Chronic electrical stimulation does not prevent spiral ganglion cell degeneration in deafened guinea pigs

Several studies have demonstrated that treatment with intracochlear chronic electrical stimulation (CES) protects spiral ganglion cells (SGCs) from degeneration in deafened animals. Other studies could not confirm this effect of CES. The present study examined whether CES in a mode as presented in cochlear implant users (amplitude modulated, high pulse rate) affects survival, morphology and functionality of SGCs in deafened guinea pigs. Eleven guinea pigs were implanted in the right cochlea with an electrode array to monitor the electrically evoked auditory brainstem responses (eABRs). The guinea pigs were deafened four weeks later. Two days after deafening, monopolar CES was started in five animals through three electrodes in the basal cochlear turn. CES lasted 4 hours per day, five days per week, for six weeks. SGC packing densities, perikaryal area, cell circularity, amplitudes of suprathreshold eABRs and eABR thresholds were not affected by CES. SGCs of all implanted cochleae were larger and more circular than SGCs in unimplanted cochleae, but this did not depend on CES treatment. Interestingly, an increase in eABR latencies observed after deafening, occurred faster in CES-treated than in untreated animals. In conclusion, amplitude-modulated chronic electrical stimulation with a high pulse rate does not affect survival, morphology and functionality of spiral ganglion cells with the exception of eABR latencies.