Exp Brain Res (2003) 152:293–306DOI 10.1007/s00221-003-1531-4

R E S E A R C H A R T I C L E

Gavin E. G. Jones · Carey D. Balaban ·Ronald L. Jackson · Kimberly A. Wood ·Richard D. Kopke

Effect of trans-bullar gentamicin treatment on guinea pig angularand linear vestibulo-ocular reflexes

Received: 14 May 2002 / Accepted: 12 May 2003 / Published online: 31 July 2003� Springer-Verlag 2003

Abstract This study provides the first systematic exam-ination of the effects of intratympanic gentamicin instil-lation on vestibulo-ocular responses of guinea pigs duringboth Earth-vertical yaw axis and off-vertical axis rotation.A scleral search coil was sutured to the right eye ofpigmented female guinea pigs prior to trans-bullarinstillation of a 0.2-ml bolus of either 20 mg/ml or40 mg/ml of gentamicin (1) into the right middle ear(unilateral treatment groups) or (2) into both ears(bilateral treatment groups). Two weeks later, eye move-ment responses were tested during yaw axis sinusoidalrotation at 7 frequencies (0.02, 0.05, 0.1, 0.2, 0.5, 1 and2 Hz, 40 deg/s peak velocity) and during off-vertical axisrotation at five constant velocities (20, 40, 60, 80 and 100deg/s), tilted 30 deg relative to the earth-vertical axis. Themain result was that unilateral trans-bullar gentamicininstillation produced almost exclusively unidirectionaldeficits in horizontal angular vestibulocular reflex(HVOR) responses and modulation and bias responsesto off-vertical axis rotation (OVAR). The HVOR gain wasreduced during rotation toward the injected ear in a dose-dependent manner for frequencies of 1 Hz and lower, butthere was no effect on responses during rotation towardthe intact ear. Further, the modulation and bias responsesto OVAR were reduced profoundly in a dose-dependentmanner during rotation toward the treated ear. It issuggested that these effects indicate selective cytotoxicand/or physiologic effects of gentamicin intoxication inthe inner ear or, possibly, the vestibular nerve and centralnervous system.

Keywords Ototoxicity · Gentamicin · Vestibulo-ocularreflexes · Off-vertical axis rotation

Introduction

Injury to the vestibular system produces acute vertigo,nausea, disorientation, nystagmus and problems in pos-tural and locomotor control. Even after central compen-sation, significant deficits in vestibulo-spinal, vestibulo-ocular and vestibulo-collic reflexes may remain (Smithand Curthoys 1989). These residual deficits can signifi-cantly erode quality of life (C 1952) and may be factors inthe development of space and motion discomfort andavoidance (Furman and Cass 1999).

Drug otoxicity is one cause of peripheral vestibulardysfunction. The ototoxic effects of aminoglycosideantibiotics have been documented for nearly a half-century (Jackson and Arcieri 1971; Aran et al. 1982;Tjernstrom et al. 1982). Until recently, vestibular toxicitywas primarily a source of concern as an unwanted sideeffect of these medications. However, transtympanicinstillation of gentamicin is currently used as an alterna-tive to surgical labyrinthectomy in patients with intrac-table vertigo (Parnes and Riddell 1993; Pyykko et al.1994; Hirsch and Kamerer 1997; Hoffer et al. 1997;Murofushi et al. 1997; Pfleiderer 1998; Minor 1999).

Gentamicin was described initially as an ototoxic drugthat preferentially damages Type I hair cells in thevestibular neuroepithelium early after administration(Watanuki et al. 1972). In the chinchilla ampulla, 95%of Type I hair cells appeared to be damaged, while only14% of Type II hair cells were destroyed 4 days afterlocal treatment by gentamicin (Lopez et al. 1997). Otherstudies have administered gentamicin locally in theguinea pig (Warchol et al. 1993; Takumida et al. 1999)or chinchilla (Chin et al. 1999; Lopez et al. 1999) andhave reported a large, persistent reduction in Type I haircells, with a smaller reduction in Type II cells. Recently,Forge et al. (1998) reported that systemic gentamicininjections produced a large reduction in the numbers of

G. E. G. Jones · R. L. Jackson · K. A. Wood · R. D. Kopke ())Department of Defense Spatial Orientation Center,Otolaryngology Department,Naval Medical Center, San Diego,34800 Bob Wilson Dr. Ste 200, San Diego, CA 92134, USAe-mail: rdkopke@nmcsd.med.navy.mil

C. D. BalabanDepartments of Otolaryngology and Neurobiology,University of Pittsburgh

Type I hair cells in the striola of the utricle (>85%reduction at 4 weeks and approximately a 70% reductionat 12 weeks), while the numbers of Type II hair cells (and“immature” cells of uncertain type) at 12 and 33 weekspost gentamicin administration were increased from thenumbers of the same types of cells recorded at 4 weeks.At 1 and 4 weeks post-administration, virtually all cellshad Type II morphology. However, other studies suggestthat gentamicin also damages Schwann cells and Scarpa’sganglion cells (Sera et al. 1987; Bareggi et al. 1990;Harada et al. 1991) in a dose-dependent manner. Inparticular, the morphologic results suggest that localgentamicin instillation should produce functional deficitsthat reflect a disproportionate loss of Type I hair cells, butrelative preservation of Type II hair cells.

Despite the relative wealth of information on morpho-logic consequences of gentamicin instillation, there is noinformation regarding the effects of local gentamicininstillation on vestibulo-ocular reflexes (VORs). Reportsof effects of gentamicin administration are limited to thehorizontal semicircular canal-mediated angular VOR inguinea pigs after systemic injections (Pettorossi et al.1986), and after transtympanic administration in humans(Allison et al. 1997). This study examined the effects ofunilateral and bilateral intratympanic gentamicin instilla-tion on vestibulo-ocular reflexes during both yaw rotation(horizontal vestibulo-ocular reflex, HVOR) and constantvelocity off-vertical axis rotation (OVAR). The lattercondition activates both static and dynamic otolithafferents, which provides the first report of the effectsof gentamicin intoxication on an otolith-ocular response.

