RESUMO
We have successfully used biocytin as a retrograde tracer in the mammalian visual system. Retinal ganglion cells, pyramidal and stellate cortical neurons were labelled. Both pressure injections and gel implants were used successfully for retrograde labelling. Biocytin was detected using avidin conjugates and horseradish peroxidase histochemistry. Retrograde filling with biocytin proved to be more reliable and to allow better morphological resolution than other commonly used neurotracers such as horseradish peroxidase. The fine details of cell morphology observable by this method are comparable in many cases to the results obtained with intracellular tracer injections. The morphological resolution obtained with this method allows the study of brain microcircuits using extracellular deposits of biocytin