Abstract
Age-related hearing loss (ARHL) is marked by audiometric hearing deficits that propagate along the auditory pathway. Neurochemical changes as a function of aging have also been identified in neurons along the auditory pathway in both rodents and carnivores, however, very little is known about how these neurochemicals change in the non-human primate. To examine how these compensatory neurochemical changes relate to normal aging and audiometric sensitivity along the auditory pathway, we collected auditory brainstem responses (ABRs) and brain specimens from seven rhesus monkeys spanning in age from 15 to 35 years old, and examined the relationship between click evoked ABR thresholds and the ABR evoked pure tone average (PTA) and changes in the number of parvalbumin and NADPH-diaphorase positive cells in the auditory midbrain. We found that the number of parvalbumin positive cells in the central nucleus and the surrounding cortex regions of the inferior colliculus were strongly correlated with advancing age and ABR PTA. We also found that the numbers of NADPHd positive cells in these same regions were not associated with normal aging or changes in the ABR thresholds. These findings suggest that the auditory midbrain undergoes an up-regulation of parvalbumin expressing neurons with aging that is related to changes in the processing of frequencies across the audiometric range.
Highlights
Age-related hearing loss (ARHL) is characterized by spectral, temporal and spatial processing deficits (Caspary et al, 1995, 2008; Frisina and Frisina, 1997; Snell, 1997; Abel et al, 2000a,b; Juarez-Salinas et al, 2010)
Auditory brainstem response (ABR) evoked by clicks are somewhat restricted across the audiometric range of monkeys, so we summarized the ABR response to tones by creating a seven frequency pure tone average (PTA)
We found that the number of PV positive cells in the inferior colliculus (ICC) (r = 0.85, p < 0.01) and the ICX (r = 0.69, p < 0.05) significantly increased as a function of increasing ABR PTA (Figure 7)
Summary
Age-related hearing loss (ARHL) is characterized by spectral, temporal and spatial processing deficits (Caspary et al, 1995, 2008; Frisina and Frisina, 1997; Snell, 1997; Abel et al, 2000a,b; Juarez-Salinas et al, 2010). Given the current interest in studying the macaque auditory system as a model for human auditory perception (e.g., Kaas and Hackett, 2000; Rauschecker and Tian, 2000) and the more recent interest in the effects of aging on macaque auditory processing (e.g., Juarez-Salinas et al, 2010; Recanzone et al, 2011; Engle and Recanzone, 2012; Engle et al, 2013) it is important to understand the similarities and differences between these two animal models Previous studies from this laboratory quantifying the numbers of neurons expressing PV and NADPHd in the macaque cochlear nucleus (Gray et al, 2014b), superior olivary complex (Gray et al, 2014a) and auditory thalamus (Gray et al, 2013) suggest that, while age-related changes are noted, they are not identical to those found in rodents. The aim of the present study is to characterize the neurochemical changes in the inferior colliculus, the obligatory relay nucleus of the ascending auditory central nervous system, of the aged macaque monkey to gain a better understanding of the anatomical correlates that accompany aging in the primate auditory system
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