Drug addiction could be genetic

Roger Highfield:

Drug addicts may be born not made, scientists suggest today.

A study has found that some people's brains are predisposed to become dependent on mind-altering substances.

As well as helping scientists understand and treat addiction, the findings could lead to a simple blood test to alert doctors and parents to whether a child has a propensity to become hooked.

Researchers agree that drug addiction and excessive sensation seeking, or impulsivity, are linked but it has been unclear which comes first – impulsivity or drug addiction.

Are the half a million people addicted to class A drugs in England and Wales predisposed to addiction because they are sensation seekers, or does chronic drug use cause the chemical changes in the brain that made them addicts?

Today, scientists at the University of Cambridge report that some individuals are indeed predisposed to drug addiction by the way their brains are wired up.

They seem to lack the docking points — receptors — for dopamine, a signalling molecule that plays a major part in what neuroscientists call the 'reward pathway' to cause the craving experienced by many smokers and addicts.

The findings, published in Science, may lead to more targeted treatments for addiction and other compulsive behaviour disorders with fewer side effects than current options.

Research in humans shows that addicts have changes in their brain chemistry.

To find out if the changes are related to the drugs or trigger dependence, Dr Jeff Dalley and colleagues, at the Behavioural and Clinical Neuroscience Institute studied spontaneously impulsive rats with a type of brain scanner.

The team found the rodents had fewer dopamine receptors of a given type in the brain. When both receptor-poor and normal rats were offered cocaine, the impulsive rodents were more likely to use the drug.

The conclusion of the study, funded by the Medical Research Council and the Wellcome Trust, is that the decrease in dopamine receptors renders an individual vulnerable to addiction and is not a consequence of chronic drug exposure.

These findings, made with colleagues in France and Spain, may have important ramifications for a range of addictive substances, including nicotine and opiates, where high consumption rates have also been linked to a similar reduction in this particular kind of brain receptor.

There is also evidence in people linking this brain receptor to certain personality traits — notably, apathy, extraversion, impulsivity — and earlier work has linked another dopamine related gene, Dat1, to attention deficit/hyper-activity disorder (ADHD).

"Much more needs to be done," he emphasised. "The real point of this study is that is shows that this particular receptor is diminished in number prior to taking to cocaine.

"This is important because much previous evidence in human cocaine addicts has likewise found reductions but it was impossible to know whether these changes pre-date cocaine use or emerge as a consequence of such use."

Dr Dalley said: "The next step is identifying the gene or genes that cause this diminished supply of brain receptors.

"This may provide important new leads in the search for improved therapies for compulsive brain disorders such as ADHD, drug addiction and pathological gambling."

Prof Colin Blakemore, chief executive of the MRC, said: "This remarkable research brings us close to answering a crucial question — whether the tendency to addiction is at least partly genetically determined.

"There have been hints of this from previous research, but now, for the first time, we have evidence of a clear linkage between an inherited brain chemistry in rats and their tendency to become addicted."

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