Intravenous nicotine self-administration. a, Mean ± SE of the daily number of responses in the device delivering the infusion of nicotine (active) and in the control device (inactive) calculated over the 42 d of testing. b, Mean ± SE of the daily intake of nicotine calculated over the 42 d of testing. Nicotine induced self-administration, as shown by the higher number of responses in the active than in the inactive device (°°°p < 0.001). In contrast, for animals having access only to vehicle (0.00 mg/kg per infusion dose), responding in the two devices did not differ and was lower than the one in the active device of all nicotine groups (***p< 0.001). The daily intake of nicotine increased across unitary doses.