The hair cell acetylcholine receptors: a synthesis

The prevalence and factors associated with hearing impairment in the Korean adults: the 2010-2012 Korea National Health and Nutrition Examination Survey (observational study)

There are few studies that have used audiometric testing to gauge the demographic characteristics and associated risk factors for hearing loss at the national-level. Here, we investigated the weighted prevalence and associated factors of hearing impairment in 16,040 Korean adult population. Subjects completed audiometric test and laboratory examination as part of the data from The 2010-2012 Korea National Health and Nutrition Examination Survey (KNHANES). In our respective study, the overall weighted (n = 33,762,584) prevalence of mild hearing impairment among the Korean adult population was 20.5% (95% clearance [CI], 19.6-21.6), whereas moderate-to-profound hearing impairment was 9.2% (95% CI, 8.6-9.9). The weighted prevalence of mild hearing impairment in younger adults (19-39 years' old) was 4.4% (3.5-5.5), in middle-age adults (40-64 years), it was 21.1% (19.8-22.5), and in older adults (≥65 years' old), it was 69.7% (67.8-71.6). Logistic regression analyses were performed for low/mid frequency or high-frequency mild hearing impairment with age, sex, tobacco use, heavy alcohol use, educational background, occupational noise exposure, obesity, hypertension, diabetes, total serum cholesterol, and estimated glomerular filtration rate (eGFR) <60 mL/min/1.73m² as covariates. The analyses revealed independent correlations between increased age, tobacco use, education, hypertension, and eGFR <60 mL/min/1.73m², and low/mid frequency and high frequency mild hearing impairment. High frequency mild hearing impairment was positively correlated with male sex, diabetes, and an increase in total serum cholesterol. Taken together, hearing impairment in Korea is highly prevalent with approximately one-fifth of Korean adult reporting mild hearing impairment. This study suggests that individuals with cardiovascular risk factors such as hypertension, diabetes, smoking, increased serum cholesterol, or decreased eGFR are at particular risk of developing hearing impairment. As such, these groups may benefit from hearing loss screening in addition to those groups typically considered to be of elevated risk including geriatrics, those of low socioeconomic status, and those with considerable occupational noise exposure.

Cigarette smoking, passive smoking, alcohol consumption, and hearing loss

The objective of this large population-based cross-sectional study was to evaluate the association between smoking, passive smoking, alcohol consumption, and hearing loss. The study sample was a subset of the UK Biobank Resource, 164,770 adults aged between 40 and 69 years who completed a speech-in-noise hearing test (the Digit Triplet Test). Hearing loss was defined as speech recognition in noise in the better ear poorer than 2 standard deviations below the mean with reference to young normally hearing listeners. In multiple logistic regression controlling for potential confounders, current smokers were more likely to have a hearing loss than non-smokers (odds ratio (OR) 1.15, 95 % confidence interval (CI) 1.09-1.21). Among non-smokers, those who reported passive exposure to tobacco smoke were more likely to have a hearing loss (OR 1.28, 95 %CI 1.21-1.35). For both smoking and passive smoking, there was evidence of a dose-response effect. Those who consume alcohol were less likely to have a hearing loss than lifetime teetotalers. The association was similar across three levels of consumption by volume of alcohol (lightest 25 %, OR 0.61, 95 %CI 0.57-0.65; middle 50 % OR 0.62, 95 %CI 0.58-0.66; heaviest 25 % OR 0.65, 95 %CI 0.61-0.70). The results suggest that lifestyle factors may moderate the risk of hearing loss. Alcohol consumption was associated with a protective effect. Quitting or reducing smoking and avoiding passive exposure to tobacco smoke may also help prevent or moderate age-related hearing loss.

Assessment of the risk factors for hearing loss in adult Nigerian population

BACKGROUND

A reduction in hearing sensitivity is common in adults and was previously considered to be normal as age increases. However, other health variables may play a role in the sensory changes. This prospective, comparative, hospital-based study assessed the risk factors (RFs) associated with sensorineural hearing losses (SNHL) in adult patients in a specialized tertiary hospital clinic in South-western Nigeria.

MATERIALS AND METHODS

Patients with clinical diagnosis of hearing impairment (bilateral SNHL) were the test subjects and age and sex-matched comparable group without SNHL were the Controls. Using a structured questionnaire, variables assessed included current and past medical history, family and social history, use of medications including ototoxic drugs, and prolonged medications.

RESULTS

One hundred and twenty-seven patients participated in the study comprising of 76 test subjects with SNHL (including 14 with suspected ARHL) and 51 controls. 59.8% of the participants were males. Univariate analysis revealed statistically-significant differences in family history, alcohol consumption, smoking, exposure to noise, previous ear discharge, previous head injury, hypertension, diabetes, osteoarthritis, ototoxic drugs usage, prolonged medication and obesity between the two categories of subjects. Logistic regression analysis revealed family history, smoking, noise exposure, head injury, hypertension had significantly increased odds of developing SNHL.

CONCLUSION

It was concluded that the RFs for SNHL in adult Nigerians were multifactorial while some of the RFs may be amenable to primary prevention. Legislation and public health education could facilitate reduction of SNHL in our community.

Responses of pigeon vestibular hair cells to cholinergic agonists and antagonists

Acetylcholine (ACh) is the major neurotransmitter released from vestibular efferent terminals onto hair cells and afferents. Previous studies indicate that the two classes of acetylcholine receptors, nicotinic (nAChRs) and muscarinic receptors (mAChRs), are expressed by vestibular hair cells (VHCs). To identify if both classes of receptors are present in VHCs, whole cell, voltage-clamp- and current-clamp-patch recordings were performed on isolated pigeon vestibular type I and type II HCs during the application of the cholinergic agonists, acetylcholine and carbachol, and the cholinergic antagonists, D-tubocurarine and atropine. By applying in different combinations, these compounds were used to selectively activate either nAChRs or mAChRs. The effects of nAChR and mAChR activation on HC currents and zero electrode current potential (V(z)) were monitored. It was found that presumed mAChR activation decreased both inward and outward currents in both type I and type II HCs, resulting in either a depolarization or hyperpolarization. Conversely, nAChR activation mainly increased both inward and outward currents in type II HCs, resulting in a hyperpolarization of their V(z). nAChR activation also increased outward currents in type I HCs resulting in either a depolarization or hyperpolarization of their V(z). The decrease of inward and outward currents and the depolarization of the V(z) in type I pigeon HCs by activation of mAChRs represents a new finding. Ion channel candidates in pigeon vestibular HCs that might underlie the modulation of the macroscopic ionic currents and V(z) by different AChR activation are discussed.

Hearing loss among licensed pesticide applicators in the agricultural health study

OBJECTIVE

We evaluated self-reported hearing loss and pesticide exposure in licensed private pesticide applicators enrolled in the Agricultural Health Study in 1993 to 1997 in Iowa and North Carolina.

