An improved method for extracellular marking of electrode tip positions in nervous tissue.

The Alcian blue method for extracellular marking of recording sites has been improved by filling the glass micropipettes with 3% Alcian blue 8GX or 8GS in 3M KCl buffered to pH 5.2 with 0.05M Potassium acetate. By filling the microelectrodes with a strong electrolyte (3M KCl) the same low resistances (5--25 M omega) and recording properties as conventional micropepettes are preserved. Currents of 0.5--1 microA for 1--3 min passed through the different electrode connected as anode, resulted in coloured spots of 10--100 micrometers in diameter. There was a positive correlation between the amount of curren passed, and the size of the spots. The localization of the electrode tips was successful in more than 80% of all cases. The dyemarks remained for at least 5 cays in vivo and resisted many histological procedures.