The brake — which balances the pros and cons of food, booze, sex and drugs — fails in heavy drinkers.
The discovery opens the door to new therapies to treat problem drinkers, with half said to be genetically prone.
In tests, experts found a third of rats continued drinking after getting an electric shock.
But their ability to refrain was restored by turning off a small group of neurons in the brain.
Professor Markus Heilig, study leader at Linkoping University in Sweden, said: “They are the difference between being able to put the brakes on and not being able to stop yourself.”
The switch is a substance called PKC found in the amygdala, an area of grey matter linked to reward.
The PKC protein fuelled drinking in susceptible rodents despite negative consequences — a phenomenon called compulsive use.