Partial Aminoglycoside Lesions in Vestibular Epithelia Reveal Broad Sensory Dysfunction Associated with Modest Hair Cell Loss and Afferent Calyx Retraction

Abstract

Although the effects of aminoglycoside antibiotics on hair cells have been investigated for decades, their influences on the dendrites of primary afferent neurons have not been widely studied. This is undoubtedly due to the difficulty in disassociating pathology to dendritic processes from that resulting from loss of the presynaptic hair cell. This was overcome in the present investigation through development of a preparation using Chinchilla laniger that enabled direct perilymphatic infusion. Through this strategy we unmasked gentamicin’s potential effects on afferent calyces. The pathophysiology of the vestibular neuroepithelia after post-administration durations of 0.5 through 6 months was assessed using single-neuron electrophysiology, immunohistochemistry, and confocal microscopy. Hair cell densities within cristae central zones (0.5-, 1-, 2-, and 6-months) and utricle peri- and extrastriola (6-months) regions were determined, and damage to calretinin-immunoreactive calyces was quantified. Gentamicin-induced hair cell loss exhibited a profile that reflected elimination of a most-sensitive group by 0.5-months post-administration (18.2%), followed by loss of a second group (20.6%) over the subsequent 5.5 months. The total hair cell loss with this gentamicin dose (approximately 38.8%) was less than the estimated fraction of type I hair cells in the chinchilla’s crista central zone (approximately 60%), indicating that viable type I hair cells remained. Extensive lesions to afferent calyces were observed at 0.5-months, though stimulus-evoked modulation was intact at this post-administration time. Widespread compromise to calyx morphology and severe attenuation of stimulus-evoked afferent discharge modulation was found at 1 month post-administration, a condition that persisted in preparations examined through the 6-month post-administration interval. Spontaneous discharge was robust at all post-administration intervals. All calretinin-positive calyces had retracted at 2 and 6 months post-administration. We found no evidence of morphologic or physiologic recovery. These results indicate that gentamicin-induced partial lesions to vestibular epithelia include hair cell loss (ostensibly reflecting an apoptotic effect) that is far less extensive than the compromise to stimulus-evoked afferent discharge modulation and retraction of afferent calyces (reflecting non-apoptotic effects). Additionally, calyx retraction cannot be completely accounted for by loss of type I hair cells, supporting the possibility for direct action of gentamicin on the afferent dendrite.

Calyx junction dismantlement and synaptic uncoupling precede hair cell extrusion in the vestibular sensory epithelium during sub-chronic 3,3′-iminodipropionitrile ototoxicity in the mouse

“…These authors used low-dose intra-labyrinth injection of gentamicin in the chinchilla and found that persistent alterations in the calyx afferents occurred together with the continued presence of the corresponding HCs. As the model and methods used by Sultemeier and Hoffman (2017) differ greatly from the ones used here, it is unclear whether the present data and their data reveal similar or distinct phenomena.…”

“…Understanding these processes should allow us to better predict the reversibility of induced functional loss, to identify candidate targets for pharmacological blockade to stop the transition from reversible to irreversible damage, or for ameliorating or accelerating the recovery processes. Traditionally, most studies on ototoxicity have focused on models causing rapid HC degeneration (e.g., Taylor et al, 2008;Li et al, 2011), but the need to understand other consequences of ototoxicity is starting to be appreciated (Hirvonen et al, 2005;Sedó-Cabezón et al, 2014Gaboyard-Niay et al, 2016;Sultemeier and Hoffman, 2017).…”

“…Future studies using other chronic ototoxicity models are needed to differentiate between the effects that are common to all models and those that are particular to IDPN. Interestingly, recent data (Sultemeier and Hoffman, 2017) demonstrate that alterations in the afferents may occur in vestibular epithelia exposed to ototoxic agents at exposure regimes milder than those causing widespread HC loss. These authors used low-dose intra-labyrinth injection of gentamicin in the chinchilla and found that persistent alterations in the calyx afferents occurred together with the continued presence of the corresponding HCs.…”

Vestibular Injury After Low-Intensity Blast Exposure

“…Even though viable type I hair cells remained, animals 1 month post gentamicin administration had severe attenuation of stimulus-evoked afferent discharge modulation. However, spontaneous discharge rates were consistent throughout all testing intervals ( 44 ). Hair cell damage caused by frontal blast-wave exposure could be significant enough to cause the vestibular pathologies observed.…”

“…Since in mammals, little to no spontaneous hair cell regeneration occurs following loss, it is likely that the stereocilia damage observed following blast in the present findings would remain permanent ( 42 ). For reference, gentamicin-induced hair cell loss in the vestibular cristae through middle ear injections typically does not result in total type I hair cell loss ( 43 , 44 ). Even though viable type I hair cells remained, animals 1 month post gentamicin administration had severe attenuation of stimulus-evoked afferent discharge modulation.…”