Do neurotrophic interactions control synapse formation in the adult rat brain?

The role of axonal transport in the regulation of synaptic contacts was studied in the adult rat dentate gyrus. Colchicine was applied to the fimbria, which includes fibers of the hippocampal commisural system, and axonal transport was measured. Axonal transport in these and other fibers of the fimbria was markedly reduced. The region of commissural termination in the molecular layer of the dentate gyrus was monitored by electron microscopy for changes in the number of synapses per unit area following cholchicine treatment, sodium chloride treatment, or fimbrial transection. Four days after colchicine treatment there was no change in the number of synapses. However, at 11 and 60-70 days after colchicine treatment the number of synapses per unit area significantly increased. This increase occurred throughout the commissural terminal zone, but it did not occur in terminal zones of other afferents in the dentate gyrus. The increased synaptic density appeared to arise from the commissural system itself because removal of commissural fibers eliminates the increase (in addition to the normal commissural input). These findings suggest a role for axonally transported trophic substances in the specific regulation of synaptogenesis in the dentate gyrus of the adult rat.