Schaal, D. W., McDonald, M. P., Miller, M. A., & Reilly, M. P. (1996).
Discrimination of methadone and cocaine by pigeons without explicit discrimination training.
Journal of the Experimental Analysis of Behavior,
66, 193-203.
Pigeons were trained to peck a key on a variable-interval 2-min
schedule of food reinforcement. Prior to each session, either 2.0
mg/kg methadone (n = 3), 3.0 mg/kg cocaine (n = 4),
or 5.6 mg/kg cocaine (n = 2) was administered. When each
pigeon's rate of pecking was stable, a range of doses of the
training drug and saline were administered prior to 20-min
extinction sessions separated by at least four training sessions.
Rate of pecking during these extinction tests was generally an
increasing function of dose, with the lowest rates obtained
following saline and low doses and the highest rates obtained
following doses near the training doses. Dose functions from
pigeons trained with 5.6 mg/kg cocaine were steeper than those
from pigeons trained with 3.0 mg/kg cocaine. Pigeons trained with
methadone or 3.0 mg/kg cocaine were then given discrimination
training, in which food reinforcement followed drug
administration and 20-min extinction sessions followed saline
administration. Rates of pecking under these conditions quickly
diverged until near-zero rates were obtained following saline and
high rates were obtained following drug. Discrimination training
steepened dose functions for the training drugs, and the effects
of several other substituted drugs depended on the pharmacology
of the training drug. The pigeons trained with 5.6 mg/kg cocaine
were tested with d-amphetamine, methadone, and morphine
prior to discrimination training. d-Amphetamine increased
rates dose dependently, and methadone and morphine did not. The
results suggest that discriminative control by methadone and
cocaine was established without explicit discrimination training.
Key words: drug discrimination, state-dependent learning,
stimulus control, methadone, cocaine, key peck, pigeons