The mammalian auditory brainstem contains specialized synapses that preserve the precise timing of action potential spikes. We propose to study two of these specialized synapses: the large calyx of Held synapse in the medial nucleus of the trapezoid body (MNTB) and the small bouton-type glycinergic synapses of the lateral superior olive (LSO), that are linked through the MNTB principal neuron. The long-term goal is to determine the biophysical properties and structure/function of these two pivotal synapses in the circuitry that computes the locus of high frequency sounds. We will perform patch clamp recordings in mouse brainstem slices from more adult-like stages of development, when mice fully acquire their fine-tuned ability to hear and localize sound. Our preliminary data show that several fundamental aspects of brainstem synapses mature only at postnatal day 30. We thus propose to study the synaptic delays, synaptic strength and short-term plasticity of adult-like auditory synapses. The first hypothesis is that adult-like calyx-type nerve terminals in the MNTB contain heterogeneous and crowded active zones (AZs) with multiple docked vesicles that produce ultrashort delays in vesicle exocytosis. We will perform detailed ultrastructural reconstructions of the AZs using high-resolution electron tomography (ET). We plan to identify the major factors that promote short exocytosis delays, such as a large vesicle pool size, crowded AZs with diffusional barriers for Ca2+ ions and tight vesicle-to-Ca2+-channel coupling. The second hypothesis is that the timing and strength of glycine release in the LSO change during postnatal development due to shifts in release probability and the size of the readily releasable pool of vesicles. We report for the first time that inhibitory postsynaptic currents from LSO neurons are preceded by a prespike waveform that reflects the synchronous arrival of the presynaptic action potentials at multiple synaptic boutons. This allowed us to quantify for the first time the synaptic delay of a glycinergic auditory synapse. We will also test the hypothesis that the temporal precision of spike-evoked glycine release relies on large multiquantal exocytosis. The third hypothesis to be tested is that during postnatal development the LSO glycinergic synapse acquires a robust Ca2+-dependent vesicle recruitment mechanism. A sustained steady- state release of glycine onto the LSO neurons thus effectively blocks their ability to fire spikes in response of excitatory inputs. Our preliminary LSO data show, surprisingly in contrast to the calyx of Held, that maturing glycinergic LSO synapses decrease their vesicle pool size and increase release probability. Using confocal microscopy and genetically encoded Ca2+ indicators, we will image Ca2+ influx at glycinergic boutons, and for the first time describe, using ET, their 3D ultrastructure at high resolution. Together with our collaborators we will further validate and study the physiological relevance of our results using in vivo recordings and computer modeling. The proposed experiments will launch new studies on mature LSO synapse structure/function using electron tomography, patch clamp recordings, and direct Ca2+ imaging of LSO bouton-type nerve terminals.
Several fundamental insights on how synapses of auditory neurons operate in the developing and mature mammalian brainstem will be obtained from the proposed studies. This information sheds light on cellular mechanisms for binaural hearing including sound localization and enlightens current and future treatment strategies of hearing disorders. These studies will lead to an improved understanding of directional hearing in maturing animals and in future studies of how deficits in auditory brainstem synaptic transmission contribute to hearing loss.
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