In vivo microdialysis coupled with electrophysiology for the neurochemical analysis of epileptic seizures

Abstract

The focus of this research has been on the development of analytical techniques for the determination of the neurochemistry pertaining to animals that model epilepsy. The utilization of microdialysis sampling coupled with electrophysiological techniques played a vital role in the understanding of these neurochemical changes.

A thorough working knowledge of epilepsy models is essential for the development of new therapies for the neurological disorder. Initial work focused on a well known convulsant, 3-mercaptopropionic acid (3-MPA). An epileptic seizure model employing 3-MPA does not exist. A constant infusion dosing scheme was employed with a steady-state concentration of 3-MPA in the brain. The ability to control the 3-MPA concentration was an excellent independent variable for further experimentation involving the correlation of neurochemical events. A PK-PD study was conducted using simultaneous ECoG recordings.

A major difficulty with microdialysis sampling of neurochemical events is the ability to fully capture the events with sufficient temporal resolution. The sampling frequency is often hindered by the analytical instrumentation commonly available. Liquid chromatography was initially utilized for the neurochemical analysis of 5 minute microdialysis collections. However, it was found that this temporal resolution was not sufficient for obtaining meaningful neurochemical data from the seizure model.

Capillary electrophoresis was also employed, allowing for the neurochemical analysis with 60 second resolution. A biphasic increase in the levels of glutamate and dopamine were observed with the enhanced temporal resolution. Moreover, the levels of glutamate and gamma-aminobutyric acid had sustained changes over extended periods of time. It was determined that glutamate receptor desensitization played a crucial role in these findings. The shortcomings of these methods include poor derivatized sample stability and decreased catecholamine sensitivity in a complex analysis of amino acid and biogenic amine neurotransmitters.

To overcome these limitations, additional instrumentation was developed using a dual-parallel electrode detection scheme for capillary electrophoresis. This design provided increased sensitivity by simultaneously operating in the series and parallel-opposed configurations, thereby permitting redox cycling. It also allowed for enhanced selectivity by operating in the parallel-adjacent configuration. This scheme would help to improve the sample stability due to the electroactive nature of the catecholamines.