Delay discounting in C57BL/6J and DBA/2J mice: adolescent-limited and life-persistent patterns of impulsivity

Abstract

Impulsivity is a defining characteristic of adolescence. Compared to adults, for example, adolescents engage in higher rates of drug and alcohol experimentation, risky sexual practices, and criminal activity. Such behavior may reflect reduced sensitivity to long-term consequences of behavior during adolescence. Recently, our lab has attempted to refine mouse procedures to study developmental trends in decision making in the laboratory. In the present experiment, we examined sensitivity to delayed rewards in C57BL/6J (B6) and DBA/2J (D2) mice during adolescence and adulthood using an adaptation of a 2-week delay discounting procedure developed by Adriani and Laviola (2003). During training, mice could choose between a 20- or 100-μl drop of milk delivered after a 1-s delay. During testing, the delay to the large drop of milk was increased from 1 to 100 seconds. As the delay to the larger volume increased, preference shifted to the smaller, more immediate option. In adolescence, both strains showed similar shifts in preference. In contrast, adult B6 mice were less sensitive to increasing delays than were adult D2 mice, who continued to perform much as their adolescent counterparts. A subsequent resistance-to-extinction test ruled out the possibility that the slower change in the adult B6 mice was due to perseverative responding. The present findings suggest that B6 and D2 strains may be differentially suited to uncovering the biological mechanism of short-term and long-term patterns of impulsive behavior.

Figure 1

Age and strain differences in tolerance to delayed to reward. The y-axis shows the percentage of large-reward choices as a function of delay. Data from one-second delay test has been separated because it also constitutes a replication of the training segment. Error bars denote ± 1 SEM.

Figure 2

Effects of extinction on fixed-ratio maintained behavior of adult mice. The top set of graphs show the effects of the removal of the reinforcement contingency on nose poking. The top left figure shows the median number of responses per minute of a 30-min extinction period for D2 (filled circles) and B6 (open circles) mice. The top right graph shows boxplots of the total number of responses emitted under extinction for each strain. The line is the median, the box indicates upper and lower quartiles, the error bars denote the 5^th and 95^th percentiles. The bottom set of graphs show data for nose poking on the inactive hole. Details are the same as in the top set of graphs.