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PubMed1991

Cochlear efferent neurones and protection against acoustic trauma: protection of outer hair cell receptor current and interanimal variability.

We have measured the changes in neural and microphonic sensitivity in the basal turn of the guinea-pig cochlea produced by intense acoustic overstimulation (10 kHz, 115 dB SPL for 60 s and 150 s). As reported previously, the drop in neural and microphonic sensitivities observed after overstimulation were highly correlated [Patuzzi et al. (1989) Hear. Res. 39, 189-202]. Presentation of a non-traumatizing pure-tone to the contralateral ear (10 kHz, 80 dB SPL) during acoustic overstimulation reduced the amount of acoustic trauma measured using the neural response or the microphonic response. Transection of the medial olivo-cochlear system of efferent fibres at the floor of the fourth ventricle abolished this protective effect of contralateral sound and dramatically reduced the variability in the data. Since the low-frequency microphonic is a simple measure of the receptor current through the outer hair cells, and this current probably plays a part in enhancing the mechanical sensitivity of the cochlea, the protection of the microphonic we have observed suggests that the efferent system protects neural sensitivity by protecting the mechano-electrical transduction of outer hair cells. The drop in variability after sectioning the efferents also suggests that inter-animal variations in susceptibility to noise trauma may be a consequence of differing tonic activity of the efferents, and/or a variation in the sensitivity of the efferent pathway.

Action Potentials↗
PubMed1989

Changes in cochlear microphonic and neural sensitivity produced by acoustic trauma.

The low-frequency (200 Hz) microphonic potentials at the round window and in the organ of Corti of the first turn of the guinea pig cochlea have been measured before and after acoustic overstimulation. Reductions in the amplitude of this microphonic after loud sound are highly correlated with neural threshold elevation in this region. The fall in the microphonic amplitude appears due to an inactivation of mechano-electrical transduction channels at the apex of the outer hair cells into a closed state. These results are consistent with the idea that the current through the outer hair cells controls the mechanical sensitivity of the organ of Corti, and that the temporary loss of mechanical and neural sensitivity following loud sound is due to a simple inactivation of the mechano-electrical transduction channels.

Action Potentials↗
PubMedDate not supplied

Inner ear pathology in the deafness mutant mouse.

A distinctive cochlear pathology was found in deafness mutant mice. There was a delay in the formation of the fluid-filled Nuel and tunnel spaces in the organ of Corti, the hair cells were distorted and degenerate, and there was poor maintenance of synapses. No hair cells appeared normal by TEM, but SEM revealed some areas where stereocilia appeared relatively normal, suggesting that SEM of the surface of the organ of Corti is not necessarily a good indicator of hair cell pathology in hereditary hearing impairment. Mutant mice show normal development of endocochlear potential, but have no measurable cochlear microphonics or compound action potential. The data suggest that the deafness gene affects the organ of Corti and that cochlear hair cells in deafness mice are never functional.

Action Potentials↗
PubMed1980

Intracochlear sound pressure measurements in guinea pigs.

The intracochlear sound pressure in guinea pigs was measured in the scala vestibuli of the first, second and third turns as well as in the scala tympani of the first and second turns. The acoustic stimuli were pure tones delivered over the frequency range 30--20 000 Hz at sound levels ranging from 60 to 100 dB. The results achieved show the sound pressure in scala vestibuli to be practically in phase in the first three turns. In scala tympani the pressure varies within wide limits when passing from the first to the second turn, but it is equal to the pressure in scala vestibuli at frequencies in excess of the best frequency of the point considered. The difference in instantaneous pressure acting on the basilar membrane exhibits the characteristics of a traveling wave. This pressure difference corresponds to the displacement of the basilar membrane evaluated from recordings of the microphonic potential.

Basilar Membrane↗
PubMed1993

Furosemide ototoxicity is enhanced in analbuminemic rats.

