RATE-DEPENDENCY HYPOTHESIS AND THE RATE-DECREASING EFFECTS OF D-AMPHETAMINE ON SCHEDULE-INDUCED DRINKING

The high levels of drinking induced by intermittent food-reinforcement schedules are dose-dependently reduced by acute doses of d-amphetamine. The present study evaluated whether the effects of d-amphetamine on this schedule-induced drinking reflect the reduction of high rates of responding. Twenty-four rats were divided into six groups (n = 4) according to the interval and time durations of a multiple fired-time (FT) fixed-interval (FT) schedule (15 s, 30 s, 60 s, 120 s, 240 s and 480 s). FT components were signalled by a tone and by lever withdrawal. Doses of 0.25 to 4.0 mg/kg of d-amphetamine were administered i.p. 10 min before test sessions. d-amphetamine produced similar dose-dependent reductions in rate of licking induced by FT and FI schedules. Rate-decreasing effects on operant lever pressing were also found after administrations of d-amphetamine. The dose-dependent decrements produced by d-amphetamine were a function of the inter-food interval length in both schedule-induced and operant behaviours. These rate-decreasing effects were rate-dependent, but d-amphetamine interacted differentially with control rates of adjunctive and operant behaviours, causing a greater suppression of the lower rates of adjunctive licking and the higher rates of operant lever pressing.