Simultaneous study of cochleotoxicity and vestibulotoxicity of 3,3’- iminodipropionitrile in rats through several experimental approaches

Many industrial chemicals and clinical compounds are toxic to the inner ear. Some preferentially target the cochlea or vestibular structures, whereas others are harmful to both. The reasons behind these distinct ototoxic profiles remain poorly understood. The lack of a clear structure-toxicity relationship means that the prediction of the ototoxic potential of new drugs or chemical compounds is challenging and that in vivo testing is necessary. Vestibular or cochlear toxicity can be readily assessed independently, but we lack a method to simultaneously evaluate both functional and histological impairments in the same animals. Here, we describe and test such a method using 3,3'-iminodipropionitrile (IDPN), a compound known to induce hair cell loss in both cochlear and vestibular epithelia in the inner ear of rodents. Female Long-Evans rats were treated with IDPN (0, 150, 200, or 300 mg/kg/day for three days, i.p.). Auditory function was assessed using distortion product otoacoustic emissions (DPOAEs), while vestibular function was evaluated by measuring post-rotatory nystagmus (PRN) and anti-gravity reflexes: the tail-lift and air-righting tests. These tests were conducted before, and four weeks after treatment. Inner ears were collected to count hair cells and to examine the cochlea, utricle, saccule, and cristae by scanning electron microscopy (SEM). Auditory and balance deficits, as well as histological damage in all epithelia, were observed from 200 mg/kg/day, with a strong correlation between functional impairments and histological findings. The method described provides a comprehensive and unbiased means to compare vestibular and cochlear toxicity.