the field of drug dependence find ourselves floating on an uncharted conceptual sea, zigging and zagging in search of a common causal process. It would be as naive to suppose that all forms of heart disease have a common cause. Instead, it is more reasonable to suppose that, Just as there are similarities in the symptoms in various forms of heart disease, there are also similarities in the symptoms in various forms of drug dependence. However, in both cases one cannot expect the normal controlling mechanisms to have gone awry in precisely the same ways. It must be assumed that a relatively limited number of variables, whose weightings differ among forms of substance abuse, interact to produce the various states of dependence (see Levison's discussion, this volume). A second problem facing the field has been the absence of a unit of analysis and a metric for assessing the control drugs exercise over the behavior of the user. It wasn't until the mid 1960's that control over Objectively measurable behavior was suggested as a criterion for assessing dependence-producing properties of drugs.
Finally, we have struggled to develop more Objective ways of assessing behavioral consequences of the drugs which are self-administered, and to provide a consistent framework within which to interpret those effects. Thus, like Janus's two faces, two oppositefacing problems of drug dependence have oriented investigators in
opposite directions. Behavioral pharmacologists have treated drug self-administration and the study of other behavioral effects of drugs as only nominally related. People in the drug treatment community have focused primarily on the adverse conseouences of drug dependence, with little interest in drug self-administration, per se. Now the two have finally come face to face (see chapters by Brady and Lasagna).
Drug dependence involves a cluster of processes in which a state is produced by repeated self-administration of the drug, such that the drug user will engage in substantial amounts of behavior leading specifically to further administration of the drug, whicn will continue even when this requires the sacrifice of other important
reinforcers and sources of satisfaction (Kalant et al. 1978). An understanding of drug dependence requires knowledge of the factors responsible for development, maintenance, and elimination of drug self-administration, and of the effects of the self-administered drug on other ongoing biobehavioral processes. We are interested, therefore, not only in how a drug comes to serve as a potent reward exercising extensive behavioral control, but how the drug influences the subject's ability to meet environmental demands. The aspects of an animal's or person's behavioral functioning which are altered by a drug are the drug's locus of action. The processes which account for the drug's behavioral effects are the mechanisms of action. BEHAVIORAL MECHANISMS OF DRUG ACTION
In the natural sciences, there is broad agreement concerning what the term "mechanism" means. For example, the mechanism by which oxygen is transferred from the atmosphere into the blood stream involves the differing gradients of partial pressure of oxygen and carbon dioxide in the alveoli of the lungs and in the bloodstream. The degree to which oxygen and carbon dioxide are exchanged has to do with differential pressure gradients. Therefore, in this case we refer to a general principle of gradients of partial pressure of gases across a membrance in specifying the mechanism.