PROJECT 2: CORTICAL GLUTAMATE SYNAPSE IN DEPRESSION

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Recent research provides evidence for a dysfunction of the glutamate system in major depressive disorder (MDD). Our postmortem studies demonstrate that MDD is associated with altered concentrations of proteins involved in glutamate signaling. Despite significant evidence for abnormal glutamatergic signaling in MDD, the molecular mechanisms that contribute to these abnormalities at the level of cortical glutamate synapses have not yet been characterized. Glutamate is a critical component of neuronal circuitry in the prefrontal cortex (PFC), a brain area where cellular pathology has been detected consistently in MDD. Previous cell counting studies revealed reductions in the density and size of neuronal and glial cells in the PFC in MDD (see Project 1- Rajkowska). Since both astrocytes and pyramidal neurons are involved in the uptake, metabolism, and recycling of glutamate we hypothesize that : 1) glutamate transporters, metabolic enzymes, and receptors will be altered in the prefrontal cortex in MDD, and 2) these alterations will correlate with reductions in the densities of astrocytes and pyramidal neurons. Consistent with our hypothesis we speculate that glutamatergic pathology in the PFC will be associated with decreased levels of glial and neuronal glutamate transporters, decreased expression of glutamine synthetase and up-regulation of postsynaptic glutamate receptors and their scaffolding protein (Aim 1and 2). To achieve these goals we will utilize frozen cortical sections from the same subjects as those used in previous studies of cell density. It has been proposed that deficit in astrocytes and pyramidal neurons reported in postmortem studies may be the consequence of enhanced glutamate signaling in MDD. Interestingly, in laboratory animals, extracellular glutamate levels are increased after exposure to stress and chronic restraint stress induces shrinkage in dendritic trees of glutamatergic pyramidal neurons in the PFC. Thus, we further hypothesize that chronic stress will lead to abnormalities in glutamate signaling markers in rats that may resemble the abnormalities observed in MDD in postmortem studies. Therefore, application of restraint stress model (Aim 3) will provide important insight into the possible basis for glutamatergic abnormalities reported in human postmortem studies. To determine that the biochemical changes in depression are not resulting from the possible confounding influence of antidepressant treatment, human glutamate markers (Aim 1 and 2) will be compared to identical biochemical measures conducted in the comparable brain region of non-human primates treated with either fluoxetine or its vehicle (Aim 4). The studies proposed here will improve our understanding of the cellular and molecular pathology in depression, particularly as it relates to glutamate and may lead to the discovery of novel antidepressant drug targets and improved treatment for depression.