The potential use of low-frequency tones to locate regions of outer hair cell loss.

Current methods used to diagnose cochlear hearing loss are limited in their ability to determine the location and extent of anatomical damage to various cochlear structures. In previous experiments, we have used the electrical potential recorded at the round window -the cochlear response (CR) -to predict the location of damage to outer hair cells in the gerbil. In a follow-up experiment, we applied 10 mM ouabain to the round window niche to reduce neural activity in order to quantify the neural contribution to the CR. We concluded that a significant proportion of the CR to a 762 Hz tone originated from phase-locking activity of basal auditory nerve fibers, which could have contaminated our conclusions regarding outer hair cell health. However, at such high concentrations, ouabain may have also affected the responses from outer hair cells, exaggerating the effect we attributed to the auditory nerve. In this study, we lowered the concentration of ouabain to 1 mM and determined the physiologic effects on outer hair cells using distortion-product otoacoustic emissions. As well as quantifying the effects of 1 mM ouabain on the auditory nerve and outer hair cells, we attempted to reduce the neural contribution to the CR by using near-infrasonic stimulus frequencies of 45 and 85 Hz, and hypothesized that these low-frequency stimuli would generate a cumulative amplitude function (CAF) that could reflect damage to hair cells in the apex more accurately than the 762 stimuli. One hour after application of 1 mM ouabain, CR amplitudes significantly increased, but remained unchanged in the presence of high-pass filtered noise conditions, suggesting that basal auditory nerve fibers have a limited contribution to the CR at such low frequencies.

Dopamine Modulates the Activity of Sensory Hair Cells.

The senses of hearing and balance are subject to modulation by efferent signaling, including the release of dopamine (DA). How DA influences the activity of the auditory and vestibular systems and its site of action are not well understood. Here we show that dopaminergic efferent fibers innervate the acousticolateralis epithelium of the zebrafish during development but do not directly form synapses with hair cells. However, a member of the D1-like receptor family, D1b, tightly localizes to ribbon synapses in inner ear and lateral-line hair cells. To assess modulation of hair-cell activity, we reversibly activated or inhibited D1-like receptors (D1Rs) in lateral-line hair cells. In extracellular recordings from hair cells, we observed that D1R agonist SKF-38393 increased microphonic potentials, whereas D1R antagonist SCH-23390 decreased microphonic potentials. Using ratiometric calcium imaging, we found that increased D1R activity resulted in larger calcium transients in hair cells. The increase of intracellular calcium requires Cav1.3a channels, as a Cav1 calcium channel antagonist, isradipine, blocked the increase in calcium transients elicited by the agonist SKF-38393. Collectively, our results suggest that DA is released in a paracrine fashion and acts at ribbon synapses, likely enhancing the activity of presynaptic Cav1.3a channels and thereby increasing neurotransmission. SIGNIFICANCE STATEMENT: The neurotransmitter dopamine acts in a paracrine fashion (diffusion over a short distance) in several tissues and bodily organs, influencing and regulating their activity. The cellular target and mechanism of the action of dopamine in mechanosensory organs, such as the inner ear and lateral-line organ, is not clearly understood. Here we demonstrate that dopamine receptors are present in sensory hair cells at synaptic sites that are required for signaling to the brain. When nearby neurons release dopamine, activation of the dopamine receptors increases the activity of these mechanosensitive cells. The mechanism of dopamine activation requires voltage-gated calcium channels that are also present at hair-cell synapses.

Effects of perilymphatic pressure, sodium nitroprusside, and bupivacaine on cochlear fluid pH of guinea pigs.

