Strain differences in delay discounting using inbred rats

A heightened aversion to delayed rewards is associated with substance abuse and numerous other neuropsychiatric disorders. Many of these disorders are heritable, raising the possibility that delay aversion may also have a significant genetic or heritable component. To examine this possibility, we compared delay discounting in six inbred strains of rats (Brown Norway, Copenhagen, Lewis, Fischer, Noble and Wistar Furth) using the adjusting amount procedure, which provides a measure of the subjective value of delayed rewards. The subjective value of rewards decreased as the delay to receipt increased for all strains. However, a main effect of strain and a strain × delay interaction indicated that some strains were more sensitive to the imposition of delays than others. Fitting a hyperbolic discount equation showed significant strain differences in sensitivity to delay (k). These data indicate that there are significant strain differences in delay discounting. All strains strongly preferred the 10% sucrose solution (the reinforcer in the delay discounting task) over water and the amount of sucrose consumed was correlated with sensitivity to delay. Locomotor activity was not correlated with delay discounting behavior. Additional research will be required to disentangle genetic influences from maternal effects and to determine how these factors influence the underlying association between heightened delay discounting and neuropsychiatric disorders.