Explanations for alcohol addiction sought

Many of us enjoy the odd drink: a beer after work or a glass of wine with dinner. But some of us like a drink more than others. It has been estimated that around one in 20 adults in the UK could be dependent on alcohol, and that alcohol is responsible for 5,000 deaths each year.

There are also around 12 million adult smokers in the UK, with smoking-related diseases killing 120,000 people a year. One question that interests psychologists is this: why is it that some people can take or leave alcohol or cigarettes, while others become addicted? If scientists can unravel this puzzle, it could provide new ways of weaning people off drink and tobacco.

New research by scientists at the universities of Bristol and Oxford has added an important piece to the complicated jigsaw of addiction, which is made up of subtle interactions between molecular, genetic, social and environmental factors.

The researchers are focusing on the neurotransmitter dopamine, a chemical that is released in the brain in response to activities that we associate with pleasure. These activities can include anything from eating when we are hungry, or drinking when we are thirsty, to having sex.

The release of dopamine is the brain's way of rewarding us for carrying out functions that are essential to our survival, and motivating us to carry on doing these things. However, some chemicals, such as nicotine, alcohol and cocaine, can also elicit a dopamine rush - and paradoxically these substances are more likely to harm us than help us to survive.

"With food and sex and other natural stimulants, the brain rewards you with a modest release of dopamine, so that you learn that these behaviours are worth persisting with," says Dr Marcus Munafo, an experimental psychologist at Bristol.

"With nicotine and alcohol you get a much more powerful release."

But why doesn't everyone who tries alcohol or cigarettes become addicted? One strand of research that scientists around the world have been following is based on a natural variation in people's dopamine systems.

When dopamine is released in the brain, it exerts its effect by docking on to specific protein molecules in nerve cells called dopamine receptors.

Several years ago scientists identified the genes that encode the dopamine receptors, of which there are five types.

Studies showed that one class of dopamine receptor, called D2, was the one involved in the effects of nicotine and alcohol.

Importantly, it turned out that there are a small number of variants of the D2 gene that occur throughout the population. Some variants, including one called Taq1A, result in a relatively lower density and distribution of the D2 receptor, while others result in a relatively higher density and distribution.

"In the population as a whole, about 30 per cent of people will have the lower-density version of Taq1A, and 70 per cent the higher-density version," Munafo says. Around 15 years ago a study in the US examined the relationship between alcoholism and the D2 gene.

The researchers found that people with the version of the gene that resulted in a lower density of receptors were more likely to become alcoholic than those with the higher density.

"One hypothesis was that the lower number of receptors meant that people get less gratification from drinking a given amount of alcohol, so to compensate they drink more," Munafo says. "This can lead to alcoholism."

- INDEPENDENT