Exp Neurobiol.  2018 Oct;27(5):397-407. 10.5607/en.2018.27.5.397.

Differential Expression of Ca²⁺-buffering Protein Calretinin in Cochlear Afferent Fibers: A Possible Link to Vulnerability to Traumatic Noise

• Sharma K
• Seo YW
• Yi E

• KMID: 2424734
• DOI: http://doi.org/10.5607/en.2018.27.5.397

Abstract

The synaptic contacts of cochlear afferent fibers (CAFs) with inner hair cells (IHCs) are spatially segregated according to their firing properties. CAFs also exhibit spatially segregated vulnerabilities to noise. The CAF fibers contacting the modiolar side of IHCs tend to be more vulnerable. Noise vulnerability is thought to be due to the absence of neuroprotective mechanisms in the modiolar side contacting CAFs. In this study, we investigated whether the expression of neuroprotective Ca²âº-buffering proteins is spatially segregated in CAFs. The expression patterns of calretinin, parvalbumin, and calbindin were examined in rat CAFs using immunolabeling. Calretinin-rich fibers, which made up ~50% of the neurofilament (NF)-positive fibers, took the pillar side course and contacted all IHC sides. NF-positive and calretinin-poor fibers took the modiolar side pathway and contacted the modiolar side of IHCs. Both fiber categories juxtaposed the C-terminal binding protein 2 (CtBP2) puncta and were contacted by synaptophysin puncta. These results indicated that the calretinin-poor fibers, like the calretinin-rich ones, were afferent fibers and probably formed functional efferent synapses. However, the other Ca²âº-buffering proteins did not exhibit CAF subgroup specificity. Most CAFs near IHCs were parvalbumin-positive. Only the pillar-side half of parvalbumin-positive fibers coexpressed calretinin. Calbindin was not detected in any nerve fibers near IHCs. Taken together, of the Ca²âº-buffering proteins examined, only calretinin exhibited spatial segregation at IHC-CAF synapses. The absence of calretinin in modiolar-side CAFs might be related to the noise vulnerability of the fibers.

Keyword

cochlear afferent fiber; calretinin; inner hair cell synapse; Ca²⁺-buffering protein

MeSH Terms

Animals
Calbindin 2*
Calbindins
Carrier Proteins
Fires
Hair Cells, Auditory, Inner
Intermediate Filaments
Nerve Fibers
Noise*
Rats
Sensitivity and Specificity
Synapses
Synaptophysin
Carrier Proteins
Synaptophysin