Research report
Gene expression of metabotropic glutamate receptor 5 and excitatory amino acid transporter 2 in the schizophrenic hippocampus

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Abstract

A disturbance in glutamatergic transmission has been suggested to contribute to the pathophysiology of schizophrenia and recent studies on ionotropic glutamate receptors are consistent with altered glutamatergic function in the hippocampus of schizophrenics. In order to investigate this hypothesis further, the expression of two ‘glutamatergic’ markers, the mRNAs of metabotropic glutamate receptor 5 (mGluR5) and human excitatory amino acid transporter (EAAT2) were compared in the hippocampus of control subjects and schizophrenics. We examined the regional/cellular mRNA expression of mGluR5 and EAAT2 in postmortem hippocampal sections from schizophrenics and control subjects, using in situ hybridization. Regions of interests were dentate gyrus, cornu ammonis 4, 3, 1 and parahippocampal gyrus. The regional/cellular mGluR5 mRNA content was not different between the two groups. The cellular EAAT2 mRNA content was significantly decreased in schizophrenic parahippocampal gyrus, but not in other hippocampal regions. Furthermore, only in the parahippocampal gyrus, schizophrenics had a significantly increased mGluR5/EAAT2 ratio at both the regional and cellular mRNA level. The results suggest that a disturbance of glutamatergic neurotransmission in schizophrenia was not apparent using these indices in the hippocampus, but ‘hypo-glutamatergic’ neurotransmission may be present in the schizophrenic parahippocampal gyrus.

Introduction

In recent years, the possible involvement of disturbances in glutamatergic function in the pathophysiology of schizophrenia has been discussed and reviewed [11], [23], [34]. The evidence implicating glutamate includes the ability of phencyclidine, an antagonist of the N-methyl d-aspartate (NMDA) glutamate receptor to induces schizophrenic-like symptoms [15], [16], and the observation that glycine, an agonist at the modulatory site of the NMDA receptor improves some symptoms in some schizophrenic patients [17], [30], [39]. The prefrontal cortex is the one of the brain regions of interest where research into pathophysiology of schizophrenia has been focused, and many studies have been carried out to attempt to verify what might be loosely termed a glutamate hypothesis of schizophrenia [23], [34]. Reports of increased NMDA and kainic acid (KA) receptor binding [9], [22], [26], [35] and decreased excitatory amino acid transporter mRNA [27] in the prefrontal cortex of schizophrenic individuals were suggested to result from compensatory up-regulation of receptor expression and down-regulation of transporter expression perhaps reflecting reduced glutamatergic function.

In contrast, in the hippocampus (HP) which has also been a focus for much research into the possible pathophysiology of schizophrenia, where morphological and cytoarchitectural abnormalities of HP have been reported [4], [5], [29], [31], [38]. Studies have also looked at glutamatergic markers on HP tissue from schizophrenic cases. Contrary to observation on the prefrontal cortex, decreased α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor binding and KA receptor binding [19], [20], decreased mRNA expression for some non-NMDA receptor [7], [10], [13] and their immuno-reactivity [6] were also reported. These results by analogy with those on the prefrontal cortex have been interpreted as reflecting compensatory down-regulatory changes caused by hyper-glutamatergic function in the HP of individuals with schizophrenia. In a previous study, we investigated the expression of metabotropic glutamate receptors (mGluR3 and 5) [3], [25], [33] and the excitatory amino acid transporter 2 (EAAT2) [1] in the prefrontal cortex of schizophrenics, and obtained evidence which would at least be potentially consistent with reduced-glutamatergic function noting particularly that comparison of the ratios of glutamatergic markers, for example the mGluRs to EAAT2 mRNA ratio, might be more significant than comparison of each individual mRNA marker in reflecting a general change in glutamatergic ‘tone’ [27]. In order to investigate the possible role of glutamate further and given our observation on the prefrontal cortex, it was of considerable interest to extend these studies to the HP of the same cases. Thus in the present study, we performed radioactive in situ hybridization on consecutive HP sections using mGluR5 and EAAT2 deoxyoligonucleotide probes and compared the content of each of these mRNAs in normal and schizophrenic HP. After this, the regional/cellular mRNA content of mGluR5 and EAAT2 and their ratio (mGluR5/EAAT2) were compared between groups, and the results discussed in relation to glutamatergic function in the HP of individuals with schizophrenia.

Section snippets

Human postmortem tissue

Subjects were from the Cambridge Brain Bank laboratory (Cambridge, UK) and the New Zealand Neurological Foundation Brain Bank (Auckland, New Zealand). The same five schizophrenic postmortem brains as our previous study of glutamatergic [27] and GABAergic transmission [28] and same six control brains with previous study of dopamine receptor [12] were used. The schizophrenics were from chronically hospitalized schizophrenic patients and diagnosed premortem according to DSM-III-R by PJM. Except

Regional and cellular gene expression of mGluR5 and EAAT2 in human HP

On film autoradiograms of control brains, the abundant β-actin mRNA signals were detected in DG and CA4-1, and strong signals were also recognized in PHG (Fig. 1A). The strong hybridization signals for mGluR5 were detected in DG and CA4-1 of Ammon’s horn. Discernible hybridization signals for mGluR5 were also detected in the subiculum and PHG, and distributed homogeneously through the cortical layer (Fig. 1B). The strong EAAT2 signal was detected in DG and CA4-1 of Ammon’s horn. Relatively

Discussion

In the present study, we studied the expression of two markers of glutamatergic function in the HP of schizophrenics, namely the mRNAs level of mGluR5 and EAAT2. The present results at the regional/cellular level can be compared with recent investigations on the expression of ionotropic glutamate receptor in schizophrenia which have reported, decreased AMPA/KA receptor binding [19], [20], decreases in some non-NMDA ionotropic receptor mRNA levels [7], [10], [13], and decreased AMPA/KA

Acknowledgements

This work was funded by grants from The Theodore and Vada Stanley Foundation (USA). The assistance of Mr. R. Hills (MRC Cambridge Brain Bank) and Mrs. K. Westmore are gratefully acknowledged.

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