Mouse saliva prevents the beta-nerve growth factor-dependent neurite formation in PC-12 cells

A highly sensitive and specific competitive beta-nerve growth factor radioimmunoassay (beta NGF-RIA) was utilized to determine the beta NGF levels in epinephrine-provoked saliva of adult male and female Swiss-Webster mice. Saliva beta NGF levels in males were higher than in females. In order to assess the immunoreactive material measured in the RIA as biologically active beta NGF, saliva of male and female mice was examined for the neurite outgrowth responsiveness both in the rat pheochromocytoma tumor cell line (PC-12) and 8-day-old chick embryo dorsal root sensory ganglion (DRG) bioassay systems. At concentrations (based on the RIA values) sufficient to saturate both high and low affinity beta NGF-receptors (100 ng and above), saliva of both male and female mice did not elicit neurite outgrowth in the PC-12 bioassay system. In contrast, similar concentrations of saliva stimulated extensive neurites in the DRG bioassay system. When serum of aggression-provoked male mice (which also contained high serum beta NGF levels as indicated by the RIA) was tested, both the PC-12 and DRG bioassay systems readily responded with neurite extension. In the PC-12 bioassay system, saliva of both sexes stimulated cell proliferation and cell aggregation, but not cell substratum-adhesion. Addition of saliva also prevented the cell substratum adhesion and the formation of neurites induced by purified submandibular gland (SMG)-beta NGF. Epidermal growth factor (EGF), a potent mitogen present in mouse saliva, did not prevent the neurite outgrowth induced by the purified SMG-beta NGF. In summary, the results suggest that saliva of adult male and female mice may contain biologically interfering substances for beta NGF-dependent neurite formation in the PC-12 bioassay system.