While the treatment of schizophrenia with antipsychotic medications revolutionized the clinical management of this illness, approximately one-third of patients with schizophrenia have persistent positive symptoms despite multiple trials of antipsychotic medicines. Recently, new strategies for the treatment of schizophrenia have emerged, including modulation of glutamate receptors, an approach which was developed, in part, based on an accumulating body of evidence of alterations in glutamate transmission from postmortem, imaging, and preclinical studies. While the initial glutamate hypothesis of schizophrenia was focused on NMDA receptor dysfunction, this hypothesis has been extended to include other glutamate receptors, transporters, and enzymes involved in glutamate transmission. Postmortem findings of changes in the expression of gluta- matergic molecules in schizophrenia may be conceptualized as functional alterations of remodeled glutamate synapses, secondary to the underlying pathophysiology of chronic severe mental illness and a lifetime of treatment with psychotropic medications. We have found decreased expression of glial glutamate transporters in this illness, suggesting that glutamate synapses have alterations in glutamate reuptake capacity. Since glutamate transporters facilitate excitatory neurotransmission by limiting glutamate spillover to adjacent synapses, we postulate that the localization of excitatory amino acid transporters (EAATs) is altered in the prefrontal cortex (PFC) in schizophrenia, and may contribute to psychopathology in this illness. Specifically, we hypothesize that perisynaptic localization of EAATs with asymmetric synapses, which are characteristic of excitatory glutamate transmission, is decreased in schizophrenia. To evaluate this hypothesis, we will assess the ultrastructural localization of EAAT isoforms using electron microscopy in postmortem tissue from subjects with schizophrenia. Our studies will focus on the middle layers of the dorsal lateral prefrontal and anterior cingulate cortices, regions with dense reciprocal thalamic innervation that are implicated in the pathophysiology of this illness. These studies will link identified changes in gene expression in the PFC in schizophrenia with circuit specific alterations in glutamate synapse composition and function. We also plan to assess the effects of chronic typical and atypical antipsychotic treatment on ultrastructural localization of glutamate transporters in the rat PFC. These rodent studies will provide novel data on the effects of chronic antipsychotic treatment on the composition of excitatory synapses, and compliment the interpretation of our postmortem findings, since most of these subjects were treated with antipsychotics. At the conclusion of this set of experiments, we will have tested the hypothesis that perisynaptic localization of glutamate transporters with asymmetric synapses is diminished in schizophrenia, suggesting decreased perisynaptic reuptake of glutamate and increased glutamate spillover. These studies will extend the glutamate hypothesis of schizophrenia beyond the NMDA receptor and provide new substrates for diagnosis and treatment of this often devastating illness.
This project will identify the critical elements of brain function that contribute to the pathophysiology of schizophrenia. Identification of the molecular elements underlying schizophrenia will provide new targets for the development of medicines to treat this illness.
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|Shan, Dan; Mount, Daniel; Moore, Stephen et al. (2014) Abnormal partitioning of hexokinase 1 suggests disruption of a glutamate transport protein complex in schizophrenia. Schizophr Res 154:1-13|
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