EPSC-enhancing effect of 8-bromo-cAMP is increased in secondary cells from morphine-tolerant rats. A, Normalized EPSC pairs in control and in 8-broma-cAMP (8-br-cAMP, 1 mM) in a saline control cell and a tolerant cell. B, Group data of 8-br-cAMP-induced decrease in the PPR in saline control (n = 4) and tolerant cells (n = 5). C, Distribution data of mEPSC frequency before and during application of 8-br-cAMP in a saline control (left) and a tolerant cell (right). D, Percent increase by 8-br-cAMP and forskolin in mEPSC frequency in saline control and tolerant cells.