Short latency vestibular evoked potentials in the Japanese quail (Coturnix coturnix japonica)

Electrophysiological Measurements of Peripheral Vestibular Function-A Review of Electrovestibulography

Electrocochleography (EcochG), incorporating the Cochlear Microphonic (CM), the Summating Potential (SP), and the cochlear Compound Action Potential (CAP), has been used to study cochlear function in humans and experimental animals since the 1930s, providing a simple objective tool to assess both hair cell (HC) and nerve sensitivity. The vestibular equivalent of ECochG, termed here Electrovestibulography (EVestG), incorporates responses of the vestibular HCs and nerve. Few research groups have utilized EVestG to study vestibular function. Arguably, this is because stimulating the cochlea in isolation with sound is a trivial matter, whereas stimulating the vestibular system in isolation requires significantly more technical effort. That is, the vestibular system is sensitive to both high-level sound and bone-conducted vibrations, but so is the cochlea, and gross electrical responses of the inner ear to such stimuli can be difficult to interpret. Fortunately, several simple techniques can be employed to isolate vestibular electrical responses. Here, we review the literature underpinning gross vestibular nerve and HC responses, and we discuss the nomenclature used in this field. We also discuss techniques for recording EVestG in experimental animals and humans and highlight how EVestG is furthering our understanding of the vestibular system.

Sustained and Transient Vestibular Systems: A Physiological Basis for Interpreting Vestibular Function

Otolithic afferents with regular resting discharge respond to gravity or low-frequency linear accelerations, and we term these the static or sustained otolithic system. However, in the otolithic sense organs, there is anatomical differentiation across the maculae and corresponding physiological differentiation. A specialized band of receptors called the striola consists of mainly type I receptors whose hair bundles are weakly tethered to the overlying otolithic membrane. The afferent neurons, which form calyx synapses on type I striolar receptors, have irregular resting discharge and have low thresholds to high frequency (e.g., 500 Hz) bone-conducted vibration and air-conducted sound. High-frequency sound and vibration likely causes fluid displacement which deflects the weakly tethered hair bundles of the very fast type I receptors. Irregular vestibular afferents show phase locking, similar to cochlear afferents, up to stimulus frequencies of kilohertz. We term these irregular afferents the transient system signaling dynamic otolithic stimulation. A 500-Hz vibration preferentially activates the otolith irregular afferents, since regular afferents are not activated at intensities used in clinical testing, whereas irregular afferents have low thresholds. We show how this sustained and transient distinction applies at the vestibular nuclei. The two systems have differential responses to vibration and sound, to ototoxic antibiotics, to galvanic stimulation, and to natural linear acceleration, and such differential sensitivity allows probing of the two systems. A 500-Hz vibration that selectively activates irregular otolithic afferents results in stimulus-locked eye movements in animals and humans. The preparatory myogenic potentials for these eye movements are measured in the new clinical test of otolith function—ocular vestibular-evoked myogenic potentials. We suggest 500-Hz vibration may identify the contribution of the transient system to vestibular controlled responses, such as vestibulo-ocular, vestibulo-spinal, and vestibulo-sympathetic responses. The prospect of particular treatments targeting one or the other of the transient or sustained systems is now being realized in the clinic by the use of intratympanic gentamicin which preferentially attacks type I receptors. We suggest that it is valuable to view vestibular responses by this sustained-transient distinction.

The adequate stimulus for mammalian linear vestibular evoked potentials (VsEPs)

Short latency linear vestibular sensory evoked potentials (VsEPs) provide a means to objectively and directly assess the function of gravity receptors in mammals and birds. The importance of this functional measure is illustrated by its use in studies of the genetic basis of vestibular function and disease. Head motion is the stimulus for the VsEP. In the bird, it has been established that neurons mediating the linear VsEP respond collectively to the rate of change in linear acceleration during head movement (i.e. jerk) rather than peak acceleration. The kinematic element of motion responsible for triggering mammalian VsEPs has not been characterized in detail. Here we tested the hypothesis that jerk is the kinematic component of head motion responsible for VsEP characteristics. VsEP amplitudes and latencies changed systematically when peak acceleration level was held constant and jerk level was varied from ∼0.9-4.6 g/ms. In contrast, responses remained relatively constant when kinematic jerk was held constant and peak acceleration was varied from ∼0.9 to 5.5 g in mice and ∼0.44 to 2.75 g in rats. Thus the mammalian VsEP depends on jerk levels and not peak acceleration. We conclude that kinematic jerk is the adequate stimulus for the mammalian VsEP. This sheds light on the behavior of neurons generating the response. The results also provide the basis for standardizing the reporting of stimulus levels, which is key to ensuring that response characteristics reported in the literature by many laboratories can be effectively compared and interpreted.

