Tonotopy of TV neuron projections from the DCN to the aVCN is also blurred.

<p>(<b>A</b>) Photomicrograph of a section of a parasagittal slice from a P18 heterozygote control animal with a biocytin injection into the aVCN (arrow). Biocytin labeled processes and cell bodies of neurons that passed through the injection site. Labeled processes included auditory nerve fibers (asterisk), the axons and terminals of TV cells whose cell bodies lie in the DCN (bracket), and the dendrites and local axonal collaterals of T stellate and D stellate cells whose cell bodies formed a halo around the injection site. Labeled TV cells clustered in a band (bracket) among terminals of auditory nerve fibers that were labeled by the same injection, showing that the TV cells lie in the same isofrequency lamina as their VCN targets. A few TV cells whose axons crossed the injection site on their way to more ventral regions were labeled ventral to (arrowhead) the labeled band of TV cells. No cell bodies were labeled in the DCN dorsal to the labeled band, indicative of the sharp tonotopic organization of the projection. (<b>B</b>) A similar injection of biocytin into the aVCN of a P19 <i>Npr2</i> mutant animal labeled a more diffuse bundle of auditory nerve fibers, a halo of neurons in the aVCN, and TV cells that were more scattered than in the heterozygote (bracket). (<b>C</b>) To compare the distribution of labeled neurons between control and mutant animals, peaks of distributions were lined up and normalized. In WT (n = 6 slices, 258 cells) and Hets (n = 20 slices, 914 cells), labeled cells were distributed in a sharp band, with no labeled cells more than 150 µm from the peak on the dorsal side. Since no clear bands were observed in <i>Npr2</i> mutants (Mut) (n = 11 slices, 858 cells), histograms were aligned along their medians, at which half of the labeled cells lay more dorsal and half more ventral. The bands were sharp in both WT and Het animals, but were significantly broader in <i>Npr2</i> mutant mice (<i>P</i><0.001, ANOVA). No differences were detected over the age range examined between P14 and P26. (<b>D</b>) Examples of reconstructed slices, with labeled cells marked with dots, gray regions denoting areas containing granule cells in the largest section, and lines indicating the location of some of the labeled fibers. Numbers of labeled TV cell bodies were plotted as a function of distance along the tonotopic axis of the DCN, as illustrated. Scale bar, 500 µm.