Sprouting of sympathetic fibers in the hippocampus in the absence of major target cell candidates.

Noradrenergic fibers of sympathetic origin enter the hippocampal formation following removal of the cholinergic septal afferents. These sympathetic fibers apparently form synaptic contacts with cells in the hippocampus, but the identity of the postsynaptic cell is unknown. As the two major cell types of the hippocampal formation are the pyramidal cells and the dentate granule cells, we sought to determine how their absence would affect the ingrowth and distribution of the sympathetic fibers following septal lesions. Hippocampal pyramidal cells were selectively removed with kainic acid and granule cells were selectively removed with colchicine 4 days before lesions were placed in the medial septal-diagonal band nuclear complex. Animals were sacrificed 4-5 weeks after the septal lesions and the brains prepared for catecholamine fluorescence histochemistry. Sympathetic fibers innervate the hippocampal formation of brains depleted of pyramidal cells or granule cells. Thus, it appears that neither cell type is an obligatory postsynaptic target or is necessary for the ingrowth of sympathetic fibers. The anomalous sympathetic fibers may be contacting interneurons or glia.