Columella footplate motion and the cochlear microphonic potential in the embryo and hatchling chicken

A piezoelectric (PZE) vibrator was used to mechanically drive the columella footplate and stimulate the cochlea of chicken embryos and hatchlings. Our objectives were to characterize the motion of the PZE driver and determine the relationship between columella footplate motion (displacement/ velocity) and the cochlear microphonic recorded from the recessus scala tympani (CMrst). At each frequency, displacement of the PZE driver probe tip was linearly related to the applied voltage over a wide range of attenuation levels (-60 to -20 dBre:50 Vp-p). The mean displacement across frequencies (100-4000 Hz) was 0.221+/-0.042 micromp-p for a constant applied voltage level of -20 dBre:50 Vp-p. Displacement was within 1.5 dB of the mean for this stimulus level at all frequencies except for 4000 Hz, where it was approximately 3 dB higher (p < 0.01). CMrst amplitudes in hatchlings were larger than amplitudes in embryos (p=0.003). For a given frequency, CM was linearly related to footplate displacement and velocity at both ages. The transform ratio of CMrst/A (CM amplitude/displacement) increased at approximately 6 dB/octave at frequencies between 100 and 1000 Hz in hatchlings suggesting that cochlear impedance (Zc) was resistive at these frequencies. In a large fraction of the embryos, Zc exhibited reactive behavior.

A comparison of vestibular and auditory phenotypes in inbred mouse strains

The purposes of this research were to quantify gravity receptor function in inbred mouse strains and compare vestibular and auditory function for strain- and age-matched animals. Vestibular evoked potentials (VsEPs) were collected for 19 inbred strains at ages from 35 to 389 days old. On average, C57BL/6J (35 to 190 days), BALB/cByJ, C3H/HeSnJ, CBA/J, and young LP/J mice had VsEP thresholds comparable to normal. Elevated VsEP thresholds were found for elderly C57BL/6J, NOD.NONH2(kb), BUB/BnJ, A/J, DBA/2J, NOD/LtJ, A/WySnJ, MRL/MpJ, A/HeJ, CAST/Ei, SJL/J, elderly LP/J, and CE/J. These results suggest that otolithic function varies among inbred strains and several strains displayed gravity receptor deficits by 90 days old. Auditory brainstem response (ABR) thresholds were compared to VsEP thresholds for 14 age-matched strains. C57BL/6J mice (up to 190 days) showed normal VsEPs with normal to mildly elevated ABR thresholds. Four strains (BUB/BnJ, NOD/LtJ, A/J, elderly LP/J) had significant hearing loss and elevated VsEP thresholds. Four strains (DBA/2J, A/WySnJ, NOD.NONH2(kb), A/HeJ) had elevated VsEP thresholds (including absent VsEPs) with mild to moderate elevations in ABR thresholds. Three strains (MRL/MpJ, Ce/J, SJL/J) had significant vestibular loss with no concomitant hearing loss. These results suggest that functional change in one sensory system does not obligate change in the other. We hypothesize that genes responsible for early onset hearing loss may affect otolithic function, yet the time course of functional change may vary. In addition, some genetic mutations may produce primarily gravity receptor deficits. Potential genes responsible for selective gravity receptor impairment demonstrated herein remain to be identified.

Gravity receptor function in mice with graded otoconial deficiencies

The purpose of the present study was to examine gravity receptor function in mutant mouse strains with variable deficits in otoconia: lethal milk (lm), pallid (pa), tilted (tlt), mocha (mh), and muted (mu). Control animals were either age-matched heterozygotes or C57BL/6J (abbr. B6) mice. Gravity receptor function was measured using linear vestibular evoked potentials (VsEPs). Cage and swimming behaviors were also documented. Temporal bones were cleared to assess the overall otoconial deficit and to correlate structure and function for lm mice. Results confirmed the absence of VsEPs for mice that lacked otoconia completely. VsEP thresholds and amplitudes varied in mouse strains with variable loss of otoconia. Some heterozygotes also showed elevated VsEP thresholds in comparison to B6 mice. In lm mice, which have absent otoconia in the utricle and a variable loss of otoconia in the saccule, VsEPs were present and average P1/N1 amplitudes were highly correlated with the average loss of saccular otoconia (R = 0.77,p < 0.001). Cage and swimming behavior were not adversely affected in those animals with recordable VsEPs. Most, but not all, mice with absent VsEPs were unable to swim. Some animals were able to swim despite having no measurable gravity receptor response. The latter finding underscores the remarkable adaptive potential exhibited by neurobehavioral systems following profound sensory loss. It also shows that behavior alone may be an unreliable indicator of the extent of gravity receptor deficits.

