June 6, 2006 — Genes and gender help determine an individual’s urge to scratch, suggests a new study on pruritus, otherwise known as itch.
The study is the first to demonstrate sex differences in itch-induced scratching behavior in animals. The researchers studied mice, but the findings could apply to humans and other mammals.
The conclusion? Females scratched themselves 23 percent more often than males did.
"We haven’t investigated the underlying mechanisms of the sex difference yet, but they usually involve gonadal (reproductive gland) hormones, such as estrogen, progesterone and testosterone, either during development or in adulthood," said Jeffrey Mogil, one of the researchers.
Mogil, a specialist in the genetics of pain at McGill University in Canada, and his colleagues induced itching in the mice by administering chloroquine, a malaria drug known to cause itchiness.
The researchers also used histamines, compounds produced by mammalian tissues to dilate small blood vessels. Histamines are largely responsible for the itchiness associated with allergies, hence the relief provided by antihistamines.
The team's findings have been accepted for publication in the journal Pain.
Mogil told Discovery News itch is the second most common reason people seek medical care.
He explained that pruritus is an important symptom of many skin, systemic and autoimmune diseases. Numerous medications, such as the malaria drug administered to the mice, cause itching as a troubling side effect. Itching can also be a chronic condition in its own right.
The most common reason people seek medical attention, of course, is pain. Interestingly enough, Mogil and his team found that individuals sensitive to pain were more resistant to itch, while those with less pain sensitivity had a greater tendency to scratch.
"I think pain and itch are wired up in the brain as opposites, in a sense, such that inheriting particular forms of pain/itch-relevant genes would simultaneously make you more sensitive to pain and less sensitive to itch, or vice versa," he speculated.
The researchers pointed out that scratching to the point of pain is a well-known strategy for alleviating itch. Conversely, pain medications and local anesthesia often intensify histamine-induced itching.
The pain/itch correlation, however, is complicated by findings from another study last year in the journal Plastic and Reconstructive Surgery. That study found that women feel pain more intensely than men do.
Bradon Wilhelmi, a plastic surgeon who authored the paper, discovered that women have, on average, 34 nerve fibers per square centimeter of facial skin while men only possess around 17 nerve fibers over the same area.
"Because women have more nerve receptors, they may experience pain more powerfully than men, requiring different surgical techniques, treatments or medicine dosages to help manage their pain and make them feel comfortable," Wilhelmi said.
It could be that women are more sensitive than men to both pain and itch, or that women have evolved some level of pain tolerance, perhaps due to giving birth.
Mogil hopes future research will show which genes turn the urge to itch up or down, how the proteins made by those genes work, and how the same genes may be linked to pain sensitivity.
