Terminal follicular growth, ovulation rate and hormonal secretion after melatonin pretreatment prior to FGA-PMSG synchronisation in Suffolk ewes at the onset of the breeding season

The first experiment was performed in order to determine the effect of exog enous melatonin followed by a fluorogestone acetate (FGA) and pregnant mare 's serum gonadotrophin (PMSG) treatment on terminal follicular dynamics, ov ulation rate and luteinizing hormone (LH) pulsatility in Suffolk ewes at th e beginning of the breeding season. Therefore, 10 ewes (MEL/FGA/PMSG) were injected melatonin 22 days before receiving sponges containing 40 mg of FGA for 14 days and 800 IU of PMSG at the removal of pessaries. Ten other ewes (FGA/PMSG) were synchronised with the same FGA/PMSG treatment without mela tonin. The second experiment was undertaken to determine the effects of exo genous melatonin followed by a FGA-PMSG synchronisation on the plasma FSH, E-2 and P-4 concentrations. The investigations on terminal follicular growt h and LH pulsatility were repeated. Six ewes were submitted to a MEL/FGA/PM SG treatment and six other to a FCA/PMSG control following the same design as in the first experiment. Melatonin treatment was effective in increasing (p < 0.05) the circulating melatonin during a period of more than 14 days but was ineffective in lowering plasma PRL concentrations. LH pulsatility d uring and after the hormonal treatment and the concentrations of plasma FSH and P-4 were comparable among MEL/FGA/PMSG and FGA/PMSG groups. Concentrat ions of plasma E-2 during the luteal phase but not during the follicular ph ase of the synchronised estrous cycle were lower (p < 0.05) in ewes treated with melatonin when compared to controls. The two groups differed only at the time of selection : the percentage of medium follicles undergoing atres ia was lower (p < 0.05) in the MEL/FGA/PMSG than in the FGA/PMSG (38% and 3 5.5% versus 65% and 61.5%); this resulted in a increased (p < 0.05) mean nu mber of large follicles (4.7 +/- 0.5 versus 3.2 +/- 0.5) and ovulation rate (2.6 +/- 0.4 versus 1.45 +/- 0.35). (C) 1999 Elsevier Science B.V. All rig hts reserved.