METHODS

Among 14,229 white male applicators in 1999 to 2003, 4926 reported hearing loss (35%). Logistic regression was performed with adjustment for state, age, and noise, solvents, and metals. We classified pesticides by lifetime days of use.

RESULTS

Compared with no exposure, the odds ratio (95% confidence interval) for the highest quartile of exposure was 1.19 (1.04 to 1.35) for insecticides and 1.17 (1.03 to 1.31) for organophosphate insecticides. Odds of hearing loss were elevated for high pesticide exposure events (1.38, 1.25 to 1.54), pesticide-related doctor visits (1.38, 1.17 to 1.62) or hospitalization (1.81, 1.25 to 2.62), and diagnosed pesticide poisoning (1.75, 1.36 to 2.26).

CONCLUSIONS

Although control for exposure to noise or other neurotoxicants was limited, this study extends previous reports suggesting that organophosphate exposure increases risk of hearing loss.

Muscarinic acetylcholine receptor subtype expression in avian vestibular hair cells, nerve terminals and ganglion cells

Muscarinic acetylcholine receptors (mAChRs) are widely expressed in the CNS and peripheral nervous system and play an important role in modulating the cell activity and function. We have shown that the cholinergic agonist carbachol reduces the pigeon's inwardly rectifying potassium channel (pKir2.1) ionic currents in native vestibular hair cells. We have cloned and sequenced pigeon mAChR subtypes M2-M5 and we have studied the expression of all five mAChR subtypes (M1-M5) in the pigeon vestibular end organs (semicircular canal ampullary cristae and utricular maculae), vestibular nerve fibers and the vestibular (Scarpa's) ganglion using tissue immunohistochemistry (IH), dissociated single cell immunocytochemistry (IC) and Western blotting (WB). We found that vestibular hair cells, nerve fibers and ganglion cells each expressed all five (M1-M5) mAChR subtypes. Two of the three odd-numbered mAChRs (M1, M5) were present on the hair cell cilia, supporting cells and nerve terminals. And all three odd numbered mAChRs (M1, M3 and M5) were expressed on cuticular plates, myelin sheaths and Schwann cells. Even-numbered mAChRs were seen on the nerve terminals. M2 was also shown on the cuticular plates and supporting cells. Vestibular efferent fibers and terminals were not identified in our studies. Results from WB of the dissociated vestibular epithelia, nerve fibers and vestibular ganglia were consistent with the results from IH and IC. Our findings suggest that there is considerable co-expression of the subtypes on the neural elements of the labyrinth. Further electrophysiological and pharmacological studies should delineate the mechanisms of action of muscarinic acetylcholine receptors on structures in the labyrinth.

Expression of nicotinic acetylcholine receptor subunit alpha9 in type II vestibular hair cells of rats

AIM

To explore the cell specific existence of alpha 9 AChR in the vestibular type II hair cells (VHC II) of rats.

METHODS

To detect the expression of alpha 9 AChR messenger RNA (mRNA) in the vestibular endorgans and single VHC II of rats by using the reverse transcription polymerase chain reaction (RT-PCR) technique and the single cell RT-PCR technique, respectively.

RESULTS

It was shown that alpha 9 AChR mRNA was detected in the vestibular endorgans. By using single-cell RT-PCR, mRNA encoding alpha 9 AChR was also detected in the VHC II of the rats. Sequence analysis of the PCR products confirmed identity to corresponding cDNA sequence in the predicted region.

CONCLUSION

We established a method which could effectively detect the cell specific expression of mRNA in an individual VHC. Present data confirm that alpha 9 AChR mRNA is expressed in the VHC II of rats and indicates that alpha 9 AChR may function as a mediator of efferent cholinergic signaling in mammalian VHC.

Irreversible cochlear damage in myasthenia gravis -- otoacoustic emission analysis

OBJECTIVE

Acetyl choline (ACh) is the main neurotransmitter of the efferent auditory system. This study is aimed to evaluate cochlear function in myasthenia gravis (MG), a neuromuscular transmission disorder caused by ACh receptor autoantibodies.

METHODS

This prospective study included 16 myasthenic patients, tested audiologically twice, first after improvement from myasthenic crisis or acute oropharyngeal dysfunction (1 week from admission) and then 2 months later. We detected the effect of contralateral acoustic stimulation (CAS) on patients' transient and distortion product otoacoustic emissions (TEOAE and DPOAE).

RESULTS

Compared with controls, patients reported significant reduction in overall echo response and amplitude of TEOAEs at 1-2 kHz and at 1-6 kHz of DPOAE with marked reduction at 5 kHz. In the control group, CAS produced amplitude reduction in TEOAEs and DPOAEs at 1-4 kHz. Utilizing masking effect, patients reported amplitude reduction in TEOAEs at 1.5-4 kHz while DPOAEs did not reach significant level except at 1.5 and 5 kHz. After 2 months, no changes were observed compared with early assessment.

CONCLUSIONS

It is clear that disease progression is associated with irreversible cochlear damage. Lack of improvement in patients' emissions despite partial non-audiometric improvement in relation to receptors needs to be considered.

CGRP Expression in the Vestibular Periphery after Transient Blockage of Bilateral Vestibular Input

This study aimed to establish an animal model of reversible bilateral vestibular disorders that is suitable for examining the mechanisms of vestibular plasticity, and to observe the changes in the plasticity of vestibular efferent systems. Tetrodotoxin (TTX) was infused continuously for 7 days into the bilateral perilymph of guinea pig cochlea. We assessed the vestibulo-ocular reflex (VOR) for evaluating the vestibular function. We also investigated the changes in calcitonin gene-related peptide (CGRP) immunoreactivity in vestibular end organs to observe the changes in the plasticity of vestibular systems. The VOR was completely eliminated by TTX administration and returned to the preoperative levels within 120 h after TTX discontinuation. An obvious increase in the number of CGRP-immunoreactive fibers was observed within the neurosensory epithelia of the maculae and cristae. An animal model of reversible bilateral vestibular disorders was established and used for investigating the plasticity of the vestibular nervous system.

