Signals that induce sprouting in the central nervous system: sprouting is delayed in a strain of mouse exhibiting delayed axonal degeneration.

This study evaluates whether CNS sprouting is initiated by signals related to the degeneration of presynaptic axons. We evaluate the time course of sprouting of cholinergic septohippocampal fibers after unilateral entorhinal cortex (EC) lesions in a substrain of mice carrying a mutation which leads to a substantial delay in the onset of Wallerian degeneration. We first verified that axonal degeneration resulting from EC lesions was delayed in mutant mice using silver-staining techniques (the Fink-Heimer method). Cholinergic sprouting was then evaluated using a histochemical technique for acetylcholinesterase (AChE) in mutant mice and normal controls. In normal control mice, both axonal degeneration and cholinergic sprouting occurred with a time course that was comparable to that described in rats. Argyrophilic degeneration debris was prominent by 4 days postlesion, and increases in AChE staining in the molecular layer of the dentate gyrus were well developed by 10 days. In mice carrying the "Ola" mutation, however, argyrophilic degeneration debris was not detectable at 4 or 6 days postlesion, began to appear in the dentate gyrus by 8 days postlesion, but did not become prominent until 12 days. Increases in AChE staining in the molecular layer of the dentate gyrus were not detectable even at 12 days postlesion, but developed gradually after 14 days. These results demonstrate that the signals which initiate at least one form of CNS sprouting are related to the degeneration of presynaptic axons.