An in Situ Measurement of Extracellular Cysteamine, Homocysteine, and Cysteine Concentrations in Organotypic Hippocampal Slice Cultures by Integration of Electroosmotic Sampling and Microfluidic Analysis

Abstract

We demonstrate an all-electric sampling/derivatization/separation/detection system for the quantitation of thiols in tissue cultures. Extracellular fluid collected from rat organotypic hippocampal slice cultures (OHSCs) by electroosmotic flow through an 11 cm (length) × 50 μm (i.d.) sampling capillary is introduced to a simple microfluidic chip for derivatization, continuous flow-gated injection, separation, and detection. With the help of a fluorogenic, thiol-specific reagent, ThioGlo-1, we have successfully separated and detected the extracellular levels of free reduced cysteamine, homocysteine, and cysteine from OHSCs within 25 s in a 23 mm separation channel with a confocal laser-induced fluorescence (LIF) detector. Attention to the conductivities of the fluids being transported is required for successful flow-gated injections. When the sample conductivity is much higher than the run buffer conductivities, the electroosmotic velocities are such that there is less fluid coming by electroosmosis into the cross from the sample/reagent channel than is leaving by electroosmosis into the separation and waste channels. The resulting decrease in the internal fluid pressure in the injection cross pulls flow from the gated channel. This process may completely shut down the gated injection. Using a glycylglycine buffer with physiological osmolarity but only 62% of physiological conductivity and augmenting the conductivity of the run buffers solved this problem. Quantitation is by standard additions. Concentrations of cysteamine, homocysteine, and cysteine in the extracellular space of OHSCs are 10.6 ± 1.0 nM (n = 70), 0.18 ± 0.01 μM (n = 53), and 11.1 ± 1.2 μM (n = 70), respectively. This is the first in situ quantitative estimation of endogenous cysteamine in brain tissue. Extracellular levels of homocysteine and cysteine are comparable with other reported values.