Compliance of the hair bundle associated with gating of mechanoelectrical transduction channels in the bullfrog's saccular hair cell

Mechanical stimuli are thought to open the transduction channels of a hair cell by tensing elastic components, the gating springs, that pull directly on the channels. To test this model, we measured the stiffness of hair bundles during mechanical stimulation. A bundle's compliance increased by about 40% at the position where half of the channels opened. This we attribute to conformational changes of transduction channels as they open and close. The magnitude and displacement dependence of the gating compliance provide quantitative information about the molecular basis of mechanoelectrical transduction: the force required to open each channel, the number of transduction channels per hair cell, the stiffness of a gating spring, and the swing of a channel's gate as it opens.

Regenerative proliferation in inner ear sensory epithelia from adult guinea pigs and humans

Supporting cells in the vestibular sensory epithelia from the ears of mature guinea pigs and adult humans proliferate in vitro after treatments with aminoglycoside antibiotics that cause sensory hair cells to die. After 4 weeks in culture, the epithelia contained new cells with some characteristics of immature hair cells. These findings are in contrast to expectations based on previous studies, which had suggested that hair cell loss is irreversible in mammals. The loss of hair cells is responsible for hearing and balance deficits that affect millions of people.

Pluripotent stem cells from the adult mouse inner ear

In mammals, the permanence of acquired hearing loss is mostly due to the incapacity of the cochlea to replace lost mechanoreceptor cells, or hair cells. In contrast, damaged vestibular organs can generate new hair cells, albeit in limited numbers. Here we show that the adult utricular sensory epithelium contains cells that display the characteristic features of stem cells. These inner ear stem cells have the capacity for self-renewal, and form spheres that express marker genes of the developing inner ear and the nervous system. Inner ear stem cells are pluripotent and can give rise to a variety of cell types in vitro and in vivo, including cells representative of ectodermal, endodermal and mesodermal lineages. Our observation that these stem cells are capable of differentiating into hair cell-like cells implies a possible use of such cells for the replacement of lost inner-ear sensory cells.

Mechanical relaxation of the hair bundle mediates adaptation in mechanoelectrical transduction by the bullfrog's saccular hair cell

Mechanoelectrical transduction by hair cells of the frog's internal ear displays adaptation: the electrical response to a maintained deflection of the hair bundle declines over a period of tens of milliseconds. We investigated the role of mechanics in adaptation by measuring changes in hair-bundle stiffness following the application of force stimuli. Following step stimulation with a glass fiber, the hair bundle of a saccular hair cell initially had a stiffness of approximately equal to 1 mN X m-1. The stiffness then declined to a steady-state level near 0.6 mN X m-1 with a time course comparable to that of adaptation in the receptor current. The hair bundle may be modeled as the parallel combination of a spring, which represents the rotational stiffness of the stereocilia, and a series spring and dashpot, which respectively, represent the elastic element responsible for channel gating and the apparatus for adaptation.

Differential distribution of stem cells in the auditory and vestibular organs of the inner ear

The adult mammalian cochlea lacks regenerative capacity, which is the main reason for the permanence of hearing loss. Vestibular organs, in contrast, replace a small number of lost hair cells. The reason for this difference is unknown. In this work we show isolation of sphere-forming stem cells from the early postnatal organ of Corti, vestibular sensory epithelia, the spiral ganglion, and the stria vascularis. Organ of Corti and vestibular sensory epithelial stem cells give rise to cells that express multiple hair cell markers and express functional ion channels reminiscent of nascent hair cells. Spiral ganglion stem cells display features of neural stem cells and can give rise to neurons and glial cell types. We found that the ability for sphere formation in the mouse cochlea decreases about 100-fold during the second and third postnatal weeks; this decrease is substantially faster than the reduction of stem cells in vestibular organs, which maintain their stem cell population also at older ages. Coincidentally, the relative expression of developmental and progenitor cell markers in the cochlea decreases during the first 3 postnatal weeks, which is in sharp contrast to the vestibular system, where expression of progenitor cell markers remains constant or even increases during this period. Our findings indicate that the lack of regenerative capacity in the adult mammalian cochlea is either a result of an early postnatal loss of stem cells or diminishment of stem cell features of maturing cochlear cells.

