Interaction of methadone and pentobarbital on chained fixed-interval performance in pigeons.

Abstract

Abstract The effects of methadone, pentobarbital, and their combination were examined in pigeons key pecking under a chained FR 3 (FI 30 sec) schedule of food presentation. The delivery of grain followed the completion of three 30-second fixed-interval components associated with different colored key lights. Intramuscular injections of methadone alone produced small response rate increases at low doses (0.25 and 0.50 mg/kg) and a dose-dependent decrease in key pecking rates at high doses (1.0 and 2.0 mg/kg). Pentobarbital (2, 4, and 8 mg/kg) produced a dose-dependent increase in response rates; 16 mg/kg pentobarbital markedly reduced or abolished responding. Combinations of these doses of methadone and pentobarbital produced increases in key pecking rates greater than the rate increases produced by methadone alone, and in four of the seven subjects, greater than the rate-increasing effects of pentobarbital alone. Changes in overall response rates were a function of changes in pause time (minutes from food delivery to the first key peck) and running response rates (total key pecks divided by session length minus pause time). Responding in the individual FI 30 sec components varied with the dose of methadone; doses greater than 0.25 mg/kg produced a dose-related decrease in key pecking rates in the initial component and small increases in the middle and terminal components. Pentobarbital (2, 4, and 8 mg/kg) produced large dose-related increases in key pecking rates in the initial and middle components. The largest increase in response rate was observed in the middle component after the combination of drugs was administered. Pentobarbital's rate-increasing effect under the chained schedule attenuated the rate-reducing effect of methadone when administered in combination.