Longitudinal distribution of cochlear potentials and the K+ concentration in the endolymph after acoustic trauma

Hearing Research, 4 (1981) 287-298 0 Elsevier~North-Ho~and Biomedical Press

287

J. SYKA, I. MELICHAR and LIBUSE GLEHLOVA Institute of Experimental Medicine, Czechoslovak Academy of Sciences, U nemocnice 2,128 08 F?ague 2, Czechoslovakia (Received X8 September 1980; accepted 23 January 1981)

Guinea pigs were exposed to 142 dB third-octave band of noise centred at 1 kHz for 1 h. At different times after exposure the endocochlear potential (EP), the anoxic negative endocochlear potential (-EP), the concentration of K+ (K:) and rni~opho~~ potentials were recorded in Scala media in four cochlear turns. The remaining hair cells were counted in each animal. immediately after the exposure9 the EP and Kl decreased evenly in all four cochlear turns and gradually returned to normal physiological values in S-20 days. When measured 20 days after the exposure, essentially normal EP and Kd values were observed, with an apicalwards decline, which was similar to that found along the cochlea in nonexposed animals. Abnormal increased EP was observed in some animals 20 days after the exposure in the first and second turns. In contrast to positiveEP and Ki values, the anoxic negative EP attained less negative values in the second turn of exposed animals, i.e., in the turn where the narrow band noise exerted the major desctructive effect. An almost normal distribution of hair cells and most negative EP values were found in the fourth turn. The distribution of persistent hair cells correlated positively with the values of the anoxic negative EP and amplitudes of the microphonic potentials. It is assumed that, in addition to the difference in K’ concentration between endolymph and perllymph, the anoxic negative EP is dependent upon the functional state of the organ of Cotti. Key words: cochlear potentials; K’ concentration;

longitudinal distribution;

acoustic trauma.

INTRODUCTION

High-intensity narrow-band noise exposure results in the destruction of the organ of Corti, The main damage is localized in the section of the organ of Corti that is related to the centre frequency of noise (One Octave higher); however, the destructive process spreads basalwards and apicalwards in the cochlea. Consequently, the basilar membrane is denuded of cells so that it no longer constitutes a barrier to ionic movements. The equilibration of ions between endolymph and perilymph, which is facilitated, may substantially influence the value of the negative anoxic EP. We have shown previously 1181 that, as the consequence of the high intensity narrow band noise exposure, the positive EP and the K’ concentration in the endolymph decreases and returns to normal values in the course of S-20 days. Furthermore, the noise exposure results in a significant decrease of the anoxic negative EP. The dependence of the negative EP on the integrity of the organ of Corti is of interest with respect to a current hypothesis, according to which the -EP is doubt to be pro-

288

duced by the leakage current of cells of the organ of Corti [ 12,9]. According to another hypothesis, this potential is considered to be a K’ diffusion potential dependent upon the chemical gradients between the endolymph and the perilymph [10,15,16,4]. If the first hypothesis is valid, we may expect lower values of the anoxic -EP at those turns of the cochlea where the narrow-band noise exposure exerts the major destructive effect. However, two other conditions have to be fulfilled: there should be a normal value of K’ in the endolymph and perilymph, and the permeability of the cochlear partition to K’ and Na’ should not change. Recording of the EP in different cochlear turns may be combined with recording of ~cropho~c potentials using the same ~croelectrode. This method, which was first used by Honntbia and Ward in 1968 [8] offers a degree of selectivity similar to the differential technique [26] ; the amplitude of microphonic potentials (CM) may be directly compared with the damage of the receptor cells. The aim of our present work was to investigate the degree of changes in co&ear potentials in four turns of the guinea pig cochlea after noise exposure and to compare it with the K* concentration in the endolymph as well as with the morpholo~c~ changes in the organ of Corti along the cochlea. METHODS