CONCLUSIONS: Hydrostatic positive pressure and vasoconstrictor acidified the cochlear fluids, whereas the vasodilator made the fluids alkaline. CBF might play a role in regulating cochlea fluid pH. OBJECTIVES: Cochlea fluid pH is highly dependent on the HCO3(-)/CO2 buffer system. Cochlear blood flow (CBF) supplies O2 and removes CO2. It is speculated that cochlear blood flow changes might affect the balance of the HCO3(-)/CO2 buffer system in the cochlea. It is known that the elevation of inner ear pressure decreases the CBF, and local application of vasodilating or vasoconstricting agents directly to the cochlea changes the CBF. The purpose of this study was to elucidate the effect of positive hydrostatic inner ear pressure and application of a vasodilator and vasoconstrictor of cochlear vessels on the pH of the endolymph and perilymph. METHODS: The authors performed animal physiological experiments on 30 guinea pigs. Hydrostatic positive pressure was infused through a glass capillary tube inserted into the scala tympani of the basal turn. The vasodilator, nitric oxide donor (sodium nitroprusside; SNP), and the vasoconstrictor, bupivacaine, were placed topically onto the round window of the guinea pig cochlea. Endolymph pH (pHe) and endocochlear potential (EP) were monitored by double-barreled ion-selective microelectrodes in the second turn of the guinea pig cochlea. During the topical application study, scala vestibuli perilymph pH (pHv) was also measured simultaneously in the second turn. RESULTS: The application of hydrostatic positive pressure caused a decrease in pHe and EP. Positive perilymphatic pressure caused the endolymph to become acidic pressure-dependently. Application of 3.0% SNP evoked an increase in both the pHe and pHv, following by a gradual recovery to baseline levels. On the other hand, 0.5% bupivacaine caused a decrease in both the pHe and pHv. The EP during topical application showed slight, non-significant changes.

Potassium ion channel openers, Maxipost and Retigabine, protect against peripheral salicylate ototoxicity in rats.

Sodium Salicylate (SS) reliably induces a sensorineural hearing loss and tinnitus when administered in high doses. Recent animal modeled studies indicate that potassium channel openers such as Maxipost and Retigabine (RTG) can block SS- or noise-induced tinnitus respectively; however, the origins and mechanisms are poorly understood. Since SS blocks the same potassium channels that Maxipost and RTG open, we postulated that these drugs might influence peripheral auditory function. To test this hypothesis Maxipost or RTG were administered alone or in combination with SS in rats. When administered alone, Maxipost and RTG had no effect on distortion product otoacoustic emissions (DPOAE) or compound action potentials (CAPs). However when Maxipost or RTG were administered with SS, Maxipost prevented the SS-reduced CAP amplitudes at high frequencies (≥20 kHz) and RTG prevented SS-reduced CAP amplitudes at low frequencies (≤8 kHz). These results suggest that Maxipost and RTG can protect against peripheral damage and therefore reduce the incidence of tinnitus.

Effects of salicylate on sound-evoked outer hair cell stereocilia deflections.

Hearing depends on sound-evoked deflections of the stereocilia that protrude from the sensory hair cells in the inner ear. Although sound provides an important force driving stereocilia, forces generated through mechanically sensitive ion channels and through the motor protein prestin have been shown to influence stereocilia motion in solitary hair cells. While a possible influence of prestin on mechanically sensitive ion channels has not been systematically investigated, a decrease in transducer currents is evident in solitary hair cells when prestin is blocked with salicylate, raising the question of whether a reduced prestin activity or salicylate itself affected the mechanotransduction apparatus. We used two- and three-dimensional time-resolved confocal imaging to visualize outer hair cell stereocilia during sound stimulation in the apical turn of cochlear explant preparations from the guinea pig. Surprisingly, following application of salicylate, outer hair cell stereocilia deflections increased, while cochlear microphonic potentials decreased. However, when prestin activity was altered with the chloride ionophore tributyltin, both the cochlear microphonic potential and the stereocilia deflection amplitude decreased. Neither positive nor negative current stimulation abolished the bundle movements in the presence of salicylate, indicating that the observed effects did not depend on the endocochlear potential. These data suggest that salicylate may alter the mechanical properties of stereocilia, decreasing their bending stiffness.

