AMPEROMETRIC MICROSENSORS FOR MONITORING CHOLINE IN THE EXTRACELLULARFLUID OF BRAIN

Selective amperometric enzyme microsensors for monitoring low micromol ar concentrations of choline in extracellular fluid of rat brain have been developed. Preparation of the choline microsensors involved the m odification of carbon fiber microcylinder electrodes (10 mu m diameter , 300-400 mu m long) with a cross-linked redox-active gel containing h orseradish peroxidase and choline oxidase. Rejection of the noise reco rded from the choline microsensors implanted in living brain tissue im proved the in vivo detection capabilities of the sensors. The microsen sors and a differential detection scheme were used to estimate the bas al concentration of choline in striatal tissue at 6.6 +/- 2.9 mu M and to measure changes in choline concentrations of 6.1 +/- 2.7 mu M in v ivo. The microsensors were also used to monitor choline produced follo wing the injections of acetylcholine in vivo. Coinjections of neostigm ine and acetylcholine significantly lowered the choline response recor ded with the microsensors, confirming that the response following the injections of acetylcholine alone was due to the activity of endogenou s acetylcholinesterase. Comparison of the maximal rate of decrease in choline concentration following the injections of 1 mM choline and 1 m M acetylcholine was used to estimate the rate of acetylcholine clearan ce from extracellular fluid through cholinesterase activity at approx. 2.5 mu M/min.