Ex Vivo Assay for In Situ Brain-Wide Mapping of Glutamate/GABA Metabolism

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DESCRIPTION (provided by applicant): Evidence of altered brain glutamatergic and GABAergic function is reported in a wide array of psychiatric and neurological disorders. Most current treatments for neuropsychiatric illness target the monoamine systems and have limited efficacy. The acknowledgement of this fact has led to an increased drive to develop novel drugs acting through alternative mechanisms. There is now intense focus on the amino acid neurotransmitter systems (glutamate/GABA/glutamine) as targets for treatment, creating the need to identify reliable biomarker assays. In vitro cell culture and brain slice preparations often fail to predict in vivo responses to glutamate- modulating drugs in humans or unanesthetized animals. There is a pressing need for quantitative assays of glutamate/GABA neurotransmission that reflect the in vivo physiological state, avoids anesthesia or postmortem effects, and can be translated more directly to humans. Recently, a novel ex vivo 3D in situ magnetic resonance spectroscopic imaging (MRSI) approach was introduced, which generates high spatial resolution quantitative maps of numerous neurochemicals from the brain's of rodents euthanized by microwave irradiation, preserving neurochemical levels and microstructure. Applied with 13C labeled tracers, high-spatial resolution 2D and 3D maps of 13C-labeled amino acids can be generated. Appropriately validated, rate maps reflecting neuronal (glutamatergic and GABAergic) and astroglial metabolism, and neurotransmitter cycling can be extracted from the data sets. Combined with other neuroimaging modalities (e.g., T1, T2 diffusion-tensor), quantitative measurements of high information content of multiple endpoints for metabolism, structure, and connectivity can be obtained. The addition of other '-omics' end-point measurements are also possible. With all neuroimaging efficiently acquired in the same brain and coordinate space, the proposed assay has significant potential to reveal altered glutamatergic/GABAergic neuronal and glial pathways, accelerating preclinical drug evaluation and treatment response. Development of this methodology would afford investigators opportunity to obtain thousands of precisely defined neurochemical and anatomical data points in a single experiment, in contrast to present methods (e.g., cell-free extracts or tissue slices) which access only one or a few regions at a time. Aim 1 will develop and validate the ex vivo metabolic flux mapping assay against 13C fractional enrichment measured in cell-free extracts, and evaluate the accuracy of the ex vivo flux measurements against in vivo MRS time courses. A double-labeling approach to increase reliability and efficiency will also be evaluated. Aim 2 will develop automated metabolite quantification and analysis methods for the ex vivo 3D MRSI/MRI high density data sets for efficient and unbiased extraction of available information, enhanced data quality, and greatly increased throughput. Aim 3 will apply the ex vivo flux mapping assay to characterize and compare the acute effects of glutamate modulating drugs with a broad range of potential therapeutic effects, on regional rates of glutamate/GABA neurotransmission with receptor activated signaling (phosphoproteins).