PubMed1992
I would like to stress several major points. The first is that CNS pharmacology is important in drug self-administration. In the case of cocaine, the necessary effect seems to be blockade of reuptake of DA rather than blockade of reuptake of NE or 5-HT or local anesthetic effects. An action of that DA on the D2 subtype of DA receptors appears to play an important role. The second point is that increased DA in synapses in the CNS is not sufficient to explain drug-maintained behavior. The schedule of reinforcement critically determines rates and patterns of drug-maintained behavior. Barrett (this volume) has presented convincing evidence that behavioral history, i.e., the sequential exposure to alternative behavioral conditions, can dramatically alter drug effects. What I hope to have made clear is that the simultaneous opportunity to engage in other behaviors can dramatically alter the reinforcing effects of drugs. Indeed, under appropriate conditions, a drug can function as both a positive and a negative reinforcer simultaneously in the same animal. The drug-choice studies I have described emphasize the dynamic nature of the interaction between behavioral and pharmacologic variables. The frequency of choosing to self-administer cocaine varied with the magnitude of the dose of cocaine (a pharmacologic variable) and the magnitude of an alternative positive reinforcer that was available (a behavioral variable). Increasing the magnitude of the alternative reinforcer decreased cocaine choice, but increasing the magnitude of the cocaine dose reestablished drug preference. In a sense, this is not a surprising result. The rhetoric of the times, however, would have it that drug self-administration, particularly cocaine self-administration, is somehow a different class of behavior that is not subject to the usual laws governing the behavior of organisms. As our understanding of drug self-administration evolves, however, that position will become increasingly untenable. These experimental findings challenge simple neurobiological assumptions about the determinants of drug self-administration. Clearly, a reinforcing effect, like any other behavioral effect of a drug, is not simply an immutable effect of a pharmacologic property of the drug. Given the pivotal role of reinforcing effects in drug abuse, this realization is critical to a comprehensive understanding of drug abuse. Any complete neurobiological model of drug abuse will have to account for these effects. Apparently, increasing the concentration of DA in synapses in the brain is not sufficient to account for drug-maintained behavior.(ABSTRACT TRUNCATED AT 400 WORDS)