Kinetic analysis of voltage- and ion-dependent conductances in saccular hair cells of the bull-frog, Rana catesbeiana

1. By the use of whole-cell and excised-patch tight-seal recording techniques, we studied ionic conductances in voltage-clamped solitary hair cells isolated from the bull-frog's sacculus. As a basis for assessing their contributions to hair cell electrical resonance, we developed kinetic models describing voltage-dependent Ca2+ and Ca2+-dependent K+ conductances. 2. A transient K+ current (IA) was activated by steps to potentials positive to -50 mV from holding potentials more negative than -70 mV. In the steady state, the current was fully inactivated at the normal resting potential. Possibly due to the dissipation of a Donnan potential between the pipette's interior and the cell, the voltage dependence of IA inactivation slowly shifted in the negative direction during whole-cell recording. 3. The voltage-gated Ca2+ current (ICa) was isolated by blocking IA with 4-aminopyridine (4-AP) and Ca2+-activated K+ current with tetraethylammonium (TEA). The ICa was activated at potentials more positive than -60 to -50 mV and was maximal at about -10 mV. Its magnitude was highly variable among cells, with an average value of -240 pA at -30 mV. Its activation could be fitted well by a third-order (m3) gating scheme. 4. A Ca2+-activated K+ current (IK(Ca)) was isolated as the component of membrane current blocked by TEA. This current was activated at potentials more positive than -60 to -50 mV and had an average value of 1.5 nA at -30 mV. The Ca2+-activated K+ conductance (gK(Ca)) showed a high apparent voltage dependence, increasing e-fold every 3 mV at potentials between -50 and -40 mV. 5. The Ca2+-activated K+ current displayed rapid activation and deactivation kinetics. The current reached half-maximal activation in 2-4 ms at voltages between -50 and -30 mV, and the tail current decayed exponentially with a time constant of 1.0 ms at -70 mV. The activation rate and magnitude of IK(Ca) were reduced by lowering the extracellular Ca2+ concentration. 6. The open probability of Ca2+-activated K+ channels was estimated by ensemble-fluctuation analysis of whole-cell currents evoked by voltage steps to -30 mV. The average open probability was estimated to be 0.8 at this potential. 7. K+-selective channels with a high conductance (140-200 pS) were examined in excised, inside-out membrane patches. The activity of these channels depended on intracellular Ca2+ and membrane potential. These properties suggest that the channels underlie the whole-cell Ca2+-activated K+ current.(ABSTRACT TRUNCATED AT 400 WORDS)

The cellular basis of hearing: the biophysics of hair cells

A crucial event in the hearing process is the transduction of mechanical stimuli into electrical signals by hair cells, the sensory receptors of the internal ear. Stimulation results in the rapid opening of ionic channels in the mechanically sensitive organelles of these cells, their hair bundles. These transduction channels, which are nonselectively permeable, are directly excited by hair-bundle displacement. Hair cells are selectively responsive to particular frequencies of stimulation, both due to the mechanical properties of their hair bundles and because of an ensemble of ionic channels that constitute an electrical resonator.

A model for electrical resonance and frequency tuning in saccular hair cells of the bull-frog, Rana catesbeiana

