Department of Obstetrics and Gynecology, Medical University Lübeck, Ratzeburger Allee 160, 23538 Lübeck, Germany
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Abstract |
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Key words: embryo implantation/GnRH antagonists/ovarian effects/ovarian stimulation protocols/steroidogenesis
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Introduction |
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GnRH analogues are able to suppress gonadotrophin release and, subsequently, gonadal function. This is the basis for their clinical application. Several agonistic or antagonistic GnRH analogues have been developed for this purpose.
Over the last two decades, GnRH agonists have been used in ovarian stimulation protocols in assisted reproductive techniques (ART) in combination with gonadotrophins to prevent a premature LH surge. GnRH agonists induce an initial rise of gonadotrophins (flare-up) before they achieve suppression through desensitization. Recently, GnRH antagonists have been introduced into ovarian stimulation protocols.
The concept of suppressing gonadotrophins by competitive receptor blockage rather than through desensitization with its inevitable flare-up is compelling. The first antagonist was synthesized >20 years ago (Karten and Rivier, 1986). Its clinical application was hampered by a high histaminergic potential. Recently, new substances were developed which lacked the histaminergic side-effects (Schally et al., 1989
; Reissmann et al., 1995
). These antagonists lead to a rapid inhibition of LH release due to competitive binding to pituitary GnRH receptors. Two of these compounds are cetrorelix and ganirelix. Cetrorelix has recently been introduced into ovarian stimulation protocols to prevent premature LH surges. Ganirelix and cetrorelix proved their safety and effectiveness in clinical trials, showing comparable results with the agonists (Diedrich et al., 1994
; Felberbaum et al., 1996
; Ganirelix Dose Finding Study Group, 1998
; Weiss et al., 1999
; Albano et al., 2000
; Borm and Mannaerts, 2000
; Olivennes et al., 2000
). No significant differences in fertilization rates, embryo quality and oocytes were found between agonist and antagonist treatment. One advantage of the antagonists seems to be a lower incidence of ovarian hyperstimulation syndrome (OHSS), a serious side-effect of ovarian stimulation treatment (De Jong et al., 1998
). There is no evident mechanism that explains this finding, but it is consistent in the studies performed so far. In a novel study (Albano et al., 2000
), the effects of cetrorelix and buserelin in patients treated in ovarian stimulation protocols were compared. Significantly lower oestradiol serum concentrations on the day of human chorionic gonadotrophin (HCG) administration were found in patients treated with GnRH antagonists, which could be due to the lower number of small follicles observed on the day of HCG administration. These observations and the shorter human menopausal gonadotrophin (HMG) treatment period could contribute to the rationale of the lower incidence of OHSS in GnRH antagonist-treated patients. Additional findings indicated that the number of HMG ampoules administered and the duration of ovarian stimulation were lower in GnRH antagonist-treated patients (Olivennes et al., 2000
). In a recent large European multicentre study, the impact of ganirelix on oocyte numbers, mean duration of stimulation, number of good quality embryos and embryos transferred compared with a long protocol using buserelin was less striking, although again the implantation (15.7 versus 21.8%) and pregnancy rates (20.3 versus 25.7%) in the ganirelix group were lower (Borm and Mannaerts, 2000
).
These findings have generated the hypothesis that GnRH antagonists might have direct ovarian actions. Although there are conflicting data, GnRH receptors have been demonstrated in the human ovary (see below). Even if the concentration of native GnRH is too low in the peripheral circulation to interfere with extrapituitary GnRH receptors, it is possible that the application of 0.25 mg GnRH antagonist daily is sufficient to induce effects on ovarian GnRH receptors. In addition, the actions of GnRH and GnRH-like peptides produced directly by the ovary might be influenced by GnRH antagonists (Aten et al., 1986; Rivier et al., 1996
). It is still unclear whether ovarian GnRH receptors are functional. Conflicting data exist on the actions of GnRH antagonist on steroidogenesis in human granulosa lutein cells.