Effects of selected pharmacological agents on avian auditory and vestibular compound action potentials

Glutamate is currently the consensus candidate for the hair cell transmitter in the inner ear of vertebrates. However, other candidate transmitter systems have been proposed and there may be differences in this regard for auditory and vestibular neuroepithelia. In the present study, perilymphatic perfusion was used to deliver prescribed concentrations of ten drugs to the interstitial fluids of the inner ear of hatchling chickens (n = 124). Dose-response curves were obtained for four of these pharmacological agents. The work was carried out in part to distinguish further the neuroepithelial chemical receptors mediating auditory and vestibular compound action potentials (CAPs). Kainic acid (KA) eliminated both auditory and vestibular responses. D-alpha-Aminoadipic acid (DAA) and dizocilpine maleate (MK-801), both NMDA-specific antagonists, failed to alter vestibular CAPs at any concentration. MK-801 significantly and selectively reduced auditory CAPs at concentrations equal to or greater than 1 mM. Similarly, kynurenic acid (4-hydroxyquinoline-2-carboxylic acid, 1 mM), a glutamate antagonist, significantly reduced auditory but not vestibular CAPs. A non-NMDA glutamate receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), reduced vestibular CAPs significantly but only at the highest concentration tested (1 mM). In contrast, CNQX reduced auditory responses at concentration as low as 1 microM. The CNQX concentration effective in reducing auditory CAPs by 50% (EC(50)) was approximately 20 microM. Glutamate (1 mM) as well as alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), a glutamate agonist, significantly reduced auditory CAPs (AMPA EC(50)=100 microM). Bicuculline, a GABA(A) receptor antagonist, and L-NAME, a nitric oxide synthase inhibitor, failed to alter responses from either modality. These findings support the hypothesis that glutamate receptors mediate auditory CAPs in birds. However, the results underscore a remarkable difference in sensitivity of the vestibular neuroepithelium (here gravity receptors) to non-NMDA receptor antagonists. The basis of the vestibular insensitivity to glutamate blockers is unknown but it may reflect differences in receptors themselves, differences in the transmission modes available to vestibular synapses or differences in the access of compounds to vestibular neuroepithelial receptors from the interstitial-perilymphatic fluid spaces.

The simultaneous in vivo perilymphatic perfusion of avian auditory and vestibular end organs

Perilymphatic perfusion is a method that allows the control of fluid parameters throughout the perilymphatic space of the inner ear. We have evaluated a new method for continuous perilymphatic perfusion of the auditory and vestibular end organs with artificial perilymph (APL) in chickens. Perfusate temperature (39.0 degrees C), pH (7.4), osmolarity (328 +/- 2 mosm), and flow rate (2 microl/min) were carefully controlled. Independent functional tests of vestibular and auditory sensory systems were made throughout perfusion periods by recording peripheral compound action potentials (CAPs). The recordings provided a means of monitoring the status of hair cell transduction, synaptic transmission and collective primary afferent activation in response to auditory or vestibular gravity receptor stimuli. Auditory and vestibular responses were stable during perfusion. No significant changes occurred in vestibular or auditory CAP amplitudes during long-term perfusion (50-80 min, n=7) and responses remained stable in one animal perfused for over 3 h. To our knowledge, there have been no reports evaluating vestibular function under these conditions. This technique enables us to systematically study receptor pharmacology in the peripheral vestibular and auditory systems virtually simultaneously in vivo. The model is well suited for use in the study of the pharmacology and toxicology of inner ear sensory systems.

