Allowing animals to bite reverses the effects of immobilization stress on hippocampal neurotrophin expression.

Acute immobilization stress alters the expression of neurotrophins, including brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), in rat hippocampus. We found that biting may be associated with reduction of systemic stress responses. The purpose of this study was to examine whether neurotrophin expression in rat hippocampus is influenced by biting. Rats were exposed to immobilization stress for 2 h (stress group without biting) or biting for the latter half of 2-hour immobilization stress (biting group). Adrenocorticotropic hormone (ACTH) and corticosterone levels were markedly elevated in the stress group, while the increases in ACHT and corticosterone were suppressed in the biting group. Decreased BDNF mRNA and increased NT-3 mRNA expression in hippocampus were detected on real-time polymerase chain reaction (PCR) in the stress group. The decrease in BDNF mRNA under acute immobilization stress was recovered by biting. In addition, the magnitude of increase in NT-3 mRNA was decreased. No changes in expression of tyrosine receptor kinase B or C, the receptors for BDNF and NT-3, respectively, were observed in this model. These findings suggest that biting influences the alterations in neurotrophin levels induced by acute immobilization stress in rat hippocampus.