A comparison of testing procedures on the discriminative morphine stimulus.

The study investigated the effects of reinforcement and non-reinforcement during test sessions, and the effects of duration of generalization test sessions on the generalization of a morphine-induced discriminative stimulus. Rats were trained to discriminate 3 mg/kg morphine from saline in a two-lever drug discrimination task and were then tested for generalization of 0, 1, 2, 3, 4, and 5 mg/kg morphine with the training drug under both reinforced and non-reinforced contingencies during 4-min test periods. The percentage of drug-appropriate responses and response rates were recorded for the first 2 min and the second 2 min of each test session. A higher proportion of drug-appropriate responding occurred with an intermediate dose of morphine when reinforcement was available during test sessions. The frequency of responding was higher during the last 2 min than during the first 2 min of reinforced test sessions. The changes in response rate observed between the first 2 min and the last 2 min of the test sessions also depended on the reinforcement contingency available and the dose of morphine administered presession. The testing parameters thus altered the degree of generalization and the shape of the generalization curve of the morphine discrimination.