Abstract:
According to the World Health Organisation, there are 360 million people worldwide (over 5% of the world’s population) with disabling hearing loss. In New Zealand, up to 25 % of the burden is generated from excessive noise exposure in occupational and leisure settings. Although oxidative stress has been postulated as the key mechanism of noise-induced hearing loss, emerging evidence suggests that cochlear inflammation may also be a major contributor. This thesis was undertaken to improve our understanding of the underlying mechanisms and dynamics of the noise-induced cochlear inflammatory response. To achieve this, the cochlear inflammatory response induced by exposure to acute and chronic noise was investigated in a mouse model. C57BL/6 mice were exposed to acute traumatic noise (100 dB SPL, 8-16 kHz for 24 h) and their cochleae collected at various intervals thereafter, up to 7 days. Using quantitative real-time RT-PCR and immunohistochemistry, changes in the expression levels of proinflammatory cytokines (TNF-α, IL-1β), chemokines (CCL2) and cell adhesion molecules (ICAM-1, PECAM-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. Also demonstrated was increased ICAM-1 immunoexpression in the inferior region of the spiral ligament, peaking 24 h post-exposure, and increased PECAM-1 immunoexpression in the cochlear vasculature, peaking 1-3 days post-exposure. Concomitant with the upregulation of these inflammatory mediators was the recruitment of adenosine A2A receptor (A2AR)-positive infiltrating cells into the cochlea, peaking 24 h post-exposure. It is speculated that inflammatory cells, recruited to the noise-exposed cochlea by cytokines/chemokines and cell adhesion molecules to clear cellular debris, may also cause significant bystander tissue injury, thus exacerbating the noise-induced damage. The occurrence of the latter peak in expression is not clear, but it is postulated that it may be associated with reparative processes. Repeated exposure to moderate noise levels over an extended period of time may also result in cochlear injury leading to permanent hearing loss, but previous studies have not investigated inflammatory processes during chronic noise exposure. Here, it was demonstrated that chronic exposure to moderate noise (90 dB SPL, 8-16 kHz, 2 h/day) also induced an inflammatory response, peaking after 2 weeks, and subsiding thereafter. This suggests that inflammatory processes may contribute to cochlear injury with chronic noise exposure even at moderate sound levels. The final study investigated the protective role of A2AR signalling in noise-induced cochlear inflammation.Adenosine is a ubiquitous signalling molecule that shows strong anti-inflammatory effects via the A2AR. The A2AR, which was mainly immunolocalised in the cochlear vasculature, showed increased immunoexpression in the inferior region of the spiral ligament following noise exposure, suggesting an endogenous protective mechanism to limit inflammation. Using A2ARKO-/- mice, it was shown that deletion of the A2AR gene differentially affected specific aspects of the noise-induced inflammatory response in the cochlea. Furthermore, it was successfully demonstrated that post-exposure treatment with the selective A2AR agonist, regadenoson, suppressed cochlear inflammation, reducing ICAM-1 immunoexpression and cellular infiltration. This thesis has thus provided fundamental novel insights into the cochlear inflammatory response following acute and chronic noise exposure, and also revealed an important role of A2AR signalling in controlling noise-induced cochlear inflammation.