A reliable technique for marking the location of extracellular recording sites using glass micropipettes

A simple and highly reliable technique is described for marking the locations of extracellularly recorded neurons using double-barreled glass micropipettes. One barrel contains 3 M NaCl for recording; the other contains horseradish peroxidase (HRP) that is iontophoretically ejected using low currents. With appropriate processing of the tissue, small well-localized spots are produced that contain a small number of HRP-filled neurons. The technique is effective in marking sequential recording sites within individual electrode tracks and in multiple penetrations using the same microelectrode.