Scripties UMCG - Rijksuniversiteit Groningen
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Cochlear anatomy in naked and Damaraland mole rats

(2018) Douma, S. (Sytze)

Compared to many other rodent species, naked mole rats (Heterocephalus glaber) have elevated
auditory thresholds, poor high frequency hearing, and limited ability to localize sound. Differences
in the peripheral and central auditory structures that underly differences in their auditory function
are poorly understood. Because the cochlea is responsible for encoding and relaying auditory
signals to the brain, we used immunofluorescence and quantitative image analysis to examine
cochlear anatomy and especially innervation in mature and developing naked mole rats compared to
mice (Mus musculus), gerbils (Meriones unguiculatus), and Damaraland mole rats (Fukomys
damarensis), another subterranean rodent. In comparison to mice and gerbils, we observed
alterations in afferent and efferent innervation as well as their patterns of developmental refinement
in naked and Damaraland mole rats; however, these alterations were not always shared similarly
between naked and Damaraland mole rats. Most conspicuously, in both naked and Damaraland
mole rats, inner hair cell afferent ribbon density was reduced, whereas outer hair cell afferent ribbon
density was increased. Naked mole rats also showed reduced lateral and medial efferent terminal
density. Developmentally, naked mole rats showed reduced and prolonged postnatal reorganization
of afferent and efferent innervation. Damaraland mole rats showed no evidence of postnatal
reorganization. Finally, BK channel expression in inner hair cells, an indicator of developmental
maturation in other rodents, was reduced and, furthermore, showed no tonotopic variation in naked
mole rats. These alterations in cochlear anatomy in naked and Damaraland mole rats may result
from neotenous maintenance of immature cochlear anatomy that serves to adapt hearing to their
unique subterranean environments. Importantly, these findings establish naked and Damaraland
mole rats as comparative species worthy of future investigation to examine cochlear mechanisms
that support high frequency hearing and sound localization.

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