Simultaneous exposure to ethyl benzene and noise: synergistic effects on outer hair cells.

The effects on hearing of simultaneous exposure to the ototoxic organic solvent ethyl benzene and broad-band noise were evaluated in rats. The effects of three ethyl benzene concentrations (0, 300 or 400 ppm) and three noise levels (95 or 105 dB(lin) SPL or background noise at 65 dB(lin) SPL) and all their combinations were investigated for a 5 day exposure at 8 h/day. Distortion product otoacoustic emissions and compound action potentials were affected after 105 dB noise alone, and after 105 dB noise in combination with ethyl benzene (300 and 400 ppm). However, the amount of loss for these combinations did not exceed the loss for 105 dB noise alone. Outer hair cell (OHC) loss after exposure to 300 ppm ethyl benzene was located in the third row of OHCs. At 400 ppm, the loss spread out to the second and first row of OHCs. Noise alone hardly affected the OHC counts except for a minor loss in the first row of OHCs after 105 dB SPL. Noise at 105 dB in combination with ethyl benzene at 300 and 400 ppm, however, showed OHC loss greater than the sum of the losses induced by noise and ethyl benzene alone.

Perilymphatic application of cisplatin over several days in albino guinea pigs: dose-dependency of electrophysiological and morphological effects.

Cisplatin, at 0, 3, 30 or 300 microg/ml in saline, was applied to the scala tympani of the cochlea of guinea pigs via osmotic mini-pumps, operating at a pump rate of 0.5 microl/h. Electrocochleographic recordings were made from an implanted round window electrode. When an electrocochleographic criterion of ototoxicity was reached (40 dB loss in compound action potential (CAP) threshold at 8 kHz), or after 1 week if this criterion was not reached, the animals were sacrificed for light microscopy. A subgroup of animals had endocochlear potentials (EPs) measured prior to sacrifice. Hearing remained stable in the 0 microg/ml control group, but a sudden drop of auditory sensitivity across the whole frequency range was observed in all other groups. It took 1-5 days before the drop occurred, dependent on cisplatin concentration. CAP and cochlear microphonics were lost simultaneously. The EP was severely depressed in the affected animals, suggesting that cisplatin effects on the EP are primary. However, histology revealed an accompanying loss of outer hair cells, primarily in the basal turn. It is concluded that if cisplatin is given until ototoxicity becomes apparent electrophysiologically, then the cochlear pathology from intrascalar cisplatin administration resembles that from daily parenteral administration at 1.5-2.0 mg/kg. The cochlear pathology from the parenteral treatment was greater than that observed with 30 microg/ml pumps, and less than that from 300 microg/ml pumps.

Summating potential in the guinea pig cochlea after perilymphatic perfusion with arginine-vasopressin.

We hypothesized that arginine-vasopressin (AVP), the natural vasopressin in the guinea pig, might increase the cochlear summating potential, indicating an increase in endolymphatic volume. Guinea pig cochleas were perfused with artificial perilymph for 15 min, with or without AVP (2 x 10(-6) M). In 1 group of animals, summating potentials (SP), compound action potentials and cochlear microphonics evoked by 2, 4 and 8 kHz tone bursts were measured with an apically placed electrode 15 min, 1 and 2 h after perilymphatic perfusion. In another group of animals the SP and endocochlear potential (EP) were measured simultaneously in the scala media during and after perfusion. In both groups the SP had increased significantly 15 min after perfusion with AVP and this increase was reversible. At the concentration of AVP used the increase in SP was not related to EP alterations. On light microscopic examination of the cochlea no evident increase in scala media volume could be detected. The increase in the SP (a sensitive indicator of acute endolymphatic hydrops) after perfusion with AVP suggests that this neuropeptide plays a role in the regulation of the ion and fluid balance in the cochlea.

Experimental autoimmune inner ear disease: an electrocochleographic and histophysiologic study.

Systemic immunization with swine inner ear antigens in complete Freund's adjuvant induces functional disturbances in the cochlea. Morphometric data indicate that an endolymphatic hydrops develops within 2 weeks. It diminishes 6 weeks after immunization. A progressive decrease in the compound action potential amplitude is observed from 2 to 6 weeks after immunization. Enhancement of the amplitude of the summating potential is present without a clear overall correlation to the presence of endolymphatic hydrops. The amplitude of the cochlear microphonics shows no significant changes after immunization. Western blot analysis of the sera performed 2 and 6 weeks after immunization shows enhanced reactivity at 68, 50, 45, and 27 kd molecular weights, as compared to controls. The same spectrum of cross-reacting antibodies is believed to be instrumental in immune-mediated sensorineural hearing loss in patients. Apparently, cross-reacting antibodies and released mediators disturb cochlear homeostasis, resulting in the observed changes in the electrophysiological responses. However, these changes are not clearly related to structural changes at the light and electron microscopic levels.

