ONLINE IN-VIVO MONITORING OF ENDOGENOUS QUINONES USING MICRODIALYSIS COUPLED WITH ELECTROCHEMICAL DETECTION

The continuous on-line monitoring of endogenous quinones has been real ized for the first time in an animal model of brain ischemia induced b y a vasoconstrictor peptide, endothelin-1. A microdialysis probe, impl anted in the striatum of a freely moving rat, was coupled online with an amperometric thin-layer cross-flow detector with a glassy carbon wo rking electrode operating at -200 mV vs Ag/AgCl. The instrumental setu p comprised a syringe pump pulse-damper consisting of an air bubble an d a silica capillary, which permitted considerable reduction of backgr ound current fluctuations and allowed improved detection limits. This original configuration allowed the quantitation of micromolar amounts of total quinones, generated from dopamine during the reperfusion peri od, to be readily monitored. Several operational parameters have been investigated: flow rate, presence of oxygen in the perfusion fluid, an d the working potential. The selectivity of the assay toward quinones was confirmed by studying possible interfering species such as ascorba te, hydrogen peroxide, riboflavin, and thiols. The results on freely m oving rats have shown that the endogenous quinone amount was directly related to dopamine concentrations. The latter was determined by HPLC from dialysate samples collected at the outlet of the on-line system. HPLC studies showed that the primary quinone, generated from dopamine by bulk electrolysis, was also found in dialysates from ischemic brain .