Scientists believe they are closer to understanding why alcohol and drugs exert an addictive effect on the brain.
A study of rats suggests that part of the brain linked to addiction produces a strong supply of morphine-like substances called endorphins in response to alcohol, cocaine or amphetamine.
These endorphins could be the means by which the brain becomes trained to crave, says the researchers.
The study, carried out at two US universities and published in the Journal of Neuroscience, is thought to be the first to show the increased presence of endorphins in this area under such circumstances.
The researchers gave the rats injections of alcohol, cocaine, amphetamine, nicotine and an inactive salt solution.
They then measured endorphin levels in brain fluids removed from awake and active rats.
They found a significant increase in endorphins in rats given the first three substances.
Despite the fact that scientists have been studying the brain pathways of addiction for more than a decade, they are still not fully understood.
About 15 years ago, it was discovered that in a region of the brain called the nucleus accumbens, "drugs of abuse" caused an elevation of the brain chemical dopamine.
The discovery that there may also be an increase in endorphins - which can lock into receptors in the nucleus accumbens - sheds more light on the process of addiction.
Dr Clyde Hodge, from the University of North Carolina at Chapel Hill, said: "We hypothesize that this drug-induced release of endorphins may contribute to the positive reinforcing and motivating properties of alcohol and psycho-stimulant drugs.
"We did not find this same boost in endorphins in this brain region with nicotine, but we don't know why.
"It could be that the dose of nicotine we used was too low or that nicotine does not cause this same effect and instead acts in some other way."
Professor David Balfour, an expert in psychopharmacology from the University of Dundee, said: "The dopamine hypothesis of dependence has been around for the last 25 years.
"But what has become clear is that it isn't enough to explain the complete biology of dependence.
"What these findings are doing is giving us another part of the jigsaw - it's beginning to fit together."
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