Expression of the beta-nerve growth factor gene correlates with the density of sympathetic innervation in effector organs.

Although beta-nerve growth factor (NGF), a protein necessary for survival and development of sympathetic neurons, is believed to be a trophic factor that is produced by sympathetic effector organs, its synthesis by these tissues has never been conclusively demonstrated. Using an assay capable of detecting 10 fg of mRNA, we measured the level of NGF mRNA in tissues innervated by sympathetic neurons. NGF mRNA was detected unambiguously in each tissue at a level that appeared to be more than enough to account for the low levels of NGF protein previously detected. Tissues that were densely innervated had comparatively high levels of NGF mRNA, while those with sparser innervation had lower levels. There was a strong positive correlation between the NGF mRNA level and norepinephrine content, a measure of the density of sympathetic innervation. NGF gene expression in one of these tissues, the iris, was shown to be induced by denervation. NGF mRNA was also found in other areas, including elements of the adult peripheral nervous system--the sciatic nerve and the sympathetic and sensory ganglia. In the central nervous system, levels of NGF mRNA were found that are too high to be attributed entirely to the vasculature, suggesting a role for NGF in adult central nervous system function.