1. Electrical resonance in solitary hair cells was examined under several experimental conditions using the tight-seal recording technique in the whole-cell current-clamp mode. 2. Resonance was characterized by the frequency and quality factor of oscillations in membrane potential evoked by depolarizing current pulses. Oscillation frequency increased with depolarization, from about 90 Hz at the resting potential to a limiting value of about 250 Hz. The quality factor of the oscillations was a bell-shaped function of membrane potential that reached a maximum of up to 12.6 at a potential slightly positive to the resting potential. 3. Pharmacological experiments were performed to assess which of three ionic currents participate in electrical resonance. Reduction of the voltage-gated Ca2+ current (ICa) and the Ca2+-activated K+ current (IK(Ca)) by lowering the extracellular Ca2+ concentration, or reduction of IK(Ca) with tetraethylammonium ion (TEA) degraded the resonance. In contrast, blockade of the transient K+ current (IA) with 4-aminopyridine (4-AP) had no significant effect. 4. To test the sufficiency of the Ca2+ and the Ca2+-activated K+ currents to account for resonance, we developed a model using mathematical descriptions of the two currents derived in the preceding paper (Hudspeth & Lewis, 1988), with additional terms for leakage conductance and membrane capacitance. The model correctly predicts the oscillatory responses to applied current pulses, including the non-linear dependences of oscillation frequency and quality factor on membrane potential. 5. Simulations of current-clamp experiments in the presence of a reduced extracellular Ca2+ concentration or of TEA were generated respectively by decreasing the model's values for the maximal Ca2+ or Ca2+-activated K+ conductances. The model's predictions of membrane-potential oscillations under these conditions agree qualitatively with experimental results, providing further support for the model as a description of the resonance mechanism. 6. To identify the factors most important in determining the hair cell's resonance properties, we systematically altered the values of selected parameters in the model. Frequency was most profoundly influenced by increasing the magnitude and activation rate of the Ca2+-activated K+ conductance, whereas the quality factor was most sensitive to increases in the level of the Ca2+ conductance. 7. By including a term describing activation of the hair cell's mechanically sensitive transduction conductance, we used the model to predict a tuning curve for responses to mechanical inputs of various frequencies.(ABSTRACT TRUNCATED AT 400 WORDS)

Steroid-Dependent Auditory Plasticity Leads to Adaptive Coupling of Sender and Receiver

For seasonally breeding vertebrates, reproductive cycling is often coupled with changes in vocalizations that function in courtship and territoriality. Less is known about changes in auditory sensitivity to those vocalizations. Here, we show that nonreproductive female midshipman fish treated with either testosterone or 17beta-estradiol exhibit an increase in the degree of temporal encoding of the frequency content of male vocalizations by the inner ear that mimics the reproductive female's auditory phenotype. This sensory plasticity provides an adaptable mechanism that enhances coupling between sender and receiver in vocal communication.

Clustering of Ca2+ channels and Ca(2+)-activated K+ channels at fluorescently labeled presynaptic active zones of hair cells

Electrical resonance, which in some hair cells provides a mechanism for frequency tuning, is mediated by clusters of Ca2+ channels and Ca(2+)-activated K+ channels that have been proposed to occur at presynaptic active zones. To localize Ca2+ channels on the cellular surface, we loaded hair cells from the frog's sacculus with the Ca2+ indicator fluo-3 and imaged them by fluorescence confocal microscopy. When a cell was depolarized, we observed on its basolateral surface several foci of transiently enhanced fluorescence due to local Ca2+ influx. After protracted recording, each cell displayed on average 18 brightly and permanently fluorescent spots at the same positions. We mapped these spots in four hair cells and compared their locations with those of presynaptic active zones, as determined from transmission electron micrographs of serial sections through the same cells. The results demonstrated that enhanced fluo-3 fluorescence marks active zones. Measurement of currents through membrane patches at fluorescently labeled active zones demonstrated that both voltage-activated Ca2+ channels and Ca(2+)-activated K+ channels occur there. These results confirm that the ion channels involved in electrical tuning and synaptic transmission by hair cells cluster together at presynaptic active zones.

Vestibular Evoked Myogenic Potentials Show Altered Tuning in Patients with Ménière’s Disease

OBJECTIVE

Acoustic stimulation of the saccule gives rise to a vestibulocollic reflex, the output of which can be measured in the neck as inhibition of activity in the ipsilateral sternocleidomastoid muscle. This vestibular evoked myogenic potential has been promoted as a means of assessing integrity of saccular function. In this study, we test the hypothesis that the cochleosaccular hydrops of Ménière's syndrome leads to alterations in saccular motion that change the dynamics of the vestibular evoked myogenic potential.

STUDY DESIGN

Prospective cohort study.

SETTING

Large specialty hospital, department of otolaryngology.

SUBJECTS

Fourteen normal adult volunteers and 34 consecutive consenting adult patients with unilateral Ménière's disease by American Academy of Otolaryngology-Head and Neck Surgery diagnostic criteria.

