Until 1903, a bottle of Coca-Cola contained around 60 mg of cocaine -- enough, it has now been shown, to trigger long-lasting changes in brain activity. According to a report in today's issue of Nature, giving a single dose of cocaine to mice changes the way that nerve connections transmit signals in a part of the brain thought to be crucial to the development of addiction. Such changes may predispose the brain to subsequent addiction, say the researchers.

Cocaine -- made from the leaves of the South American plant Erythroxylon coca -- produces its powerful psychostimulant effect by blocking the reuptake of dopamine in the brain. Dopamine is a neurotransmitter affecting brain processes that control movement, emotional response and the ability to feel pleasure and pain -- people with Parkinson's disease, for example, have almost no dopamine. Cocaine changes the flow of dopamine by preventing it from returning to storage in the neurons in the midbrain. Nerve impulses in the pleasure centers of the brain are therefore repeated, producing a "high."

Until 1916, cocaine could be bought over the counter at Harrods: Many Victorians, it seems, had brains altered like the mice in the new study. Now, despite its illegality, use of the drug remains widespread. In 1997, 1.5 million people in the United States aged 12 and above were chronic cocaine users, and in Britain cheap prices have meant the drug has become popular with students and other groups besides the traditional wealthy users.