Neurotrophic factors regulate neuronal differentiation, as well as cell
survival and proliferation.1 A lack of neurotrophic
support can result in cell death and atrophy of surviving neurons
and their processes.12 Studies of
rabbit and rat olfactory systems have shown that removal of the olfactory epithelium
causes degeneration of some neurons in the olfactory bulbs.13 Additional
studies of young rodents indicate that depriving the olfactory bulbs
of active nerve input by naris occlusion results in reduced bulb
size.8 Restricting
olfactory nerve activity by naris occlusion also results in increased
levels of cell death in the
olfactory bulbs.14 It has recently been shown that neurotrophins can be
released from the neurons, which synthesize them in an activity-dependent manner.15
Since neurotrophins can be released from neurons during stimulation, and some olfactory
bulb neurons can die from a lack of olfactory nerve stimulation, it may be that
neurotrophic factors are transported to the bulb and released by active olfactory nerve
afferents. Cell death in the deprived olfactory bulb therefore
may be due to a lack of trophic support from the olfactory nerve.
This study demonstrates that neurotrophins are expressed by the developing
rat olfactory epithelium, and NT3 immunoreactivity can be detected
in olfactory nerve axons.
Neurotrophin receptors are expressed in the developing forebrain
and olfactory bulb. Based on our results, we propose that, as has been seen in other parts of the
brain, there is anterograde transport of neurotrophins in the rat olfactory system. Future
studies will examine transport of radiolabeled neurotrophins injected
into the olfactory epithelium.
Acknowledgements
The author thanks Dr. Kathleen Guthrie for
her assistance, guidance, and patience throughout this project. The author also
acknowledges Dr. Christine Gall for the opportunity to participate in the research
symposium.
Works Cited
1Yuen,
Eric C., et al. "Neurotrophins and the neurotrophic factor hypothesis." Neural Notes 1 (1996):
3-7.
2Lipton, Peter, and Ronald Kalil. "Neurotrophic
factors-an overview." Promega Notes 50 (1995): 18-22.
3Johnson, Frank, et al. "Neurotrophins
suppress apoptosis induced by deafferentation of an avian motor-cortical
region." Journal
of Neuroscience 17:6 (1997): 2101-11.
4Altar, Anthony C., et al. "Anterograde
transport of brain-derived neurotrophic factor and its role in the brain." Nature 389
(1997): 856-60.
