A note on the transneuronal transport of wheat germ agglutinin-conjugated horseradish peroxidase in the avian and rodent visual systems.

While using horseradish peroxidase conjugated to the plant lectin wheat germ agglutinin (WGA-HRP) as an anterograde marker to label the developing retinofugal projection in the chick, we have found that a significant amount of the tracer can escape from the axons and axon terminals of retinal ganglion cells and be subsequently taken up and transported both anterogradely and retrogradely by neighboring neurons and/or axon terminals. The release and uptake of the tracer appears to be nonspecific, and is particularly striking at embryonic stages; at these stages there is also appreciable uptake of the WGA-HRP by ependymal cells and by radial glial processes. Subsequent experiments in rats have shown that as early as 2 days after an injection of WGA-HRP into an eye there is clear labeling of geniculo-cortical axons in the striate cortex. Since WGA-HRP is being used increasingly in neuroanatomical studies, it is now clear that when interpreting the results of experiments with this marker the possibility of transneuronal labeling must be borne in mind, and especially after relatively long post-labeling survival periods. At the same time the secondary transport of WGA-HRP may, in certain neural systems, provide a useful tool for analyzing second-order connections.