Evidence for a Vestibular Contribution to Auditory Processing via the LVN–IC Pathway

초록

Background: The vestibular nuclei (VN) are brainstem structures essential for balance and posture. They also play an important role in natural behavior by broadcasting vestibular signals across the brain and thereby interacting with other sensory systems. When this multisensory coordination is disrupted, neurological disorders may arise, often resulting in combined vestibular and auditory symptoms. This study aims to determine whether the neurons of the VN project to the inferior colliculus (IC). Methods: C57BL/6J mice (7–9 weeks old) were anesthetized and injected with CTB-Alexa568, anterograde AAV, or retrograde AAV into the IC or lateral vestibular nucleus (LVN). For electrophysiology, mice were implanted with a headpost before recording, and two small cranial windows were made over the IC and LVN for neural recording and optogenetic stimulation, respectively. Neural signals were acquired using a 6-channel tungsten electrode array (~5 MΩ, FHC). To test sound responses, animals were placed in a sound booth and exposed to 70 dB pure tones (8, 32 kHz) for 2 hours following 90 min of silence. Brains were subsequently fixed, sectioned, and processed for c-Fos histology to verify sound and stimulation-evoked activity. Results: Retrograde virus injection into the IC resulted in labeled neurons in the LVN, and anterograde virus injection into the LVN revealed labeled fibers in the ventromedial region of the IC. In addition, retrograde virus injection into the LVN led to labeling in the uvula, suggesting that the LVN receives afferent input from the uvula. Virus tracing revealed double-labeled neurons in the LVN following retrograde injection into the IC and anterograde injection into the uvula. Optogenetic stimulation of the LVN, followed by electrophysiological recordings from the IC, revealed a strong modulation in the firing rate of IC neurons with a short latency. To investigate whether IC-projecting LVN neurons respond to auditory stimuli in addition to vestibular input, we conducted a sound exposure experiment. After sound stimulation, c-Fos expression in the LVN was observed in response to 8 and 32 kHz sounds. This result indicates that IC-projecting LVN neurons are also activated by auditory stimulation. Conclusions: Our results demonstrate that the LVN and IC are interconnected through both anatomical projections and functional responses, with the LVN receiving motor-related input from the uvula and directly modulating IC activity. Additionally, sound-evoked c-Fos expression in the LVN indicates that IC-projecting LVN neurons respond not only to vestibular input but also to auditory input, suggesting that the LVN may serve as a site where motor- and auditory-related inputs converge before reaching the IC. 3 keywords: vestibular system, auditory, lateral vestibular nucleus