Central labeling of a sacculo-cochlear neuronal connection

Previous HRP, WGA-HRP and fluorescent tracer studies led to the assumption that primary afferent saccular fibers project to the cochlear nucleus. The aim of this study was to confirm the existence of these fibers by retrograde labeling after intracochlear tracer placement. The fluorescent carbocyanine dye DiI (1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbo-cyanine perchlorat) was chosen as the neuronal tracer. Guinea pigs weighing between 100-150g were perfused transcardially with 0.9% NaCl (37 deg C) followed by 4% paraformaldehyde in 0.1 M phosphate buffer (pH 7.4; 4 deg C). The animals were decapitated, the brainstem was exposed and cut transversely at the height of the cochlear nuclear complex. Crystals of DiI were carefully placed on the surface of the cochlear nucleus as lateral as possible. After 2-4 months, the brainstems were dissected free, embedded in tissue-tek and cut transversely at 40 μm on a cryocut. The sections were immediately mounted on glass-slides, allowed to dry and examined using an epifluorescent microscope equipped with a rhodamine filter. Labeled axons branching off the dorsal part of the inferior vestibular nerve were observed. These fibers may correspond to the sacculo-cochlear fiber projection. (author)