Effects of complete Freund’s adjuvant (CFA), formalin, or respective controls on intracranial self-stimulation (ICSS). Panels A and B: Abscissae show hours or days following bilateral 100 ul injection of CFA (gray bars, Panel A), formalin (filled bars, Panel B), or respective saline controls (open bars, both panels). Ordinates show ICSS rate expressed as total stimulations per component relative to pre-injection baseline. Dollar signs ($) indicate a significant within-group difference from the respective baseline, and asterisks (*) indicate a significant between-group difference at that time point. Panels C and D show full frequency-rate ICSS curves for selected time points from A and B. Abscissae show frequency of electrical brain stimulation (Log Hz). Ordinates show ICSS rate expressed as percent maximum control rate (%MCR). Filled points indicate statistical significance of treatment effects relative to the pre-injection baseline. All statistical analyses were performed using two-way Repeated Measures ANOVA followed by the Holm-Sidak post hoc test (p < 0.05). All points show mean ± SEM from 8 rats. See Additional file 1 for detailed statistical results.