Effects of methamphetamine on delay discounting in rats using concurrent chains.

Research has examined how stimulants affect impulsive choice in delay-discounting tasks, but little is known about whether such drugs influence how discounting varies with reward magnitude. This study sought to investigate the effects of acute and chronic methamphetamine administration on rats' responding in a rapid acquisition choice task in which reward delays were changed unpredictably across sessions. In each group of four sessions, delays were unequal (1 s/8 s, or 8 s/1 s) or equal (1 s/1 s, or 8 s/8 s) while reward magnitudes were constant and unequal (one dipper cycle/four dipper cycles). This enabled us to obtain both estimates of delay discounting (i.e., sensitivity to delay) and the magnitude effect (in which larger rewards are discounted at a lower rate). Methamphetamine was administered in increasing doses acutely and chronically. Baseline results showed that rats reliably preferred the alternative with a shorter delay and that choice for the larger reward was greater when the delays were long, consistent with the magnitude effect. Acute methamphetamine dose dependently reduced both sensitivity to delay and the magnitude effect, but not sensitivity to magnitude. Chronic administration had no systematic effect on choice. This study is the first to report a magnitude effect with rats in a rapid acquisition choice procedure similar to that found in delay discounting research with humans, and suggests that acute methamphetamine administration reduces control by contingencies that change across sessions.