Development of N-methyl-D-aspartate receptor subunit immunoreactivity in the neonatal gerbil cochlear nucleus

The distribution of immunoreactivity for the ionotropic N-methyl-D-aspartate (NMDA) receptor subunits was mapped in the cochlear nucleus of postnatal day (P) 7, P14, P21, and P28 gerbils. Frozen sections and serial plastic sections of tissue were incubated with antibodies to NMDAR1 (NR1), NMDAR2A (NR2A), NMDAR2A/B (NR2A/B), and NMDAR2B (NR2B). An overall diffuse stain was noted at P7 for NR1 and NR2A/B. Staining of neuronal somata in the dorsal cochlear nucleus molecular layer and fusiform cell layer, the posteroventral cochlear nucleus octopus cell area, and the anteroventral cochlear nucleus increased from P7 to P28. Staining of the neuropil (the unresolved mass of processes and axons, excluding only neuronal somata and distinctly stained proximal dendrites) of the deep dorsal cochlear nucleus and posteroventral cochlear nucleus showed a steady decrease, while molecular layer neuropil remained moderately stained. The NR2A antibody produced a distinctive staining of dendrites in the dorsal cochlear nucleus deep and fusiform cell layers seen first at P14 with increasing dendritic lengths stained at P21 and P28. Giant neurons of the deep dorsal cochlear nucleus were the most conspicuous somata stained by the NR2A. Their stained dendrites spanned much of the dorsal cochlear nucleus deep and fusiform cell layers and even extended into the octopus cell area of the posteroventral cochlear nucleus. Dendritic staining was also present in caudal and rostral posteroventral cochlear nucleus, first distinguishable at P14 and becoming increasingly strong. The Chemicon polyclonal NR2B antibody produced glial staining especially prominent in the caudal posteroventral cochlear nucleus and the dorsal cochlear nucleus fusiform cell layer, most intense at P7 and subsequently decreasing, although not disappearing, in all areas through P28. The Molecular Probes (Eugene, OR) polyclonal NR2B produced a light granular staining pattern over a number of somata but no glial staining. Neuropil staining was not prominent with either NR2B antibody. Differences in changes of neonatal immunoreactivity patterns in different populations of cochlear nucleus neuronal somata and dendrites for NR1, NR2A, NR2A/B, and NR2B suggest that alterations in some receptor composition is occurring over the period spanning the onset of hearing.