Immunohistochemical localization of endogenous nerve growth factor

Nerve growth factor (NGF) has been proposed as a trophic molecule essential for the development of sympathetic and primary sensory neurones^1,2. In newborn mice and rats, administration of nerve growth factor results in an increase in the number of surviving neurones^3,4, whereas administration of antiserum to NGF decreases neuronal survival^5,6. Thus it has been proposed that the factor is produced and secreted by the relevant target tissues to provide trophic support for the ingrowing nerves ^1,7. The site of synthesis of nerve growth factor is still unknown, and it has been emphasized that a precise physiological role for the molecule cannot be ascribed until the cell types that produce it are known^7-9. I report here the use of immunohistochemistry to localize endogenous NGF in the rat iris, a tissue in which there is sound biochemical evidence for the production of NGF activity¹⁰. Surprisingly, the results reveal that NGF can be detected readily in Schwann cells, but not in smooth muscle cells of the iris when it is sympathetically denervated or cultured.