Several nitriles have been demonstrated to cause hair cell loss in the inner ear of the rat, but the susceptibility of other species to this toxic effect has not been investigated. Adult male Swiss mice were administered (po) control vehicle, cis-crotononitrile (2.75 mmol/kg), or 3,3'-iminodipropionitrile (IDPN, at 8, 16, and 24 mmol/kg), and the changes in vestibular function were assessed by behavioral endpoints. In addition, surface preparations of the vestibular sensory epithelia were examined for hair cell loss using scanning electron microscopy (SEM). IDPN, in a dose-dependent manner, and cis-crotononitrile induced both vestibular dysfunction and loss of hair bundles. Male Dunkin Hartley guinea pigs were administered IDPN (0, 1.6, 2.4, or 3.2 mmol/kg, ip), and their vestibular and auditory sensory epithelia were examined by SEM. The guinea pigs developed behavioral abnormalities indicative of vestibular dysfunction, with more overt effects observed in the animals treated with larger doses, and displayed a dose-dependent loss of hair bundles in both the vestibular and the auditory epithelia. Frogs (Rana perezi) were administered IDPN (0, 16, 24, or 32 mmol/kg, ip), and their sensory epithelia in the inner ear were examined by SEM. IDPN caused behavioral abnormalities indicative of vestibular dysfunction and loss of hair bundles. We conclude that some nitriles are thorough ototoxic compounds affecting hair cells in a wide range of species. This conclusion highlights the potential interest of this toxic effect and offers new animal models in which to decipher its basis.

Allylnitrile, cis-crotononitrile, and 3,3'-iminodipropionitrile are known to cause vestibular toxicity in rodents, and evidence is available indicating that cis-2-pentenenitrile shares this effect. We evaluated nineteen nitriles for vestibular toxicity in wild type (129S1) and CYP2E1-null mice, including all the above, several neurotoxic nitriles, and structurally similar nitriles. A new acute toxicity test protocol was developed to facilitate evaluation of the vestibular toxicity by a specific behavioral test battery at doses up to sub-lethal levels while using a limited number of animals. A mean number of 8.5+0.3 animals per nitrile, strain and sex was necessary to obtain evidence of vestibular toxicity and optionally an estimation of the lethal dose. For several but not all nitriles, lethal doses significantly increased in CYP2E1-null mice. The protocol revealed the vestibular toxicity of five nitriles, including previously identified ototoxic compounds and one nitrile (trans-crotononitrile) known to have a different profile of neurotoxic effects in the rat. In all five cases, both sexes were affected and no decrease in susceptibility was apparent in CYP2E1-null mice respect to 129S1 mice. Fourteen nitriles caused no vestibular toxicity, including six nitriles tested in CYP2E1-null mice at doses significantly larger than the maximal doses that can be tested in wild type animals. We conclude that only a subset of low molecular weight nitriles is toxic to the vestibular system, that species-dependent differences exist in this vestibular toxicity, and that CYP2E1-mediated metabolism is not involved in this effect of nitriles although it has a role in the acute lethality of some of these compounds.

Ototoxicity is a major cause of the loss of hearing and balance in humans. Ototoxic compounds include pharmaceuticals such as aminoglycoside antibiotics, anti-malarial drugs, loop diuretics and chemotherapeutic platinum agents, and industrial chemicals including several solvents and nitriles. Human and rodent data indicate that the main target of toxicity is hair cells (HCs), which are the mechanosensory cells responsible for sensory transduction in both the auditory and the vestibular system. Nevertheless, the compounds may also affect the auditory and vestibular ganglion neurons. Exposure to ototoxic compounds has been found to cause HC apoptosis, HC necrosis, and damage to the afferent terminals, of differing severity depending on the ototoxicity model. One major pathway frequently involved in HC apoptosis is the c-jun N-terminal kinase (JNK) signaling pathway activated by reactive oxygen species, but other apoptotic pathways can also play a role in ototoxicity. Moreover, little is known about the effects of chronic low-dose exposure. In rodent vestibular epithelia, extrusion of live HCs from the sensory epithelium may be the predominant form of cell demise during chronic ototoxicity. In addition, greater involvement of the afferent terminals may occur, particularly the calyx units contacting type I vestibular HCs. As glutamate is the neurotransmitter in this synapse, excitotoxic phenomena may participate in afferent and ganglion neuron damage. Better knowledge of the events that take place in chronic ototoxicity is of great interest, as it will increase understanding of the sensory loss associated with chronic exposure and aging.