The experiments were performed on 31 young guinea pigs weighting 200-300 g. The guinea pigs were placed in a specially constructed box with a relatively homogeneous intense noise field and exposed for 1 h at 142 dB to l/3-octave-band noise with the centre frequency at 1 kHz. Control measurements were performed on 15 nonexposed guinea pigs. Measurements of the EP and Ki were performed in exposed animals either on the same day as the noise exposure or after a delay ranging from 1 to 20 days. CM were measured 20 days after the exposure. The animals were anaesthetized with a 20% solution of urethane (0.8 ~/l~ g body weight) injected ~trape~tone~y. They then received Tricurane (gallamine iodide) and were maintained by artificial respiration. The cochlea was exposed by the submandibular approach. After fixation in a head holder (with the aid of acrylic cement), small fenestrae (100 pm) were made in the bony otic capsulae over the Scala media in four cochlear turns. Special care was taken not to damage the spiral ligament. The fenestrae were always made in the same regions, Based on the anatomical maps of von BCk&y [2], Femandez [7], Ann&o [l] and personal experience (fiehlovri, unpublished data), the fenestrae were located in the first, second, third and fourth turns at approximately 4.5, 11.0, 14.5 and 18.0 mm, respectively, from the base of the cochlea. Either 3 M KCI-filled glass microelectrodes (with the impedance less than 4 ML?) were introduced through the fenestrae for the measurement of the EP and CM or double-barreled K’ selective microelectrodes were used for the measurement of the Kg and EP (for preparation see previous papers [16,17]). The K’-selective microelectrodes had the tip of one barrel filled with liquid ionexchanger (Coming 477 3 17) and the second barrel with 150 mM NaCl. Electrodes were calibrated in standard solutions containing 20-150 mM KCl at a temperature of 25°C f. 3°C. The potential change per decalog change in potassium concentration was 50-58 mV at this temperature. Potassium changes have been expressed in terms of concentration, assuming identical activity coefflcients of endolymph and calibration solutions. The selectivity of K’-specific microelec-

289

trodes was about SO : 1 for I(’ : Na’, respectively. The electrodes were moved with the aid of an electronically driven microdrive through the stria vascularis into the Scala media. In some experiments all four fenestrae were drilled and then potentials were measured. In other experiments, EP, CM and G were measured after opening one fenestra, after which the next fenestra was drilled. The electrodes were connected to a d.c. amplifier with a capacitance-compensated network, and the potentials were recorded against a nonpolarizable Ag-AgCl electrode located in the muscles of the head. Tone pips with a frequency of 100 Hz to 5 kHz were used as the acoustic stimuli. The duration, repetition rate, rise time, decay and intensity of pips were controlled with an electronic switch. As a rule, the CM amphtudes were measured at the frequencies 300, 600, 1200 and 2400 Hz but the responses to other frequencies were also investigated. The same frequencies and intensities were applied while recording from the different turns. The stimuli were delivered to the external ear through a sealed sound system [21] with a wide-frequency band (20 Hz to 45 kHz) provided by a miniature piezoelectric transducer. The actual sound pressure in front of the ear drum was measured with the aid of a probe tube connected to a Brtiel and Kjaer condenser microphone (type 4134) and a Brtiel and Kjaer precision sound-level meter (type 2603). The whole probe-tube microphone ensemble was calibrated before the experiment with an “artificial ear’ for the guinea pig [21]. The CM and the electrode impedance were displayed on the d.c.coupled 5103 N Tektronix storage oscilloscope. The CM were evaluated by measuring the peak-to-peak voltage of the recorded deflections. The EP and K’, were recorded on two linear recorders and evaluated later. The temperature of the animal was maintained at 37°C with the aid of a heating pad, and anoxia was induced by ~ter~p~g the artificial respiration. All measurements of the anoxic EP were performed within one hour after interruption of art% cial respiration. Because the anoxic EP changed continuously after the interruption of respiration it was necessary to repeat the measurements in individual turns and to calculate the correction factors. At the end of each experiment the cochleae were removed and perfused with 10% neutral formaldehyde solution. Under a stereomicroscope, the inner ear tissues in all guinea pigs were dissected out and the surface specimen technique was used for examination of the inner ear structure. In 12 animals the number of inner and outer hair cells was assessed in four co&ear turns. The obtained values were compared with the data published by StockweB et al. [24] and with our own control data from normal animals (~ehlov~, unpub~~ed data) and expressed as a percentage of normal values for each co&ear turn. RESULTS

Values of EP and G in four cochlear turns were measured in four nonexposed animals. Fig. IA shows a typical example; q differences between the individual turns did not exceed 8 mmol/l and the decrease of the EP from the basal to the apical turn was gradual. In 14 animals values of EP in four cochlear turns were measured l-5 days after the exposure. The aim of this measurement was to investigate whether the noise, which has a localized deleterious effect on the sensory epithelium of the organ of Corti (with the

290 EP(mVl 90

EP(mVl

A

K’lmmd r160

80 70 +

C---_-C----

-&

K’lrnrn0l II r 160

I)

140

----* o

t

-140

--_c-__-4 + EP -

60 i

EP K’,_-_-_

K,-__

EP(mV) 60+ SO403020.

