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Nicotine challenge increases the reinforcing efficacy of the lever-cue in paired and unpaired groups. Active port entries are compared in rats previously treated with nicotine or saline, subdivided into rats receiving a nicotine or saline challenge. Dotted lines indicate mean inactive port responses. Asterisks denote significant increases in nicotine- vs. saline-challenged groups (p < 0.05). Data are presented as means +/− SEM.