OBJECTIVE: The purpose of this study was to investigate the effect of furosemide on the endocochlear potential (EP) of Sprague-Dawley rats and rats that lack albumin in their serum (Nagase analbuminemic rats [NAR]). DESIGN: Group comparisons between analbuminemic rats and normal Sprague-Dawley rats was carried out, with statistical evaluation using the Student's t test. SETTING: Experiments were carried out in a sound-attenuated booth in a research laboratory. SUBJECTS: Young adult Sprague-Dawley and analbuminemic rats (NAR) 50 to 80 days of age were used as experimental animals. INTERVENTIONS: Subjects were anesthetized with ketamine and xylazine. Furosemide, 35 mg/kg, was injected intravenously in each of three groups: NAR rats, NAR rats pretreated with albumin and normal Sprague-Dawley rats. MAIN OUTCOME MEASURES: Endocochlear potential was measured via the round window membrane approach. Urine samples were collected with a metabolic cage, and volumes were recorded. RESULTS: Sprague-Dawley rats had a very slight EP reduction following furosemide. The NAR rats, however, were found to have an extremely large and rapid reduction of the EP one order of magnitude greater. The NAR rats pretreated with albumin had a significantly smaller reduction of EP than NAR rats not receiving albumin. However, NAR rats pretreated with albumin had a significantly greater urine output than control NAR rats. CONCLUSIONS: These findings support the hypothesis that the access of furosemide to its site of ototoxic action in the cochlea depends on the quantity of unbound furosemide in the serum.

Action Potentials↗
PubMed1990

Effect of Escherichia coli endotoxin on cochlear potentials following its application to the chinchilla middle ear.

The compound action potential (CAP) of the eighth nerve and the endocochlear potential (EP) were examined in the chinchilla as an animal model when Escherichia coli endotoxin (100 micrograms) was applied to the middle ear cavity. A significant elevation of the CAP threshold at 2, 3, and 4 kHz was observed 48 h after the instillation of endotoxin, but this hearing loss was thought to be caused by a conductive component. No significant change in the CAP threshold was recognized 30 days after instillation. The EP in either period showed no significant difference. These findings indicate that the application of endotoxin at the concentration used in the present study does not cause a cochlear disturbance.

Action Potentials↗
PubMed1982

Supporting-cell and extracellular responses to acoustic clicks in the free-standing region of the alligator lizard cochlea.

1. Acoustic clocks were delivered to the tympanic membrane of anesthetized alligator lizards, and electric responses were measured within the free-standing region of the cochlea using glass micropipettes. Responses were recorded intracellularly in supporting cells and extracellularly in the receptor organ and in the scalae. Gross responses were also recorded with wire electrodes in scala tympani. 2. Intra- and extracellular responses contain two components: (1) an early (with latent period less than 0.15 ms after the onset of the click), rate-independent component presumed to originate in the receptor cells, which we call the 'receptor component'; and (2) a later (with latent period from 2 to 5 ms after the onset of the click), rate-dependent component presumed to originate in the primary neurons, which we call the 'neural component'. 3. The receptor component consists of a positive, slow, polarity-independent potential which is superimposed on a small, oscillatory, polarity-dependent potential. The average magnitude of the receptor component in supporting cells (0.72 mV for -20 dB clicks) is about 10 times that in the extracellular spaces and about 1/5 of that recorded in receptor cells. This component depends nonlinearly on the sound stimulus in the -20 to -55 dB range of click levels. 4. The average magnitude of the neural component in supporting cells (0.3 mV for clicks at -20 dB and 10 clicks/s) is about 5-10 times larger than that in the extracellular spaces. 5. The receptor and neural components have different distributions within the cochlea. The slow potential of the receptor component has positive polarity within the receptor organ and in scala tympani, and negative polarity in scala media. In contrast, the neural component has approximately the same biphasic (negative then positive) waveform in all extracellular compartments where it was detected. However, the neural component has a larger magnitude and an inverted (positive then negative) waveform in supporting cells. The neural component has not been detected in receptor cells.

Acoustic Stimulation↗
PubMed1986

Single-unit response at the round window of the guinea pig.

