Schizophrenia involves disruptions across at least half a dozen neurotransmitter systems, not just the single “chemical imbalance” many people assume. Dopamine has dominated the conversation since the 1960s, but decades of research have drawn glutamate, GABA, serotonin, acetylcholine, noradrenaline, and several less familiar signaling molecules into the picture. The interplay among these systems, rather than a fault in any one of them, is what makes schizophrenia so difficult to treat and so varied in its symptoms.
Dopamine and the Hypothesis That Started It All
The dopamine hypothesis emerged from a simple observation: drugs that block dopamine receptors reduce psychotic symptoms, and drugs that flood the brain with dopamine can produce them. That core insight still holds. A meta-analysis of brain-imaging studies using radiolabeled tracers found that people with schizophrenia show roughly a 14% increase in presynaptic dopamine synthesis capacity in the striatum compared to healthy controls.1Schizophrenia Bulletin. Striatal Presynaptic Dopamine in Schizophrenia, Part II: Meta-Analysis of [18F/11C]-DOPA PET Studies That elevation has been confirmed in both the caudate and the putamen, two subregions of the striatum involved in motivation, reward, and motor planning.
The revised version of the dopamine hypothesis adds a crucial twist: the problem is not simply “too much dopamine everywhere.” Instead, dopamine appears to be overactive in mesolimbic pathways, which connect deep brain structures to the limbic system, while being underactive in prefrontal regions that support planning and working memory.2PubMed Central. The role of dopamine in schizophrenia from a neurobiological and evolutionary perspective: old fashioned, but still in vogue The mesolimbic excess is thought to drive positive symptoms like hallucinations and delusions, while the prefrontal deficit contributes to negative symptoms such as flattened emotion, social withdrawal, and difficulty organizing thoughts. PET imaging has confirmed elevated dopamine synthesis in both schizophrenia and bipolar disorder with psychosis, with similarly large effect sizes in each group, suggesting the dopamine surge is linked to psychosis broadly rather than being unique to schizophrenia alone.3PubMed Central. A test of the trans-diagnostic dopamine hypothesis of psychosis, using PET imaging in bipolar affective disorder and schizophrenia
Every first-line antipsychotic medication on the market works at least partly by blocking dopamine D2 receptors. That makes dopamine the best-validated target, but it also explains why current treatments are much better at controlling hallucinations and delusions than at improving cognitive deficits or emotional blunting. The other neurotransmitter systems fill in those gaps.
Glutamate and the NMDA Receptor
Glutamate is the brain’s primary excitatory neurotransmitter, and its role in schizophrenia became hard to ignore once researchers noticed that drugs blocking a specific glutamate receptor, the NMDA receptor, produce a remarkably convincing imitation of schizophrenia in healthy people. Ketamine and PCP both act on NMDA receptors, and both can trigger not just hallucinations but also the flat affect, cognitive fog, and social withdrawal that dopamine-focused models struggle to explain. Two decades of converging evidence from genetics, pharmacology, and biochemistry point to NMDA receptor underactivity as a core contributor to the disease process.4PubMed Central. The NMDA Receptor and Schizophrenia: From Pathophysiology to Treatment
The NMDA hypofunction appears to hit certain brain cells especially hard. Preclinical work using genetically modified mice has shown that when NMDA receptors malfunction specifically on a class of fast-spiking inhibitory neurons called parvalbumin interneurons, the animals develop behaviors that resemble schizophrenia symptoms.5PubMed Central. The origin of NMDA receptor hypofunction in schizophrenia This detail matters because it connects the glutamate story directly to the GABA story described below.
Clinically, researchers have tried boosting NMDA receptor activity by supplying its co-activators, glycine and D-serine. A meta-analysis found that these co-agonists produced a moderate reduction in negative symptoms, offering some proof-of-concept that the glutamate pathway is a real therapeutic target.6Schizophrenia Research. Glutamatergic drugs for schizophrenia: a systematic review and meta-analysis Still, no glutamate-based medication has yet reached mainstream clinical use for schizophrenia, so this remains an active frontier.
