Abstract
Age-related hearing loss (ARHL) is a public health problem that has been associated with negative health outcomes ranging from increased frailty to an elevated risk of developing dementia. Significant gaps remain in our knowledge of the underlying central neural mechanisms, especially those related to the efferent auditory pathways. Thus, the aim of this study was to quantify and compare age-related alterations in the cholinergic olivocochlear efferent auditory neurons. We assessed, in young-adult and aged CBA mice, the number of cholinergic olivocochlear neurons, auditory brainstem response (ABR) thresholds in silence and in presence of background noise, and the expression of excitatory and inhibitory proteins in the ventral nucleus of the trapezoid body (VNTB) and in the lateral superior olive (LSO). In association with aging, we found a significant decrease in the number of medial olivocochlear (MOC) cholinergic neurons together with changes in the ratio of excitatory and inhibitory proteins in the VNTB. Furthermore, in old mice we identified a correlation between the number of MOC neurons and ABR thresholds in the presence of background noise. In contrast, the alterations observed in the lateral olivocochlear (LOC) system were less significant. The decrease in the number of LOC cells associated with aging was 2.7-fold lower than in MOC and in the absence of changes in the expression of excitatory and inhibitory proteins in the LSO. These differences suggest that aging alters the medial and lateral olivocochlear efferent pathways in a differential manner and that the changes observed may account for some of the symptoms seen in ARHL.
Highlights
As social animals, any circumstance that disrupts our ability to communicate can have profound consequences on our health
In the ventral nucleus of the trapezoid body (VNTB) we found larger cells that we classified as neurons of the medial olivocochlear (MOC) system, whereas those found in the lateral superior olive (LSO) were classified as part of the lateral olivocochlear (LOC) system (Brown and Levine, 2008; RadtkeSchuller et al, 2015)
We found a significant correlation between the age of the animals and the number of OC neurons (Figure 2A, R-square = 0.3344, F = 20.60, DFn = 1, DFd = 41, p < 0.0001), with aging associated with decreased number of cells
Summary
Any circumstance that disrupts our ability to communicate can have profound consequences on our health. ARHL is defined as a progressive loss of hearing ability that is most pronounced at high frequencies (Bowl and Dawson, 2019). Despite its prevalence and the negative health outcomes associated with ARHL, our understanding of the biological mechanisms and processes that explain this condition is still incomplete, with respect to our knowledge of how the central auditory pathways are altered and the role they play in aging. These gaps in our knowledge of ARHL are even more prominent with respect to the efferent auditory pathways, which are not usually the focus of aging research
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