Understanding the biological basis of drug addiction is an important link to understanding drug abuse behaviors, and unlike the biology of sexuality has been the object of a great deal of research. (However, it may be less useful in understanding highrisk drug use such as sharing injection equipment.) The possible link between alcohol and highrisk sexual behaviors has also become a subject of more intense research in recent years. The addiction syndrome consists of physical dependence, psychic dependence, and tolerance (Koob and Bloom, 1988). Physical dependence is considered to be an adaptive state resulting in profound physiological disturbances upon withdrawal of drug administration. Psychic dependence has been associated with the behaviorally reinforcing properties of the drug, resulting in a sense of satisfaction and a drive requiring continued administration to produce pleasure and avoid discomfort (Koob and Bloom, 1988). Tolerance is the requirement for progressively higher drug doses for a given effect with chronic use and appears to have a major learned component (Chen, 1979; LeBlanc, Gibbins, and Kalant, 1973; Schuster, Dockens, and Woods, 1966; Siegel, 1976, 1978; Siegel and Sdao-Jarvie, 1986; Wenger et al., 1981). Recent studies on the cellular and molecular basis of dependence and tolerance suggest that the processes are separate and distinct and are mediated by different brain systems (Koob and Bloom, 1988).

Traditional models of addiction suggest that one set of unspecified brain mechanisms mediate the primary, reinforcing, hedonic (pleasure-seeking) aspects of drug abuse and that, with time, a second ''adaptive" set of brain mechanisms antagonizes the first, necessitating higher doses to get the same subjective effect. The brain's adaptive response, however, also leads to a physiological reaction if the drug is withdrawn (Collier, 1980; Himmelsbach, 1943; Jaffe and Sharpless, 1968; Martin and Sloan, 1977; Solomon, 1977; Tabakoff and Hoffman, 1988). Contemporary studies are beginning to define the molecular and cellular bases of some of these well-known clinical phenomena.

Identification of opioid peptides (short proteins produced by nerve cells that bind to the same receptors as heroin and other opiate drugs) and mapping of their pathways in the brain have contributed an enormous amount of new data about the biology of opiates, including heroin (Bjorklund and Lindvall, 1984; Bloom, 1983; Khachaturian et al., 1985; Merchenthaler and Maderdrut, 1985). Some of the structures associated with addiction are now