Open access publications in the SLU publication database

Abstract

The general aim of this research was to gain knowledge of oestrogen and progesterone receptor (ER and PR) expression in the uterus and pituitary gland of the ewe in different reproductive stages (postpartum period, seasonal anoestrus and oestrous cycle), as well as in experimentally induced subnormal vs. normal luteal phases in anoestrous ewes. Single, saturable and high-affinity binding sites for both oestrogen (E) and progesterone (P) were demonstrated in all of the tissue samples of the pituitary and the uterus. The values of the apparent dissociation constants (Kd) of ER and PR did not differ between the different postpartum days examined. Likewise the Kd values of ER and PR did not differ between anoestrous ewes, anoestrous treated ewes and cyclic ewes. The similar Kd values found during the different reproductive stages suggest that variations in the sensitivity of these target tissues to the ovarian hormones may not depend on changes in receptor affinity but rather on the binding capacity (number of receptors). During the postpartum period of ewes lambing in the breeding season, both ER and PR concentrations in the uterus were significantly lower in early than in late postpartum. The correlation between PR and ER concentration was positive, while the correlation between uterine weight and the concentration of either steroid receptor was negative. During the late postpartum period the number of ewes with follicles larger than 4 mm (presumptive oestrogen-active follicles) increased. Therefore, the restoration of uterine ER and PR concentrations was temporally associated with the presence of E-active follicles in the ovary. Overall results suggest that E up-regulated the uterine steroid receptor concentrations and these molecular events may be involved in the uterine remodelling in the late postpartum period during the breeding season. In seasonal anoestrous ewes, low pituitary ER and PR concentrations were found; in contrast with the high receptor concentrations found in the uteri of the same animals. However, the ERα mRNA concentrations in both the pituitary gland and the uterus were similar. While P treatment did not affect the pituitary receptor concentrations, it did decrease the uterine receptor concentrations, but it did not affect ERα mRNA concentrations in either the pituitary or the uterus. Treatment with gonadotrophin-releasing hormone (GnRH), with or without P in the anoestrous ewes, increased the pituitary ER and PR concentrations ten fold without affected the uterine receptor concentrations. GnRH treatment (with or without P) increased ERα mRNA concentrations in both the pituitary gland and the uterus. The decreases of uterine steroid receptor concentrations with P treatment, without affecting the ERα mRNA concentrations, suggest that P down-regulation occurs at posttranscriptional level. The results show that regulation of ER and PR concentration by P and GnRH is tissue specific in anoestrous ewes. During the normal oestrous cycle in the breeding season, both pituitary and uterine ER and PR concentrations were higher on day 1 than on days 6 and 13 after oestrus. This higher steroid receptor concentration at the expected time of ovulation than in the luteal phase of the oestrous cycle is consistent with the known up- and down-regulation exerted by E and P respectively on receptor expression. The high pituitary steroid receptor expression found in cyclic and GnRH treated ewes as compared with anoestrous ewes suggest that this increase of sensitivity to the steroid hormones is needed for the pituitary gland to control the cyclic function. Experimental subnormal or normal luteal phases were induced by GnRH or P + GnRH-treatments in anoestrous ewes. In all treated ewes, a synchronised surge of luteinizing hormone and follicle-stimulating hormone was found. The control animals treated with P + GnRH developed normal luteal phases and the GnRH-treated ewes developed subnormal luteal phases. The pattern of pituitary steroid receptor concentrations in the P + GnRH-treated ewes resembled the pattern found during the normal oestrous cycle, with ER and PR concentrations decreasing from the expected time of ovulation (Day 1) to the early luteal phase (day 5 or 6). In contrast, in ewes treated with GnRH alone, pituitary ER and PR concentrations increased in the early luteal phase suggesting that this impaired expression of steroid receptors may be involved, in the development of subnormal luteal phases. In the uterus, whereas in the GnRH-treated ewes the receptor concentrations increased from days 1 to 5, in the P + GnRH-treated ewes as well as in cyclic ewes the receptor concentrations decreased. On day 5, the GnRH-treated ewes had lower progesterone concentrations, and higher uterine ERα mRNA, ER and PR concentrations than the P + GnRH-treated ewes did. The results suggest that the induction of steroid receptor expression in the uterus and the hormonal environment found in the GnRH-treated ewes at the expected time of premature luteolysis may be involved in the mechanisms causing subnormal luteal phases.