, I

,I

111

IV

TURN

T-

7~

I

;

111

----

I”

--

TURN

Fig. 1 Endocochlear potential and K+ concentration in endolymph recorded in four cochlear turns. A. In a nonexposed animal. B. Summarized values in animals l-5 days after the noise exposure. Numerals at each point indicate the number of experiments. Fig. 2. A. Endocochlear potential and K+ concentration in endolymph measured in four cochlear turns in an animal 20 days after the noise exposure. B. Values of anoxic negative endocochlear potential found in a nonexposed animal.

damage in the second and third turn), would decrease the positive EP in individual turns differently. As is evident from the data presented in Fig. IB, this was not the case. The decline of EP towards the apical turn was in agreement with results from nonexposed animals. Similarly, the values of G, obtained from co&ear turns in 5 exposed animals (l-5 days postexposure), do not reveal significant differences between the Ki levels in individual turns, with the exception of a decline apicalwards similar to the decline of the EP. Measurement of the EP, performed 20 days postexposure, has shown that the positive EP attained normal values (Figs. 3,4). In some animals a high value of the EP (from t90 mV to t100 mV) was observed in the first and second turns. Data from 15 animals 20 days postexposure are summarized in the upper part of Fig. 5. As in the case of nonexposed animals, there exists an apicalwards decrease of the positive EP values, the difference between the first and fourth turn being approximately 13 mV. In guinea pigs 20 days after the exposure the EP and the Kd values were measured simultaneously in four turns. An example is shown in Fig. 2A. Corresponding to the differences in the EP values, the K: level shows slight differences between the individual turns. Whereas the positive EP recovered to normal values 5 days postexposure, the anoxic negative potential (-EP) in exposed animals never decreased to negative values found in nonexposed animals (-8.5 mV on the average in animals after the exposure or -35 mV in nonexposed animals - cf. [ 181). The rate of decline of the EP in exposed animals was 2.9 mV/min when measured at l-10 min of anoxia, whereas in nonexposed animals the EP decreased by about 5.6 mV/min during the same interval. It is demonstrated in Fig. 2B main

291

mV

I

Of0

GNP- 79 -10.11

---\

HAIR

100

POTENTIAL

ENDOCOCHLEAR

CELLS

COCHLEAR

MICROPHONICS

80 60 40 20 A BCD

I A=300

ABC0

II Hz. 8x600

Hz.

ABC Ill C:1200 Hz,

ABC IV TURN 03 24OOHz

Fig. 3. Endocochlear potential, negative anoxic endocochlear potential, dis~ibution of inner and outer hair cells and relative amplitudes of co&ear microphonics te 300, 600, 1200 and 2400 Hz tones (expressed as perckntage of amplitudes found in control nonexposed animals) in four co&ear turns in an animal 20 days after the noise exposure.

in a normal animal the negative EP may amount to -36 mV, with slight differences between individual turns. In Fig. 3 an example is shown of -EP values in four co&ear turns in one animal, where the deleterious effect of the noise exposure was moderate. In this animal it was possible to evoke the Preyer reflex after the noise exposure; the average counts of hair cells (expressed as the percentage of the counts found in nonexposed anim&) were relatively high, in comparison with other ~stolo~c~y controlled guinea pigs (Fig. 5). In this animal, however, the values of the EP in turns II, III and IV were less negative than -30 mV, that

292

mV

I

POTENTIAL

ENDOCOCHLEAR

GNP-79-S-24

0

20 -1

3 A

,I

HAIR 0

II

ill

IV

Ill

IV

CELLS INNER

OUTER

60 40 20 II

I COCHLEAR mtensity

‘A’BCD I A=jOOHz.

MICROPHONICS

80 dB

ABCD !’ 0=600H2.

ABC0 III C*1200H2.

ABC0 IV D= 2400H2

TURN

Fig. 4. Endocochlear potential, negative anoxic endocochlear potential, distribution of inner and outer hair cells and relative amplitudes of cochlear microphonics to 300, 600, 1200 and 2400 Hz tones (expressed as percentage of amplitudes found in control nonexposed animals) in four cochlear turns in an animal 20 days after the noise exposure.

