Decreased glutamic acid decarboxylase67 mRNA expression in multiple brain areas of patients with schizophrenia and mood disorders

https://doi.org/10.1016/j.jpsychires.2009.02.005Get rights and content

Abstract

Reduced levels of glutamic acid decarboxylase67 (GAD67), an essential enzyme for GABA synthesis, is one of the most consistent gene expression changes found in the frontal cortex of patients with schizophrenia. Recently this reduction has been shown to extend to other areas including primary sensory, primary motor and anterior cingulate (ACC) cortices. To determine the extent to which additional cortical and subcortical regions may be affected in schizophrenia, we measured the level of GAD67 mRNA in previously unexplored areas including the orbitofrontal (OFC) and superior temporal (STG) cortices as well as the caudate, putamen, nucleus accumbens, medial dorsal thalamus and anterior thalamus using in situ hybridization. We also examined GAD67 mRNA levels in all these regions in individuals with bipolar disorder and major depression. ANCOVA comparing GAD67 mRNA levels in all four diagnostic groups revealed a significant reduction (∼30%) in layers III and IV of the OFC of patients with schizophrenia and bipolar disorder. A priori t-tests comparing GAD67 mRNA levels between the schizophrenia and control groups revealed significant reductions in the ACC, STG, striatum and thalamus. These findings suggest that there may be a widespread reduction in GABA neurotransmission due to a decrease in the synthesis of GAD67 in subjects with psychiatric disorders. The resulting decrease in inhibitory tone across multiple brain areas may contribute to the psychotic behavior observed in patients with schizophrenia and bipolar disorder.

Introduction

Schizophrenia is a disabling psychiatric disorder characterized by social withdrawal, hallucinations, working memory impairments, and attentional deficits (Harrison, 1999, Hirsch and Weinberger, 1995). Due to these diverse symptoms, multiple brain areas have been implicated in the disorder, particularly associative areas of neocortex, such as the dorsolateral prefrontal cortex (DLPFC), orbitofrontal cortex (OFC), anterior cingulate cortex (ACC) and superior temporal gyrus (STG). Structural abnormalities seen in subcortical regions, such as the striatum and thalamus which communicate and integrate signals across cortical domains, may also contribute to the aberrant processing detected in schizophrenia (Harms et al., 2007, Mamah et al., 2007). Convergent evidence from several studies suggest that abnormalities in cortical inhibition, specifically involving inhibitory interneurons containing γ-aminobutyric acid (GABA) (Benes and Berretta, 2001, Benes et al., 1991, Lewis et al., 2005, Wassef et al., 2003) exist in the brains of patients with schizophrenia. Numerous studies have documented abnormalities affecting these cortical GABAergic neurons particularly within the DLPFC (Akbarian et al., 1995, Guidotti et al., 2000, Hashimoto et al., 2008a, Ohnuma et al., 1999, Woo et al., 2008), ACC (Benes et al., 2001, Cotter et al., 2002, Hashimoto et al., 2008b, Woo et al., 2004) and hippocampus (Heckers et al., 2002, Reynolds et al., 1990, Zhang et al., 2002) of subjects with schizophrenia and related disorders.

