Fragment C tetanus toxin: a putative activity-dependent neuroanatomical tracer.

The non-toxic C fragment of tetanus toxin fused to the beta-galactosidase enzyme was analysed as a neuroanatomical tracer. After intramuscular injection in rat tongue, its location in the hypoglossal network was compared with other classic tracers such as neurotropic viruses. The hybrid protein reached second and higher-order neurons after crossing several synapses. It appears to be a powerful tool to map neuronal circuits since the protein is easy to handle and detect and its transsynaptic transport is potential activity-dependent.