Aging inhibits olfactory recovery from traumatic olfactory system injury

Although recent advances in treatment of traumatic olfactory dysfunction, which had a low rate of improvement through treatment, have increased the improvement rate in younger patients, it remains low in middle-aged and older patients. Although olfactory function declines with age, its impact on traumatic dysfunction recovery remains unclear. This study was conducted to investigate the effects of aging on olfactory nerve regeneration and olfactory function recovery in an animal model of traumatic olfactory injury. Behavioral experiment using olfactory preference and morphological measurements were performed on OMP-tau-lacZ mice at 3 months, 1.0 year and 1.5 years old, in which the olfactory nerve can be visualized after X-gal staining. In the behavioral experiments, we measured the time required for mice to search for and find a piece of potato chip hidden under the cage bedding. The head trauma model included a bilateral olfactory nerve transection (BNTx) group and a control craniotomy alone without BNTx group. In each age and surgery group, behavioral olfactory tests were performed 6 weeks (42 days) and 1.0 year (365 days) after surgery. The reinnervation of olfactory nerve axons to the olfactory bulb was then assessed in histological tissue samples from the mice. The older BNTx group performed worse in behavioral tests than the younger BNTx group, and reinnervation of olfactory nerve axons to the olfactory bulb was lower in the older BNTx group. These findings suggest that aging leads to poor olfactory nerve axon regeneration and poor recovery of olfactory function after olfactory nerve transection injury.