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GnRH receptors in human ovary |
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The rodent ovary contains GnRH receptors. Most of the studies were performed in rat ovaries where high affinity binding sites for GnRH have been demonstrated (Clayton et al., 1979; Jones et al., 1980
; Pieper et al., 1981
). In rats, GnRH has inhibitory effects on granulosa and luteal cell functions. The peptide reduces LH-induced steroidogenesis and agonist-induced cAMP-production (Hsueh and Jones, 1981
).
However, conflicting data exist on the presence of GnRH receptors in the human ovary, their gene expression and function. Binding studies with human luteal tissue or cells revealed no, low or high affinity binding sites for GnRH. GnRH receptors have been found in oocytes (Dekel et al., 1988). High affinity GnRH receptors have been shown by quantitative autoradiography in human granulosa cells (Latouche et al., 1989
). Binding sites with low affinity have been found in the corpus luteum (Bramley et al., 1985
), whereas other authors (Clayton and Huhtaniemi, 1982
) detected no GnRH binding in human corpus luteum.
The human GnRH receptor was cloned in 1992 (Kakar et al., 1992). Since then molecular approaches have become possible. GnRH receptor mRNA was detected in the human ovary using reverse transcriptasepolymerase chain reaction (RTPCR) (Kakar et al., 1992
; Peng et al., 1994
; Minaretzis et al., 1995b
). The level of GnRH receptor mRNA in the ovary is ~200-fold lower than in the pituitary. Another author (Fraser et al., 1996
) found very low levels of GnRH receptor mRNA in the corpus luteum by in-situ hybridization. Studies on the regulation of the GnRH receptor mRNA found that GnRH up-regulates and HCG down-regulates GnRH receptor and GnRH receptor gene expression (Peng et al., 1994
). A more recent study (Brus et al., 1997
) found GnRH receptors predominantly after the LH surge. They could not detect GnRH receptors in preovulatory follicles. Other authors (Kang et al., 2000
) showed that GnRH and GnRH receptor mRNA are expressed in human ovarian surface epithelium. In human granulosaluteal cells it was shown (Nathwani et al., 2000
) that oestradiol (24 h) inhibits GnRH mRNA and regulated its receptor gene expression in a biphasic way such as short-term oestradiol treatment (6 h) increases and long-term (48 h) decreases GnRH mRNA levels. Their finding indicates an autocrine role for GnRH in the human ovary and that gonadal steroids might be important regulators of the ovarian GnRH/GnRH-receptor system.
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Actions of GnRH agonists in the human ovary |
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Actions of GnRH antagonists on steroidogenesis in the human ovary |
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The finding that antagonists do not alter the steroidogenesis of granulosa lutein cells, does not exclude the existence of other effects on ovarian functions. It has been established that cAMP-mediated signalling is of importance in granulosa lutein cells. Early studies (Ranta et al., 1983) suggested that GnRH reduces ovarian adenylate cyclase, while a later study (Furger et al., 1996
) indicated that the GnRH agonist, triptorelin, interacts with FSH-induced cAMP-dependent signal transduction. Therefore, we investigated whether GnRH antagonists exert any effects on cAMP accumulation. We found that ganirelix did not affect basal or HCG-stimulated cAMP accumulation in human granulosa and cumulus cells (Demirel et al., 2000
).
In conclusion, there is little evidence that antagonists exert direct effects on ovarian steroidogenesis. This assumption is mainly derived from extensive in-vitro work with granulosa lutein cells. The action of gonadotrophins is not altered. Both stimulated steroid secretion and cAMP accumulation were not significantly influenced by antagonists. Our observations do not support the hypothesis that lower concentrations of serum oestradiol during ovarian stimulation in combination with a GnRH antagonist are due to direct ovarian action of the peptide analogue. However, other ovarian functions may be altered by direct GnRH analogue effects.
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Notes |
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This debate was previously published on Webtrack (www.oup.co.uk/humrep/comment) on December 7, 2000
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References |
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