The discriminative stimulus effects of tripelennamine in humans

Twenty volunteers were trained to discriminate between 75 mg tripelennamine (TP) and placebo. During the first four sessions, the drugs were identified prior to ingestion by letter code. During the next six sessions, the procedure was the same except the capsules were not identified. At the end of the 3-h session, participants indicated which capsule they believed they received using the letter codes. When correct, they received a monetary bonus. If they were correct on five sessions, they entered the third phase which had ten additional training and 12 test sessions. During tests, participants received capsules that contained other drugs, including diphenhydramine (50 and 75 mg), chlorpheniramine (4 and 6 mg), diazepam (5 and 10 mg), d-amphetamine (5 and 10 mg), as well as tripelennamine (25, 50 and 75 mg) and placebo. Thirteen participants learned the discrimination and nine entered the third phase. Except for placebo, most participants identified the test compounds as TP and labeled them as sedatives. TP produced significant changes on several subjective and physiological measures. The test compounds produced varied effects which were neither clearly dose-related nor related to the identification as TP or placebo. These results indicate that tripelennamine can function as a discriminative stimulus, but with little evidence of pharmacological specificity.