Exacerbation of noise-induced hearing loss in mice lacking the glutamate transporter GLAST

Abstract

Acoustic overstimulation is one of the major causes of hearing loss. Glutamate is the most likely candidate neurotransmitter for afferent synapses in the peripheral auditory system, so it was proposed that glutamate excitotoxicity may be involved in noise trauma. However, there has been no direct evidence that noise trauma is caused by excessive release of glutamate from the inner hair cells (IHCs) during sound exposure because studies have been hampered by powerful glutamate uptake systems in the cochlea. GLAST is a glutamate transporter highly expressed in the cochlea. Here we show that after acoustic overstimulation, GLAST-deficient mice show increased accumulation of glutamate in perilymphs, resulting in exacerbation of hearing loss. These results suggest that GLAST plays an important role in keeping the concentration of glutamate in the perilymph at a nontoxic level during acoustic overstimulation. These findings also provide further support for the hypothesis that IHCs use glutamate as a neurotransmitter.

Fig. 1.

Images by confocal laser scanning microscopy.A, B, Immunofluorescence for GLAST in the cochlea in wild-type (A) and GLAST-deficient (B) mice. The interference image is superimposed on the immunofluorescent image. C, Double immunofluorescence for GLAST (red, rabbit antibody) and glutamine synthetase (green, mouse) in the cochlea in wild-type mice. Green areas (indicating single labeling for glutamine synthetase) occur in the central parts of the supporting cells, whereas yellow areas (indicative of colocalization) occur along the margins of these cells.D, A bright field of C.IHC, Inner hair cell; OHC, outer hair cell; LIM, limbus; SG, spiral ganglion;TM, tectorial membrane; BM, basilar membrane. Scale bars: A, B, 100 μm;C, D, 50 μm.

Fig. 2.

Changes in glutamate concentration in the perilymph before, during, and after 105 dB sound exposure in wild-type (n = 8, open circles) and GLAST-deficient mice (n = 8, closed circles). Data are mean ± SEM (bars) values.

Fig. 3.

Shifts of the ABR thresholds after 105 dB sound exposure in wild-type (n = 8, open circles) and GLAST-deficient (n = 8,closed circles) mice. Data are mean ± SEM (bars) values. (* p < 0.005; **p < 0.05; data were analyzed by ttest).

Fig. 4.

Effect of 105 dB sound exposure on the auditory dendrites below the IHCs in wild-type (A,C, E) and GLAST-deficient (B, D, F) mice. Typical transmission electron micrographs showing IHC region, presound exposure (A, B), 5 min (C, D), and 120 min (E, F) after sound exposure.Arrows indicate massive swelling of structures in dendrite region. All micrographs are taken from the basal turn of the cochlea. Scale bars, 2 μm.