al., 1999; Kendler et al., 1999; Bucholz et al., 2000). Similarly, the processes that influence relapse are not necessarily identical to either of these earlier processes (e.g., see Siegel, 1984; Childress et al., 1993; Brewer et al., 1998; Robbins et al., 2000).

Advances in the neuroscience of addiction are beginning to provide a strong scientific basis for drug abuse treatment and prevention programs as well as other drug control policies (Institute of Medicine, 1996). Neuroscientists have long linked drug addiction with some disruption of the brain reward system (Olds and Milner, 1954; Wise, 1978; Cooper et al., 1996). Many drugs, including illegal drugs, can change feeling-states and may induce pleasurable feelings through actions within the central nervous system. This can happen in a number of different ways, depending on the drug, and often involves the neurotransmitter dopamine (Institute of Medicine, 1997). Normally, after release, a “transporter” returns dopamine back to the neuron that released it.1 Cocaine causes a buildup of dopamine by effectively blocking the transporter and preventing the neurotransmitter from deactivating. Amphetamine causes the neuron to release more dopamine by essentially putting the transporter into “reverse.” Heroin, alcohol, and other drugs also affect an array of reward and neurotransmitter pathways, some (but not all) linked to the dopamine transporter mechanisms (e.g., see Ritz et al., 1987, 1988; Maldonado et al., 1997; Self, 1998; Yoshimoto et al., 2000).

An important feature of the addictive process is that the feeling-states induced by drug use can be affected by past consumption. The mechanisms are different for each category of drug: nicotine, opioids (heroin), sedatives (alcohol), and stimulants (cocaine, amphetamines). Some research on the stimulant drugs suggests a sensitization model. According to this model, repeated use of stimulants sensitizes certain aspects of the reward system so that a small amount of the drug or even an environmental cue previously associated with the drug can precipitate renewed drug use. Desensitization, or tolerance, involves a different model, in which exposure to a drug causes less response than was previously caused. This phenomenon is particularly prominent with opiates, such as heroin, and sedatives, such as alcohol. Tolerance can be explained in part by the response of the nervous system acting to maintain a constant balance of neural activity in spite of major changes in stimulation. The nervous system integrates attempts to keep the body in a state of equilibrium.

These neuro-adaptive changes are critical for producing addiction. Taken with adequate dose and frequency, addictive drugs produce long-


Recent evidence points to other potential pathways of reinforcement-related neurotransmission (e.g., see Cornish et al., 1999; Cornish and Kalivas, 2000; Koob, 2000).

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