Timing and space usage are disrupted by amphetamine in rats maintained on DRL 24-s and DRL 72-s schedules of reinforcement

Abstract

RATIONALE: A differential-reinforcement-of-low-rate schedule (DRL) delivers reinforcement only when the interresponse time (IRT) exceeds a fixed time interval, thereby shaping rats to discriminate the timing of their responses. However, little is known about the motor behavior and location of the rats in the chamber during the IRTs that lead to reinforcement. Although amphetamine is known to disrupt DRL timing behavior, the effects of this drug on non-operant motor behavior during DRL performance has not yet been quantified. OBJECTIVE: The purpose of this research was to measure the motor behavior (movement trajectories in the horizontal plane and spatial location in the plane) during longer IRT’s after either vehicle or amphetamine treatment. METHOD: Experimental chambers were constructed with a force-plate actometer as the floor, and while performing the operant task, the rats’ motor behaviors were measured continuously with high temporal and spatial resolution. Separate groups of 8 male Sprague Dawley rats were maintained on either DRL 24-s or DRL 72-s schedules of water reinforcement in 4-hr recording sessions. RESULTS: Analyses of IRT distributions showed that the rats’ timing behavior conformed to their respective DRL requirements. In the absence of drug, analysis of motor behavior in pre-reinforcement intervals showed that rats located themselves away from the operandum, and exhibited very low levels of movement. Rats exhibited a significant temporal diminution of horizontal movement that reached a minimum 4–8 s before the rats moved to the operandum to execute operant responses. Amphetamine treatment increased locomotion, abolished the temporal movement gradient, and brought the rats closer to the operandum compared to vehicle treatment. Movement changes induced by amphetamine were accompanied by degraded timing behavior. CONCLUSIONS: Taken together, the data show that DRL training induced rats to locate themselves away from the operandum and to remain nearly motionless during longer IRTs, and that amphetamine treatment interfered with this complex of behavioral features.