Materials and methods

For this study, a total of 27 pigmented female guinea pigs (Caviaporcellus) were used with weights ranging between 450 and 700 g.All surgical procedures and recording protocols were reviewed andapproved by the Institutional Animal Care and Use Committee(IACUC) at the Naval Medical Center, San Diego, for compliancewith NIH guidelines for animal utilization. For the control group,recordings were made from 15 animals (no drug treatment). Threeanimals were used for each of four experimental groups. Animalswere anesthetized with a mixture of ketamine (100 mg/ml, 13 mg/kg) and xylazine (100 mg/ml, 1 mg/kg). The dorsal portion of eitherthe right tympanic bulla (unilateral injection groups) or bothtympanic bullae (bilateral injections groups) was exposed. A smallhole was drilled through the bone, and a 0.2-ml bolus of either 20 or40 mg/ml gentamicin solution was administered (without airbubbles). The hole was carefully sealed with Duralon to ensurecomplete filling of the bulla. A scleral search coil was fitted to theright eye, beneath the conjunctiva. It was fixed to the eye with fourapproximately equidistant anchoring sutures (8–0 vicryl coatedmonofilament). The eye coil was a three-turn, 7-mm coil madefrom 0.0048-inch inner diameter, 0.0100-inch outer diameter,Teflon-coated, stainless steel wire. A head fixation device wasplaced on the bregma. Placement of the head post was such that theplane of rotation was coplanar with the horizontal semicircularcanal plane (rotated 40 deg nose-up from the stereotaxic horizontalplane). At this angle, the vertical semicircular canals were close toorthogonal to the rotation plane (Curthoys et al. 1975); hence therewould be little contamination of the recorded responses fromHVOR trials by vertical canal input.

Recordings were made using the scleral search coil/dual fieldcoil (CNC Engineering) technique (Robinson 1963). Data wereacquired 14 days after surgical preparation. Two pairs of orthog-onal electromagnetic field coils generated the magnetic field. Theeye coil voltage was separated into horizontal and verticalcomponents by a phase detector and then passed through a pre-amplifier. A computer with a 12-bit analog-digital card was used tosample horizontal and vertical eye and head position on indepen-dent channels at a 200-Hz rate, using VRF NASA Eye MovementData Collection & Data Analysis Program, version 1.06 (providedgraciously by Dr. David Tomko). Eye coils were calibrated beforeimplantation by attaching an identical dummy coil to a rotatingprotractor and pointer (the calibrating device). The calibration wasset such that €2.0 volts equaled €10 deg. Eye velocity wascalculated as the first derivative of position.

Alert animals were first habituated to the device and were laterrestrained on a servo-controlled rate table (Neurokinetics) whichhoused the animal, its restraining devices and the field coils. TheHVOR performance was assessed during sinusoidal Earth-verticalaxis rotation (EVAR) in darkness at frequencies of 0.02, 0.05, 0.1,0.2, 0.5, 1 and 2 Hz and a peak velocity of 40 deg/s. For off-verticalaxis rotation (OVAR), the rotation axis was tilted 30 deg fromEarth-vertical. Constant velocity rotation was utilized in both theclockwise and counterclockwise directions at speeds of 20, 40, 60,80 and 100 deg/s in the dark. During the constant velocity OVARtrials, linear acceleration along the naso-occipital axis wasmeasured with a Neurokinetics SK0077–1 DC accelerometer. Theaccelerometer was mounted directly below the animal’s head andwas aligned parallel to the animal’s naso-occipital axis. Modulationof the accelerometer signal was positive when the linear acceler-ation vector was towards the animal’s nose, and negative when thelinear acceleration vector was directed caudally. Horizontal andvertical eye position (re: head), turntable velocity (tachometer) andlinear accelerometer signals were digitized at a rate of 200 samples/sand analyzed off-line.

The analyses in this study focused on only the horizontal eyevelocity during sinusoidal earth-vertical axis and constant velocityoff-vertical axis rotation. During earth-vertical axis rotation, theHVOR is purely horizontal. As reported in other species (e.g.,Darlot and Denise 1988; Hess and Dieringer 1990; Maruta et al.2001), the major component of eye movements during constantvelocity OVAR was a horizontal nystagmus. During constantvelocity OVAR, the horizontal eye velocity represents the responseto a sinusoidally varying linear acceleration in the horizontal planeof magnitude 0.5 g (for 30-deg tilt). This horizontal response wasused as a metric to assess the effects of gentamicin on otolith-ocularresponses. As in rabbits (Maruta et al. 2001), the verticalcomponent of nystagmus during OVAR was variable in guineapigs. Therefore, it was not analyzed.

Two different methods were used to analyze HVOR data. First,the VRF NASA Eye Movement Data Collection & Data AnalysisProgram (version 1.06) was used to calculate the gain and phase ofthe HVOR. Briefly, this program calculates horizontal eye velocityas a derivative of the eye position data, desaccades the velocitydata, and then averages at least eight cycles of stimulus presentationto produce a single average cycle of slow phase eye velocity.Regression methods are then used to estimate gain and phaseparameters to fit the data with an averaged head velocity cycle fromthe tachometer signal. Sinusoids at the same frequency as that ofthe head velocity were fitted to the raw eye velocity tracing and thehead velocity signal. Gain was defined as the ratio of the peak-to-peak modulation of the sinusoid fitted to eye velocity relative to thepeak-to-peak modulation of the sinusoidal head velocity. Phaseestimates (measured in degrees) for HVOR trials were referenced tocontralateral head velocity. Phase leads occurred when the eyevelocity peaked before the head velocity and resulted in positivephase values.

A second method was used to assess the directional symmetryof the HVOR. This method views the HVOR as a piece-wise linearresponse. The horizontal eye position data were first low passfiltered (eighth order Chebyshev type I lowpass filter with cutofffrequency of 40 Hz) and resampled at half the sampling frequency,

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using the ‘decimate.m’ function in MATLAB. The slow phasehorizontal eye velocity was then estimated by a simple difference(divided by the sampling interval), using the ’diff.m’ subroutine.Epochs containing rapid eye movements (�2.5 standard deviationsfrom mean velocity) were identified and truncated at the limitingvalue. The directional symmetry of the HVOR responses was thenassessed by regression analysis to estimate the leftward (Gleft) andrightward (Gright) gains of the slow phase eye velocity responses tothe measured head velocity. For the estimation procedure, therelative phase shifts of the horizontal slow phase eye velocity andhead velocity data (with polarity reversed) were determined bycross-correlation analysis and the head velocity data were shifted toeliminate the phase difference. The alignment of the head velocityand eye velocity traces was verified visually and adjusted, whennecessary, to completely eliminate a phase difference. A Mar-quardt-Levenberg optimization algorithm (‘leastsq.m’ routine forMATLAB, Mathworks) was then used to estimate the directionalgains for the slow phase eye velocity with respect to the reversed-polarity head velocity by minimizing the least-squares errorbetween the slow phase eye data and the function

Grightmax H�; 0

� �þ Gleftmin H

�; 0

� �ð1Þ

where H represents head velocity (with polarity reversed relative tothe slow phase eye velocity).

