Drug Discrimination Under a Concurrent Fixed‐interval Fixed‐interval Schedule

Abstract

Pigeons were trained to discriminate 5.0 mg/kg pentobarbital from saline under a concurrent fixed-interval (FI) FI schedule of food presentation on which, after pentobarbital administration, responses on one key were reinforced with food under an FI 60-s component and responses on the other key were reinforced under an FI 240-s component. After saline administration, the schedule contingencies on the two keys were reversed. After both pentobarbital and saline, pigeons responded more frequently on the key on which responses had been programmed to produce the reinforcer under the FI 60 component of the concurrent schedule. The schedule was changed to concurrent FI 150 FI 150 s for drug-substitution tests. In each bird, increasing doses of pentobarbital, ethanol, and chlordiazepoxide produced increases in the proportion of responses on the key on which responses had been reinforced under the FI 60 component after pentobarbital administration during training sessions. The proportion of responses on that key was slightly lower for ethanol than for chlordiazepoxide and pentobarbital. At a dose of pentobarbital higher than the training dose, responding decreased on the key that had been reinforced under the FI 60 component during training sessions. Phencyclidine produced less responding on the key programmed under the FI 60-s component than did pentobarbital. Methamphetamine produced responding primarily on the key on which responses had been reinforced under the FI 60-s component after saline administration.

Drug Discrimination in Rats Under Concurrent Variable‐interval Variable‐interval Schedules

“…In general, increasing doses of pentobarbital produced increased responding on the pentobarbital-biased lever; however, at the higher doses of pentobarbital the dose-response curve appeared to be descending for several rats. This tendency for the pentobarbital dose-response curve to ascend and then descend has been observed previously with pigeons when responding was maintained under concurrent interval schedules (McMillan et al, 1997). Similar effects have been seen with chlordiazepoxide, both in the current experiments and in experiments with pigeons using concurrent interval schedules of reinforcement (McMillan et al, 1997).…”

“…This tendency for the pentobarbital dose-response curve to ascend and then descend has been observed previously with pigeons when responding was maintained under concurrent interval schedules (McMillan et al, 1997). Similar effects have been seen with chlordiazepoxide, both in the current experiments and in experiments with pigeons using concurrent interval schedules of reinforcement (McMillan et al, 1997). Why the dose-response curve often descends after high doses under these schedules is not clear.…”

“…The training dose of the drug produces interoceptive stimuli that become established as discriminative stimuli through differential reinforcement. We have shown recently that drugs can be established as discriminative stimuli in pigeons under conditions in which both response alternatives produce the reinforcer (McMillan et al, 1997;Snodgrass & McMillan, 1996). In the first of these experiments, we showed that pentobarbital could be established as a discriminative stimulus in pigeons using a concurrent VI VI schedule, in which reinforcers were available for responses on one key four times more frequently than for responses on the other key during training sessions.…”

“…Drug discrimination is under the joint control of the training drug and the schedule of reinforcement that maintains responding (Holloway & Gauvin, 1989;Massey, McMillan, & Wessinger, 1992;McMillan & Li, 1999a;Mc-Millan, Li, & Hardwick, 1997;McMillan & Wenger, 1984;Snodgrass & McMillan, 1991Stolerman, 1991;Young, 1991), as well as the training history (McMillan & Li, 1999b;McMillan, Sun, & Hardwick, 1996). Although the role of the schedule of reinforcement has only recently been investigated extensively in drug-discrimination research, evidence is accumulating to show that if drug-discrimination responding is maintained by interval schedules of reinforcement, drugs that substitute for the training drug produce generalization curves that are graded (Massey et al, 1992;McMillan et al, 1997;Snodgrass & McMillan, 1991). In contrast, when responding is maintained by fixed-ratio (FR) schedules, these dose-response curves are quantal (Massey et al, 1992;McMillan & Li, 1999a;Snodgrass & McMillan, 1991).…”

“…Recently, we have extended these observations that graded responding occurs when drug-discrimination responding is maintained under interval schedules to experiments with concurrent interval schedules. Not only did we establish drug discrimination under concurrent reinforcement schedules on which responding on both operanda could produce the reinforcer, but also the generalization curves for the training drug and for other drugs that generalized to the training drug were graded under two concurrently available variable-interval (VI) schedules (Snodgrass & McMillan, 1996) and two concurrently available fixed-interval (FI) schedules (McMillan et al, 1997). In contrast, when responding was maintained under concurrently available FR schedules, the dose-response curves were quantal (McMillan & Li, 1999a).…”