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Effect of naltrexone on cocaine self-administration under a progressive-ratio schedule. Data are expressed as the number of injections on the left axis and ratio/injection on the right axis. Error bars are SEM values. The abscissa represents the dose of naltrexone. Doses of naltrexone were subcutaneously injected into rats 30 min before each test session. *p<0.05, **p<0.01, ***p<0.001 compared with vehicle
