Corticorubral connections: ultrastructural evidence for homotypical synaptic reinnervation after developmental...

Corticorubral (CR) connections are reorganized after the red nucleus is deafferented by neonatal hemispherectomy. Our objective was to determine if the new decussating CR axons establish synaptic junctions with their target neurons. The ultrastructure of CR axonal terminal arbors was identified by the anterograde transport and histochemical labeling of wheat germ agglutinin lectin-bound horseradish peroxidase injected unilaterally into the pericruciate neocortex of adult cats that received unilateral neonatal hemispherectomy (HEMI cases) and normal adult cats (control cases). The regional organization of undecussated and decussated CR arbors was nearly equivalent in the HEMI cases. The fine structure of both connections was uniform. Asymmetrical synaptic junctions were observed in frequent axodendritic and infrequent axosomatic boutons en passage and boutons terminaux. The undecussated arbors formed the principal connection in the control cases. However, the fine structures of the decussated and undecussated arbors were comparable and matched the arrangements in the HEMI cases. We conclude that developmental deafferentation redirects CR axons to provide stable synaptic afferents to new targets. The reinnervation is homotypical in terms of regional and fine structure organization. It is likely to be an important factor in chemically mediated neurotransmission that contributes to the sparing/recovery of function after brain damage.