INTERVENTIONS

All subjects underwent vestibular evoked myogenic potential testing using ipsilateral broadband click and short tone-burst stimuli at 250, 500, 1,000, 2,000, and 4,000 Hz.

MAIN OUTCOME MEASURES

Threshold, amplitude, and latency of vestibular evoked myogenic potential responses in normal and Ménière's affected and unaffected ears.

RESULTS

Vestibular evoked myogenic potential was present in all ears tested. Normal subjects show a frequency-dependent vestibular evoked myogenic potential threshold, with best response ("frequency tuning") at 500 Hz. Compared with normal subjects and unaffected ears of Ménière's subjects, affected Ménière's ears had significantly increased vestibular evoked myogenic potential thresholds. Affected Ménière's ears showed threshold shifts at all frequencies and there was less tuning apparent at 500 Hz. Unaffected ears of Ménière's subjects also showed significantly elevated vestibular evoked myogenic potential thresholds compared with normal subjects. Analyses of vestibular evoked myogenic potential thresholds for effects of age, hearing loss, and audiometric configuration showed no significant differences.

CONCLUSIONS

Ménière's ears display alterations in vestibular evoked myogenic potential threshold and tuning, supporting our hypothesis of altered saccular motion mechanics arising from hydropic distention. Unaffected ears of unilateral Ménière's subjects show similar changes, though to a lesser degree. This finding may be because of occult saccular hydrops in the asymptomatic ear or binaural interactions in the vestibular evoked myogenic potential otolith-cervical reflex arc.

The transduction channel of hair cells from the bull-frog characterized by noise analysis

Receptor currents in response to mechanical stimuli were recorded from hair cells in the excised epithelium of the bull-frog sacculus by the whole-cell, gigohm-seal voltage-clamp technique. The stimulus-dependent transduction current was separated from the cell's stimulus-independent K+ and Ca2+ currents; the K+ currents were blocked with an internal solution containing Cs+ while the Ca2+ current was reduced by holding the membrane potential below -70 mV. The temperature of the preparation was maintained at about 10 degrees C to slow the kinetics of the cells' transduction channels. Calibrated displacements of hair bundles of individual hair cells were made with a probe coupled by suction to the kinociliary bulb and moved with a piezoelectricbimorph stimulator. The root mean square noise of probe motion was less than 2 nm. The mean, I, and the variance, sigma 2, of the receptor current were measured from the response to saturating (+/- 0.5 micron) displacements of the hair bundle. I was corrected for current offsets and sigma 2 for the transduction-independent background variance. The relation between sigma 2 and I is consistent with the predictions of a two-conductance-state model of the transduction channel, a model having only one non-zero conductance state. The relation between sigma 2 and I was fitted by the equation sigma 2 = Ii-I2/N, where N is the number of transduction channels in the cell and i is the current through a single open channel. The conductance of the transduction channel is approximately ohmic with a reversal potential near 0 mV. The estimated conductance of a single transduction channel, gamma, is 12.7 +/- 2.7 pS (mean +/- S.D.; n = 18) at 10 degrees C. gamma is independent of the maximum transduction conductance of the cell, Gmax. The number of transduction channels, N, is proportional to Gmax. N ranges from 7 to 280 in cells with Gmax ranging from 0.08 to 2.48 nS. The largest values of N correspond to a few, perhaps four, active transduction channels per stereocilium. Control experiments show that transduction by the hair cell of two artifactual sources of hair-bundle stimulation, noisy or discontinuous motion of the probe, do not contribute substantially to the measured variance, sigma 2. Displacement-response curves are generally sigmoidal and symmetrical; they reasonably fit the predictions of a two-kinetic-state model, comprising one open state and one closed state. The estimated displacement-sensitive free energy, Z, is 5.7 +/- 1.1 kcal/mol micron (mean +/- S.D., n = 18).(ABSTRACT TRUNCATED AT 400 WORDS)

The goldfish ear codes the axis of acoustic particle motion in three dimensions

Auditory and vestibular nerve fibers of the goldfish are strongly directionally sensitive to whole-body acceleration at audio frequencies. The three-dimensional pattern of sensitivity shows that input from a receptor ensemble (hair cells) is essentially equivalent to that expected from a single hair cell having a given three-dimensional orientation of best sensitivity. Fibers from the sacculus, lagena, and utriculus differ with respect to distributions of directional orientation, but are similar in best threshold (less than 1 nanometer, root mean square, at 140 hertz). In combination with other mechanisms for detection of sound pressure, this directionality is a likely basis for directional hearing in fishes, and it could allow the determination of underwater acoustic intensity.

