Formalin induced-depression of intracranial self-stimulation (ICSS) is dose and time related. Abscissa: Days after varying doses of intraplantar formalin or saline administration. Ordinate: ICSS rate expressed as percent baseline stimulations per component. Statistical analysis was performed using two-way Repeated Measures ANOVA followed by the Holm-Sidak post hoc test (p < 0.05). Dollar signs ($) indicate a significant within-group difference from the pre-injection baseline, and filled points indicate a significant between-group difference at that time point relative to saline treatment. All data show mean ± SEM from 6 rats per treatment. See Additional file 1 for detailed statistical results.