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Abstract:
Pharmaco Magnetic Resonance Imaging (phMRI) offers great potential in non-invasive monitoring of synaptic events in the living brain. Synaptic responses to pharmacological manipulations of entire neuronal networks can be studied with high spatial and temporal resolution. However, the judicious interpretation of the BOLD signal (Blood Oxygenation Level Dependent) recorded in phMRI is often hampered by the indirect measurement of neuronal activity by its metabolism. It would be therefore convenient to directly monitor synaptic events and their mediators, neurotransmitters and neuromodulators, in combination with
phMRI. We developed a phMRI compatible tool for simultaneous monitoring of relevant keyplayers in synaptic transmission to study their interplay in response to pharmacological manipulations. The spatial image resolution of phMRI delivers the corresponding spatial pattern of the
synaptic events. We simultaneously analyze acetylcholine, choline, serotonin, dopamine, c-aminobutyric acid (GABA), glutamate and aspartate in samples of the extracellular brain fluid (EBF). EBF was collected from primary visual cortex (V1) of anesthetized monkeys and from extrastriate (V4) and prefrontal cortex of awake animals. We used hydrophilic interaction chromatography coupled to tandem masspectrometry, phMRI was performed in a 4.7 Tesla Scanner. A push-pull sampling method was developed with flow rates of tens of nl/min yielding a temporal resolution of 5 -10 minutes and the following detection limits: acetylcholine,
serotonin, dopamine, GABA, glutamate and aspartate were 0.015, 0.15, 0.3, 1.2, 6 and 15 femtomoles, respectively. The quantitative determination of synaptic keyplayers from the brain combined with phMRI will open a new diagnostic window for synaptic events.