The rat cochlea in the absence of circulating adrenal hormones: an electrophysiological and morphological study.

Circulating adrenal hormones affect strial function. Removal of endogenous levels of adrenal steroids by bilateral adrenalectomy (ADX) in rats causes a decrease of Na(+)/K(+)-ATPase activity in the cochlear lateral wall [Rarey et al., 1989. Arch. Otolaryngol. Head Neck Surg. 115, 817-821] and a decrease of the volume of the marginal cells in the stria vascularis [Lohuis et al., 1990. Acta Otolaryngol. (Stockh.) 110, 348-356]. To study further the effect of absence of circulating adrenocorticosteroids on cochlear function, 18 male Long Evans rats underwent either an ADX or a SHAM operation. Electrocochleography was performed 1 week after surgery for tone bursts in a frequency range of 1-16 kHz. Thereafter, the cochleas were harvested and examined histologically. No significant changes in the amplitude growth curves of the summating potential (SP), the compound action potential (CAP) and the cochlear microphonics (CM) were detected after ADX. However, visually, there appeared to be a decrease of endolymphatic volume (tentatively called imdrops). Reissner's membrane (RM) extended less into scala vestibuli in ADX animals than in SHAM-operated animals. The ratio between the length of RM and the straight distance between the medial and lateral attachment points of RM were used as an objective measure to quantify this effect in each sub-apical half turn of the cochlea. The decrease in length of RM was statistically significant. Thus, circulating adrenal hormones appear to be necessary for normal cochlear fluid homeostasis. Absence of one or more of these hormones leads to shrinkage of the scala media (imdrops). However, the absence of adrenal hormones does not affect the gross cochlear potentials. Apparently, the cochlea is capable of compensating for the absence of circulating adrenal hormones to sustain the conditions necessary for proper cochlear transduction.

Reversible cisplatin ototoxicity in the albino guinea pig.

Guinea pigs implanted with round window electrodes received daily doses (2.0 mg/kg) of cisplatin until a profound hearing loss occurred (> 40 dB at 8 kHz). Afterwards, pronounced recovery occurred. Recovery progressed over intervals up to 3 weeks before it saturated. Loss and recovery involved both the compound action potential and, less pronounced, the cochlear microphonics. Cochlear potentials evoked by lower frequencies recovered more fully than those evoked by higher frequencies. Loss and recovery was found also in the endocochlear potential. Outer hair cell counts did not change over the recovery period. These findings confirm our previously reported results on the reversibility of cisplatin damage. Further, they implicate the vascular stria as an important target for cisplatin in the cochlea.

Ethyl benzene-induced ototoxicity in rats: a dose-dependent mid-frequency hearing loss.

Rats were exposed to ethyl benzene at 0, 300, 400 and 550 ppm for 8 hours/day for 5 consecutive days. Three to six weeks after the exposure, auditory function was tested by measuring compound action potentials (CAP) in the frequency range of 1-24 kHz and 2f1-f2 distortion product otoacoustic emissions (DPOAEs) in the frequency range of 4-22.6 kHz. In addition, outer hair cell (OHC) loss was quantified by histological examination. The lowest concentration ethyl benzene had no effect on any of the above measures. At 400 ppm, auditory thresholds were increased by 15 and 16 dB at 12 and 16 kHz, respectively, and at 550 ppm by 24, 31, and 22 dB at 8, 12, and 16 kHz, respectively. DPOAE amplitude growth with stimulus level was affected only after 550 ppm at 5.6, 8, and 11.3 kHz. OHC loss was found in two of the five examined locations in the cochlea. At 400 ppm, 25% OHC loss was found at the 11- and 21-kHz region. The highest concentration evoked 40% and 75% OHC loss at the 11- and 21-kHz location, respectively. Thus, the mid-frequency region of rats is affected after exposure to relatively low concentrations of ethyl benzene (400-550 ppm). These results indicate that ethyl benzene is one of the most potent ototoxic organic solvents known today.

