The presynaptic strength of GABA neurotransmission is partially determined by the amount of terminal GABA available for release. Terminal GABA is synthesized locally by GAD67 and GAD65 protein. Reduced expression of the transcript for GAD67, the principal synthesizing enzyme for GABA, is perhaps the most replicated pathological disturbance in schizophrenia. Reductions in GAD67 mRNA expression may lead to reduced GABA synthesis, resulting in weaker GABA neurotransmission, and hence oscillatory impairment, in schizophrenia. However, it is unknown if the deficit in GAD67 mRNA is accompanied by a comparable decrease in GAD67 protein, particularly at the major site of function (e.g. the terminal). In addition, the dependency of interneurons, particularly those in which reductions of GAD67 mRNA are proposed to occur in schizophrenia, on GAD67 versus GAD65 for terminal GABA synthesis is unknown. Therefore, we have developed a novel high-throughput fluorescence imaging methodology that allows for the accurate quantification of fluorescently-labeled puncta (putative terminals), the colocalization of different labels in the same terminal, and the quantification of fluorescence intensity in these same structures. Using this methodology, we will perform studies that will be the first to compare the level of GAD67 and GAD65 protein in the terminals of different interneurons that are relevant to schizophrenia in the non-human and human (schizophrenia and matched control subjects) primate dorsolateral prefrontal cortex (DLPFC). At the completion of the proposed studies we will know: 1) if interneuron subpopulation specific differences in terminal GAD67/GAD65 ratios are present in the non-human primate DLPFC;2) whether observed differences are conserved between non-human and human primates;3) if GAD67 is reduced in subpopulations of GABA terminals in the DLPFC of subjects with schizophrenia. This acquired knowledge will allow us to: 1) formulate hypotheses about the cell type specific consequences of the GAD67 mRNA reduction in schizophrenia;2) interpret findings in the context of the rich body of physiologic, pharmacologic, and anatomic data that exists for non-human primate;3) provide data necessary to optimize the design of future studies in postmortem human tissue.
Deficits in GABA neurotransmission associated with schizophrenia are believed to contribute to the impairments in certain cognitive functions that are core features of the illness. For the most part, current pharmacological treatments for schizophrenia are ineffective at improving cognitive function. These studies will provide much needed information about the key cells, potential pharmacological targets, involved in GABA neurotransmission and how they are affected in schizophrenia.
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