A behavioral economic analysis of cocaine and remifentanil self-administration in rhesus monkeys.

RATIONALE: Behavioral economics can be used to evaluate the relative reinforcing effectiveness of drugs and the economic interaction between drugs, information which may help to explain patterns of polydrug abuse in humans.
OBJECTIVES: In phase 1, the reinforcing effectiveness of the opiate remifentanil and the stimulant cocaine was compared using a demand-curve analysis. In phase 2, the economic relation between these drugs was determined.
MATERIALS AND METHODS: Rhesus monkeys pressed levers according to fixed-ratio schedules for intravenous drug infusions. A demand-curve analysis was conducted (phase 1) in which drug consumption was measured as the response requirement, or price, was increased, and the rate at which consumption decreased with increases in price (demand elasticity) provided an index of the reinforcing effectiveness of each drug. Cocaine and remifentanil were then available concurrently (phase 2), and the price of one drug was increased (the manipulated-price alternative) while the price of the other drug was held constant (the fixed-price alternative). Consumption of the fixed-price alternative was measured as a function of increases in the price of the manipulated-price alternative, and demand for the manipulated-price alternative was assessed.
RESULTS: The reinforcing effectiveness of cocaine and remifentanil did not significantly differ, and these drugs functioned as economic substitutes. As the price of the manipulated-price alternative increased, consumption of the fixed-price alternative increased. In addition, demand for the manipulated-price alternative became more elastic with the concurrent availability of the fixed-price alternative.
CONCLUSION: Polydrug use involving stimulants and opiates may occur because these drugs are highly substitutable.