Apomorphine-like effect evoked by exposure to environmental cues associated with drug administration

1. Over 12 days, two groups of mice were given one daily injection of apomorphine 2 mg/kg in a distinctive environment. One group received the drug in the experimental room where the apomorphine-induced climbing was evaluated. The second group was given apomorphine in the colony room. 2. On the placebo test day, all mice were injected with saline in the experimental room and climbing behaviour was scored. 3. Results showed that mice previously receiving apomorphine in the experimental room displayed under placebo an enhancement in climbing qualitatively similar to that observed following apomorphine administration. 4. This placebo effect is attributed to classical conditioning of apomorphine-induced climbing (unconditioned effect) to the environmental cues present at the time of drug exposure (conditioned stimulus).