Incremental repeated acquisition in the rat: acute effects of drugs

Rats lever pressed for food and learned new response sequences on three levers. At the beginning of each daily session, responses on only one of the levers produced food. After meeting criterion on one lever, the task was "incremented" so that sequential responses on two levers were required and so on up to five sequential responses. Each new required response was added in front of the previously performed sequence. Sequences of lever presses required to produce food changed each session. Following establishment of stable acquisition behavior, the acute effects of d-amphetamine (0.30-3.0 mg/kg), diazepam (0.125-4.0 mg/kg), morphine (0.30-10.0 mg/kg), pentobarbital (1.0-17.5 mg/kg), and chlorpromazine (0.10-3.0 mg/kg) were examined. All drugs decreased the number of response sequences completed in a dose-dependent fashion. Response rates generally decreased at or below those doses that caused an increase in errors. For d-amphetamine, the profound disruption of incremental repeated acquisition behavior was primarily due to drug-induced perserverative responding. Pentobarbital and chlorpromazine increased errors both when the sequence was incremented and within the sequence whereas diazepam only increased errors when the sequence was incremented. Morphine generally increased within sequence errors without affecting errors when the sequence was incremented.