Orthograde axonal and transcellular transport of different fluorescent tracers in the primary visual system of the rat

The differential labeling properties of various fluorescent tracers injected intraocularly were investigated using as a model the rat primary visual system. All of the tracers tested (Fast Blue, FB; True Blue, TB; Nuclear Yellow, NY; bisbenzimide, BB; Evans Blue, EB; propidium iodide, PI) produced a retrograde neuronal labeling of oculomotor neurons. However, no such labeling was observed in the medial pretectal nucleus (NPM) considered to be the site of origin of the rat centrifugal visual pathway. Orthograde transport within the axons of the optic tract and their terminal arborizations were visualized directly with FB and TB. No evidence of EB or PI orthograde transport was demonstrated. Furthermore, FB, TB, NY and BB displayed varying degress of leakage from the optic axons and terminals into the extracellular space, there to be taken up by glial (FB, TB, NY, BB) and neuronal (NY, BB) somas of the primary optic system or in adjacent structures including NPM (BB). The neuronal labeling with NY or BB does not result from the retrograde axonal transport but appears to involve an orthograde transneuronal process transport. Some limitations in the use of different fluorescent tracers for determining neuronal connections are discussed.