Synaptic ribbon dynamics after noise exposure in the hearing cochlea

The Size and Localization of Ribeye and GluR2 in the Auditory Inner Hair Cell Synapse of C57BL/6 Mice Are Affected by Short-Pulse Corticosterone in a Sex-Dependent Manner

Background: Inner hair cell (IHC) ribbon synapses are the initial synapses in the auditory pathway, comprising presynaptic ribbons and postsynaptic glutamate receptors on the peripheral afferent fibers. The excitatory neurotransmitter glutamate primarily signals through AMPA-type heterotetrameric receptors (AMPARs), composed of GluR1, GluR2, GluR3, and GluR4 subunits. Research shows that corticosterone affects AMPA receptor subunits in the central nervous system. The present study investigates the effects of corticosterone on AMPA receptor subunits in the murine cochlea. Methods: Cochlear explants were isolated from male and female C57BL/6 pups (postnatal days 4-5), treated for 20 min with 100 nM corticosterone, and cultured for an additional 24 h. The concentration of AMPAR protein subunits was quantified using an ELISA assay, while gene expression was analyzed using RT-PCR. The synaptic localization patterns of GluR2 and Ribeye were examined using immunofluorescence and confocal microscopy. Results: Male C57BL/6 mice have a significantly greater basal concentration of the GluR2 subunit than females and more GluR2 puncta per IHC than females. Corticosterone increases the size of Ribeye in males and increases twofold GluR2/Ribeye colocalization in the apical region of females. Conclusions: Exposure of membranous cochleae to corticosterone induces changes consistent with neuroplasticity in the auditory periphery. The observed effect is sex-dependent.

Canonical MAPK signaling in auditory neuropathy

Auditory neuropathy (AN) is an under-recognized form of hearing loss characterized by lesions in inner hair cells (IHCs), ribbon synapses and spiral ganglion neurons (SGNs). The lack of a targeted therapy for AN has increased the need for a better understanding of the pathogenic mechanism of AN. As mitogen-activated protein kinase (MAPK) signaling is ubiquitous in many biological processes, its alteration may facilitate the pathogenesis of multiple sites in AN. Here, we summaries the characteristics of AN under different molecular bases and first explore the mechanism of MAPK at different lesion sites. Alterations of extracellular signal-regulated kinase (ERK)/MAPK occur in IHCs and SGNs, whereas modulations of p38 and c-Jun NH2-terminal kinase (JNK) were found in ribbon synapses and SGNs. In conclusion, inductive MAPK alterations in the pathogenesis and development of AN are likely to represent a potential therapeutic target to guide the development of treatments.

Noise-induced ribbon synapse loss in the mouse basal cochlear region does not reduce inner hair cell exocytosis

Noise-induced hearing loss is one of the most common forms of hearing loss in adults and also one of the most common occupational diseases. Extensive previous work has shown that the highly sensitive synapses of the inner hair cells (IHCs) may be the first target for irreparable damage and permanent loss in the noise-exposed cochlea, more precisely in the cochlear base. However, how such synaptic loss affects the synaptic physiology of the IHCs in this particularly vulnerable part of the cochlea has not yet been investigated. To address this question, we exposed 3-4-week-old C57BL/6J mice to 8-16 kHz noise for 2 h under isoflurane anesthesia. We then employed hearing measurements, immunohistochemistry and patch-clamp to assess IHC synaptic function. Two noise sound pressure levels (SPLs) were used to evoke acute hearing threshold elevations with different levels of recovery 2 weeks post-exposure. Regardless of noise intensity, the exposure resulted in a loss of approximately 25-36% of ribbon synapses in the basal portions of the cochlea that persisted 2 weeks after exposure. Perforated patch-clamp recordings were made in the IHCs of the basal regions of the cochlea where the greatest synaptic losses were observed. Depolarization-evoked calcium currents in IHCs 2 weeks after exposure were slightly but not significantly smaller as compared to controls from age-matched non-exposed animals. Exocytic changes monitored as changes in membrane capacitance did not follow that trend and remained similar to controls despite significant loss of ribbons, likely reflecting increased exocytosis at the remaining synapses. Additionally, we report for the first time that acute application of isoflurane reduces IHC calcium currents, which may have implications for noise-induced IHC synaptic loss.

Noise induces Ca2+ signaling waves and Chop/S-Xbp1 expression in the hearing cochlea

Exposure to loud noise is a common cause of acquired hearing loss. Disruption of subcellular calcium homeostasis and downstream stress pathways in the endoplasmic reticulum and mitochondria, including the unfolded protein response (UPR), have been implicated in the pathophysiology of noise-induced hearing loss. However, studies on the association between calcium homeostasis and stress pathways have been limited due to limited ability to measure calcium dynamics in mature-hearing, noise-exposed mice. We used a genetically encoded calcium indicator mouse model in which GCaMP6f is expressed specifically in hair cells or supporting cells under control of Myo15Cre or Sox2Cre, respectively. We performed live calcium imaging and UPR gene expression analysis in 8-week-old mice exposed to levels of noise that cause cochlear synaptopathy (98 db sound pressure level [SPL]) or permanent hearing loss (106 dB SPL). UPR activation occurred immediately after noise exposure, and the pattern of UPR activation was dependent on noise level, with the proapoptotic pathway upregulated only after 106 dB noise exposure. Spontaneous calcium transients in hair cells and intercellular calcium waves in supporting cells, which are present in neonatal cochleae, were quiescent in mature-hearing cochleae but reactivated upon noise exposure. Noise exposure of 106 dB was associated with more persistent and expansive intercellular Ca2+ signaling wave activity. These findings demonstrate a strong and dose-dependent association between noise exposure, UPR activation, and changes in calcium homeostasis in hair cells and supporting cells, suggesting that targeting these pathways may be effective to develop treatments for noise-induced hearing loss.