# Microdialysis coupled with droplet microfluidics and mass spectrometry for determination of neurotransmitters in vivo with high temporal resolution

## TL;DR

A new method combines microdialysis, droplet microfluidics, and mass spectrometry to monitor multiple neurotransmitters in the brain with high temporal resolution.

## Contribution

The novel approach enables real-time monitoring of up to six neurotransmitters with second-level temporal resolution using droplet ESI-MS/MS.

## Key findings

- The method achieves a temporal resolution of 11 seconds for neurotransmitter monitoring in vivo.
- Limits of detection down to 2 nM were achieved for adenosine, dopamine, and serotonin.
- The system successfully monitored neurotransmitter dynamics in response to 100 mM K+ and amphetamine.

## Abstract

Monitoring the concentration fluctuations of neurotransmitters in vivo is valuable for elucidating the chemical signals that underlie brain functions. Microdialysis sampling is a widely used tool for monitoring neurochemicals in vivo. The volume requirements of most techniques that have been coupled to microdialysis, such as HPLC, result in fraction collection times of minutes, thus limiting the temporal resolution possible. Further the time of analysis can become long for cases where many fractions are collected. Previously we have used direct analysis of dialysate by low-flow electrospray ionization-tandem mass spectrometry (ESI-MS/MS) on a triple quadrupole mass spectrometer to monitor acetylcholine, glutamate, and γ-amino-butyric acid to achieve multiplexed in vivo monitoring with temporal resolution of seconds. Here, we have expanded this approach to adenosine, dopamine, and serotonin. The method achieved limits of detection down to 2 nM, enabling basal concentrations of all these compounds, except serotonin, to be measured in vivo. Comparative analysis with LC-MS/MS showed accurate results for all compounds except for glutamate, possibly due to interference for this compound in vivo. Pairing this analysis with droplet microfluidics yields 11 s temporal resolution and can generate dialysate fractions down to 3 nL at rates up to 3 fractions per s from a microdialysis probe. The system is applied to multiplexed monitoring of neurotransmitter dynamics in response to stimulation by 100 mM K+ and amphetamine. These applications demonstrate the suitability of the droplet ESI-MS/MS method for monitoring short-term dynamics of up to six neurotransmitters simultaneously.

Samples collected from brain of living subjects are segmented to nanoliter droplets and infused to the mass spectrometer. Temporal resolution for in vivo concentration dynamics of key neurotransmitters of 10 s is achieved. Created with Biorender.com.