Figure 5.

Synaptic localization of NR2A and NR2B subunits demonstrated by postembedding immunogold electron microscopy at developing synapses in VP nucleus of thalamus. A-C, Progressive changes in the synaptic distribution of NR2A subunit at developing thalamic synapses. A, Few, if any, NR2A immunogold particles are seen at PSDs of asymmetrical synapses at P4 (A), but by P15 (C), a number of NR2A gold particles are found concentrated at the PSD of an asymmetrical synapse formed by a terminal (t). Scale bars, 0.2 μm. D, Graph illustrating the increased number of NR2A particles at single PSD profiles of thalamic synapses as a function of postnatal age. A significant increase occurs between P4 and P9 (Student's t test; p ≤ 0.001). E-I, Electron micrographs showing synaptic localization of NR2B subunits at developing synapses in VP nucleus. At P2 (E, F), NR2B immunogold particles are specifically associated with PSDs of thalamic synapses contacting dendritic profiles (d). Inset in E is a high-magnification image showing immunogold particles associated with a PSD profile. At P7 (G, H), electron micrographs show NR2B immunogold particles concentrated at the PSDs of asymmetrical synapses (t). By P15 (I), few gold particles are detected at thalamic PSDs. The inset in I shows a high-power image of two gold particles at a PSD profile. Scale bars: E-I, 0.2 μm; E, G, insets, 0.1 μm; I, inset, 0.05 μm. J, Graph showing the progressive change in the mean number of NR2B particles at single PSD profiles of thalamic synapses as a function of postnatal age; a significant decrease occurs from P7 to P15 (Student's t test; p ≤ 0.001). K, Graph showing changes in the normalized number of NR2A (filled black circle) and NR2B (filled black square) particles at thalamic synapses at different postnatalages. The mean numbers of particles at each age is normalized to the mean numbers at P2. Note the reciprocal nature of changes in NR2A and NR2B subunits at single PSD profiles after P7.