Spinal cord mediation of the opioid analgesia of pregnancy.

It has been demonstrated that during pregnancy and labor in rats and humans there is an opioid-mediated elevation in the threshold for responsiveness to aversive stimuli which reaches a maximum at term. Acute administration of the opiate antagonist, naltrexone, into the lumbar intrathecal (i.t.) space of pregnant rats (day 20 of gestation) significantly reduces the threshold for reflexive jumping in response to electric footshock. The i.t. administration of the inactive stereoisomer of a closely related narcotic antagonist, (+)-naloxone, is devoid of any effect on pain threshold. No effect on the pain threshold is observed following intrathecal saline administration to pregnant rats, i.t. naltrexone administration to non-pregnant rats or following systemic administration of an intrathecally effective dose of naltrexone to pregnant rats. These data indicate that the analgesia observed during gestation is mediated, at least in part, via a spinal opioid pathway which is activated by some aspect of the pregnant condition.