Dose-dependent effects of recombinant human NGF on grafted adult adrenal medullary tissue

It has previously been shown that the chromaffin cells of the adrenal medulla respond to nerve growth factor (NGF) with neurite outgrowth and increased cell survival in tissue culture or after grafting. In the present study we evaluated the dose dependency in neurite outgrowth from chromaffin tissue to recombinant human NGF (rhNGF). Therefore, pieces of adrenal medullary tissue from adult rat were grafted into the anterior chamber of the eye of previously sympathectomized recepients. Survival time was 4 weeks. At grafting and at Days 7, 14, and 21 postgrafting, the eyes were injected with 5 microliters of rhNGF at concentrations of 10, 30, 60, 100, 150, and 200 micrograms/ml, or with a control solution. All grafts, including the controls, survived well and became vascularized. At the low doses of rhNGF, 10 and 30 micrograms/ml, a small area of the irides was reinnervated and the density of the nerve fiber network was low. The maximal response was obtained at 100 micrograms rhNGF/ml. Using larger concentrations of 150 and 200 micrograms rhNGF/ml, the density of the nerve fiber network did not change, but the reinnervated area of the irides was significantly decreased compared to the outgrowth seen in irides treated with 100 micrograms/ml. In conclusion, adult rat chromaffin tissue responds to rhNGF in a dose-dependent manner. However, at the highest doses used, the outgrowth area was suboptimal, although nerve fiber density was maximal. These results indicate that to obtain maximal effects, the dose of NGF is critical.