For OVAR experiments, the first 25 s of the horizontal eyemovement data were discarded to eliminate contributions ofsemicircular canal activation (Angelaki and Hess 1996). TheVRF NASA Eye Movement Data Collection & Data AnalysisProgram (version 1.06) was used to decompose the desaccadedhorizontal eye velocity responses to OVAR into two components,modulation sensitivity and bias velocity, by fitting the data to amodel of an offset sinusoid of the form:

a þ b sinpw180

t þ q� �

ð2Þ

where a represents the bias velocity (a constant), b represents themodulation sensitivity, t represents time in seconds, w representsthe constant angular head velocity (in deg/s), and q represents thephase (in radians) relative to right ear down. Hence, the modulationcomponent is a sinusoidal change in velocity at the same frequencyas the oscillation of the head from nose-down position to nose-down position. Modulation sensitivity (in deg/s/g) was estimated asthe magnitude of a sinusoidal fit of the eye velocity to this rotationfrequency divided by the peak linear acceleration. Bias was theconstant velocity offset (in degrees per second) from zero velocity.Statistical comparisons were made using repeated measuresANOVA using SYSTAT 8.0 (SPSS, Evanston, IL). Post-hoccomparisons were made using the Least-significant differences(LSD) test.

Results

The results were obtained from recordings of the right eyeonly.

Horizontal vestibulo-ocular reflex (HVOR)

Normal HVOR responses

Sinusoidal rotation about an Earth-vertical axis produceda symmetric sinusoidal modulation of horizontal slowphase eye velocity at all tested frequencies. The gainincreased monotonically between 0.02 and 0.5 Hz(F(5,80)=196.37, P<0.01), but did not differ significantlyat 1 and 2 Hz (repeated measures ANOVA, F(1,16)=2.75,

NS). The phase lead decreased monotonically for thefrequencies between 0.02 and 0.5 Hz (F(4,64)=197.4,P<0.01, individual P<0.01 differences by Bonferroni-corrected t-tests), but it did not change significantly froma slight phase lead at 1 and 2 Hz (paired comparisonswere not significant). Repeated measures ANOVA re-vealed that there were no significant differences in theleftward versus rightward HVOR gains in controls (maineffect, F(1,13)=0.41, NS; direction � frequency interactioneffect, F(6,78)=1.83, NS). Hence, HVOR gain was direc-tionally symmetric in the intact animal.

Effects of unilateral-gentamicin instillation on the HVOR

Gentamicin injections produced clear decrements investibular responses. Figure 1 shows HVOR eye move-ment recordings (0.1 Hz) from an intact animal (Fig. 1A),an animal treated unilaterally with 40 mg/ml gentamicin(Fig. 1B), and an animal treated bilaterally with 40 mg/mlgentamicin (Fig. 1C). There was no spontaneous nystag-mus in the unilaterally treated animals. However, theHVOR was asymmetric in the unilaterally treatedanimals. These guinea pigs displayed a marked decreasein eye velocity during rotation to the right (i.e., rotatedtowards the injected ear). The slow-phase eye velocitywas reduced bidirectionally after bilateral gentamicininstillation. Plots of the gains and phases of theseresponses are shown for control and gentamicin treatedgroups in Fig. 2. As summarized in Fig. 2A, unilateralgentamicin instillation produced a dose-dependent reduc-tion in HVOR gain (two-way ANOVA; repeated mea-sures factor of frequency and a between groups factor ofdose, main effect of dose when all three doses werecompared, F(2,20)=6.12, P<0.01). When individual com-parisons between the control and a treated group weremade using LSD tests, the 20-mg/ml group gain differedsignificantly from the control group only at 0.05 Hz(P<0.05), but gains from the 40-mg/ml group weresignificantly lower than those of the controls at allfrequencies (P<0.05 at 0.02 Hz, P<0.01 at 0.05, 0.1, 0.2,0.5, 1 and 2 Hz). Two-way ANOVA (within-groupsfactor: frequency, between-groups factor: dose) compar-ing gains from the group instilled unilaterally with 20 mg/ml gentamicin and controls showed that gains did notdiffer significantly (F(1,9)=0.621, NS). There was also adose-dependent increase in phase lead (dose main effect:F(2,20)=47.57, P<0.01, comparing the controls and thegroup instilled with 20 mg/ml gentamicin) in the unilat-erally treated animals. Multiple range tests (LSD criteri-on) showed that the 40-mg/ml group had a greater phaselead than controls at all frequencies less than 2 Hz(P<0.05 at 0.5 Hz; P<0.01 at 0.02, 0.05, 0.1, 0.2 and1 Hz), and that the phase was advanced relative tocontrols in the 20-mg/ml group at 0.05, 0.1 and 0.2 Hz(P<0.01). The phase (shown in Fig. 2C) also differedsignificantly between the 20- and 40-mg/ml groups at0.02, 0.05, 0.1 and 0.2 Hz (LSD test, P<0.05). Both thegain and phase findings from this analysis (data from

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animals instilled with 40 mg/ml gentamicin) are similar tothe short-term data from guinea pigs after surgicallabyrinthectomy (Vibert et al. 1993).

Because there was an obvious asymmetry in the slowphase eye velocity records from the unilaterally treatedanimals (Fig. 1), gains were analyzed separately forleftward and rightward slow phase responses. Figure 3shows the leftward and rightward slow phase eye velocitygains in treated groups and controls. Unilateral instillationof 40 mg/ml gentamicin produced a significant decreasein the HVOR gain for rotation in the direction of thetreated ear (the leftward slow phase gain), but nosignificant decrement for rotation toward the intact ear.The significant effects from analysis of variance thatsupport this conclusion are summarized in Table 1. Inaddition, separate ANOVA and LSD pair-wise compar-isons were used to compare control and the two treatmentgroup responses at each frequency during leftwardrotation (hence, there were seven ANOVA and LSDcomparisons). These analyses indicated that the doseeffect was produced by a significant gain reduction(P<0.05) from control gains in only the 40-mg/ml groupduring 0.02, 0.1, 0.2, 0.5 and 1 Hz stimulation. The

leftward slow phase eye velocity gains from the 20-mg/mlgroup did not differ significantly from either controls orthe 40-mg/ml group for any frequency, suggesting a dose-dependent intermediate effect that did not reach statisticalsignificance.

Effects of bilateral gentamicin instillation on the HVOR

Bilateral instillation of gentamicin (Figs. 1C, 2B and 3C,D) produced a large and directionally symmetric HVORgain decrement. Repeated measures ANOVA (gains fromwhole cycle analysis) was used with dose (levels: 0, 20and 40 mg/ml) as the between groups factor and withstimulus frequency (0.05, 0.1, 0.2, 0.5, 1 and 2 Hz) andslow phase direction as within groups factors. There weresignificant main effects of dose (F(2,17)=6.5, P<0.01) andfrequency (F(5,85)=17.19, P<0.01). For direction-specificanalysis, separate ANOVAs were performed (with stim-ulus frequency and slow phase direction as within groupsfactors) to compare each treatment group with the controlgroup and the two treatment groups. Significant reduc-tions from control responses were present in both the 20-

