Apomorphine-induced pecking in pigeons classically conditioned to environmental cues.

The dopamine agonist apomorphine elicits protracted pecking when injected systemically (1 mg/kg) into pigeons. In two experiments it was investigated whether apomorphine would function as an unconditioned stimulus in the classical conditioning of pecking in these animals. An experimental design based on a differentiation procedure was used so that possible pseudoconditioning effects were controlled. Two differently coloured test chambers served as negative (CS-) and positive conditioned (CS+) stimuli. During the training phase the subjects experienced the former while injected with saline, and the latter while injected with apomorphine. In later tests not involving any injections the pigeons made significantly more pecks (conditioned response) in the CS+ chamber than in the CS- chamber. In the first and second experiments the conditioned stimuli were, respectively, discrete and diffuse visual cues, but both had similar effects. The conditioning obtained may explain sensitization effects that are observed with repeated apomorphine injections. Apomorphine probably also functions as a positive reinforcer for instrumental conditioning in pigeons.