In guinea pigs the unit contribution (unit action potential, ap) to the response of the round window was computed following the method used by Kiang and co-workers (1976), i.e., fibre discharges registered by a microelectrode in the nerve were used as trigger pulses for the averaging process of the corresponding ap registered with a gross electrode at the round window. Normally the ap was independent of the fibre-CF, had a diphasic waveform, and its amplitude was about 0.1 microV. Small inter-animal differences were found in waveform and amplitude of the ap. In a pilot experiment exploring pathological influences on ap, the auditory nerve was stretched to mimic the effect of some acoustic nerve tumors. We found that the waveforms of both ap and compound action potential (CAP) changed. The results indicate that in normal guinea pig cochleas the existence of an elementary unit waveform can be used in the convolution of the CAP [Goldstein, M.H. Jr. and Kiang, N.Y.S. (1958) J. Acoust. Soc. Am. 30, 107-114]. In abnormal cochleas, however, deteriorated aps may disturb the simple convolution concept of the CAP.

Action Potentials↗
PubMed1987

Kynurenic acid and gamma-D-glutamylaminomethylsulfonic acid suppress the compound action potential of the auditory nerve.

Kynurenic acid and gamma-D-glutamylaminomethylsulfonic acid, two excitatory amino acid antagonists, were perfused through the guinea pig cochlea while monitoring various cochlear potentials. Both drugs (0.6-10 mM) reduced the magnitude of the compound action potential (CAP) of the cochlear nerve without much effect on other potentials. The results are consistent with the hypothesis that the hair cell transmitter is an excitatory amino acid, possibly L-glutamate.

Action Potentials↗
PubMed1988

The active process is affected first by intense sound exposure.

Evidence exists to suggest that intense sound releases excess neurotransmitter from the inner hair cells. However, it has been previously reported that intense sound affects the cochlear micromechanics by altering the stereocilia. Therefore, we tested the hypothesis that intense sound affects structures involved in transduction before it affects the nerve endings. In order to test this hypothesis, we examined the interaction of intense sound with kynurenate which blocks the action of the neurotransmitter on the afferent nerve endings. Intracochlear perfusion of artificial perilymph containing 5 mM kynurenate did not reduce the effect of intense sound when we compare the results with a control group perfused with artificial perilymph alone. These results show that blockade of afferent transmitter receptors did not reduce the effect of acoustic trauma, and the acoustic trauma used herein affected structures involved in transduction before it affected the postsynaptic structures. We speculate that the active process is affected first during acoustic trauma. This interpretation is consistent with the notion that stereocilia are structures that make up part of the active process.

Action Potentials↗
PubMed1994

Effects of endolymphatic and perilymphatic application of salicylate in the pigeon. I: Single fiber activity and cochlear potentials.

The effects of salicylate on the mammalian cochlea function are well documented. However, there is a lack of reports on salicylate effects on the avian auditory periphery and it might well be that salicylate is not ototoxic at all in submammalian vertebrates. We therefore recorded single fiber activities, compound action potential (CAP) and endocochlear potential (EP) during application of salicylate (calculated final concentration of about 2-18 mmol/l) into the scala media of pigeons. We furthermore recorded CAP and EP during perilymphatic perfusion of salicylate (2-20 mmol/l). Salicylate applied into the scala media led to an elevation of tip threshold in single fibers ranging from 5 to 35 dB. The characteristic frequencies of the fibers were not changed. This effect on auditory nerve fibers was reflected in an elevation of CAP thresholds. The mean spontaneous discharge rate was either slightly increased or remained unchanged in the majority of fibers. Perilymphatic salicylate perfusion also led to an elevation of CAP thresholds that was reversible following subsequent perfusion with artificial perilymph. The EP remained unchanged in both application modes. The effects of salicylate were dose dependent and more pronounced in the mid- to high-frequency range. These results are consistent with an action of salicylate on the process (electrical or mechanical, or both) responsible for the sensitivity and frequency selectivity in the avian peripheral hearing organ.

Action Potentials↗
PubMedDate not supplied

Effects of acute styrene and simultaneous noise exposure on auditory function in the guinea pig.

