MOSSY FIBER PROJECTIONS FROM THE CUNEATE NUCLEUS TO THE COCHLEAR NUCLEUS IN THE RAT

A reciprocal connection is known to exist between the cuneate nucleus, which is a first-order somatosensory nucleus, and the cochlear nucleu s, which is a first-order auditory nucleus. We continued this line of study by investigating the fiber endings of this projection in the coc hlear nucleus of rats using the neuronal tracer Phaseolus vulgaris leu coagglutinin in combination with ultrastructural and immunocytochemica l analyses. In the cochlear nucleus, messy fiber terminals had been de scribed and named for their morphologic similarity to those in the cer ebellum, but their origins had not been discovered. In the present stu dy, we determined that the axonal projections from the cuneate region gave rise to messy fiber terminals in the granule cell regions of the ipsilateral cochlear nucleus. The cuneate messy fibers appear to be ex citatory in nature, because they are filled with round synaptic vesicl es, they make asymmetric synapses with postsynaptic targets, and they are labeled with an antibody to glutamate. The postsynaptic targets of the messy fibers include dendrites of granule cells. This projection onto the granule cell interneuron circuit of the cochlear nucleus indi cates that somatosensory cues are intimately involved with information processing at this early stage of the auditory system. (C) 1996 Wiley -Liss, Inc.