Melatonin deprival modifies folliclar and corpus luteal growth dynamics in a sheep model

This study assessed the effect of melatonin deprival on ovarian status and function in sheep. Experimental procedures were carried out within two consecutive breeding seasons. Animals were divided into two groups: pinealectomized (n=6); sham-operated (n=6). The completeness of the pineal gland removal was confirmed by the plasma concentration of melatonin. Ovarian status was monitored by ovarian ultrasonography during one year to study reproductive seasonality. Follicles and corpora lutea growth dynamics were assessed during an induced oestrous cycle. Since melatonin effects on the ovary may be also mediated by its antioxidant properties, plasma trolox equivalent antioxidant capacity (TEAC) was determined monthly during one year. Pinealectomy significantly extended the breeding season (310 ± 24.7 vs 217.5 ± 24.7 days in controls; p<0.05). Both pinealectomized and sham-operated ewes showed a well-defined wave-like pattern of follicle dynamics; however, melatonin deficiency caused fewer waves during the oestrous cycle (4.3 ± 0.2 vs. 5.2 ± 0.2; p<0.05) because waves were one day longer when compared with the controls (7.2 ± 0.3 vs. 6.1 ± 0.3; p<0.05). The mean area of the corpora lutea (105.4 ± 5.9 vs. 65.4 ± 5.9 mm2; p<0.05) and progesterone plasma levels (7.1 ± 0.7 vs 4.9 ± 0.6 ng/mL; p<0.05) were significantly higher in sham-operated compared with pinealectomized ewes. In addition, TEAC values were significantly lower in pinealectomized ewes compared to control ones. These data suggest that melatonin, besides exerting its well-known role in the synchronization of seasonal reproductive fluctuations, also influences the growth pattern of the follicles and the steroidogenic capacity of the corpus luteum.