Assessment of motivational aspects involved in initiation of cocaine and heroin self-administration in rats.

A behavioral paradigm was explored to assess the motivational aspects involved in drug-taking behavior during initiation of drug self-administration. In separate saline-controlled experiments, naive animals were allowed to self-administer either cocaine or heroin (0.16 and 0.32 mg/kg per infusion) during five consecutive daily 3-h sessions by pressing one of two levers present in the test cage. During 15 min preceding the last four self-administration sessions, the animals had access to the levers but pressing the reinforcement lever did not result in a drug infusion. The animals properly self-administered both doses of cocaine and heroin, because the amount of self-infusions was higher than their saline control groups. Animals self-administering the high dose of cocaine and either dose of heroin performed lever-press behavior during the preceding period in a similar fashion as during the self-administration sessions, suggesting that this behavior during the preceding period was performed in the absence of the primary reinforcer, this behavior likely reflects the motivational state of animals to obtain the drug reinforcer, and thus may serve as a measure of the motivational aspects involved in the initiation of drug self-administration.