An improved HRP method for the study of central nervous connections.

The HRP tract-tracing method has been modified by the introduction of cryoprotective agents, thereby allowing the enzymatic reaction to take place at sub-zero temperatures. By using cold temperatures the enzymatic reaction can be better controlled, which in turn makes it possible to "push" the method to its maximum without obtaining excessive crystallization. The addition of antifreeze agents, however, changes the acid-base phenomena, which in turn have to be adjusted by the use of appropriate buffers. The overall result is a considerable increase in efficiency as compared with the commonly used HRP procedure.