An electron microscopic study of lesion-induced synaptogenesis in the dentate gyrus of the adult rat. II. Reappearance of morphologically normal synaptic contacts.

Intact synapses in the denervated area of the rat dentate gyrus are reduced to 14% of those normally present 2-4 days following a unilateral entorhinal lesion. By 160-240 days after lesion, the former entorhinal terminal zone is repopulated with new synapses. In all, there is more than a 5-fold increase in the density of intact synapses in the denervated zone between 2 and 240 days post-lesion, and the denervated zone of the molecular layer is restored to 80% of control values. The synapses are Gray type I and are formed on simple and complex spines which closely resemble those normally present. A few boutons have an abnormally large number of synaptic junctions. Reinnervation seems to progress at differential rates. Synapses are rapidly regained up to 30 days after operation, but thereafter the reacquisition of synaptic connections is much slower. Reinnervation is more rapid in the portion of the denervated zone nearest the granule cells, where the maximal densities are attained within 30 days. The time course of reinnervation differed from that of degeneration. A portion of the new synapses in the reinnervated molecular layer appear to arise by the assembly of new synaptic junctions. Over time, the number of post-synaptic contact sites along a given length of dendritic surface recovers, suggesting the formation of new synaptic sites. Our data indicate that granule cells retain a capacity even into adulthood to manufacture, position and assemble postsynaptic components of a synapse and, in concert with reactive afferents, form normal-appearing synapses.