An animal model of ocular vestibular-evoked myogenic potential in guinea pig

Institute of Toxicology, College of Medicine, National Taiwan University, Taipei, Taiwan.
Experimental Brain Research (Impact Factor: 2.04). 08/2010; 205(2):145-52. DOI: 10.1007/s00221-010-2346-8
Source: PubMed


This study aimed to establish an animal model of ocular vestibular-evoked myogenic potential (oVEMP) in guinea pigs. Ten healthy and 10 gentamicin-treated guinea pigs underwent oVEMP test using a hand-held bone-conducted vibrator placed on the animal's forehead. All 10 healthy animals exhibited bilateral oVEMPs at the stimulus intensity of 139 dB force level (FL), with a mean threshold and latencies of peak nI and pI of 130 +/- 4 dBFL, 3.17 +/- 0.37 ms and 4.72 +/- 0.38 ms, respectively. Similar to response rate, the nI-pI amplitude decreased markedly in magnitude as stimulus intensity decreased. Another 10 animals administered with gentamicin (2 mg) on the left ear 1 week after surgery had 100% clear oVEMPs beneath the left eye (ipsilateral to the lesion side), whereas oVEMPs were absent and reduced beneath the right eye (opposite to the lesion side) in 7 and 3 animals, respectively. Morphological study of animals with absent oVEMPs identified substantial damage to the hair cells of the utricular macula. Quantitative analysis revealed that histological density of intact hair cells of the utricular macula from control and lesion ears were 194 +/- 15 and 66 +/- 9 per 130 x 130 microm(2) field, respectively, showing a 68% reduction in the latter. Further, the stereocilia of the residual hair cells were either fused or deformed, and pointed outward randomly. In conclusion, this study establishes the animal model of oVEMP in guinea pigs using bone-conducted vibration stimuli, which sets the stage for investigating the pathophysiology of the utricular disorders.

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    ABSTRACT: The aim of the study was: (a) to test whether short duration (6 ms) 500 Hz bone-conducted vibration (BCV) of the skull in alert head free guinea pigs would elicit eye movements; (b) to test whether these eye movements were vestibular in origin; and (c) to determine whether they corresponded to human eye movements to such stimuli. In this way we sought to establish the guinea pig as an acceptable model for testing the mechanism of the effect BCV on the vestibulo-ocular reflex. Consistent short-latency stimulus-locked responses to BCV were observed. The magnitude of eye displacement was directly related to stimulus intensity as recorded by accelerometers cemented onto the animal's skull. The strongest and most consistent response component was intorsion of both eyes. In lateral-eyed animals intorsion is produced by the combined contraction of the inferior rectus and superior oblique muscles. In humans the same pair of muscles acts to cause depression of the eye. To test whether the movements were vestibular we selectively ablated the vestibular endorgans: 3 of the 8 animals underwent a bilateral intratympanic injection of gentamicin, an ototoxic aminoglycoside antibiotic, to ablate their vestibular receptors. After ablation there was an overall reduction in the magnitude of eye displacement, as well as a reduction in the effectiveness of the BCV stimulus to elicit eye movements. The animals' hearing, as measured by the threshold for auditory brainstem responses, remained unchanged after gentamicin, confirming that the cochlea was not affected. The reduced magnitude of responses after vestibular receptor ablation demonstrates that the eye-movement responses to BCV are probably caused by the stimulation of vestibular receptors, which in turn activate the vestibulo-ocular reflex.
    No preview · Article · Aug 2011 · Brain research bulletin
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    ABSTRACT: Extracellular single neuron recordings of primary vestibular neurons in Scarpa's ganglion in guinea pigs show that low-intensity 500 Hz bone-conducted vibration (BCV) or 500 Hz air-conducted sound (ACS) activate a high proportion of otolith irregular neurons from the utricular and saccular maculae but few semicircular canal neurons. In alert guinea pigs, and humans, 500 Hz BCV elicits otolith-evoked eye movements. In humans, it also elicits a myogenic potential on tensed sternocleidomastoid muscles. Although BCV and ACS activate both utricular and saccular maculae, it is possible to probe the functional status of these two sense organs separately because of their differential neural projections. Saccular neurons have a strong projection to neck muscles and a weak projection to the oculomotor system. Utricular afferents have a strong projection to eye muscles. So measuring oculomotor responses to ACS and BCV predominantly probes utricular function, while measuring neck muscle responses to these stimuli predominantly probes saccular function.
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    ABSTRACT: This study compared cervical and ocular vestibular-evoked myogenic potentials (cVEMPs and oVEMPs) between air-conducted sound (ACS) and bone-conducted vibration (BCV) modes to determine whether these two stimulation modes activate the same population of primary vestibular afferents. Fifteen healthy subjects underwent cVEMP and oVEMP tests using ACS stimuli at 127 dB pe SPL and BCV stimuli at 128 dB force level. The characteristic parameters of cVEMPs and oVEMPs were compared between ACS and BCV modes. The mean p13 and n23 latencies of ACS-cVEMPs were significantly longer than those of BCV-cVEMPs. Likewise, the mean nI and pI latencies for ACS-oVEMPs were also significantly longer than those for BCV-oVEMPs. There was no significant difference in the mean amplitude of cVEMPs between the ACS and BCV modes. However, comparing the oVEMP amplitude, a relationship: (Amplitude of BCV-oVEMP) = 2.3 x (Amplitude of ACS-oVEMP) was demonstrated. In conclusion, the population of primary vestibular afferents activated by ACS and BCV stimuli is similar for cVEMPs. In contrast with oVEMPs, BCV mode activates more number of primary vestibular afferents than ACS mode does. In interpreting oVEMP and cVEMP results, stimulation mode should be checked first.
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