WASHINGTON (Reuters) - Just the sight of a crack pipe or other drug paraphernalia can send a cocaine addict into a frenzy of craving and now doctors know why — it causes physical changes in their brains. The same changes in brain chemistry that occur with actual drug use happen when addicts see something they associate with cocaine, a team at the Yerkes Primate Centre in Georgia found. Cocaine affects dopamine, a neurotransmitter or message-carrying chemical linked with pleasure and addiction, and Howell found that, after a while, the lights alone could affect dopamine levels in the test subjects. Doctors want to develop a drug blocking the dopamine rush. There are drugs that do this, but they work only once the cocaine has been taken."