Threshold dose of cocaine-induced CPP is obtained by CPP test and nicotine enhances cocaine-induced CPP (A and B).

<p>(A) To explore threshold dose of cocaine-induced CPP, we designed two doses of cocaine (5 mg/kg and 2 mg/kg) to treat mice once a day for 3 days (n = 12 in each group). (B) To detect effects of nicotine on cocaine, four experimental groups were designed (n = 8 for all groups).