A delayed sprouting response to partial hippocampal deafferentation: time course of sympathetic ingrowth following fimbrial lesions.

Overview

Sympathetic, noradrenaline-containing fibers grow into the hippocampal formation following lesions of the medial septum or fimbria/fornix. Fluorescent histochemical analysis reveals that these fibers begin to arise as collateral sprouts of the normal sympathetic innervation of the internal and external transverse hippocampal arteries at 9 days post-lesion. These initial fibers are oriented orthogonally to the septo-temporal axis of the hippocampal formation. They grow towards the granule cells of the fascia dentata and the CA3 pyramidal cells, where they begin to proliferate at 14 days post-lesion. This process continues until 29 days, resulting in a final distribution of fibers in areas of septal deafferentation: stratum lucidum, the inner one-third of stratum oriens and stratum pyramidale of CA3; and the hilus, the inner one-third of stratum moleculare and stratum granulosum of the fascia dentata. The time course of this sprouting response is relatively late in onset and slow in completion when compared to sprouting responses of intrinsic afferent systems of the hippocampal formation following entorhinal cortical or commissural deafferentation.