[Thyroid and ovarian hormones in ewes treated with gestagens and PMSG in the spring season]

Recent experimental observations have shown that the thyroid gland plays a dominant part in the induction and maintenance of anoestrus in ewes. The mechanisms of the anoestrous effects of the thyroid gland are still unclear. On the basis of experiments, in which after thyroidectomy at the onset of sexual activity LH production was maintained also during the spring months, iodothyronines have been supposed to stimulate the inhibitory effects of oestrogens upon the neuroendocrine centres that generate pulsatile LH secretion (Moenter et al., 1991; Webster et al; 1991). However, in our previous work (Bekeová et al., 1995) we observed significant changes in iodothyronine levels, mainly T3, in ewes treated with FSH, LH-RH and oxytocin-based preparations in 24 and 72 h after parturition in the spring. Having made the above observations we suppose seasonal anoestrus to result rather from changes in thyroid and ovarian hormone interactions or from a decrease in thyroid hormone levels that is induced by a temporary decrease in sexual hormones in this phase of the year. Within investigations into the effects of thyroid hormones and their interactions in spring this study focused on the response of the thyroid gland and ovaries in anoestrous ewes to chlorsuperlutin and PMSG treatment in the second half of May. Eighteen Slovak Merino ewes were divided into an experimental and a control group counting 15 and 3 animals, respectively. The experimental animals were each treated with 20 mg chlorsuperlutin (Agelin Spofa vaginal inserts) for 12 days. On day 12 the inserts were removed and each animal was given 500 IU PMSG. In the same time intervals the controls were treated with a placebo (sterile polyurethane, saline). Blood samples were obtained prior to swab insertion (day 0) and in 4-day intervals under chlorsuperlutin treatment (days 4, 8 and 12). For the first 24 h after PMSG-treatment blood samples were taken in 2-hour intervals and then in 48 and 72 h. For radioimmunological determination of T4, T3, E2 and P4 levels the RIA-test-T4, RIA-test-T3, RIA-test-Estra and RIA-test-Prog commercial kits (manufacturer: URVJT Kosice, Slovak Republic) were used, respectively. When compared to the almost constant but significantly lower T4 values in the controls (P < 0.05; P < 0.01; Tab. II, Fig. 1), a repeated massive release of T4 occurred in the experimental animals (Tab. I, Fig. 1). Its first peak observed 4 h after PMSG was significant in comparison both to Day 0 and the controls (P < 0.05 and P < 0.01, respectively). The same was true for the 2nd peak observed 20 h after PMSG-treatment (P < 0.001 and P < 0.01, respectively). The dynamics of T3 was similar in both groups. The transitory increase in T3 levels observed in the controls (Tab. II, Fig. 2) on day 4 of chlorsuperlutin treatment was insignificant when compared to day 0. Both the decrease observed between day 8 and of chlorsuperlutin treatment and 20 h after PMSG gavage, and the increase between 24 and 72 h appeared to be insignificant. Comparison to day 0 revealed increased T3 levels in the experimental group (Tab. I, Fig. 2) on days 4 and 8 of chlorsuperlutin treatment, the levels of significance being P < 0.01 and 0.05, respectively. Between 8 and 24 h after PMSG-gavage, in contrast to the controls, T3 levels in the experimental animals acquired the character of a slowly increasing rhythmic pulsation. At 72 h after PMSG a significant decrease occurred (P < 0.05). In the control animals (Tab. II, Fig. 3) E2 levels revealed interchanging episodes of insignificant increase and decrease beneath test sensitivity. In the experimental ewes (Tab. I, Fig. 3) a double-peaked elevation of E2 could be observed, the first (insignificant) peak occurring 18 and 20 h and the second (significant) one 48 and 72 h following PMSG treatment (P < 0.05 and 0.01, respectively). The inter-group differences were significant at the level of P < 0.05 in each case.