1973, Laverty & Taylor 1970) retain the ability to shake their heads, groom, and attempt to escape from restraint if placed in deep water, they swim and if placed in an ice bath, they escape ( Keefe et al. They have adequate motor capacity: They respond to footshock ( Cooper et al. If not artificially fed, they die of starvation ( Anand & Brobeck 1951, Stricker & Zigmond 1976, Ungerstedt 1971). Dopamine-Depleted Animals Have Only Unconditioned ReflexesĪdult animals with selective damage to their dopamine system are akinetic.
Discussed in the following sections, four aspects of dopamine function are related to addiction: ( a) Animals lacking dopamine have only unconditioned reflexes, ( b) phasic firing of dopamine neurons stamps in (reinforces) learning, ( c) tonic firing of dopamine neurons controls or contributes to motivation, and ( d) habit reliance decreases interest in other incentives. Dopamine facilitates the development of long-lasting cellular modifications that either potentiate or depress the influence of glutamate, and these adaptations determine the effectiveness of reward predictors to control subsequent search behaviors. In the striatum, where it has its most obvious, but not only, motivational functions, dopamine modulates the excitatory control of glutamate-releasing input neurons on GABA-releasing output neurons. It is sometimes identified as a neuromodulator-a subtype of neurotransmitter-because it acts more to modulate the sensitivity to other neurotransmitters rather than substituting for them.
THE DOPAMINE SYSTEM AND BEHAVIORĭopamine is a neurotransmitter: a chemical synthesized by neurons and released in ways that influence the activity of other neurons. In this review, we first discuss the role of dopamine in physiology and behavior and then describe how addictive drugs and substances activate the dopamine system to perpetuate maladaptive behavior. Thus, arguments develop around issues such as whether marijuana, gambling, or high-calorie food is addictive.Īlthough the direct effects and withdrawal symptoms vary across addictive drugs, all such drugs and some other rewards impact the brain reward system and the extracellular fluctuations of the implicated neurotransmitter dopamine. One problem with these definitions is that they do not apply equally across the range of addictive drugs and they do not apply equally to other powerful rewards. This notion of addiction has narrowed the discussion to drug addiction and links the phenomenon to drugs like heroin and alcohol that, when withdrawn, have prominent aversive withdrawal symptoms. Rather, they are the imagined future consequences, read about or heard about, such as possible or probable cancer, heart problems, or incarceration. However, the idea of negative consequences is a human issue, and for humans, negative consequences are rarely experienced at the time of drug taking. 2004) or a tone previously associated with footshock ( Vanderschuren & Everitt 2004). 46), head of the National Institute on Drug Abuse, describing addiction as “compulsive drug seeking and use even in the face of negative health and social consequences.” Negative consequences have been added to the discussion of addiction, as modeled by animals pressing a lever for a cocaine reward accompanied by either footshock ( Deroche-Gamonet et al. The latter notion has been inferred from a statement by Alan Leshner (1997, p. Often used pejoratively, the term refers to drug taking done by other people or to drug taking despite negative side effects. As a result of habitual intake of addictive drugs, dopamine receptors expressed in the brain are decreased, thereby reducing interest in activities not already stamped in by habitual rewards.Īddiction refers to the compulsive nonmedical self-administration of drugs. Independent discharges of dopamine neurons (tonic or pacemaker firing) determine the motivation to respond to such cues.
Burst discharges (phasic firing) of dopamine-containing neurons are necessary to establish long-term memories associating predictive stimuli with rewards and punishers. Animals unable to synthesize or use dopamine lack the conditioned reflexes discussed by Pavlov or the appetitive behavior discussed by Craig they have only unconditioned consummatory reflexes. Such addictions can also be defined in terms of the brain mechanisms they activate most addictive drugs cause elevations in extracellular levels of the neurotransmitter dopamine.
It is usually defined by characteristics of intoxication or by characteristics of withdrawal symptoms. Addiction is commonly identified with habitual nonmedical self-administration of drugs.