Pain follows cycle.

The rise and fall of estrogen during a female's menstrual cycle may change her perception of pain, according to an experiment on rats.

Studies have shown that women tend to report more-intense and longer-lasting pain than men do, but the reason for this difference hasn't been clear. Nicole Amador of the City University of New York and her colleagues suspected that female-sex hormones could make women more sensitive to pain.

To test their hypothesis, the researchers tracked the estrous cycle in adult female rats. It's analogous to a woman's menstrual cycle. On the same day, regardless of what part of the cycle the animals were experiencing, the scientists injected one of each animal's hind paws with a chemical that causes painful inflammation. Each rat was then monitored for an hour for signs of pain.

Amador and her colleagues found that the animals' responses differed depending on what phase of the estrous cycle they were in. Pain perception was consistently high in animals in the half of the cycle when estrogen concentrations were low. Pain seemed to be less intense to animals in the high-estrogen part of the cycle, which precedes ovulation.

The results suggest that estrogen might relieve pain, says Amador. She adds that if these results extend to people, doctors may eventually want to take women's menstrual phases into account when determining dosages for analgesic drugs.--C.B.