Behavioral response to apomorphine and its interaction with opiates in domestic pigeons.

Domestic pigeons received peripheral injections of saline or the dopamine agonist apomorphine (AM) at doses of 0.025, 0.05, 0.1, 0.25, 0.5 or 1 mg and their behavior was studied for 30 min after these treatments. Given at a dose of 0.025 mg, AM decreased pecking, whereas doses ranging from 0.1 to 1 mg strongly stimulated this behavior. The frequency of headshaking was enhanced by the administration of each dose of AM; at the 3 higher doses, the drug also attenuated the frequency of preening. In another experiment, AM was administered 40 min after the injection of either naloxone (0.5, 1 or 4 mg), the opiate agonist levorphanol (0.25, 0.5 or 1 mg) or its dextroisomer, dextrorphan (0.25, 0.5 or 1 mg), while the birds were observed as before. No interaction between AM and either naloxone or dextrorphan was detected. By contrast, injection of each dose of levorphanol attenuated preening, and completely antagonized the stimulating effect of AM treatment on headshaking. At a dose of 1 mg, levorphanol also slightly decreased the frequency and increased the latency of occurrence of pecking. It is concluded that in pigeons, opiates modulate the behavioral response to apomorphine in a complex fashion.