The ototoxic effects of ethyl benzene in rats.

Exposure to organic solvents has been shown to be ototoxic in animals and there is evidence that these solvents can induce hearing loss in humans. In this study, the effects of inhalation of the possibly ototoxic solvent ethyl benzene on the cochlear function and morphology were evaluated using three complementary techniques: (1) reflex modification audiometry (RMA), (2) electrocochleography and (3) histological examination of the cochleas. Rats were exposed to either ethyl benzene (800 ppm, 8 h/day for 5 days) or to control conditions. The RMA threshold increased significantly by about 25 dB, 1 and 4 weeks after the exposure, irrespective of the stimulus frequency tested (4-24 kHz). Electrocochleography was performed between 8 and 11 weeks after exposure to the organic solvent. The threshold for the compound action potential increased significantly by 10-30 dB at all frequencies tested (1-24 kHz). Histological examination of the cochlea showed outer hair cell (OHC) loss, especially in the upper basal and lower middle turns (corresponding to the mid-frequency region) to an extent of 65%. We conclude that exposure to 800 ppm ethyl benzene for 8 h/day during 5 days induces hearing loss in rats due to OHC loss.

Signs of endolymphatic hydrops after perilymphatic perfusion of the guinea pig cochlea with cholera toxin; a pharmacological model of acute endolymphatic hydrops.

There are indications that endolymph homeostasis is controlled by intracellular cAMP levels in cells surrounding the scala media. Cholera toxin is a potent stimulator of adenylate cyclase, i.e. it increases cAMP levels. We hypothesized that perilymphatic perfusion of cholera toxin might increase endolymph volume by stimulating adenylate cyclase activity, providing us with a pharmacological model of acute endolymphatic hydrops (EH). Guinea pig cochleas were perfused with artificial perilymph (15 min), with or without cholera toxin (10 microg/ml). The endocochlear potential (EP) was measured during and after perfusion. The summating potential (SP), evoked by 2, 4 and 8 kHz tone bursts, was measured via an apically placed electrode 0, 1, 2, 3 and 4 h after perfusion. Thereafter, the cochleas were fixed to enable measurement of the length of Reissner's membrane, reflecting EH. After perfusion the EP increased significantly over time in the cholera toxin group as compared to the controls. Also, the SP increased gradually at all frequencies in the cholera toxin group. Comparison within animals showed that the increase in SP became significant after 2 h at 4 kHz, after 3 h at 2 kHz and after 4 h at 8 kHz. In the control group the SP did not change significantly. The compound action potential (CAP) amplitude decreased monotonically over time at all frequencies in both the cholera toxin group and the control group, but it decreased faster in the cholera toxin group. Also, the cochlear microphonics amplitude decreased over time at all frequencies in both groups, but the decrease was significant only in the cholera toxin group after 3 h at 2 and 4 kHz. Quantification of the length of Reissner's membrane showed a small but insignificant enlargement in the cholera toxin treated animals compared to controls. These results are in accord with our view that EH is accompanied by an increase in SP and a decrease in CAP. Our results partially confirm previous results of Feldman and Brusilow (Proc. Natl. Acad. Sci. USA (1973) 73, 1761-1764). New aspects in relation to that study are the significantly increased EP and SP. In the classical EH model, based on obstruction of the absorptive function of the endolymphatic sac, increased SPs are accompanied by decreased EPs. In this cholera toxin model of EH, it is unlikely that the endolymphatic sac is involved. Apparently, EH can be based on mechanisms located in the cochlea itself as opposed to mechanisms located in the endolymphatic sac.

Cisplatin ototoxicity and the possibly protective effect of alpha-melanocyte stimulating hormone.

