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Keywords:

  • auditory system;
  • electron microscopy;
  • hearing;
  • multipolar cells

Abstract

Ventrotubercular cells are multipolar cells in the ventral cochlear nucleus (VCN) that project a collateral axon to the ipsilateral dorsal cochlear nucleus (DCN). These cells are thought to be involved in sensitizing DCN output neurons to spectral shapes that represent the location of a sound source in space. The present report focused on the neuronal composition of this pathway. Intracellular labeling studies in cats and mice have described two types of ventrotubercular cells (Smith and Rhode [1989] J Comp Neurol. 282:595–626; Oertel et al. [1990] J Comp Neurol. 295:136–154). In cats, one difference between the two classes is that type I multipolar neurons have fewer than 35% of their somata apposed by terminals, whereas type II cells have greater than 70% apposition values. Intracellular recordings from single cells, however, are difficult and thus limit the yield of data. We investigated whether a two-component description of the ventrotubercular pathway was representative of a larger population. This issue was addressed by retrogradely labeling ventrotubercular neurons with an extracellular injection of biotinylated dextran amine into the DCN of rats. These injections labeled many VCN neurons, thus providing a more complete view of the pathway than previous studies. Thirty-eight labeled cells were selected for electron microscopic analysis with respect to their location, cell body size, and ultrastructural morphology. We observed labeled type I and type II neurons, but unlike ventrotubercular cells in cats, many of these neurons in rats (17 of 38 cells) had appositions between 35% and 70%. On the basis of this analysis, a third class of ventrotubercular cell, called the adendritic neuron, was revealed. Adendritic neurons have small somata with many filopodial appendages, no observable dendrites, and high percentage of terminal appositions (>80%). The results demonstrated that the ventrotubercular pathway in the rat is diverse. J. Comp. Neurol. 459:77–89, 2003. © 2003 Wiley-Liss, Inc.