Discriminative stimulus properties of intravenous morphine in the rat.

Using a single-lever operant task, rats were trained to discriminate between the effects of 1 mg/kg morphine and saline administered intravenously 15 min prior to sessions. Tests of stimulus control indicated that responding appropriate to the morphine training treatment was present at 5 min and persisted for at least 60 min following intravenous administration. Morphine administered intraperitoneally 15 min prior to sessions resulted in dose-related responding with 6 mg/kg producing stimulus control not significantly different from the intravenous training treatment. These results suggest that the discriminative stimulus properties of morphine in the rat are qualitatively similar following intraperitoneal and intravenous administration.