Nerve growth factor influences the distribution of sympathetic sprouting into the hippocampal formation by implanted superior cervical ganglia.

Following cholinergic denervation of the hippocampal formation, peripheral sympathetic fibers from the superior cervical ganglion (SCG) sprout into hippocampal tissue. The molecular mechanism controlling this process is unknown, although a role by trophic factors seems likely. In the present study, neonatal SCG were used as biological probes to investigate the association between NGF-immunoreactive regions in the hippocampal formation and areas innervated by regrowing sympathetic fibers. Cholinergic deafferentation of the hippocampal formation was achieved by a fimbria-fornix transection and neonatal SCG were placed into the lesion cavity, abutting the rostral pole of the hippocampus. At 16 days following ganglia transplantation, NGF immunoreactivity within the hippocampal formation appeared indistinguishable from unlesioned controls and was localized within the mossy fiber region of the dentate gyrus and CA3 and CA2 hippocampal subfields. Sympathetic innervation, revealed in adjacent sections by dopamine beta-hydroxylase or p75LNGFR immunoreactivity, was also restricted primarily to the mossy fiber region. Ablation of the entorhinal cortex at the time of transplantation resulted in the appearance of an additional discrete band of NGF immunoreactivity within the outer molecular layer of the dentate gyrus. In animals receiving an entorhinal lesion concurrent with the SCG transplant, sympathetic fibers were observed not only within the mossy fiber region but also within the outer molecular layer of the dentate gyrus where the lesion-induced NGF immunoreactivity appeared. These results support the hypothesis that topographic distributions of NGF may be used in the hippocampal formation to define terminal fields for sprouting NGF-sensitive neuronal populations.