ReviewIn vivo neurochemical monitoring by microdialysis and capillary separations
Introduction
One of the greatest challenges facing science is understanding the brain, how it controls behavior, how it malfunctions in mental illness and how such malfunctions might be corrected. As the brain is composed of neurons and glial cells that communicate via chemicals released into the extracellular space, it is of interest to monitor these chemical messengers in vivo. This is a daunting task as over 100 compounds including amino acids, simple amines and peptides have been identified as neurotransmitters and their levels can change rapidly in localized brain regions. Methods that enable in vivo neurotransmitter monitoring include positron emission tomography, sensors and sampling methods such as microdialysis. These methods have been influential in brain research because they enable identification of intracranial chemical changes associated with behavior and facilitate discovery of targets for pharmacological interventions. This article provides an overview of in vivo neurochemical monitoring using microdialysis and reviews recent advances based on coupling high sensitivity techniques such as capillary electrophoresis (CE) and capillary liquid chromatography (LC) to microdialysis probes.
Section snippets
Examples of microdialysis studies
Microdialysis is one of the most popular approaches for studying in vivo brain chemistry because of its convenience and high informing power. In microdialysis, a semi-permeable membrane, typically 1–4 mm long and 0.2–0.4 mm in diameter, is fashioned into a probe such that the interior of the probe can be perfused at 0.1 to 2 μl/min. When the probe is implanted into the brain, analytes present in the extracellular space diffuse to the probe according to their concentration gradient and become
Microdialysis with capillary electrophoresis
CE has been recognized as an attractive analytical method to couple to microdialysis mainly because of the potential for improved temporal resolution. Temporal resolution is important as neurochemical levels and behaviors can change rapidly. Indeed, the dopamine study discussed above was unable to distinguish among some roles for dopamine because temporal resolution was limited to 5 min. With microdialysis, temporal resolution is usually limited by mass sensitivity of the analytical method
CE–LIF coupled on-line to microdialysis
Although CE–LIF has the sensitivity to monitor analytes in nanoliter fractions, routine use of such small fractions is problematic because of the tremendous throughput needed to analyze the samples. Consider that at 6 s fraction collection intervals, 600 samples would be generated during 1 h of experimentation. One way to avoid this problem is to take advantage of the automatability and high-speed capability of CE to perform on-line measurements.
Successful design of an on-line method requires
Microdialysis with capillary LC–MS2: peptides, neuroproteomics and more
One of the most challenging problems in neurochemical monitoring in vivo is detecting neuropeptides. The difficulty stems from the low in vivo concentration of peptides (1–100 pM), which is ∼106-fold lower than amino acid neurotransmitters. Thus, the detection method for peptides must be sensitive enough to quantify attomolar levels in microliter sample volumes.
Analytical methods developed to address this challenge include LC–radioimmunoassay [20], capillary LC–electrochemistry [21], CE [22]
Conclusion
In vivo neurochemical monitoring is a fertile field for research. Continued advances in analytical techniques have enabled significant improvements in temporal resolution, spatial resolution, multi-analyte capability and compound discovery. The present state of development of these methods is such that they require a substantial investment in training and novel instrumentation to be implemented for neuroscience applications. An important goal over the next few years will be to develop the
Acknowledgements
Our work in this area was supported by NIH NS38476.
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest
•• of outstanding interest
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