Transient alteration of the vestibular calyceal junction and synapse in response to chronic ototoxic insult in rats.

Abstract

Ototoxicity is known to cause permanent loss of vestibule function through degeneration of sensory hair cells (HCs). However, functional recovery has been reported during washout after chronic ototoxicity, although the mechanisms underlying this reversible dysfunction are unknown. Here, we study this question in rats chronically exposed to the ototoxic compound 3,3′-iminodipropionitrile (IDPN). Pronounced alterations in vestibular function appeared before significant loss of HCs or stereociliary coalescence became evident by ultrastructural analyses. This early dysfunction was fully reversible if the exposure was terminated promptly. In cristae and utricles, the distinct junctions formed between type I HCs (HCI) and calyx endings were completely dismantled at these early stages of reversible dysfunction, and completely rebuilt during washout. Immunohistochemical observations revealed loss and recovery of the junction proteins CASPR1 and tenascin-C and RT-PCR indicated that their loss was not due to decreased gene expression. KCNQ4 was mislocalized during intoxication and recovered control-like localization after washout. At early stages of the intoxication, the calyces could be classified as showing intact or lost junctions, indicating that calyceal junction dismantlement is triggered on a calyx-by-calyx basis. Chronic toxicity also altered the presence of ribeye, PSD-95 and GluA2 puncta in the calyces. These synaptic alterations varied between the two types of calyx endings (formed by calyx-only or dimorphic afferents) and some persisted at the end of the washout period. The present data reveal new forms of plasticity of the calyx endings in adult mammals, including a robust capacity for rebuilding the calyceal junction. These findings contribute to a better understanding of the phenomena involved in progressive vestibular dysfunction and its potential recovery during and after ototoxic exposure.

Quantitative assessment of anti-gravity reflexes to evaluate vestibular dysfunction in rats

“…Acute and chronic exposure of male adult Long-Evans rats to IDPN caused effects on body weight matching the data reported previously (Llorens et al, 1993;Llorens and Rodríguez-Farré, 1997;Sedó-Cabezón et al, 2015). Acute exposure to 400 mg/kg IDPN caused no significant effects on body weight, while exposure to 600 or 1000 mg/kg caused a decrease in body weight that progressed for 4-5 days, after which animals resumed body weight gain at a similar rate as control animals.…”

“…The loss of vestibular function was assessed by three means. First, a measure was obtained using a semi-quantitative behavioral test battery that has been proven to be sensitive and specific to vestibular function, as reported previously (Llorens et al, 1993;Llorens and Rodríguez-Farré, 1997;Boadas-Vaello et al, 2005;Sedó-Cabezón et al, 2015). In this battery, 6 items are rated from 0 (normal behavior) to 4 (highest score of behavioral deficiency) to obtain a sum score of 0 to 24 (Vestibular Dysfunction Rating, VDR).…”

“…Precise recordings of spontaneous motor behavior to robustly assess vestibular function have been reported, but they also require the surgical fixation of a device in the skull of the rat (Pasquet et al, 2016). For more than 25 years, our laboratory has been using a behavioral test battery to obtain a semiquantitative evaluation of vestibular dysfunction in rats and mice exposed to ototoxic chemicals or other vestibular-damaging conditions (Llorens et al, 1993;Llorens and Rodríguez-Farré., 1997;Boadas-Vaello et al, 2005;Soler-Martín et al, 2007;Saldaña-Ruíz et al, 2013;Sedó-Cabezón et al, 2015). This test battery has proven to be reliable, robust and specific to vestibular dysfunction.…”

Correlation between afferent rearrangements and behavioral deficits after local excitotoxic insult in the mammalian vestibule: an animal model of vertigo symptoms?

“…Vestibular hair cell loss is a typical pathophysiology induced by aminoglycosides, such as gentamicin, or nitriles such as 3,3′-iminodipropionitrle (IDPN). Recent studies have demonstrated that in addition to hair cell impairment, the calyx innervation of type I hair cells is also altered by gentamicin or IDPN administration, suggesting that excitotoxicity and damage to the afferents might also be involved in the pathophysiology of drug-induced ototoxicity (Hirvonen et al, 2005; Sedo-Cabezon et al, 2015). Finally, the sole correlation between structural excitotoxic damages within the vestibule and altered vestibular behavior has rarely been studied.…”

Development and regeneration of vestibular hair cells in mammals

“…Hair cells appear to be one of the more susceptible vestibular cell types; however, loss of vestibular ganglion neurons and damage to supporting cells has also been observed [52, 96, 97]. For example, chronic exposure to 3,3′-iminodipropionitrile can lead to transient disassembly of calyceal synapses in the absence of any overt hair cell or ganglion damage [98]. Finally, several studies have found differential susceptibility of hair cell subtypes – Type I versus Type II, macular versus ampullary, and central versus peripheral – to various ototoxins [4, 49, 99–104].…”