Tuning of synapse number, structure and function in the cochlea

Alexander C Meyer, Thomas Frank, Darina Khimich, Gerhard Hoch, Dietmar Riedel, Nikolai M Chapochnikov, Yury M Yarin, Benjamin Harke, Stefan W Hell, Alexander Egner, Tobias Moser
2009 Nature Neuroscience  
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
more » ... ar IHCs. Synapses were distributed in the subnuclear portion of IHCs. High-resolution imaging of IHC synapses provided insights into presynaptic Ca 21 channel clusters and Ca 21 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 Ca 21 signals, even within individual IHCs, providing a candidate presynaptic mechanism for the divergent dynamics of spiral ganglion neuron spiking. After processing by the mammalian ear's exquisite micromechanics and mechanoelectrical transduction, acoustic information is encoded at the afferent synapses of IHCs with high temporal precision 1,2 . Presynaptic active zones of IHCs contain a synaptic ribbon, a multiprotein structure that tethers synaptic vesicles 3,4 and ensures a large pool of readily releasable vesicles 5-8 . Stimulus-secretion coupling is governed by Ca V 1.3 L-type Ca 2+ channels 9,10 that tightly control the release of glutamate from nearby fusion-competent vesicles 2,8,11 onto postsynaptic AMPA receptors 12,13 on the unbranched peripheral axon of the bipolar spiral ganglion neuron (SGN) 14 . As a result, each SGN receives input from only one IHC active zone, while each IHC drives several SGNs. Whether and how the number and properties of afferent synapses of the cochlea are 'tuned' for optimal sound encoding remains an important question (refs. 15,16, for example). The frequency selectivity of SGNs is primarily determined by the location of the innervated IHC on the cochlea's tonotopic axis, providing a place code for frequency (refs. 17,18, for example). In addition, studies of small samples of synapses from distinct cochlear regions have indicated that the innervation density varies along the length of the cochlea 19-21 . Moreover, it has been shown that SGNs covering a narrow frequency range differ markedly in spontaneous and evoked firing rates, sound threshold and dynamic range (for example, refs. 22,23) and that they collectively encode a large range of sound pressures. It is generally believed, but not yet directly proven, that each IHC makes contact with such physiologically diverse SGNs. If true, the heterogeneity of SGN dynamics could be caused by pre-and postsynaptic mechanisms 7,19,24,25 . Pioneering work on the cat cochlea suggested that lowspontaneous-rate SGNs preferentially contact active zones with large or even multiple synaptic ribbons at the neural side of IHCs (toward the modiolus), whereas high spontaneous rate SGNs are driven by small, 'simple' synapses at the abneural IHC side (toward the outer hair cells) 24 . Here we have used patch-clamp, confocal imaging of IHC presynaptic Ca 2+ signals; confocal, 4Pi 26,27 and stimulated emission depletion (STED) 28 microscopy of immunolabeled synapses; and electron microscopy to characterize the distribution of afferent synapses as well as their structure and function at different tonotopic regions of the cochlea. Having investigated thousands of synapses in hundreds of IHCs, we provide a continuous representation of synapse number per IHC along the entire mouse cochlea, and we show that synapse density parallels the neuronal population audiogram. Using STED microscopy, we provide optical, nanometer-scale measurements of individual clusters of presynaptic Ca 2+ channels and postsynaptic AMPA receptors. Whereas average structural and functional synapse properties varied only slightly along the cochlea's tonotopic axis, we found considerable heterogeneity of presynaptic Ca 2+ signals among the synapses in IHCs in a given region.
doi:10.1038/nn.2293 pmid:19270686 fatcat:22ciufefxrfczeqaxajotd4eeu