Delay Discounting in Lewis and Fischer 344 Rats: Implications for the Use of an Adjusting-amount Procedure to Detect Between-strain Differences

Abstract

Strain-related behavioral differences may facilitate investigation of the genetic and neurochemical determinants of delay discounting. Previous researchers report that Lewis rats make more impulsive choices than Fischer 344 rats, although the size of this difference appears to vary depending on the procedure used. Additionally, researchers using a rapid-determination adjusting-amount procedure (in which reinforcer delay is manipulated between sessions regardless of within-session trend or variability) report no difference in delay discounting between strains. The present study was designed to compare degree of delay discounting between strains using a steady-state version of the adjusting-amount procedure in which delay was manipulated between conditions following satisfaction of experimenter-defined stability criteria. Using such a procedure, Lewis rats exhibited modestly steeper delay discounting for food reinforcers than Fischer 344 rats. However, under the rapid-determination procedure, no difference was observed; further, among Lewis rats, discounting was not correlated between assessments. These results suggest the rapid-determination procedure may be insensitive to modest differences otherwise detected using steady-state methods. Differences in discounting are discussed in the context of neurochemical and neuroendocrine differences between strains.