Muscarinic ACh Receptor Activation Causes Transmitter Release From Isolated Frog Vestibular Hair Cells

In the frog, vestibular efferent fibers innervate only type-II vestibular hair cells. Through this direct contact with hair cells, efferent neurons are capable of modifying transmitter release from hair cells onto primary vestibular afferents. The major efferent transmitter, acetylcholine (ACh), is known to produce distinct pharmacological actions involving several ACh receptors. Previous studies have implicated the presence of muscarinic ACh receptors on vestibular hair cells, although, surprisingly, a muscarinic-mediated electrical response has not been demonstrated in solitary vestibular hair cells. This study demonstrates that muscarinic receptors can evoke transmitter release from vestibular hair cells. Detection of this release was obtained through patch-clamp recordings from catfish cone horizontal cells, serving as glutamate detectors after pairing them with isolated frog semicircular canal hair cells in a two-cell preparation. Although horizontal cells alone failed to respond to carbachol, application of 20 μM carbachol to the two-cell preparation resulted in a horizontal cell response that could be mimicked by exogenous application of glutamate. All of the horizontal cells in the two-cell preparation responded to 20 μM CCh. Furthermore, this presumed transmitter release persisted in the presence of d-tubocurarine at concentrations that block all known hair cell nicotinic ACh receptors. The effect on the detector cell, imparted by the carbachol application to the hair cell-horizontal cell preparation, was blocked both by 2-amino-5-phosphonopentanoic acid, a selective N-methyl-d-aspartate antagonist, and the muscarinic antagonist, atropine. Thus vestibular hair cells from the frog semicircular canal can be stimulated to release transmitter by activating their muscarinic receptors.

A transiently expressed SK current sustains and modulates action potential activity in immature mouse inner hair cells

From just after birth, mouse inner hair cells (IHCs) expressed a Ca(2+)-activated K(+) current that was reduced by intracellular BAPTA at concentrations >or= 1 mM. The block of this current by nifedipine suggests the direct involvement of Ca(v)1.3 Ca(2+) channels in its activation. On the basis of its high sensitivity to apamin (K(D) 360 pM) it was identified as a small-conductance Ca(2+)-activated K(+) current (SK), probably SK2. A similar current was also found in outer hair cells (OHCs) from the beginning of the second postnatal week. In both cell types the appearance of the SK current coincided with their becoming responsive to acetylcholine (ACh), the main efferent neurotransmitter in the cochlea. The effect of ACh on IHCs was abolished when they were simultaneously superfused with strychnine, consistent with the presence of nicotinic ACh receptors (nAChRs). Extracellular Ca(2+) either potentiated or blocked the nAChR current depending on its concentration, as previously reported for the recombinant alpha9alpha10 nAChR. Outward currents activated by ACh were reduced by blocking the SK current with apamin or by preventing SK current activation with intracellular BAPTA (>or= 10 mM). The endogenous mobile Ca(2+) buffer concentration was estimated to be equivalent to about 1 mM BAPTA, suggesting that in physiological conditions the SK channel is significantly activated by Ca(2+) influx through both Ca(v)1.3 Ca(2+) channels and alpha9alpha10 nAChRs. Current clamp experiments showed that in IHCs the SK current is required for sustaining a train of action potentials and also modulates their frequency when activated by ACh.

Mouse outer hair cells lacking the alpha9 ACh receptor are motile

Efferent nerve fibers form chemical synapses at the bases of outer hair cells (OHC), with acetylcholine (ACh) being their principal neurotransmitter. The activation of ACh receptors on OHCs is known to influence cochlear function. These efferent effects exhibit an unusual pharmacology and are generally known to be inhibitory. Recent evidence suggests that an ACh receptor subunit, known as alpha9, plays a dominant role in mediating the olivocochlear neurotransmission to OHCs. In this investigation, we attempt to determine the possible role(s) of the alpha9 subunit in regulating OHC function by examining OHC electromotility and compound action potentials (CAP) in mice carrying a null mutation for the alpha9 gene. Results indicate that cochlear sensitivity, based on CAP thresholds, is similar for homozygous mutant and wild-type mice. Electromotility is also present in OHCs, independent of whether the alpha9 subunit is present or absent.

Cloning and characterization of α9 subunits of the nicotinic acetylcholine receptor expressed by saccular hair cells of the rainbow trout (Oncorhynchus mykiss)

alpha9/alpha10 Subunits are thought to constitute the nicotinic acetylcholine receptors mediating cholinergic efferent modulation of vertebrate hair cells. The present report describes the cloning and sequence analysis of subunits of the alpha9-containing receptor of a hair-cell layer from the saccule of the rainbow trout (Oncorhynchus mykiss). A major alpha9 subunit, termed alpha9-I, displayed typical features of a nicotinic alpha subunit, with total coding sequence of 572 amino acids including a 16 amino-acid signal peptide. It possessed an extended cytoplasmic loop between membrane-spanning regions M3 and M4, compared with mammalian homologs. Transcript for alpha9-I was robustly expressed in the saccular hair cell layer and less prominently in trout olfactory mucosa, spleen, pituitary gland, and liver, as determined by reverse transcription-polymerase chain reaction. alpha9-I cDNA was not detected in trout brain, skeletal muscle, retina, and kidney. The alpha9-I nicotinic receptor protein was immunolocalized, with an affinity-purified antibody directed against a trout alpha9-I epitope, to hair-cell and neural sites in the saccular hair-cell layer. Foci were found at basal and basolateral membrane sites on hair cells as well as on afferent nerve. Receptor clustering was observed in hair cells bordering non-sensory epithelium. Since in higher vertebrates the alpha9 is reported to associate with another nicotinic subunit, alpha10, we examined the possibility of expression of additional nicotinic subunits in trout saccular hair cells. Message for another nicotinic subunit, termed alpha9-II, was found to be expressed in the hair cells, although more difficult to amplify than alpha9-I. In contrast to alpha9-I, alpha9-II was expressed in brain, as well as in olfactory mucosa, less prominently in pituitary gland and liver, but not in spleen, skeletal muscle, retina, or kidney. The cloned alpha9-II had a total coding sequence of 550 amino acids, which included a 17-amino-acid signal peptide, and an extended M3-M4 loop. A third nicotinic subunit message, termed alpha9-III, was PCR-amplified from trout olfactory mucosa where it was strongly expressed. However, message for alpha9-III was not detected in hair cells. Message for alpha9-III was moderately expressed in trout brain, retina, and pituitary gland but not in trout spleen, skeletal muscle, liver, and kidney. Thus, alpha9-I and alpha9-II may together contribute to the formation of the hair-cell nicotinic receptor of teleosts, where no ortholog of alpha10 appears to exist. The current work is, to our knowledge, the first description of alpha9 coding sequences directly from a vertebrate hair cell source. Further, the generality of hair cell expression of subunits for the alpha9-containing nicotinic cholinergic receptor has been extended to fishes, suggesting a similar efferent mechanism across all vertebrate octavolateralis sensory systems.

