Efferent projections of a physiologically characterized region of the inferior colliculus of the young adult CBA mouse

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Abstract
The present investigation is part of an ongoing series of studies aimed at discerning the neural bases of presbycusis. Presbycusis is a sensory perceptual disorder involving loss of high-pitch hearing and reduced ability to process biologically relevant acoustic signals in noisy environments. The purpose of the present experiment was to delineate the efferent projections of a functionally characterized region of the dorsomedial inferior colliculus (IC, auditory midbrain) in young, adult CBA mice. The CBA strain’s progressive loss of hearing over its lifespan approximates many aspects of the mild-to-moderate hearing loss experienced by a significant number of humans suffering from presbycusis. Focal, iontophoretic injections of HRP were made in the 18–24 kHz region of dorsomedial IC of the CBA strain following physiological mapping experiments. Serial sections were reacted with a chromagen, counterstained and examined for anterogradely labeled fibers and boutons. Efferent projections were observed ipsilaterally in: medial and ventral divisions of the medial geniculate body (MGB); middle layers of the superior colliculus; central gray; and external nucleus (E), dorsal cortex (DC) and central nucleus of IC. Contralaterally, labeled fibers and boutons were seen in the IC at a location homologous to the injection site, as well as in E and DC. A small projection was noted in contralateral MGB. These findings in young, adult mice with normal hearing can now serve as a baseline for similar experiments being conducted in mice and animals of other species of older ages and with varying degrees of hearing loss.

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