Functional Development of the Auditory Brainstem Nuclei During Embryogenesis of the Mouse Revealed by Optical Recording With a Voltage‐Sensitive Dye

A fundamental question in auditory neuroscience is when and how complex connections between the nuclei/centers are organized during ontogenesis. In the present study, we applied a multiple-site optical recording technique with a voltage-sensitive dye and surveyed the developmental organization of the auditory system in the mouse embryo. Stimulation of the cochlear (auditory) nerve elicited excitatory postsynaptic potential (EPSP)-related optical responses in the lateral brainstem, which corresponded to the auditory sensory nucleus, namely, the cochlear nucleus. The EPSP was mediated by glutamate and mainly dependent on N-methyl-d-aspartate (NMDA) receptors. The EPSP was first detected at E (embryonic day) 12, indicating that functional connections between the periphery and neurons in the cochlear nucleus are established at this stage. At later developmental stages, cochlear nerve stimulation elicited additional postsynaptic responses in the ipsilateral and contralateral ventral brainstem, which corresponded to the higher center of the auditory pathway, the superior olivary complex (SOC). The EPSP in the SOC was detected from E15 in normal physiological solution and E14 in a Mg²⁺-free solution. Thus, the synaptic connections between the cochlear nucleus and the SOC are established by E14, but they are suppressed by the Mg²⁺ block on the NMDA receptors before E15. Overall, our results suggest that postsynaptic responses in the cochlear nucleus and the SOC exhibit much earlier than previously reported, and functional synapses are generated soon after the arrival of afferent fibers and before morphological differentiation of the brainstem nuclei has been completed.