Some behavioral effects of repeated d ‐amphetamine administrations

Effects of daily administrations of d ‐amphetamine were studied on key peck responses of pigeons maintained under a multiple fixed‐interval 2‐min, fixed‐ratio 30‐responseschedule. Under the fixed‐interval schedule, a pause was followed by a transition to increasing rates of responding until food presentation. Under the fixed‐ratio schedule, higher sustained rates of responding were maintained. Low to intermediate doses (0.3‐1.0 mg/kg) of d ‐amphetamine changed the temporal patterns and occasionally increased rates of responding under the fixed‐interval schedule. Higher doses decreased rates of responding under bothschedules. With daily injections of 1.0 mg/kg d ‐amphetamine prior to experimental sessions, the effects of this dose on rates and patterns of responding were attenuated, and d ‐anphetamine dose‐effect curves were shifted to the right, primarily under the fixed‐ratio schedule. Similar results were obtained with daily presession injections of 5.6 mg/kg d ‐amphetamine in a second group of pigeons, except that rates of responding under both schedules were decreased by this daily dose, and did not return completely to control values with repeated injections. In a third group of pigeons, 1.0 mg/kg d ‐amphetamine administered daily, after experimental sessions, did not alter dose‐effect functions for d ‐amphetamine. In a second experiment, pigeons were trained to peck one response key when given 1.0 mg/kg d ‐amphetamine and a different key when given presession water injections. Increasing doses of d‐amphetamine produced incresing percentages of d ‐amphetamine‐key responses. Repeated administration of 5.6 mg/kg d‐amphetamine shifted these dose‐effect functions to the right one‐half log unit. Results suggested that decreases in reinforcement frequency are not a necessary condition for the development of behavioral tolerance to d‐amphetamine.