Histological preparation of developing vestibular otoconia for scanning electron microscopy

The unique nature of vestibular otoconia as calcium carbonate biominerals makes them particularly susceptible to chemical deformation during histological processing. We fixed and stored otoconia from all three otolith endorgans of embryonic, hatchling and adult Japanese quail in glutaraldehyde containing either phosphate or non-phosphate buffers for varying lengths of time and processed them for scanning electron microscopy. Otoconia from all age groups and otolith endorgans processed in 0.1 M phosphate buffer (pH 7.4) showed abnormal surface morphology when compared to acetone fixed controls. Otoconia processed in 0.1 M sodium cacodylate or HEPES buffered artificial endolymph (pH 7.4) showed normal morphology that was similar to controls. The degree of otoconial deformation was directly related to the time exposed to phosphate buffer. Short duration exposure produced particulate deformations while longer exposures resulted in fused otoconia that formed solid sheets. Otoconial surface deformation and fusing was independent of the glutaraldehyde component of the histological processing. These findings should help vestibular researchers to develop appropriate histological processing protocols in future studies of otoconia.

Malleal processus brevis is dispensable for normal hearing in mice

The mammalian middle ear cavity contains a chain of three ossicles (the malleus, incus, and stapes), which develop from the mesenchyme of the first two branchial arches. Mice deficient in the Msx1 homeobox gene exhibit craniofacial abnormalities, including the absence of the malleal processus brevis that is normally attached to the upper part of the tympanic membrane. Here, we show that the expression of Msx1 and Msx2 overlaps in the malleal primordium during early embryonic development. A functional redundancy of Msx1 and Msx2 in the development of the middle ear is suggested by the stronger hypomorphism in the malleus of Msx1(-/-)/Msx2(-/-) embryos, including the absence of the malleal manubrium and the malleal processus brevis. The expression of Bmp4, a known downstream target of Msx1 in several developing craniofacial organs, was down-regulated in the malleal primordium, particularly in the region of the developing malleal manubrium, of Msx1 and Msx1(-/-)/Msx2(-/-) embryos. Msx genes, thus, appear to act in a cell autonomous manner, possibly by regulating Bmp4 expression, in the formation of the malleus. Transgenic rescue of the cleft palate of Msx1(-/-) mice overcame the neonatal lethality and allowed Msx1(-/-) mice to grow into adulthood but retain the phenotype of the absence of the malleal processus brevis. The availability of this animal model for the first time allowed us to measure auditory evoked potentials to assess the functional significance of the malleal processus brevis. The results demonstrated unimpaired auditory function in Msx1(-/-) mice. In addition, mutant mice appeared normal in balance behavior and in the vestibular evoked potential screening test. These results indicate that the malleal processus brevis is not necessary for sound transmission and seems dispensable for normal hearing and balance in mammals.

Stimulus and recording variables and their effects on mammalian vestibular evoked potentials

Linear vestibular evoked potentials (VsEPs) measure the collective neural activity of the gravity receptor organs in the inner ear that respond to linear acceleration transients. The present study examined the effects of electrode placement, analog filtering, stimulus polarity and stimulus rate on linear VsEP thresholds, latencies and amplitudes recorded from mice. Two electrode-recording montages were evaluated, rostral (forebrain) to 'mastoid' and caudal (cerebellum) to 'mastoid'. VsEP thresholds and peak latencies were identical between the two recording sites; however, peak amplitudes were larger for the caudal recording montage. VsEPs were also affected by filtering. Results suggest optimum high pass filter cutoff at 100-300 Hz, and low pass filter cutoff at 10,000 Hz. To evaluate stimulus rate, linear jerk pulses were presented at 9.2, 16, 25, 40 and 80 Hz. At 80 Hz, mean latencies were longer (0.350-0.450 ms) and mean amplitudes reduced (0.8-1.8 microV) for all response peaks. In 50% of animals, late peaks (P3, N3) disappeared at 80 Hz. The results offer options for VsEP recording protocols.

Compound gravity receptor polarization vectors evidenced by linear vestibular evoked potentials

