Maintenance of sympathetic innervation into the hippocampal formation requires a continuous local availability of nerve growth factor.

Abstract

The sprouting of peripheral sympathetic fibers into the septally denervated hippocampal formation is a well-characterized model of lesion-induced plasticity. While various studies have demonstrated the importance of nerve growth factor for evoking sympathetic sprouting, little is known concerning whether nerve growth factor continues to be required for maintaining innervation once it has occurred. In the present study we have addressed this point by (i) investigating the consequences of withdrawing exogenous nerve growth factor support from rats in which sympathetic innervation was enhanced by a nerve growth factor infusion and (ii) using blocking antibodies to interfere with the actions of endogenous nerve growth factor. The results of this investigation clearly indicate that a continuous supply of nerve growth factor (either exogenous or endogenous) is required to maintain sympathetic innervation within the hippocampal formation. Evidence is also provided demonstrating that the nerve growth factor must be made available locally within a given region to evoke and maintain the sympathetic innervation within this location. Axonal rearrangement within the developing and adult brain is believed to be an important mechanism underlying learning and memory is crucial for lesion-related plasticity. In various experimental paradigms, nerve growth factor has been shown to be an important cue for initiating axonal remodeling. In the current study, we have demonstrated that once such rearrangements have taken place, nerve growth factor may also be required to maintain them.