For most of the twentieth century, schizophrenia research was a story about dopamine. That focus was understandable: every effective antipsychotic blocked dopamine receptors, and stimulants that boosted dopamine could provoke psychotic symptoms. But by the 1990s, a quiet, parallel literature was building around a very different neurotransmitter — glutamate, the brain's primary excitatory chemical messenger — and a particular receptor it acts on, called the NMDA receptor. The glutamate hypothesis has not replaced the dopamine model, but it now sits alongside it as one of the two pillars of modern schizophrenia neuroscience.
Reduced activity of the NMDA glutamate receptor on key inhibitory neurons may be one of the upstream problems that ultimately drives the dopamine dysregulation seen in schizophrenia.
The clue: ketamine and PCP
The strongest clinical clue came from anaesthesia. In the 1950s and 60s, doctors began using phencyclidine (PCP) as a surgical anaesthetic. It worked, but patients emerging from PCP often experienced something that looked uncannily like a full psychotic episode — hallucinations, paranoia, disorganised thought, flat affect, and cognitive slowing. PCP was eventually withdrawn from medical use, but its successor, ketamine, remained.
What pharmacologists eventually realised was that PCP and ketamine share a single primary mechanism: they block the NMDA receptor. And unlike amphetamine-induced psychosis (which mostly produces positive symptoms), ketamine produces something that looks much more like the full schizophrenia syndrome — including the negative and cognitive symptoms that dopamine drugs do not cause. This was the first major sign that something other than dopamine was at the heart of the disorder.
What NMDA receptors do
The NMDA receptor (named for one of the molecules used to identify it, N-methyl-D-aspartate) is one of the principal receivers of glutamate signals. It plays a central role in:
- Synaptic plasticity — the cellular basis of learning and memory
- Coordinating timing of brain rhythms, particularly fast gamma oscillations
- Regulating other neurotransmitter systems, including dopamine, GABA, and serotonin
- Sculpting cortical circuits during adolescent brain development
Crucially, NMDA receptors sit on a class of inhibitory neurons called parvalbumin-positive (PV) interneurons. These small, fast-firing cells use the inhibitory neurotransmitter GABA to keep larger excitatory neurons in line. If NMDA receptors on PV interneurons under-perform, those inhibitory cells go quiet. The excitatory neurons they were supposed to be restraining then over-fire, producing a state of cortical disinhibition.
How the hypothesis links to dopamine
Here is where the glutamate model connects to the older dopamine story. Cortical glutamate circuits, when working normally, send projections that dampen dopamine release in the striatum. If those circuits are disinhibited because of NMDA hypofunction, the brake comes off — and striatal dopamine release goes up. This is one of the leading explanations for why imaging studies consistently find elevated striatal dopamine in schizophrenia despite no obvious problem with dopamine neurons themselves. The dopamine excess is downstream; the upstream problem is glutamatergic.
What supports the glutamate hypothesis
- Ketamine and PCP reproduce a remarkably full schizophrenia-like syndrome in healthy volunteers, including negative and cognitive symptoms.
- Genetic studies — including the large SCHEMA and PsychENCODE consortia — repeatedly implicate genes involved in glutamate signalling and synaptic pruning.
- Postmortem brain studies show reduced markers of NMDA receptor subunits, parvalbumin interneurons, and dendritic spines in cortical regions.
- Magnetic resonance spectroscopy studies have shown altered glutamate concentrations in several brain regions in schizophrenia, particularly in the early phase of illness.
- Anti-NMDA receptor encephalitis, an autoimmune disease in which antibodies attack NMDA receptors, often produces a syndrome that initially looks like schizophrenia.
What it does not yet do well
The glutamate hypothesis remains, in important respects, harder to translate into treatment than dopamine. Several efforts to develop drugs that boost NMDA function — glycine, D-serine, sarcosine, and a much-watched mGluR2/3 agonist called pomaglumetad — have produced mixed or disappointing trial results. The biology is more complex than the dopamine system: glutamate is everywhere, NMDA receptors come in multiple subtypes, and direct stimulation can be neurotoxic. Researchers are still working out which subpopulation of NMDA receptors needs to be modulated, in which brain region, and at what stage of illness.
Where this is heading
Several promising directions sit in current research:
- Targeting GluN2C/GluN2D NMDA subunits, which are enriched on parvalbumin interneurons and may avoid the neurotoxic risks of broad NMDA enhancement.
- Ketamine analogues that paradoxically improve depression at sub-anaesthetic doses and might shed light on glutamate plasticity.
- Combining glutamate-modulating drugs with antipsychotics to address negative and cognitive symptoms.
- Early intervention research exploring whether NMDA-related changes are present before the first psychotic episode.
Why it matters for patients
For now, the glutamate hypothesis does not directly change anyone's treatment plan — antipsychotics remain dopamine-focused. But it changes the conversation. It means schizophrenia is increasingly understood as a disorder of cortical circuit timing and synaptic regulation, with dopamine excess as one prominent symptom of that deeper problem. It also explains why ketamine, when used in psychiatric settings for treatment-resistant depression, has to be approached with extra care in people with schizophrenia or strong family history.
Most importantly, the glutamate model is the reason there is realistic hope, for the first time in decades, that drugs with genuinely new mechanisms will eventually emerge. The 2024 approval of xanomeline-trospium, which works through cholinergic-glutamatergic interactions rather than direct dopamine blockade, is a small but encouraging early sign.
This article is for educational purposes only and is not medical advice, diagnosis, or treatment. Always consult a qualified mental health professional. If you or someone you know is in crisis, call or text 988 in the US, or your local emergency number.