Although styrene has been demonstrated to disrupt vestibular function acutely, parallel studies have not been conducted in the auditory system. This article presents data on the effects of acute styrene administration by injection and inhalation on cochlear function. No deleterious effect of the maximally tolerated styrene dose on hearing was identified when cochlear function was assessed using a within-subjects design. When guinea pigs were administered styrene by inhalation during a single 7-h period, normal auditory function was observed both 1 and 7 days later as compared to chamber controls which did not receive styrene. In some instances, the interactive effects of noise and simultaneous styrene inhalation were studied to determine whether chemical exposure might enhance the disruptive effects of noise on hearing. While a persistent noise-induced hearing loss was observed 1 day following exposure, subjects administered styrene simultaneously did not show a greater hearing loss than those receiving noise alone. Finally, when a 7-day recovery period for noise-induced hearing loss was interposed before audiometric testing, the combined exposure to styrene and noise was not more potent than noise alone in elevating auditory thresholds. Although auditory dysfunction has been reported following subchronic styrene administration, the current results do not support an ototoxic effect of styrene at the level of the cochlea with short-term exposure.

Action Potentials↗
PubMed1983

Relation between discharges in auditory nerve fibers and the whole-nerve response shown by forward masking: an empirical model for the AP.

Whole-nerve action potentials evoked by a standard click were recorded from a gross electrode on the RW and the discharges of auditory nerve fibers to the same standard click were recorded from micropipette electrodes in the auditory nerve. The effect of a preceding tone burst (2, 4, or 8 kHz) upon the responses was measured for forward-masker intensities from 20 to 80 dB SPL. All forward maskers reduced the discharges of auditory nerve fibers to the standard click with the greatest reduction occurring for fibers with characteristic frequencies (CFs) near the masker frequency. The 4- and 8-kHz forward maskers produced similar effects on N1, P1, and N2 of the RW response. However, the 2-kHz forward masker produced enhancement at P1 and N2 in the RW response to the standard click. An empirical summation model of estimated elemental waveforms at latencies dependent on CF produces synthesized AP response waveforms which are similar to the observed RW response. The model permits the exclusion of discharges from segments along the cochlear partition in a manner similar to the effect of a forward masker. Alterations in the synthesized waveforms due to the exclusion of segments are similar in direction to the alterations observed in the RW responses for forward masking.

Acoustics↗
PubMed1985

Influence of direct current on dc receptor potentials from cochlear inner hair cells in the guinea pig.

Inner hair cell responses to sound were monitored while direct current was applied across the membranous labyrinth in the first turn of the guinea pig cochlea. The current injection electrodes were positioned in the scala vestibuli and on the round window membrane. Positive and negative current (less than 100 microA) caused changes in the sound-evoked dc receptor potentials which were dependent on the sound frequency and intensity. The frequencies most affected by this extracellular current were those comprising the "tip" portion of the inner hair cell frequency tuning characteristic (FTC). The influence of current increased with increasing frequency. Positive current increased the amount of dc receptor potential for the affected frequencies while negative current decreased the potential. Current-induced changes (on a percentage basis) were greater for low intensity sounds and the negative current direction. These frequency specific changes are evidenced as a loss in sensitivity for the tip area of the FTC and a downward shift of the inner hair cell characteristic frequency. Larger current levels (greater than 160 microA) cause more complex changes including unrecoverable loss of cell performance. In separate experiments positive and negative currents (less than 1.1 microA) were injected into the inner hair cell from the recording electrode during simultaneous measurement of the sound-evoked dc receptor potential. This condition caused a shift in IHC sensitivity that was independent of sound frequency and intensity. Positive current decreased the sensitivity of the level of the cell while negative current increased the responses. The effect of current level on sound-evoked dc receptor potential was nonlinear, as comparatively greater increases in cell response were observed for negative than decreases for positive current. The intracellular current injection results are accounted for by the mechano-resistive model of hair cell transduction, where nonlinear responses with current level may reflect outward rectification. Response changes induced by extracellular current are evidence of current effects on both inner and outer hair cells. The frequency and intensity dependences are hypothesized to represent voltage mediated control of inner hair cell response by the outer hair cells.

