Project Summary/Abstract Bipolar disorder (BD) is a devastating condition affecting many Veterans. Lithium and valproate are the drugs of choice for the manic phase of BD. While therapeutically effective, their mechanisms of action are not fully understood, and undesirable side effects are common. Lithium inhibits brain inositol monophosphatase (IMPase), thereby depleting the available pool of free inositol, and resulting in the correction of the manic phase of BD. Inositol synthase (IPS) is a key enzyme that regulates inositol levels by generating the precursor, inositol phosphate (IP). Our preliminary data demonstrate that rat brain IPS is up-regulated by chronic lithium treatment, which would have the same net effect as inositol depletion relative to inositol phosphate. We hypothesize that lithium and other mood stabilizers exert their therapeutic effects by up-regulating IPS expression, increasing the available inositol phosphate pool. This implies that the pathophysiological mechanism underlying BD is a disturbance in maintaining a proper balance in the ratio of free inositol to inositol phosphate pools. Our long-term objective is to define how mood stabilizers work, and to find common therapeutic targets in the pathophysiology of BD so new medications can be designed. Our specific aims are: 1) Determine the changes in abundance and activity of the newly-identified isoforms of IPS in brain regions after chronic lithium and valproate; 2) Determine the effects of lithium and valproate on IPS binding to clathrin and receptor-mediated endocytosis; 3) Establish the link between inositol metabolism and behavior in IPS-deficient mice; and 4) Identify drug-targetable convergent pathways for lithium, valproate and lamotirgine responsive genes. To accomplish Aim 1, IPS and its isoforms will be quantified in drug-treated rat brain regions by enzyme activity assay, Western blot, and Q-PCR methods. To accomplish Aim 2, brain IPS and its complexes will be analyzed by Western and mass spectral analyses. Effects on receptor-mediated endocytosis will be monitored by following transferrin receptor trafficking and fluorescence resonance energy transfer (FRET) analysis. For Aim 3, lithium and valproate treated and untreated IPS knock-out mice will be subjected to standardized behavioral tests and their brains assayed for key enzymes in inositol metabolism. Aim 4 will be addressed by microarray and pathway analyses to map and compare the chronic lithium, valproate and lamotrigine transcriptomes in human neuronal cell lines and in rat primary cortical neuronal cell cultures. Defining the mechanisms of lithium action on inositol metabolism in the brain from the perspective of IPS function may establish a direct connection between inositol regulation and behavior. Thus advancing our understanding of the pathophysiology of BD and disclosing new therapeutic targets for treating BD.