Effects of d-amphetamine and diazepam on fixed-interval, fixed-ratio responding in humans

Human operant responding, maintained under a multiple fixed-ratio (FR) 30 fixed-interval (FI) 5 min schedule of point presentation was shown to be differentially sensitive to the effects of d-amphetamine and diazepam. d-Amphetamine (5-20 mg) increased rates of FR responding, whereas 10 and 20 mg of diazepam generally decreased FR rates. FI rates, in general, were unaffected by d-amphetamine, with consistent increases occurring in only one of four subjects. Diazepam decreased FI rates. The number of reinforcers obtained in the FR and the FI was increased by d-amphetamine and decreased by diazepam for two of the four subjects and was unaffected by either drug in the other two subjects. In addition, physiological measures, including heart rate and blood pressure, and verbal reports of drug effects as measured by the Profile of Mood Scales and the Addiction Research Center Inventory were generally differentially affected by the two drugs. These results suggest that a multiple FR 30 FI 5 min schedule of point presentation is capable of distinguishing behavioral effects of two different classes of drugs, the psychomotor stimulants and the minor tranquilizers. In addition, the results extend previously reported effects of diazepam on responding maintained under similar schedules from nonhuman subjects to humans. The effect of d-amphetamine in increasing rates of FR 30 responding, however, was unexpected in that amphetamine is generally reported to produce decreases in the high rates of responding generated by FR schedules in nonhuman subjects.