Production and transport of endogenous trophic activity in a peripheral nerve following target removal

High concentrations of trophic factor for sympathetic neurones were found in the discrete nerve which innervates the expansor secundariorum muscle of the chicken wing. Mouse nerve growth factor (NGF) was additive with nerve extract in allowing survival of sympathetic neurones. Antiserum to NGF, while inhibiting outgrowth in response to NGF, only partially blocked survival promoted by the nerve extract. In these characteristics, the nerve extract resembled the previously characterized activity of its target organ. However, after surgical removal of the muscle there was no decrease in the concentration of trophic factors in the nerve 7 days later. Likewise sectioning, crushing or removing a piece of nerve 1 cm from the muscle had little effect on trophic levels after muscle removal. Retrograde transport of the activity occurred in the nerve even in the absence of the muscle. Distal to ligatures placed on the nerve stump 24 h earlier, 60% of the trophic activity was inhibited by an antiserum to mouse NGF suggesting that a chicken form of the molecule is present and transported. In culture, cells of the nerve sheath produced trophic factors, and the rate of production increased greatly during the first 24 h of incubation. Two conclusions are made: the major source of trophic activity in the nerve is Schwann cells and at least two molecules are present, one of which is a chicken form of NGF.