Uptake of the retrograde fluorescent tracer fast blue from the cerebrospinal fluid of the rat.

Injections of the fluorescent dye Fast blue (FB) into the subarachnoid space result in the retrograde labeling of cells in many brainstem and forebrain structures. These nuclei include precerebellar nuclei, cranial nerve nuclei, raphe nuclei, reticular formation and regions giving rise to tracts traveling near the surface of the brainstem. These results present a serious source of potential error (false positives) in retrograde tracing studies using FB and perhaps other fluorescent dyes. More importantly, they demonstrate possible potential routes, i.e. across the axonal membrane, or uptake by axon terminals at the brain surface, by which compounds, including toxins, can gain entrance into the central nervous system via the cerebrospinal fluid.