Lgr5+ cells regenerate hair cells via proliferation and direct transdifferentiation in damaged neonatal mouse utricle

Recruitment of endogenous progenitors is critical during tissue repair. The inner ear utricle requires mechanosensory hair cells (HCs) to detect linear acceleration. After damage, non-mammalian utricles regenerate HCs via both proliferation and direct transdifferentiation. In adult mammals, limited transdifferentiation from unidentified progenitors occurs to regenerate extrastriolar Type II HCs. Here we show that HC damage in neonatal mouse utricle activates the Wnt target gene Lgr5 in striolar supporting cells. Lineage tracing and time-lapse microscopy reveal that Lgr5+ cells transdifferentiate into HC-like cells in vitro. In contrast to adults, HC ablation in neonatal utricles in vivo recruits Lgr5+ cells to regenerate striolar HCs through mitotic and transdifferentiation pathways. Both Type I and II HCs are regenerated, and regenerated HCs display stereocilia and synapses. Lastly, stabilized ß-catenin in Lgr5+ cells enhances mitotic activity and HC regeneration. Thus Lgr5 marks Wnt-regulated, damage-activated HC progenitors and may help uncover factors driving mammalian HC regeneration.

Using the zebrafish lateral line to screen for ototoxicity

The zebrafish is a valuable model for studying hair cell development, structure, genetics, and behavior. Zebrafish and other aquatic vertebrates have hair cells on their body surface organized into a sensory system called the lateral line. These hair cells are highly accessible and easily visualized using fluorescent dyes. Morphological and functional similarities to mammalian hair cells of the inner ear make the zebrafish a powerful preparation for studying hair cell toxicity. The ototoxic potential of drugs has historically been uncovered by anecdotal reports that have led to more formal investigation. Currently, no standard screen for ototoxicity exists in drug development. Thus, for the vast majority of Food and Drug Association (FDA)-approved drugs, the ototoxic potential remains unknown. In this study, we used 5-day-old zebrafish larvae to screen a library of 1,040 FDA-approved drugs and bioactives (NINDS Custom Collection II) for ototoxic effects in hair cells of the lateral line. Hair cell nuclei were selectively labeled using a fluorescent vital dye. For the initial screen, fish were exposed to drugs from the library at a 100-muM concentration for 1 h in 96-well tissue culture plates. Hair cell viability was assessed in vivo using fluorescence microscopy. One thousand forty drugs were rapidly screened for ototoxic effects. Seven known ototoxic drugs included in the library, including neomycin and cisplatin, were positively identified using these methods, as proof of concept. Fourteen compounds without previously known ototoxicity were discovered to be selectively toxic to hair cells. Dose-response curves for all 21 ototoxic compounds were determined by quantifying hair cell survival as a function of drug concentration. Dose-response relationships in the mammalian inner ear for two of the compounds without known ototoxicity, pentamidine isethionate and propantheline bromide, were then examined using in vitro preparations of the adult mouse utricle. Significant dose-dependent hair cell loss in the mouse utricle was demonstrated for both compounds. This study represents an important step in validating the use of the zebrafish lateral line as a screening tool for the identification of potentially ototoxic drugs.

Depolarization redistributes synaptic membrane and creates a gradient of vesicles on the synaptic body at a ribbon synapse

We used electron tomography of frog saccular hair cells to reconstruct presynaptic ultrastructure at synapses specialized for sustained transmitter release. Synaptic vesicles at inhibited synapses were abundant in the cytoplasm and covered the synaptic body at high density. Continuous maximal stimulation depleted 73% of the vesicles within 800 nm of the synapse, with a concomitant increase in surface area of intracellular cisterns and plasmalemmal infoldings. Docked vesicles were depleted 60%-80% regardless of their distance from the active zone. Vesicles on the synaptic body were depleted primarily in the hemisphere facing the plasmalemma, creating a gradient of vesicles on its surface. We conclude that formation of new synaptic vesicles from cisterns is rate limiting in the vesicle cycle.

