The reason why some individuals may be predisposed to drug addiction has been discovered by scientists from the universities of Bristol and Cambridge.
The new findings, published in the latest edition of Science, one of the world's most prestigious journals, may lead to more targeted treatments for addiction and other compulsive behaviour disorders, with less side effects than current alternatives.
Certain changes in brain chemistry have been linked with drug addiction in humans. However, previous studies were unable to conclude whether individuals were predisposed to drug addiction because of these chemical changes or if chronic drug use itself caused the chemical changes in the brain.
Dr Emma Robinson from the University of Bristol and her colleagues at the Cambridge Behavioural and Clinical Neuroscience Institute, may have resolved this debate.
Dr Robinson said "The findings may have important ramifications for a range of addictive substances and could help us to understand why some individuals are more susceptible to drug addiction."
Using positron emission tomography (a PET scan), the team discovered that rats that were behaviourally impulsive, but which had not been exposed to drugs, had significantly less brain dopamine receptors than their more restrained counterparts.
Additionally, these same impulsive rats were found to be considerably more likely to self-administer cocaine intravenously, thus linking impulsive behaviour with drug addiction vulnerability. This demonstrates that the changes in dopamine receptors and impulsivity pre-date drug use, and do not emerge as a result of prolonged addiction.
Dr Dalley, lead author on the paper, said, "The next step is to identify 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 attention deficit/hyperactivity disorder (ADHD) and compulsive brain disorders such as drug addiction and pathological gambling".
Government reports estimate there are between 281,000 and 506,000 individuals addicted to Class A drugs (to include ecstasy, LSD, heroin, cocaine, crack, mushrooms and injectable amphetamines) in England and Wales.