A typical case of negative EP values in a noise exposed animal is shown in Fig. 4. The EP values are distributed from -9to -20 mV, with the least negative value in the second turn. This distribution correlates with the percentage of persistent hair cells; in the second turn outer hair cells were not found and the number of inner hair cells was low. The general validity of the relationship between the density of hair cells and the value of the anoxic negative EP in individual turns is demonstrated in Fig. 5. This figure summarizes values of the positive EP, -EP, distribution of hair cells and microphonic potentials from 15 animals 20 days after the noise exposure. The lowest value of the anoxic negative EP, -11.0 i: 10.1 mV, is present in the second turn; the highest value, -15.1 f

293 POTENTIAL

ENDOCOCHLEAR

II

I HAIR

0’0

III

IV

CELLS (n:12)

100 80 60 40 20 II

I COCHLEAR 300 Hz.

M~CRO~ONICS

111

(n ~9)

BOdB

20 10 I

II

Ill

IV

TURN

Fig. 5. Average EP, anoxic EP values, number of inner and outer hair cells and relative amplitudes of the CM to 300 Hz, 80 dB tones in four c~MGear turns in animals 20 days after the noise exposure. Numerals near the mean of the EP values indicate the number of experiments.

7.9 mV, in the fourth turn. In the second turn, where the destructive effect of the 1 ~Hz narrow band noise was greatest, approximately 40% imrer hair cells remained, when compared with their distribution in normal animals, whereas almost all outer hair cells were absent. 97% inner haii cells and 70% outer hair cells were found on average in the fourth turn. In nine animals, where mo~holo~c~ and elec~ophysiolo~c~ data for all turns were available, the correlation coefficient between the number of hair cells’and the values of the anoxic -EP for individual animals and individual turns was calculated. The resulting co~elation coefficient, 0.82, demonstrates a tight relation~p between these measures. The development of pathological changes in the cochlea, after the acoustic trauma has

294

been described previously [ 181. Primary pathological changes, observed immediately postexposure, were combined with subsequent posttraumatic changes, which spread from the second and the beginning of the third turn apicalwards and basalwards. The spread of necrotic changes was usually completed by the fifth day when parts of the denuded basilar membrane were already covered with migrating flattened epithelial cells. The process of substitution of the degenerated cells of the organ of Corti with migrating flattened epithelial cells started on the second day postexposure and was practically terminated by the fifth day after the exposure. In several cases perforations were observed in the Reissner’s and basilar membranes; some perforations were not healed even 20 days after the exposure. In these cases the positive EP was mostly lower than in other cases, but the correlation between morphological findings and physiological parameters was not unequivocal. In five control nonexposed animals distribution of the cochlear microphonics (CM) was measured in the Scala media at four co&ear turns. Four frequencies (300,600, 1200 and 2400 Hz) and intensities of 40-90 dB SPL were used. The amplitudes and spatial patterns of the CM were essentially the same as described by Honrubia and Ward [8]. In the upper part of Fig. 6 typical results are shown from a nonexposed guinea pig for frequencies 1200 and 300 Hz, plotted on semilogarithmic coordinates. In accordance with the travelling-wave envelope, the longitudinal distribution of the CM maximum amplitude is different for different frequencies and, with increasing intensity, the maximum

Fig. 6. Longitudinal distribution of the CM recorded inside the scala media in four cochlear turns. Upper graphs: in a nonexposed animal; lower graphs: in an animal 20 days after the exposure (GNP 7% 10-11). Responses were obtained for two different frequencies at the indicated intensities. The CM voltage is plotted on semilogarithmic coordinates. Abscissa: millimeters along the cochlear duct.