It is known that adrenocorticotrophic hormone (ACTH)-derived peptides, the so-called melanocortins, can reduce cisplatin-induced neurotoxicity. Recently, our group has found that cisplatin-induced ototoxicity can also be reduced or prevented by treatment with the synthetic melanocortin-like peptide, ORG 2766 (Hamers et al., 1994; De Groot et al., 1997). The present study was designed to investigate the possibly ameliorating effects of the physiologically more relevant naturally occurring neuropeptide alpha-melanocyte stimulating hormone (alpha-MSH) upon cisplatin ototoxicity and to compare its protective effects to those of ORG 2766. For eight consecutive days guinea pigs were treated with cisplatin at a concentration of either 1.5 mg/kg/day or 2 mg/kg/day. Animals were co-treated with either alpha-MSH (75 microg/kg/day), ORG 2766 (75 microg/kg/day), or a sham injection containing physiological saline. Electrocochleography and hair cell counts were performed. Treatment with 1.5 mg/kg/day cisplatin resulted in a large variability of the morphological and electrophysiological data, a variability that might have masked possible effects of ORG 2766 and alpha-MSH. Treatment with 2 mg/kg/day cisplatin caused less variable, severe reductions in the compound action potentials and cochlear microphonics combined with basal and middle-turn outer hair cell loss in five out of six animals. However, in the alpha-MSH co-treated groups, two out of six animals could be classified as normal, two animals as moderately affected and two animals as severely affected. In the ORG 2766 co-treated group we found three animals that were not affected and three animals that were severely affected. We conclude that the protective effects of alpha-MSH and ORG 2766 co-treatment are comparable and that alpha-MSH might be clinically useful in protecting against cisplatin-induced ototoxicity.

Protection and spontaneous recovery from cisplatin-induced hearing loss.

Cisplatin [cis-diamminechloroplatinum(II)] has proved itself as a potent antineoplastic agent. However, nephrotoxicity, neurotoxicity, gastrointestinal toxicity, myelosuppression, and ototoxicity interfere with its therapeutical efficacy. Forced diuresis reduces nephrotoxicity, effectively leaving neurotoxicity and ototoxicity as the major side effects of concern, and gastrointestinal toxicity and myelosuppression as the secondary side effects. So far, attempts to reduce these side effects by developing equally potent platinum analogs have been unsuccessful. Some success has been achieved, however, by co-treatment with protective agents. Nearly all these agents are sulfur- or sulfhydryl-containing compounds (thio compounds), known as antioxidants and potent heavy metal chelators. These thio compounds may provide protection from cisplatin toxicity either (1) by direct interaction between the cisplatin and the thio moiety, (2) by displacing platinum from its site of toxic action, (3) by preventing platinum from interfering with superoxide dismutase, or (4) by scavenging of cisplatin-induced free radicals. In particular the first two protective mechanisms bear the risk of reducing the antineoplastic activity of cisplatin. Since nephrotoxicity can be controlled effectively by forced diuresis, a more specific approach of coping with ototoxicity by focusing on protection at the sensorineural level was chosen. Being familiar with the neuro-protective and neurotrophic properties of ACTH-related neuropeptides, specifically against cisplatin-induced peripheral neuropathies, it was judged expedient to test for a possible otoprotective action of these neuropeptides. The results were positive, although tainted with high interanimal variability. When testing for the possibility that the neuropeptides would merely delay cisplatin-induced ototoxicity rather than reduce it, it was discovered in control series without neuropeptide co-treatment that the ear can recover spontaneously from cisplatin-induced hearing loss. This was found both electrophysiologically and in outer hair cell (OHC) counts. Although these preliminary findings require further investigation, they strongly suggest that spontaneous recovery of cochlear injury can occur in the mature mammalian cochlea. Moreover, the otoprotective action of the ACTH-related neuropeptides suggests that it may be possible to stimulate recovery from acute hearing loss using neuropeptides.

Cisplatin ototoxicity. An electrophysiological dose-effect study in albino guinea pigs.

Recently, the effect of the ACTH(4-9) analog, ORG2766, on cisplatin ototoxicity was studied by Hamers et al. (1994). This study showed that the ACTH(4-9) analog partially prevents the ototoxicity of cisplatin. The authors suggested that the daily dose of 2.0 mg/kg/day for 8 days might have been too high to obtain full protection. Knowledge about dose-effect relations for cisplatin ototoxicity is rather meager. Therefore, we conducted a basic dose-effect study for cisplatin without any concomitant additives. A follow-up of the Hamers et al. (1994) study, based on dose-effect data from this paper, is presented in a companion paper. The effects of cisplatin on the compound action potential (CAP), cochlear microphonics (CM) and summating potential (SP) were determined in acute experiments, in different groups of albino guinea pigs, each group injected with a different dose of cisplatin. Daily doses ranged from 0.7 to 2.0 mg/kg/day cisplatin (i.p.) for 8 consecutive days. Electrocochleography was performed at day 10. The measurements were performed over a broad range of frequencies (0.5-16 kHz). The results showed clustering of the data in two groups, the first group concerning the treatments of 1.5 and 2.0 mg/kg/day with large frequency-dependent losses in the three cochlear potentials, the second group concerning the treatments with lower doses of cisplatin (0.7, 1.0 and 1.25 mg/kg/day) where almost no losses in the three cochlear potentials were found. The threshold curves regarding the lower doses (0.7-1.25 mg/kg/day) were almost indistinguishable from the control threshold curve except at the higher frequencies (12 and 16 kHz). Thus, a marked transition from almost no ototoxic effect to a large effect seems to occur between cisplatin doses of 1.25 and 1.5 mg/kg/day for 8 days. The small difference between the effects found for 1.5 mg/kg/day and 2 mg/kg/day suggests that a smaller dose than the one of 2 mg/kg/day for 8 days used previously (Hamers et al., 1994) might better suit research into protection against cisplatin ototoxicity.

