OPIOID-PEPTIDES MEDIATE HEAT STRESS-INDUCED IMMUNOSUPPRESSION DURING PREGNANCY

To clarify the involvement of the opioid system in enhanced immunosupp ression induced by heat stress during pregnancy, we examined the effec ts of heat exposure and intraperitoneal administration of opioid recep tor antagonist naloxone on beta-endorphin (beta-EP) in blood, pituitar y lobes, and placenta as well as splenic natural killer cell activity (NKCA) and placental steroids in pregnant rats at 15-16 days gestation . Two-way analysis of variance revealed significant increases in blood beta-EP induced by heat and naloxone and a significant interaction be tween heat and naloxone on blood beta-EP and progesterone (P). Whereas heat reduced NKCA, intraperitoneal administration of naloxone reverse d it. Significant increases in blood and placental beta-EP induced by both heat and naloxone administration and a significant interaction on blood and placental beta-EP was observed. These results suggest that immunosuppression produced by heat stress during pregnancy is mediated by the opioid system. A positive correlation between beta-EP in blood and placenta during heat and naloxone administration suggests that in creased placental beta-EP during heat results in hypersecretion of bet a-EP into blood. P increased by heat during pregnancy may be involved in the immunosuppression.