Persistence of sympathetic ingrowth fibers in aged rat hippocampus.

Sympathetic fibers appear in the rat hippocampus following interruption of the septal-hippocampal pathway. The current study examined the relative ability of young (3 month) and old (24 month) rats to produce sympathetic ingrowth fibers. Sympathetic ingrowth was present in the hippocampus of young adult rats 2 months after fimbria/fornix transection. The aged rats either lacked, or had very limited, hippocampal ingrowth two months after fimbria/fornix transections. In contrast, aged rats which had fimbria/fornix transections as young adults had extensive hippocampal ingrowth fibers. The topographical distribution of ingrowth fibers in the hippocampal formation was similar in young 3-month-old rats and rats which were lesioned young and sacrificed when aged. Counts of sympathetic fibers found similar numbers of hippocampal ingrowth fibers at both of the post-operative ages. Thus, although aged animals were acutely impaired in their ability to initiate the ingrowth response, the sympathetic fibers which were established at a young age persisted into senescence.