Role of the endogenous kappa opioid system in pain-related depression of ICSS. Panels A and B: Transcript levels of PDYN (A) or KOR (B) mRNA as measured by qRT-PCR in brain regions implicated in mood disorders (CPu: caudate/putamen, NAc: nucleus acumbens core, NAcSh: nucleus acumbens shell, PFC: prefrontal cortex, VTA: ventral tegmental area). Abscissae: Brain area evaluated. Ordinates: Transcript levels expressed as “Fold-Induction” relative to saline controls. “ND” in Panel B signifies “Not Determined” due to low transcript levels below the level of detection in some rats. Panel C-D: Effects of the kappa antagonist norBNI (32 mg/kg) on ICSS in rats treated with intraplantar saline (C) or formalin (D). Abscissae show frequency of electrical brain stimulation (Log Hz). Ordinates show ICSS rate expressed as percent maximum control rate (%MCR). “BL” shows the frequency-rate curve determined on Day 7 after intraplantar saline or formalin and immediately before norBNI treatment. ICSS was then redetermined 24 hr after norBNI. Summary data are shown in Panel E, where the abscissa shows norBNI dose in mg/kg in rats that received intraplantar saline (open bars) or formalin (filled bars), and the ordinate shows ICSS rate expressed as total stimulations per component relative to the pre-formalin baseline. The asterisk (*) indicates a significant between-group difference at a given dose. All data show mean ± SEM from 6 rats. See Additional file 1 for detailed statistical results.