Physiological aspects of cochlear excitation and neurotransmitter release

Using an in vitro model of the guinea pig, the temporal bone preparation, cochlear physiology was studied. By perfusion of the scala tympani with oxygenated culture medium, the responses of the hearing organ to sound stimuli were maintained over several hours and the functional state was followed by monitoring the electrophysiological pararneters cochlear microphonics (CM) and summating potential (SP). The apical turn of the organ of Corti was viewed in incident light and the images were further improved by availability of a confocal microscope with fluorescent probes staining the sensory cells and other structures of the organ of Corti. With the use of a laser heterodyne interferometer, the mechanical characteristics were investigated by recording the vibratory responses from the apical turns. The frequency tuning and the shapes of the mechanical and electrical responses of the apical turn were very similar to each other. As a measure of functional integrity of the preparation, nonlinearities were demonstrated as saturation of the responses at the best frequency and by the presence of higher frequency harmonic components. The local anaesthetic, tetracaine, reduced the CM, increased the positive SP and changed the polarity of a negative SP to a positive SP. The high frequency slope of the second harmonic of the mechanical response was shifted to a lower frequency. All the effects of tetracaine were reversible. There were no measurable structural changes of the hair cells. Tetracaine's mechanism of action in the cochlea is suggested to be by lowering the stiffness of the stereocilia bundle or of the body of the outer hair cells. The release of aspartate increased in a stimulus-dependent manner at physiological sound levels whereas glutamate increased only at the highest level. Together with immunohistochemistry findings of aspartate in the inner hair cell region where the afferent transmission takes place, this suggests that aspartate is involved in cochlear afferent transmission. A 3 fold increase in aspartate and 1.5 fold increase in glutamate was demonstrated to occur upon high noise stimulation using in vivo microdialysis of the guinea pig cochlea. This high level noise also caused a permanent ABR threshold shift, swelling of the afferent dendrites under the inner hair cell and a small degree of outer hair cell loss. Pre-treatment with 1 mg/kg MK 801, an NMDA antagonist, resulted in nearly complete protection of the afferent dendrites of the inner hair cells, suggesting the possible involvement of aspartate in the excitotoxic mechanism underlying noise trauma