The pleasure centers of a rat's brain feel the aftereffects of morphine as much as a day later, scientists say.

The affected area of the brain was the ventral tegmental area, a section of the midbrain's reward system. In addition to reinforcing rewarding behaviors such as eating and having sex, this area of the brain plays a role in addiction, the researchers wrote. A single dose of morphine was enough to affect the rats for more than 24 hours.

"Natural stimuli don't have this persistent effect," said head researcher Julie Kauer, a professor in the department of molecular pharmacology, physiology and biotechnology at Brown University in Providence, R.I.

The paper, co-written by Freseshteh Nugent, Esther Penick and Kauer, was published in Nature.

The area changed in the rats' brains was an inhibitor, which limit the release of pleasure chemicals, Kauer said. By preventing the inhibitors from working properly, morphine increases the brain's response to rewards, which may fuel addiction. Addicts don't feel rewarded by the drugs anymore, though they continue to put themselves at risk to get them.

"This work is quite novel and contains really good ideas about how the brain helps a person get addicted to a drug," said Jose Moron-Concepcion, an assistant professor at the University of Texas Medical Branch's Pharmacology and Toxicology department in Galveston, Texas. He wasn't affiliated with the research.

The rats were very young, which means their brains might still have been developing, said John Williams, a senior scientist at the Vollum Institute, a privately endowed research unit of the Oregon Health and Science University in Portland. He wasn't involved in the study.

"We could be seeing a developmental effect," he said.

More research needs to be done to determine the significance of the effect, he said.

By Elizabeth Lopatto Bloomberg News