The glutamate (Glu) and gamma aminobutyric acid (GABA) hypotheses of schizophrenia were proposed in the 1980s. However, current findings on those metabolite levels in schizophrenia have been inconsistent, and the relationship between their abnormalities and the pathophysiology of schizophrenia remains unclear. To summarize the nature of the alterations of glutamatergic and GABAergic systems in schizophrenia, we conducted meta-analyses of proton magnetic resonance spectroscopy (1H-MRS) studies examining these metabolite levels.
A systematic literature search was conducted using Embase, Medline, PsycINFO, and PubMed. Original studies that compared four metabolite levels (Glu, glutamine [Gln], Glx [Glu+Gln], and GABA), as measured by 1H-MRS, between individuals at high risk for psychosis, patients with first-episode psychosis, or patients with schizophrenia and healthy controls (HC) were included. A random-effects model was used to calculate the effect sizes for group differences in these metabolite levels of 18 regions of interest between the whole group or schizophrenia group and HC. Subgroup analysis and meta-regression were performed based on the status of antipsychotic treatment, illness stage, treatment resistance, and magnetic field strength.
One-hundred-thirty-four studies met the eligibility criteria, totaling 7993 participants with SZ-spectrum disorders and 8744 HC. 14 out of 18 ROIs had enough numbers of studies to examine the group difference in the metabolite levels. In the whole group, Glx levels in the basal ganglia (g = 0.32; 95% CIs: 0.18–0.45) were elevated. Subgroup analyses showed elevated Glx levels in the hippocampus (g = 0.47; 95% CIs: 0.21–0.73) and dorsolateral prefrontal cortex (g = 0.25; 95% CIs: 0.05–0.44) in unmedicated patients than HC. GABA levels in the MCC were decreased in the first-episode psychosis group compared with HC (g = −0.40; 95% CIs: −0.62 to −0.17). Treatment-resistant schizophrenia (TRS) group had elevated Glx and Glu levels in the MCC (Glx: g = 0.7; 95% CIs: 0.38–1.01; Glu: g = 0.63; 95% CIs: 0.31–0.94) while MCC Glu levels were decreased in the patient group except TRS (g = −0.17; 95% CIs: −0.33 to −0.01).
Increased glutamatergic metabolite levels and reduced GABA levels indicate that the disruption of excitatory/inhibitory balance may be related to the pathophysiology of schizophrenia-spectrum disorders.