The accessibility of macromolecules to the postnatal mammalian central
nervous system is a subject of controversy and importance. One of the
biologically important macromolecules in neural development, nerve gr
owth factor (NGF), is thought to be unable to cross the blood-brain ba
rrier in postnatal mammals. In this study, I-125 labeled 7S NGF was sy
stemically administered to postnatal rats and was found to accumulate
in the brain in significant levels. In addition, brains from saline pe
rfused rats contained intact 7S NGF as determined by gel autoradiograp
hy. Radiolabeled NGF was found to be localized to specific areas corre
sponding to p75 NGF receptor distribution in the hippocampus and corte
x by autoradiography. Circulating plasma NGF has access to the develop
ing postnatal neuraxis where it may exert a humoral trophic function.