Effects of ERK kinase (MEK) blockade on extracellular signal regulated kinases (ERK) and cAMP-response element binding protein (CREB) phosphorylation. (A,B) Quantitative analysis and representative immunoblots of pERK1/2 and pCREB (respectively) in hypothalamic paraventricular nucleus tissue isolated from placebo or morphine-dependent rats after administration of naloxone in absence or presence of SL-327. Each bar represents mean ± SEM (% of controls). Post hoc analysis revealed that the increase in CREB phosphorylation during morphine withdrawal was not attenuated by SL-327 (B), whereas both ERK1 and ERK2 phosphorylation was attenuated by SL-327 (A). *P < 0.05; **P < 0.01 versus placebo + dimethyl sulphoxide (DMSO) + naloxone; +++P < 0.001 versus morphine + DMSO + naloxone; #P < 0.05; ##P < 0.01; ###P < 0.001 versus placebo + DMSO + naloxone. m, morphine pellets; n, naloxone; p, placebo pellets.