Transient expression by hair cells, increasing levels of FGF-1 mRNA in neon
atal rat spiral ganglion neurons and strong expression in adulthood, make F
GF-1 a candidate to be associated with development and maintenance of the m
ammalian spiral ganglion.
To test this hypothesis, dissociated spiral ganglion cells from 5 day old r
ats were cultured in the presence of FGF-1 at 100 ng/ml plus heparan sulfat
e proteoglycans (HSPG) at 500 ng/ml for 72 hours. Spiral ganglion cells inc
ubated with FGF-1/HSPG achieved an average neurite length of 323 mu m while
control cells gained an average neurite length of 203 mu m.
The results of this study are consistent with our previous findings in whol
e spiral ganglion explants (3) where FGF-1 incubation significantly stimula
ted neurite outgrowth at about the same range. However, stimulation of neur
ite outgrowth in dissociated spiral ganglion cells suggests that FGF1 direc
tly binds to FGF receptors on the surface of spiral ganglion neurons and/or
neurites instead of acting via intermediate cells such as glia. Since FGF
receptor mRNA was found to be expressed only at very low levels in neonatal
spiral ganglion neurons (7) it is possible that the receptors are highly l
ocalized, perhaps to neurite growth cones. Alternatively,an unknown FGF rec
eptor or splice variant may be expressed in these cells. Adequate FGF-1 app
lication to the human inner ear may stimulate spiral ganglion cell survival
and neurite extension after hair cell loss in patients suitable for cochle
ar implant treatment. By creating a closer contact between spiral ganglion
cells and the electrode, FGF-1 might also improve the efficacy of cochlear
implants.