Effects of delay to reinforcement on the choice between cocaine and food in rhesus monkeys

Rationale: Although a delay between behavior and reinforcer has been shown to weaken behavior, little is known about the effects of delay on drug choice. Objectives: The present study examined effects of delay between lever press and reinforcer presentation on the choice between a drug and non-drug reinforcer and between different drug doses. Materials and methods: Monkeys (n=4) were allowed to choose 32 times/day between cocaine and four food pellets. The delay between lever press and a preferred dose of cocaine (0.05 mg/kg/injection) was increased systematically from 0 to 240 s, while the delay to food remained at 0 s. A second group of monkeys (n=4) was allowed to choose between 0.05 mg/kg/injection and a lower dose of cocaine (0.025 mg/kg/injection). Next, a delay that resulted in less than 20% choice of 0.05 mg/kg/injection cocaine was selected and delay to the alternative was varied. Results: Results were similar across groups. The choice of 0.05 mg/kg/injection approximated 100% at 0 delay and decreased to near 0 as delay increased. As the delay to alternative was subsequently increased from 0 to 240 s, choice of 0.05 mg/kg/injection increased, though full cocaine choice was not generally restored. The delay estimated to maintain 50% choice (indifference point) was lower for the cocaine-food choice (mean=64 s) than for the cocaine-cocaine choice (mean=207 s). Conclusions: This experiment demonstrates that the choice between cocaine and a non-drug or drug alternative can be modified by increasing the interval between behavior and drug injection. Overall, the results are consistent with a temporal discounting model of drug choice.