Alterations in synaptic NMDAR subunit expression after extinction of morphine CPP. A, B, After extinction of morphine CPP, NR2b NMDAR subunit expression was increased at the PSD (n = 4 per group; unpaired t test; *p < 0.05). C, D, In contrast, mice showed no changes in NMDAR subunit expression after extinction from a design of unpaired morphine treatment.