Predicting the location of missing outer hair cells using the electrical signal recorded at the round window.

The electrical signal recorded at the round window was used to estimate the location of missing outer hair cells. The cochlear response was recorded to a low frequency tone embedded in high-pass filtered noise conditions. Cochlear damage was created by either overexposure to frequency-specific tones or laser light. In animals with continuous damage along the partition, the amplitude of the cochlear response increased as the high-pass cutoff frequency increased, eventually reaching a plateau. The cochlear distance at the onset of the plateau correlated with the anatomical onset of outer hair cell loss. A mathematical model replicated the physiologic data but was limited to cases with continuous hair cell loss in the middle and basal turns. The neural contribution to the cochlear response was determined by recording the response before and after application of Ouabain. Application of Ouabain eliminated or reduced auditory neural activity from approximately two turns of the cochlea. The amplitude of the cochlear response was reduced for moderate signal levels with a limited effect at higher levels, indicating that the cochlear response was dominated by outer hair cell currents at high signal levels and neural potentials at low to moderate signal levels.

Regulation of dopamine D2 receptors in the guinea pig cochlea.

CONCLUSION: Our study demonstrates that the regulation of D2 receptors may be frequency specific. The reduction in cochlear microphonics (CM) and distortion product otoacoustic emission (DPOAE) amplitudes after perfusion with a D2 antagonist suggests that this receptor plays a role in the regulation of cochlear hair cell activation. OBJECTIVES: Dopaminergic terminals are subject to negative feedback from dopamine D2 receptors. In the present study we investigated whether the regulation of dopamine D2 receptor is frequency specific and evaluated changes in CM in guinea pig cochlea. METHODS: A total of 30 male guinea pigs were randomly assigned to 3 groups and perfused with artificial perilymph (AP), AP containing ethanol (0.1%), or a D2 antagonist (L741626) for 2 h. In each group, compound action potentials (CAPs) evoked by a 1, 2, 4, 8, 16 or 24 kHz tone pip, CM evoked by 4 kHz tone bursts, and DPOAEs were measured before and 2 h after perilymphatic perfusion. RESULTS: Perfusion with the D2 antagonist resulted in increased CAP thresholds compared with the other two groups at high frequencies (4, 8, 16, 24 kHz, p < 0.05); however, no significant increase was observed at low frequencies (1, 2 kHz, p > 0.05). There was a significant reduction in DPOAEs and CM amplitudes after the 2 h perfusion with the D2 antagonist. A CM input/output (I/O) function curve plotted with the stimulating level as input and the CM relative amplitude as output indicated obvious nonlinearity after the 2 h perfusion in all three groups.

Large endolymphatic potentials from low-frequency and infrasonic tones in the guinea pig.

Responses of the ear to low-frequency and infrasonic sounds have not been extensively studied. Understanding how the ear responds to low frequencies is increasingly important as environmental infrasounds are becoming more pervasive from sources such as wind turbines. This study shows endolymphatic potentials in the third cochlear turn from acoustic infrasound (5 Hz) are larger than from tones in the audible range (e.g., 50 and 500 Hz), in some cases with peak-to-peak amplitude greater than 20 mV. These large potentials were suppressed by higher-frequency tones and were rapidly abolished by perilymphatic injection of KCl at the cochlear apex, demonstrating their third-turn origins. Endolymphatic iso-potentials from 5 to 500 Hz were enhanced relative to perilymphatic potentials as frequency was lowered. Probe and infrasonic bias tones were used to study the origin of the enhanced potentials. Potentials were best explained as a saturating response summed with a sinusoidal voltage (Vo), that was phase delayed by an average of 60° relative to the biasing effects of the infrasound. Vo is thought to arise indirectly from hair cell activity, such as from strial potential changes caused by sustained current changes through the hair cells in each half cycle of the infrasound.

Membrane cholesterol modulates cochlear electromechanics.

