In Vivo Neurochemical
Monitoring Using Benzoyl Chloride
Derivatization and Liquid Chromatography–Mass Spectrometry
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Abstract
In vivo neurochemical monitoring using microdialysis
sampling is
important in neuroscience because it allows correlation of neurotransmission
with behavior, disease state, and drug concentrations in the intact
brain. A significant limitation of current practice is that different
assays are utilized for measuring each class of neurotransmitter.
We present a high performance liquid chromatography (HPLC)–tandem
mass spectrometry method that utilizes benzoyl chloride for determination
of the most common low molecular weight neurotransmitters and metabolites.
In this method, 17 analytes were separated in 8 min. The limit of
detection was 0.03–0.2 nM for monoamine neurotransmitters,
0.05–11 nM for monoamine metabolites, 2–250 nM for amino
acids, 0.5 nM for acetylcholine, 2 nM for histamine, and 25 nM for
adenosine at sample volume of 5 μL. Relative standard deviation
for repeated analysis at concentrations expected in vivo averaged
7% (<i>n</i> = 3). Commercially available <sup>13</sup>C
benzoyl chloride was used to generate isotope-labeled internal standards
for improved quantification. To demonstrate utility of the method
for study of small brain regions, the GABA<sub>A</sub> receptor antagonist
bicuculline (50 μM) was infused into a rat ventral tegmental
area while recording neurotransmitter concentration locally and in
nucleus accumbens, revealing complex GABAergic control over mesolimbic
processes. To demonstrate high temporal resolution monitoring, samples
were collected every 60 s while neostigmine, an acetylcholine esterase
inhibitor, was infused into the medial prefrontal cortex. This experiment
revealed selective positive control of acetylcholine over cortical
glutamate