Analyzing the acquisition of drug self-administration using growth curve models

Current approaches to studying acquisition of drug self-administration have modest power to detect individual differences in the pattern of acquisition or to efficiently and accurately describe trajectories of behavior change. Methodological advances in human research have elucidated approaches to describing repeated measure data that focus on modeling the behavior of individual subjects. In this article, we re-analyzed data published in [Psychopharmacology 136 (1998) 83] using growth curve modeling to characterize the acquisition of nicotine-taking in rats. Change over time in the infusion rate was examined, revealing that the acquisition process could be described with a quadratic equation represented by intercept, slope, and acceleration parameters. Unit dose of nicotine, sex and fixed ratio (FR) schedule of reinforcement had significant effects on the acquisition curves. Dose altered the absolute rate of infusions, but not the slope or acceleration, indicating that, when an effective dose was available, the shape of acquisition trajectories was not affected by dose. In addition, dose impacted acquisition by moderating the disruption in infusion rates after an increase in the response requirement. Thus, the role of a higher dose may not be to accelerate the acquisition process but to lead to behavior that is more resistant to change. Trajectories differed between males and females at the smallest dose, but these differences dissipated by the end of acquisition. Growth curve modeling captures the process of acquisition of drug self-administration and facilitates a greater understanding of the individual differences in change in drug-taking behavior over time.