Emory University researchers reported Tuesday what they call the strongest evidence yet that cocaine depletes the brain's supply of the chemical dopamine - which might explain the craving the users experience during withdrawal.

Dr. J.B. Justice Jr., in a presentation to the American Chemical Society, said Emory researchers have used a tiny probe - about the size of a human hair - to conduct molecular studies on the brains of rats given cocaine daily for 10 days.After 10 days off the drug, Justice said, "one does see a depletion in dopamine," the chemical that brain cells use to signal each other. Prior research has suggested that cocaine blocks dopamine from returning to its home cell after it signals a target cell.

"Every day that an animal or an individual takes these drugs, you see this buildup of dopamine, day after day," Justice said. "Well, the brain responds to that; it adjusts. And then when you take the drug away, you have a system that's altered from what it was before" - and in dopamine depletion.

The Emory researchers used ultra-thin microprobes of cellulose acetate to withdraw molecules from active areas of rats brains, so they could study dopamine levels. Through those tests, they confirmed the dopamine depletion.

"People have seen hints of it, but we think we have fairly definitive results that show this," Justice told reporters Monday in advance of his presentation.

Researchers believe the dopamine depletion may be to blame for the cravings cocaine addicts experience in withdrawal, but that hasn't been proven yet.