A review of the status of the horseradish peroxidase method in neuroanatomy

The purpose of this review is to evaluate several aspects of the retrograde transport of horseradish peroxidase as a neuroanatomical tracing technique. These include (1) methods for the delivery of tracer, (2) determination of the effective injection site, (3) perfusion techniques and histological processing, (4) identification of labelled cells, (5) the role of axonal injury in the uptake and accumulation of enzyme, (6) representative species and neural systems in which the method has been used, (7) comments on anterograde transport, (8) conjunctive or multiple tracing methods, and (9) a comparison of the retrograde transport method with other tracing techniques. The recent literature pertaining to these areas is reviewed, and where necessary, appropriate future experiments are proposed. It is concluded that the retrograde transport method offers a number of conspicuous advantages to tracing methods based on neuronal degeneration. Many unresolved issues merit further study, especially numbers (2) and (5). The retrograde transport method is compatible with and complements many histochemical and degeneration methods as well as autoradiography and electron microscopy.