They studied rats, not people, to see how one dose of morphine affects the brain's ventral tegmental area (VTA).
Kauer and colleagues focused on certain nerve cell connections in the VTA that usually limit levels of a brain chemical called dopamine.
One dose of morphine blocked those nerve cell connections in the rats' brains. That created a surge in dopamine levels, throwing off the brain's chemical balance.
"It's as if a brake were removed," Kauer says in a Brown University news release.
The activity in the rats' brains after one shot of morphine "could increase vulnerability to addiction," Kauer says. "The brain may, in fact, be learning to crave drugs."
The dopamine-inhibiting nerve cell connections in the VTA may be good targets for new drugs to treat addiction, note Kauer and colleagues.
Their study appears in Nature.