EC, IC or SC rats were administrated nicotine (Nic, 0.35 mg/kg, s.c.) or saline (Sal) on Days 1–15. Panels A and B show the total horizontal activity (mean ± SEM) in saline control and nicotine-treated groups during the 30 min pre-injection habituation period. Panels C and D shows the total horizontal activity (mean ± SEM) during the 60 min following saline or nicotine injection with robust behavioral sensitization observed under all housing conditions (panel D). Total horizontal activity after a subcutaneous injection of saline or nicotine revealed a significant effect of housing condition (F(2, 67) = 104.29, p<0.001), treatment (F(1, 67) = 333.57, p<0.001) and day (F(7, 469) = 139.97, p<0.001). A significant interaction of housing condition × treatment × day (F(14, 469) = 2.76, p<0.05) and treatment × day (F(7, 469) = 115.42, p<0.05) were found. n = 12 rats/group.