The medial olivocochlear (MOC) reflex arc is probably a three-neuron pathway consisting of type I spiral ganglion neurons, reflex interneurons in the cochlear nucleus, and MOC neurons that project to the outer hair cells of the cochlea. We investigated the identity of MOC reflex interneurons in the cochlear nucleus by assaying their regional distribution using focal injections of kainic acid. Our reflex metric was the amount of change in the distortion product otoacoustic emission (at 2f 1 -f 2 ) just after onset of the primary tones. This metric for MOC reflex strength has been shown to depend on an intact reflex pathway. Lesions involving the posteroventral cochlear nucleus (PVCN), but not the other subdivisions, produced long-term decreases in MOC reflex strength. The degree of cell loss within the dorsal part of the PVCN was a predictor of whether the lesion affected MOC reflex strength. We suggest that multipolar cells within the PVCN have the distribution and response characteristics appropriate to be the MOC reflex interneurons. Keywordshearing; outer hair cell; otoacoustic emission; superior oliveThe auditory system contains several descending pathways (Spangler and Warr, 1991), of which the best-studied is the olivocochlear (OC) efferents. OC neurons reside in the superior olivary complex and project to the cochlea (Fig. 1). The OC system is divided into two subsystems (reviewed by Warr, 1992;. Medial olivocochlear (MOC) neurons have cell bodies in the medial part of the superior olivary complex and project to outer hair cells. Lateral olivocochlear (LOC) neurons have cell bodies in or around the lateral superior olive and project to afferent fibers near inner hair cells (Warr and Guinan, 1979;Liberman, 1980;Liberman and Brown, 1986;Brown, 1987;Vetter and Mugnaini, 1992;Maison et al., 2003). Cholinergic MOC endings in the cochlea exert their effects by means of a nicotinic receptor (Vetter et al., 1999;Elgoyhen et al., 2001Elgoyhen et al., , 2003 that influences outer hair cell function and alters cochlear responses (Wiederhold and Kiang, 1970;Mountain, 1980;Siegel and Kim, 1982;Brown and Nuttall, 1984). For instance, activation of MOC neurons alters distortion product otoacoustic emissions (DPOAEs), causing rapid amplitude changes in the first several hundred milliseconds after primary-tone onset (Liberman et al., 1996;Kujawa and Liberman, 2001). The functional role of this sound-evoked feedback system may include adjusting *Correspondence to: M. Christian Brown, Eaton-Peabody Laboratory, Massachusetts Eye and Ear Infirmary, 243 Charles Street, Boston, MA 02114. E-mail: mcb@epl.meei.harvard.edu. Grant sponsor: National Institute on Deafness and Other Communication Disorders; Grant number: RO1 DC 01089; Grant number: RO1 DC 00188; Grant number: P30 DC05209; Grant number: T32 DC00020-11; Grant sponsor: Triological Society. NIH Public Access NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript cochlear gain, reducing effects of masking noise, and protecting the ear from acous...

The complementary pattern of immunohistochemical staining for the calcium-binding proteins parvalbumin (PV) and calbindin D-28k (CB) was used to delineate four major subdivisions of the rabbit medial geniculate body (MGB). PV immunoreactivity predominates in the ventral and medial divisions, whereas CB-immunoreactive cells characterize the dorsal and internal divisions. The ventral nucleus is strongly PV+ due to dense neuropil labeling and moderately labeled somata. The medial nucleus contains both medium-sized and large PV+ somata, as well as thick PV+ axons and terminals. The wedge-shaped internal nucleus composed of densely labeled CB+ cells, separates the dorsal and ventral nuclei rostrally, and expands caudally to encapsulate the posterior MGV. Large multipolar CB+ neurons with radiate dendrites characterize the dorsal nucleus. The differential expression of PV and CB also distinguishes the deep dorsal and superficial dorsal subnuclei in the dorsal division and a ventrolateral component in the ventral division. A comparison with studies of MGB connectivity in a variety of species suggests that PV immunoreactivity is highest in subdivisions that receive a substantial input from the central nucleus of the inferior colliculus and that project to primary auditory cortex. In contrast, CB immunoreactivity characterizes nuclei that receive input primarily from other sources, such as the paracentral nuclei of the inferior colliculus, the lateral tegmentum, and the spinal cord, and that project to secondary auditory areas. The ability of calcium-binding protein immunohistochemistry to delineate neuronal compartments across indistinct cytoarchitectonic borders makes it a powerful tool for guiding future connectional and physiological studies of the MGB.

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