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Introduction: Hearing decline stands as the most prevalent single sensory deficit associated with the aging process. Giving compelling evidence suggesting a protective effect associated with the efferent auditory system, the goal of our study was to characterize the age-related changes in the number of efferent medial olivocochlear (MOC) synapses regulating outer hair cell (OHC) activity compared with the number of afferent inner hair cell ribbon synapses in CBA/J mice over their lifespan. Methods: Organs of Corti of 3-month-old CBA/J mice were compared with mice aged between 10 and 20 months, grouped at 2-month intervals. For each animal, one ear was used to characterize the synapses between the efferent MOC fibers and the outer hair cells (OHCs), while the contralateral ear was used to analyze the ribbon synapses between inner hair cells (IHCs) and type I afferent nerve fibers of spiral ganglion neurons (SGNs). Each cochlea was separated in apical, middle, and basal turns, respectively. Results: The first significant age-related decline in afferent IHC-SGN ribbon synapses was observed in the basal cochlear turn at 14 months, the middle turn at 16 months, and the apical turn at 18 months of age. In contrast, efferent MOCOHC synapses in CBA/J mice exhibited a less pronounced loss due to aging which only became significant in the basal and middle turns of the cochlea by 20 months of age. Discussion: This study illustrates an age-related reduction on efferent MOC innervation of OHCs in CBA/J mice starting at 20 months of age. Our findings indicate that the morphological decline of efferent MOC-OHC synapses due to aging occurs notably later than the decline observed in afferent IHC-SGN ribbon synapses. [ABSTRACT FROM AUTHOR]

IntroductionHearing decline stands as the most prevalent single sensory deficit associated with the aging process. Giving compelling evidence suggesting a protective effect associated with the efferent auditory system, the goal of our study was to characterize the age-related changes in the number of efferent medial olivocochlear (MOC) synapses regulating outer hair cell (OHC) activity compared with the number of afferent inner hair cell ribbon synapses in CBA/J mice over their lifespan.MethodsOrgans of Corti of 3-month-old CBA/J mice were compared with mice aged between 10 and 20 months, grouped at 2-month intervals. For each animal, one ear was used to characterize the synapses between the efferent MOC fibers and the outer hair cells (OHCs), while the contralateral ear was used to analyze the ribbon synapses between inner hair cells (IHCs) and type I afferent nerve fibers of spiral ganglion neurons (SGNs). Each cochlea was separated in apical, middle, and basal turns, respectively.ResultsThe first significant age-related decline in afferent IHC-SGN ribbon synapses was observed in the basal cochlear turn at 14 months, the middle turn at 16 months, and the apical turn at 18 months of age. In contrast, efferent MOC-OHC synapses in CBA/J mice exhibited a less pronounced loss due to aging which only became significant in the basal and middle turns of the cochlea by 20 months of age.DiscussionThis study illustrates an age-related reduction on efferent MOC innervation of OHCs in CBA/J mice starting at 20 months of age. Our findings indicate that the morphological decline of efferent MOC-OHC synapses due to aging occurs notably later than the decline observed in afferent IHC-SGN ribbon synapses.

The lateral superior olive (LSO) is a key structure in the central auditory system of mammals that exerts efferent control on cochlear sensitivity and is involved in the processing of binaural level differences for sound localization. Understanding how the LSO contributes to these processes requires knowledge about the resident cells and their connections with other auditory structures. We used standard histological stains and retrograde tracer injections into the inferior colliculus (IC) and cochlea in order to characterize two basic groups of neurons: (1) Principal and periolivary (PO) neurons have projections to the IC as part of the ascending auditory pathway; and (2) lateral olivocochlear (LOC) intrinsic and shell efferents have descending projections to the cochlea. Principal and intrinsic neurons are intermixed within the LSO, exhibit fusiform somata, and have disk-shaped dendritic arborizations. The principal neurons have bilateral, symmetric, and tonotopic projections to the IC. The intrinsic efferents have strictly ipsilateral projections, known to be tonotopic from previous publications. PO and shell neurons represent much smaller populations (<10% of principal and intrinsic neurons, respectively), have multipolar somata, reside outside the LSO, and have nontopographic, bilateral projections. PO and shell neurons appear to have widespread projections to their targets that imply a more diffuse modulatory function. The somata and dendrites of principal and intrinsic neurons form a laminar matrix within the LSO and share quantifiably similar alignment to the tonotopic axis. Their restricted projections emphasize the importance of frequency in binaural processing and efferent control for auditory perception. This study addressed and expanded on previous findings of cell types, circuit laterality, and projection tonotopy in the LSO of the mouse. [ABSTRACT FROM AUTHOR]

The lateral superior olive (LSO) is a key structure in the central auditory system of mammals that exerts efferent control on cochlear sensitivity and is involved in the processing of binaural level differences for sound localization. Understanding how the LSO contributes to these processes requires knowledge about the resident cells and their connections with other auditory structures. We used standard histological stains and retrograde tracer injections into the inferior colliculus (IC) and cochlea in order to characterize two basic groups of neurons: (1) Principal and periolivary (PO) neurons have projections to the IC as part of the ascending auditory pathway; and (2) lateral olivocochlear (LOC) intrinsic and shell efferents have descending projections to the cochlea. Principal and intrinsic neurons are intermixed within the LSO, exhibit fusiform somata, and have disk-shaped dendritic arborizations. The principal neurons have bilateral, symmetric, and tonotopic projections to the IC. The intrinsic efferents have strictly ipsilateral projections, known to be tonotopic from previous publications. PO and shell neurons represent much smaller populations (