Loss of alpha CGRP reduces sound-evoked activity in the cochlear nerve

alpha-Calcitonin gene-related peptide (alphaCGRP) is one of several neurotransmitters immunolocalized in the unmyelinated component of the cochlear efferent innervation, the lateral olivocochlear (OC) system, which makes axo-dendritic synapses with cochlear sensory neurons. In rodents, CGRP is also immunocolocalized in the myelinated medial OC system, which contacts cochlear outer hair cells (OHCs). To understand the role(s) of this neuropeptide in the OC system, we characterized the auditory phenotype of alphaCGRP-null mice. Cochlear threshold sensitivity was normal in mutant mice, both via a neural metric, the auditory brain stem response (ABR), and an OHC-based metric, distortion product otoacoustic emissions (DPOAEs). Medial OC function and resistance to acoustic injury were also unaffected by alphaCGRP deletion: the former was assessed by measuring cochlear response suppression with electrical stimulation of the OC bundle, the latter by measuring temporary threshold shifts after exposure to high level sound. However, significant abnormality in alphaCGRP-null mice was seen in the growth of cochlear neural responses with increasing stimulus level. This observation, contrasted with normal amplitude-versus-level functions for DPOAEs, is consistent with a selective, postsynaptic effect on cochlear neurons via alphaCGRP release from lateral OC terminals. This constitutes the most direct evidence to date for a functional role of the lateral OC system in the auditory periphery.

Muscarinic receptor subtypes are differentially distributed in the rat cochlea

Five different genes encode the muscarinic acetylcholine receptors. The muscarinic receptor subtypes M1, M3, and M5 are typically coupled to activation of the Galpha(q/11)-phosphatidyl inositol pathway, whereas the M2 and M4 subtypes are typically linked to Galpha(i) and adenylyl cyclase inhibition. In order to localize muscarinic receptors in the rat cochlea, we applied polyclonal antibodies for subtypes M1, M2, M3, and M5, and monoclonal antibody for subtype M4 to paraffin sections. In the organ of Corti, outer hair cells exhibited strong immunoreactivity for M3 and weak immunoreactivity for M1. Deiters' cells were strongly immunoreactive to antibodies for the M1 and M2 subtypes, with weak staining observed for M3, and weaker yet for M5. Inner hair cells showed moderate immunoreactivity for the M1 subtype, weaker staining for the M5 subtype, and slight staining for the M3 subtype. Among the spiral ganglion neurons, weak to moderate immunoreactivity was detected for M3 and M5 subtypes and weak staining was observed for the M1 subtype. The efferent fibers of the intraganglionic spiral bundle were positive for M2 and M5. In the lateral wall, weak to moderate staining was detected for M5 in the stria vascularis corresponding in position to the basolateral extensions of marginal cells. Staining for M3 was observed associated with capillaries. Fibrocytes of the spiral ligament exhibited limited but selective subtype immunoreactivity. No immunoreactivity was detected in the cochlea for the M4 subtype. From the present findings we suggest that M3 is the primary muscarinic receptor subtype in outer hair cells mediating a postsynaptic response to the medial olivocochlear cholinergic efferent input. The muscarinic receptor subtypes M1, M3, and M5 appear to subserve the action of cholinergic lateral olivocochlear efferent stimulation on postsynaptic responses in type I afferents. Whether M1, M3, and M5 protein in inner hair cells indicates constitutive or vestigial expression remaining from development is unknown. M2 and M5 muscarinic receptors expressed presynaptically may modulate the efferent signal. Finally, expression by Deiters' cells of several muscarinic subtypes raises the possibility that cholinergic efferents couple to these non-sensory cells through muscarinic receptors.

A novel human nicotinic receptor subunit, alpha10, that confers functionality to the alpha9-subunit

We present herein the cloning of the human nicotinic acetylcholine receptor alpha9-ortholog and the identification of a new alpha-like subunit (alpha10) that shares 58% identity with alpha9. Whereas alpha10 fails to produce functional receptors alone, it promoted robust acetylcholine-evoked currents when coinjected with alpha9. The presence of alpha10 modifies the physiological and pharmacological properties of the alpha9 receptor indicating that the two subunits coassemble in a single functional receptor. Fusing the N-terminal domain of alpha9 with the rest of the alpha10-cDNA yielded a functional alpha9:alpha10-chimera that displays the acetylcholine binding properties of alpha9 and ionic pore characteristics of alpha10-containing receptors. In addition, alpha9- and alpha10-subunit mRNAs show limited similar tissue distribution patterns and are expressed in cochlea, pituitary gland, and keratinocytes. These data suggest that, in vivo, alpha9-containing receptors coassemble with alpha10-subunit.

Effects of contralateral white noise stimulation on distortion product otoacoustic emissions in myasthenic patients

Myasthenia gravis (MG) induces a reduction of transient evoked otoacoustic emissions (TEOAEs) and distortion product otoacoustic emissions (DPOAEs) that reverses partially after administration of an acetylcholinesterase (AChE) inhibitor. In normal subjects a contralateral acoustic stimulation (CAS) produces an amplitude reduction of TEOAEs and DPOAEs. This effect, called contralateral suppression (CS), is mediated by the efferent auditory system. Twenty subjects affected by MG underwent DPOAE recording with and without contralateral white noise in a drug-free baseline period ('basal') and 1 h ('post') after administration of a reversible AChE inhibitor. In 'basal' condition CAS did not induce significant DPOAE amplitude changes but a paradoxical slight increase was observed. After drug administration, CAS produced a significant decrease of DPOAE amplitudes for middle frequencies (f(2) between 1306 and 2600 Hz). In normal controls CAS caused a significant decrease (P<0.001) for all frequencies. The amount of CS in controls and in the MG 'post' condition was not significantly different. The increased acetylcholine (ACh) availability following drug consumption seems to partially restore outer hair cell function and enhances their electromotility; a further influx of ACh due to CAS yields to restoration of the CS. These findings also suggest that DPOAEs may be useful in the diagnosis of MG and for monitoring the effectiveness of treatment.

Cochlear outer-hair-cell efferents and complex-sound-induced hearing loss: protective and opposing effects

Centrifugal crossed and uncrossed medial olivocochlear systems (CMOCS and UMOCS) terminate on cochlear outer hair cells (OHCs) and exercise effects through a nicotinic cholinergic receptor. Hence their cochlear effects have not been differentiated. Recent work on protection from loud-sound-induced temporary threshold shifts (TTSs) in hearing sensitivity suggest the two OHC efferent systems may act differently. This was tested, using traumatic complex sound, to determine if such sound could activate both MOCS components and then reveal whether they exerted different effects on TTSs to such stimuli. Traumatic noise bands activated crossed and uncrossed MOCS efferents. Two different CMOCS effects were observed. For frequencies in the noise (within-band frequencies), it protected hearing sensitivity as expected. Novel findings were that at frequencies higher than the noise band range (high-side frequencies), it acted to worsen hearing sensitivity and that this was opposed by a UMOCS effect generally targeted to these frequency regions. It is proposed that the two crossed MOCS actions are extensions of a contrast-enhancement action for low-level noise bands. It is also proposed that the UMOCS plays a state-restoration role to prevent an undesired CMOCS side-effect of exacerbation of high-side TTSs to high-level noise bands.

