Nicotine doesn't just stimulate the brain's "reward" centre, it also shuts down the system that limits how long those rewards last.

The research helps explain why addiction to cigarettes takes hold so quickly and is so hard to break, said the study's authors.




"It would be difficult to design a better drug to promote addiction to this horrible habit," study director Daniel McGehee said in a statement.

"It takes only a few exposures to create a lasting memory of the rewards of smoking, which are reinforced by each cigarette smoked," said McGehee, a neurobiologist at the University of Chicago.

The research on rat brain tissue, published in Thursday's issue of the journal Neuron, could lead to drugs that block nicotine's effects, said the authors.

The brain's reward centres normally reinforce behaviours that are good for you, such as eating when you're hungry.

Nicotine hijacks the reward system by attaching to receptors on nerve cells and triggering the release of dopamine, a neurotransmitter which causes pleasant feelings.

Nicotine also attaches to another receptor that triggers the release of a chemical called GABA, which stops dopamine.

The receptors keep releasing GABA until they run out and they can't produce more for up to an hour after being exposed to nicotine.

Without GABA, the body can't stop the pleasure signal caused by nicotine.

"As a result, the reward system is turned on right away and it keeps sending reward signals for 60 minutes even though nicotine levels drop off 15 minutes after smoking," said McGehee.