Opioid peptides mediate heat stress-induced immunosuppression during pregnancy.

To clarify the involvement of the opioid system in enhanced immunosuppression induced by heat stress during pregnancy, we examined the effects of heat exposure and intraperitoneal administration of opioid receptor antagonist naloxone on β-endorphin (β-EP) in blood, pituitary 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 β-EP induced by heat and naloxone and a significant interaction between heat and naloxone on blood β-EP and progesterone (P). Whereas heat reduced NKCA, intraperitoneal administration of naloxone reversed it. Significant increases in blood and placental β-EP induced by both heat and naloxone administration and a significant interaction on blood and placental β-EP was observed. These results suggest that immunosuppression produced by heat stress during pregnancy is mediated by the opioid system. A positive correlation between β-EP in blood and placenta during heat and naloxone administration suggests that increased placental β-EP during heat results in hypersecretion of β-EP into blood. P increased by heat during pregnancy may be involved in the immunosuppression.