Average (± 1 S.E.M.) number of head twitches observed in a 20-minute observation period in rats that received cumulative doses of DOM (left and right panels, open circles) or lorcaserin (middle panel, open squares). DOM was also studied in combination with 0.01 mg/kg MDL 100907 (left panel, closed circles) and in combination with 3.2 mg/kg lorcaserin (right panel, closed diamonds). Lorcaserin was examined in combination with 1.0 mg/kg SB 242084 (middle panel, closed squares). The average number of head twitches (ordinate) is expressed as a change from a saline test (eight saline injections spaced 25 minutes apart to match the temporal conditions of the drug tests) and plotted as a function of dose (abscissae; V, vehicle).