Adaptive rundown of excitatory post-synaptic potentials at synapses between hair cells and eight nerve fibres in the goldfish

1. The excitatory post-synaptic potentials (e.p.s.p.s.) evoked by sound stimuli were recorded intracellularly from large afferent eight nerve fibres in the sacculus of the goldfish (S1 fibres). The fish were anaesthetized with MS-222 and spike potentials were suppressed with locally applied tetrodotoxin. 2. The e.p.s.p.s. successively evoked in response to each wound wave showed a marked rundown in size, while no reduction was observed in the microphonic potentials. The amplitude of successive e.p.s.p.s was reduced keeping approximately a fixed ratio to the preceding ones, suggesting that the rundown is attributable to a depletion of transmitter quanta from the release sites. 3. The rate of rundown of successive e.p.s.p.s, however, remained almost unchanged when the intensity of the stimulus sound was changed. It was also observed that, even after the e.p.s.p.s had been completely adapted to a continuous sound, a vigorous discharge of new e.p.s.p.s was observed when the intensity of the sound was increased. 4. These findings seem to indicate that it is the size of the readily available store and not the release fraction that is changed by a change in the sound intensity. 5. The saccular macula was superfused with solutions different in Ca and Mg ion concentrations. High Ca ion concentration brought about an increase in the size of the readily available store as well as the release fraction. 6. Mechanisms underlying these observations were discussed in terms of the quantal release mechanism as well as the morphology of the release sites.

Spontaneous hair cell regeneration in the mouse utricle following gentamicin ototoxicity

Whereas most epithelial tissues turn-over and regenerate after a traumatic lesion, this restorative ability is diminished in the sensory epithelia of the inner ear; it is absent in the cochlea and exists only in a limited capacity in the vestibular epithelium. The extent of regeneration in vestibular hair cells has been characterized for several mammalian species including guinea pig, rat, and chinchilla, but not yet in mouse. As the fundamental model species for investigating hereditary disease, the mouse can be studied using a wide variety of genetic and molecular tools. To design a mouse model for vestibular hair cell regeneration research, an aminoglycoside-induced method of complete hair cell elimination was developed in our lab and applied to the murine utricle. Loss of utricular hair cells was observed using scanning electron microscopy, and corroborated by a loss of fluorescent signal in utricles from transgenic mice with GFP-positive hair cells. Regenerative capability was characterized at several time points up to six months following insult. Using scanning electron microscopy, we observed that as early as two weeks after insult, a few immature hair cells, demonstrating the characteristic immature morphology indicative of regeneration, could be seen in the utricle. As time progressed, larger numbers of immature hair cells could be seen along with some mature cells resembling surface morphology of type II hair cells. By six months post-lesion, numerous regenerated hair cells were present in the utricle, however, neither their number nor their appearance was normal. A BrdU assay suggested that at least some of the regeneration of mouse vestibular hair cells involved mitosis. Our results demonstrate that the vestibular sensory epithelium in mice can spontaneously regenerate, elucidate the time course of this process, and identify involvement of mitosis in some cases. These data establish a road map of the murine vestibular regenerative process, which can be used for elucidating the molecular events that govern this process.

Aging Effect on Vestibular Evoked Myogenic Potential

OBJECTIVE

Vestibular evoked myogenic potential (VEMP) is applied to explore the integrity of sacculocollic reflex. Although tests to evaluate vestibular-ocular reflex pathway have shown that vestibular function is adversely affected by aging, VEMP, in this study, is used as a novel test to define how aging influences sacculocollic reflex pathway.

STUDY DESIGN

Prospective study.

SETTING

Academic tertiary referral center.

SUBJECTS

Eighty normal subjects, equally divided into four groups according to their age, were enrolled to this study. Group I included patients aged <20 years, Group II patient ages ranged from 21 to 40 years, Group III patients were 41 to 60 years, and Group IV included patients older than 60 years.

