Endorphin-mediated increases in pain threshold during pregnancy.

Maternal pain thresholds in rats were determined during various stages of pregnancy and parturition by measuring the intensity of electric shock that elicited reflexive jumping. There was a gradual rise in the pain threshold between 16 and 4 days prior to parturition and a more abrupt rise 1 to 2 days before that event. This increase was abolished by long-term administration of the narcotic antagonist naltrexone. The endorphin system is thus an important component of intrinsic mechanisms that modulate responsiveness to aversive stimuli. The data also demonstrate the activation during pregnancy of an endorphin system that is apparently quiescent in nonpregnant female rats treated the same way.