Glutamate induces apoptosis in cultured spiral ganglion explants

https://doi.org/10.1016/j.bbrc.2007.03.098Get rights and content

Abstract

Traumatic sound exposure, aminoglycoside antibiotics, cochlea ischemia or traumatic stress leads to an excessive release of glutamate from inner hair cells into the synaptic cleft. The high glutamate concentration can cause a swelling and destruction of the dendrites of spiral ganglion neurons of type I as well as a reduction in the number of neurons. This may be a cause of hearing loss. The mechanism causing the reduction of neurons is still not known. Apoptosis, also called programmed cell death, could be involved. In this study, cultured spiral ganglion explants were incubated with glutamate in high concentrations. Neurite outgrowth was determined and additionally a new method was established for studying the morphology of single spiral ganglion neurons. For the first time it was shown that glutamate induces apoptosis of spiral ganglion neurons, which could be blocked selectively by a caspase-3 inhibitor. This could offer a new therapeutic strategy for hearing disorders.

Section snippets

Materials and methods

Preparation of spiral ganglion explants. All animal experiments were approved by the local administration (Regierung Oberbayern, Munich, No. 211–25314/2002). The preparation of the spiral ganglion explants was performed as described [14], [15], [16] using gerbils (Charles River, Sulzfeld, Germany) at day 4 post-natal. After cervical transfection the heads of the gerbils were skinned, the skulls were opened midsagittally, and the brain halves were removed. The following steps were carried out

Glutamate induces apoptosis in cultured spiral ganglion explants

Fig. 1a shows a typical spiral ganglion explant incubated with culture medium for 48 h and stained by immunohistochemistry for detecting neurofilament. The neurites extend from the spiral ganglion explant edge. Near the edge they form fascicles and further away they split into individual fibers. The neurites are adjacent to non-neuronal cells, which seem to be glial cells and fibroblasts. If the spiral ganglion explants are incubated with 5 mM glutamate for 48 h, the extension of the neurites as

Discussion

Glutamate exposure to spiral ganglion explants reduced the length of neurites and led to apoptosis of the spiral ganglion neurons depending on the glutamate concentration. This is the first study that directly shows that apoptosis is induced by the neurotoxic effect of high concentrations of glutamate on neurites and spiral ganglion cells.

This is in congruence with observations of Janssen et al. [4]. In his study post-natal treatment with glutamate in rats (4 g glutamate/kg/day, i.p., post-natal

Acknowledgments

We gratefully thank Professor Dazert for showing his spiral ganglion explant preparation. The authors were funded by KKF (a hospital internal funding organization of the Faculty of Medicine of the Technical University of Munich).

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