Changing the concentration of cholesterol in the plasma membrane of isolated outer hair cells modulates electromotility and prestin-associated charge movement, suggesting that a similar manipulation would alter cochlear mechanics. We examined cochlear function before and after depletion of membrane cholesterol with methyl-ß-cyclodextrin (MßCD) in an excised guinea pig temporal bone preparation. The mechanical response of the cochlear partition to acoustic and/or electrical stimulation was monitored using laser interferometry and time-resolved confocal microscopy. The electromechanical response in untreated preparations was asymmetric with greater displacements in response to positive currents. Exposure to MßCD increased the magnitude and asymmetry of the response, without changing the frequency tuning of sound-evoked mechanical responses or cochlear microphonic potentials. Sodium salicylate reversibly blocked the enhanced electromechanical response in cholesterol depleted preparations. The increase of sound-evoked vibrations during positive current injection was enhanced following MßCD in some preparations. Imaging was used to assess cellular integrity which remained unchanged after several hours of exposure to MßCD in several preparations. The enhanced electromechanical response reflects an increase in outer hair cell electromotility and may reveal features of cholesterol distribution and trafficking in outer hair cells.

New insights into glutamate ototoxicity in cochlear hair cells and spiral ganglion neurons.

CONCLUSION: Excess glutamate (Glu) exposure (20 mM) in the cochlear perilymph affects the physiological function of outer hair cells (OHCs) within a 2 h period and induces apoptosis in the modiolus spiral ganglion neurons (SGNs) in an apoptosis-inducing factor (AIF)-dependent manner. OBJECTIVES: To determine whether high-dose Glu affects the function of OHCs and whether it induces AIF- and caspase-3-dependent apoptosis in the cochlear SGNs. METHODS: Perilymphatic perfusions of Glu (20 mM) and artificial perilymph (AP) solutions were performed in adult guinea pig cochleae. Both cochlear microphonics (CM) and electrical auditory brainstem response (eABR) were measured before and 2 h after perfusions. The hair cell morphologies were examined using transmission electron microscopy. The expression of two apoptotic indicators, AIF and caspase-3, was examined 8 h after perfusions. RESULTS: In contrast to AP perfusions, the perfusion of 20 mM Glu caused significant reduction in the CM and eABR amplitudes. Inner hair cells (IHCs) after Glu perfusion were deformed and exhibited vacuolization in the postsynaptic region, whereas the OHC system appeared unaffected. AIF expression was detected in the nuclei of SGNs 8 h after Glu exposure, but the expression of caspase-3 was not shown in any cochlear tissues.

Too much of a good thing: long-term treatment with salicylate strengthens outer hair cell function but impairs auditory neural activity.

Aspirin has been extensively used in clinical settings. Its side effects on auditory function, including hearing loss and tinnitus, are considered as temporary. A recent promising finding is that chronic treatment with high-dose salicylate (the active ingredient of aspirin) for several weeks enhances expression of the outer hair cell (OHC) motor protein (prestin), resulting in strengthened OHC electromotility and enhanced distortion product otoacoustic emissions (DPOAE). To follow up on these observations, we carried out two studies, one planned study of age-related hearing loss restoration and a second unrelated study of salicylate-induced tinnitus. Rats of different strains and ages were injected with salicylate at a dose of 200 mg/kg/day for 5 days per week for 3 weeks or at higher dose levels (250-350 mg/kg/day) for 4 days per week for 2 weeks. Unexpectedly, while an enhanced or sustained DPOAE was seen, permanent reductions in the amplitude of the cochlear compound action potential (CAP) and the auditory brainstem response (ABR) were often observed after the chronic salicylate treatment. The mechanisms underlying these unexpected, permanent salicylate-induced reductions in neural activity are discussed.

Effects of isoflurane on auditory evoked potentials in the cochlea and brainstem of guinea pigs.

