Dopamine Nation: Finding Balance in the Age of Indulgence

by Anna Lembke

Researchers explored the effects of cocaine exposure on rats by injecting them with the same amount of cocaine on successive days for a week and measuring how much they ran in response to each injection. A rat injected with cocaine will run across the cage instead of keeping to the periphery like normal rats do. The amount of running can be measured by using beams of light that project across the cage. The more times the rat breaks the beams of light, the more it’s running. The scientists found that with each successive day of cocaine exposure, the rats progressed from a lively jog on the first day, to an outright running frenzy on the last, showing a cumulative sensitization to the effects of cocaine. Once the researchers stopped administering cocaine, the rats stopped running. One year later—a veritable lifetime for a rat— the scientists reinjected the rats with cocaine one time, and the rats were immediately running as they had on the final day of the original experiment. When the scientists examined the rats’ brains, they saw cocaine-induced changes in the rats’ reward pathways consistent with persistent cocaine sensitization. These findings show that a drug like cocaine can alter the brain forever.