COMPARISON OF THE LUTEOLYTIC ACTION OF GONADOTROPIN-RELEASING-HORMONE ANTAGONIST AND CLOPROSTENOL, AND THE ABILITY OF HUMAN CHORIONIC-GONADOTROPIN AND MELATONIN TO OVERRIDE THEIR LUTEOLYTIC EFFECTS IN THE MARMOSET MONKEY

The effects of the luteolytic and luteotrophic agents cloprostenol, human chorionic gonadotrophin (hCG) and melatonin on the corpus luteum have been investigated in marmoset monkeys treated with an LHRH antagonist to reduce endogenous LH secretion. This has allowed the effects of these agents to be investigated in the absence of the principal endogenous luteotrophin. Administration of the LHRH antagonist ([N-acetyl-D betaNal1-D-pCl-Phe2-D-Phe3-D-Arg6-Phe7-Arg8-D-Ala10] NH2-LHRH) or cloprostenol between days 7 and 11 after ovulation (preimplantation) resulted in luteolysis. A significant (P < 0.05) decrease in progesterone concentrations had occurred by 4 h after administration of the LHRH antagonist and was indeed preceded by a fall in LH concentrations. Co-administration of hCG with the LHRH antagonist prevented the fall in progesterone. In contrast, administration of cloprostenol resulted in an immediate fall in progesterone concentrations, to less than half the initial level within 1 h, and co-administration with hCG did not prevent the fall. Administration of hCG stimulated progesterone production when given 8 h after the LHRH antagonist but not after 24 h. Cloprostenol prevented the stimulation by hCG. Co-administration of melatonin with the LHRH antagonist did not prevent the decrease in progesterone concentrations. Melatonin was also not effective in preventing the fall in progesterone induced by cloprostenol. However, co-administration of melatonin and cloprostenol between days 17 and 21 after ovulation (post-implantation) significantly (P < 0.05) delayed the fall in progesterone seen with cloprostenol alone. These results suggest that while the LHRH antagonist and cloprostenol have different sites of action their effect is similar at the corpus luteum, that is in depriving the corpus luteum of luteotrophic support. The results also suggest that melatonin may be able to influence the luteolytic action of cloprostenol but that its effect varies with the stage of the cycle. The physiological role for such an action, if any, remains unknown.