Ouabain Induces Apoptotic Cell Death in Type I Spiral Ganglion Neurons, but not Type II Neurons

Abstract

Application of ouabain to the intact round-window (RW) membrane of the gerbil cochlea induces apoptosis in most spiral ganglion neurons (SGNs), leaving a few neurons intact (Schmiedt et al. 2002). Here, physiological measures and immunostaining were used to examine the process of SGN degeneration at 3, 6, 12, and 24 h, 4 days, and 1 and 5 months after ouabain treatment. The few remaining neurons surviving up to 5 months after ouabain treatment were immunoreactive for peripherin, a type II neuron marker. Peripherin-positive cell counts indicate that about 7% of the SGNs in the gerbil cochlea are type II neurons, and these neurons survive intact after ouabain treatment. Ouabain exposure had little effect on the outer hair cell and lateral wall systems, even after a 5 month loss of auditory-nerve function. The cellular locations of cytochrome c, poly (ADP-ribose) polymerase (PARP), and activated caspase 3 were examined in control and ouabain-treated cochleas. A redistribution of cytochrome c in peripherinnegative (type I) neurons was observed at 3 h after ouabain exposure. Degraded PARP and activated caspase 3 were also detected in peripherin-negative SGNs at 6 and 24 h after treatment, respectively. These results suggest that the redistribution of cytochrome c is an early event during apoptosis in type I SGNs and that activation of PARP and caspase 3 are associated with apoptosis in these cells. Calcineurin and NF-kB are two important signaling pathways that may modulate cell survival in the central nervous system. Here, we found that calcineurin and NF-kB selectively labeled type II neurons. It is speculated that the high levels of calcineurin and NF-kB in type II SGNs, as compared with type I SGNs, may play protective roles in enhancing the survival of type II neurons exposed to ouabain.

Distribution of the Na,K-ATPase α Subunit in the Rat Spiral Ganglion and Organ of Corti

“…Specifically, these experiments show threshold increases or complete abolishment of the compound action potential without a reduction of the endocochlear potential (Schmiedt et al 2002;Wang et al 2006). Moreover, chronic exposure to ouabain is associated with degeneration of type I but not type II SGNs in the gerbil cochlea (Lang et al 2005). These findings are consistent with our observation of the NKAα3 in type I but not type II spiral ganglion afferent terminals.…”

Transplantation of Mouse Embryonic Stem Cells into the Cochlea of an Auditory-Neuropathy Animal Model: Effects of Timing after Injury

“…Our previous studies have shown that application of ouabain to the RW niche of the gerbil inner ear results in degeneration of most SGNs without cell losses within the organ of Corti and cochlear lateral wall Lang et al 2005). In this model, a series of apoptotic events, including a redistribution of cytochrome c and poly (ADP-ribose) polymerase and activation of caspase 3, occurs in most ganglion cells within 2 days (Lang et al 2005).…”

“…In this model, a series of apoptotic events, including a redistribution of cytochrome c and poly (ADP-ribose) polymerase and activation of caspase 3, occurs in most ganglion cells within 2 days (Lang et al 2005). Most spiral ganglion cells along with their peripheral and central processes were absent 1 week after ouabain exposure .…”

“…The methods for recording the compound action potential (CAP) response, distortion product otoacoustic emissions (DPOAEs), and endocochlear potential (EP) were similar to those previously described (Lang et al 2005(Lang et al , 2006. Physiological data were obtained from ESC-implanted ears and a group of untreated ears.…”

“…Further understanding of the host environmental changes that regulate the survival and differentiation of transplanted cells is crucial in the development of effective and safe cell replacement treatment for SNHL. The ouabain-induced auditory neuropathy model has been shown to selectively destroy most spiral ganglion neurons (SGNs) with little effect on the morphology and function of the sensory hair cells and cells in the cochlear lateral wall Lang et al 2005;Corrales et al 2006). Thus, this model is ideal for investigating the fate of stem/progenitor cells in the inner ear as well as the effects of the host microenvironment on survival and differentiation of the transplanted cells.…”