Effects of Paired Housing and Acute d-Amphetamine on Delay Discounting in Lewis and Fischer 344 rats

Lewis (LEW) and Fischer 344 (F344) rats are two genetically different strains that differ in impulsive choices made in a delay-discounting task. An operational definition of impulsive choice is a choice of a smaller-sooner reinforcer over a larger-later reinforcer. Research using LEW and F344 rats has housed rats singly. Studies have shown that paired housing influences impulsive choice, but it is not known if paired housing affects impulsive choice in the LEW and F344 strains specifically. The present study compared impulsive choice in pair-housed LEW and F344 rats to archival data from singly housed LEW and F344 rats. Impulsive choices were measured at baseline and after d-amphetamine (d-AMP) administration. In singly housed rats, LEW rats made more impulsive choices at baseline compared to F344 rats. In addition, impulsive choices decreased for LEW rats but increased or did not change for F344 rats when d-AMP was administered. The present study found the reverse in pair-housed rats: F344 rats made more impulsive choices at baseline than LEW rats. In addition, d-AMP either increased or did not affect impulsive choices in LEW rats, but decreased impulsive choices in F344 rats. Though a small sample size limits the conclusions that may be drawn from the data, this study demonstrates that environmental variables, particularly social conditions, may affect impulsive choice in the LEW and F344 strains differently. In addition, effects of d-AMP on impulsive choice are likely baseline-dependent, evidenced by the occurrence of decreases in impulsive choice in response to d-AMP for rats making larger numbers of baseline impulsive choices in both studies

Temporal Bisection and Effects of d-Amphetamine Administration in Lewis and Fischer 344 Rats

Temporal control concerns the discrimination of intervals of time. Individuals with various psychological disorders have shown differences in temporal control when compared to control populations. It is unknown whether and how temporal control might be linked with impulsivity, another measure that predicts problem behavior such as attention-deficit/hyperactivity disorder. Because Lewis (LEW) rats reliably show greater impulsive choice relative to Fischer 344 (F344) rats, the present study examined responding in these two strains under a temporal-bisection procedure that assesses temporal control. In the temporal-bisection procedure, rats are trained to discriminate between two durations. Intermediate durations are subsequently presented to determine the duration at which responding is nearly equally distributed between the original training durations. This duration is the bisection point. Because LEW rats show greater impulsive choice, it was hypothesized that LEW rats would show overestimation of duration (shorter bisection points). This effect would indicate that the subjective experience of duration, or delay, is longer for LEW rats. Results indicated that LEW rats\u27 bisection points were shorter on average than F344 rats\u27 bisection points in the baseline phase of the experiment. The baseline difference between LEW and F344 was not replicated during a subsequent phase of the experiment, but LEW and F344 rats did show differential effects of d-amphetamine on behavior during select conditions and doses. Effects of d-amphetamine replicated previous studies that showed apparent loss of stimulus control with increasing d-amphetamine doses, as opposed to underestimation or overestimation of temporal duration. The results generally add to the evidence supporting a link between impulsivity and temporal control