Assessing Animal Models of Impulsive Behavior: Inter-Model Congruity, Relationship to Demand for Cocaine, and Effects of Selective Dopaminergic Compounds.

Abstract

Valuing immediate rewards and acting quickly to obtain those rewards is often the optimal strategy. It is when such opportunistic strategies become excessive and disadvantageous that behavior is deemed impulsive. Psychological disorders such as attention-deficit/hyperactivity disorder and substance abuse are considered impulse-control disorders, and people that have been diagnosed with these disorders have been shown to exhibit impulsive behavior patterns. People with impulse-control disorders also have abnormal levels of neural activity in key areas of the brain that use the neurotransmitter dopamine. Numerous models exist for the study of impulsive behavior in laboratory animals, but in many cases the relevance of these models to the human condition is unclear. A multi-faceted approach was taken to assess the construct and predictive validity of chosen animal models of impulsivity.

Assessed first was whether individual differences in the performance of rats on the delay discounting task, a model of impulsive choice, was associated with individual differences in self-administered cocaine injections. Since substance abusers are known to be more impulsive on the version of the delay discounting task used in humans, it was predicted that impulsive choice in rats would be associated with drug demand, a measure of reward value. Results confirmed that impulsive choice was associated with demand for cocaine, but not for sucrose in a control condition.

The delay discounting task, a paced fixed consecutive number schedule, and the uncertain visual discrimination task are purported models of impulsive choice, impulsive action, and impulsive preparation, respectively. Drugs that preferentially act at different populations of dopamine receptors were administered to separate groups of rats performing on each of these tasks to assess the theoretical and clinical relevance of each. Receptor-specific modulation of impulsive behavior was found on each task.

Finally, inter-model congruity of the same three tasks was assessed in a within-subjects design to determine if individual differences in performance correlated across tasks. Selected drugs were also given to the same subjects trained on multiple tasks to assess whether individual variation in drug effects correlated across tasks. Results did not fully support the classification of these tasks into three distinct subtypes of impulsivity.