Effects of d-amphetamine and of beta-phenylethylamine on fixed interval responding maintained by self-regulated lateral hypothalamic stimulation in rats.

d-Amphetamine sulphate (0.25-4.0 mg/kg) and beta-phenylethylamine hydrochloride (1.25-50 mg/kg) respectively were administered IP to two groups of rats (R1-R4 and R5-R8 respectively) responding on a fixed interval (FI) schedule (R1-R4 FI 60 sec, R5-R8 FI 30 sec) of electrical hypothalamic stimulation. The duration of each train of stimulation was controlled by the duration of each lever press that initiated stimulation. Under these conditions administration of d-amphetamine resulted in a marked increase in overall response rates on the FI 60 sec schedule. This effect was significant at 1, 2 and 4 mg/kg. d-Amphetamine had not significant effect on the duration of hypothalamic stimulation or on the duration of responses occurring during the FI. Administration of beta-phenylethylamine resulted in a decrease in overall response rates on the FI 30 sec schedule. This effect was significant at 50 mg/kg. beta-Phenylethylamine increased the duration of responses occurring during the FI, this effect being significant at 25 and 50 mg/kg, but had no significant effect on the duration of hypothalamic stimulation. These results indicate that the systemic effects of d-amphetamine on response rate, and of beta-phenylethylamine on both response rate and response duration, are dissociable from changes in the self-regulated duration of lateral hypothalamic stimulation.