Role of nerve growth factor in the olfactory system

Olfactory neurons are unique in the mammalian nervous system because of their capacity to regenerate in adult animals. It has been shown that olfactory receptor cells located in the olfactory epithelium are replaced on a continuous basis and in response to injury throughout the life span of most species. NGF, which is one of the neurotrophic factors, is present in many areas of the central and peripheral nervous system. It has been shown that NGF in the olfactory bulb plays a role in the survival of cholinergic neurons in the horizontal limb of the diagonal band (HDB). Recent studies of NGF in the olfactory bulb suggest that it is involved in the development, maintenance, and regeneration of olfactory receptor cells. In this study, we review reports examining the relationship between NGF in the olfactory bulb and neuronal regeneration and development in the mammalian olfactory systems. Low- and high-affinity NGF receptor immunoreactivity is markedly expressed during regeneration and at different stages of development in the mouse olfactory system. This level of immunoreactivity is no longer present after completion of regeneration and at maturation. Other findings indicate that NGF injected into the olfactory bulb is transported retrogradely to the olfactory epithelium. It has also been shown that continuous anti-NGF antibody injection into the olfactory bulb causes degeneration and olfactory dysfunction. Administration of NGF directory into nasal cavity results in an increase in the expression of olfactory marker protein within the olfactory epithelium in axotomized rats. These findings suggested that the presence of NGF in the olfactory bulb plays an essential role in regeneration, maintenance, and development in the olfactory system of mammals.