Synaptic input to cochlear nucleus dendrites that receive medial olivocochlear synapses

Abstract

Axons of olivocochlear neurons originate in the superior olivary complex and project to the cochlea. Along their course, medial olivocochlear axons give off branches to the cochlear nucleus. We labeled these branches with horseradish peroxidase and used electron microscopy to determine their target dendrites. Target dendrites were of two classes: "large" dendrites and "varicose" dendrites. Using serial sections, we reconstructed the dendrites and, in addition to the labeled olivocochlear input, we determined the synaptic profile of unlabeled inputs onto the dendrites. We classified the terminals on the basis of the shape and size of their synaptic vesicles. On large dendrites, the predominant type of unlabeled terminal had small round (SmRnd) vesicles. These terminals are likely to be excitatory, and some of them may originate from unlabeled medial olivocochlear branches. On varicose dendrites, the predominant type of terminal had pleomorphic vesicles. These terminals are likely to be inhibitory. They may be from descending inputs that arise in higher centers. A final type of terminal onto large dendrites exhibited signs of neuronal degeneration, possibly because the cell body of origin was damaged during the injection procedure. These terminals often had long, perforated synaptic densities and may originate from type II primary afferents. Thus, medial olivocochlear efferents and type II afferents, which both contact outer hair cells in the periphery, appear to synapse onto the same targets in the cochlear nucleus. In contrast, where examined, the target dendrites did not receive terminals with large vesicles from afferents that contact inner hair cells. Thus, target neurons appear to function in a neural circuit associated more closely with outer than with inner hair cells.

Functional organization of the dorsal cochlear nucleus of the horseshoe bat ( Rhinolophus rouxi ) studied by GABA and glycine immunocytochemistry and electron microscopy

“…The superficial layers of the horseshoe bat DCNv also contain mossy fiber glomeruli, the typical synaptic input structures of granule cells. Such endings are known to be derived from various sources including descending auditory projections from the olivocochlear system Benson et al 1996), from the IC (Caicedo and Herbert 1993) and the primary auditory cortex (Feliciano et al 1995;Weedman and Ryugo 1996), input from the vestibular (Burian and Gstoettner 1988) and somatosensory (Itoh et al 1987;. Our finding of colocalization of dense core vesicles with electron-lucent vesicles in some mossy fiber endings also argues for a heterogeneity of origin (McDonald and Rasmussen 1971;Vetter et al 1993;Dunn et al 1996;Petralia et al 1996).…”

“…The granule/cartwheel cell system of the superficial layers (layers I and II) targets the apical dendrites of the fusiform cells and provides multimodal and non-primary auditory inputs. Its excitatory interneurons, the granule cells, receive inputs from various sources, e.g., type II auditory nerve fibers (Berglund and Brown 1994;Berglund et al 1996), descending projections from the olivocochlear system Benson et al 1996), the inferior colliculus (Caicedo and Herbert 1993) and the primary auditory cortex (Feliciano et al 1995;Weedman and Ryugo 1996) as well as non-auditory inputs from the somatosensory Itoh et al 1987) and the vestibular system (Kevetter and Perachio 1989). The inputs are mediated via mossy fibers in typical, glomerular-like arrangements (review: Dunn et al 1996).…”

Ultrastructure, synaptic organization, and molecular components of bushy cell networks in the anteroventral cochlear nucleus of the rhesus monkey

“…Ultrastructurally, these endings resemble cholinergic endings in the rodent cochlear nucleus and suggest that they contribute to the modulation of BC output (Benson et al, 1996; Gómez-Nieto et al, 2008a). …”

Golgi cells in the superficial granule cell domain overlying the ventral cochlear nucleus: Morphology and electrophysiology in slices

“…The granule cell domains receive auditory input from many levels. Unmyelinated, primary auditory nerve afferents terminate in or near the granule cell domains (Brown and Ledwith, 1990;Benson et al, 1996;Berglund et al, 1996). Two types of cells from the magnocellular VCN terminate in granule cell domains, octopus cells (Golding et al, 1995), and D stellate cells .…”

“…Two types of cells from the magnocellular VCN terminate in granule cell domains, octopus cells (Golding et al, 1995), and D stellate cells . There are descending projections from the olivocochlear efferents (Brown et al, 1988b;Benson et al, 1996), from the inferior colliculus (Caicedo and Herbert, 1993), and from the auditory cortex (Feliciano et al, 1995;Weedman and Fig. 5.…”