For all mice that successfully acquired the discrimination, stimulus substitution tests were conducted on Tuesdays and Fridays during 15 min test sessions, with maintenance of training continuing on intervening days. During test sessions, responses on either manipulandum delivered reinforcement according to an FR10 schedule. To be tested in either experiment, mice must have completed the first FR10 on the correct manipulandum and ≥ 80% of the total responding must have occurred on the correct manipulandum during the prior day’s training session. In addition, the mouse must have met these same criteria during previous training sessions with the alternate training compound (training drug or vehicle). A dose-effect curve determination with THC was conducted in each group followed by tests with the other compounds. Throughout both mouse drug discrimination studies, doses of all drugs were administered at a volume of 10 ml/kg and in ascending order for each dose-effect curve. THC was administered intraperitoneally (i.p., Experiment 1) or subcutaneously (s.c., Experiment 2) 30 min prior to the test session. In Experiment 1, anandamide (0.1–10 mg/kg) and 2-AG (0.1–30 mg/kg) were administered i.p.
