ABSTRACT
This chapter describes the procedures for in vivo tracing using horseradish peroxidase (HRP) and in vivo and in vitro tracing using carbocyanine dyes. Non-fluorescent tracers such as HRP require further procedures to be visualized in the tissue. In the case of HRP this is a histochemical method dependent on the enzyme activity of HRP to produce a visible reaction-product. HRP is supplied as a powder. It is generally injected as a 30% solution in distilled water or physiological saline, using a microsyringe and volumes in the order of 0.01-0.1 μl. HRP can be taken up by damaged axons passing through the injected area so this should be kept in mind when interpreting the results. Extremely small deposits in developing animals can be obtained by drying a concentrated solution of HRP onto the tip of a microelectrode which is then inserted into the tissue to dissolve the HRP.
