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Hear Res. 2015 Dec;330(Pt B):191-9. doi: 10.1016/j.heares.2015.02.009. Epub 2015 Mar 11.

Synaptopathy in the noise-exposed and aging cochlea: Primary neural degeneration in acquired sensorineural hearing loss.

Author information

1
Department of Otology and Laryngology, Harvard Medical School, Boston, MA, USA; Eaton-Peabody Laboratory, Massachusetts Eye & Ear Infirmary, Boston, MA, USA; Department of Audiology, Massachusetts Eye and Ear, Boston, MA, USA.
2
Department of Otology and Laryngology, Harvard Medical School, Boston, MA, USA; Eaton-Peabody Laboratory, Massachusetts Eye & Ear Infirmary, Boston, MA, USA. Electronic address: Charles_Liberman@meei.harvard.edu.

Abstract

The classic view of sensorineural hearing loss (SNHL) is that the "primary" targets are hair cells, and that cochlear-nerve loss is "secondary" to hair cell degeneration. Our recent work in mouse and guinea pig has challenged that view. In noise-induced hearing loss, exposures causing only reversible threshold shifts (and no hair cell loss) nevertheless cause permanent loss of >50% of cochlear-nerve/hair-cell synapses. Similarly, in age-related hearing loss, degeneration of cochlear synapses precedes both hair cell loss and threshold elevation. This primary neural degeneration has remained hidden for three reasons: 1) the spiral ganglion cells, the cochlear neural elements commonly assessed in studies of SNHL, survive for years despite loss of synaptic connection with hair cells, 2) the synaptic terminals of cochlear nerve fibers are unmyelinated and difficult to see in the light microscope, and 3) the degeneration is selective for cochlear-nerve fibers with high thresholds. Although not required for threshold detection in quiet (e.g. threshold audiometry or auditory brainstem response threshold), these high-threshold fibers are critical for hearing in noisy environments. Our research suggests that 1) primary neural degeneration is an important contributor to the perceptual handicap in SNHL, and 2) in cases where the hair cells survive, neurotrophin therapies can elicit neurite outgrowth from spiral ganglion neurons and re-establishment of their peripheral synapses. This article is part of a Special Issue entitled <Auditory Synaptology>.

PMID:
25769437
PMCID:
PMC4567542
DOI:
10.1016/j.heares.2015.02.009
[Indexed for MEDLINE]
Free PMC Article

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