Menstrual cycle changes the brain.

Hormone fluctuations over the course of a woman's menstrual cycle change the abundance of a particular type of receptor on the surface of nerve cells, say scientists. The finding may explain why some women with neurological disorders experience flare-ups of their conditions at the same time most months.

Previous research showed that nearly 80 percent of epileptic women have more seizures than usual during the phase of the menstrual cycle when their blood concentration of progesterone declines and that of estrogen increases. Other studies showed that women with a condition called premenstrual dysphoric disorder experience severe anxiety and depression during the same phase.

Istvan Mody and his colleagues at the University of California, Los Angeles examined brain slices from female mice. In search of changes in cells during the animals' 6-day menstrual cycles, the researchers looked at nerve-cell receptors for gamma aminobutyric acid, a chemical that inhibits neurons from firing.

The scientists found that the prevalence of a receptor subtype called delta was high when progesterone concentrations were up and estrogen concentrations were down. The same subtype was less prominent during the rest of the cycle, when the relative hormone concentrations are reversed. Nerve cells with more delta receptors were less likely to fire when stimulated with electricity than were cells with fewer delta receptors.

In a separate experiment, live mice in the high-progesterone phase of their cycle were less likely to have a seizure when given convulsion-inducing drugs than were mice in their high-estrogen phase. The high-progesterone mice also displayed less anxious behaviors than high-estrogen mice did.

Mody and his colleagues say that these results, published in the June Nature Neuroscience, suggest that a shortage of delta receptors may increase nerve cell activity during the low-progesterone phase of the menstrual cycle, in turn increasing anxiety and seizure susceptibility.--C.B.