A new study demonstrating that the brain of a male rat produces a higher level of morphine-binding receptors than the brain of a female rat provides a mechanism that might explain sex differences in morphine potency found in humans (Loyd DR et al. J Neurosci. 2008;28:14007-14017). Previous research has suggested that opioid-based narcotics are more potent in men than in women.
Researchers at Georgia State University in Atlanta found that mu-opioid receptor expression in the periaqueductal gray area of the midbrain is significantly higher in male rats compared with female rats. This region contains a large number of neurons expressing mu-opioid receptors, to which morphine and other opiates primarily bind, instructing the brain to cease responding to pain signals. The researchers also found that the stage of estrous in the female rats affected the level of mu-opioid expression.
Friedrich MJ. Pain and the Brain. JAMA. 2009;301(5):477. doi:10.1001/jama.2009.39