Morphological and physiological development of auditory synapses

Abstract

Acoustic communication requires gathering, transforming, and interpreting diverse sound cues. To achieve this, all the spatial and temporal features of complex sound stimuli must be captured in the firing patterns of the primary sensory neurons and then accurately transmitted along auditory pathways for additional processing. The mammalian auditory system relies on several synapses with unique properties in order to meet this task: the auditory ribbon synapses, the endbulb of Held, and the calyx of Held. Each of these synapses develops morphological and electrophysiological characteristics that enable the remarkably precise signal transmission necessary for conveying the miniscule differences in timing that underly sound localization. In this article, we review the current knowledge of how these synapses develop and mature to acquire the specialized features necessary for the sense of hearing.

Fig. 1. Overview of the synapses that are specialized for transmission of signals along the auditory pathway

Peripheral processes of spiral ganglion neurons (SGN) receive input from inner hair cells in the cochlea via the ribbon synapse. Central projections of SGNs bifurcate upon entering the brainstem. The ascending branch extends toward the anterior ventral cochlear nucleus (AVCN) and forms an endbulb of Held synaptic contact on spherical bushy cells (SBC) or smaller, modified endbulbs of Held on globular bushy cells (GBC). SBCs send bilateral projections to terminate on neurons of the contralateral and ipsilateral medial superior olive (MSO), which forms a pathway crucial for determining interaural time differences (ITD). Axons of GBCs project contralaterally to the medial nucleus of the trapezoid body (MNTB) and elaborate the calyx of Held synapse on principal neurons. The principal neurons of the MNTB provide glycinergic inhibitory inputs to neurons in the lateral superior olive (LSO), converging with excitatory inputs from SBCs of the ipsilateral AVCN. LSO neurons use contralateral inhibition from GBCs by way of the MNTB and ipsilateral excitation from SBCs to compute interaural level (intensity) differences (ILD). Computation of ITD and ILD permits binaural sound localization.

Fig. 2. Development of the mouse inner hair cell (IHC) ribbon synapse

A. Presynaptic ribbon complexes are formed in IHCs and descend toward the basal cell membrane. Presynaptic densities are anchored by two rodlets to the membrane thickening. Synaptic clefts begin to be filled with a dense filamentous matrix. Subsequently, postsynaptic densities (PSDs) are assembled at sites apposing presynaptic active zones (Sobkowicz et al., 1986). In the mature ribbon synapse, the presynaptic complex is attached to the active zone by a single curved density and the PSD forms a concave shape that exceeds the territory of the presynaptic active zone (Sobkowicz et al., 1982). During early development, ribbons are round and tend to occur in clusters. Mature ribbon synapses are elliptical, and each individual afferent is juxtaposed to a single ribbon. This maturation process depends on thyroid hormone. B. Ribbons are gradually localized to the basolateral surface of the IHC in response to the innervation of SGN neurites during perinatal stages. After the first postnatal week, pruning, retraction and refinement of afferent fibers result in reduction of ribbon synapse number. Concurrently, clusters of ribbons consolidate. After hearing onset, each individual afferent terminal is apposed to a single ribbon.

Fig. 3. Morphological changes in the endbulb of Held in normal developing or deaf adult mice (adapted from (Limb et al., 2000) with permission)

The endbulb of Held in mice is initially small and simple but gradually grows to form a highly branched and intricate structure. Endbulb complexity is severely reduced in adult mice that are congenitally deaf.

Fig. 4. Major stages in the development of the calyx of Held synapse in mice

A. Around E13 to E14.5, GBC axons are attracted to the midline by Netrin-1/DCC signaling. B. By 14.5, the majority of GBC axons have crossed the midline of the brainstem due to the expression of Robo3, which prevents premature Slit responsiveness in pre-crossing axons. In parallel, EphB2/Ephrin-B2 signaling suppresses the formation of aberrant ipsilateral VCN-MNTB projections and is required for strictly contralateral VCN-MNTB projections. C. By E17, the earliest synaptic contacts between GBC axons and MNTB principal neurons are established. At P0 and P1, only a few small axosomatic contacts are formed on MNTB neurons. During this time, the presynaptic action potential (AP) has a long duration and small amplitude. D. Around P2-P3, presynaptic endings have formed large cup-shaped swellings called protocalyces, which contain many collaterals and are structurally dynamic. MNTB neurons receive inputs from several calyces at this stage. E. During the first and second postnatal weeks, collaterals are retracted and parts of the calyx become thinner and more intricate. Synaptic competition between multiple calyceal inputs is largely resolved, such that the strongest calyceal input wins, with monoinnervation of nearly all MNTB neurons by this stage. F. Adult calyx of Held synapses show a highly elaborate structure. Presynaptic AP kinetics are faster, with a shorter duration and larger amplitude. In addition, the readily releasable vesicle pool (RRP) size is increased and release probability (Pr) is decreased. Both the morphological and functional maturation of the calyx of Held require BMP signaling and Robo3-mediated axon midline crossing. The timeline is indicated as embryonic and postnatal ages in mice.