The monthly mood swings experienced by many women may serve an evolutionary purpose, researchers say, by helping to get them pregnant.

Levels of sex hormones such as oestrogen and progesterone fluctuate throughout a woman's monthly menstrual cycle. During the follicular phase at the start of the cycle, the egg is maturing and the body releases oestrogen, while during the luteal phase, when a fertilised egg might implant, progesterone is secreted.

To see how these influence the brain, Jean-Claude Dreher and colleagues at the National Institute of Mental Health in Bethesda, Maryland, US, used functional magnetic resonance (fMRI) imaging to examine the changes in brain activity over the course of the month.

The team scanned the brains of 15 women at different stages of menstruation as they played a game with hypothetical prizes of money at the end. During the follicular phase, both the orbitofrontal cortex and the amygdala showed higher activity both when the women were anticipating a reward and when the reward was delivered. The orbitofrontal cortex is associated with decision making, reward and emotion processing, and the amygdala mediates emotional reactions.

This means the women were probably experiencing greater feelings of reward during the first half of their menstrual cycles than during the second half, although they were not specifically asked to report this. "Our work specifies the brain networks that are modulated by the menstrual cycle," says Dreher.
Pleasure booster

It is unwise to speculate whether women also get more pleasure from activities such as sex, shopping or eating chocolate during the first half of their menstrual cycle. So says Emily Stern at Cornell University's Weill Medical College, New York, US, whose own work has shown how women use different parts of the brain at different stages of their menstrual cycle. "However, certain behaviours that are known to involve reward systems, such as drug addiction, might be enhanced during the follicular phase," she says.

Indeed, previous experiments have shown that women report getting more pleasure from cocaine and amphetamine use during the follicular phase compared to the luteal phase, says Dreher. He believes his findings may therefore help treat women with drug abuse problems, or those with mood disorders.

Dreher also speculates that increased feelings of reward during the follicular phase - when a woman is ovulating and therefore most likely to get pregnant - may have an evolutionary benefit. "It is interesting to note that, from an evolutionary perspective, the increased availability, receptivity and desire that may occur during the ovulatory period has been thought to facilitate procreation," he says.

Journal reference: Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.0605569104)