EPR Imaging of Brain O2 in Drug Abuse

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DESCRIPTION (provided by applicant): Molecular oxygen plays a central role in the control of brain physiology, if for no other reason than O2 is the terminal electron acceptor for enzymes that are crucial to the biosynthesis of the important neurotransmitters: dopamine, serotonin and norepinephrine. And, thus changes in O2 levels directly impact homeostasis. Surprisingly, given the significance of O2 in brain physiology, the quantitation of O2 in the brain and changes thereof as the result of drug abuse, such as methamphetamine, with different imaging modalities has not been achieved. Molecular oxygen, being paramagnetic, broadens the EPR spectral lines of other paramagnetic species, such as nitroxides. Measured changes in EPR spectral line width of these compounds have been used to estimate O2 concentrations in homogenous solutions. With development of low-frequency EPR spectroscopy with imaging capability and the appropriate spin probes; it is feasible to reliably estimate local O2 concentrations in vivo, in situ and in real time. An obstacle in the development of minimally invasive EPR imaging to quantitate O2 in the brain is the difficulty of transporting O2-sensitive probes to this organ. Herein, we propose to synthesize O2-sensitive spin probes, e.g., nitroxides, that can be delivered to the brain and distributed throughout, which can be used to map and quantitate O2 in this tissue. Based on a series of pilot studies, we demonstrated the feasibility of transporting a nitroxide across a mouse blood-brain barrier and into the brain. The current proposal will focus on a proof-of-principle with methamphetamine, an increasingly abused stimulant that is also leading to an increase in unsafe sexual practices. Methamphetamine is known to lead to neurotoxicity on chronic administration, and that the toxicity involves free radicals and O2 reduction species.6-9 Thus, the Objective of this grant is to develop nitroxides and EPR as an imaging modality to map and quantitate O2 in mouse brain and to study the effects methamphetamine has on O2 levels therein, thereby allowing a complimentary method of studying its mechanism of action and neurotoxicity. To accomplish the Objective of this grant, three Specific Aims will be addressed. 1. Synthesize nitroxides specifically designed that will allow their transport across the blood-brain barrier into the brain. 2. Determine the pharmacokinetic and pharmacodynamic profiles of compounds synthesized in Specific Aim #1, determining what structural features on the nitroxyl ring promote high concentrations into brain, sufficient to image and quantitate O2 by low-frequency EPR spectroscopy. 3. Obtain changes in pO2 distribution in mouse brain acutely treated with methamphetamine, using the optimized parameters from Specific Aim #1 and #2. Molecular oxygen plays a central role in the control of brain physiology, as it crucial to the biosynthesis of the important neurotransmitters: dopamine, serotonin and norepinephrine. This grant is designed to develop EPR imaging as a tool to study the effects of drugs of abuse on brain oxygen use, which may be critical to understanding brain dysfunction in drug abusing humans.