Electrophysiological recordings of the auditory system are commonly performed in deeply anesthetized animals. This study evaluated the effects of various concentrations of the volatile anesthetic isoflurane (1-3%) on the compound action potential (CAP), cochlear microphonic (CM) and auditory brainstem response (ABR). Recordings were initiated in the awake, lightly restrained animal. Anesthesia was induced with a single dose of Hypnorm (fentanyl and fluanisone). After tracheostomy increasing isoflurane concentrations were applied in N(2)O/O(2) via controlled ventilation. Data were compared to recordings in the awake animal using repeated measures ANOVA and Dunnett's post hoc test. On average, isoflurane dose-dependently suppressed the amplitude and increased the latency of the CAP. CM amplitude was suppressed. These effects were most profound at high frequencies and were typically significant at isoflurane concentrations of 2.5% and 3%. Amplitude and latency of the second negative peak of the CAP (N(2)) were affected to a greater extent compared to the first peak (N(1)). On average, isoflurane dose-dependently reduced the amplitude and increased the latency of the ABR. These effects were typically significant at an isoflurane concentration of 2%. Effects on peak IV and V were more pronounced compared to the early peaks I and III.

Post exposure administration of A(1) adenosine receptor agonists attenuates noise-induced hearing loss.

Adenosine is a constitutive cell metabolite with a putative role in protection and regeneration in many tissues. This study was undertaken to determine if adenosine signalling pathways are involved in protection against noise injury. A(1) adenosine receptor expression levels were altered in the cochlea exposed to loud sound, suggesting their involvement in the development of noise injury. Adenosine and selective adenosine receptor agonists (CCPA, CGS-21680 and Cl-IB-MECA) were applied to the round window membrane of the cochlea 6h after noise exposure. Auditory brainstem responses measured 48h after drug administration demonstrated partial recovery of hearing thresholds (up to 20dB) in the cochleae treated with adenosine (non-selective adenosine receptor agonist) or CCPA (selective A(1) adenosine receptor agonist). In contrast, the selective A(2A) adenosine receptor agonist CGS-21680 and A(3) adenosine receptor agonist Cl-IB-MECA did not protect the cochlea from hearing loss. Sound-evoked cochlear potentials in control rats exposed to ambient noise were minimally altered by local administration of the adenosine receptor agonists used in the noise study. Free radical generation in the cochlea exposed to noise was reduced by administration of adenosine and CCPA. This study pinpoints A(1) adenosine receptors as attractive targets for pharmacological interventions to reduce noise-induced cochlear injury after exposure.

A mouse model for degeneration of the spiral ligament.

Previous studies have indicated the importance of the spiral ligament (SL) in the pathogenesis of sensorineural hearing loss. The aim of this study was to establish a mouse model for SL degeneration as the basis for the development of new strategies for SL regeneration. We injected 3-nitropropionic acid (3-NP), an inhibitor of succinate dehydrogenase, at various concentrations into the posterior semicircular canal of adult C57BL/6 mice. Saline-injected animals were used as controls. Auditory function was monitored by measurements of auditory brain stem responses (ABRs). On postoperative day 14, cochlear specimens were obtained after the measurement of the endocochlear potential (EP). Animals that were injected with 5 or 10 mM 3-NP showed a massive elevation of ABR thresholds along with extensive degeneration of the cochleae. Cochleae injected with 1 mM 3-NP exhibited selective degeneration of the SL fibrocytes but alterations in EP levels and ABR thresholds were not of sufficient magnitude to allow for testing functional recovery after therapeutic interventions. Animals injected with 3 mM 3-NP showed a reduction of around 50% in the EP along with a significant loss of SL fibrocytes, although degeneration of spiral ganglion neurons and hair cells was still present in certain regions. These findings indicate that cochleae injected with 3 mM 3-NP may be useful in investigations designed to test the feasibility of new therapeutic manipulations for functional SL regeneration.

Toluene can perturb the neuronal voltage-dependent Ca2+ channels involved in the middle-ear reflex.

