Lithium (Li) is the treatment of choice for acute mania and for relapse prevention in bipolar disorder. Despite its widespread clinical use and extensive research into its mechanisms of action, there is little agreement about how Li provides therapeutic benefit. Li treatment is usually monitored by measuring the serum concentration. Serum concentrations are relatively easy and inexpensive to measure, but treatment failures in the therapeutic range are common and neurotoxicity is a constant concern. Measurement of brain Li concentration should better reflect its biochemical availability, clinical therapy, and toxicity and help identify Li's mechanisms of therapeutic action and neurotoxicity. In vivo 7Li magnetic resonance spectroscopy (MRS) is a noninvasive method for measuring directly the brain concentration of Li. But measurement of the overall brain Li concentration is only a first step. Little is known about the environment of Li in the brain, particularly its intracellular-to-extracellular (i/e) ratio. Because most biochemical processes occur within the cell, it is often presumed that Li exerts its clinically relevant effects in the intracellular space of neurons in the brain. Intra- and extracellular Li cannot normally be distinguished by 7Li MRS. But localized transverse (T2) spin relaxation measurements can distinguish intra- and extracellular Li in vivo by observing biexponential relaxation curves, where the two components represent cations with different T2s in the intra- and extracellular environments. We hypothesize that the intracellular Li T2 in vivo is significantly lower than the extracellular Li T2;measurement of the 7Li T2 decay in vivo will result in biexponential behavior due to compartmentation and permit the determination of the Li i/e ratio. Our Specific Aims are to: 1) use localized 7Li MRS measurements of T2 to estimate the ratio of intra- to extracellular Li in rat brain in vivo directly from the observed biexponential behavior;2) confirm the identification of intracellular and extracellular signals by measuring the separate intra- and extracellular 7Li signals after infusion of mannitol to disrupt the blood-brain barrier and a thulium-based compound to separate the two signals. The results of the proposed study will provide the first in vivo measurements of the i/e ratio of Li in the brain. The long-term goal is to measure the i/e ratio of Li in human brain. The results of this study will supply data for demonstrating proof-of-principle for using an approximate interpolation method that could be extended to estimate the i/e ratio of Li in human brain. Knowledge of the i/e ratio of Li in human brain will provide insight into Li's mechanism(s) of action in bipolar disorder and potential causes of neurotoxicity. PUBLIC HEALTH RELEVANCE: Lithium (Li) is an important treatment for bipolar disorder, a devastating mental illness that occurs in about 3% of the population. Understanding how lithium works biochemically can lead to more effective treatment. This project studies lithium in brain cells to understand better how it works.