Protective effects of a neurotrophic ACTH(4-9) analog on cisplatin ototoxicity in relation to the cisplatin dose: an electrocochleographic study in albino guinea pigs.

Cisplatin is a potent cell cycle non-specific chemotherapeutic agent that produces side effects including high-frequency hearing loss. Hamers et al. (1994) studied electrophysiologically the effect of an ACTH(4-9) analog, also known as ORG2766, on the ototoxicity of cisplatin (administered at 2 mg/kg/day for 8 days) in guinea pigs. ORG2766 was given concomitantly with cisplatin during the 8 day period and an additional dose was given on day 9. The conclusion of this study was that ORG2766 might partially prevent cisplatin ototoxicity, but that the chosen cisplatin dose (2 mg/kg/day; 8 days) might have been too high. Because of the high cisplatin dose the protective power of the co-treatment with ORG2766 might not have stretched to all animals. In this study the results of co-treatment with the same dose and daily schedule of ORG2766 and cisplatin doses of 1.0 mg/kg/day and 1.5 mg/kg/day for 8 days are presented. The measurements were performed over a broad range of frequencies (0.5-16 kHz). Electrocochleography was performed at day 10. In the 1.0 mg/kg/day group there was no beneficial effect of ORG2766, although a tendency towards a division between a subgroup resembling control animals and a subgroup with severe cisplatin effects was noted in the co-treated group. In the 1.5 mg/kg/day co-treated group three animals showed compound action potential (CAP) amplitudes close to those of the controls at all frequencies except the very highest (12 and 16 kHz), the remaining three had CAP amplitudes comparable to those of animals in the cisplatin alone group. The effect of ORG2766 on the latter group of six animals taken together was statistically significant. The dichotomy in the results for the 1.5 mg/kg/day group co-treated with ORG2766 suggests that ORG2766 may have a protective effect against cisplatin ototoxicity which, however, depends on a factor currently unknown.

Induction of endolymphatic hydrops in the guinea pig by perisaccular deposition of sepharose beads carrying and not carrying immune complexes.

We tried to induce endolymphatic hydrops in guinea pig cochleas by unilateral, perisaccular deposition of sepharose beads carrying immune complexes. Controls consisted of the deposition of sepharose beads without immune complexes and the contralateral, untreated ear. The effects of the treatment were studied by light microscopy and electrophysiological recordings of the gross cochlear potentials 1, 2, and 6 weeks after treatment. Each condition included six animals. Analysis of variance of the morphometric data concerning the ears treated with deposition of the beads showed a statistically significant difference (P = 0.04) between the degree of hydrops found for the beads with immune complexes and for those without. The difference between the treated ears and the contralateral untreated ears was significant (P = 0.01) for the beads with immune complexes and not significant (P = 0.8) for those without immune complexes while there was no significant effect of post-treatment time interval. Analysis of variance of the electrophysiological data, collected in response to tone bursts at the apex of the cochlea, showed no significant differences between the results for the beads with and without immune complexes. Therefore these results were pooled. One week after treatment the pooled results for the compound action potential showed a small decrease in amplitude, just significant at 2 kHz, but not at 4 and 8 kHz. This decrease disappeared completely after 6 weeks. The pooled results for the negative summating potential (SP) showed a significant increase in magnitude at all frequencies decreasing with post-treatment interval. The cochlear microphonics did not demonstrate any change in amplitude after treatment. The results indicate that deposition of sepharose beads with immune complexes induces endolymphatic hydrops. Also, deposition of the sepharose beads itself may have induced hydrops together with enhancement of the SP. SP enhancement may be related to the development of endolymphatic hydrops rather than to the presence of hydrops as such. This may be based on pressure build-up while hydrops develops.

