The nature of dopamine dysfunction in schizophrenia and what this means for treatment

Abstract

Context: 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.

Data sources: The MEDLINE, EMBASE, and PsycINFO databases were searched for studies from January 1, 1960, to July 1, 2011.

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.

Figures

Figure 1

Studies of presynaptic dopaminergic function: Forrest plot showing the effect size and 95% confidence intervals of the difference between patients with schizophrenia and controls by study. There was evidence of a significant elevation in schizophrenia with a summary effect size of d=0.79.

Figure 2

Studies of presynaptic dopaminergic function: the effect sizes for studies by antipsychotic treatment history (in the boxplot the band is the median and the whiskers indicate the lowest and highest data points that are within 1.5 * the interquartile range, and data outside this range (circles if present) are regarded as potential outliers)

Figure 3

Studies of dopamine transporter availability: Forrest plot showing the effect size and 95% confidence intervals of the effect sizes by study. The 95% confidence interval for the summary effect size (lozenge, d=0.34) includes 0, indicating no significant difference between patients with schizophrenia and controls.

Figure 4

Studies of D2/3 receptor availability: Forrest plot showing the effect size and 95% confidence intervals of the effect sizes by study. There was evidence of a small increase in D2 receptor availability in schizophrenia with a summary effect size (lozenge) of d=0.26.

Figure 5

Schematic diagram summarising the findings from our meta-analyses of dopamine imaging findings in schizophrenia showing that the major abnormality is increased presynaptic dopamine synthesis capacity and release (not shown to scale).