The inferior colliculus (IC) is partitioned into three subdivisions: the dorsal and lateral cortices (DC and LC) and the central nucleus (ICC), and serves as an integration center of auditory information. Recent studies indicate that a certain population of IC neurons may represent the non-GABAergic phenotype, while they express well-established cortical/hippocampal GABAergic neuron markers. In this study we used the optical disector to investigate the phenotype of IC neurons expressing parvalbumin (PV) and/or nitric oxide synthase (NOS) in C57BL/6J mice during the late postnatal period. Four major types of IC neurons were defined by the presence (+) or absence (–) of PV, NOS, and glutamic acid decarboxylase 67 (GAD67): PV+/NOS−/GAD67+, PV+/NOS+/GAD67+, PV+/NOS−/GAD67−, and PV−/NOS+/GAD67−. Fluorescent in situ hybridization for vesicular glutamate transporter 2 mRNA indicated that almost all GAD67− IC neurons represented the glutamatergic phenotype. The numerical densities (NDs) of total GAD67+ IC neurons remained unchanged in all subdivisions. The NDs of PV+/NOS−/GAD67+ neurons and PV−/NOS+/GAD67− neurons were reduced with age in the ICC, while they remained unchanged in the DC and LC. By contrast, the NDs of PV+/NOS+/GAD67+ neurons and PV+/NOS−/GAD67− neurons were increased with age in the ICC, although there were no changes in the DC and LC. The cell body size of GAD67+ IC neurons did not vary according to the expression of PV with or without NOS. The present findings indicate that the expression of PV and NOS may shift with age within the GABAergic and glutamatergic phenotypes of IC neurons during the late postnatal period. J. Comp. Neurol. 525:868–884, 2017.
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