Absence of sympathetic sprouting in the rat olfactory bulb after cholinergic denervation

Sympathetic axons have been shown to invade the rat hippocampal formation after lesions of the septohippocampal (cholinergic) projection. To test the generality of this type of transmitter-specific sprouting within the central nervous system, lesions were placed within the horizontal limb of the diagonal band in order to denervate the main olfactory bulb (MOB) of its cholinergic innervation. Some animals also received autologous transplants of the superior cervical ganglion to the region of the MOB. There was no evidence of sympathetic axon growth into the MOB under any circumstance as assessed with qualitative fluorescence histochemistry. In addition, retrograde fluorescent dye studies demonstrated that the basal forebrain neurons projecting to the neocortex, where sympathetic sprouting has also been reported, are a separate population from those which project to the MOB. Although it is difficult to unequivocally interpret negative results, these data indicate that cholinergic denervation does not induce sympathetic sprouting throughout the central nervous system. The availability of a brain region where sympathetic sprouting does not occur may provide clues to understanding the conditions which permit such neuronal plasticity in the hippocampal formation.