Pain thresholds are elevated during gestation and following the simulation of pregnancy blood levels of estrogen and progesterone (hormone simulated pregnancy; HSP). The analgesia associated with both conditions is opioid-mediated and results from the activation of spinal cord kappa and delta (but not mu) opiate receptors. Blockade of spinal kappa or delta opiate receptors, individually, can abolish the antinociception associated with either gestational day 20 or day 19 of HSP. Surprisingly, during either physiological pregnancy or HSP, the magnitude of reduction in the increment in jump thresholds following the combined intrathecal application of suboptimum concentrations of kappa and delta antagonists is indistinguishable from that observed following their individual intrathecal application. These data indicate that gestational and ovarian sex steroid-induced antinociception is not simply the sum of the independent analgesic effects of spinal kappa and delta opioid systems but requires their coincident activation. It is suggested that the synergy that has been reported following the exogenous intrathecal application of kappa and delta opioids also occurs between their endogenous counterparts and underlies the intrinsic analgesia associated with each condition. Utilization of such a mechanism allows for significant physiological effects (analgesia) to be achieved with doses of relevant substrates (dynorphin and enkephalin) which alone would produce minimal receptor activation (and analgesia). This would minimize tolerance and dependence formation.
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