Anatomy of olivocochlear neurons in the hamster studied with FluoroGold
Introduction
In nearly all mammals, the olivocochlear (OC) neurons form two distinct efferent systems, lateral and medial, that originate in the lateral and medial regions of the superior olivary complex and that innervate, respectively, the inner versus outer hair cells of the cochlea (for review, see Warr, 1992). The functional role of these systems is, in general terms, to reflexively modulate the sensitivity of receptor mechanisms operating in relation to each type of hair cell within the organ of Corti (for reviews, see Guinan, 1996, Veuillet et al., 1999, Le Prell et al., 2001, Fuchs, 2002, Puel et al., 2002).
Lateral olivocochlear (LOC) neurons are situated close to or within the lateral superior olivary nucleus (LSO) and project by means of unmyelinated axons mainly to the ipsilateral cochlea where they synapse primarily with radial afferent fibers beneath the inner hair cells. Medial olivocochlear (MOC) neurons are situated in the medial region of the superior olivary complex and project by means of myelinated axons mainly to the contralateral cochlea, where they synapse directly with outer hair cells. In addition to their projections to the cochlea, MOC neurons (Brown et al., 1991) send axon collaterals into the ventral cochlear nucleus, which appear to compensate for the suppressive effects that MOC neurons exert in the periphery (Benson and Brown, 1990, Kim et al., 1995, Mulders et al., 2003).
The inputs to LOC neurons originate almost exclusively from the ipsilateral cochlear nucleus. Those to MOC neurons are somewhat more complex, and include predominantly contralateral projections from the cochlear nuclei as well as descending fibers from the inferior colliculus and the auditory cortex (Mulders and Robertson, 2000, Xiao and Suga, 2002). The inputs descending from higher levels are thought to underlie the possible involvement of the MOC system in selective attention (Dewson, 1968, Oatman and Anderson, 1977, Scharf et al., 1997). Vetter and co-workers discovered a new type of OC neuron in the rat, shell neurons, which surround the LSO (Vetter et al., 1991, Vetter and Mugnaini, 1992). The existence of a similar cell type has been confirmed in the chinchilla (Azeredo et al., 1999) and the cat (Warr et al., 2002). These neurons, considered a subgroup of LOC neurons since they innervate the inner hair cell region (Warr et al., 1997), have recently also been shown to send collaterals into the ventral cochlear nucleus of the rat (Horvath et al., 2000).
The efferent innervation in the golden hamster has been the subject of a series of studies aimed at determining the temporal sequence of neurodevelopment of the LOC and MOC systems (Simmons et al., 1990, Simmons et al., 1996a, Simmons et al., 1996b, Simmons et al., 1999, Simmons and Raji-Kubba, 1993, Simmons, 1994, Raji-Kubba et al., 2002). These studies have provided presumptive evidence concerning the location and types of OC neurons in the hamster, but these cells have yet to be positively labeled, counted, measured, and their respective patterns of ipsilateral versus contralateral projections determined. A need for such information became apparent in our own laboratory in connection with studies of a strain of hamsters which are subject to audiogenic seizures and exhibit morphological changes both in the cochlea and in the cell body of MOC neurons (Cantos et al., 1996, López et al., 1999). Accordingly, we undertook a comprehensive morphological analysis of the OC neurons in the hamster based on labeling with the sensitive retrograde tracer FluoroGold® (FG). Our report also includes a three-dimensional (3D) reconstruction of the topographic distribution of the OC neurons in the brainstem.
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Animals and surgical procedures
Five adult golden hamsters (Mesocricetus auratus) (Charles River Labs, Barcelona, Spain) were used in this study. Animals were handled and experiments were conducted according to the guidelines of the European Communities Council Directive (86/609/EEC) and the current Spanish legislation for the use and care of laboratory animals (BOE 67/8509-12, 1998).
The hamsters were anesthetized with an intramuscular injection of a mixture of ketamine chlorohydrate (Imalgene®) and xylazine (Rompun®). The
Results
The injection of FG into the left cochlea of the hamsters resulted in the labeling of all three types of OC neurons bilaterally in the superior olivary complex. Sections of the injected cochlea confirmed that, in the two cochleae examined, FG had reached the entire basal–apical extent of the organ of Corti. As anticipated from the results in other rodents, we found the two fundamental types of OC neurons: (a) intrinsic LOC neurons, situated within the neuropil of the LSO, mainly (almost
Discussion
We have described the morphology and number of OC neurons in the hamster and created a 3D reconstruction of their distribution in the superior olivary complex. Overall, our findings confirm, by means of specific retrograde labeling from the cochlea, that the hamster does possess the same three basic types of OC neurons found in several other rodent and non-rodent species, and that they have rather similar locations and proportional distributions as found in other rodents. However, it was
Acknowledgements
The authors wish to thank Mr. Ignacio Plaza for excellent technical assistance in the histological procedures, and Mr. Juan Carlos Andaluz, who kindly helped with the 3D reconstruction software. M.A.S.G. was supported by a scholarship from the Instituto de Neurociencias de Castilla y León. W.B.W. was supported by NIH Grant NIDCD DC00215. D.E.L. was supported by Grants FIS PI021697 and JCyL-UE SA084/01.
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2023, Journal of Chemical NeuroanatomyMorphofunctional alterations in the olivocochlear efferent system of the genetic audiogenic seizure-prone hamster GASH:Sal
2017, Epilepsy and BehaviorCitation Excerpt :We also obtained a distribution pattern of olivocochlear neurons at the ipsicontralateral and rostrocaudal levels, in the GASH:Sal and control hamsters, which was similar to that reported in other rodents [21,46–49]. In our material, the total number of olivocochlear neurons in the control and GASH:Sal hamsters was higher than that obtained in our previous study using FG injections into the hamster's cochlea [21]. In fact, our quantification was very similar to that obtained in the mouse [47].
Dendrites of medial olivocochlear neurons in mouse
2008, NeuroscienceCitation Excerpt :Secondly in favor of an MOC designation, DPO neurons occur on both sides of the brainstem, with the majority on the opposite side (Fig. 3B). This pattern is like other MOC neurons and unlike the mainly ipsilateral LOC neurons, in particular, the shell neurons that are mostly on the same side of the brain as the innervated cochlea (Vetter and Mugnaini, 1992; Azeredo et al., 1999; Sánchez-González et al., 2003). DPO neuron axons in our material were observed to be as thick in diameter as VNTB axons.
Efferent System
2008, The Senses: A Comprehensive Reference