The utricle and saccule are gravity receptor organs of the vestibular system. These receptors rely on a high-density otoconial membrane to detect linear acceleration and the position of the cranium relative to Earth's gravitational vector. The linear vestibular evoked potential (VsEP) has been shown to be an effective non-invasive functional test specifically for otoconial gravity receptors (Jones et al., 1999). Moreover, there is some evidence that the VsEP can be used to independently test utricular and saccular function (Taylor et al., 1997; Jones et al., 1998). Here we characterize compound macular polarization vectors for the utricle and saccule in hatchling chickens. Pulsed linear acceleration stimuli were presented in two axes, the dorsoventral (DV, +/- Z axis) to isolate the saccule, and the interaural (IA, +/- Y axis) to isolate the utricle. Traditional signal averaging was used to resolve responses recorded from the surface of the skull. Latency and amplitude of eighth nerve components of the linear VsEP were measured. Gravity receptor responses exhibited clear preferences for one stimulus direction in each axis. With respect to each utricular macula, lateral translation in the IA axis produced maximum ipsilateral response amplitudes with substantially greater amplitude intensity (AI) slopes than medially directed movement. Downward caudal motions in the DV axis produced substantially larger response amplitudes and AI slopes. The results show that the macula lagena does not contribute to the VsEP compound polarization vectors of the sacculus and utricle. The findings suggest further that preferred compound vectors for the utricle depend on the pars externa (i.e. lateral hair cell field) whereas for the saccule they depend on pars interna (i.e. superior hair cell fields). These data provide evidence that maculae saccule and utricle can be selectively evaluated using the linear VsEP.

Short latency compound action potentials from mammalian gravity receptor organs

Gravity receptor function was characterized in four mammalian species using far-field vestibular evoked potentials (VsEPs). VsEPs are compound action potentials of the vestibular nerve and central relays that are elicited by linear acceleration ramps applied to the cranium. Rats, mice, guinea pigs, and gerbils were studied. In all species, response onset occurred within 1.5 ms of the stimulus onset. Responses persisted during intense (116 dBSPL) wide-band (50 to 50,000 Hz) forward masking, whereas auditory responses to intense clicks (112 dBpeSPL) were eliminated under the same conditions. VsEPs remained after cochlear extirpation but were eliminated following bilateral labyrinthectomy. Responses included a series of positive and negative peaks that occurred within 8 ms of stimulus onset (range of means at +6 dBre: 1.0 g/ms: P1=908 to 1062 micros, N1=1342 to 1475 micros, P2=1632 to 1952 micros, N2=2038 to 2387 micros). Mean response amplitudes at +6 dBre: 1.0 g/ms ranged from 0.14 to 0.99 microV. VsEP input/output functions revealed latency slopes that varied across peaks and species ranging from -19 to -51 micros/dB. Amplitude-intensity slopes also varied ranging from 0.04 to 0.08 microV/dB for rats and mice. Latency values were comparable to those of birds although amplitudes were substantially smaller in mammals. VsEP threshold values were considerably higher in mammals compared to birds and ranged from -8.1 to -10.5 dBre 1.0 g/ms across species. These results support the hypothesis that mammalian gravity receptors are less sensitive to dynamic stimuli than are those of birds.

Vestibular responses to linear acceleration are absent in otoconia-deficient C57BL/6JEi-het mice

Vestibular evoked potentials (VsEPs) were measured in normal mice and in mice homozygous for the head tilt mutation (het/het, abbr. het). The het mice lack otoconia, the inertial mass critical for natural stimulation of inner ear gravity receptors. Our findings demonstrate that vestibular neural responses to pulsed linear acceleration are absent in het mice.

THE RESULTS

(1) confirm that adequate sensory stimuli fail to activate gravity receptors in the het model; and (2) serve as definitive evidence that far-field vestibular responses to pulsed linear acceleration depend critically on otolith end organs. The C57BL/6JEi-het mouse may be an excellent model of gravity receptor sensory deprivation.

Vestibular function in Belgian Waterslager canaries (Serinus canarius)

The purpose of this study was to measure vestibular function in Belgian Waterslager canaries using short latency vestibular evoked potentials (VsEPs) elicited by linear acceleration stimuli. Responses were recorded with vertex to mastoid leads using traditional signal averaging. Response thresholds, latencies, and amplitudes were quantified and compared to non-Waterslager controls. Cochlear and vestibular organs were also processed for scanning electron microscopy. Results indicated that vestibular response thresholds were slightly, but significantly, higher for Belgian Waterslager canaries and response amplitudes at 0 dBre: 1.0 g/ms were significantly reduced compared to non-Waterslagers. Response peak latencies were not significantly different. The most striking morphological finding was that the stereociliary bundles of Waterslager saccular hair cells showed no common orientation. Previous reports have also found significantly less hair cells in Waterslager saccules (Weisleder and Park, Hear. Res. 80 (1994) 64-70); however, the present study did not confirm this finding. The utricle and ampullae appeared normal. The present results indicate that vestibular neural function, as measured with VsEPs, is affected in Belgian Waterslager canaries. The results also suggest that one structural correlate of the functional loss is the disordered stereociliary bundles in the sacculus.