Vesicular Glutamatergic Transmission in Noise-Induced Loss and Repair of Cochlear Ribbon Synapses

Noise-induced excitotoxicity is thought to depend on glutamate. However, the excitotoxic mechanisms are unknown, and the necessity of glutamate for synapse loss or regeneration is unclear. Despite absence of glutamatergic transmission from cochlear inner hair cells in mice lacking the vesicular glutamate transporter-3 (Vglut3(KO)), at 9-11 weeks, approximately half the number of synapses found in Vglut3(WT) were maintained as postsynaptic AMPA receptors juxtaposed with presynaptic ribbons and voltage-gated calcium channels (Ca(V)1.3). Synapses were larger in Vglut3(KO) than Vglut3(WT). In Vglut3(WT) and Vglut3(+/-) mice, 8-16 kHz octave-band noise exposure at 100 dB sound pressure level caused a threshold shift (similar to 40 dB) and a loss of synapses (>50%) at 24 h after exposure. Hearing threshold and synapse number partially recovered by 2 weeks after exposure as ribbons became larger, whereas recovery was significantly better in Vglut3(WT). Noise exposure at 94 dB sound pressure level caused auditory threshold shifts that fully recovered in 2 weeks, whereas suprathreshold hearing recovered faster in Vglut3(WT) than Vglut3(+/-). These results, from mice of both sexes, suggest that spontaneous repair of synapses after noise depends on the level of Vglut3 protein or the level of glutamate release during the recovery period. Noise-induced loss of presynaptic ribbons or postsynaptic AMPA receptors was not observed in Vglut3(KO), demonstrating its dependence on vesicular glutamate release. In Vglut3(WT) and Vglut3(+/-), noise exposure caused unpairing of presynaptic ribbons and presynaptic Ca(V)1.3, but not in Vglut3(KO) where Ca(V)1.3 remained clustered with ribbons at presynaptic active zones. These results suggest that, without glutamate release, noise-induced presynaptic Ca2+ influx was insufficient to disassemble the active zone. However, synapse volume increased by 2 weeks after exposure in Vglut3(KO), suggesting glutamate-independent mechanisms.