Researchers at Massachusetts General Hospital, in Boston, US, found addicts' amygdalas are significantly smaller.

The amygdala is an almond-shaped section within the brain.

The researchers, writing in the journal Neuron, say their findings indicate those with reduced amygdala volume may be vulnerable to addiction.

But they added that family-based and longitudinal studies of cocaine addicts would reveal whether that was the case.

Predisposition

The researchers carried out magnetic resonance imaging (MRI) scans of the brains of 27 cocaine addicts and a group of non-addicts of the same age and background.

The data from the scans was used to create 3D images of the brain, which then allowed the researchers to determine the volume of the amygdalas in the subjects.

It was found the cocaine users had amygdalas that were on average 13% smaller on the left side of the brain and 23% smaller in structure on the right side.

The volume of the amygdala in the addicts did not correlate with measures of anxiety or depression, cocaine use, or age at which cocaine use began.

Addicts' brains, compared with normal subjects, also showed a loss of "laterality", in which the amygdala in the two sides of the brain was normally approximately equal in volume.

The size of the hippocampus, which lies next to the amygdala, was also examined.

However, researchers found no significant differences between the structure in cocaine users' brains and those of non-users.

The changes in amygdalas seen in cocaine users were also different to the pattern seen in manic-depression and Alzheimer's disease, in which both the hippocampus and amygdala show a loss in volume.

'Addiction genes'

The team, led by Nikos Makris, said: "These observations are relevant because cocaine-dependent subjects have significant difficulty identifying the potential negative outcomes of their behaviour or acknowledging that these outcomes could transpire."

They added that some animal studies had suggested the amygdala changes could indicate changes in the brain or damage resulting from drug use.

But they added: "An alternative hypothesis is that the observed amygdala abnormalities are a developmental condition that predisposes subjects to cocaine abuse or addiction."

They pointed out that the normal relationship in amygdala size between the two sides of the brain was not seen in the cocaine addicts.

In addition, amygdala volumes did not correlate with any measure of drug use in the addicts.

They added that further research could examine if decreased amygdala volume in cocaine addicts represents a marker for illness that could lead to the identification of genes that modify the risk for addiction.

Dr Michael Isaac, a consultant in psychiatry at the Institute of Psychiatry, said: "This study points the way to what could well be a fruitful avenue for research.

"We are learning more and more that addiction has a physical basis, and the amygdala - a small paired structure in the brain - had been implicated in basic drives such as craving, fear, anger and other emotional responses."

He added: "That the amygdala in cocaine addicts is smaller than in non-addicts is very interesting. But there is something of the 'chicken-and-egg' about the finding.

"Does the amygdala shrink under the influence of cocaine, or does a small amygdala pre-dispose to addictive behaviour? The bulk of the evidence suggests the former, but part of this study indicates a familiar or genetic smallness of the amygdala before cocaine use.

"What we don't know, and what this study doesn't seem to give us, is the clinical significance of a smaller amygdala, nor which of the many kinds of nerve cells in the amygdala are fewer in cocaine addiction."