Monitoring extracellular amino acids in the brain by microdialysis coupled on-line to capillary electrophoresis.

Abstract

Capillary electrophoresis with laser-induced fluorescence (CE-LIF) is an analytical technique that offers high speed, high resolution and high sensitivity. These attributes make CE-LIF an ideal separation technique to use for on-line monitoring in microenvironments. As an example of this concept, microdialysis is coupled on-line to CE-LIF to monitor extracellular amino acids in vivo. Dialysate is derivatized on-line and then electrokinetically injected onto the separation capillary via a flow-gated interface. After a high-speed separation, the dialysate is detected off-column using a sheath-flow cuvette interface. Previous attempts to characterize extracellular compounds by microdialysis coupled to HPLC have relied on relatively long sampling times (5--15 min). With CE-LIF, short sampling times (12 seconds or less) allow the monitoring of glutamate, aspartate, GABA, taurine, glutamine, serine, and glycine on more physiologically relevant time-scales. The sensitivity and resolution of this technique allow for the detection of dynamic changes in extracellular levels that would be missed during longer sampling times. Experiments investigating rodent models of disease, drugs of abuse, hormonal regulation, and glutamatergic regulation of GABA release demonstrate the versatility of utilizing this analytical technique for in vivo monitoring. Corticostriatal glutamatergic regulation of GABA release in the striatum of anesthetized rats using various glutamate receptor antagonists and agonists suggested that AMPA/kainate receptors induce initial depolarization of GABA neurons while NMDA receptors maintain the depolarization. Our instrumental design for the first time allowed for on-line monitoring of extracellular levels in freely moving animals, thus eliminating the confounds introduced by the presence of anesthesia. Experiments using freely moving animals included dose dependent ethanol effects on amino acids in the dorsal and ventral striatum, cocaine effects on extracellular compounds in the striatum when infused at different rates, estradiol effects on basal and potassium induced levels of GABA and taurine in the striatum, and extracellular changes in mouse models of stereotypy and Huntington's disease. Taken together, these experiments have demonstrated the versatility of on-line CE-LIF for in vivo monitoring.