"With the dopamine system, in particular, there are gender differences as to how the brain responds to situations," he says, pointing particularly to stress.

The more advanced the disease, the dimmer the dopamine system appears.

According to the study published in Science, cannabinoids, through a complex chain of events, rev up the dopamine system.

But they simply kept the thing in check and had the side effect of enlarging the dopamine system.

"You see the lowest levels of activity in the mesolimbic dopamine system during that time."

Inside the dopamine system, the researchers' attention has lighted on D2.

They had started out as occasional users but over time the drug hijacked their natural dopamine systems.

Some people seem to be born with vulnerable dopamine systems that get hijacked by social rewards.

But compulsive gamblers seem to have vulnerable dopamine systems, he said.

Inherent differences in the dopamine system between individuals may also increase the vulnerability of some to become and addict.