INTERVENTIONS

Recordings of VEMP responses.

MAIN OUTCOME MEASURES

The response rate and parameters of VEMP, including p13 latency, n23 latency, amplitude, and interaural difference ratio.

RESULTS

The VEMP response rates from Groups I to IV was 98%, 98%, 90%, and 60%, respectively, disclosing a significant difference only between Group IV and other groups (p < 0.05). The amplitude was negatively correlated with age in contrast to the n23 latency, correlating positively with age; both reached a significant difference (p < 0.05). Although the p13 latency had a trend to prolong as age increased, no significant correlation existed (p < 0.06). Moreover, the interaural difference ratio was also not significantly correlated with age.

CONCLUSIONS

As age increased over 60 years, the VEMP response rate decreased dramatically. While age increased, the VEMP amplitude decreased in comparison to n23 latency prolonged. These findings might suggest that aging could deteriorate the saccular and corresponding neural functions. When interpreting the VEMP parameters, it should be kept in mind that aging could affect VEMP responses. Based on this study, we suggest establishing different reference values according to different age groups when evaluating VEMP response in patients with vestibular diseases.

The ocular vestibular-evoked myogenic potential to air-conducted sound; probable superior vestibular nerve origin

OBJECTIVE

Intense air-conducted sound (ACS) elicits an ocular vestibular-evoked myogenic potential (oVEMP), and it has been suggested that it does so by stimulating saccular receptors and afferents in the inferior vestibular nerve and so activating a crossed sacculo-ocular pathway. Bone conducted vibration (BCV) also elicits an oVEMP probably by activating utricular receptors and a crossed utriculo-ocular pathway. Are there two separate pathways mediating oVEMPs for ACS and BCV? If saccular receptors and afferents are primarily responsible for the oVEMP to ACS, then the oVEMP to ACS should be normal in patients with reduced or absent utricular function--unilateral superior vestibular neuritis (SVN). If utricular receptors and afferents are primarily responsible for oVEMP n10, then oVEMP to ACS should be reduced or absent in SVN patients, and in these patients there should be a close relationship between the size of the oVEMP n10 to BCV and to ACS.

METHODS

The n10 component of the oVEMP to 500 Hz BCV and to 500 Hz ACS was recorded in 10 patients with unilateral SVN but who had saccular and inferior vestibular nerve function preserved, as shown by their normal cVEMP responses to ACS.

RESULTS

In SVN patients with normal saccular and inferior vestibular nerve function, the oVEMP n10 in response to ACS was reduced or absent. Across SVN patients there was a very close correspondence between the size of oVEMP n10 for ACS and for BCV.

CONCLUSIONS

The n10 component of the oVEMP to ACS is probably mediated predominantly by the superior vestibular nerve and so most likely by utricular receptors and afferents.

SIGNIFICANCE

The n10 component of the oVEMP to either ACS or BCV probably indicates mainly superior vestibular nerve function.

A saccular origin of frequency tuning in myogenic vestibular evoked potentials?: implications for human responses to loud sounds

Previous research has indicated that an early component of click-evoked myogenic potentials in the sternocleidomastoid muscle is vestibularly mediated, since it can be obtained in subjects with loss of cochlear function, but is absent in subjects with loss of vestibular function (Colebatch et al., 1994). We report here the results of an experiment to investigate whether this response shows any tuning properties. In a sample of 11 subjects, we obtained acoustically evoked EMG from the sternocleidomastoid muscle in response to 110 dB SPL 10 ms tone pips with frequencies of 100 Hz, 200 Hz, 400 Hz, 800 Hz, 1600 Hz and 3200 Hz. The results of this experiment indicate that this response does indeed have a well-defined frequency tuning which may be modelled as a resonance with a maximum response at frequencies between 300-350 Hz. The possible saccular origin of the tuning response and the consequences that this may have in human responses to loud sounds is discussed. Also discussed are the consequences of particular electrode arrangements in relation to the innervation and anatomy of sternocleidomastoid.

