Fibroblast growth factor-1 (FGF-1) effects on spiral ganglion cells in themammalian

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.