Transneuronal transport in the vestibular and auditory systems of the squirrel monkey and the arctic ground squirrel. I. Vestibular system

Abstract

Transneuronal transport of [ 3H]proline, [ 3H]fucose, and [ 3H]leucine in various combinations from pledgets implanted in the ampulla of a single semicircular duct was studied in the squirrel aonkey and arctic ground squirrel after long survival periods. Triated amino acids implanted in any single ampulla resulted in labeling of nearly all vestibular and auditory receptors, nearly all cells of the vestibular and spiral ganglia and central transport via nearly all root fibers of both nerves. Primary vestibular fibers were distributed to the vestibular nuclei (VN) and specific parts of the cerebellum in the pattern previously described 11. Transneuronal transport of [ 3H]proline by vestibular neurons was present in all known secondary pathways, except those projecting to thalamic nuclei. Observations were similar in both species, except for small differences in commissural vestibular projections. Major commissural transport was to all parts of the opposite medial vestibular nucleus (MVN) and to peripheral parts of the superior vestibular nucleus (SVN), but some transport was present in all contralateral VN, including ventral cell group y. Descending transneuronal transport was evident in vestibulospinal tract (VST) ipsilaterally and in the medial longitudinal fasciculus (MLF) bilaterally. Both [ 3H]proline and [ 3]fucose were transported transneuronally to the ipsilateral abducens nucleus (AN); with long survivals [ 3H]proline was transported preipherally via the ipsilateral abducens nerve root. Ascending transport in the MLF was bilateral, asymmetric and greatest contralaterally. Fibers entered the contralateral MLF near the AN and the lateral wing of the ipsilateral MLF rostral to most of the VN. Terminals in the trochlear nuclei (TN) were bilateral and greatest contralaterally. In the monkey terminals in ipsilateral oculomotor complex (OMC) were distributed uniformly in all subdivisions, except for the medial rectus subdivision (MRS), where terminals were more numerous. The greatest density of terminals was present contralaterally in the superior rectus subdivision (SRS) of the OMC; only spare terminals were present in the MRS on that side. Transport in the ipsilateral abducens nerve roots in the monkey and the virtual absence of transport to the MRS of the contralateral OMC suggested transneuronal transport to abducens motor neurons, but not to internuclear neurons (AIN). The AIN project only the MRS of the contralateral OMC and do not appear to receive vestibular input. Comparable observations were made in the AN, TN and OMC of the ground squirrel, although the representation of the extraocular muscles in the OMC is unknown. Fibers by-passing the OMC terminated bilaterally in the interstitial nucleus of Cajal (INC), the nucleus of Darkschewitsch ( (ND) and in the rostral interstitial nucleus of the MLF (RiMLF); terminals in INC and RiMLF were greatest contralaterally. Contralateral dominance in the transneuronal transport to the RiMLF suggests that secondary vestibular projections to this nucleus may arise from nuclei other than SVN which gives rise only to uncrossed projections. Failure of vestibulothalamic fibers to show transneuronal transport did not appear to be due to their length, since terminals were found in the RiMLF.