The scientists wanted to test whether the fact that women often require two-to-three times the amount of morphine as men to achieve the same result might be something to do with microglia activating in women’s brains. What they found was incredibly interesting: when they knocked out the actions of microglia in female mice, they discovered that their responsiveness to opioid pain relief suddenly reached the same level as males’. At one stroke, they reduced the doubling or tripling of morphine to the same amount.
Male brains have microglia too, so what the hell is going on here? The scientists discovered something else, too, which may prove to be the explanation we’re looking for. “While no overall sex differences in the density of microglia were reported,” they said, “microglia exhibited a more “activated” phenotype in females.” And the more activated the microglia were, the poorer the female mice responded to morphine. This is an extremely odd thing on the face of it. Why would the female immune system get involved in pain processing? But as you explore it a bit, things start to get a little clearer.