AN EXTENDED STUDY - PROTECTIVE EFFECTS OF NERVE GROWTH-FACTOR IN NEOMYCIN-INDUCED AUDITORY NEURAL DEGENERATION

Intracochlear survival of auditory neurons is thought to be a factor c ontributing to the success of cochlear implantations. Damage to the co chlear end-organ is frequently associated with loss of neurites in the osseous spiral lamina. The authors have previously reported a model f or long-term intracochlear administration of pharmacologic agents usin g a technique and device analogous to a cochlear implant. These studie s suggest that nerve growth factor (NGF) protects the auditory nerve f rom the immediate neuro-toxic effects of neomycin. The intent of the p resent investigation is to determine whether NGF can protect auditory neural elements from neomycin-induced degeneration after several weeks have elapsed between the neomycin perfusion and the administration of NGF. Neomycin was infused over 24 hours through an indwelling, intrac ochlear cannula attached to a mini osmotic pump to unilaterally destro y the cochlear end-organ. The pump perfused the cochlea with artificia l perilymph for 2 weeks. Then, the pump reservoirs were surgically rep laced; the new reservoirs delivered either NGF or artificial perilymph for an additional 2 weeks. Spiral ganglion cell densities were measur ed along Rosenthal's canal in the basal, middle, and apical cochlear t urns. This investigation revealed that NGF prevented auditory nerve de generation over the 2-week period, when compared to controls, and that the protective effect was greatest in the neural elements closest to the source of NGF.