Short- and Long-term Effects of Nerve Growth Factor on the Sympathetic Nervous System in the Adult Mouse

Nerve growth factor (NGF) induced a marked increase in the adrenergic nerve terminal networks of several peripheral organs in intact adult and young adult mice. The animals were investigated at 3 days and at 2 months after 6 daily subcutaneous injections of the 7S species of NGF (1 mug/g body weight each dose). At 3 days after the end of the treatment an increased fluorescence intensity, as well as an increased density of the adrenergic terminal plexuses was observed by fluorescence histochemical techniques in iris, salivary glands, heart, intestine, spleen and pancreas. These changes were paralleled by significant increases (up to 55%) in the endogenous NA levels. Increased NA levels were also detected in the brain of the NGF-treated animals. At 2 months after treatment the effects had almost totally disappeared, demonstrating that the NGF-induced overgrowth of the sympathetic axons was only temporary. The increased adrenergic innervation probably resulted from a stimulatory effect of NGF on collateral sprouting from the intact adrenergic axons. In line with the idea that the terminals of peripheral axons are continuously renewed, it is suggested that endogenous NGF could play a regulatory role in such a continuous growth process of the fully developed, adult sympathetic nervous system, directed at the maintenance of an adequate adrenergic terminal network.