The stimulus control of drug abuse: Additive summation of cocaine self-administration
From a behavioral viewpoint, drugs of abuse are seen as reinforcers that increase behavior, much like conventional reinforcers such as food, water, and shock avoidance. To the extent that drugs are similar to other reinforcers, the theoretical framework already devised to account for conventionally reinforced behavior can be applied to the problem of drug abuse. Drugs tend to be self-administered in specific environmental contexts, and the features of these contexts that influence the seeking and taking of the drug are called discriminative stimuli. The effects of presenting independently-established discriminative stimuli in compound have been well documented for behavior maintained by conventional reinforcers. In one of these phenomena, known as additive summation, compounding discriminative stimuli dramatically increases rates of behavior. To determine whether drug self-administration is subject to additive summation, rats were trained to press a lever to receive cocaine intravenously. The drug was only made available when either a tone or a light was present. Once the tone and light had been independently established as discriminative stimuli, they were presented in combination in a stimulus-compounding test. Compared to tone and light alone, the tone-plus-light compound stimulus increased responding approximately three-fold when cocaine was withheld during testing, and it increased drug intake approximately two-fold when cocaine was made available during testing. Thus, additive summation was observed as robust increases in both the seeking and taking of cocaine. In a truly-random control condition where the availability of cocaine was not correlated with tone and light, additive summation did not occur. These results systematically replicated earlier experiments that used conventional reinforcers, supporting the incorporation of drug reinforcers into general-process learning theory. The demonstration of additive summation in this animal model of drug abuse allows the specification of situations where combinations of environmental stimuli might substantially increase human drug use. Such information could have implications for behaviorally-based treatment and prevention strategies as well as research into the biological bases of drug craving and addiction. The techniques developed in these experiments provide the groundwork for an extensive program of research investigating motivational systems and the stimulus control of drug abuse.