Objectives/Hypothesis: Clinical and basic studies have correlated tinnitus with stress. Although the etiology of tinnitus is unknown, the cochlear nucleus (CN) appears to play a role. To better understand the potential impact of stress on tinnitus and modulation of the central auditory system in general, the goal of the current study was to examine the presence and distribution of axon terminals containing urocortin in the CN of the mouse.
Study Design: Prospective description of histological findings.
Methods: Three different forms of urocortin were labeled in brainstem sections collected from 10 wild-type mice by immunohistochemistry. Immunoreactive terminal fibers in the CN were digitally photographed, as well as reconstructed in the CN under a drawing tube attached to a light microscope.
Results: Specific staining was found in en passant type fibers scattered throughout the CN but situated mostly within the granule cell domains. Clusters of labeled fibers were observed in the nerve root. Labeled axons were observed in the three tracts known to carry olivocochlear fibers to the CN, as well as in the olivocochlear bundle itself. As the axons within the olivocochlear bundle departed the brainstem in the vestibular nerve, numerous labeled en passant fibers were observed among somata of the vestibular ganglion (Scarpa's). Centrally, labeled axons were followed from the CN to the lateral superior olive, an established source of urocortin-positive efferents to the cochlea. These findings indicate that lateral olivocochlear efferents innervate the CN and Scarpa's ganglion, and also that urocortin is likely a neuromodulator in particular CN circuits.
Conclusions: The current study supports innervation of specific regions of the mouse CN and Scarpa's ganglion by neurons expressing urocortin. The innervation may be one substrate by which stress modulates particular CN processes. Further studies are necessary to establish the role of urocortin in CN models of tinnitus.