Cisplatin-induced ototoxicity; electrophysiological evidence of spontaneous recovery in the albino guinea pig.

For 8 days albino guinea pigs (n = 48) were treated with cisplatin (cis-diamminedichloroplatinum(II), 1.5 mg/kg body weight/day). Compound action potentials (CAP), cochlear microphonics (CM) and summating potentials (SP) were recorded from the apical surface of the cochlea in response to tone bursts ranging in frequency from 0.5 to 16 kHz. The recordings were collected in different groups of animals, 1 day, 1 week, 2, 4, 8 and 16 weeks after cisplatin treatment, respectively. One day after the 8-day treatment we found frequency-dependent loss in the amplitudes of the three cochlear potentials, with the larger losses occurring at the higher frequencies. In terms of threshold shift the losses were larger for the CAP than for the hair cell-related potentials SP and CM. A salient improvement in both CAP and CM amplitude occurred over the next 8 weeks. Also, the SP showed improvement. These results indicate that guinea pig cochlear transduction recovers spontaneously after cisplatin injury. Recovery of the hair cell-related potentials suggests that recovery occurs already at the hair cell level. The question whether this recovery originates with the formation of new hair cells or with repair of damaged hair cells should be answered on the basis of subsequent morphological investigations.

Identification of the nonlinearity governing even-order distortion products in cochlear potentials.

In order to characterize the cochlear transducer nonlinearities which are involved in the generation of the summating potential (SP), we investigated the effect of a change in the electrical operating point of the cochlear transducer on the SP. The electrical operating point of the cochlear transducer was affected by suppressing reversibly the endocochlear potential (EP). This was realized by intravenous injection of furosemide in guinea pig. A differential recording technique was used in the basal turn of the cochlea to measure locally generated even-order distortion products: the SP and the second harmonic component (2F0) of the cochlear microphonics (CM). These potentials were evoked by 2 and 8 kHz stimuli presented at 60 dB SPL. Following furosemide injection, the SP changed polarity twice over time. The zero crossings of the SP coincided with a minimum in the amplitude of 2F0. Concomitantly, the phase of 2F0 shifted about 120 degrees. The changes in the electrical even-order products were comparable to the changes that occurred in a mechanical even-order intermodulation distortion product (the difference tone F2-F1 otoacoustic emission) after furosemide application (Mills et al., J. Acoust. Soc. Am. 94 (1993) 2108-2122). The combined results suggest that only one sigmoidal transfer function may account for the SP, 2F0, and the emission of the difference tone F2-F1, and that shifts in the operating point of the transfer function would be the major cause behind the furosemide-induced changes in the even-order distortion products. The sigmoidal transfer function is likely associated with the mechano-electrical transducer channel at the apical pole of the outer hair cell.

4-Aminopyridine effects on summating potentials in the guinea pig.

DC receptor potentials measured in hair cells, and the associated extracellular DC potential known as the summating potential (SP), originate with nonlinear elements in the mechanoelectric transduction chain. Nonlinear electric conductance has been demonstrated in the basolateral membrane of the hair cell, and is commonly attributed to the presence of voltage- and time-dependent K+ conductances in this part of the hair cell membrane. To study a possible contribution of these K+ channels to the SP we perfused the perilymphatic spaces of the guinea pig cochlea with the K+ channel blocker 4-aminopyridine (4-AP). Since 4-AP might also affect the afferent fibers and, thus, interfere with SP measurement, we added tetrodotoxin (TTX) to the perfusion solutions to block the neuronal discharges. Sound-evoked (2-12 kHz) intracochlear potentials were recorded from the basal turn of both scala vestibuli and scala tympani. The results showed a frequency- and level-dependent effect of 4-AP on the magnitude of the SP. At low and moderate levels of 8 and 12 kHz stimuli 4-AP mostly reduced the SP amplitude, while at high levels of these stimuli and at all levels of 2 and 4 kHz stimuli 4-AP enlarged the SP amplitude. These effects were reversible and occurred in both scala vestibuli and scala tympani. We attribute these bi-directional effects on the SP amplitude to a differential effect of 4-AP on inner hair cell (IHC) and outer hair cell (OHC) physiology. The decrease in SP was found for stimulus conditions where the SP presumably depends mainly on contributions from basal turn IHCs. Blocking the 4-AP-sensitive K+ channel in the IHC membrane should lead to a reduced contribution from the IHCs to the SP, because of an increase in basolateral membrane resistance. The increase in SP was found for stimulus conditions where the SP is assumed to depend mainly on contributions from basal turn OHCs. In this case the OHCs seemed to respond to blocking of the 4-AP-sensitive K+ channel in the basolateral membrane with an increased contribution to the nonlinearity of the transduction chain. Administration of 4-AP did not affect the endocochlear potential. Light microscopic examination revealed no apparent changes in morphology after 4-AP perfusion.

