AN EXAMINATION OF THE SPIRAL GANGLION NEURONS OF THE COCHLEA FOLLOWING CHRONIC USE OF A COCHLEAR IMPLANT.
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Cochlear implants have helped many profoundly deaf individuals regain auditory communication. However, the extent to which hearing is restored is variable, and may depend on fundamental changes to the auditory pathways following deafness. In the present study, I used the congenitally deaf white cat to explore cochlear and auditory nerve structures following cochlear implant use. Many white cats are congenitally deaf from birth, and this was confirmed using auditory brainstem potentials in cats used for this study. These congenitally deaf cats exhibit a lack of spike activity in the auditory nerve, and abnormalities in a specialized auditory nerve ending called the endbulb of Held (Ryugo et al 1996, Ryugo et al 1998). Cochlear implants (Clarion 1.2 or II, Advanced Bionics) were placed in six congenitally deaf cats, and auditory nerve endings and spiral ganglion neurons were examined using light and electron microscopy. Auditory nerve endings originating in the stimulated cochlea appeared similar to those from normal hearing cats. This result suggests that auditory nerve activity early in life can restore normal synaptic function in the congenitally deaf animal. Surprisingly, auditory nerve endings originating in the contralateral cochlea, which were not directly stimulated, exhibited an intermediate phenotype, similar to that found in cats with elevated hearing thresholds. This intermediate phenotype was not found in a cat who received a non-functional implant. This result suggests widespread, top-down, bilateral effects of the cochlear implant, perhaps lengthening the efficacy window for a second implant. The spiral ganglion neurons of the cochlea did not show an effect of stimulation in terms of total neuron number or neuron size. This is consistent with the iii finding that degree of spiral ganglion neuron survival is not closely related to clinical benefit from a cochlear implant. Taken together, these results demonstrate that positive effects of cochlear implantation occur at the earliest auditory synapse in the central nervous system. The implication for clinicians is that a single cochlear implant is an intervention which prevents abnormal central development. The bilateral effects imply that a single implant primes the contralateral nerve for a second implant.