Fig. 1A–C Depicted are theHVOR eye velocity tracingsfrom a control animal (A) andfrom animals instilled unilater-ally (B) and bilaterally (C) with40 mg/ml gentamicin. Thestimulus was a 40-deg/s veloc-ity sinusoid at 0.2 Hz. Gain andphase values are listed to theright of each tracing. The hor-izontal line depicts zero veloc-ity. Rightward eye velocity isdefined as positive. In B, notethe asymmetric response. Theeye velocity modulation is de-creased when the head is rotatedto the right (i.e., leftward slowphase eye velocity), comparedto the response for leftwardhead rotation (rightward slowphase eye velocity). The eyevelocity modulation is de-pressed profoundly and bidi-rectionally in the animal thatreceived bilateral injections ofgentamicin (C)

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Fig. 2A–D Plots of the HVORslow phase eye velocity re-sponse for controls and gen-tamicin-treated groups. A, BFrequency vs. gain after (A)unilateral and (B) bilateral ab-lation with gentamicin. C, DFrequency vs. phase after uni-lateral (C) and after bilateral(D) ablation with gentamicin.For the control group, n=15. Foreach of the treatment groups,n=3. Error bars are SEM

Table 1 ANOVA of leftwardand rightward HVOR gains.Overall ANOVA: unilateralgentamicin

Factors between groups Factors within groups Significant effects

Dose (0, 20 and 40 mg/ml) 1. Frequency Main effects:2. Directional gain Dose (F(2,17)=3.87, P<0.05)

Frequency (F(6,102)=24.97, P<0.01)Direction (F(1,13)=6.99, P<0.05)Interaction effect:Frequency � Direction(F(6,102)=1.04, H-F corrected P<0.05)

20-mg/ml unilateral group only1. Frequency Main effect:2. Directional gain Frequency (F(6,42)=5.107, P<0.01)

40-mg/ml unilateral group only1. Frequency Main effect:2. Directional gain Frequency (F(6,12)=12.36, P<0.01)

Interaction effect:Frequency � Direction(F(6,12)=5.1, H-F P<0.01)

Rightward HVOR slow phase gainDose (0, 20 and 40 mg/ml) Frequency None

Leftward HVOR slow phase gainDose (0, 20 and 40 mg/ml) 1. Frequency Main effect:

2. Directional gain Dose (F(2,17)=7.88, P<0.01)

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mg/ml (F(1,15)= 5.28, P<0.05) and the 40-mg/ml(F(1,15)=9.97, P<0.01) groups, but there was no significantdifference in leftward versus rightward responses. Thetwo treatment groups did not differ significantly. Thus,both bilaterally treated groups showed equivalent gaindecrements.

Comparison between unilateraland bilateral gentamicin instillation on the HVOR

A direct comparison between the leftward eye velocityresponses of the unilaterally and bilaterally treated groupswas performed on data from the half-cycle analyses to testthe hypothesis that input from the intact ear contributed tothe high frequency responses in the unilaterally injectedanimals. Repeated measures three-way ANOVA [betweengroups factors: laterality (bilateral vs. unilateral) and dose(20 vs. 40 mg/ml); within groups factor: frequency] of theleftward eye velocity gains indicated that there was nosignificant main effect of bilateral versus unilateralinfusions. There was a significant laterality � frequencyinteraction effect (F(5,40)=4.01, P<0.01), which indicatedthat when the data are pooled across doses, the bilateralinfusions produced an increased impairment at higherfrequencies. However, LSD tests indicated that there wereno significant differences between unilateral and bilater-ally treated animals at any frequency or dose, comparingvalues from the same direction and dose. Hence, the highfrequency gains from unilaterally instilled animals were

not significantly greater than the gains acquired from therespective bilaterally instilled group. Therefore, it appearsthat the high frequency input from the left (intact) ear isnot great enough to significantly change the HVOR gainon the damaged side.

Responses during OVAR

Normal responses: directional asymmetryin control guinea pigs

The raw data from an OVAR trial from a control animalduring 80 deg/s OVAR stimulation are shown in Fig. 4.Only movements of the right eye were recorded and onlythe horizontal eye velocity analyses (during constantvelocity head-referenced yaw rotation) will be described.During ipsilateral (clockwise) rotation (Fig. 4A), thesensitivity of the modulation component of horizontal eyevelocity was 9.0 deg/s/g, and the bias component of thehorizontal eye velocity was �22.3 deg/s, i.e., in thecontralateral (counterclockwise) direction. During contra-laterally directed rotation, there was a marked reductionin the magnitude of the modulation sensitivity ofhorizontal eye velocity (4.7 deg/s/g), but a similarmagnitude of the bias component of horizontal eyevelocity [18.9 deg/s, directed ipsilaterally (clockwise)](Fig. 4B).

The horizontal eye velocity modulation and biasresponses during OVAR (Fig. 5) were dependent on both

Fig. 3 Illustrated is a plot offrequency vs. directional gainfor treated animals and controls.The leftward gain is for left-ward eye modulation duringrightward head motion. Theconverse is true for rightwardgain. For the control group,n=15. For each of the treatmentgroups, n=3. Error bars areSEM

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the velocity and the direction of rotation. The modulationcomponent sensitivity and the bias component magnitudeincreased with rotational velocity during rotation directedeither ipsilaterally (clockwise) or contralaterally (coun-terclockwise). This finding was indicated in repeatedmeasures one-way ANOVA by a significant main effectof rotation speed for both the horizontal eye velocitymodulation sensitivity (F(4,64)=162.69, P<0.01) and thebias horizontal eye velocity (F(4,64)=199.07, P<0.01)magnitude. Two-way repeated measures ANOVA wasalso performed using both rotation speed and direction aswithin-groups factors. There was a significant difference

between responses to ipsilaterally versus contralaterallydirected rotation (i.e., a main effect of direction ofrotation) for both the horizontal eye velocity modulationsensitivity (F=13.72, P<0.01) and the horizontal eyevelocity bias magnitude (F(1,16)=9.82, P<0.01), indicatinga direction asymmetry of these responses. There were alsosignificant rotation speed � direction interaction effectsfor both the modulation (F(4,64)=7.71, P<0.01) and bias(F(4,64)=9.74, P<0.01) components of horizontal eyevelocity responses, which indicated that the degree ofdirectional asymmetry of these eye velocity responsecomponents varied across stimulus speeds. The direction-al differences were significant (P<0.05) at all but thelowest rotational velocity (20 deg/s) for the horizontal eyevelocity modulation sensitivity (LSD tests comparingsensitivity values in both directions at a particular speed).However, the horizontal eye velocity bias magnitude

Fig. 4A, B Examples of the horizontal eye movements producedby OVAR. The horizontal line depicts zero velocity. The uppertraces show the output of a linear accelerometer mounted beneaththe animal’s head and oriented along the animal’s naso-occipitalaxis. ‘Nose Down’ (ND), ‘Nose Up’ (NU), ‘Right Ear Down’(RED), and ’Left Ear Down’ (LED) positions are indicated in thefigure. When the tracing is at zero position, the animal’s Naso-occipital axis is Earth-horizontal (either ‘Right Ear Down’ or ‘LeftEar Down’). A The stimulus is clockwise constant velocity rotationat 80 deg/s. The upper tracing shows table (head) velocity (degreesper second), and the lower tracing depicts eye velocity, in degreesper second. B Results from a counterclockwise OVAR stimulationin the same animal as A, also at a constant velocity of 80 deg/s

