Effects of clomipramine on self-control choice in Lewis and Fischer 344 rats

Abstract

Rates of delay discounting (impulsive choice) have been shown to vary among individuals, particularly people who abuse drugs relative to those who do not, but factors that may contribute to these differences have not been identified. To explore a role for possible genetic and neurochemical determinants, Lewis (n = 8) and Fischer 344 (n = 8) rats were allowed to choose between one food pellet delivered immediately and three food pellets delivered after increasing delays. The delays to the large reinforcer (0, 10, 20, 40, 60 s) were increased across five blocks of trials in daily experimental sessions. For both groups of rats, choice for the larger reinforcer decreased as the delay to presentation increased. However, the Lewis rats were more likely to choose the smaller, immediate reinforcer earlier in the session, i.e., at shorter large-reinforcer delays, than the Fisher 344 rats. This difference in choice was statistically significant. Repeated administration of 3.0 mg/kg, i.p. clomipramine (mean of last five sessions) did not significantly alter choice, relative to baseline, for either strain. The present findings suggest that differences in delay discounting/impulsive choice may involve genetic, e.g., neurochemical, differences.

Fig. 1

Percent choice for the larger food reinforcer as the delay to its presentation was increased across the session. Mean choices for the Lewis rats (n=7) are identified by filled circles. Mean choices for the Fischer 344 rats (n=8) are identified by filled squares. Vertical bars represent the standard errors of the means. Dotted drop lines indicate the delays to the large reinforcer when choice was indifferent, i.e., 50% choice for either alternative.

Fig. 2

Percent choice for the larger food reinforcer as the delay to its presentation was increased across the session during baseline (no-drug) conditions is identified by filled symbols for the Fischer 344 rats (n=8; upper panel) and the Lewis rats (n=7; lower panel). Means from the last five sessions of repeated 3.0 mg/kg, i.p. clomipramine are represented by open symbols. Vertical bars indicate the standard errors of the means.