Strain differences in delay discounting between Lewis and Fischer 344 rats at baseline and following acute and chronic administration of d-amphetamine

Stimulant drugs have been shown either to increase or decrease rates of delay discounting (impulsive choice). These mixed findings may result from genetic, neurochemical, or environmental factors. Lewis (LEW) and Fischer 344 (F344) rats have neurochemical and behavioral differences that may be relevant to delay discounting and were used to examine effects of acute and chronic administration of d-amphetamine (d-AMP) on impulsive choice using a within-session delay-discounting procedure. Male LEW (n = 8) and F344 (n = 8) rats chose between one food pellet delivered immediately and three food pellets delivered after an increasing delay. Saline and d-AMP (0.1, 0.3, 1.0, and 1.7 mg/kg) were tested acutely and during chronic d-AMP exposure. Choice for the larger reinforcer decreased as the delay to its presentation increased for both strains at baseline. LEW rats made more impulsive choices than F344 rats as indicated by shorter indifference points, and this is consistent with previous research. Acute administration of d-AMP dose dependently increased larger-reinforcer choice and area under the curve (AUC) for LEW, but not F344 rats. During chronic exposure to d-AMP, larger-reinforcer choice and AUC increased relative to acute administration for F344 rats responding in shorter delay series, but not for F344 rats responding in longer delay series or for LEW rats. Differential effects of acute and chronic administration of d-AMP on impulsive choice in LEW and F344 rats may be a result of various factors, including genetic, neurochemical, and environmental variables. Future research should attempt to tease apart the relative contribution of each of these factors on impulsive choice.