In vivo chemical monitoring using capillary separations

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Our overall objective is to develop new bioanalytical methods for exploring brain chemistry dynamics in vivo. Neurons and glia in the brain communicate by releasing neurotransmitters that interact with receptors on neighboring cells. Monitoring the concentration dynamics of neurochemicals and metabolites in vivo is a vital tool in the effort to understand brain function, diseases, and treatments. A versatile and effective approach for in vivo monitoring of chemical messages is to couple sampling methods, such as microdialysis, to analytical measurements. Although this approach has proven invaluable, its utility is limited by poor temporal resolution, poor spatial resolution, poor results for neuropeptide monitoring, and application to only acute measurements. In this project, we will develop technology and methods to solve these problems. Temporal resolution is important because concentrations of transmitters can change rapidly during behavior and experimental maneuvers. Temporal resolution is often limited by dispersion of concentration pulses as they are transported to the analytical system. We will develop a microfluidic sampling system whereby the aqueous sample stream is segmented into droplets within a stream of oil and the droplets subsequently analyzed by rapid chip-based electrophoresis assays. Sample stream segmentation will prevent dispersion during mass transport and allow temporal resolution of 10 s or better for many neurotransmitters. This system will be coupled to miniaturized sampling probes to improve spatial resolution and allow access to small brain regions. Neuropeptides regulate many brain functions; however, monitoring them in vivo is limited by the sensitivity of current methods so that samples must be collected for ~30 min resulting in poor temporal resolution. We will develop high sensitivity neuropeptide assays based on capillary liquid chromatography and microfluidic immunoassays. The assays will have detection limits of 1 pM for 1 µL samples allowing an unprecedented 10-fold improvement in temporal resolution for neuropeptide monitoring. In vivo chemical measurements are nearly always performed acutely; however, it would be extremely useful to be able to monitor neurochemistry over a period of weeks to monitor progressive changes associated with diseases, like addiction, or normal function, like learning. Long term monitoring is typically prevented by reactive gliosis, a tissue reaction that results in encapsulation of the probe and prevents sampling from active neural tissue. We will explore the use of pharmacological interventions with compounds known to suppress reactive gliosis and support neuroregeneration to prolong in vivo monitoring. Finally, we will perform fundamental neuroscience studies as a means of testing the methods and demonstrating their utility to the broader neuroscience community. These applications include determining: 1) the role of leptin receptors in regulating dopamine and feeding behavior; 2) the effect of psychostimulants on opioid peptides, and 3) neurochemical differences underlying distinct behavioral phenotypes that are a model for vulnerability to drug addiction.