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Paris Hilton stresses mice out--and makes them forget their pain.

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Stress, Pain, and Paris Hilton

SAN DIEGO, CALIFORNIA--Paris Hilton: Actress, author, ... analgesic? Neuroscientists have found that a cardboard cutout of the ubiquitous Hilton Hotel heiress has a painkilling effect on mice. But don't expect clinical trials to begin anytime soon: Paris works only for males, and it may be only because she stresses them out.

The idea for the unconventional experiment arose when Jeffrey Mogil of McGill University in Montreal, Canada, and his colleagues noticed that male mice spent less time licking the site of a painful injection--indicating that they had less pain--when a scientist was present. To investigate whether it was the sight or smell of a human that caused the effect, the researchers acquired a promotional cardboard cutout of Hilton from her television show The Simple Life ("A special order," says Mogil's collaborator Leigh MacIntyre).

As in humans, Paris's effect appears to be gender-specific. Male mice spent less time licking their wounds when fake Paris was in sight, but females showed no such effect, the researchers reported here Saturday at the annual meeting of the Society for Neuroscience. When the team put up a screen to block the rodents' view, the effect went away. Following a Paris Hilton encounter, male mice--but not females--also had lower-than-usual expression of a gene called c-fos in a part of the spinal cord that transmits pain signals to the brain, suggesting reduced neural activity in this pain pathway.

Mogil suspects the analgesic effect has something to do with stress. He notes that other researchers have reported that stress can reduce pain in laboratory rodents exposed to cats or cat odors. (If a predator lurks nearby, taking time to lick your wounds may not be the best idea.) Mice probably see humans as potential predators, Mogil says, and for some reason, males stress out about it more than females do. Indeed, in another presentation Saturday, Tara Perrot-Sinal of Dalhousie University in Halifax, Canada, reported that a 60-minute exposure to a well-worn cat collar boosted levels of the stress hormone corticosterone in male rats but not in females.

The findings add to a growing body of evidence that males and females respond differently to stressors, says psychopharmacologist Rebecca Craft of Washington State University in Pullman. The work also offers a note of caution for researchers who study sex differences in a variety of behaviors, Craft says: "If you're comparing males and females in your experiment, you may be measuring sex differences in stress responses rather than, or in addition to, what you think you're measuring." Meanwhile, the effect of a Paris Hilton video on male mice has yet to be studied.