They studied mice that were administered nicotine, the active constituent of cigarettes, for several weeks. The researchers then withheld the nicotine and measured the subsequent alterations in dopamine signaling during the withdrawal period.
They reported that withdrawal from nicotine produced a deficit in dopamine in which the basal dopamine concentration and tonic dopamine signals were disproportionately lower than the phasic dopamine signals. Re-exposure to nicotine reversed the hypodopaminergic state.