Spiral ganglion cell morphology in guinea pigs after deafening and neurotrophic treatment

Abstract

It is well known that spiral ganglion cells (SGCs) degenerate in hair-cell-depleted cochleas and that treatment with exogenous neurotrophins can prevent this degeneration. Several studies reported that, in addition, SGC size decreases after deafening and increases after neurotrophic treatment. The dynamics of these cell size changes are not well known. In a first experiment we measured size, shape (circularity) and intracellular density of SGCs in guinea pigs at various moments after deafening (1, 2, 4, 6, and 8 weeks) and at various cochlear locations. In a second experiment, the effect of treatment with brain-derived neurotrophic factor (BDNF) on SGC morphology was investigated at various cochlear locations in deafened guinea pigs. We found that SGC size gradually decreased after deafening in the basal and middle cochlear turns. Already after one week a decrease in size was observed, which was well before the number of SGCs started to decrease. After BDNF treatment SGCs became noticeably larger than normal throughout the cochlea, including the middle and apical turns, whereas an effect on survival of SGCs was primarily observed in the basal turn. Thus, both after deafening and after neurotrophic treatment a change in size occurs before survival is affected. Morphological changes were not restricted to a subpopulation of SGCs. We argue that although changes in cell size and changes in survival might be manifestations of two separate mechanisms, morphological measures such as size, circularity and intracellular density are indicative for survival and degeneration.

Introduction

Degeneration and protection of the auditory nerve have been studied extensively over the past decades. An important impetus for these studies is the cochlear implant, which relies on a viable nerve in order to be effective in providing electrical hearing in patients with profound sensorineural hearing loss. It is well known that spiral ganglion cells (SGCs) degenerate after loss of cochlear hair cells (Ylikoski et al., 1974; Spoendlin, 1975; Webster and Webster, 1981; Koitchev et al., 1982; Leake and Hradek, 1988; McFadden et al., 2004; Shepherd et al., 2004; Versnel et al., 2007). There is a gradual loss of SGCs, and the cells that survive are smaller and less ovoid-shaped than SGCs in normal cochleas (Staecker et al., 1996; Leake et al., 1999; Richardson et al., 2005; Shepherd et al., 2005; Glueckert et al., 2008; Agterberg et al., 2008). This degeneration of SGCs is thought to be a consequence of loss of neurotrophic support by the hair cells, as the absence of endogenous neurotrophins, such as brain-derived neurotrophic factor (BDNF) or neurotrophin-3 (NT-3), leads to a loss of SGCs (Ernfors et al., 1995; Fritzsch et al., 1999; Schimmang et al., 2003). Administration of exogenous neurotrophins can prevent SGC loss (Ernfors et al., 1996; Staecker et al., 1996; Miller et al., 1997; Shinohara et al., 2002; Gillespie et al., 2004; Shepherd et al., 2005; Glueckert et al., 2008; Agterberg et al., 2008; Song et al., 2009; Leake et al., 2011). An additional effect of such treatment is that cells become larger than normal (McGuinness and Shepherd, 2005; Shepherd et al., 2005; Glueckert et al., 2008; Agterberg et al., 2008; Leake et al., 2011).

A significant reduction of SGC perikaryal area (by more than 20%) was observed as early as two weeks after deafening, while the number of SGCs remained near-normal (Agterberg et al., 2008; their Fig. 6). Neurotrophic treatment of deafened animals might lead to larger surviving SGCs without enhancement of survival (Richardson et al., 2005). Considering these observations one might say that a change in cell size occurs prior to cell death or cell rescue. In this paper, we address this notion in two experiments in deafened guinea pigs. First, the morphological changes of SGCs after deafening were examined as a function of both time after deafening and cochlear location. Specifically, it was examined whether SGC size or shape changed after one week, when the number of cells is not yet decreased (Versnel et al., 2007). Second, the morphological changes of the SGCs after deafening and subsequent neurotrophic treatment were examined as a function of cochlear location. Dependence on cochlear location is expected for two reasons. First, gradients along the cochlea are found with respect to, among others, expression levels of BDNF and NT-3, and with respect to discharge properties of SGCs (Adamson et al., 2002; Schimmang et al., 2003; Davis and Liu, 2011). Second, delivery of drugs through the round window membrane or through a cochleostomy in the basal turn will allow the drugs to reach the apical turn, albeit in lower concentrations than in the basal turn (Salt and Plontke, 2009; Hahn et al., 2012). Also, the effect of BDNF treatment is often more pronounced in the basal turn (Shepherd et al., 2005; Glueckert et al., 2008; Agterberg et al., 2008); therefore, we examined here whether the effect of BNDF on SGC morphology depends on location in a similar fashion.