Fig. 5A, B Plots of OVAR modulation sensitivity (A) and theabsolute value of bias eye velocity (B) as a function of constantrotation velocity (30-deg OVAR) for untreated (control) guineapigs (n=15). Recordings are from the right eye only. A Modulationsensitivity (units: deg/s/g) increased monotonically with thevelocity of rotation (deg/s). Note that the modulation sensitivitywas greater for clockwise (i.e., ipsilateral) rotation. B Bias eyevelocity was rightward for counterclockwise rotation and leftwardfor clockwise rotation. However, bias velocity magnitudes arepresented as absolute values for ease of comparison. Error bars areSEM

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showed significant directional differences at only twostimulus speeds, 80 and 100 deg/s (LSD tests). It wasparticularly noteworthy, though, that the direction ofmaximal response was opposite for the modulation andthe bias components. Modulation sensitivity was greaterfor ipsilateral (clockwise) rotation at all but the lowestrotation velocity (Fig. 5A), while the horizontal eyevelocity bias response magnitude was greater for contra-lateral (counterclockwise) rotation at the two highestrotation velocities (Fig. 5B).

Effects of unilateral gentamicin instillationon OVAR responses

Sample records of the horizontal eye velocity and headlinear velocity during OVAR stimulation (rotated at 80-deg/s constant velocity) are shown in Fig. 6 for aunilaterally and bilaterally treated animal. Figure 6A, Bshows the horizontal eye velocity and head linear velocitydata from a unilaterally injected animal (40 mg/ml).Figure 6A shows the horizontal eye velocity record fromthe right eye for one animal during horizontal rotation inthe direction of the injected (ipsilateral) ear. There was amarked reduction in the number of quick phases, thehorizontal eye velocity modulation sensitivity and thehorizontal eye velocity bias magnitude when compared toFig. 6B, which shows the large modulation component,

bias component, and robust leftward beating quick phasesduring rotation toward the intact (contralateral) ear.

Modulation component sensitivity

Unilateral instillation of gentamicin into the right earproduced dose-dependent changes in both the horizontaleye velocity modulation (Fig. 7A, C) and bias (Fig. 8A)components of responses to OVAR of the right eye. Themodulation sensitivity of the right eye after unilateralgentamicin instillation (Fig. 7A, B) was reduced signif-icantly in a dose-dependent manner during OVAR towardthe injected ear (clockwise rotation; within groups factor:speed; between groups factor: drug dose, main effect,F(2,20)=13.34, P<0.01). Separate comparisons demonstra-ted that the effects on the horizontal eye velocitymodulation component were dose-dependent during rota-tion toward the injected ear: the main effect of dose wassignificant for separate comparisons between the controlgroup and the 20-mg/ml group (between groups factor:dose; within groups factor: speed, F(1,18)=5.32, P<0.05),between the control and the 40-mg/ml group (F=22.59,P<0.01), and between the 20-mg/ml and 40-mg/ml group(F(1,4)=9.87, P<0.05). The horizontal modulation velocitysensitivity of the right eye was significantly lower for the40-mg/ml treated animals than for control animals at allspeeds tested during OVAR toward the injected ear (LSD

Fig. 6A–D Illustrations of rawhead linear acceleration and eyevelocity tracings from repre-sentative OVAR trials. Therecords show the steady-stateresponse after decay of thesemicircular canal-related ini-tial responses. Modulation sen-sitivity and bias eye velocityvalues are listed below eachrecord. The horizontal line de-picts zero velocity. The upperrecord shows linear accelera-tion of the head along the naso-occipital axis. The lower recordshows the eye velocity. A, BThe nose-down (ND), right-eardown (RED), left-ear down(LED) and nose-up (NU) orien-tations are indicated. A, C Re-sults from clockwise rotation.B, D Results from counter-clockwise rotation. A, B Re-cords from a unilaterallydamaged animal. C, D Resultsfrom an animal instilled bilat-erally with gentamicin. In A–D,animals were instilled with40 mg/ml gentamicin. Animalswere rotated at 80 deg/s

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tests, P<0.01). Similarly, LSD comparisons of horizontalmodulation velocity of the right eye showed significantdecrements during 60 (P<0.05) and 100 (P<0.01) deg/sOVAR rotation toward the injected (ipsilateral) ear in the20-mg/ml group (re: controls). Thus, a major effect ofunilateral instillation of gentamicin on OVAR responseswas a dose-dependent decrement of the right eye velocitymodulation response during rotation toward the injected(right) ear.

During OVAR stimulation toward the left intact ear,though, the main effect of drug dose on the right eyemodulation sensitivity was not significant (F(2,20)=1.56,NS). However, there was a significant speed � doseinteraction effect (F(8,80)=2.69, P<0.05), which was pro-duced by significant decrements in right eye velocitymodulation sensitivity in the 40-mg/ml treated animals atonly 20- and 100-deg/s stimulus speeds (LSD tests,P<0.05). Thus, only a minor component of the OVARhorizontal right eye velocity modulation response duringcontralateral rotation is affected by gentamicin instillationin the ipsilateral ear.

The net result of these directionally selective effects ofunilateral gentamicin treatment was the elimination of thenormal directional asymmetry of the horizontal eyevelocity modulation component of the OVAR response.As described above, control guinea pigs showed greaterright horizontal eye velocity modulation sensitivity valuesfor ipsilateral (clockwise) than contralateral (counter-clockwise) rotation. The reduction in the response toipsilateral (clockwise) rotation after instillation of 20 mg/

ml gentamicin in the right ear eliminated the differencebetween the modulation responses to ipsilateral (clock-wise) versus contralateral (counterclockwise) rotation(within-group comparison, using both speed and directionas factors, for rotation direction main effect, F(1,2)=0.09,NS). The decrements in horizontal eye velocity modula-tion during both ipsilateral (clockwise) and contralateral(counterclockwise) OVAR in the 40-mg/ml infusiongroup also eliminated the directional difference in mod-ulation sensitivity observed in control guinea pigs (with-in-group comparison for rotation direction main effect,F(1,2)=1.95, NS). These findings suggest that gentamicin-sensitive signals from the ipsilateral vestibular apparatusare an important drive for the horizontal eye velocitymodulation component of guinea pig OVAR responses.