Action Potentials↗
PubMedDate not supplied

The effect of vasopressin upon the cochlear potentials in the guinea pig.

The change in the EP, CM, SP and AP during perilymphatic perfusion of vasopressin (antidiuretic hormone) was examined in the guinea pig. The EP was recorded with a microelectrode through the spiral ligament of the second turn. The CM, SP and AP were measured with the differential electrodes in the basal turn. The perfusion of vasopressin at concentration of more than 10(-5)M produced a reversible decrease in the EP. The extent of the EP decline was dependent upon the concentration of vasopressin. Abolition of the effect of vasopressin upon the EP by the resumption of respiration after transient asphyxia was observed. During the perfusion of vasopressin, the CM and AP decreased, while the negative component of the SP increased. The mechanism causing the effect of vasopressin upon the cochlear potentials is discussed.

Action Potentials↗
PubMed1987

Effect of hypothermia on the cochlear potentials.

The effects of hypothermia upon the cochlear potentials were studied in 30 guinea pigs. Particular attention was paid to the transient increase in AP amplitude at a moderate temperature. CM and AP were recorded via an Ag-AgCl lead placed on the round window membrane. Although the CM amplitude and threshold, and the AP latency and threshold were both impaired concomitant with the lowering of body temperature, the AP amplitude exhibited a transient increase at the beginning of cooling (peak at 30 degrees C) and a decrease thereafter. Narrow-band analysis of AP revealed that this phenomenon was most prominent in the high-frequency region. When the cochlear efferent fibers (crossed and uncrossed olivocochlear bundle) were cut, this transient increase in AP amplitude disappeared. This phenomenon seems to be due to the difference in sensitivity to hypothermia between the efferent and afferent fiber systems.

Action Potentials↗
PubMedDate not supplied

Electrochemical composition of the cochlear fluids in the early experimental hydrops. Preliminary results.

The composition of endolymph and perilymph was studied in the guinea pig cochlea after 2 and 6 weeks of blockage of the vestibular aqueduct in an experimental model of hydrops. Compound action potential was monitored several times in the observation period. The endocochlear potential was measured and the endolymph was sampled at the first and third turns of the scala media. The Na, K, and Cl concentrations were determined in nanolitre aliquots of endolymph and of perilymph, the latter sampled from the basal scala vestibuli. After 2 weeks, no change in endolymphatic electrochemical composition was observed. After 6 weeks, endocochlear potential was decreased by 25% at both cochlear turns; K concentration was decreased in endolymph of the basal turn and Cl concentration was decreased in both turns; the calculated osmolality (Na + K + Cl) was decreased in both turns. These results indicate that the blockage of the vestibular aqueduct induced early auditory dysfunction whereas alterations of the electrochemical composition of endolymph occurred later after a time lag of more than 2 and less than 6 weeks.

Action Potentials↗
PubMed1992

[Effects of furosemide on endocochlear potentials, auditory action potentials and summating potentials and the changes of inner ear pathology].

Guinea pigs were injected with furosemide 50 mg/kg (group A) and 25mg/kg (group B). Two minutes after injection, EP of group A decreased to -13.9mv while that of group B decreased to +65 mv. Also, AP of group A disappeared, and recovered at 8.5 mins. while AP amplitude of group B decreased to 78%. The SP value of group A changed from -14.5mv to +23.4mv 1 min after injection and returned to negative polarity in 12 min. Edema of stria vascularis was observed under light microscope. Transmission electron microscope showed edema between marginal cells and intermedia cells, cytoplasm of the marginal cell protruded to the cochlear duct, and cell membrane of outer hair cell folded. The finding of this study illustrates that furosemide inhibits the transportation of the active ions of cochlear duct tissue resulting in decrease of EP and alters the function of hair cells causing the change of AP amplitude. -SP depends on the ion transportation, the polarity can be inversed while large dosage of furosemide was used.

Action Potentials↗