Dopamine and glutamate in schizophrenia: biology, symptoms and treatment

  • Full Title: Dopamine and glutamate in schizophrenia: biology, symptoms and treatment
  • Authors: Robert A McCutcheon, John H Krystal, Oliver D Howes
  • Year: 2020
  • Source URL: https://doi.org/10.1002/wps.20693

Overview

This review synthesizes the evidence for dopaminergic and glutamatergic dysfunction in schizophrenia, exploring how these two systems interact to produce the disorder’s diverse symptoms. It moves beyond single-transmitter models to describe schizophrenia as a result of complex circuit disruptions involving both dopamine (primarily in the striatum) and glutamate (primarily in cortical and hippocampal regions).

Core Thesis

Schizophrenia involves a “dual-hit” of neurotransmitter dysfunction: elevated presynaptic dopamine synthesis and release in the striatum (driving positive symptoms) and NMDA receptor hypofunction in cortical regions (driving cognitive and negative symptoms). These systems are interlinked, as cortical glutamatergic deficits can lead to striatal dopaminergic hyperactivity through disinhibition of midbrain dopamine neurons.

Key Findings

1. Striatal Dopaminergic Hyperactivity

  • What was found: Consistently elevated presynaptic dopamine synthesis capacity and release in the striatum, particularly the dorsal striatum (Hedges’ g ≈ 0.7).
  • How it was measured: PET and SPECT imaging using radiolabeled L-DOPA and D2/3 ligands.
  • Strength of evidence: Strong (Consistently replicated across multiple meta-analyses).

2. Glutamatergic Dysfunction (NMDA Hypofunction)

  • What was found: Evidence for NMDA receptor hypofunction, especially on GABAergic interneurons, leading to cortical disinhibition and impaired synchronized neuronal oscillations.
  • How it was measured: Pharmacological challenge (ketamine/PCP), post-mortem studies of receptor density/subunits, and 1H-MRS imaging.
  • Strength of evidence: Moderate to Strong (Supported by robust pharmacological models and emerging imaging data).

3. Dopamine-Glutamate Interaction

  • What was found: Cortical glutamate levels are elevated in treatment-resistant schizophrenia, suggesting a primary glutamatergic pathology in these cases. Conversely, midbrain dopamine neurons are regulated by descending glutamatergic projections, where cortical NMDA hypofunction can trigger striatal dopamine surges.
  • How it was measured: Integrated PET/MRS studies and animal models of circuit disruption.
  • Strength of evidence: Moderate (Theoretical models are well-supported, but direct human evidence of circuit interaction is still maturing).

4. Treatment Implications

  • What was found: While current antipsychotics effectively block D2 receptors, they fail to address the underlying presynaptic dopamine elevation or the glutamatergic deficits. Novel treatments targeting glutamate (e.g., GlyT1 inhibitors, mGluR2/3 agonists) are in development but have shown mixed results in clinical trials.
  • How it was argued: Review of clinical trial data and pharmacological mechanisms of first-line and experimental agents.
  • Strength of evidence: Strong (Clinical reality of treatment-resistant symptoms is well-documented).

Mechanisms Proposed

Circuit Disinhibition

The “Glutamate-Dopamine” circuit model: NMDA receptor hypofunction on cortical GABAergic interneurons leads to disinhibition of glutamatergic pyramidal neurons. These overactive neurons then over-stimulate midbrain dopamine neurons (either directly or via the striatum), causing excessive striatal dopamine release and “aberrant salience.”

Impaired Synchrony

NMDA hypofunction disrupts the precise timing of neuronal firing (gamma oscillations) required for working memory and executive function, explaining the cognitive deficits of schizophrenia.

What this paper adds

It provides the most up-to-date and comprehensive integration of the two leading chemical hypotheses of schizophrenia. It specifically highlights the regional specificity of these dysfunctions (striatal DA vs. cortical Glutamate) and the clinical relevance of treatment-resistant cases where glutamate may be the primary driver.

Limitations and Caveats

  • 1H-MRS cannot distinguish between intracellular and extracellular glutamate pools.
  • Most human studies are cross-sectional; the temporal evolution of these transmitter changes is still being mapped.
  • Significant heterogeneity exists among patients, suggesting that schizophrenia may involve multiple distinct neurochemical subtypes.

Open Questions

  • Is NMDA receptor hypofunction the primary “insult,” or is it secondary to other developmental changes?
  • Can we use neuroimaging (PET/MRS) to predict treatment response and stratify patients?
  • Why have glutamate-modulating treatments consistently failed in large-scale clinical trials despite strong preclinical support?

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