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Tuning of synapse number, structure and function in the cochlea

Abstract

Cochlear inner hair cells (IHCs) transmit acoustic information to spiral ganglion neurons through ribbon synapses. Here we have used morphological and physiological techniques to ask whether synaptic mechanisms differ along the tonotopic axis and within IHCs in the mouse cochlea. We show that the number of ribbon synapses per IHC peaks where the cochlea is most sensitive to sound. Exocytosis, measured as membrane capacitance changes, scaled with synapse number when comparing apical and midcochlear IHCs. Synapses were distributed in the subnuclear portion of IHCs. High-resolution imaging of IHC synapses provided insights into presynaptic Ca2+ channel clusters and Ca2+ signals, synaptic ribbons and postsynaptic glutamate receptor clusters and revealed subtle differences in their average properties along the tonotopic axis. However, we observed substantial variability for presynaptic Ca2+ signals, even within individual IHCs, providing a candidate presynaptic mechanism for the divergent dynamics of spiral ganglion neuron spiking.

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Figure 1: The number of afferent synapses per IHC co-varies with ABR threshold along the tonotopic axis.
Figure 2: Spatial distribution of afferent synapses within IHCs of one tonotopic region.
Figure 3: Ribbon synapse morphology in the apical cochlea (100–400 μm) and mid-cochlea (1,300–2,100 μm) of the mouse (P16–P21).
Figure 4: Exocytosis scales with the number of afferent synapses per IHC.
Figure 5: Synaptic Ca2+ signals are comparable at different tonotopic locations.
Figure 6: Intracellular variability of synaptic Ca microdomains.

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Acknowledgements

We thank E. Neher, M. Göpfert, R. Nouvian, N. Strenzke, M. Müller and A. Lysakowski for comments on the manuscript; K. Wadel and C. Henrich for participation in an early stage of the project; A. Neef for image analysis support; J. Hegerman and S. Eimer for support with high-pressure rapid freeze and freeze substitution; and C. Rüdiger and M. Köppler for technical assistance. This work was supported by grants from the Deutsche Forschungsgemeinschaft (Center for Molecular Physiology of the Brain; T.M., A.E. and S.W.H), a Lichtenberg Fellowship from the state of Lower Saxony (T.F.), the European Commission (Eurohear, T.M.), the Max-Planck-Society (Tandemproject, T.M.), BMBF (Bernstein Center for Computational Neuroscience Göttingen, T.M.) and an intramural grant from the University of Göttingen Medical School (A.C.M.).

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Contributions

The study was designed by T.M., A.C.M., A.E. and T.F. The experimental work was performed by A.C.M., T.F., D.K., D.R., G.H., N.M.C., Y.M.Y. and B.H. S.W.H. co-developed the super-resolution microscopes.

Corresponding authors

Correspondence to Alexander Egner or Tobias Moser.

Supplementary information

Supplementary Text and Figures

Supplementary Figures 1–6, Supplementary Tables 1–3, and Supplementary Methods (PDF 5062 kb)

Supplementary Movie 1

3D STED imaging of postsynaptic AMPA receptor clusters. Left, Voltex-rendering of a 3D STED image of a postsynaptic AMPA receptor cluster (immunolabeled for GluR2/3, resolution 150 nm, raw data). The gradient of the fluorescence intensity as well as the off-center maximum are clearly visible. Right, in order to emphasize the “ring-like” appearance and the curved shape the voltex-rendering is combined with an isosurface view. The isosurface has been generated by using a seed fill algorithm (magic wand, Amira, Visage Imaging) utilizing 64% and 100% of the maximum intensity as the lower and upper threshold respectively. (MOV 1680 kb)

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Meyer, A., Frank, T., Khimich, D. et al. Tuning of synapse number, structure and function in the cochlea. Nat Neurosci 12, 444–453 (2009). https://doi.org/10.1038/nn.2293

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