Septohippocampal cholinergic axonal regeneration through peripheral nerve bridges: quantification and temporal development.

Axons of the adult mammalian CNS have been shown to regrow vigorously into peripheral nerve grafts. Using a cholinergic septohippocampal model for adult CNS regeneration, involving complete denervation of the hippocampal formation from its basal forebrain cholinergic afferents, this study has established quantitative parameters and a temporal baseline of cholinergic fiber regeneration into the dorsal hippocampal tissue through a peripheral sciatic nerve graft. In nerve-implanted animals (i) the nerve grafts are maximally invaded by AChE-positive fibers between 2 weeks and 1 month postlesion, (ii) the fibers entering the hippocampal formation from the graft show a peak numerical increase and rate of elongation around the first month and/or in the proximal hippocampal region, (iii) an apparently normal innervation pattern and fiber density in the most rostral 1.5 mm of the dorsal hippocampal formation is reached by 6 months postlesion. The present study provides a basis for future quantitative comparisons of manipulations of different components of the system, e.g., the contributing neurons, the bridging material, and the receiving central nervous tissue. The temporal/spatial pattern of fiber regeneration suggests that the hippocampal CNS tissue can be a good axonal growth-promoting environment, albeit with temporal and/or spatial limitations, and is therefore not an immutably restrictive environment for axonal regeneration.