Women appear to manufacture much less of a key mood-regulating brain chemical than do men, which could help explain why women are more likely to suffer from certain types of psychological problems such as depression, Canadian researchers reported yesterday.

The McGill University researchers used new imaging techniques to measure serotonin secreted in the brains of eight healthy men and seven healthy women. As a group, the men produced 52 percent more of the important neurotransmitter than did the women.

"We were surprised," said Mirko Diksic, a McGill neuroscientist who led the study, which is being published in today's issue of the Proceedings of the National Academy of Sciences. "This is the first time this has been found in humans."

Questions remain about whether the researchers accurately measured serotonin levels and what exactly the findings could mean. But if the results are confirmed by additional research, they could provide an important insight into mental differences between men and women.

"Many psychiatric disorders have a gender difference in incidence, and this could be on the basis of the different chemistry in the male and female," said George R. Heninger, a professor of psychiatry at Yale University, who wrote a commentary accompanying the article.

Researchers long have been puzzled by the fact that women tend to be much more prone to certain mental health problems such as depression, eating disorders including anorexia, and schizophrenia.

"People have argued whether this is due to the female social role and other sort of cultural and psychosocial dimensions, versus some vulnerability in their biological systems," Heninger said. "Obviously there's a powerful and complex interaction of many factors. But this could be evidence for the vulnerability factor."

If women produce relatively low levels of serotonin, under certain circumstances they may be more likely to experience a shortage of the critical chemical, which helps modulate emotions and moods. Anti-depressants such as Prozac work by boosting serotonin levels.

"It could be that females have less of a reserve of serotonin, and we know that when you have any stress, serotonin is used more," Diksic said. "So if females have less of a reserve, there is a possibility they go into depression faster because their brains cannot accommodate any more increased demand."

Diksic and Heninger cautioned that the findings must be confirmed with additional research, and there's a chance that women simply need less serotonin to function normally.

"Different cars utilize gasoline in different rates," Heninger said. "So even though they are producing less serotonin, women could utilize it more efficiently than men and still get as much bang for the buck in situations where they need it."

To follow up on the findings, Diksic is studying depressed people to see if their brains produce abnormally low levels of serotonin.