Expression and Distribution of Tumor Necrosis Factor-Alpha in Mice Cochlea Exposed to Noise

Intracochlear Perfusion of Tumor Necrosis Factor-Alpha Induces Sensorineural Hearing Loss and Synaptic Degeneration in Guinea Pigs

Tumor necrosis factor-alpha (TNF-α) is a proinflammatory cytokine that plays a prominent role in the nervous system, mediating a range of physiologic and pathologic functions. In the auditory system, elevated levels of TNF-α have been implicated in several types of sensorineural hearing loss, including sensorineural hearing loss induced by vestibular schwannoma, a potentially fatal intracranial tumor that originates from the eighth cranial nerve; however, the mechanisms underlying the tumor's deleterious effects on hearing are not well-understood. Here, we investigated the effect of acute elevations of TNF-α in the inner ear on cochlear function and morphology by perfusing the cochlea with TNF-α in vivo in guinea pigs. TNF-α perfusion did not significantly change thresholds for compound action potential (CAP) responses, which reflect cochlear nerve activity, or distortion product otoacoustic emissions, which reflect outer hair cell integrity. However, intracochlear TNF-α perfusion reduced CAP amplitudes and increased the number of inner hair cell synapses without paired post-synaptic terminals, suggesting a pattern of synaptic degeneration that resembles that observed in primary cochlear neuropathy. Additionally, etanercept, a TNF-α blocker, protected against TNF-α-induced synaptopathy when administered systemically prior to intracochlear TNF-α perfusion. Findings motivate further investigation into the harmful effects of chronically elevated intracochlear levels of TNF-α, and the potential for etanercept to counter these effects.

Characterisation of cochlear inflammation in mice following acute and chronic noise exposure

Oxidative stress has been established as the key mechanism of the cochlear damage underlying noise-induced hearing loss, however, emerging evidence suggests that cochlear inflammation may also be a major contributor. This study aimed to improve our understanding of the cochlear inflammatory response associated with acute and chronic noise exposure. C57BL/6 mice were exposed to acute traumatic noise (100 dBSPL, 8-16 kHz for 24 h) and their cochleae collected at various intervals thereafter, up to 7 days. Using quantitative RT-PCR and immunohistochemistry, changes in expression levels of proinflammatory cytokines (TNF-α, IL-1β), chemokines (CCL2) and cell adhesion molecules (ICAM-1) were studied. All gene transcripts displayed similar dynamics of expression, with an early upregulation at 6 h post-exposure, followed by a second peak at 7 days. ICAM-1 immunoexpression increased significantly in the inferior region of the spiral ligament, peaking 24 h post-exposure. The early expression of proinflammatory mediators likely mediates the recruitment and extravasation of inflammatory cells into the noise-exposed cochlea. The occurrence of the latter expression peak is not clear, but it may be associated with reparative processes initiated in response to cochlear damage. Chronic exposure to moderate noise (90 dBSPL, 8-16 kHz, 2 h/day, up to 4 weeks) also elicited an inflammatory response, reaching a maximum after 2 weeks, suggesting that cochlear damage and hearing loss associated with chronic environmental noise exposure may be linked to inflammatory processes in the cochlea. This study thus provides further insight into the dynamics of the cochlear inflammatory response induced by exposure to acute and chronic noise.

Noise-induced cochlear inflammation

Hearing loss is the most common sensory disability with considerable social and economic implications. According to recent World Health Organization estimates, 360 million people worldwide suffer from moderate to profound hearing loss. Exposure to excessive noise is one of the major causes of sensorineural hearing loss, secondary only to age-related hearing loss (presbyacusis). Since cochlear tissues have limited abilities of repair and regeneration, this damage can be irreversible, leading to cochlear dysfunction and permanent hearing loss. Recent studies have shown that cochlear inflammation can be induced by noise exposure and contribute to the overall pathogenesis of cochlear injury and hearing loss. The cochlea is separated from the systemic circulation by the blood-labyrinth barrier, which is physiologically similar to the blood-brain barrier of the central nervous system. Because of this feature, the cochlea was originally considered an immunologically privileged organ. However, this postulate has been challenged by the evidence of an inflammatory response in the cochlea in the presence of bacterial or viral pathogens or antigens that can cause labyrinthitis. Although the main purpose of the inflammatory reaction is to protect against invading pathogens, the inflammatory response can also cause significant bystander injury to the delicate structures of the cochlea. The cochlear inflammatory response is characterised by the generation of proinflammatory mediators (cytokines, chemokines and adhesion molecules), and the recruitment of inflammatory cells (leukocytes). Here, we present an overview of the current research on cochlear inflammation, with particular emphasis on noise-induced cochlear inflammation. We also discuss treatment strategies aimed at the suppression of inflammation, which may potentially lead to mitigation of hearing loss.