Embryonic assembly of auditory circuits: spiral ganglion and brainstem

During early development, peripheral sensory systems generate physiological activity prior to exposure to normal environmental stimuli. This activity is thought to facilitate maturation of these neurons and their connections, perhaps even promoting efficacy or modifying downstream circuitry. In the mammalian auditory system, initial connections form at embryonic ages, but the functional characteristics of these early neural connections have not been assayed. We investigated processes of embryonic auditory development using a whole-head slice preparation that preserved connectivity between peripheral and brainstem stations of the auditory pathway. Transgenic mice expressing fluorescent protein provided observation of spiral ganglion and cochlear nucleus neurons to facilitate targeted electrophysiological recording. Here we demonstrate an apparent peripheral-to-central order for circuit maturation. Spiral ganglion cells acquire action potential-generating capacity at embryonic day 14 (E14), the earliest age tested, and action potential waveforms begin to mature in advance of comparable states for neurons of the ventral cochlear nucleus (VCN) and medial nucleus of the trapezoid body (MNTB). In accordance, auditory nerve synapses in the VCN are functional at E15, prior to VCN connectivity with the MNTB, which occurs at least 1 day later. Spiral ganglion neurons exhibit spontaneous activity at least by E14 and are able to drive third-order auditory brainstem neurons by E17. This activity precedes cochlear-generated wave activity by 4 days and ear canal opening by at least 2 weeks. Together, these findings reveal a previously unknown initial developmental phase for auditory maturation, and further implicate the spiral ganglion as a potential controlling centre in this process.