Histograms represent the average relative frequencies of synapses lacking GluN1, GluN2B and GluN2A (0 particles) or containing 1 to ≥ 18 immunogold particles. The fraction of synapses lacking GluN1- or GluN2B-immunogold particles was significantly higher in FZP-withdrawn rats (p<0.05 and p<0.01, respectively, Mann-Whitney U test). A significant decrease was also observed in the fraction of FZP-withdrawn synapses containing 8 GluN1 immunogold particles, that was parallel to increases in the fraction of synapses containing 2 immunogold particles (p<0.05, Mann-Whitney U test). No significant changes in either the fraction of synapses lacking immunogold particles or the fraction of synapses labeled with different numbers of immunogold particles were detected in the case of GluN2A subunit labeling (p>0.05, Mann-Whitney U test).