Regulation of GABA neurotransmission is partly due to its rate-limiting synthesizing enzyme glutamate decarboxylase (GAD), which synthesizes GABA from glutamate (Akbarian and Huang, 2006). GAD exists as 2 separate isoforms, GAD65 and GAD67, encoded by two independent genes located on chromosomes 2 and 10, respectively, which differ in their subcellular distribution and biological properties (Bu et al., 1992, Soghomonian and Martin, 1998). Mice mutant for either GAD isoform demonstrate that each plays a role in specific aspects of GABA neurotransmission. GAD65 is associated with GABA packaging and release due to its localization to axon terminals and synaptic vesicle membranes, whereas GAD67 is associated with GABA synthesis and non-vesicular associated release due to its presence in the cytosol (Kaufman et al., 1991, Soghomonian and Martin, 1998, Tian et al., 1999). Of the two isoforms GAD67 is the more interesting with respect to schizophrenia and other mental illnesses because of its ability to be regulated by environmental stimuli (Benson et al., 1994, Soghomonian and Martin, 1998), its consistent alteration in subjects with mental disorders (Akbarian and Huang, 2006, Akbarian et al., 1995, Dracheva et al., 2004, Fatemi et al., 2005, Guidotti et al., 2000, Hashimoto et al., 2003, Heckers et al., 2002, Knable et al., 2002, Torrey et al., 2005, Volk et al., 2000, Woo et al., 2004), and its genetic link to schizophrenia (Addington et al., 2005, Straub et al., 2007).

In this study, we investigated whether the reductions in GAD67 mRNA previously described in schizophrenia extend beyond the DLPFC, ACC, primary sensory and primary motor cortex to other integrative regions of the telencephalon such as the OFC, STG, thalamus, and striatum. We also inquired as to whether the reductions in GAD67 mRNA were diagnostically specific by measuring GAD67 mRNA levels in patients with bipolar disorder and major depression as well as in those with schizophrenia. An understanding of the regional, laminar and diagnostic specificity of GAD67 deficits in schizophrenia and other psychiatric disorders may be a useful guide when considering future therapeutic strategies aimed at the inhibitory system.

Section snippets

Brain cohort

Frozen, 14 μm thick coronal sections from the OFC, ACC, and STG were obtained from the Stanley Foundation Neuropathology Consortium (SFNC; Fig. 1). The OFC sections were from Brodmann area (BA) 45 of the orbital gyrus, the ACC sections were from BA24 of the cingulate gyrus at the level of the genu of the corpus callosum and the STG sections were from BA22 at the level of the geniculate bodies. Coronal sections of the striatum, at the level of the nucleus accumbens (NA) and containing the head of

Orbitofrontal cortex

All subjects showed robust but punctate GAD67 mRNA expression throughout all layers of the OFC (Fig. 1A). The punctate (or spotted) pattern of GAD67 mRNA label seen at the level of autoradiographic film reflected the fact that many neurons were heavily labeled with GAD67 mRNA, but many neurons did not appear to contain GAD-67 mRNA (Fig. S2). Two dark bands of increased GAD67 mRNA expression, one superficial and one deep, were apparent on the films. Subjects with mental illness showed a 30–50%

Discussion

We show that the reduction in GAD67 mRNA is not specific to schizophrenia but can also be detected in patients with bipolar disorder and major depression. This study shows for the first time that GAD67 mRNA levels are reduced in the OFC in patients with schizophrenia, and are also reduced in patients with bipolar disorder and major depression. These findings support and extend results from previous studies that consistently show a decrease of GAD67 mRNA expression in neocortical areas and in

Disclosure/conflicts of interest

The authors declare that, except for income received from my primary employer, no financial support or compensation has been received from any individual or corporate entity over the past three years for research or professional service and there are no personal financial holdings that could be perceived as constituting a potential conflict of interest.

Contributors

Author Mia Thompson performed the statistical analysis and wrote the manuscript. Authors Cynthia Shannon Weickert and Maree Webster designed the study and contributed to composing the manuscript. Author Eugene Wyatt performed the study, quantified the data and performed the preliminary statistical analysis of the data. In addition, all authors contributed to and have approved the final manuscript.

Role of funding source

Funding for this study was provided by the Stanley Medical Research Institute (SMRI); the SMRI had no further role in study design; in the collection, analysis and interpretation of data; in the writing of the report; and in the decision to submit the paper for publication.

Acknowledgements

The authors thank Laura Zwolinski for her technical expertise and the staff of the Stanley Medical Research Institute (SMRI) Laboratory of Brain Research for their assistance. The research was funded by SMRI.

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