The effects of long-acting oxytocin, GnRH and FSH administration on thyroxin, triiodothyronin, oestradiol 17-β and progesterone levels as well as conception rates in post-partum ewes

This work was a first attempt to discover whether the suppression of adenopituitary gonadotropin secretion might be responsible for a decrease of systemic T4 and T3 levels in the post-partum period in ewes, and also whether the depression of T4 and T3 levels might be retroactively responsible for a depression of sexual activity in the post-partum period and in spring. T4, T3, E2 and P4 levels were observed in four groups of ewes. After parturition (lambings took place in the first 10 days of February) the animals were treated with an oxytocin-based preparation (/2-0-methyltyrosin/deamino-1-carba-oxytocinum, Depotocin injection, Leciva), synthetic LH-RH (Dirigestran injection, Spofa) or pituitary gonadotropin FSH (Folicotropin injection, Spofa). Group 1 was the control group (n = 10), and each animal received 2 ml saline. The ewes in Group 2 (experimental; n = 9) were treated with 0.14 mg Depotocin per animal, the ewes in Group 3 (experimental; n = 10) were given 200 μg Dirigestran per animal, and the ewes in Group 4 (experimental; n = 5) received 160 IU Folicotropin. All preparations were administered in two doses: an intramuscular dose at 24 h post-partum and a subcutaneous dose at 72 h post-partum. On Day 51, oestrus was induced in all animals by combined Agelin (6-chloro-6-dehydro-16 methylene-17 α-hydroxyprogesterone) and PMSG treatment. Blood samples were obtained from the jugular vein at 24 h ante-partum (a.p.) (Day -1), at 36 h post-partum and on Days 4, 7, 14, 17, 21, 25, 34, 42 and 51 post-partum. When compared with the controls, altered T4 and T3 dynamics and significantly increased levels of T3 (between Days 4 and 25, P < 0.05, P < 0.01, except for Days 7 and 14), E2 (at 36 h, P < 0.05, and on Days 4, P < 0.05, and 25, P < 0.05) and P4 (at 36 h) were observed in the Depotocin-treated animals. In this group, conception rates and births were increased by 44.5% and 111.1%, respectively. Altered T4 and T3 dynamics significantly increased levels of T4 (Day 21, P < 0.01), T3 (between 36 h and Day 25, P < 0.05, P < 0.01, P < 0.001), E2 (between 36 h and Day 34, P < 0.05, P < 0.01, P < 0.001, except for Days 7 and 17) and P4 (at 36 h) were recorded in the Dirigestran-treated ewes in which conception rates and births were increased by 22.2% and 33.4%, respectively. In the Folicotropin-treated group, T3 levels were significantly increased (between Days -1 and 21, P < 0.05, P < 0.01, P < 0.001, except for Day 14 post-partum). In these animals, conception rates and births were increased by 4.5% and 13.4%, respectively. On the basis of the above results, we suggest that the causes of depression of T4 and T3 levels after parturition in spring might be a lack of gonadotropins. Decreased T4 and T3 secretion in certain phases of the post-partum period might be retroactively responsible for the decline in post-partum sexual activity in ewes.