Simultaneous rather than retrograde spiral ganglion cell degeneration following ototoxically induced hair cell loss in the guinea pig cochlea.

Severe damage to the organ of Corti leads to degeneration of the spiral ganglion cells (SGCs) which form the auditory nerve. This degeneration starts at the level of synaptic connection of the peripheral processes (PPs) of SGCs with the cochlear hair cells. It is generally thought that from this point SGC degeneration progresses in a retrograde fashion: PPs degenerate first, followed by the SGC soma with a delay of several weeks to many months. Evidence for this course of events, both in animals and in humans, is not unambiguous, while this knowledge is important since the presence or absence of the different neural elements may greatly influence the response to electrical stimulation with a cochlear implant (CI). We therefore aimed to provide a comprehensive account of the course of SGC degeneration in the guinea pig cochlea after ototoxic treatment. Histological analysis of eighteen healthy and thirty-three deafened cochleas showed that the degeneration of SGCs and their peripheral processes was simultaneous rather than sequential. As the site of excitation for electrical stimulation with a CI may depend on the course of degeneration of the various neural elements, this finding is relevant both for understanding the electrophysiological mechanisms behind cochlear implantation and for recent efforts to induce PP resprouting for improved electrode-neural interface. Since excitation of the PPs is often thought to result in (secondary) longer-latency activity, we tested the hypothesis that having relatively many PPs produces a larger N2 peak in the electrically evoked compound action potential (eCAP); the present findings however do not support this theory. The course of the degeneration process may vary among species, and may depend on the cause of deafness, but the present findings at least indicate that gradual retrograde degeneration of the auditory nerve is not an elemental process following severe damage to the organ of Corti.