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-admini stration. In separate saline-controlled experiments, naive animals wer e allowed to self-administer either cocaine or heroin (0.16 and 0.32 m g/kg per infusion) during five consecutive daily 3-h sessions by press ing one of two levers present in the test cage. During 15 min precedin g the last four self-administration sessions, the animals had access t o the levers but pressing the reinforcement lever did not result in a drug infusion. The animals properly self-administered both doses of co caine 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 d uring the preceding period in a similar fashion as during the self-adm inistration sessions, suggesting that this behavior is reinforcement-r elated. Because the lever-press behavior during the preceding period w as performed in the absence of the primary reinforcer, this behavior l ikely reflects the motivational state of animals to obtain the drug re inforcer, and thus may serve as a measure of the motivational aspects involved in the initiation of drug self-administration.