Neuronal Connections Between the Dorsal Cochlear Nucleus and the Inferior Colliculus in the Mouse

摘要

The central auditory system is comprised of numerous auditory nuclei that are connected by ascending and descending projections. The cochlearnucleus (CN) and inferior colliculus (IC) are two major structures with complementary interconnections whose detailed nature remains to bedetermined. The cochlear nucleus (CN) gives rise to all ascending auditory pathways that eventually converge in the inferior colliculus (IC). The ICintegrates ascending and descending auditory information: it sends ascending projections to higher auditory structures and descending projectionsto brainstem structures. One of the targets of these collicular projections is the dorsal CN (DCN). We propose that feedback connections betweenthe DCN and the IC may be involved in signal extraction in noisy backgrounds.We injected a cocktail of anterograde and retrograde tracer dyes in the DCN or the IC of mice to label axons and neurons involved in the DCN-ICcircuit. Brains were processed using standard histologic methods for visualization of neuronal tracer dyes using fluorescence, bright field, andelectron microscopy. Cell locations were mapped in 3-dimensions.Descending projections from the IC to the DCN had a strong ipsilateral bias and majority of them originated from the central nucleus of the IC inboth ipsilateral and contralateral IC. Ipsilateral descending projections were topographic, frequency-specific, and excitatory in nature. Theseprojections contacted excitatory projection neurons (pyramidal and giant cells) in the DCN as well as multimodal granule cells. Additionally,descending projections to the DCN were observed to arise from a novel nucleus – the ventral tectal longitudinal column – which receives auditoryand vision-related inputs.Excitatory feedback from the IC to the projection neurons in the DCN suggest the establishment of a reverberating circuit that could serve as amechanism for frequency-specific enhancement of acoustic signals. The DCN has a role in sound source localization in the vertical plane.Therefore, input to multimodal granule cells in the DCN may direct the attention towards a sound source based on position cues from the head,neck and pinna. Together, these connections may direct our attention to the relevant sound signal, boost its representation, and facilitate itslocalization.