Development of acetylcholine receptors in cultured outer hair cells

Efferents, originating in the superior olivary complex, preferentially synapse with cochlear outer hair cells (OHCs), with acetylcholine (ACh) as their primary neurotransmitter. The OHC ACh receptors (AChRs), which have unusual pharmacology, have been cloned and identified as a new subunit (alpha9) of the nicotinic AChR family. The expression of alpha9 AChRs is first detected before birth and peaks between 6 and 10 days after birth (DAB) in developing mice and rats, while functional maturation of the receptor, as determined by measuring the ACh-induced currents, takes place between 6 and 12 DAB. In this study we attempted to examine the development of AChRs in OHCs grown in explanted cultures, deprived of efferent innervation. ACh-induced currents were used as an assay. Reverse transcription-PCR analysis was also performed to detect the expression of alpha9 subunit from cultured OHCs. PCR study indicates that mRNA of the alpha9 subunit was expressed in primary cochlear cultures, similar to that seen in the cochleae of developing animals. Measurement of whole-cell currents showed that ACh-induced outward current was first detected around 5 days in a fraction of cultured OHCs. The number of responsive cells increased between 5 and 12 days in culture. The size of ACh-induced currents also increased during this period. These results suggest that the development of AChRs in cultured OHCs is not affected by removal of efferent innervation.

Noise priming and the effects of different cochlear centrifugal pathways on loud-sound-induced hearing loss

Priming/conditioning the cochlea with moderately loud sound can reduce damage caused by subsequent loud sound. This study examined immediate effects of short-term priming with monaural broadband noise on temporary threshold shifts (TTSs) in hearing caused by a subsequent loud high-frequency tone and the role of centrifugal olivocochlear pathways. Priming caused delay-dependent changes in tone-induced TTSs, particularly or only at frequencies higher than the peak tone-affected frequency, through two general effects: a short-lasting increase in cochlear susceptibility to loud sound and longer-lasting complex end effects of centrifugal pathways. The results indicated the following points. Priming noise had "pure" cochlear effects, outlasting its presentation and declining with delay, that exacerbated tone-induced TTSs at frequencies higher than the peak tone-affected frequency. The centrifugal uncrossed medial olivocochlear system (UMOCS) could prevent this noise exacerbation and as this noise effect declined, could even reduce tone-induced TTSs below those to the unprimed tone. For longer delays, when priming noise no longer had any exacerbative "pure" cochlear effects on TTSs, UMOCS exacerbated TTSs above those to the unprimed tone. The crossed medial olivocochlear system (CMOCS) appeared to show a gradual "build-up" of effects postpriming. A parallel study showed it exercised no end effect on TTSs when noise and tone were concurrent. With priming, CMOCS effects were observed. For the shortest priming delay, the CMOCS blocked a UMOCS effect preventing noise exacerbation of tone-induced TTSs. For longer delays, CMOCS end effects, when present, reduced tone-induced TTSs below those to the unprimed tone. The CMOCS may oscillate between producing these effects and exerting no end-effect. With increasing delay CMOCS protection occurred in a greater proportion of animals. Finally, with a delay of 600 s between primer and loud tone, all these systems appeared to have reset to normal so that TTSs were similar to those in the unprimed condition. Thus the effects of short-term priming are not simple and do not suggest that centrifugal pathways act automatically as a protective system during such priming.

Development of the rat efferent vestibular system on the ground and in microgravity

We investigated whether plastic changes occurred in the organization of the vestibular efferent network in the rat utricle during a 17-day episode of microgravity, from postnatal (PN) day 8 to PN23, and on return to earth on PN25. We also determined the normal pattern of efferent development from birth to PN25. Immunofluorescence experiments were performed with a specific biochemical marker of the efferent system, the calcitonin gene-related peptide (CGRP), and vibratome sections of the utricles were analyzed by laser scanning confocal microscopy. At birth, a few efferent fibers were detected beneath the sensory epithelium. These then massively invaded the epithelium between PN2 and PN4. At the time of launch, PN8, most fiber paths in the utricular epithelium, after following transient courses (towards the epithelial surface for example) returned to the base and were stabilized in the lower part of the epithelium, in which they established synaptic contacts with sensory cells, except at a few immature locations. The main difference between this stage (on PN8) and subsequent more mature stages was the lower density of fibers and synapses in the utricle. The maturation of the vestibular efferent system was similar in microgravity and on the ground. Thus, maturation of the efferent system between PN8 and PN23 was not sensitive to a change in gravitational environment. These results suggest that periods of microgravity at earlier stages are required to identify critical periods in peripheral vestibular system development.

alpha10: a determinant of nicotinic cholinergic receptor function in mammalian vestibular and cochlear mechanosensory hair cells

We report the cloning and characterization of rat alpha10, a previously unidentified member of the nicotinic acetylcholine receptor (nAChR) subunit gene family. The protein encoded by the alpha10 nAChR subunit gene is most similar to the rat alpha9 nAChR, and both alpha9 and alpha10 subunit genes are transcribed in adult rat mechanosensory hair cells. Injection of Xenopus laevis oocytes with alpha10 cRNA alone or in pairwise combinations with either alpha2-alpha6 or beta2-beta4 subunit cRNAs yielded no detectable ACh-gated currents. However, coinjection of alpha9 and alpha10 cRNAs resulted in the appearance of an unusual nAChR subtype. Compared with homomeric alpha9 channels, the alpha9alpha10 nAChR subtype displays faster and more extensive agonist-mediated desensitization, a distinct current-voltage relationship, and a biphasic response to changes in extracellular Ca(2+) ions. The pharmacological profiles of homomeric alpha9 and heteromeric alpha9alpha10 nAChRs are essentially indistinguishable and closely resemble those reported for endogenous cholinergic eceptors found in vertebrate hair cells. Our data suggest that efferent modulation of hair cell function occurs, at least in part, through heteromeric nAChRs assembled from both alpha9 and alpha10 subunits.

The effects of efferent activation on the acoustically and electrically evoked otoacoustic emission

The effects of efferent activation on the otoacoustic emission were measured in anesthetized guinea pigs. The otoacoustic emission (2F(1)-F(2)) was evoked by the conventional method of presenting either two continuous tones or a sinusoidal current to the round window (RW) of the cochlea. The efferent effects on the acoustically evoked emission are greatest at low stimulus levels and least for high levels. The efferent effects on the electrically evoked emission (EEOAE) are relatively constant across current levels. In each case, efferent activation resulted in an initial large reduction in the emission amplitude followed by a smaller and more constant reduction. Strychnine eliminated the efferent effects independent of the method of emission activation. Strychnine had no effect on the EEOAE, suggesting that the RW current did not evoke a local efferent effect. Slow versus fast efferent effects were observed in the recovery of the emission amplitude at the termination of efferent activation. Only a fast recovery in the emission amplitude was observed for stimuli below 10 kHz while the amplitude recovery had fast and slow components for stimuli presented above 10 kHz.

