Effects of melatonin implants in pony mares. 1. Acute effects

Melatonin concentration in follicular fluid is correlated with antral follicle count (AFC) and in vitro fertilization (IVF) outcomes in women undergoing assisted reproductive technology (ART) procedures

The aim of the present study was to evaluate the possible relationship between melatonin levels in the follicular fluid (FF) and in vitro fertilization (IVF) outcomes in women undergoing assisted reproductive treatment. Sixty-three females (20 to 40 years old) scheduled for IVF were divided into three groups based on their antral follicle count (AFC). We determined FF melatonin concentrations in group A (AFC≦6, n = 21), group B (7≦AFC≦14, n = 22), group C (AFC≧15, n = 20) on oocyte retrieval day. Patients in group C had significantly higher melatonin levels as compared to patients in groups A and B (p < .001). Melatonin levels of the patients were significantly positively correlated with antral follicle count (AFC, p < .001), serum anti-Müllerian hormone(AMH) levels (p =.001), serum estradiol (E₂) levels on human chorionic gonadotropin (HCG) administration day (p = .001), total follicle-stimulating hormone (FSH) dose (p = .002), starting FSH dose (p = .035), number of retrieved oocytes (p < .001), total fertilized oocytes (p < .001), normally fertilized oocytes (p < .001), cleaved oocytes (p < .001), number of high-quality day 3 embryos (p = .004), blastocysts obtained (p = .007) and total embryos obtained (day3 embryos + day5/6 blastocysts) (p = .005). The levels were significantly negatively correlated with age (p < .001), basal serum FSH levels (p = .003), serum FSH (p = .001) and serum luteinizing hormone (LH) levels (p = .003) on HCG administration day. This is the first demonstration of a significant positive correlation of melatonin concentrations with AFC in patients undergoing IVF. We propose that FF melatonin levels may influence the IVF outcomes.

Effect of Melatonin Implants during the Non-Breeding Season on the Onset of Ovarian Activity and the Plasma Prolactin in Dromedary Camel

To examine a possible control of reproductive seasonality by melatonin, continual-release subcutaneous melatonin implants were inserted 4.5 months before the natural breeding season (October-April) into female camels (Melatonin-treated group). The animals were exposed to an artificial long photoperiod (16L:8D) for 41 days prior to implant placement to facilitate receptivity to the short-day signal that is expected with melatonin implants. The treated and control groups (untreated females) were maintained separately under outdoor natural conditions. Ovarian follicular development was monitored in both groups by transrectal ultrasonography and by plasma estradiol-17β concentrations performed weekly for 8 weeks and then for 14 weeks following implant insertion. Plasma prolactin concentrations were determined at 45 and 15 days before and 0, 14, 28, 56, and 98 days after implant insertion. Plasma melatonin concentration was determined to validate response to the artificial long photoperiod and to verify the pattern of release from the implants. Results showed that the artificial long photoperiod induced a melatonin secretion peak of significantly (P < 0.05) shorter duration (about 2.5 h). Melatonin release from the implants resulted in higher circulating plasma melatonin levels during daytime and nighttime which persisted for more than 12 weeks following implants insertion. Treatment with melatonin implants advanced the onset of follicular growth activity by 3.5 months compared to untreated animals. Plasma estradiol-17β increased gradually from the second week after the beginning of treatment to reach significantly (P < 0.01) higher concentrations (39.2 ± 6.2 to 46.4 ± 4.5 pg/ml) between the third and the fifth week post insertion of melatonin implants. Treatment with melatonin implants also induced a moderate, but significant (P < 0.05) suppressive effect on plasma prolactin concentration on the 28th day. These results demonstrate that photoperiod appears to be involved in dromedary reproductive seasonality. Melatonin implants may be a useful tool to manipulate seasonality and to improve reproductive performance in this species. Administration of subcutaneous melatonin implants during the transition period to the breeding season following an artificial signal of long photoperiod have the potential to advance the breeding season in camels by about 2.5 months.

Reproductive seasonality in the mare: neuroendocrine basis and pharmacologic control

Reproductive seasonality in the mare is characterized by a marked decline in adenohypophyseal synthesis and secretion of LH beginning near the autumnal equinox. Thus, ovarian cycles have ceased in most mares by the time of the winter solstice. Endogenous reproductive rhythms in seasonal species are entrained or synchronized as a result of periodic environmental cues. In the horse, this cue is primarily day length. Hence, supplemental lighting schemes have been used managerially for decades to modify the annual timing of reproduction in the mare. Although a full characterization of the cellular and molecular bases of seasonal rhythms has not been realized in any species, many of their synaptic and humoral signaling pathways have been defined. In the mare, neuroendocrine-related studies have focused primarily on the roles of GnRH and interneuronal signaling pathways that subserve the GnRH system in the regulatory cascade. Recent studies have considered the role of a newly discovered neuropeptide, RF-related peptide 3 that could function to inhibit GnRH secretion or gonadotrope responsiveness. Although results that used native peptide sequences have been negative in the mare and mixed in all mammalian females, new studies that used an RFRP3 antagonist (RF9) in sheep are encouraging. Importantly, despite continuing deficits in some fundamental areas, the knowledge required to control seasonal anovulation pharmacologically has been available for >20 yr. Specifically, the continuous infusion of native GnRH is both reliable and efficient for accelerating reproductive transition and is uniquely applicable to the horse. However, its practical exploitation continues to await the development of a commercially acceptable delivery vehicle.

Expression of functional melatonin MT1 receptors in equine luteal cells: in vitro effects of melatonin on progesterone secretion

In the present study, we analysed the molecular mechanism(s) by which melatonin directly affects ovarian function in the mare. In Experiment 1, follicles and corpora lutea (CL) were collected from slaughterhouse ovaries and analysed for melatonin (MT1) receptor mRNA and protein. In Experiment 2, CL were collected from slaughterhouse ovaries and cultured in Dulbecco’s modified Eagle’s medium-F12 medium (control medium) supplemented with 50 ng mL–1 equine chorionic gonadotrophin (eCG), 1 nM–1 μM melatonin, 1 μM forskolin or 1 μM luzindole. Explants were cultured for 3 h in the presence of these drugs. Conditioned media were analysed for progesterone production; luteal cells were analysed for cholesterol side-chain cleavage enzyme (P450scc), a steroidogenic enzyme that converts cholesterol into pregnenolone. Both MT1 receptor mRNA and protein were expressed in follicles and CL. Melatonin inhibited both the eCG- and forskolin-stimulated production of progesterone, as well as the forskolin-stimulated expression of P450scc, in equine luteal cells and the effect was dose-dependent. The inhibitory effect of melatonin was blocked by luzindole, a non-selective melatonin MT1 and MT2 receptor antagonist. The data support the presence of functional melatonin receptors in luteal cells and a regulatory role for melatonin in the endocrine function of the equine CL.