Chronic proximal axonopathy in rats is associated with long-standing neurofilament depletion in neuromuscular junctions and behavioral deficits

Upregulation of Vesicular Glutamate Transporter 2 and STAT3 Activation in the Spinal Cord of Mice Receiving 3,3'-Iminodipropionitrile

Chronic administration of 3,3'-iminodipropionitrile (IDPN) causes axonal impairment. Although controversy still remains, it has been suggested that IDPN intoxication mimics the axonopathy of amyotrophic lateral sclerosis (ALS). Interestingly, recent studies including our own showed that signal transducer and activator of transcription 3 (STAT3) in spinal α-motoneurons was activated in both IDPN-treated mice and SOD1 ^G93A mice, a genetic model of familial ALS. Because activation of STAT3 occurs in response to various stimuli, such as axonal injury, ischemia, and excessive glutamate, here we focused on a potential link between phosphorylated STAT3 (pSTAT3, an active form) and vesicular glutamate transporter 2 (VGluT2, a regulator of glutamate storage and release) in IDPN-treated mice and SOD1 ^G93A mice. Impairment of axonal transport was confirmed by western blot analysis: the expression levels of phosphorylated neurofilament H were elevated in both models. As shown in SOD1 ^G93A mice, the expression frequencies of VGluT2 in synaptophysin-positive (SYP)⁺ presynaptic terminals around spinal α-motoneurons were significantly higher in IDPN-treated mice than in vehicle controls. The coverages of spinal α-motoneurons by VGluT2⁺ presynaptic terminals were more elevated around pSTAT3⁺ cells than around pSTAT3^- cells in IDPN-treated mice and SOD1 ^G93A mice. Considering that excessive glutamate is shown to be involved in axonal impairment and STAT3 activation, the present results suggest that IDPN-induced upregulation of VGluT2 may result in an increase in glutamate, which might cause axonopathy and induction of pSTAT3. The link between upregulation of VGluT2 and activation of STAT3 via glutamate may represent a common pathological feature of IDPN-treated mice and SOD1 ^G93A mice.

Dose-dependent cochlear and vestibular toxicity of trans-tympanic cisplatin in the rat

In vivo studies are needed to study cisplatin ototoxicity and to evaluate candidate protective treatments. Rats and mice are the preferred species for toxicological and pharmacological pre-clinical research, but systemic administration of cisplatin causes high morbidity in these species. We hypothesized that trans-tympanic administration of cisplatin would provide a good model for studying its auditory and vestibular toxicity in the rat. Cisplatin was administered by the trans-tympanic route in one ear (50μl, 0.5-2mg/ml) of rats of both sexes and two different strains. Cochlear toxicity was corroborated by histological means. Vestibular toxicity was demonstrated by behavioral and histological analysis. Cisplatin concentrations were assessed in inner ear after trans-tympanic and i.v. administration. In all experiments, no lethality and only scant body weight loss were recorded. Cisplatin caused dose-dependent cochlear toxicity, as demonstrated by hair cell counts in the apical and middle turns of the cochlea, and vestibular toxicity, as demonstrated by behavioral analysis and hair cell counts in utricles. High concentrations of cisplatin were found in the inner ear after trans-tympanic administration. In comparison, i.v. administration resulted in lower inner ear concentrations. We conclude that trans-tympanic administration provides an easy, reproducible and safe model to study the cochlear and vestibular toxicity of cisplatin in the rat. This route of exposure may be useful to address particular questions on cisplatin induced ototoxicity and to test candidate protective treatments.

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

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.

Summary: New forms of damage and repair have been identified in the vestibular sensory epithelium using a rat model of chronic ototoxicity and recovery that causes reversible vestibular dysfunction.