Morphine remains one of the most widely used drugs to alleviate severe persistent pain and doctors have noticed that it frequently does not work as well in women. However, the study from Georgia State University claims to be the first to pinpoint the reason why. It looked closely at a tiny area of the brain called the periaqueductal grey area (PAG), which is important in the way that pain signals are interpreted. Many neurons in this region have, on their surface, "receptors" designed to receive and lock onto the molecules found in opioid drugs. These "mu-opioid receptors", when locked onto an opioid drug, send a message telling the brain to stop responding to pain signals, reducing the sensation of pain.
The Georgia State team found that, in the rat brain, females had a lower level of mu-opioid receptors in this part of the brain, suggesting that the potential potency of morphine is much reduced.
Additional tests suggested that the response to morphine varied depending on which part of the menstrual cycle the female rat was in.