Effects of apomorphine on elicited and operant pecking in pigeons

The effects of apomorphine (0.001-32.0 mg/kg) on elicited and operant pecking were studied in pigeons. Elicited pecking was measured in a 1-h observation test. Apomorphine caused dose-related increases in the pecking elicited by the drug in all the subjects, with maximal responding at 3.2 mg/kg. In contrast, operant responding on a multiple, 5 min fixed interval, 30 response fixed-ratio schedule revealed individual differences in sensitivity to the drug. A dose of 0.32 mg/kg eliminated key pecking in fixed-interval and fixed-ratio components in 4 (group 1) of the 15 subjects while 3.2 mg/kg eliminated responding in 9 other subjects (group 2), and 2 of the subjects (group 3) required 32.0 mg/kg to eliminate responding. The 13 birds in groups 1 and 2 showed decreases in operant responding with concomitant increases in elicited pecking. For the 2 remaining birds, increases in operant behavior were highly correlated with increased stereotypy. The effects of apomorphine on operant behavior appeared to depend on induced stereotypy, with rate-decreasing effects resulting from the disruption of ongoing behavior by stereotyped pecking aimed else-where in the chamber, and rate increases resulting from the redirection of elicited pecking towards the operant key.