Two mathematical frameworks for vesicle release from a ribbon synapse of a retinal bipolar cell

Objective:bipolar cells(BCs)communicate with amacrine and ganglion cells of the retina via both transient and sustained neurotransmitter release in ribbon synapses.Reconstructing the published quantitative release data from electrical soma stimulation(voltage clamp experiments)of rat rod BCs were used to develop two simple models to predict the number of released vesicles as time series.In the experiment,the currents coming to the All amacrine cell originating from releasing vesicles from the rod BC were recorded using paired-recordings in whole-cell voltage-clamp method.Method:one of the models is based directly on terminal transmembrane voltage,so-called ‘model’,whereas the temporally exacter modelCa includes changes of intracellular calcium concentrations at terminals.Result:the intracellular calcium concentration method replicates a 0.43 ms signal delay for the transient release to pulsatile stimulation as a consequence of calcium channel dynamics in the presynaptic membrane,while the modelV has no signal delay.Both models produce the quite similar results in low stimuli amplitudes.However,for large stimulation intensities that may be done during extracellular stimulations in retinal implants,the modelCa predicts that the reversal potential of calcium limits the number of transiently released vesicles.Adding sodium and potassium ion channels to the axon of the cell enable to study the impact of spikes on the transient release in BC ribbons.Conclusion:a spike elicited by somatic stimulation causes the rapid release of all vesicles that are available for transient release,while a non-spiking BC with a similar morphometry needs stronger stimuli for any transient vesicle release.During extracellular stimulation,there was almost no difference in transient release between the active and passive cells because in both cases the terminal membrane of the cell senses the same potentials originating from the microelectrode.An exception was found for long pulses when the spike has the possibility to generate a higher terminal voltage than the passive cell.Simulated periodic 5 Hz stimulation showed a reduced transient release of 3 vesicles per stimulus,which is a recovery effect.

THE COCHlEAR RIBBON SYNAPTIC RESPONSE TO AMINOGIYCOSIDE OTOTOXICITY IN C57BL/6J MICE

Objective To investigate the early change of cochlear ribbon synapses on inner hair cells in response to aminoglycoside ototoxicity. Methods C57BL/6J mice received intraperitoneal injection of gentamicin(100mg/kg/day), and the apical coil organ of Corti was examined on the 4th, 7th and 10 th day(n=10). Littermates without gentamicin treatment served as controls(n=10). RIBEYE on the presynaptic membrane and AMPA receptors on the postsynaptic membrane were labeled with CtBP2 or GluR2/3 respectively. Three dimension reconstruction was conducted using the 3DS MAX 8.0 software. Results There were no disruptions of outer or inner hair cells in all groups. However, the number of ribbon synapses on cochlear inner hair cells increased significantly within 7 days after gentamicin exposure(P<0.01), followed by a significant decrease after 7 days.Conclusion During the early stage of aminoglycoside ototoxicity, increased population of cochlear ribbon synapses may indicate a significant down-regulation of synaptic function.

LOWER DOSE OF AMINOGLYCOSIDE OTOTOXIC EXPOSURE CAUSES PRESYNAPTIC ALTERATIONS ASSOICATED WITH HEARING LOSS

Objective To study presynaptic alternations of cochlear ribbons arising from aminoglycoside ototoxic stimuli in C57BL/6J mice. Methods Animals were injected with low dose gentamicin(100 mg/kg/day) for14 days, From the 14 th to 28 th days, the mice were maintained free of gentamicin treatment. Immunohistochemistry labeling was employed to trace RIBEYE, a major presynaptic componment of ribbon synapses.RIBEYE/CtBP2 expression levels were assessed and compared with hearing threshold shifts. Auditory function was assessed by auditory brainstem responses. The stereocilia of outer hair cells(OHCs) and IHCs was examined by scanning electron microscopy(SEM). Results Hearing thresholds were elevated with peak hearing loss observed on the 7th day after gentamicin exposure, followed by improvement after the 7th day.RIBEYE/CtBP2 expression directly correlated with observed hearing threshold shifts. Strikingly, we did not see any obvious changes in stereocilia in both OHCs and IHCs until the 28 th day. Mild changes in stereocilia were only observed in OHCs on the 28 th day. Conclusions These findings indicate that presynapse cochlear ribbons, rather than stereocilia, may be sensitive to aminoglycoside ototoxic exposure in mice cochleae. A pattern of RIBEYE/CtBP2 expression changes seems to parallel hearing threshold shifts and suggests presynaptic response properties to lower dosage of aminoglycoside ototoxic stimuli.

Aligned Organization of Synapses and Mitochondria in Auditory Hair Cells

Recent studies have revealed great functional and structural heterogeneity in the ribbon-type synapses at the basolateral pole of the isopotential inner hair cell(IHC).This feature is believed to be critical for audition over a wide dynamic range,but whether the spatial gradient of ribbon morphology is fine-tuned in each IHC and how the mitochondrial network is organized to meet local energy demands of synaptic transmission remain unclear.By means of three-dimensional electron microscopy and artificial intelligence-based algorithms,we demonstrated the cell-wide structural quantification of ribbons and mitochondria in mature mid-cochlear IHCs of mice.We found that adjacent IHCs in staggered pairs differ substantially in cell body shape and ribbon morphology gradient as well as mitochondrial organization.Moreover,our analysis argues for a location-specific arrangement of correlated ribbon and mitochondrial function at the basolateral IHC pole.