GABA and the Brain’s Inhibitory Balance
GABA is the brain’s main inhibitory neurotransmitter, essentially the counterweight to glutamate’s excitatory push. In schizophrenia, the disruption centers on the same parvalbumin-positive basket cells mentioned above. These fast-firing inhibitory neurons normally impose tight rhythmic control on the pyramidal cells that form the backbone of cortical circuits. When they malfunction, the result is weakened inhibitory control and disrupted gamma oscillations, the rapid brain rhythms tied to attention, perception, and working memory.7PubMed Central. Cortical parvalbumin interneurons and cognitive dysfunction in schizophrenia
Post-mortem brain studies have revealed multiple abnormalities in these parvalbumin basket cells in people who had schizophrenia, including changes on both sides of the synapse. The emerging picture is one where NMDA receptor underactivity on these GABA neurons leads to weakened inhibition, which in turn disrupts the glutamate-GABA balance across the cortex. This interconnection makes it misleading to think of the glutamate hypothesis and the GABA hypothesis as competing explanations. They are closer to two chapters of the same story.
Serotonin and Atypical Antipsychotics
Serotonin entered the schizophrenia conversation largely because of the drugs used to treat it. The so-called atypical antipsychotics, medications like clozapine, risperidone, and olanzapine, differ from older drugs partly through their strong activity at serotonin receptors, particularly the 5-HT2A subtype. Research has shown that most atypical antipsychotics act as inverse agonists at both 5-HT2A and 5-HT2C receptors, meaning they do not just block the receptor but actively suppress its baseline signaling.8PubMed Central. Atypical antipsychotics and inverse agonism at 5-HT2 receptors A more recent pharmacological study confirmed that risperidone, clozapine, olanzapine, and even the older drug haloperidol all function as inverse agonists at the 5-HT2A receptor through specific intracellular signaling pathways.9Molecular Psychiatry. Pharmacological fingerprint of antipsychotic drugs at the serotonin 5-HT2A receptor
Whether serotonin itself is fundamentally dysregulated in schizophrenia, or whether these receptors just happen to be useful drug targets, remains debated. The clinical benefit of targeting 5-HT2A receptors is clear, especially for reducing some of the motor side effects associated with pure dopamine blockade, and potentially for improving negative symptoms. But the serotonin piece is less well established as a core disease mechanism than the dopamine or glutamate pieces.
Acetylcholine and the First Truly New Mechanism in Decades
Acetylcholine has long been a quiet player in schizophrenia research, but it made headlines recently. In September 2024, the FDA approved xanomeline combined with trospium chloride for the treatment of schizophrenia in adults, making it the first approved antipsychotic that works primarily through muscarinic acetylcholine receptors rather than dopamine receptors.10PubMed Central. From theory to therapy: unlocking the potential of muscarinic receptor activation in schizophrenia with the dual M1/M4 muscarinic receptor agonist xanomeline and trospium chloride and insights from clinical trials The drug activates M1 and M4 muscarinic receptors, and in clinical trials it reduced symptoms without the weight gain, motor problems, elevated prolactin, or sexual side effects commonly seen with dopamine-blocking drugs. That is a genuinely new treatment approach for a disorder where the pharmaceutical toolkit had been essentially unchanged for decades.
A separate branch of the acetylcholine story involves nicotinic receptors, particularly the alpha-7 subtype. The strikingly high smoking rate among people with schizophrenia, estimated at over 80% compared to about 25% of the general population, has been interpreted as a form of self-medication.11PubMed Central. Smoking, Genetics and Schizophrenia: Evidence for Self Medication Nicotine activates alpha-7 receptors, which are involved in sensory filtering and cognition, both of which are impaired in schizophrenia. People with the disorder often report that smoking helps them think more clearly, and genetic studies have linked the alpha-7 receptor gene to schizophrenia risk. This connection has spurred development of selective alpha-7 agonists, though none have yet reached approval.
Noradrenaline and the Stress-Response System
Noradrenaline (also called norepinephrine) is the brain’s arousal and alertness signal, produced primarily by a small brainstem structure called the locus coeruleus. Dysfunction in this system appears to contribute to cognitive impairments in schizophrenia. Brain imaging during working-memory tasks has shown increased activation of the locus coeruleus in people with schizophrenia compared to healthy controls, suggesting an abnormal noradrenergic response during demanding cognitive operations.12PubMed Central. Working memory in schizophrenia: The role of the locus coeruleus and its relation to functional brain networks
Separately, a study measuring pupil dilation, a physiological readout of noradrenergic activity, found that people with schizophrenia who had more severe symptoms also showed a more blunted pupil response. The correlation was substantial, especially with overall symptom severity.13PubMed Central. Dysregulated noradrenergic response is associated with symptom severity in individuals with schizophrenia Noradrenaline does not get as much attention as dopamine or glutamate, but these findings suggest it plays a real role, particularly in the cognitive symptoms that existing drugs manage poorly.
