Intravitreal injections of horseradish peroxidase in the rat consistently resulted in the labelling of contralateral and ipsilateral efferents of the retina. Simultaneous intravitreal injections of horseradish peroxidase and tritiated amino acids yielded identical projection patterns. Intravitreal injections of colchicine and pentobarbital reversibly blocked this transport of horseradish peroxidase from the eye to the brain. Furthermore, silver impregnation procedures indicated that this transport occurs in the absence of significant neurol damage. The anterograde transport of horseradish peroxidase along the optic pathways occurs at a rate between 288 and 432 mm per day. These observations show that the anterograde migration of horseradish peroxidase is subserved by fast axonal transport and that its occurrence does not depend on neurol injury or passive diffusion. The anterograde transport of horseradish peroxidase thus offers a valid and reliable anatomical method for tracing efferent connections in the nervous system.
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