'Uppers' Affect Men More Powerfully

New Scientist ^ | 4-10-2006 | Roxanne Khamsi

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'Uppers' affect men more powerfully

20:30 10 April 2006
NewScientist.com news service
Roxanne Khamsi

The stimulating drugs known as amphetamines have a stronger effect on men than women, a new study reveals.

The study is the first to reveal how the release of dopamine, an important nerve signalling chemical, differs between the sexes in humans, says Gary Wand of the Johns Hopkins University School of Medicine in Baltimore, Maryland, US. He says the findings could help researchers understand why some disorders such as Parkinson's disease and Tourette's syndrome more commonly afflict men than women.

Research suggests that men are more prone to certain types of drug addictions. And according to a recent US health survey, 6.0% of males above the age of 12 illegally used amphetamines, also known as 'uppers', while only 3.8% of females did so. The new work showed amphetamines caused a greater surge of the pleasure-causing chemical dopamine in men than in women.

"The fact that they responded differently to the drug suggests that there's an underlying biological sex difference in how the brain's reward centre responds to amphetamines," says Wand.

Good reception

The team recruited 43 healthy volunteers between the ages of 18 and 29 years of whom about a third were women. The participants did not have a history of drug abuse. The volunteers were given relatively low doses of amphetamines, and their dopamine response was measured with the help of a second chemical.

This second compound, called Raclopride C11, binds with the dopamine receptors on the outside of cells, but relinquishes its hold on the receptors if dopamine is present. So, as more dopamine is released, levels of Raclopride C11 drop in that part of the brain. Raclopride C11 is mildly radioactive and can therefore its distribution in the body be detected with scanning technology.

After the amphetamines were taken, the pleasure centre in the men's brains showed lower levels of Raclopride C11 than the women's brains, indicating higher levels of dopamine. The men in the study showed a 60% greater dopamine response in this brain region than the women.

The researchers ruled out the possibility that the men and women had different numbers of dopamine receptors by scanning the volunteers after they had been given Raclopride C11 but before they received amphetamines.

Rush and fidgeting

Wand's team also asked the participants to describe how the amphetamines made them feel. Men reported significantly higher responses in terms of both positive and negative effects - such as rush and fidgetiness - than women did in all but one category, dizziness.

The results could help to explain why men are more likely than women to abuse amphetamines. Wand says that animal studies have shown dopamine-producing neurons are more prone to damage in males than females. He speculates that the surge of dopamine in men may over-stimulate and injure these cells, perhaps leading to greater drug dependence.

The findings may also provide insight into Parkinson's disease, which is believed to be caused by a deficiency of dopamine and into Tourette's syndrome, which appears to involve abnormal dopamine signaling.

Journal reference: Journal of Biological Psychiatry (DOI: 10.1016/j.biopysch.2006.01.08