That TMD predominantly strikes women in their childbearing years has long intrigued scientists. But studies of the effects of female hormones—exactly how they affect TMD symptoms—remain controversial.
Now scientists in China, in an experiment on female rats with inflamed jaws, suggest that estradiol (the most potent of naturally occurring estrogens in mammals) increases pain sensitivity. It does this by stimulating nerve cells in a part of the brain called the hippocampus to express a specific type of “vanilloid” receptor that responds to pain associated with acid, hot peppers, and heat (including the heat associated with inflammation). This expression of vanilloid receptors amplifies the pain signals coming into the brain from nerves in the inflamed jaws.
The studies involved female rats whose jaws had become painfully inflamed as a result of a chemical injection. The ovaries of the rats had been surgically removed (ovariectomy), but the rats were then given varying doses of replacement estradiol. Compared to a control group of ovariectomized rats without replacement hormones, the estradiol rats proved more sensitive to normally painless pressures applied to the jaw, which they showed by quickly withdrawing their heads to mild pressures. As a further confirmation of their conclusions, the experimenters injected a substance into the hippocampus that blocked the actions of the vanilloid receptors and showed that it significantly reduced the rat’s sensitivity to mild pressures.