Bias component magnitude

The bias component of horizontal eye velocity wasalso affected in a dose-dependent manner by unilateralgentamicin instillation (Fig. 8A). Repeated measuresANOVA [between groups factor: dose (control, 20 mg/ml or 40 mg/ml); within groups factors: rotation direction(clockwise vs. counterclockwise) and speed (20, 40, 60,80 or 100 deg/s)] indicated that there were significantmain effects of dose (F(2,20)=4.92, P<0.05), rotationdirection (F(1,20)=9.27, P<0.01), and speed (F(4,80)=59.1,P<0.01). Two interactions were also significant: speed �

Fig. 7A–D Effects of gentami-cin instillation on the modula-tion component of OVARresponses. The modulation sen-sitivity of horizontal eye move-ments (deg/s/g along naso-occipital axis) is plotted as afunction of tilt table velocity. A,B Results from rotation in theclockwise direction (towardsthe lesion for unilaterally ab-lated animals). C, D Modula-tion sensitivity to counterclock-wise rotation (away from thelesion for unilaterally ablatedanimals). In compiling aver-ages, n=15 for the control ani-mals, while n=3 for the fourgentamicin-treated groups.Error bars are SEM

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dose (F(8,80)=3.32, P<0.01) and speed � rotation direction(F(4,80)=7.30, P<0.01).

Least-significant differences tests comparing the treat-ed groups and controls revealed that these effects weredue to a marked decrement in the horizontal eye velocitybias responses of the group given 40mg/ml gentamicinunilaterally to rotation toward the gentamicin-treated ear(clockwise rotation). These tests for dose-related effectsat individual stimulus speeds revealed that the 40-mg/mldose produced a significant decrement (P<0.01) fromcontrols during 40-, 60-, 80- and 100-deg/s clockwisestimulation. This decrement was so profound that itabolished stimulus speed-related differences in the biascomponent (one-way repeated measures ANOVA for thegroup instilled unilaterally with 40 mg/ml gentamicin,

factor: speed, F(4,8)=1.15, NS) and produced the speed �dose and speed � rotation direction interaction effects inthe main ANOVA described above. Hence, the effect ofgentamicin on bias values for rotation towards the treatedear was greatly depressed. However, for counterclockwisestimulation, only the decrement in the bias response to100-deg/s stimulation approached significance (P=0.056),and the relationship to stimulus speed was preserved(one-way repeated measures ANOVA, factor: speed,F(4,8)=5.79, P<0.05). Although there was a slight decre-ment in the bias component response magnitude duringclockwise rotation in the group receiving 20 mg/mlgentamicin, it was not statistically significant at any speedof rotation. These data indicate that there is a stronglateralization of inner ear input for the bias component ofOVAR in guinea pigs. During clockwise rotation, the biascomponent of the right eye appears to reflect primarilyinput from the ipsilateral vestibular apparatus that issusceptible to gentamicin intoxication. During leftward(counterclockwise) rotation, though, the bias componentof the right eye appears to be driven primarily by the left(intact) vestibular apparatus.

Effects of bilateral gentamicin instillationon OVAR responses

Figure 6C, D shows examples from a bilaterally ablatedanimal (40 mg/ml). Figure 6C shows the horizontal eyevelocity record from the right eye during horizontalOVAR rotation (80 deg/s) in the direction of theipsilateral ear. Figure 6D shows the horizontal eyevelocity record for the right eye during contralateral(counterclockwise) rotation. Quick phases, a modulationcomponent, and a slight bias component can be seen forboth directions of rotation. For this example, the modu-lation sensitivity was greater in magnitude for contralat-eral rotation, while the bias velocity had a greatermagnitude for ipsilateral rotation. The ipsilaterally(rightward) beating horizontal component of nystagmusshown in the eye velocity tracing is a common charac-teristic seen during ipsilateral (clockwise) rotation in thecontrols and bilaterally ablated animals. During contra-lateral (counterclockwise) rotation, the quick phases weredirected contralaterally (leftward).

Bilateral instillation of gentamicin had effects similarto unilateral instillation on both the horizontal eyevelocity modulation sensitivity (Fig. 7B, D) and themagnitude of the bias velocity (Fig. 8B). Repeatedmeasures ANOVA comparisons between the controlsand each bilateral treatment group showed significantreduction in horizontal eye velocity modulation sensitiv-ity for all four comparisons [20-mg/ml group:F(1,18)=4.94, P<0.05 for ipsilateral (clockwise) rotationand F(1,18)=6.13, P<0.05 for contralateral (counterclock-wise) rotation; 40-mg/ml group: F(1,18)=16.72, P=0.01 foripsilateral (clockwise) rotation and F(1,18)=15.17, P<0.01for contralateral (counterclockwise) rotation]. For thehorizontal bias eye velocity, three of the four comparisons

Fig. 8A, B The effects of gentamicin instillation on the bias eyevelocity (degrees per second) for OVAR trials. Negative velocityvalues denote leftward rotation. A Results from unilateral instilla-tion of gentamicin. B Results from bilateral instillation ofgentamicin. The data are displayed for control guinea pigs andguinea pigs treated unilaterally or bilaterally with 40 mg/mlgentamicin. The data from the group given 20 mg/ml bilaterally(not shown) did not differ from the 40-mg/ml group. For the controlgroup, n=15. For each of the treatment groups, n=3. Error bars areSEM

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showed significant decrements in the bilateral treatmentgroup relative to controls [20-mg/ml group: F(1,18)=6.94,P<0.05 for ipsilateral (clockwise) rotation andF(1,18)=6.65, P<0.05 for contralateral (counterclockwise)rotation; 40-mg/ml group: F(1,18)=7.84, P<0.05 for con-tralateral (counterclockwise) rotation], but the bias ve-locity did not differ from controls during ipsilateral(clockwise) rotation in the 40-mg/ml group ( F(1,18)=2.87,P=0.108). Neither the horizontal eye velocity modulationsensitivity (F(1,4)=7.418, P=0.053 for clockwise rotationand F(1,4)=2.16, P>0.2 for counterclockwise rotation) northe horizontal bias eye velocity (F(1,4)=0.45, P>0.5 forclockwise rotation and F(1,4))=0.12, P>0.7 for counter-clockwise rotation) differed significantly between the 20-and 40-mg/ml bilateral groups.

Comparison between effects of unilateraland bilateral gentamicin instillation on OVAR responses

Separate repeated measures ANOVAs were used tocompare the responses between the unilateral and bilat-eral instillation groups [between groups factor: laterality(unilateral vs. bilateral); within groups factor: speed ofrotation] for the horizontal eye velocity responses duringipsilaterally (clockwise) and contralaterally (counter-clockwise) directed OVAR. Separate analyses wereperformed for the 20- and 40-mg/ml per ear doses. Withthe exception of significant main effects of speed (i.e., thehorizontal eye velocity modulation sensitivity and biaseye velocity changed with stimulus speed), there were nosignificant main or interaction effects for horizontal eyevelocity responses during ipsilateral (clockwise) or con-tralateral (counterclockwise) OVAR. The only interactioneffect that approached significance was the speed �direction of rotation interaction for the modulationsensitivity of responses to counterclockwise rotation ofthe 40-mg/ml groups (F(4,16)=2.72, P=0.067). This mar-ginal interaction effect reflected a significant differencebetween horizontal eye velocity modulation sensitivitybetween the bilateral and unilateral instillation groupsduring contralaterally directed (counterclockwise) 60-deg/s OVAR (for LSD test comparisons, bilateral vs.unilateral vs. control, P<0.05) and a marginal difference(LSD test, P=0.16) in horizontal eye velocity modulationsensitivity during 80-deg/s contralateral OVAR.

