H89 reverses the chronic morphine effect on glutamate EPSCs in tolerant secondary cells. A, Normalized EPSC pairs in tolerant cells without and after treatment with H89 (10 μM), a protein kinase A inhibitor. B, Group data of the H89 effect on PPR in tolerant (n = 13) and saline control cells (n = 6). C, A plot of mEPSC frequency distribution in tolerant cells with or without H89 treatment. D, Group data of the H89 effect on mEPSC frequency in tolerant and saline control cells (n = 5 in each group).