Rate-dependent effects of d-and l-amphetamine on schedule-controlled responding in pigeons and squirrel monkeys.

The effects of d- and l-amphetamine were studied on key-pressing responses of squirrel monkeys maintained under fixed-interval schedules of electric shock presentation, and on key-pecking responses of pigeons maintained under multiple fixed-ratio, fixed-interval schedules of food presentation. Under the fixed-interval schedules, responding followed a pause and occurred at increasing rates as the interval elapsed. Both isomers produced comparable increases in rates of responding under relatively long fixed-interval schedules with small to intermediate doses; maximal effects of the d-isomer were obtained at doses one-half log unit smaller than the doses of the l-isomer. Responding of pigeons maintained under relatively short-fixed-interval schedules was only decreased by either amphetamine isomer. Responding of pigeons maintained under fixed-ratio schedules occurred at the highest rates and was also only decreased by either amphetamine isomer. Decreased in response rate produced by the d-isomer were generally obtained at doses one-half log unit smaller than doses of the l-isomer than produced comparable effects. Both isomers increased responding that occurred at low rates early in the fixed-interval to a proportionally greater extent than thigh rates from later in the interval. The highest rates in the fixed-interval was generally decreased. Differences in potency between the two isomers in producing rate-dependent effects were small, most noticable with larger doses, and less than the potency differences between the two isomers in producing changes in response rate.