Monitoring Hydrogen Peroxide in the Extracellular Space of the Brain with Amperometric Microsensors

Interest in the detection of hydrogen peroxide in living brain tissue is growing for several reasons. Peroxide and other reactive oxygen species are implicated in neurodegenerative disorders and appear to have neuromodulatory functions in the brain. Also, there is a need to measure peroxide levels as a companion to measurements with amperometric sensors that rely on enzymes to generate peroxide for the detection of glutamate, choline, and glucose. Herein, we report on measurements performed in the brain of anesthetized rats with carbon fiber amperometric sensors coated with a cross-linked redox polymer film that contains horseradish peroxidase. Prior work with these sensors has established that they are both sensitive and selective toward hydrogen peroxide. When implanted in the striatal region of the rat brain, a biphasic response is observed upon electrical stimulation of the dopaminergic pathway that innervates the striatal tissue. No response is observed at sensors lacking HRP, which are not sensitive to peroxide, suggesting that the biphasic response is due to the production of hydrogen peroxide by two separate mechanisms. Additional measurements of dopamine and oxygen, and the administration of two drugs with well-known effects on the biochemical kinetics of the dopamine neurons, are used to identify those mechanisms. One appears to be the production of peroxide upon the oxidation of dopamine by molecular oxygen. This occurs during the electrical stimulation itself, which elevates both dopamine and oxygen levels in the extracellular space. The other appears to be the production of peroxide as a byproduct in the oxidative metabolic conversion of dopamine to DOPAC by the mitochondrial enzyme, monoamine oxidase. The production of peroxide due to dopamine metabolism is also observed after rats receive a dose of L-DOPA, a drug used in the treatment of Parkinsonʹs disease.