Hair Cell Electromotility Research Articles

Effect of salicylate on electrically evoked otoacoustic emissions elicited in the first and third turns of the guinea pig cochlea

Objective—Electrically evoked otoacoustic emissions (EEOAEs) are sounds emitted in the ear canal when an alternating current is delivered to the cochlea. Salicylate, which causes reversible hearing loss, decreases both the electromotility of the outer hair cells (OHCs) and the EEOAE output. The frequency response of EEOAEs is known to depend on the location of the stimulating electrode. Using electrodes placed in the first or third turn of the cochlea, we measured EEOAEs before and after salicylate administration in order to clarify the frequency range of EEOAEs from which exact information can be acquired.Material and Methods—A sinusoidal electrical signal (160 μA root-mean-square) at frequencies ranging from 500 Hz to 16 kHz was delivered to the first or third turn of the scala tympani of the guinea pig cochlea in order to determine EEOAEs. The EEOAE outputs were measured before and every 5 min after administration of salicylate (500 mg/kg) for 60 min. The threshold of the compound action potential (CAP) was measured before and 60 min after salicylate administration.Results—CAP thresholds were reduced at frequencies ranging from 2 to 12 kHz after salicylate administration. The maximum EEOAE output recorded with the first-turn electrode was obtained at 12 kHz with a gentle slope of low cutoff, whereas the frequency response of the EEOAE recorded with the third-turn electrode demonstrated a steep dip at 2 kHz.Conclusion—EEOAE reduction after salicylate administration was restricted to frequencies close to that of the characteristic frequency (CF) of the electrode position. This was probably related to the fact that the EEOAE is the sum of waves from the CF of the electrode position and the stimulus frequency. In terms of studying the electromotility of OHCs, useful information obtained from EEOAEs is restricted to the narrow frequency band close to the CF of the electrode location.

Elastic moduli of the piezoelectric cochlear outer hair cell membrane

The outer hair cell is a specialized cell in the mammalian cochlea, believed to amplify incoming sound waves. This amplification is associated with the outer hair cell's electromotility, a unique cellular phenomenon of voltage-dependent length changes. Outer hair cell properties can be described in terms of the piezoelectric relationships, and the elastic moduli are a key part of them. We revisit the problem of estimating the elastic moduli of the outer hair cell composite membrane (wall) where two methods have previously been proposed. We analyze the two methods, while taking into account experimental ranges of the measured parameters. We have shown that cell stiffness is the critical parameter that determines the difference between the method predictions, and we have found a range of stiffness where the results are reasonably close. The elastic moduli corresponding to this range can be recommended for estimation of the characteristics of the piezoelectric model.

The effects of auditory research on clinical practice

Recent research has shown that only the inner hair cells pass information on to the brain while the outer hair cells serve as an active amplifier and thus stimulate the inner cells. The electromotility of the outer hair cells is very vulnerable. If it is lacking, sensorineuronal hearing loss occurs. Substances, that compete for the chloride combining site of the motor protein prestin, such as salicylate, might have a blocking effect on the regulation of electromotility. On the other hand, the control of the intracellular Ca(2+) level and the intracellular combination of ions in the outer hair cells might protect their electromotile properties against damage caused by harmful substances (ototoxic drugs) or mechanisms (exposition to noise), which would otherwise lead to irreversible sensorineuronal hearing loss. This is because an increase in the intracellular Ca(2+) level activates phosphorylating enzymes. Thus the stiffness of the outer hair cells is reduced, causing an increase in electromotility.

Voltage responses to tones of outer hair cells in the basal turn of the guinea-pig cochlea: significance for electromotility and desensitization

Voltage responses were recorded from outer hair cells (OHCS) in the basal coil of the guinea-pig cochlea in response to tones at frequencies above the characteristic frequency (CF) presented together with a 100 Hz tone at 80 dB or 85 dB sound pressure level (SPL). The amplitude and polarity of voltage responses to a 100 Hz, 85 dB SPL tone were altered when presented together with tones at frequencies above CF according to the frequency and level of the high-frequency tone, OHC phasic (ac) (greater than 500 microV) but not tonic (dc) voltage responses were elicited by the high-frequency tone. Thus the responses of OHCS to low-frequency tones can be altered when presented together with a high-frequency tone without an apparent dc change in membrane potential. Recordings were made from an OHC during cochlear desensitization through exposure to an intense tone. The maximum voltage response to high-level low-frequency tones remained unchanged, although the OHC response to high-frequency tones became less sensitive to low-level stimuli and more linear as a function of level. It is suggested that desensitization is associated with a change in the mechanical properties of the cochlea, possibly associated with the OHCS themselves, and not with inactivation of the transducer channels. The amplitude of the OHC ac voltage response was measured at neural threshold, and the consequences of these measurements on hair cell electromotility are considered.