Study on Role of Glutamate and its Protection of Neural Toxity in Inner Ear Damage

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Glutamate is a neurotransmitter at the synapse of the primary afferent auditory neuron. When the inner ear is damaged, it is released excessively to the surroundings and aggravates hearing loss by working toxic to them. In the first experiment, we investigated the mechanisms of hearing loss due to transient cochlear ischemia in gerbils. The results showed that cochlear ischemia for 5 min causes delayed neuronal death of the inner hair cells probably due to excitotoxicity of glutamate. This was supported by remarkable increase of glutamate concentration in the perilymph. In the second experiment, effects of hypothermia were investigated in gerbils with cochlear ischemia for 15 min. Hypothermia was very effective to prevent hearing loss due to cochlear anoxia. Loss of the hair cells in hyopthermic group was very few compared to those in normothermic group. In the third experiment, role of glutamate in noise induced hearing loss was studied in mouse which genetically lacks GLAST, a glutamate transporter. In GLAST lacking mouse, hearing loss due to continuous exposure to loud noise was greater than that in wild mouse. As GLAST resorbs glutamate in the synaptic space, metabolism of glutamate in the synapse of the primary afferent auditory neuron is important to prevent hearing loss by noise.