Implications for clinical trials evaluating immunometabolic targeted therapies
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.26Signal Transduction and Targeted Therapy. Epigenetic regulation in major depression and other stress-related disorders: molecular mechanisms, clinical relevance and therapeutic potential
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.27PubMed Central. Epigenetics and depression 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.28PubMed Central. The Role of Trace Amine-Associated Receptor 1 (TAAR1) in the Pathophysiology and Treatment of Depression 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.7PubMed. All roads lead to glutamate: NMDA and AMPA receptors as targets for rapid-acting antidepressants
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.29PubMed Central. Targeting the Neuropeptide Y System in Stress-related Psychiatric Disorders 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.