Changes in estradiol, progesterone, melatonin, prolactin and thyroxine concentrations in blood plasma of goats following induced estrus in and outside the natural breeding season

The aim of this trail was to determine and compare the profiles of changes in progesterone (P4), 17- -estradiol (E2), melatonin (MEL), prolactin (PRL) and free thyroxine (FT4) in the peripheral blood following induced estrus in and outside the natural breeding season in goats. Observations were made on Anglo-Nubian does in the autumn (n = 12) and spring (n = 12). Does were treated with intravaginal progestagen sponges (40 mg FGA) for 12 days, and 500 IU PMSG i.m. at sponge withdrawal whereafter blood samples were collected. Samples were collected from 24 to 68 h following sponge withdrawal and the occurrence of estrous symptoms was recorded. Estrus occurred in both groups (season) of animals. However, in autumn (breeding season) estrus occurred earlier and persisted longer than in spring (outside the breeding season). Also in autumn plasma concentration of progesterone (0.42±0.11 versus 0.27 ± 0.08 ng/ml; P < 0.01) and estradiol (15.3 ± 5.0 versus 12.2 ± 3.8 pg/ml; P < 0.01) were higher. Moreover, in autumn significant differences in MEL concentration were recorded at night, 16:00 and 06:00 h, between the first (30–44 h after sponge withdrawal) and the second dark phase (54–68 h after sponge withdrawal) (22.5±16.8 versus 70.1±27.0 pg/ml; P < 0.01). Time was found to have a negative correlation between changes in concentrations of MEL and E2 (r = −0.33;P < 0.01) and betweenMELand FT4 (r = −0.39;P < 0.01). In spring these correlations were positive and the differences between MEL concentration at night (18:00 and 04:00 h) in the first (34–42 h after sponge withdrawal) and the second dark phase (58–66 h after sponge withdrawal) were not significant (46.3±9.3 and 46.5±11.3 pg/ml, respectively). In all does an increase of PRL concentration was recorded, several times higher than the basal one, which occurred a few hours earlier in autumn than in spring. In spring mean PRL concentration was higher than in autumn (103.0±58.7 versus 23.0±20.3 ng/ml, respectively; P < 0.01). Similar differences that refer to FT4 concentration (spring 14.4±4.1 versus autumn 8.9±2.3 pg/ml, respectively; P < 0.01) were recorded. In conclusion, the results show seasonal differences in the secretion of the ovarian steroid hormones, melatonin and thyroxine, following induced estrus, which suggests a different reproduction capacity in the two seasons. Seasonal differences in the profile of melatonin release during estrus may be related to the estradiol and thyroxine secretions. © 2003 Elsevier B.V. All rights reserved