Significant differences in the retrograde labeling of spinothalamic tract cells by horseradish peroxidase and the fluorescent tracers fast blue and diamidino yellow.

The laminar distributions of spinothalamic tract cells retrogradely labeled by the fluorescent tracers Fast Blue and Diamidino Yellow and by free or lectin-coupled horseradish peroxidase have been found to be significantly different. The total numbers of cells labeled by each method are similar, but nearly twice as many lamina I cells are labeled by the fluorescent tracers and more lamina V cells are labeled by peroxidase. Injection site spread and spurious labeling due to leakage or fibers of passage do not account for these differences. These results indicate that both horseradish peroxidase and fluorescent tracers may be selectively transported and, thus, that the cautious use of both methods should be recommended for analyses of afferent populations.