Trace Amines and TAAR1
Trace amines are a group of signaling molecules found in small amounts throughout the brain that act on their own receptor, the trace amine-associated receptor 1, or TAAR1. This receptor sits in cortical, limbic, and midbrain regions and appears to modulate dopamine, serotonin, and glutamate circuits simultaneously.14PubMed Central. Ulotaront: A TAAR1 Agonist for the Treatment of Schizophrenia That multi-system reach made it an appealing drug target. Preclinical work and early clinical trials suggested that TAAR1 agonists can normalize neurochemical changes relevant to schizophrenia without producing the side effects common to standard antipsychotics.15Trends in Neurosciences. Trace amine-associated receptor 1 in schizophrenia
The lead compound, ulotaront, received FDA Breakthrough Therapy Designation and advanced into Phase 3 trials. However, its late-stage trials did not meet their primary endpoints, and development has stalled. The biology remains interesting, as ulotaront was shown to bidirectionally modulate excitatory signaling in the striatum in ways that differ between subtypes of striatal neurons.16Neuropsychopharmacology. TAAR1 agonist ulotaront modulates striatal and hippocampal glutamate function in a state-dependent manner Other TAAR1-targeting drugs are still in development, so this pathway has not been abandoned.
The Kynurenine Pathway and Neuroinflammation
Not every molecule involved in schizophrenia fits neatly into the classic neurotransmitter categories. Kynurenic acid is a metabolic byproduct of tryptophan breakdown that happens to block both NMDA receptors and alpha-7 nicotinic receptors, two systems already implicated in the disorder. A systematic review and meta-analysis found that kynurenic acid levels are elevated in people with schizophrenia, specifically in the central nervous system (cerebrospinal fluid and brain tissue) rather than in the blood.17PubMed Central. Kynurenic Acid in Schizophrenia: A Systematic Review and Meta-analysis Elevated brain kynurenic acid tightly controls both glutamate and dopamine signaling and has been linked to psychotic symptoms and cognitive impairments.18PubMed. The kynurenine pathway in schizophrenia and bipolar disorder
The immune system connects to this pathway because inflammation activates the enzyme that starts the kynurenine cascade. Immune activation in schizophrenia appears to drive an imbalance in tryptophan-to-kynurenine metabolism, pushing the system toward greater kynurenic acid production and, consequently, greater NMDA receptor blockade.19PubMed. Kynurenine pathway in schizophrenia: pathophysiological and therapeutic aspects This is one of the clearest bridges between the neuroinflammation findings in schizophrenia and the neurotransmitter disturbances described elsewhere in this article.
The Endocannabinoid System
The brain’s endocannabinoid system, which uses internally produced cannabis-like molecules to fine-tune neural signaling, also shows disruption in schizophrenia. Evidence from post-mortem studies, cerebrospinal fluid analysis, blood samples, and brain imaging consistently points to altered cannabinoid CB1 receptor levels and changed endocannabinoid concentrations in people with psychotic disorders.20PubMed Central. Endocannabinoid system in psychotic and mood disorders, a review of human studies
PET imaging in people experiencing their first episode of psychosis revealed a widespread decrease in CB1 receptor availability across frontal, temporal, and parietal brain regions, with large effect sizes. The reduction correlated with the severity of psychotic symptoms, and the finding was replicated in an independent sample of medication-free patients.21PubMed Central. 42.4 THE ENDOCANNABINOID SYSTEM IN FIRST-EPISODE PSYCHOSIS Researchers interpret the lower receptor availability as a sign of increased endocannabinoid activity, since heavy use of a signaling system typically leads to receptor downregulation. This finding also adds context to the well-documented epidemiological link between heavy cannabis use and psychosis risk: external cannabinoids may be pushing an already stressed system further out of balance.
