Chronic morphine decreases the paired-pulse ratio (PPR) of glutamate EPSCs in μ-opioid receptor-containing cells (termed secondary cells) in the NRM from morphine-tolerant rats. A, Representative EPSC pairs in secondary cells from a saline-treated rat, a morphine-treated tolerant rat and the same two EPSC pairs normalized to the first EPSC. B, Representative EPSC pairs in the other type of NRM cells (termed primary cells) lacking the μ receptor from two similar groups of rats. C, Group data of the PPR in the two cell types from saline control and morphine-tolerant rats. Stimulus artifacts are blanketed. Numbers in columns indicate cell numbers. ** p < 0.01.