DRUG DISCRIMINATION IN RATS UNDER CONCURRENT VARIABLE-INTERVAL VARIABLE-INTERVAL SCHEDULES
Eight rats were trained to discriminate pentobarbital from saline under a concurrent variable-interval(VI) VI schedule, on which responses on the pentobarbital-biased lever after pentobarbital were
reinforced under VI 20 s and responses on the saline-biased lever were reinforced under VI 80 s.
After saline, the reinforcement contingencies programmed on the two levers were reversed. The rats
made 62.3% of their responses on the pentobarbital-biased lever after pentobarbital and 72.2% on
the saline-biased lever after saline, both of which are lower than predicted by the matching law.
When the schedule was changed to concurrent VI 50 s VI 50 s for test sessions with saline and the
training dose of pentobarbital, responding on the pentobarbital-biased lever after the training dose
of pentobarbital and on the saline-biased lever after saline became nearly equal, even during the
first 2 min of the session, suggesting that the presence or absence of the training drug was exerting
minimal control over responding and making the determination of dose–effect relations of drugs
difficult to interpret. When the pentobarbital dose–response curve was determined under the con-
current VI 50-s VI 50-s schedule, responding was fairly evenly distributed on both levers for most
rats. Therefore, 6 additional rats were trained to respond under a concurrent VI 60-s VI 240-s sched-
ule. Under this schedule, the rats made 62.6% of their responses on the pentobarbital-biased lever
after pentobarbital and 73.5% of their responses on the saline-biased lever after saline, which also
is lower than the percentages predicted by perfect matching. When the schedule was changed to a
concurrent VI 150-s VI 150-s schedule for 5-min test sessions with additional drugs, the presence or
absence of pentobarbital continued to control responding in most rats, and it was possible to gen-
erate graded dose–response curves for pentobarbital and other drugs using the data from these 5-
min sessions. The dose–response curves generated under these conditions were similar to the dose–
response curves generated using other reinforcement schedules and other species.
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