295

response moves towards the base. In twelve animals explored 20 days after the exposure, large individual differences were found in the longitudinal distribution of CM. In some

animals the destructive effect of the noise exposure was relatively mild and the loss of CM voltage, especially in the fourth turn, was small. Fig. 3 shows that in the animal GNP 79. 10.11, where the outer hair cell loss in the second, third and fourth turns was not severe, it was possible to evoke CM at 80 dB intensity; however, with amplitudes lower than those found in normal nonexposed animals. In the fourth turn almost normal amplitudes of the CM were observed. ‘Ihe longitudinal distribution of the CM for frequencies 1200 and 300 Hz in this an& mal is illustrated in the lower part of Fig. 6. In comparison with the nonexposed animals, the slope of distribution curves is shallow and the abnormality is also evident from the movement of the m~um response with increasing intensity toward the apex. It should be noted that the largest outer hair cell loss was found in this animal in the frst turn (Fig. 3). 300 Hz stimulation evoked almost normal CM amplitudes in the fourth turn, whereas the CM amplitude decreased by about 30-35 dB in the second and the third tum. ‘Ihe output at 80 dB in the first turn was greater than that in the second and third turns, which demonstrates that the remaining sensory epithelium in the first turn generated a low frequency CM. A const~bution of the remote source from the fourth turn is improbable because of the low output of the second and third turns. In Fig. 4 another example is shown of an animal (GNP-79-S-24) with relatively little damage of the cochlea, which is mainly localized in the second and third turn (with total loss of outer hair cells in the second turn and with 18% of the normal population in the third turn). Consequently, the CM were observed at 80 dB SpL in the fmt turn (less than 10% of amplitude found in normal animals) and almost normal values were found in the fourth turn. In this animal the distribution of CM roughly corresponds with the density of outer cells in individual turns. The reduction of CM amplitude was, however, more remarkable in the majority of noise-exposed animals than in the animals GNP-79-10-1 1 and GNP79-5-24. As a rule, the 80 or 90 dB s~u~tion at frequencies 150-2400 Hz did not evoke any CM, with the exception of the fourth turn, where in some animals very low amplitudes of the CM (with the loss of about 30 dB) were observed. Surprisingly, in some animals, where a great number of outer hair cells was found histologically in the fourth turn, we did not record any CM response at.BO-90 dB. In Fig. 5 the lower graph summarizes the results of measurements of the CM in seala media in nine noise-exposed animals. The amplitude of the CM to 300 Hz s~~a~on at 80 dB is compared with the CM amplitude, found in five nonexposed animals under the same stimulus conditions. With the exception of the fourth turn, the output from the cochlea is very low, in correspondence with the extensive degeneration of outer hair cells. DISCUSSION

The present results demonstrate that, with exception of the decline of the positive EP towards the fourth turn, which was also found in nonexposed control animals, the narrow-band noise exposure at 142 dB did not cause a localized effect on the positive EP in the preferentially impaired cochlear turns (second and third). Immediately after the expo-

296

sure, a uniform decrease of the EP and < in all four cochlear turns was observed and the EP returned evenly in all the four turns, when measured during 20 days after the exposure. The function of the whole stria vascularis, which is the source of the positive EP, was at first diminished and became totally restored at later stages. This explanation finds support in th.e even distribution of G values after the exposure, which is related to the corresponding EP values in individual turns. In 1972, Benitez et al. [3] recorded the EP in three co&ear turns in chinchilla after noise exposure (95 dB for 48 and 72 h) and did not find significant differences in comparison with control nonexposed animals. The reason for the systematic apicalward decline of the positive EP is not known, although this decline has been recognized by many authors [20,25,5,3]. The lower values of the EP in more apical turns may be, for example, a consequence of diminished oxygen supply and ATPase activity in the more apical turns [14,19]. In contra~stinction to the even apicalward decline of the positive EP along the cochlea, the negative anoxic EP was to be decreased unevenly after the narrow-band noise exposure, depending on the maximum energy of the noise. Minimal values of the -EP were found in the second turn (-11.0 + 10.1 mv), whereas maximal negative values were found in the fourth turn (-15.1 ?r7.9 mV). The destructive effect of the noise was mostly expressed in the second turn, and minimal changes were found in the fourth turn, judging from the morphological controls of the cochlear partitions and distribution of microphonic potentials. According to the generally accepted hypothesis, the recorded EP is the sum of a positive electrogenic potential generated by stria vascularis and a negative potential which replaces the EP in anoxia (-EP). Therefore, EP values in the second turn are expected to be higher with respect to the other turns in animals 20 days after exposure. However, we found few animals that showed a higher EP that was connected with small negative anoxic EP in the same (i.e., second) turn. In nonexposed animals there is a striking apicalward decrease of values of -EP (Fig. 2B) instead of an increase, which would be expected according to the hypothesis of generation of the EP. It has been shown that the value of the -EP correlates with the number of persisting hair cells in individual cochlear turns. In this connection there arises the question about the nature of the -EP. We have to bear in mind, that 20 days postexposure, when the EP measurements were performed, the structural state of cochlear partitions was different from that found in normal animals. The organ of Corti was mostly destroyed and was replaced by flattened epithelial cells with an unknown permeability for K’. It may be assumed from our previous data [18], that the permeability in this case would be decreased, because the rate of EP decline during anoxia was found to be decreased. A similar slow decline of the EP during anoxia was observed in waltzing guinea pigs and in kanamycin-treated guinea pigs [l 11; and also after administration of an ototoxic diuretic - ethacrynic acid [ 17,4] _ Bosher. [4] quantitatively estimated the reduced rate of ionic changes during anoxia after ethacrynic acid which indicated the decrease of permeability of cochlear partitions for Na’ and K’. The decreased rate of K’ exchange between endolymph and perilymph in noiseexposed animals was recently confirmed [ 13,221. For interpretation of the reduced values of anoxic -EP after the noise exposure, data from the kanamycin-treated guinea pigs and waltzing guinea pigs [ 1 l] are especially important. In both cases severe degeneration or absence of the organ of Corti were present and the values of anoxic -EP were reduced. Furthermore, it was noted that the post-

