Absence of transsynaptic transport in cerebello-thalamo-cortical path of the rat.

Attempts to visualize the cerebello-thalamo-cortical path in the rat were made with different approaches. We tried (1) double labelling with somatopetal tracing from the motor cortex and somatofugal from the cerebellar nuclei, (2) transmembrane labelling by depositing biocytin or wheat germ agglutinin (WGA) into the motor cortex or cerebellum. WGA was either iodinated with 125I or conjugated with horseradish peroxidase (HRP). The double labelling technique showed an overlap of the tracers in the same thalamic region but no evidence of transsynaptic transport in either direction was obtained. Our results indicate a difference in the organization of this system in primates and rodents, since transsynaptic labelling in the cerebellar nuclei after injections of WGA-HRP conjugate in the monkey motor cortex has been found.