Nature of Dopamine Dysfunction in Schizophrenia and What This Means for Treatment?

Current drug treatments for schizophrenia are inadequate for many patients, and despite 5 decades of drug discovery, all of the treatments rely on the same mechanism: dopamine D(2) receptor blockade. Understanding the pathophysiology of the disorder is thus likely to be critical to the rational development of new treatments for schizophrenia.

OBJECTIVE:
To investigate the nature of the dopaminergic dysfunction in schizophrenia using meta-analysis of in vivo studies.

STUDY SELECTION:
A total of 44 studies were identified that compared 618 patients with schizophrenia with 606 controls, using positron emission tomography or single-photon emission computed tomography to measure in vivo striatal dopaminergic function.

DATA EXTRACTION:
Demographic, clinical, and imaging variables were extracted from each study, and effect sizes were determined for the measures of dopaminergic function.
Studies were grouped into those of presynaptic function and those of dopamine transporter and receptor availability. Sensitivity analyses were conducted to explore the consistency of effects and the effect of clinical and imaging variables.

DATA SYNTHESIS:
- There was a highly significant elevation (P < .001) in presynaptic dopaminergic function in schizophrenia with a large effect size (Cohen d = 0.79).
- There was no evidence of alterations in dopamine transporter availability.
- There was a small elevation in D(2/3) receptor availability (Cohen d = 0.26), but this was not evident in drug-naive patients and was influenced by the imaging approach used.

CONCLUSIONS:
- The locus of the largest dopaminergic abnormality in schizophrenia is presynaptic, which affects dopamine synthesis capacity, baseline synaptic dopamine levels, and dopamine release.
- Current drug treatments, which primarily act at D(2/3) receptors, fail to target these abnormalities. Future drug development should focus on the control of presynaptic dopamine synthesis and release capacity.





Important Facts from this Article:
- These studies were focused on Striatum , so future work needs to evaluate extrastriatal dopaminergic system.
- Drugs like Reserpine and alpha- methylparatyrosine depletes the stores of presynaptic dopamine, and are associated with rapid & profound reduction in psychotic symptoms.
- Dopamine and Nor-Epinephrine shares the same synthetic pathway (check the diagram below), so treatments affecting dopamine will also risk affecting NE leading to undesirable adverse events. Future studies should focus on this issue.
- Treatment resistant patients shows normal normal dopamine synthesis capacity.