THYROID AND OVARIAN HORMONES IN EWES TREA TED WITH GESTAGEN AND PMSG IN SPRING

Recent experimental observations have shown that the thyroid gland pla ys a dominant part in the induction and maintenance of anoestrus in ew es. The mechanisms of the anoestrous effects of the thyroid gland are still unclear. On the basis of experiments, in which after thyroidecto my at the onset of sexual activity LH production was maintained also d uring the spring months, iodothyronines have been supposed to stimulat e the inhibitory effects of oestrogens upon the neuroendocrine centres that generate pulsatile LH secretion (Moenter et al., 1991; Webster e t al., 1991). However, in our previous work (Bekeova al., 1995) we obs erved significant changes in iodothyronine levels, mainly T-3, 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 hormon e levels that is induced by a temporary decrease in sexual hormones in this phase of the year. Within investigations into the effects of thy roid hormones and their interactions in spring this study focused on t he response of the thyroid gland and ovaries in anoestrous ewes to chl orsuperlutin and PMSG treatment in the second half of May. Eighteen Sl ovak Merino ewes were divided into an experimental and a control group counting 15 and 3 animals, respectively. The experimental animals wer e each treated with 20 mg chlorsuperlutin (Agelin Spofa vaginal insert s) 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 treat ed with a placebo (sterile polyurethane, saline). Blood samples were o btained prior to swab insertion (day 0) and in 4-day intervals under c hlorsuperlutin treatment (days 4, 8 and 12). For the First 24 h after PMSG-treatment blood samples were taken in 2-hour intervals and then i n 48 and 72 h. For radioimmunological determination of T-4, T-3, E(2) and P-4 levels the RIA-test-T-4, RIA-test-T-3, RIA-test-Estra and RlA- test-Prog commercial kits (manufacturer: URVJT Kosice Slovak Republic) were used, respectively. When compared to the almost constant but sig nificantly lower T-4 values in the controls (P < 0.05; P < 0.01; Tab. II, Fig. 1), a repeated massive release of T-4 occurred in the experim ental 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 < 0.0 5 and P < 0.01, respectively). The same was true for the 2nd peak obse rved 20 h after PMSG-treatment (P < 0.001 and P < 0.01, respectively). The dynamics of T-3 was similar in both groups. The transitory increa se in .T-3 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 of chlorsuperlutin treatment and 20 h after PMSG gavage, and the increase between 24 and 72 h appe ared to be insignificant. Comparison to day 0 revealed increased T-3 l evels in the experimental group (Tab. I, Fig. 2) on days 4 and 8 of ch lorsuperlutin 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, T-3 levels in the experimental animals acquired the character of a slowly increasing rhythmic pulsation. At 72 h after PMS G a significant decrease occurred (P < 0.05). In the control animals ( Tab. II, Fig. 3) E(2) levels revealed interchanging episodes of insign ificant increase and decrease beneath lest sensitivity. In the experim ental ewes (Tab. I, Fig. 3) a double-peaked elevation of E(2) could be observed, the first (insignificant) peak occurring 18 and 20 h and th e second (significant) one 48 and 72 h following PMSG treatment (P < 0 .05 and 0.01, respectively). The inter-group differences were signific ant at the level of P < 0.05 in each case. P-4 levels in the controls (Tab. II, Fig. 4) ranged within 1.2 and 3.4 nmol/l serum, except of a decrease in 10 and 48 h that was insignificant when compared to day 0. In the experimental ewes (Tab. I, Fig. 4) Pq levels revealed a gradua l decrease, reaching their lowest values (significant in comparison to Day 0 at the level of P < 0.001) between 22 and 24 h after PMSG treat ment. Comparison to the controls revealed the lowest P between 12 and 20 h after PMSG-treatment (P < 0.05, P < 0.01; P < 0.001): The changes in both iodothyronine levels that could be observed after chlorsuperl utin and PMSG treatment were partly different from those published by Moenter et al. (1991) and Webster et al. (1991). In agreement with the above authors we suppose the changes in thyroid and ovarian hormone i nteractions to be the cause of seasonal anoestrus induction and mainte nance. On the basis of the obtained results we suppose that the depres sion of the secretion of iodothyronines is the basic mechanism respons ible for the onset of the seasonal anoestrus as well as for its durati on. We suppose that the changes in the secretion of sexual hormones as well as their controlling elements in spring are responsible for the decreased activity of the thyroid gland.