Addiction to drugs, such as heroin and cocaine, often leads to cycles of detoxification and then relapse. A big problem for addicts – even those who have abstained from drug use for many years – is the temptation of small visual cues, such as seeing a small pile of white powder. These cues can stimulate a very strong craving for the drug. Finding a way to block the craving would be helpful for treatment.

Pierre Sokoloff from the Centre Paul Broca, Paris, France, and colleagues, have found a substance, called BP 897, that is effective at decreasing cocaine cravings in rats. Reporting in the 22 July issue of Nature, the researchers found that unlike substances such as methadone (used to help treat heroin addicts), BP 897 does not act as a replacement for cocaine, and does not cause addiction.

Once someone has taken cocaine there are two competing factors that stimulate him or her on to take more. One is the miserable feeling of withdrawal as the drug leaves the body. The other is the expectation – called the "reinforcing effect" – of the feeling to come from taking more cocaine.

This effect occurs because one of the actions of cocaine is to keep the brain from re-absorbing a substance called dopamine. This is a substance, secreted by brain cells, one of whose effects is to promote the sensation of pleasure. Dopamine works by binding to a number of different receptors, one of which is known as D₃. Cocaine works by blocking the re-absorption of dopamine, which accumulates to a far higher degree than it would normally, binding to the D₃ receptor to an excessive degree. It is this effect that seems to be responsible for the "reinforcing" effect of cocaine addiction – the expectation of further pleasure from taking more of the drug.

Crucially, BP 897 binds specifically to the D₃ receptor, ignoring the several other varieties of dopamine receptor. What makes BP 897 unusual is that it is a "partial agonist." This means that if there is no cocaine around, BP 897 binds to the D₃ receptor and stimulates the cells, decreasing the craving for the actual drug. But, if cocaine is present, BP897 competes with the dopamine for binding to the receptor and dampens the cocaine's effect on craving.

In their experiments with rats, Sokoloff's group conditioned rats to associate a light with the availability of cocaine, and thus the feeling of taking the drug: in the same way humans might associate the drug with the sight of white powder, or a hypodermic needle. Once the rats were conditioned, they were given BP 897 before the light was turned on. These rats showed a decreased interest in the cocaine.

In an accompanying commentary in Nature, Gary Aston-Jones and Jonathan Druhan from the University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania, write that the researchers "provide a potentially powerful new means of treating cocaine addiction."

But BP 897 had a very specific effect – decreasing only the initial craving for cocaine that comes from seeing the associated cue. The substance was not effective in rats that had already been given cocaine. Aston-Jones and Druhan point out that this means that BP 897 might only be effective for helping those people who have already quit taking cocaine, and not for people trying to break their addiction, or dealing with a relapse.