Effects of the mu opioid agonist morphine (0.32-3.2 mg/kg) on ICSS eight days after bilateral intraplantar saline or 5% formalin. Panels A and B: Abscissae show frequency of electrical brain stimulation (Log Hz) in rats that received intraplantar saline (A) or formalin (B). Ordinates show ICSS rate expressed as percent maximum control rate (%MCR). “BL” shows the baseline frequency-rate curve determined before intraplantar treatment, “0.0” shows the frequency-rate curve determined on Day 8 after intraplantar treatment but before morphine treatment. Filled points in panel B show significant morphine effects relative to “0.0.” Panel C: The abscissa shows morphine dose in mg/kg in rats that received intraplantar saline (open bars) or formalin (filled bars). The ordinate shows ICSS rate expressed as total stimulations per component relative to baseline. Dollar signs ($) indicate a significant within-group difference from “0.0”, and asterisks (*) indicate a significant between-group difference at a given morphine dose. All statistical analyses were performed using a two-way Repeated Measures ANOVA followed by the Holm-Sidak post-hoc test (p < 0.05). All data show mean ± SEM from 8 rats per treatment. See Additional file 1 for detailed statistical results.