Discriminable effects of antihistamine drugs.

Rats were trained to discriminate drug vs. no-drug in a shock-escape T-maze task; a right turn was required when rats were drugged and a left turn when undrugged. The three antihistamine drugs pyrilamine, dimenhydrinate (Dramamine) and diphenhydramine (Benadryl) were discriminated after an average of 20 training sessions indicating that their effects were only moderately discriminable. After criterion performance was achieved under the training conditions, substitution tests were conducted during which rats received a novel drug and were then run in the maze with both goals accessible. Rats trained with antihistamine drugs made drug choices during tests with other antihistamines. Rats trained with other types of drugs made no-drug choices during tests with antihistamines. The results suggest that the antihistamines share a discriminable effect which is relatively unique to that class of drugs.