Glial cell changes in the central auditory system through the lifespan of rhesus macaques – a potential mechanism for inflammation in the progression of age-related hearing loss

Age-related hearing loss (ARHL) is a common condition, affecting half of adults over the age of 75. It has a large impact on quality of life, contributing to social isolation and depression. Despite ARHL being common, it is not universal, suggesting that its progression may be amenable to modulation or therapy - this would allow a level of hearing adequate for social interaction to be maintained for longer. Pathology in the cochlea is the main focus of current presbycusis (ARHL) research. Due to a loss of input from the cochlea, there is also degeneration of the central auditory system, which causes auditory processing to be impaired. An association between raised inflammatory status and decreased hearing ability has been shown in an aging human population. It is feasible that systemic inflammation could be contributing to hearing loss by increasing degeneration of the central auditory pathway. During aging there are changes to the immune system, including a tendency towards chronic low grade inflammation.The link between systemic inflammation and neurodegenerative diseases is well established. Given that there is also neurodegeneration occurring in ARHL, it is possible that hearing loss progression could be intensified by systemic inflammation via interaction with the glia. Perineuronal nets (PNN) are specialised extracellular matrix structures and have an important role in the auditory system as they aid intregration of sound inputs. Therefore, alongside glial changes, an alteration in PNN structure could contribute to hearing impairment. The rhesus macaque is an important model in understanding a possible role for inflammation in ARHL: similar to humans, non-human primates have an unusually long lifespan compared to mammals of a similar size, they develop chronic inflammation with age and they variably develop ARHL