Regeneration of olfactory sensory neurons and reconnection in the aging hamster central nervous system

Olfactory neuron recovery and axon growth was studied in 12-24 month old hamsters after unilateral olfactory nerve transection. At recovery times ranging from 4 to 126 days olfactory nerve regeneration and axon reconnection to the olfactory bulb were examined by anterograde horseradish peroxidase (HRP) neurohistochemistry and electron microscopy. Nerve transection produced immediate retrograde neuron degeneration and there was no HRP reaction product in the bulb at 4 days post transection. By day 35, centrally growing olfactory axons had reached the bulb. Axons formed glomeruli smaller than those in the unoperated control bulb and they were not always confined to the glomerular layer of the bulb. Some animals showed robust fiber growth with axon fascicles penetrating the different layers of the bulb and forming ectopic glomeruli along their path. Second order bulb neurons contained wheat germ agglutinin HRP reaction product, indicating that transneuronal transport had occurred. Electron microscopy confirmed transneuronal transport between olfactory axons and second order bulbar neurons. These results show that the capacity for olfactory neuron recovery and reconnection persists in the hamster well into old age.