MARKING MICROELECTRODE PENETRATIONS WITH FLUORESCENT DYES

Fluorescent dyes were used to mark and identify the tracks left by ext racellular microelectrodes in neurophysiological experiments. Forty-tw o penetrations were made into the postcentral gyrus of 3 Macaque monke ys with electrodes coated with 1 of 5 fluorescent dyes (DiI, DiO, DiI- C5, PyPO, and Fast Blue). The electrodes were driven at rates ranging from 10 to 1000 mu m/min, to a depth of about 4000 mu m, where a small electrolytic lesion was made. Histological sections were viewed under fluorescent optics and the electrode tracks were reconstructed from t he dye traces. Fluorescent traces (width 50-400 mu m) were observed in 41 of 42 penetrations with 24 traces extending to the lesion site. Of the electrodes driven in less than 3 h, those coated with DiI (8/8) a nd DiI-C5 (8/8) left a trace to the lesion site, while 57% (4/7) of th e DiO, 40% (2/5) of the Fast Blue and only 11% (1/9) of the PyPO track s were fully marked. This method of marking penetrations can be used w ith any extracellular recording configuration, does not require tissue sections to be processed or stained, does not require electrical lesi ons, and causes no detectable tissue damage. Because the dyes fluoresc e at different wavelengths, closely spaced tracks can be uniquely iden tified.