Fig. 1.

Results of experiments 1 and 2. In experiment 1, rats were first trained to self-administer sucrose and nicotine with only one of the levers available during each session (a, Acquisition Sessions). Extinction training was then conducted, in which there was still only one lever available at a time, but no sucrose or nicotine reinforcement was delivered (a, Extinction). In the final phase of experiment 1, rats were allowed to self-administer sucrose and nicotine with both levers concurrently available (b). In experiment 2, rats were initially trained to self-administer nicotine for 10 sessions, but were not given access to sucrose pellets (c, Acquisition Sessions). Then, the nicotine lever remained available but nicotine delivery was discontinued for eight sessions (c, Extinction). Finally, the rats of experiment 2 were given concurrent access to the sucrose and nicotine levers (d). All points represent mean (±sem). All sessions lasted for 1 h or 100 sucrose deliveries. Asterisks indicate significant differences (p<0.05) between sucrose and nicotine lever responding within specific sessions (or between each sucrose session and the corresponding two-session block of nicotine sessions during single-lever training in (a))