Contribution of the otoliths to the calculation of linear displacement

1. The present work is a quantitative study of the eye movements induced by linear translation when the subject is instructed to stabilize his gaze on a memorized earth-fixed target. These experiments may allow a better understanding of the central processing of otolithic signals. 2. Human subjects were submitted to either sinusoidal or step-like horizontal linear displacements along the interaural (Y)-axis in darkness, seated in a cart moving along a linear track. Each subject's head was fixed by a helmet secured to the cart. They were asked to keep their eyes on an earth-fixed memorized target at 63 cm from them on the X-axis. 3. During sinusoidal motion, a combination of low smooth compensatory eye movements and of compensatory saccades allowed the subjects to track the memorized target. The linear model of the responses of five subjects (seven sessions) exhibited a near-ideal slope of 1.14 (range 0.84-1.58). Two subjects did not compensate properly for their displacement. The mean "vestibular-saccadic" (VS) gain (ratio of overall eye movement peak-peak amplitude versus head displacement amplitude) was 1.52 +/- 0.80 (SD), showing an overestimation of head displacement. 4. The otolith-ocular reflex (OOR) mean gain values (ratio of slow phase cumulated peak-peak amplitude versus head displacement amplitude) were about 0.13 degrees/cm. This value is 5 times higher than what has been reported in the literature, probably due to the fact that the target was at a short distance. 5. The number of saccades occurring during sinusoidal stimulations varied according to the different subjects. They were obviously compensatory saccades and not quick phases. They indicate that although the gain of the OOR was small, the brain has computed the adequate desired eye position. 6. During steplike head displacements in darkness, although the OOR gain was also small, seven of the eight subjects could stabilize their gaze with a mean VS gain of 1.01 +/- 0.70. The linear model for the pooled responses of these subjects exhibited a slope of 0.82. 7. When subjects were instructed not to move their eyes during the translation, three of the five examined could still correctly reproduce the head movement amplitude with saccades, even as late as 50 s after motion had stopped. This indicates that head displacement was stored with the adequate metrics and could be used to drive the saccadic system. 8. Bilabyrinthectomized patients could not perform any adequate gaze stabilization. This shows that the observed performance was of vestibular origin.

Calretinin modifies presynaptic calcium signaling in frog saccular hair cells

To determine whether the concentrations of calcium-binding proteins present in some neurons and sensory cells are sufficient to influence presynaptic calcium signaling, we studied the predominant calcium-binding protein in a class of sensory hair cells in the frog ear. Based on antibody affinity and molecular weight, we identified this protein as calretinin. We measured its cytoplasmic concentration to be approximately 1.2 mM, sufficient to bind approximately 6 mM Ca2+. Calcium signaling was altered when the diffusible cytoplasmic components were replaced by an intracellular solution lacking any fast calcium buffer, and was restored by the addition of 1.2 mM exogenous calretinin to the intracellular solution. We conclude that calretinin, when present at millimolar concentration, can serve as a diffusionally mobile calcium buffer/transporter capable of regulating calcium signaling over nanometer distances at presynaptic sites.

Stiffness of sensory hair bundles in the sacculus of the frog

The stiffness of individual hair bundles of hair cells from the frog sacculus was measured using calibrated quartz probes. For displacements of up to 1 micron in either direction (angular deflections up to +/- 0.13 rad) the stiffness was constant. The stiffness did not depend on whether the bundle was in compression or tension. At first approximation, the stiffness was inversely proportional to the square of the height of application of the force above the apical surface of the hair cell. This is consistent with pivoting of the stereocilia within the hair bundle about their points of insertion into the cuticular plate. The pivotal stiffness of the bundle was approximately proportional to the bundle's cross-sectional area and hence to the number of stereocilia of which it is composed. It is inferred that the contribution of the kinocilium to the total bundle stiffness is small. It is concluded that applied forces are shared almost equally amongst all stereocilia, that there is relative shear between neighbouring stereocilia during bundle deflection and that each stereocilium contributes a pivotal stiffness of 0.49 +/- 0.15 X 10(-15) N X m X rad-1. The measured stiffness of the stereocilium is consistent with a structure which bends mainly at the tapering insertion point. The data are also consistent with little cross-linking here between actin filaments. The nature of the links between stereocilia in the hair bundle is also discussed.