Transganglionic transport of D-aspartate from cochlear nucleus to cochlea--a quantitative autoradiographic study

This study concerns the connections of the inner and outer hair cells and the different types of ganglion cells of the mammalian cochlea with the central nervous system by making use of their putative neurotransmitters. D-[3H]Aspartate (D-ASP), a putative marker for glutamatergic neurons, was injected into the cochlear nucleus of cats and guinea pigs and the cochleas prepared for light microscopic autoradiography after varying survival times. A quantitative, statistical autoradiographic method is described. Grain counts per unit area were made for each of 14 tissue compartments in the cochlea and normalized to permit comparisons between cases. An operationally defined background labeling level was computed for each case and a statistical test for significance applied to the neuron-containing tissue compartments. With increasing survival times, significant labeling appeared successively in the cochlear nerve root, in each type of spiral ganglion cell, and in the neuron-containing tissue compartments of the organ of Corti. The findings are consistent with uptake of D-ASP and retrograde transport by cochlear nerve axons from the cochlear nucleus to the perikarya and peripheral processes of the spiral ganglion. We conclude that axons of all spiral ganglion cells project to the cochlear nucleus and that this nucleus is directly connected with both the inner and outer hair cells. Transganglionic transport of D-ASP from the cochlear nucleus is consistent with the hypothesis that the cochlear nerve axons use glutamate or aspartate as a neurotransmitter.