Conditioned drug effects to d-amphetamine- and morphine-induced motor acceleration in mice: experimental approach for placebo effect.

Ninety-two mice were divided into 2 groups and 2.5 mg/kg of d-amphetamine sulfate was given to one group, and 10 mg/kg of morphine hydrochloride to the other, at daily intervals. These two groups were then further divided into flicking light-presented and non-presented groups. The acute effects of the drugs on the ambulatory activity of each mouse were observed for 180 min after the drug administration. The motor-accelerating effect of both drugs was progressively enhanced with repeated administration. After the drugs were administered 10 times, physiological saline was given subcutaneously to the mice and the motor-accelerating effect of the drugs could be observed for a short time, especially in the flicking light-presented groups. This effect disappeared after repeated administration of saline solution at daily intervals for 6 days. Thus, the motor-accelerating effect produced by saline is considered to have been elicited as the result of conditioning between the conditioned stimuli (flicking light and other environmental factors) and drug effects.