Recovered drug addicts often continue to crave the drug for many years, and such cravings are linked to the long-term risk of relapse. These 'automatic' responses to drug use may develop more quickly than previously thought, suggests a paper in the May issue of Nature Neuroscience. Learning in response to a single cocaine exposure was more persistent than learning in response to a more conventional reward, a tasty food.
Roberto Ciccocioppo and colleagues trained rats to press a lever for food, then one day surprised the animals by delivering intravenous cocaine instead of food when they pressed the lever. This unusual condition was signaled by a loud noise that occurred when cocaine became available, and the rats learned to press the lever more vigorously when the noise occurred because they associated it with the drug. After a single exposure to cocaine, the rats continued to respond to the noise by increasing their lever pressing for nearly a year. In contrast, when the unexpected reward of sweetened condensed milk was signaled by a similar procedure in a different group of rats, it took less than three months for the animals to stop responding to the signal that indicated its availability. The authors conclude that learned responses to drug-related environmental stimuli may be important not only in relapse risk, but also in the motivation to repeat an initial drug experience.
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