Numerous laboratory-based data have shown the ability of toluene (Tol) to exacerbate noise-induced hearing loss. However, the mechanism responsible for the synergistic effects of a coexposure to noise and Tol has not yet been completely elucidated. Recent investigations in rats have focused on quantifying the anticholinergic effects of certain aromatic solvents and have demonstrated that these solvents can cancel the protective role played by the middle-ear reflex (MER). Voltage-dependent Ca(2+) channels (VDCCs) regulate acetylcholine release in the central synaptic network and control muscular excitation/contraction processes as well. In order to identify the prevailing action of Tol in the central or peripheral compartment of the MER arc, two VDCC antagonists were injected into the common carotid trunk: omega-conotoxin MVIIC, which blocks only the neuronal N- and P/Q-type Ca(2+) channels, or verapamil, which inhibits the muscular L-type Ca(2+) channels. Rats were also implanted with an electrode on the round window membrane to measure the cochlear microphonic potential (CMP) elicited with a band noise centered at 4 kHz and emitted at 85 dB sound pressure level. The variations in CMP recorded during the test compound injection showed that Tol has similar effects to those induced by omega-conotoxin, the neuronal VDCC blocker. The response obtained with the verapamil injection was broader than those obtained with Tol or conotoxin. This investigation therefore revealed that Tol can mimic the effects of VDCC blockers. The antagonist effects of Tol would be closer to neuronal than to muscular blockers and would be presumably located at the level of the integrator centers of the reflex.

Relation between outer hair cell loss and hearing loss in rats exposed to styrene.

The relationship between outer hair cell (OHC) loss and cochlear sensitivity is still unclear, because in many animal models there exist surviving but dysfunctional OHCs and also injured/dead inner hair cells (IHC). Styrene is an ototoxic agent, which targets and destroys OHCs starting from the third row to the second and first rows depending on the exposure level. The remaining cells may be less affected. In this experiment, rats were exposed to styrene by gavage at different doses (200-800 mg/kg/day) for varying periods (5 days/week for 3-12 weeks). An interesting finding was that the cochlear sensitivity was not affected in a few rats with all OHCs in the third row being destroyed by styrene. A further loss of OHCs was usually accompanied with a linear input/output (I/O) function of cochlear compound action potentials (CAP), indicating the loss of cochlear amplification. However, normal CAP amplitudes at the highest stimulation level of 90 dB SPL were often observed when all OHCs were destroyed, indicating normal function of the remaining IHCs. The OHC-loss/hearing-loss relation appeared to be a sigmoid-type function. Initially, styrene-induced OHC losses (<33%) did not result in a significant threshold shift. Then CAP threshold shift increased dramatically with OHC loss from 33% to 66%. Then, CAP threshold changed less with OHC loss. The data suggest a tri-modal relationship between OHC loss and cochlear amplification. That is, under the condition that all surviving OHCs are ideally functioning, the cochlear amplifier is not affected until 33% of OHCs are absent, then the gain of the amplifier decreases proportionally with the OHC loss, and at last the amplifier may fail completely when more than 67% of OHCs are lost.

Blebs in inner and outer hair cells: a pathophysiological hypothesis.

INTRODUCTION: The ototoxic effects of cisplatin include loss of outer hair cells, degeneration of the stria vascularis and a decrease in the number of spiral ganglion cells. Scanning microscopy has shown balloon-like protrusions (blebs) of the plasma membrane of inner hair cells following cisplatin administration. The present study was undertaken to identify the possible role of inner and outer hair cell blebs in the pathogenesis of cisplatin-induced ototoxicity. MATERIALS AND METHODS: Twenty-five guinea pigs were injected with cisplatin and their hearing tested at different time-points, before sacrifice and examination with scanning electron microscopy. RESULTS AND ANALYSIS: Seven animals showed blebs in the inner hair cells at different stages. Hearing thresholds were lower in animals showing blebs. DISCUSSION: Cisplatin seems to be able to induce changes in inner hair cells as well as in other structures in the organ of Corti. Blebbing observed in animals following cisplatin administration could play a specific role in the regulation of intracellular pressure.

