Performance during the test for conditioned reinforcement. During this test a nose poke into the active port resulted in presentation of the cue either previously paired or unpaired with non-contingent nicotine delivery for 2 sec. Data represent the mean ± SEM difference in nose pokes into the Active minus Inactive port during the conditioned reinforcement test where each data point represents an independent group of rats (UP STs n=8, GTs n=8; 7.5 μg/kg nicotine STs n= 12, GTs n= 12; 15 μg/kg nicotine STs n= 14, GTs n= 15; 25 μg/kg nicotine STs n= 15, GTs n= 16). UP= Unpaired. *, indicates a significant group difference between STs and GTs. p< 0.05.