A new approach to intramuscular placement of horseradish peroxidase.

A new method of intramuscular placement of a semisolid paste of horseradish peroxidase (HRP) using a root canal file is described. This method has been demonstrated to produce consistent and reproducible numbers of labeled motoneurons in the mouse extensor digitorum longus muscle (EDL). While the number of labeled cells is similar to that found with optimal intramuscular injection of liquid HRP, this new technique provides more reproducible results by eliminating the reflux often associated with intramuscular injection of liquid HRP.