Effect of β-endorphin on human chorionic gonadotrophin secretion by placental explants

In the present study the effect of physiological concentrations of β-endorphin was examined upon human chorionic gonadotrophin (HCG) secretion by first trimester placental explants. Results show that at 7-9 weeks of gestation, β-endorphin inhibited HCG secretion; a maximal suppression of 60% was noted at 5×10^-10 M concentrations, while fivefold lower or higher doses were less effective. This inhibitory effect was completely reversed by naloxone, an opiate receptor antagonist, indicating involvement of an opiate receptor in the action of β-endorphin. The opioid peptide specificity was demonstrated by the failure of N-acetyl-β- endorphin, a non-opiate analogue used at the same concentration range, to affect HCG secretion. Following the HCG peak, at 11 weeks however, the effect of β-endorphin was stimulatory on HCG secretion, which suggests a gestational age-dependent effect of the opioid peptide. In conclusion, these data indicate that β-endorphin, a μ and δ opioid receptor ligand, has a modulatory effect on HCG secretion in vitro in the young placenta.