Cochlear Inner Hair Cell Ribbon Synapse is the Primary Target of Ototoxic Aminoglycoside Stimuli

Abstract

The ribbon synapses of inner hair cells (IHCs) play an important role in sound encoding and neurotransmitter release. However, it remains unclear whether IHC ribbon synapse plasticity can be interrupted by ototoxic aminoglycoside stimuli. Here, we report that quantitative changes in the number of IHC ribbon synapses and hearing loss occur in response to gentamicin treatment in mice. Using 3D reconstruction, we were able to calculate the number of IHC ribbon synapses after ototoxic gentamicin exposure. Mice were injected intraperitoneally with a low dose of gentamicin (100 mg/kg) once a day for 14 days. Double immunostaining was used to identify IHC ribbon synapses; histopathology and scanning electron microscopy were used to observe the morphology of cochlear hair cells and spiral ganglion neurons (SGNs), the hearing threshold shifts were recorded by auditory brainstem response examinations. Our study shows that the maximal number of IHC ribbon synapses appeared at the 7th day after treatment, followed by a significant reduction after the 7th day regardless of ongoing treatment. Correspondingly, the maximal elevation of hearing threshold was observed at the 7th day after treatment. Meanwhile, additional cochlear components included OHCs, IHCs, and SGNs were unaffected, suggesting that IHC ribbon synapses are more susceptible to ototoxic aminoglycoside stimulation. Our study indicated that quantitative changes in the number of IHC ribbon synapses is critical response to lower dose of ototoxic stimulation, and may contribute to moderate hearing loss. Additionally, our data indcated that ribbon synaptic plasticity may require the quantitative changes to play self-protective role adapted to ototoxic aminoglycoside stimuli.

Changes in the Numbers of Ribbon Synapses and Expression of RIBEYE in Salicylate-Induced Tinnitus

“…6, our results demonstrated that the IHC profiles were intact in all of the mice, but the numbers of IHC ribbon synapses were different between the salicylate and control groups. The numbers of synaptic ribbons in IHCs were similar between the salicylate and control groups on the 1 st and 3 rd d. The number and distribution of ribbon synapses peaked on the 7 th d and decreased from the 8 th d to the 10 th d, which was consistent with the gentamicin study [21]. In particular, the salicylate group results were significantly lower than the control group results on the 10 th d. Changes in the numbers of ribbons seemed to lag behind behavioral measures and molecular changes in RIBEYE.…”

“…ABRs may not precisely reflect ribbon synaptic function, which occurred in the local region between IHCs and SGNs because ABRs are the far-field reflections of potentials arising in the cochlea, auditory nerve, and auditory brainstem pathway. Other targets in the cochlea, such as OHCs, IHCs, and SGNs may also be involved [21]. Combined with the unchanged ABR, it might be reasonable to infer that the administration of salicylate does not cause any obvious damage to the auditory nucleus in the brain stem.…”

“…IHCs are responsible for transmitting signals of sound frequency and intensity to the central nervous system (CNS) in mammals [21]. The synaptic terminals of IHCs are ribbon synapses, a type of special proteinaceous organelle that tethers large numbers of synaptic vesicles near the active zone where neurotransmission occurs [22].…”

Biomarkers for Hearing Dysfunction: Facts and Outlook

“…In conclusion, an increasing number of studies have now demonstrated that the counting of IHC ribbons after hearing loss, for example, in aged specimens [62,64] , or after an acoustic trauma [22,31,32,61,75,76] , or after intake of aminoglycoside [77,78] , can be a first indication of the loss of IHC synapses and AN integrity, independent of (elevated) hearing thresholds.…”

Aminoglycoside-Induced Oxidative Stress: Pathways and Protection

“…Aminoglycosides also have effects on other structures, including thinning of the stria vascularis (Ruedi et al 1952 ) with fewer marginal cells (Hawkins 1973 ). More recent studies suggest that the ribbon synapses of inner hair cells are particularly susceptible to aminoglycosides and may be an early target of these drugs (Liu et al 2013 ). We are not aware of studies showing decreased auditory brainstem response amplitude in the absence of threshold shift at low doses or at low test frequencies in animal studies with aminoglycosides.…”