Cocaine self-administration on a hold-down schedule of reinforcement in rats

Although many contingencies operating in the natural environment include continuous dimensions of responses and reinforcers, previous studies of drug self-administration have almost exclusively used discrete dimensions of responses (e.g., a lever press) and reinforcers (e.g., 1.0 mg/kg/injection cocaine). Therefore, the present study provides an initial examination under experimental conditions with both responses and reinforcers measured along continuous dimensions.

Cocaine-maintained responding was studied in rats under a novel, hold-down schedule of reinforcement wherein the duration of the response was directly related to the magnitude of the reinforcer. These conditions were established by activating the syringe pump when the lever was pressed down and turning the pump off when the lever was released. The concentration of cocaine available in the syringe was varied across sessions.

Cocaine self-administration was readily maintained under these conditions and remained stable across sessions. Responding was concentration dependent, with the number of responses and total duration of the response inversely related to concentration, and overall session intake of cocaine was stable across concentrations. In general, the duration of the responses were less than 0.5 s and did not vary as a function of concentration.

Stability of responding under these schedule conditions was acquired quickly. This schedule of reinforcement may be useful for comparing across drug classes, can be extended for use with other types of responses and reinforcers, and may be more representative of the natural world where response-reinforcer contingencies are more likely to be experienced along continuous, rather than discrete, dimensions.