Introduction:
The peripheral auditory system in rodents reaches its functional maturity within the postnatal days 12 – 14. This maturation involves the structural and functional remodeling of the inner hair cell (IHC) presynaptic active zones (AZ). This latter process is essentially required for the exquisite temporal precision and exceedingly high neurotransmission rates observed at this synapse and fundamental for synaptic sound encoding. To date, the molecular characterization underlying these developmental processes remain poorly understood.
Methods:
Throughout this study, organs of Corti (OC) from neonatal C57BL/6J mice were used. After decapitation, cochlear microdissection with OC extraction was followed by an immediate fixation process. OC were immunohistochemically stained for various presynaptic proteins (including Bassoon, RIM2, RIM-BP1/2, etc.), which were characterized using super-resolution stimulated emission depletion (STED) microscopy. Subsequently, the subsynaptic localization and 3D orientation of each protein within the AZ were quantitatively analyzed at regular intervals along the developmental process.
Results:
Essential presynaptic proteins within the first auditory synapse could be consequently analyzed during its postnatal development, forming the basis of a molecular distribution atlas of the IHC AZ. Our findings document the developmental re-organization of IHC key presynaptic proteins involved in hearing.
Conclusion:
This study conveys a thorough ultrastructural characterization, as well as a quantitative analysis of the maturation process of various essential molecules responsible for the presynaptic architecture and function of the first auditory synapse between IHC and afferent spiral ganglion neurons.
