A CONSTANT-CURRENT SOURCE FOR EXTRACELLULAR MICROIONTOPHORESIS

A sophisticated constant-current source suitable for extracellular mic roiontophoresis of tract-tracing substances, such as Phaseolus vulgari s leucoagglutinin, Biocytin or Fluoro-Gold, is described. This design uses a flyback switched-mode power supply to generate controllable hig h-voltage and operational amplifier circuitry to regulate current and provide instrumentation. Design features include a fast rise time, +/- 2000 V supply (stable output in < 250 ms), simultaneous load current and voltage monitoring, and separate pumping and holding current setti ngs. Three features of this constant-current source make it especially useful for extracellular microiontophoresis. First, the output voltag e monitor permits one to follow changes in the microelectrode resistan ce during current injection. Second, the voltage-limit (or out-of-comp liance) indicator circuitry will sound an alarm when the iontophoretic pump is unable to generate the desired current, such as when the micr opipette is blocked. Third, the high-compliance voltage power supply i nsures up to +/- 20 mu A of current through 100 M Ohm resistance. This device has proven itself to be a reliable constant-current source for extracellular microiontophoresis in the laboratory.