Endogenous analgesia in the pregnant rat: An artifact of weight-dependent measures?

It has been reported that pregnancy proceduce an opioid-mediated, endogenousa in the rats. In an attempt to confirm this finding, we used 5 different analgesic measures to compare the responsivensss of pregnant and non-pregnant female rats to painful stimuli. Pregnant and non-pregnant rats differed only when assessed by measures that were highly correlated with body weight. Furthermore, the reduced pain responsiveness of pregnant rats was not prevented by administration of the opioid antagonists, naloxone or naltrexone. We can find no evidence for an endogenous analgesia of pregnancy; instead, our results suggest that findings of a diminished response to painful stimuli in pregnant rats may be an artifact related to the greater body mass/weight of the pregnant animals.