Synaptic properties of layer 6 auditory corticothalamic inputs in normal hearing and noise-induced hearing loss

Layer 6 corticothalamic neurons (CTs) provide strong feedback input that is crucial to perception and cognition in normal and pathological states; however, the synaptic properties of this input remain largely unknown, especially in pathology. Here, we examined the synaptic properties of CT axon terminals in the medial geniculate body (MGB), the auditory thalamus, in normal hearing mice and in a mouse model of noise-induced hearing loss. In normal hearing mice, we found that the synaptic strength of CTs to the core-type ventral subdivision of the auditory thalamus (MGv), which mainly conveys rapid sensory information, is stronger than the synaptic strength of CTs to the matrix-type dorsal subdivision of the auditory thalamus (MGd), which likely conveys higher-order internal state information. This is due to increased functional release sites (n) in CT→MGv compared to CT→MGd synapses. After noise trauma, we observed enhanced short-term facilitation in CT→MGd but not CT→MGv synapses. Our findings reveal a previously unknown mechanism of short-term synaptic plasticity after noise-induced hearing loss via which CTs enhance the throughput of matrix-type thalamus, likely to improve perceptual recovery via higher-order contextual modulation.