Correlation among effects of gentamicin instillationon HVOR and OVAR responses

Since gentamicin instillation produced directional re-sponse deficits in both HVOR gain and OVAR horizontaleye velocity modulation and bias responses, correlationsamong the responses were examined to determine thedegree of dependence between the observed performancedecrements. Pair-wise correlations (with Bonferroni-cor-rected P values) were tested among the directional HVORgain at 2-Hz yaw rotation, OVAR modulation sensitivity

at 100 deg/s and OVAR bias magnitude for unilaterallyablated animals. Three correlations were highly signifi-cant across the control and gentamicin-treated groups. Astrong correlation between the horizontal right eyevelocity modulation sensitivity and bias magnitude duringipsilateral (clockwise) OVAR (r=�0.70, Bonferroni-cor-rected P<0.001; negative correlation convention of des-ignating leftward as the negative direction) indicated thatduring OVAR toward an injected ear, gentamicin-sensi-tive components in the ipsilateral ear make proportionalcontributions to the horizontal eye velocity modulationand bias responses. In addition, there were strong positivecorrelations between (1) the contralaterally directed(counterclockwise) OVAR horizontal eye velocity biasresponse magnitude and the HVOR gain during contra-lateral (counterclockwise rotation (r=0.68, Bonferroni-corrected P<0.005) and (2) the horizontal eye velocitybias response magnitude (of the right eye) duringipsilaterally directed (clockwise) OVAR and the HVORgain of the right eye during contralateral (clockwise)rotation (r=�0.67, Bonferroni-corrected P<0.005). Thesecorrelations are consistent with the parallel dependence ofboth the horizontal eye velocity bias responses and theipsilaterally directed HVOR gain on the functional statusof the ipsilateral ear. The lack of a significant correlationbetween the ipsilaterally directed (clockwise) horizontaleye velocity modulation sensitivity and the HVOR gainduring either ipsilateral (clockwise) rotation (r=0.53,Bonferroni-corrected P>0.1) or contralateral (counter-clockwise) rotation (r=0.48, Bonferroni-corrected P>0.2)is consistent with the independence of horizontal eyevelocity modulation sensitivity bias responses duringOVAR from HVOR gain.

Discussion

Overview

This study revealed that unilateral trans-bullar gentamicininstillation produces almost exclusively unidirectionaldeficits in HVOR responses and horizontal eye velocitymodulation and bias responses during OVAR. The HVORgain was reduced during rotation toward the injected earin dose-dependent manner for frequencies of 1 Hz andlower, but there was no effect on responses duringrotation toward the intact ear. Further, the horizontal eyevelocity modulation and bias responses during OVARwere reduced profoundly in a dose-dependent mannerduring rotation toward the treated ear. There was only aminor reduction at two OVAR speeds in horizontal eyevelocity modulation responses to rotation toward theintact ear in the high dose group and no effect on biasresponses during rotation toward the intact ear. Bilateralinstillation, on the other hand, produced bidirectionaldeficits in these responses. However, the significantresidual HVOR and horizontal eye velocity OVARresponses in the bilaterally-treated animals indicate thata component of vestibular function was resistant to the

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high dose of gentamicin. Three factors are germane to theinterpretation of these findings. First, the responses willindicate the degree of lateralization of the effects ofgentamicin to an injected ear. Second, they will reflect thecellular targets of gentamicin in the inner ear. Finally,they will indicate the contribution of these cellular targetsto central pathways that mediate angular and linearvestibulo-ocular reflexes. The unidirectional effects afterunilateral instillation suggest both a confinement ofeffects to the injected ear and directionally selectiveconnections in both angular and linear vestibulo-ocularreflex pathways in guinea pigs. The lack of a significantHVOR deficit at 2 Hz in unilaterally injected animals andthe residual HVOR and OVAR modulation responses inthe bilaterally injected animals, on the other hand, reflectperipheral vestibular activation that is insensitive togentamicin intoxication. Hence, these findings can beattributed to selective cytotoxic and/or physiologic effectsof gentamicin intoxication in the inner ear, or possibly,the vestibular nerve and central nervous system.

Effects of trans-bullar gentamicin

The limited data from previous studies suggest that trans-bullar instillation of gentamicin produces strictly ipsilat-eral cytotoxic effects within the inner ear of guinea pig(Bareggi et al. 1990; Sera et al. 1987; Forge et al. 1998).Both the low (Sera et al. 1987) and high doses (Bareggi etal. 1990; Harada et al. 1991) damaged sensory and non-sensory cells in the vestibular neuroepithelia within 3–4days, with signs of vestibular nerve Schwann cell damageand damaged organelles in ganglion cells at longer timepoints. The same structures in the contralateral ear werereported to be normal. Other studies raise the possibilitythat the direct effects of local gentamicin application mayextend centrally. For example, Rosen et al. (1998)reported increased astrocytic glial fibrillary acidic ex-pression and decreased cytochrome oxidase activity in thecochlear nucleus of guinea pigs given intratympanic bolusinjections of gentamicin. It is well known that perilymphis confluent with cerebrospinal fluid in the subarachnoidspace. Stover et al. (2000) presented convincing evidencethat adenoviral injections into one cochlea result intransfection of cells in the brain stem and the contralateralcochlea within 6 h. This effect was dependent on theinjection volume, and cochlear infection was also pro-duced by intrathecal adenovirus administration. Since theeffect is too rapid for transneuronal transport, thesefindings show that there is a potential perilymph-cerebrospinal fluid route for diffusion of drugs that enterthe perilymph through the round window, such as withtrans-bullar injected aminoglycosides. However, it seemsmost likely that the observed changes in HVOR andOVAR responses primarily reflect direct effects in theinjected ear and/or vestibular nerve.

One remarkable finding in recent years is that mam-malian vestibular hair cell numbers seem to recover fromtoxic effects of gentamicin (Forge et al. 1998; ; Warchol

et al. 1993). As a result, the magnitude and cellularappearance of the “lesion” will vary dynamically withtime after aminoglycoside administration. Forge et al.(1998) reported that the numbers of utricular hair cellbundles decreased by approximately 60% in the striolarregion 1 week after topical gentamicin, but reported onlya 30% reduction at 12 weeks after administration. Aftersystemic gentamicin instillation, a large, persistent re-duction in Type I hair cells was recorded in the striola ofthe utricle (>85% reduction at 4 weeks, and approxi-mately 70% reduction from control values at 12 weeks).Type I hair cells showed limited recovery at 12 and 33weeks post-gentamicin instillation. However, the numberof Type II hair cells increased significantly (>50%) fromcontrol counts. These limited findings suggest thatvestibular nerve signals driving residual responses of atreated ear are derived predominantly from hair cells witha Type II morphology. The gentamicin-insensitive com-ponents of HVOR and OVAR responses, then, willrepresent the net result of central compensation utilizingthese signals and information from the intact ear.Gentamicin-sensitive components of responses, though,are likely to reflect an inability of central mechanisms tocompensate for disruption of ganglion cells and both haircell types in treated ears.

