State versus trait: Evaluating the stability of neurocognitive functioning over time in cocaine-dependent participants

Abstract

Aims: A drug-versus-drug choice procedure was used to examine the impact of reinforcement delay on responding maintained by the mu opioid receptor agonist remifentanil. Methods: Rhesusmonkeys (n=4) lever pressed under a concurrent fixed-ratio 30 schedule. Responses on either lever delivered an i.v. infusion, either remifentanil or saline. After dose–effect curves were determined when remifentanil was available on one lever and salinewas available on the other, monkeys chose between two doses of remifentanil, and the delay to delivery of the larger dose was varied systematically across sessions. Results: Remifentanil (0.01–1.0 g/kg/infusion) dose dependently increased responding on the drug lever when the alternative was saline. When given a choice between two doses of remifentanil that maintained responding, monkeys chose the larger dose. Whengivenachoicebetweena smallerdosedelivered immediately (0.032–0.01 g/kg), and a larger dose (0.32–1.0 g/kg) delivered after a delay (30–240 s), responding for the larger dose decreased, and responding for the smaller dose increased, as a function of delay. Conclusions: Responding under this choice procedurewas sensitive to both reinforcer amount and delay. In some cases, when the larger dose was delayed, monkeys responded for smaller doses of remifentanil that otherwise (i.e., during the single-lever selfadministration study) did not maintain levels of responding above that maintained by saline, suggesting that the context in which drug taking occurs (e.g., changes in the availability of other reinforcers) can influence the reinforcing effectiveness of drugs. The imposition of a delay not only reduces the value of the delayed reinforcers but also increases the relative value of other immediately available commodities (e.g., smaller doses of drug). Enhancement of the reinforcing effectiveness of drugs in the context of other delayed reinforcers might contribute to increased vulnerability for drug abuse among individuals that are more sensitive to reinforcer delay. Financial support: Supported by USPHS Grants R01DA029254, K05DA017918 (CPF), and T32DA031115 (DRM).