A role for chloride in the hyperpolarizing effect of acetylcholine in isolated frog vestibular hair cells

Acetylcholine (ACh) is the dominant transmitter released from inner ear efferent neurons. In frog vestibular organs, these efferent neurons synapse exclusively with type II hair cells. Hair cells isolated from the frog saccule hyperpolarize following the application of 50 microM ACh, thereby demonstrating the presence of an ACh receptor. A role for Cl(-) in the response of hair cell-bearing organs to efferent nerve activation or ACh application was suggested some years ago. Perfusion with solutions in which most of the Cl(-) was replaced by large impermeant anions decreased the cholinergic inhibition of afferent firing in the cat and turtle cochleas, and frog semicircular canal. Our previous work in the intact organ demonstrated that substitution of large impermeant anions for Cl(-) or use of Cl(-) channel blockers reduced the effect of ACh on saccular afferent firing. Using the perforated-patch clamping technique, replacement of Cl(-) by methanesulfonate, iodide, nitrate, or thiocyanate attenuated the hyperpolarizing response to ACh in hair cells isolated from the frog saccule. The chloride channel blockers picrotoxin and 4,4'-dinitrostilbene-2,2'-disulfonic acid were also tested and found to inhibit the ACh response. Thus, the present work demonstrates that the effects of Cl(-) substitutions or Cl(-) channel blockers on the ACh response in the intact saccule can be explained completely by effects on the hair cell. Evidence is also presented for the presence of the messenger RNA for a calcium-dependent chloride channel in all hair cells but especially saccular hair cells. This channel may be involved in the response to ACh. The precise role for chloride in this response, whether as a distinct ion current, as a transported ion, or as a permissive ion for other components, is discussed.

Alpha and beta subunits of acetylcholine receptors in the human inner ear

The localization and distribution of nicotinic acetylcholine receptors (n-ACh-r) was characterized by studying alpha and beta subunits in the adult human inner ear by FITC fluorescence technique. In the cochlea, distinct fluorescence staining occurred for beta subunits in outer hair cells (OHCs), but no alpha subunits were identified. Beta subunits differ quantitatively between the three rows of OHCs, decreasing along a base-to-apex gradient in the cochlea. Both alpha and beta subunits were identified on spiral ganglion cells, adjacent nerve fibres and in vestibular hair cells (HCs). It would appear that they form an active complex in n-ACh-r at these locations.

Glycine induced calcium concentration changes in vestibular type I sensory cells

Glutamate is the neurotransmitter of the synapse between vestibular type I hair cells and the afferent nerve calyx. This calyx may also be involved in local feedback, which may modify sensory cell activity via N-methyl-D-aspartate (NMDA) receptors. Glycine is the co-agonist of glutamate in NMDA receptor activation. Both agents have been detected by immunocytochemistry in the nerve calyx. Glutamate and NMDA stimulations cause changes in the intracellular calcium concentration ([Ca(2+)](i)) of isolated type I sensory cells. We investigated the effect of glycine stimulation on [Ca(2+)](i) in guinea pig type I sensory cells by spectrofluorimetry with fura-2. Glycine application to isolated type I sensory cells induced a rapid and transient increase in [Ca(2+)](i). The fluorescence ratio increased by 55% above the resting level. The peak was reached in 9 s and the return to basal level took about 20 s. A specific antagonist of the glycine site on NMDA receptors, 7-chlorokynurenate (10 microM), decreased the calcium response to glycine by 60%. Glycine may activate NMDA receptors. Glycine may also activate the strychnine-sensitive glycine receptor-gated channel. Strychnine (50 microM) decreased the calcium response to glycine by 60%. Thus, glycine probably induces calcium concentration changes in type I vestibular sensory cells via NMDA receptors and/or glycine receptors.

Autoradiographic demonstration of quinuclidinyl benzilate binding sites in the vestibular organs of the gerbil

Gerbil vestibular tissues were isolated by microdissection and incubated in vitro with 3H-quinuclidinyl benzilate (3H-QNB). Control tissues were incubated in medium containing unlabeled atropine to differentiate non-specific from specific binding. Autoradiographic grain densities were determined by morphometric techniques and evaluated by two-tailed t-test. The label densities of sensory epithelia from experimental preparations of ampulla, utricle and saccule were found to be significantly higher than those in the adjacent endolymphatic compartment and also higher than those of adjacent stromal tissue comprising connective tissue, nerve fibers and capillaries. In contrast, no tissue region in atropine controls showed label density significantly above that of the endolymphatic compartment. Label density of ampullar sensory epithelium incubated with 3H-QNB alone was significantly higher than that of sensory epithelium from utricle or saccule. Grain density was greater in the peripheral regions of the ampullar crista compared to the vertex. Appreciable label was also present in nerve bundles beneath the sensory epithelium of the ampulla. The current study demonstrates the existence of putative muscarinic neurotransmitter/neuromodulator receptor sites in mammalian vestibular sense organs at locations corresponding to efferent innervation, with particularly significant concentrations in the ampulla.

Development of acetylcholine-induced responses in neonatal gerbil outer hair cells

Cochlear outer hair cells (OHCs) are dominantly innervated by efferents, with acetylcholine (ACh) being their principal neurotransmitter. ACh activation of the cholinergic receptors on isolated OHCs induces calcium influx through the ionotropic receptors, followed by a large outward K+ current through nearby Ca2+-activated K+ channels. The outward K+ current hyperpolarizes the cell, resulting in the fast inhibitory effects of efferent action. Although the ACh receptors (AChRs) in adult OHCs have been identified and the ACh-induced current responses have been characterized, it is unclear when the ACh-induced current responses occur during development. In this study we attempt to address this question by determining the time of onset of the ACh-induced currents in neonatal gerbil OHCs, using whole cell patch-clamp techniques. Developing gerbils ranging in age from 4 to 12 days were used in these experiments, because efferent synaptogenesis and functional maturation of OHCs occur after birth. Results show that the first detectable ACh-induced current occurred at 6 days after birth (DAB) in 12% of the basal turn cells with a small outward current. The fraction of responsive cells and the size of outward currents increased as development progressed. By 11 DAB, the fraction of responsive cells and the current size were comparable with those of adult OHCs. The results indicate that the maturation of the ACh-induced response begins around 6 DAB. It appears that the development of ACh-induced responses occur during the same time period when OHCs develop motility but before the onset of auditory function, which is around 12 DAB when cochlear microphonic potentials can first be evoked with acoustic stimulation in gerbils.

