Fluorescent tracers as potential candidates for double labeling of descending brain neurons in larval lamprey

In larval lamprey, seven fluorescent tracers were tested as potential candidates for retrograde double labeling of descending brain neurons: Fluoro Gold (FG); fluorescein dextran amine (FDA); True Blue (TB); cascade blue dextran amine (CBDA); Fast Blue (FB); Texas red dextran amine (TRDA); and tetramethylrhodamine dextran amine (RDA). The first tracer (FG, TB, FB, or CBDA) was applied to the spinal cord at 40% body length (BL). In separate experiments, the second tracer (TRDA or RDA) was applied to the spinal cord at 20% BL. The tracer combination FG/TRDA was found to have the best optical properties for double labeling. However, application of FG to the spinal cord with the method used for the other tracers resulted in labeling of 'lateral cells' along the sides of the rhombencephalon that were presumed to be non-neuronal and that obscured some of the descending brain neurons. Control experiments suggested that FG was transported in the circulation to the brain, where the tracer was taken up by lateral cells. Therefore, a special technique was developed for applying FG to the spinal cord without allowing the tracer to enter the circulation. In larval lamprey, this important double-labeling technique that was developed for TRDA and FG is being used to examine axonal regeneration and projection patterns of descending brain neurons.