els of drug-seeking behavior, that have, for example, yielded objective measures of initiation and repeated administration of drugs, thereby providing the scientific foundation for assessments of "abuse liability" (i.e., the potential for abuse) of specific drugs (see Chapter 2). This information is an essential predicate for informed regulatory decisions under the Food, Drug and Cosmetic Act and the Controlled Substances Act. Taking advantage of technological advances in molecular biology, neuroscientists have identified receptors or receptor types in the brain for opioids, cocaine, benzodiazepines, and marijuana and have described the ways in which the brain adapts to, and changes after, exposure to drugs. Those alterations, which may persist long after the termination of drug use, appear to involve changes in gene expression. They may explain enhanced susceptibility to future drug exposure, thereby shedding light on the enigmas of withdrawal and relapse at the molecular level (see Chapter 3). Epidemiologists have designed and implemented epidemiological surveillance systems that enable policymakers to monitor patterns of drug use in the population (Chapter 4) and that enable researchers to investigate the causes and consequences of drug use and abuse (Chapters 5 and 7, respectively). Paralleling broader trends in health promotion and disease prevention in the past 20 years, the field of drug abuse prevention has made significant progress in evaluating the effectiveness of interventions implemented in a range of settings including communities, schools, and families (see Chapter 6).

Marked gains have also been made in treatment research, including improvements in diagnostic criteria; development of a wide range of treatment interventions and sophisticated methods to assess treatment outcome; and development and approval of Leo-alpha-acetylmethadol (LAAM), a medication for the treatment of opioid dependence. Pharmacological and psychosocial treatments, alone or in combination, have been shown to be effective for drug dependencies, and treatment has been shown to reduce drug use, HIV (human immunodeficiency virus) infection rates, health care costs, and criminal activity (see Chapter 8).

Drug abuse researchers have also made major contributions to knowledge in adjacent fields of scientific inquiry. For example, NIDA-sponsored research was the driving force in the identification of morphine-like substances that serve as neurotransmitters in specific neurons located throughout the central and peripheral nervous systems (Orson et al., 1994). Identification of these substances represents a dramatic breakthrough in understanding the mechanisms of pain, reinforcement, and stress. Additionally, the discovery of opioid peptides as neurotransmitters played a key role in the identification of numerous other peptide neurotransmitters (Cooper et al., 1991; Goldstein, 1994; Hokfelt et al., 1995). These discoveries have broadened the understanding of brain function and now

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