An improved method for the injection of crystalline horseradish peroxidase in neuroanatomical studies

A simple, inexpensive method for the injection of crystalline horseradish peroxidase is described that uses glass micropipettes. The horseradish peroxidase is expelled from the pipette by a fine wire inserted to the end of the pipette. A clay plug prevents diffusion of HRP during the needle's descent. The technique enables the investigator to reliably produce small, densely labeled injection sites with minimal diffusion. Both the retrograde and anterograde transport seen using this method compare favorably with that seen using iontophoretic or pressure injection methods.