The anterograde and retrograde transport of neurobiotin in the central nervous system of the rat: comparison with biocytin.

In order to test whether neurobiotin, an analogue of biotin, is transported by neurones in the central nervous system, injections of varying volumes of a 5% solution were made into different regions of the rat brain. Following perfusion-fixation, the sites of injection and possible sites of transport were sectioned and incubated with an avidin-biotin-peroxidase complex and then subjected to a peroxidase reaction. All injection parameters and sites were directly compared to equivalent injections of the closely related substance, biocytin, which is effectively transported in an anterograde fashion. The sites of injection of neurobiotin were characterised by large areas of labelling that were more extensive than those produced by equivalent injections of biocytin but with less intense labelling of individual neurones. Each of the injections of neurobiotin gave rise to marked anterograde labelling, the characteristics of which were similar to those produced by biocytin, but due to the larger injection sites was heavier than that produced by biocytin. Each of the injections of biocytin or neurobiotin also gave rise to retrograde labelling; the degree of labelling was far greater with neurobiotin but varied between pathways. Retrograde labelling only occurred to a minor degree in some pathways but was particularly marked in others, e.g., the striatonigral pathway. As with biocytin, tract-tracing with neurobiotin can be applied to electron microscopy and can readily be combined with immunocytochemistry for endogenous substances. It is concluded that neurobiotin is an effective anterograde and retrograde marker that may be of use in studies in the central nervous system, particularly in large homogeneous structures such as cortical fields and the striatum.