α2δ-1 mediates chronic morphine exposure–induced potentiation of presynaptic N-methyl-d-aspartate receptor activity in the spinal dorsal horn. (A) Representative recording traces and cumulative plots show the effect of bath application of 50 μM 2-amino-5-phosphonopentanoic acid (AP5) on the frequency and amplitude of miniature excitatory postsynaptic currents (mEPSCs) of a lamina II neuron from a morphine-treated rat. (B) Summary data show the effect of 50 μM AP5 on the mean frequency and amplitude of mEPSCs of lamina II neurons (n = 11 neurons) from morphine-treated rats. (C) Representative recording traces and cumulative plots show that bath application of 50 μM AP5 had no effect on the frequency or amplitude of mEPSCs of a lamina II neuron pretreated with 100 µM gabapentin from a morphine-treated rat. (D) Summary data show no effect from 50 μM AP5 on the mean frequency or amplitude of mEPSCs of lamina II neurons (n = 10 neurons) pretreated with 100 µM gabapentin from morphine-treated rats. Data are shown as means ± SD. **P < 0.01 versus the baseline. ###P < 0.001 versus the baseline in the morphine + vehicle group.