Transneuronal transport of WGA-HRP in immature rat visual pathways

Wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) was used to study transneuronal transport in the developing rat visual pathways. Intraocular injections of WGA-HRP were made in neonatal albino rat pups at different ages from the day of birth, postnatal day 0 (P0), to one month of age. Transneuronal labeling in geniculostriate fibers and in tectoparabigeminal terminals was observed as early as P1 and showed little change with eye-opening. However, at early ages, consistent transneuronal labeling was found to require injection of up to 4 times the amount of tracer (0.18 mg WGA-HRP) as adults (0.04 mg WGA-HRP), delivered in two injections. Control injections of HRP alone produced heavy anterograde labeling at all ages, without requiring increased injections. The results suggest that transneuronal transport precedes synaptic transmission, and may illustrate a mechanism for exchanging molecules between neurons. One explanation for the requirement of increased tracer is that axonal and/or transneuronal transport of WGA-HRP may be selectively limited to certain cells in the postnatal retina.