In their experiments, researchers led by Regina M. Carelli of the University of North Carolina in Chapel Hill used infinitesimally small recording electrodes in the brains of rats to detect the release of the neurotransmitter dopamine from the region called the nucleus accumbens. Almost all drugs of abuse cause release of dopamine from this region--part of the brain's reward system. Such release triggers neurons in other brain regions, generating the pleasurable sensation associated with taking such drugs.
The researchers first taught rats to self-administer cocaine by pressing a lever when a light in their cage came on. They next extinguished this behavior by substituting saline solution for the cocaine. Finally, they reinstated the lever-pressing behavior by restoring the cocaine.
Their measurements revealed three distinct types of transient bursts of dopamine from the nucleus accumbens. One dopamine signal that occurred immediately before a lever press continued to occur, even after the rats ceased to receive cocaine. This signal, concluded the researchers, could reflect the motivation to obtain the drug.
However, another dopamine signal that occurred immediately after the lever press declined rapidly after the cocaine ceased. This signal could encode the learned association between environmental cues and cocaine, theorized the researchers. A third spontaneous signal, unlike the second type, was associated with the levels of cocaine in the rats' systems. This signal could
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