Individual Differences in Delay Discounting and Nicotine Self-administration in Rats

2008 iv Delay discounting—a behavioral measure of impulsivity defined as a tendency to prefer a small, immediate reward over a larger reward delayed in time—has been extensively linked with tobacco smoking. However, the causal direction of this relationship remains unclear. One possibility is that delay discounting may be a marker for an underlying vulnerability to nicotine reinforcement—a possibility which can be isolated using an animal model. In the current study, we investigated whether indifference points derived using an adjustable delay procedure of delay discounting predicted several indices of nicotine reinforcement in rats, including rate of acquisition of nicotine self-administration, break point reached on a progressive ratio schedule of reinforcement, or a shift in the dose-response curve. Stable indifference points were assessed for 63 male Sprague-Dawley rats, and extreme groups of highly impulsive (HI; n=15) and low impulsive (LI; n=11) rats were selected to self-administer nicotine. Rats responded by nose poking for infusions of 0.03 mg/kg nicotine during 1 hour daily sessions. After a 20 session acquisition period, rats completed 3 4-hour progressive ratio sessions, during which the response requirement was increased after each infusion earned. This was followed by 3 1-hour fixed ratio sessions at each of 3 nicotine doses, presented in ascending order (0.015, 0.03, and 0.09 mg/kg). All but one rat (HI group) acquired stable nicotine self-administration; however, no group differences in rate of acquisition were observed. HI and LI rats did not differ in their responses on a progressive ratio schedule or infusions earned at any dose of nicotine, although a significant v dose-response effect was observed overall. Indifference points reassessed after self-administration were highly correlated with original indifference points, and mean indifference points for each group at the second assessment did not differ significantly from baseline assessment. These results suggest that delay discounting is a highly reliable measure, but may not be a predictive marker for increased vulnerability to nicotine self-administration in rats. Table 4. Mean (and SD) indifference points assessed during delay discounting at Time 1 and Time 2, and t statistics for paired samples comparisons among high impulsivity rats, low Figure 4. Mean active and inactive nose pokes (with standard errors) for each day during acquisition for rats high and low in impulsivity.. Figure 6. Mean infusions earned (with standard errors) during the last two sessions at each dose for rats high and low in impulsivity.. 1.0 INTRODUCTION Tobacco smoking is a behavioral risk …