Sex and drugs looks at how men and women perceive pain, and how drugs affect the two sexes differently:
Dr Mogil is now convinced that the pain response in men and women is mediated by different brain circuits — and not only because of his own observations. Obstetricians and gynaecologists have long known that certain drugs are particularly effective in women. Mothers in childbirth prefer nalbuphine to morphine, for instance. Men, however, report the opposite preference when they are in pain.Both nalbuphine and morphine work by stimulating the brain’s endogenous-opioid receptors (endogenous opioids are the molecules that opium-derived drugs mimic). But opioid receptors come in several varieties, two of the most important of which are known as mu and kappa. Morphine binds to the mu receptors, while nalbuphine stimulates the less well-studied kappa receptors. Kappa-receptor agonists, as molecules such as nalbuphine are known, appear to have little or no pain-relieving effect in men.
Two years ago, Dr Mogil identified the first gene known to be involved in modulating pain thresholds in women. Variations in this gene have no effect on men’s responses to a kappa-receptor agonist called pentazocine, but they do affect the response in women. The protein produced by this gene, melanocortin-1 receptor, also affects hair and skin colour. Working in collaboration with Roger Fillingim of the University of Florida, Gainesville, Dr Mogil found that redheaded women with fair skin — who have a particular version of the receptor — have a heightened response to pentazocine.
Jon Levine and Robert Gear, of the National Institutes of Health Pain Centre at the University of California, San Francisco, also think that there are fundamental differences between the sexes when it comes to pain. They have explored the effects of nalbuphine on post-operative pain in men and women who have had their wisdom teeth removed. The results suggest that kappa-opioid agonists not only fail to alleviate pain in men, they can actually make it worse.
Incidentally, men resist pain better than women. I love the way this puzzles scientists:
The evolutionary reason why men resist pain better than women is, however, a mystery. After all, pain is there to stop you doing bad things to yourself. Perhaps it is because males and females are exposed to different sorts of pain. Males, for instance, get into fights much more often than females do, and thus get wounded more often. On the other hand, they do not have to undergo the visceral pain of childbirth. And perhaps a willingness to tolerate less pain than men do helps to explain why women live longer than their menfolk.