Genetics Tying the Systems Together
If you are wondering how all these separate neurotransmitter systems can be disrupted in the same disorder, the genetic data offer a partial answer. Large-scale genomic studies of schizophrenia have now identified hundreds of risk-associated genetic regions, and many of them code for receptors or other proteins involved in glutamate, GABA, dopamine, serotonin, acetylcholine, and even opioid signaling.22PubMed Central. Genetic evidence for role of integration of fast and slow neurotransmission in schizophrenia In other words, the genetic blueprint for schizophrenia risk is scattered across multiple neurotransmitter systems rather than concentrated in one.
Functional genomic analyses further support the view that synaptic dysfunction within glutamatergic and GABAergic neurons of the cortex and hippocampus is a central component of the disease.23PubMed Central. Schizophrenia Genomics: Convergence on Synaptic Development, Adult Synaptic Plasticity, or Both? The genetic architecture suggests that many small-effect variants collectively impair the way neurons develop, form synapses, and maintain synaptic plasticity across the lifespan. Major modifications in GABAergic, glutamatergic, and dopaminergic neurotransmission during adolescence may go awry in schizophrenia, potentially leading to excess synaptic pruning and the reductions in gray matter volume that often appear around the time symptoms first emerge.24Schizophrenia Bulletin. The Adolescent Brain: Implications for the Understanding, Pathophysiology, and Treatment of Schizophrenia
Emerging and Less Familiar Signals
Beyond the major systems, several other signaling molecules are drawing attention. Adenosine, the molecule that accumulates during wakefulness and makes you feel sleepy, also influences both dopamine and glutamate activity in the brain. An “adenosine hypothesis” of schizophrenia proposes that dysfunction in adenosine signaling could contribute to the multi-system neurotransmitter imbalances that characterize the disorder, essentially acting as an upstream regulator of the better-known systems.25PubMed Central. Adenosine hypothesis of schizophrenia–opportunities for pharmacotherapy
Histamine, more commonly associated with allergies, also operates as a brain neurotransmitter. PET imaging has identified a role for the histamine H3 receptor in executive function, with evidence that this role is disrupted in schizophrenia. The disruption appears related to cognitive impairment even when the overall number of H3 receptors looks relatively normal, suggesting a functional rather than structural deficit.26PubMed Central. The histamine system and cognitive function: An in vivo H3 receptor PET imaging study in healthy volunteers and patients with schizophrenia
Oxytocin, a neuropeptide traditionally associated with social bonding, has also been explored in schizophrenia because of the disorder’s prominent social deficits. Dysregulated oxytocin may influence how schizophrenia symptoms are expressed, and some clinical studies have examined whether supplemental oxytocin improves social cognition.27PubMed Central. Oxytocin in Schizophrenia: Pathophysiology and Implications for Future Treatment One prospective study found that baseline oxytocin levels in male patients were negatively correlated with positive symptom severity, and that increases in oxytocin during treatment tracked with improvements in social cognitive functioning.28PubMed. Oxytocin, social cognition, and neurocognitive function in male patients with schizophrenia: A prospective study Results are still preliminary, but oxytocin adds yet another dimension to the neurochemistry of the disorder.
The Gut-Brain Connection
One of the more surprising recent findings is that the gut microbiome can influence brain neurotransmitter levels in ways relevant to schizophrenia. When researchers transplanted fecal matter from people with schizophrenia into germ-free mice, the animals showed lower hippocampal glutamate, higher glutamine and GABA, and behavioral changes resembling those seen in other mouse models of schizophrenia involving glutamate underactivity.29PubMed Central. The gut microbiome from patients with schizophrenia modulates the glutamate-glutamine-GABA cycle and schizophrenia-relevant behaviors in mice A separate study found that the gut bacteria’s ability to produce tyrosine, a building block for dopamine, was positively associated with cognitive performance in people with schizophrenia.30Biological Psychiatry Global Open Science. Alteration of Gut Microbiome in Patients With Schizophrenia Indicates Links Between Bacterial Tyrosine Biosynthesis and Cognitive Dysfunction
These findings are still at an early stage and do not yet translate into clinical treatments, but they suggest that the neurochemical disruptions in schizophrenia are not confined to the brain alone. The gut microbiome may be one more upstream influence shaping the neurotransmitter landscape that goes awry in the disorder.