EFFERENT PROJECTIONS OF A PHYSIOLOGICALLY CHARACTERIZED REGION OF THEINFERIOR COLLICULUS OF THE YOUNG-ADULT CBA MOUSE

The present investigation is part of an ongoing series of studies aime d at discerning the neural bases of presbycusis. Presbycusis is a sens ory perceptual disorder involving loss of high-pitch hearing and reduc ed ability to process biologically relevant acoustic signals in noisy environments. The purpose of the present experiment was to delineate t he efferent projections of a functionally characterized region of the dorsomedial inferior colliculus (IC, auditory midbrain) in young, adul t CBA mice. The CBA strain's progressive loss of hearing over its life span approximates many aspects of the mild-to-moderate hearing loss ex perienced by a significant number of humans suffering from presbycusis . Focal, iontophoretic injections of HRP were made in the 18-24 kHz re gion of dorsomedial IC of the CBA strain following physiological mappi ng experiments. Serial sections were reacted with a chromagen, counter stained and examined for anterogradely labeled fibers and boutons. Eff erent projections were observed ipsilaterally in: medial and ventral d ivisions of the medial geniculate body (MGB); middle layers of the sup erior colliculus; central gray; and external nucleus (E), dorsal corte x (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 contrala teral MGB. These findings in young, adult mice with normal hearing can now serve as a baseline for similar experiments being conducted in mi ce and animals of other species of older ages and with varying degrees of hearing loss. (C) 1997 Acoustical Society of America.