The metabotropic glutamate receptors of the vestibular organs

This research sought to test the presence and function of metabotropic excitatory amino acid receptors (mGluR) in the frog semicircular canal (SCC). The mGluR agonist +/- 1-aminocyclopentane-trans-1,3-dicarboxylate (ACPD) produced an increase in afferent firing rates of the ampullar nerve of the intact posterior canal. This increase was not due to a stimulation of cholinergic efferent terminals or the acetylcholine (ACh) receptor, since atropine, in concentrations which blocked the response to exogenous acetylcholine, did not affect the response to ACPD. Likewise, ACPD effects were not due to stimulation of postsynaptic NMDA receptors, since the NMDA antagonist D(-)-2-amino-5-phosphonopentanoate (AP-5) did not affect the response to ACPD, reinforcing the reported selectivity of ACPD for mGluRs. When the SCC was superfused with artificial perilymph known to inhibit hair cell transmitter release (i.e. low Ca-high Mg), ACPD failed to increase afferent firing. This suggests that the receptor activated by ACPD is located on the hair cell. Pharmacological evidence suggested that the mGluRs involved in afferent facilitation belong to Group I (i.e. subtypes 1 and 5). In fact, the Group III agonist AP-4 had no effect, and the ACPD facilitatory effect was blocked by the Group I mGluR antagonists (S)-4-carboxyphenylglycine (CPG) and (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA). Additional pharmacological evidence supported the presence of Group I mGluRs. Interestingly, the mGluR antagonists, AIDA and 4CPG, by themselves did not affect the resting firing rates of ampullar afferents. This may suggest that the mGluRs are not involved in resting activity but perhaps only in evoked activity (as suggested in Guth et al. (1991) Hear. Res. 56, 69-78). In addition, the mRNA for the mGluR1 has been detected in hair cells of both SCC, utricle, and saccule. In summary, the evidence points to an mGluR localized to the hair cell (i.e. an autoreceptor) which may be activated to produce a positive feedback augmentation of evoked but not resting transmitter release and thus affect afferent activity.

Two distinct nicotinic receptors, one pharmacologically similar to the vertebrate alpha7-containing receptor, mediate Cl currents in aplysia neurons

Ionotropic, nicotinic receptors have previously been shown to mediate both inhibitory (Cl-dependent) and excitatory (cationic) cholinergic responses in Aplysia neurons. We have used fast perfusion methods of agonist and antagonist application to reevaluate the effects on these receptors of a wide variety of cholinergic compounds, including a number of recently isolated and/or synthesized alpha toxins [alpha-conotoxin (alphaCTx)] from Conus snails. These toxins have been shown in previous studies to discriminate between the many types of nicotinic receptors now known to be expressed in vertebrate muscle, neuroendocrine, and neuronal cells. One of these toxins (alphaCTx ImI from the worm-eating snail Conus imperialis) revealed that two kinetically and pharmacologically distinct elements underlie the ACh-induced Cl-dependent response in Aplysia neurons: one element is a rapidly desensitizing current that is blocked by the toxin; the other is a slowly desensitizing current that is unaffected by the toxin. The two kinetically defined elements were also found to be differentially sensitive to different agonists. Finally, the proportion of the rapidly desensitizing element to the sustained element was found to be cell-specific. These observations led to the conclusion that two distinct nicotinic receptors mediate Cl currents in Aplysia neurons. The receptor mediating the rapidly desensitizing Cl-dependent response shows a strong pharmacological resemblance to the vertebrate alpha-bungarotoxin-sensitive, alpha7-containing receptor, which is permeable to calcium and mediates a rapidly desensitizing excitatory response.

Cholinergic modulation of extracellular ATP-induced cytoplasmic calcium concentrations in cochlear outer hair cells

Outer hair cells (OHC) of the mammalian cochlea modulate the inner hair cell (IHC) mechanoelectrical transduction of sound. They are contacted by synapsing efferent neurons from the CNS, their main efferent neurotransmitter being acetylcholine (ACh). OHC function and in particular their control of [Ca2+]i is highly important and is modulated by ACh and also by other substances including extracellular (EC) ATP. OHC carry at their efferent synapse a not yet completely identified neuronal type of ionotropic ACh receptor (AChR), with an unusual pharmacology, which is, in vivo and in vitro, reversibly blocked by alpha-bungarotoxin (alpha-bgtx). The AChR mediates a fast influx of Ca2+ into OHC which, in turn, activates a closeby located outwardly-directed Ca(2+)-dependent K(+)-channel, thus shortly hyperpolarizing the cell. A cloned homomeric alpha 9 nAChR mimicks the function and pharmacology of this receptor. We here report results from a study designed to observe only slower effects triggered by EC ATP and the ACh-AChR system. EC presence of ATP at OHC increases [Ca2+]i by activating both P2x and P2y purinoceptors and also by indirect activation of OHC L-type Ca(2+)-channels. The L-type channel activation is responsible for a large part of the [Ca2+]i increase. Simultaneous EC presence of ACh and ATP at OHC was found to depress ATP-induced effects on OHC [Ca2+]i, an effect that is completely blocked in the presence of alpha-bgtx. Our observations suggest that the ACh-AChR system is involved in the modulation of the observed EC ATP-triggered events; possibly the OHC AChR is able to act both in its well known rapid ionotropic way, but also, perhaps after modification in a slower, metabotropic way interfering with the EC ATP-induced [Ca2+]i increase.

Expression of small-conductance calcium-activated potassium channels (SK) in outer hair cells of the rat cochlea

Physiological evidence suggests that SK-type Ca2+-activated K+ channels participate in ACh-induced hyperpolarization of OHCs (outer hair cells). Based on the sequences published by Kohler et al. [(1996), Science, 273: 1709), we designed degenerated primers recognizing cDNA subunits of rSK1, rSK2 and rSK3. Using this consensus set of primers, we probed by PCR a rat organ of Corti cDNA library. Two PCR products of 707 base pairs with sequence identical to rSK3 and rSK2 were obtained and cloned to generate RNA probes for in situ hybridization in the rat cochlea. The subunit rSK2 showed hybridization in the organ of Corti, at the location of the OHCs. The expression of rSK2 by OHCs was confirmed by probing with PCR a poly(A) amplified OHC cDNA library. During development, rSK2 hybridization in the organ of Corti was negative at embryonic days E16, E18 and at P0, weak at P4 and stronger from P8 to adulthood. The subunit rSK2 could also be detected in the spiral ganglion from P4 to the adult stage. Contrary to rSK2, the subunit rSK3 did not show specific hybridization in the organ of Corti at the adult stage (P120) and only a weak expression was observed at P10 and P21. Our study demonstrates expression of rSK2 in OHCs. These potassium channels are good candidates to underlie the ACh-activated K+ currents recorded during patch-clamp recordings in isolated OHCs. The expression of rSK2 in the cochlear ganglion at the adult stage suggests that SK Ca2+-activated K+ channels may also participate in the repolarization of the auditory neurons after the action potential and may influence their firing patterns.

