The role of neurotrophins related to stress in saliva and salivary glands.

Nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) are well-studied neurotrophins involved in neurogenesis, differentiation, growth, and maintenance of selected peripheral and central populations of neuronal cells during development and adulthood. Neurotrophins, in concert with the hypothalamic-pituitary-adrenal (HPA) axis, play key roles in modulating brain plasticity and behavioral coping, especially during ontogenetic critical periods, when the developing brain is particularly sensitive to external stimuli. Early life events, such as psychophysical stress, affect NGF and BDNF levels and induce dysregulation of the HPA axis, thereby affecting brain development and contributing to inter-individual differences in vulnerability to stress or psychiatric disorders. Immobilization stress modifies BDNF mRNA expression in some organs. We studied the effect of immobilization stress on BDNF and its receptor tyrosine receptor kinase B (TrkB) in rat submandibular glands, and found increased BDNF expression in duct cells under immobilization stress. Upon further investigation on the influence of salivary glands on plasma BDNF using an acute immobilization stress model, we found that acute immobilization stress lasting 60 min significantly increases the plasma BDNF level. However, plasma BDNF elevation is markedly suppressed in bilaterally sialoadenectomized rats. This suggests that salivary glands may be the primary source of plasma BDNF under acute immobilization stress. This report reviews the structure of salivary glands, the role of neurotrophins in salivary glands, and the significance of BDNF in saliva and salivary glands, followed by a summary of the evidence that indicates the relationship between immobilization stress and BDNF expression within salivary glands.