Transport of radioactivity from primary auditory neurons beyond the cochlear nuclei

Transneuronal degeneration in secondary brainstem auditory nuclei has been described following cochlear lesions, and Marchi studies have revealed degeneration in the trapezoid body after such lesions. These findings support the view of some early neuroanatomists that primary auditory afferents project beyond the cochlear nuclei. Silver impregnation studies of degenerating cochlear afferents have failed to support this claim, leaving the question unresolved. Using [3H] amino acids auditory projections in the monkey were traced autoradiographically from labeled ganglion cells: (1) in all portions of the spiral ganglion, (2) in apical and middle turns of the cochlea and (3) primarily in the basal turn of the cochlea. Labeling of the entire spiral ganglion resulted in profuse transport of isotope to parts of all cochlear nuclei [anteroventral (AVCN), posteroventral (PVCN), and dorsal (DCN) cochlear nuclei]. Labeled axons entering the trapezoid body (TB) projected ipsilaterally to: (1) the lateral trapezoid nuclei (LTN), (2) the lateral superior olivary nucleus (LSO), (3) the dorsal dendritic zone of the medial superior olivary nucleus (MSO), and (4) the pericollicular or cortical nucleus of the inferior colliculus (IC). Auditory fibers crossing the midline in the TB projected contralaterally to: (1) the medial trapezoid nucleus (MTN), (2) the ventral dendritic zone of the MSO, (3) the ventral nucleus of the lateral lemniscus (VNLL), and (4) the central nucleus of the inferior colliculus (CNIC). Selective labeling of cells in the apical and middle ganglionic turns ofthe cochlea resulted in a similar pattern of centripetal transport. Selective [3H] uptake primarily by ganglion cells in the basal turn of the cochlea resulted in: (1) labeling of restricted parts of all cochlear nuclei, and (2) a considerable reduction of isotope transported beyond the cochlear nuclei. Labeled fibers in the TB were distributed sparsely in the ipsilateral LTN, LSO and dorsal dendritic zone of the MSO. Fibers in the TB were not labeled contralaterally in these animals. Interpretation of these data considers the possibility of transneuronal isotope transport. The pattern of isotope transported beyond the cochlear nuclei does not correspond to the secondary projections of any individual cochlear nucleus, although it bears some resemblance to that of the AVCN. The current study suggests that some primary auditory afferents project beyond the cochlear nuclei.