291

mortem negative EP did not show close correlation with K’ concentration in the endolymph. All these data indicate that the -EP, which is assumed to be a K’ diffusion potential [ 10,15,16,4], is largely dependent upon the permeability of cochlear partitions for K’ and Na’ ions. The assumed lower permeability for K’ of the layer of epithelial cells which replaces the organ of Corti in some parts of the cochlea in noise-exposed animals may account for the reduced -EP found especially in the second turn. That the changes in the ionic permeability may substantially influence the value of the -EP follows from the data obtained in the fourth turn, where relatively small morphological changes were combined with reduced -EP values (in comparison with control nonexposed animals). On the other hand, the data do not contradict the alternative hypothesis, that the -EP depends upon the intracellular potentials of the cells of the organ of Corti [9,23]. Reduced values of the -EP were found at parts of the cochlea which were deprived of the organ of Corti. Almost normal values of the -EP (which may be generated by leakage currents’) were found at regions of the cochlea that had an almost normal pattern of hair cells. We may expect, however, much larger differences between the values of -EP in the second turn, which is practically without the organ of Corti, and the values found in the fourth turn. The longitudinal distribution of microphonic potentials in control nonexposed animals was the same as originaIly described by Homubia and Ward [8]. Small differences, especially in CM amplitudes, may account for differences in the calibration of the sound pressure: Hom-ubia and Ward used open-field measurements, whereas in our experiments the sound pressure was measured in the close vicinity of the ear-drum in a closed acoustic system. After the exposure (20 days) we observed a wide variety of reductions of the CM in individual animals ranging from total unresponsiveness to 90 dB SPL sounds to a relatively mild decrease in the CM values, which were dependent upon the distribution of persisting outer hair cells along the cochlea. Animals with mild hearing losses yielded interesting CM data, which mainly concerned the spatial pattern of the CM. It was shown (Fig. 6) that the basal turn generates not only high-frequency CM but also low-frequency CM. This problem has been discussed at length by Dallos [6]. The CM curves in the fourth turn in some animals with relatively small hair celI losses were essentially the same as in normal nonexposed animals; however, the CM pattern from other turns showed distinct abnormal features. Whereas in normal animals the increase in the sound intensity under 90 dB evoked a relatively linear increase in the CM amplitude, in noiseexposed animals the increase was progressively larger with increasing sound intensity, comparable with the recruitment phenomenon. In a few cases, where it was possible to observe the CM in the first and second turns, the point of maximum CM voltage shifted with increasing intensity not towards the base but towards the apex. We may assume that the exposure to a very intensive noise may result in serious disturbances of both mechanical and electrical properties of the cochlear partitions. REFERENCES [l] Anniko, M. (1976): The cytocochleogram in atoxyl-treated guinea pigs. Acta Oto-Laryngol. 70-81. [ 21 Bekesy, G. von (1960): Experiments in Hearing. McGraw-Hill, New York.

