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Drug abuse can be conceptualized as excessive choice of drug over other reinforcers, and factors that affect drug taking can be examined experimentally using choice procedures. This study examined the impact of reinforcer delay on self-administration of the μ-opioid receptor agonist remifentanil in rhesus monkeys (n = 4) lever pressing under a concurrent fixed-ratio 30 schedule. Responding on either lever delivered an intravenous infusion of either remifentanil or saline. Dose-effect curves were first determined when responding on one lever delivered remifentanil and responding on a second lever delivered saline. Monkeys then chose between two doses of remifentanil, and delay to delivery of the large dose was varied systematically. Responding for remifentanil (0.01-1.0 µg/kg/infusion) increased dose-dependently when the alternative was saline or a dose of remifentanil. Delaying delivery of the large dose of remifentanil by 30, 60, 120, or 240 seconds increased responding for smaller, immediately available doses (0.01-0.1 µg/kg/infusion) and, in some cases, increased responding for doses of remifentanil that did not maintain responding when the alternative was saline. These data demonstrate that delaying the delivery of an opioid receptor agonist can significantly affect its reinforcing effectiveness. The imposition of a delay reduces the effectiveness of large doses of drug to maintain responding and increases the effectiveness of immediately available commodities, including smaller doses of drug. Increased reinforcing effectiveness of smaller doses of drug in the context of other delayed reinforcers might contribute to the development and maintenance of opioid abuse.

Effect of Delay on Self-Administration of Remifentanil under a Drug Versus Drug Choice Procedure in Rhesus Monkeys