Extracellular nucleotide signaling in the inner ear

Extracellular nucleotides, particularly adenosine 5'-triphosphate (ATP), act as signaling molecules in the inner ear. Roles as neurotransmitters, neuromodulators, and as autocrine or paracrine humoral factors are evident. The diversity of the signaling pathways for nucleotides, which include a variety of ATP-gated ion channels (assembled from different subtypes of P2X-receptor subunit) and also different subtypes of G protein-coupled nucleotide receptors (P2Y receptors) supports a major physiological role for ATP in the regulation of hearing and balance. Almost invariably both P2X and P2Y receptor expression is apparent in the complex tissue structures associated with the inner-ear labyrinth. However P2X-receptor expression, commonly associated with fast neurotransmission, is apparent not only with the cochlear and vestibular primary afferent neurons, but also appears to mediate humoral signaling via ATP-gated ion channel localization to the endolymphatic surface of the cochlear sensory epithelium (organ of Corti). This is the site of the sound-transduction process and recent data, including both electrophysiological, imaging, and immunocytochemistry, has shown that the ATP-gated ion channels are colocalized here with the mechano-electrical transduction channels of the cochlear hair cells. In contrast to this direct action of extracellular ATP on the sound-transduction process, an indirect effect is apparent via P2Y-receptor expression, prevalent on the marginal cells of the stria vascularis, a tissue that generates the standing ionic and electrical gradients across the cochlear partition. The site of generation of these gradients, including the dark-cell epithelium of the vestibular labyrinth, may be under autocrine or paracrine regulation mediated by P2Y receptors sensitive to both purines (ATP) and pyrimidines such as UTP. There is also emerging evidence that the nucleoside adenosine, formed as a breakdown product of ATP by the action of ectonucleotidases and acting via P1 receptors, is also physiologically significant in the inner ear. P1-receptor expression (including A1, A2, and A3 subtypes) appear to have roles associated with stress, acting alongside P2Y receptors to enhance cochlear blood flow and to protect against the action of free radicals and to modulate the activity of membrane conductances. Given the positioning of a diverse range of purinergic-signaling pathways within the inner ear, elevations of nucleotides and nucleosides are clearly positioned to affect hearing and balance. Recent data clearly supports endogenous ATP- and adenosine-mediated changes in sensory transduction via a regulation of the electrochemical gradients in the cochlea, alterations in the active and passive mechanical properties of the cells of the sensory epithelium, effects on primary afferent neurons, and control of the blood supply. The field now awaits conclusive evidence linking a physiologically-induced modulation of extracellular nucleotide and nucleoside levels to altered inner ear function.

Frequency-dependent enhancement of basilar membrane velocity during olivocochlear bundle stimulation

Basilar membrane (BM) velocity responses were measured in the presence of olivocochlear bundle (OCB) stimulation. Frequency threshold tuning curves (FTCs) were derived from tone-evoked input-output (I/O) functions. Efferent nerve activation produced decreases in velocity amplitude for frequencies around best frequency (BF) at low stimulus levels with little or no effect for stimuli well below the BF. A level-dependent efferent reduction/enhancement of BM velocity was found for certain stimulus frequencies above the BF. Efferent activation either had no effect or caused small reductions in the velocity response produced by low level sound, whereas, at higher stimulus levels, efferent activation increased the velocity response. The derived FTCs, therefore, showed criterion-dependent changes with efferent activation. For low BM criterion velocities, FTCs showed the classic desensitization of the tip region without a shift of BF. Some BM velocity criterion values showed FTCs with an expanded high-frequency response area, also without a shift of BF. The results suggest that the effect of OCB activation changes the gain of the voltage-dependent outer hair cell motility such that BM velocity response near BF is decreased while increasing the response for tones well above BF.

Chronic strychnine administration into the cochlea potentiates permanent threshold shift following noise exposure

To investigate whether elimination of the medial efferent system influences permanent threshold shift following noise exposure, we developed an animal model in which strychnine was chronically delivered into the cochlea via an osmotic pump. Pigmented female guinea pigs were allocated into three groups: group I was treated with strychnine (50 microM, 0.5 microl/h, 14 days) in the left ear and exposed to noise (105 dB SPL broadband, 3 h) 3 weeks after the cessation of the strychnine perfusion; group II received strychnine in the left ear but no noise exposure; group III was treated with Ringer's solution in the left ear and exposed to noise. Animals in group II developed no hearing loss after the strychnine perfusion. The operated ears in group I demonstrated greatest hearing threshold shift 3 h after noise exposure. Hearing recovered during 2 weeks after noise exposure in both operated and non-operated ears in groups I and III. Two weeks after noise exposure, the operated ears in group I showed significantly greater threshold shift at 12, 16, and 20 kHz compared to the operated ears in group III and non-operated ears in groups I and III. These findings suggest that chronic strychnine administration into the cochlea inactivates the medial efferents without changing hearing threshold and that the medial efferents help to protect against permanent threshold shift following noise exposure.

A role for chloride in the suppressive effect of acetylcholine on afferent vestibular activity

Afferents of the frog semicircular canal (SCC) respond to acetylcholine (ACh) application (0.3-1.0 mM) with a facilitation of their activity while frog saccular afferents respond with suppression (Guth et al., 1994). All recordings are of resting (i.e., non-stimulated) multiunit activity as previously reported (Guth et al., 1994). Substitution of 80% of external chloride (Cl-) by large, poorly permeant anions of different structures (isethionate, methanesulfonate, methylsulfate, and gluconate) reduced the suppressive effect of ACh in the frog saccular afferents. This substitution did not affect the facilitatory response of SCC afferents to ACh. Chloride channel blockers were also used to test further whether Cl- is involved in the ACh suppressive effect. These included: niflumic and flufenamic acids, picrotoxin, 5-nitro-2-(-3-phenylpropylamino)benzoic acid (NPPB), and 4,4'-dinitrostilbene-2,2'-disulfonic acid (DNDS). As with the Cl- substitutions, all of these agents reduced the suppressive response to ACh in the saccule, but not the facilitatory response seen in the SCC. The suppressive effect of ACh on saccular afferents is considered to be due to activation of a nicotinic-like receptor (Guth et al., 1994; Guth and Norris, 1996). Taking into account the effects of both Cl- substitutions and Cl- channel blockers, we conclude that changes in Cl- availability influence the suppressive effect of ACh and that therefore Cl- may be involved in this effect.

Reversible hearing impairment induced by lithium in the guinea pig

Lithium salts remain one of the most widely used treatments for depressive illness. The mechanisms involved probably include reduction in free inositol. Visual perceptive disturbances can be a side effect of the treatment. We report here for the first time that chronic lithium treatment in the guinea pig induces a predominantly low frequency hearing loss and, in the longer term, loss of sensitivity is observed across the whole audiogram. The changes are reversed when treatment is arrested. The observations could be accounted for, at least partially, by a lithium-induced perturbation of the phosphoinositide cascade within the inner ear.