A gene associated with red hair and fair skin may be responsible for how females respond to painkillers, as reported in the April issue of the Proceedings of the National Academy of Sciences. This is a new finding from Canada that adds to other research on female-specific pain pathways in the brain.

"While we believe pain is the same in all women of all hair colors, our study shows women with red hair respond better to the pain-killing drug we tested than anyone else - including men," explains the study's lead researcher, Jeffrey Mogil, a McGill University psychology professor in Montreal.

Analgesics that target receptors on a female-specific pain pathway -- called kappa-opioid receptors -- have been reported to work only in women. In previous laboratory studies, Mogil and his colleagues have identified a gene that may be responsible for this sex-specific difference in pain response. The gene, called Mc1r, was first linked with pigmentation -- not with pain response. Variations of this gene cause red hair and fair skin in humans.

In the recent study, researchers tested the effects of the kappa-specific analgesic on laboratory mice that have an inactive version of Mc1r similar to the "redhead variation" in humans. The normal mice had the sex-specific pain response to these kappa-specific analgesics, but the mutant mice did not show sex-specific responses to the painkiller.

They then tested another analgesic on both men and women, all with different variations of the Mc1r gene and with different hair colors and skin types.

No differences in response were seen in men whether they had the Mr1 or not. For women, only the redheaded, fair-skinned participants showed a greater response.

The researchers suggest that there are sex differences in pain, but that analgesic sensitivity also shows promise in identifying new areas for drug development.