82,

298 131 Benitez, L.D., Eldredge, D.H. and Templer, J.W. (1972): Temporary threshold shifts in chinchilla: electrophysiological correlates. J. Acoust. Sot. Am. 52, 1115-l 123. [4] Bosher, S.K. (1979): The nature of the negative endocochlear potentials produced by anoxia and ethacrynic acid in the rat and guinea pig. J. Physiol. (London) 293, 329-345. [S J Bosher, S.K. and Warren, R.L. (1971): A study of the electrochemistry and osmotic relationships of the cochlear fluids in the neonatal rat at the time of development of the endocochlear potential. J. Physiol. (London) 212,739-X1. [6] Dallos, P. (1973): The auditory periphery. Academic Press, London. [7] Fernrindez, C. (1952): Dimensions of the cochlea (guinea pig). J. Acoust. Sot. Am. 24, 519-523. [8] Honrubia, V. and Ward, P.H. (1968): Longitudinal distribution of the cochlear microphonics inside the cochlear duct (guinea pig). J. Acoust. Sot. Am. 44,951-958. [9] Honrubia, V., Strelioff, D. and Sitko, S.T. (1976): Physioiogical basis of cochlear transduction and sensitivity. Ann. Otol. Rhinol. Laryngol. 85,697-710. [ 101 Johnstone, B.M. (1965): The relation between endolymph and the endocochlear potential during anoxia. Acta Oto-Laryngol. 60, 113-120. [ 111 Konishi, T. (1979): Some observations on negative endocochlear potential during anoxia. Acta Oto-Laryngol. 87,506~51.6. fl2] Konishi, T., Kelsey, E. and Singleton, G.T. (1967): Negative potential in Scala media during early stage of anoxia. Acta Oto-Laryngol. 64, 107-l 18. [ 131 Konishi, T., Salt, A.N. and Hamrick, P.E. (1979): Effects of exposure to noise on ion movement in guinea pig cochlea. Hearing Res. 1, 325-342. [14] Kuijpers, W. and Bonting, S.L. (I 970): The cochlear potentials I. The effect of ouabain on cochlear potentials of the gumea pig. Pfliigers Arch. 320,348-358. f’5 1 Kuijpers, W. and Bonting, S.L. (1970): The cochlear potentials 11. The nature of the cochlear resting potential. Pfhigers Arch. 320,359-372. 116 Melichar, 1. and Syka, J. (1977): Time course of anoxia-induced K+ concentration changes in the cochlea measured with K* specific microelectrodes. Pfliigers Arch. 372, 207-213. 117 1 Melichar, I. and Syka, J. (1978): The effects of ethacrynic acid upon the potassium concentration in guinea pig cochlear fluids. Hearing Res. 1, 35-4 1. [18] Melichar, I., Syka, J. and Ulehlova, L. (1980): Recovery of the endocochlear potential and the K* concentrations in the cochlear fluids after acoustic trauma. Hearing Res. 2, 55-63. [19] Meyer zum Gottesberge, A., Rauch, S. and Koburg, E. (196.5): Untersclliede in ~etabolismus der einzelnen Schneckenwindungen. Acta Oto-Laryngol. 59,116-123. 1201 Misrahy, G.A., Hildreth, K.M., Shinaberger, E.W. and Cannon, W.J. (1958): Electrical properties of wall of endolymphatic space of the cochlea (guinea pig). Am. J. Physiol. 194, 396-402. 121] Salava, T., Syka, J. and Pope%, J. (1979): A sealed sound system for small animal auditory research. Physiol. Bohemoslov. 28,271. ,] Salt, A.N. and Konishi, T. (1979): Effects of noise on cochlear potentials and endolymph potas122 sium con~ntration recorded with potassium-selective electrodes. Hearing Res. I, 343-363. ]23 ] Sitko, S.T., Strelioff, D. and Honrubia, V. (1976): Source and maintenance of the endocochlear potential. Trans. Am. Acad Ophthalmol. Otolaryngol. 82, 328-335. 124 ] Stockwell, Ch.W., Ades, H.W. and Engstrom, H. (1969): Pattern of hair cell damage after intense auditory stimulation. Ann. Otol. Rhinol. Laryngol. 78, 1144-l 168. ]25 ] Suga, F., Morimitsu, T. and Matsuo, K. (1964): Endocochlear DC potential: how is it maintained along the cochlear turns? Ann. Otol. Rhinol. Laryngol. 73,924-929. 126 ] Tasaki, I. and Fernindez, C. (1952): Modi~cations of cochlear microphonics and action potentials by KC1 solution and by direct currents. J. Neurophysiol. 15,497-512.