Morphine as a conditioned stimulus in a conditioned emotional response paradigm.

A Pavlovian conditioning experiment was conducted to determine whether morphine (6 mg/kg, IP) could act as a conditioned stimulus (CS) when paired with an electric shock unconditioned stimulus (US), and later produce a conditioned suppression of drinking (CR) in water deprived rats. Seven groups were tested for conditioning after exposure to one of the following conditioning procedures: (1) morphine paired with shock; (2) morphine alone with no shock; (3) shock but no morphine; (4) no shock and no morphine; (5) morphine paired with vocalizations of shocked rats; (6) saline paired with shock; (7) saline alone with no shock. Groups 1 and 2 tested whether morphine could act as a CS. Groups 3 and 4 tested for sensitization. Group 5 tested whether exposure to the vocalizations of other rats could act as a US when paired with a morphine CS. Groups 6 and 7 tested whether cues associated with the injection procedure could act as a CS. Only subjects in group 1 showed conditioned suppression of drinking, when compared to control groups. Overall, the results indicate that morphine could act as a conditioned stimulus and that several of the more obvious possible sources of artifact did not significantly contribute to the CR that is produced.