Salicylate ototoxicity and its implications for cochlear microphonic potential generation.

Salicylic acid causes a reversible sensori-neural hearing loss. Its ototoxicity is probably related to its effect on prestin, the motor protein of the outer hair cells. In order to gain further insight into the mechanism and implications of its ototoxicity, auditory nerve brainstem evoked responses, compound action potentials of the auditory nerve, distortion product otoacoustic emissions, and cochlear microphonic potentials (CM) and vestibular evoked potentials were recorded before and after systemic salicylate administration. These responses were depressed, except for the CM and the vestibular evoked potential. This result and additional considerations provide evidence that the extracellularly recorded CM does not represent the summation of intracellular outer hair cell receptor potentials. It is possible that the CM reflects an early stage of mechano-electrical transduction by the outer hair cells, before the activation of the cochlear amplifier and the later stages of transduction.

Effects of aromatic solvents on acoustic reflexes mediated by central auditory pathways.

From previous in vivo investigations, it has been shown that toluene can mimic the effects of cholinergic receptor antagonists and may thereby modify the response of protective acoustic reflexes. The current study aimed to define the relative effects of aromatic solvents on the middle ear and inner ear acoustic reflexes. Toward this end, the cochlear microphonic (CMP) elicited with a band noise centered at 4 kHz, and the compound action potential (CAP) elicited with 4-kHz tone pips was measured in rats. Both potentials were recorded before, during, and after triggering the protective reflexes by a 110-dB SPL contralateral octave band noise centered at 12.5 kHz (12.5 kHz-OBN). In several rats, the middle ear muscles were severed to identify the relative effects of toluene on the two reflexes. While the reflex elicitor was capable of decreasing both the CMP and CAP amplitudes, an injection of 116.2 mM toluene cancelled this suppressor effect induced by the contralateral sound. In the rats with nonfunctional middle ear muscles, a solvent injection did not modify the electrophysiological responses of the cochlea. Different solvents were tested to study the relationship of the chemical structure of the solvents on the acoustic reflexes. The present study showed that aromatic solvents can inhibit the action of the middle ear reflex by their anticholinergic effect on the efferent motoneurons. An aromatic nucleus and the presence of one side chain of no more than 3 C seem to be required in the solvent structure to inhibit the efferent motoneurons.

Increase in cochlear microphonic potential after toluene administration.

Human and animal studies have shown that toluene can cause hearing loss. In the rat, the outer hair cells are first disrupted by the ototoxicant. Because of their particular sensitivity to toluene, the cochlear microphonic potential (CMP) was used for monitoring the cochlea activity of anesthetized rats exposed to both noise (band noise centered at 4 kHz) and toluene. In the present experiment, the conditions were specifically designed to study the toluene effects on CMP and not those of its metabolites. To this end, 100-microL injections of a vehicle containing different concentrations of solvent were made into the carotid artery connected to the tested cochlea. Interestingly, an injection of 116.2-mM toluene dramatically increased in the CMP amplitude (approximately 4 dB) in response to an 85-dB SPL noise. Moreover, the rise in CMP magnitude was intensity dependent at this concentration suggesting that toluene could inhibit the auditory efferent system involved in the inner-ear or/and middle-ear acoustic reflexes. Because acetylcholine is the neurotransmitter mediated by the auditory efferent bundles, injections of antagonists of cholinergic receptors (AchRs) such as atropine, 4-diphenylacetoxy-N-methylpiperidine-methiodide (mAchR antagonist) and dihydro-beta-erythroidine (nAchR antagonist) were also tested in this investigation. They all provoked rises in CMP having amplitudes as large as those obtained with toluene. The results showed for the first time in an in vivo study that toluene mimics the effects of AchR antagonists. It is likely that toluene might modify the response of protective acoustic reflexes.