Localization of nerve growth factor bound to neurons growing nerve fibers in culture.

The peptide hormone, nerve growth factor, is necessary for the normal growth and development of sensory and sympathetic neurons. In culture, this hormone is also need for the regeneration of fibers from sensory and sympathetic neurons. We have needed for the regeneration of fibers from sensory and sympathetic neurons. We have determine the distribution of the nerve growth factor receptors on cells cultured from embryonic dorsal root ganglia. The binding of the radiolabeled nerve growth factor is highly selective for neurons, though some saturable binding is present on non-neuronal cells, the density of binding is less than 10% of that on neurons. By incubating cultures with different concentrations of radiolabeled nerve growth factor, we were able to quantify the type I (high affinity) and type II (low affinity) sites on neuronal somata and along growing nerve fibers. We estimate that there are 5 times more type I sites on the nerve fibers than on the cell bodies. On the other hand, type II sites appear to be uniformly distributed on nerve fibers and cell bodies. The amount of radiolabeled nerve growth factor binding was directly correlated with the extent of nerve fiber growth. However, binding to growth cones was variable and was not obviously different from the main axis cylinder of the growing nerve fiber. Finally, the saturable binding found on non-neuronal cells appears to be the same as the type II sites on neuronal cells. Bases upon our calculations, the apparent equilibrium dissociation constants for the two types of cells are the same (approximately 2 X 10(-9) M).