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Self-administration dose-response curves for CP 55 940 (filled circles), THC (closed triangles) and Δ9-tetrahydrocannabinol (THC) after the development of tolerance to rate-decreasing effects on food-maintained responding (open triangles). Abscissae: Unit dose (μg/kg/injection) available for self-administration. Ordinate: Number of injections earned in the 60-min session. Data represent mean±SD of last three sessions. *p<0.05, **p<0.01, ***p<0.001, ****p<0.0001 compared with vehicle.
