Synaptic density and axonal sprouting in rat hippocampus: stability in adulthood and decline in late adulthood.

Synaptic density and axonal sprouting remain stable in the rat dentate gyrus up to 12 months of age. At 18-24 months, an 8-10% loss in synaptic density occurs, and synaptic reinnervation could not be clearly detected at 15 days postlesion, compared to a 40% synaptic recovery in rats lesioned at 6 or 12 months postnatal. Since the amount of synaptic degeneration remaining in the denervated zone was the same at 15 days postlesion in all rats examined, synaptic reinnervation is not primarily dependent on the rate at which degeneration is removed. The results indicate that the decline in the growth response in the rat hippocampus is not a progressive trend from early adulthood into old age, but appears after the midpoint of life.