Tetraethylammonium effects on cochlear potentials in the guinea pig.

Voltage-dependent K+ channels in the basolateral membrane of hair cells in guinea-pig cochlea might contribute to the non-linear current-voltage relationships in these hair cells and, thereby, to generation of the extracellular summating potential (SP). To evaluate the role of K+ channels in the generation of the SP the perilymphatic perfusion technique was used to introduce the K(+)-channel blocker tetraethylammonium (TEA) into the cochlea. Sound-evoked cochlear potentials were measured subsequently. Without blocking nerve activity TEA induced reversible shifts of the SP in the negative direction, irrespective of whether we recorded from scala vestibuli or scala tympani. Shifts in the negative direction were probably due to TEA acting directly on the afferent fibres, since removal of nerve activity by the potent Na(+)-channel blocker tetrodotoxin (TTX) prevented TEA from shifting the SP in the negative direction. Once nerve activity had been removed by TTX, administration of TEA caused a small decrease in the magnitude of the SP, both in scala vestibuli and in scala tympani, irrespective of its polarity. The decrease was significant for the highest test frequencies only (8-12 kHz), and completely reversible. The rapidly activated K+ channel in the inner hair cell (IHC) is probably blocked by TEA and this blocking might be responsible for the small decrease in magnitude of the SP. The asymmetric contribution from this K+ channel to the IHC's current-voltage relationship seems to be only partly responsible for the generation of the SP, since blocking of this K+ channel with TEA caused relatively small decreases in the amplitude of the SP. TEA did not affect the endocochlear potential.

A comparison of the effect of cochlear perfusion with ouabain on summating potentials and distortion product otoacoustic emissions in the guinea pig.

In order to investigate whether or not the summating potential (SP) and the 2f1-f2 distortion product otoacoustic emission (DPOAE) are due to related cochlear non-linearities, their behavior was studied in the guinea pig after intracochlear perfusion with ouabain and subsequent rinsing. The SP was evoked with either 4 or 8 kHz tone bursts, and the 2f1-f2 DPOAE was evoked with simultaneous presentations of 6.6 and 8 kHz continuous tones. After ouabain perfusion, DPOAE was dramatically reduced while the SP underwent only a small reduction. After rinsing out the ouabain with artificial perilymph, the DPOAE showed partial recovery while the SP displayed a large and long-lasting increase when compared to its a initial value. These results suggest that the non-linear processes giving rise to the SP and DPOAE are not identical.

Adaptation in the compound action potential response of the guinea pig VIIIth nerve to electric stimulation.

An experimental study, carried out in guinea pigs, was designed to investigate whether forward masking measured psychophysically in 3M-House cochlear implant users might have a correlate in VIIIth nerve activity. The study was based on electrically evoked VIIIth nerve compound action potentials (ECAPs), using a masking paradigm comparable to the one used in the psychophysical study. Trains of 50 maskers with inter-masker-intervals of 509 ms appeared to induce a long-term fatigue effect that could influence the recovery from adaptation measurements. Fatigue stabilized within about 1 to 3 min when masker trains were repeated with intervening silent intervals of 10.5 s. The change in amplitude of probe-evoked ECAPs with increasing masker-probe delays was determined within the steady fatigue state. The recovery-from-adaptation functions obtained from these measurements resembled the forward masking functions found in 3M-House cochlear implant users. No correlate of psychophysical backward masking was found at the VIIIth nerve level. To examine whether hair cells were involved in fatigue and recovery from adaptation, the measurements described above were carried out in intact cochleas and in cochleas without hair cells. Results were essentially the same in the different preparations. The results suggest that processes at the level of the VIIIth nerve could, at least partly, account for forward masking found in 3M-House cochlear implant users. Backward masking must be attributed to mechanisms located centrally to the VIIIth nerve.