Probability Discounting of Lewis and Fischer 344 rats: Strain Comparisons at Baseline and Following Acute Administration of d-Amphetamine

Risky choice can be defined as choice for a larger, uncertain reinforcer over a smaller, certain reinforcer when choosing the smaller alternative maximizes reinforcement. Risky choice is studied using various procedures in the animal laboratory; one such procedure is called probability discounting. There are many variables that contribute to risky decision-making, including biological and pharmacological determinants. The present study assessed both of these variables by evaluating dose-response effects of d-amphetamine on risky choice of Lewis (LEW) and Fischer 344 (F344) rats. The probability-discounting procedure included discrete-trials choices between one food pellet delivered 100% of the time and three food pellets delivered following one of varying probabilities. The probability of three food pellets being delivered decreased systematically across blocks within each session. At baseline, risky choice did not differ between LEW and F344. However, choice for LEW became significantly less risky throughout extended training while choice for F344 remained relatively stable over time. d-Amphetamine significantly increased risky choice for both rat strains at low-to-moderate doses (0.1 and 0.3 mg/kg), although it did so at a lower dose for F344 (0.1 and 0.3 mg/kg) than LEW (0.3 mg/kg only), suggesting greater behavioral sensitivity to effects of d-amphetamine for F344. High doses of d-amphetamine (1.0 and 1.8 mg/kg) produced overall disruptions in choice for both strains, indicated by reductions in choice for the larger, uncertain alternative when the probability of delivery was relatively high and increases when the probability was relatively low. Results from the current study stand in contrast to previous reports investigating impulsive choice (i.e., choice involving temporal delays rather than uncertainty) of LEW and F344. Thus, the present work underscores the importance of considering risky and impulsive choice as two separate, but related, behavioral processes.