HVOR responses

The responses of the control group to horizontal angularrotation were consistent with previous studies in guineapigs (Pettorossi et al. 1986; Escudero et al. 1993;Andrews et al. 1997). As reported by others (Andrewset al. 1997; Gilchrist et al. 1998), the gains weresymmetric for leftward and rightward rotation. Unilateralgentamicin instillation produced a dose-dependent deficitin HVOR gain during rotation toward the damaged ear.The results of the 40-mg/ml unilateral dose were similarto the results of surgical unilateral labyrinthectomy afterlong-term (>35 day) recovery (Vibert et al. 1993).Bilateral gentamicin instillation also produced a dose-dependent gain reduction, which, as expected, wassymmetric. The HVOR gain in the animals given40 mg/ml gentamicin bilaterally, including the residualhigh frequency gain, was remarkably similar to the effectsof an 18-day systemic course (100 mg, twice daily) ofgentamicin (Pettorossi et al. 1986). It is suggested that theresidual high frequency gain is mediated by both centralcompensation and vestibular signals from spared Type IIhair cells in bilateral gentamicin-treated animals (seediscussion above).

It is of interest to note that the gain decrement in the40-mg/ml bilateral injection group (relative to controls)showed a profile similar to the transfer function of guineapig irregular afferents (Curthoys 1982). However, thissimilarity is deceptive. Unlike the effects of gentamicin,galvanic inactivation of irregular afferents does not affectHVOR responses to sinusoidal oscillation in squirrelmonkeys (Minor and Goldberg 1991; Angelaki and

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Perachio 1993). Thus, the effects of gentamicin instilla-tion on the HVOR cannot be attributed purely todifferential ablation of Type I hair cells and resultingchanges in responses of irregular afferents. Rather, theresults must reflect limitations of compensation in theabsence of Type I hair cell function.

Response to OVAR

Normal responses

The modulation and bias components of responses ofcontrol guinea pigs during OVAR stimulation werestrongly dependent upon the direction of rotation. Themodulation sensitivity was greater for rotation toward therecorded eye, while the bias magnitude was greater forrotation in the opposite direction. Similar horizontal eyevelocity responses have been reported during constantvelocity OVAR stimulation of pigmented rats by Hessand Dieringer (1990) and Dutch belted rabbits by Marutaet al. (2001). Hess and Dieringer (1991) later assumedthat the dependence of the responses on rotation directionis a central mechanism to account for expected changes inviewing distance due to head translation. By contrast,ocular responses during OVAR are conjugate in humans(J.M. Furman, personal communication).

Effects of gentamicin on modulation sensitivity

The modulation component responses of guinea pig eyemovements during OVAR are normally asymmetric, suchthat each eye shows a larger response during ipsilateralrotation. The effects of unilateral gentamicin instillationsuggest that this directional asymmetry is produced byextraocular muscle activity that reflects a predominantinput from the ipsilateral ear during ipsilateral rotation.Unilateral instillation of either dose of gentamicin elim-inated the directional asymmetry in modulation responsesensitivity due to the effects of the recorded eye. Despite aprofound reduction in modulation component sensitivityto ipsilateral rotation, the higher dose (40 mg/ml)produced only a minor effect on responses to contralateralrotation. These results suggest that the OVAR horizontaleye velocity modulation sensitivity of the “leading eye”reflects at least three components: (1) gentamicin-sensi-tive utricular and saccular signals from the ipsilateral(“leading”) ear, (2) gentamicin-sensitive utricular andsaccular signals from the contralateral (“trailing”) ear and(3) gentamicin-insensitive utricular and saccular signals.The results of bilateral instillation of the high gentamicindose indicate that approximately 60% of this responsedepends upon gentamicin-sensitive afferents. Based onhistopathologic results reviewed above, the residualresponse likely reflects almost exclusively input fromsurviving Type II hair cells and unaffected ganglion cells,which probably constitute a subset of dimorphic andbouton-type vestibular afferents. The decrements in

function, conversely, likely represents the loss of Type Ihair cell-calyx complexes. Thus, these findings areconsistent with results of galvanic stimulation (Angelakiet al. 1992), which suggest that OVAR horizontal eyevelocity modulation responses are highly dependent onintact inputs from irregular afferents.

Effects of gentamicin on the bias response

The effects of gentamicin on OVAR horizontal eyevelocity bias responses show two differences from theeffects on modulation responses. First, the bias responsesappear to have a different absolute sensitivity to gentam-icin. Unlike the modulation component, there were nosignificant effects of unilateral or bilateral 20-mg/mldoses on bias responses; significant effects emerged onlyat the 40-mg/ml dose level. The second major differenceis that the bias response of an eye during ipsilateralrotation, like the HVOR, is dependent predominantly (ifnot completely) upon gentamicin-sensitive vestibularmechanisms in the ipsilateral (“leading”) ear. The factorsresponsible for these differences in responses, particularlythe dose-dependence, are unclear, but they may rangefrom differential sensitivity of afferent populations togentamicin intoxication to differences in central process-ing for controlling the modulation and bias componentsduring OVAR.

In conclusion, this study provides the first documen-tation of specific effects of unilateral gentamicin-intox-ication on eye movement responses during OVARstimulation. It is important to acknowledge that theseeffects were assessed on only the horizontal eye velocitycomponent of the responses of the eye ipsilateral to theinjected ear. It is also clear that these findings haveimplications that require further testing with binocular,three-dimensional eye movement recording techniques.For example, Hess and Dieringer (1990) demonstratedthat the differences between the horizontal eye velocitymodulation sensitivities of the “leading” and “trailing”eyes resulted in a cyclic modulation of the horizontalvergence angle of rats during OVAR stimulation. Ourmonocular recording data predict a similar vergencemodulation in normal guinea pigs and that the vergencemodulation is likely to be affected profoundly (and,possibly eliminated) after unilateral gentamicin instilla-tion. However, our data indicate that monocular record-ings of the horizontal slow phase velocity of nystagmusduring OVAR provide a simple metric for verifyingeffects of unilateral gentamicin infusion in guinea pigs.

Acknowledgements The authors wish to thank Robert H. Schor,Ph.D., for computer programming assistance and helpful commentson an earlier version of the manuscript. The Office of NavalResearch supported this research with ongoing funding of thisproject.

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