Abstract
Epidemiological evidence has shown that smoking is associated with an increased risk of hearing loss. However, the underlying mechanisms regarding the impact of nicotine on the cochlea remain unclear. This study aimed to investigate the cytotoxic effects of nicotine on cochlear cells using cultured cochlear basilar membranes. Cochlear basilar membranes were isolated from newborn rats, cultured, and treated with 1–100 ng/mL nicotine for 48 h. Cuticular plates and stereocilia bundle staining were used to evaluate hair cell (HC) loss. Spiral ganglion neuron and acoustic nerve fiber staining were assessed to evaluate cochlear neural injury. Scanning electron microscopy and transmission electron microscopy imaging were employed to examine cochlear ultrastructural changes. Our results showed that compared to spiral ganglia and nerve fibers, HCs are more susceptible to nicotine-induced toxicity. HC loss was more severe in the basal turn than in the middle and apical turns, while nerve fibers and spiral ganglion cells were morphologically maintained. Ultrastructural changes revealed disordered and damaged stereocilia, swelling and decreased mitochondrial density, swelling, and degranulation of the endoplasmic reticulum. Our results suggest that nicotine causes HCs’ degeneration and loss and may have implications for smoking-related hearing loss.
Highlights
Epidemiological evidence has shown that smoking is associated with an increased risk of hearing loss [1–3]
When the nicotine concentration increased to 10 ng/mL, the stereocilial bundles of both the inner hair cells (IHCs) and outer hair cells (OHCs) collapsed, and loss of hair cell (HC) was evident at the basal and middle turns (Figure 2)
Statistics based on different cochlear segments revealed that HC loss was proportional to the logarithm of nicotine concentration, and the injury started at the base and developed to the apex (Figure 4a and b)
Summary
Epidemiological evidence has shown that smoking is associated with an increased risk of hearing loss [1–3]. Smoking has been reported to worsen auditory thresholds at high frequencies, lower response levels in transient evoked otoacoustic emissions, and increase the incidence of tinnitus [1,4,5], suggesting that smoking might be associated with cochlear injuries. A previous in vivo study demonstrated that nicotine administration caused damage to outer hair cells (OHCs), disorganization of stereocilia, and expansion of the surrounding supporting cells [9]. It has not yet been entirely established that nicotine undermines the cochlear HCs and the relevant afferent neural structure. This study aimed to investigate the characteristic injury patterns of cochlear HCs and the relevant afferent neural structure that is affected using nicotine in cochlear organotypic cultures
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