Estrogen Induces Estrus Unaccompanied by a Preovulatory Surge in Luteinizing Hormone in Suckled Sows1

Abstract

The objective was to determine if progressive changes occurred in incidence of estrus and patterns of luteinizing hormone (LH) after estradiol benzoate (EB) administration at three stages of lactation. Estradiol benzoate (800 micrograms) was injected at the beginning of the second (7.8 +/- 0.3 days, range 7-8, n = 4), third (15.6 +/- 0.3 days, range 15-16 days, n = 5), or fourth (23.3 +/- 0.5 days, range 22-24, n = 4) wk of lactation. Interval to estrus (h) and proportion in estrus (in parentheses) were 72 (1/4), 88.5 (4/5), and 99 (4/4; pooled SEM = 3.5) for the second, third, and fourth weeks, respectively. Only one animal ovulated during lactation (third week). This animal had a progesterone concentration of 17 ng/ml 1 wk after estrus and an LH concentration above 2.0 ng/ml for 72 through 90 h after EB. In other sows, LH remained less than 1.0 ng/ml after EB. Patterns of LH after EB in sows treated during the fourth week of lactation were increased to a maximum of 0.76 ng/ml by 120 h after EB, which was greater than for those treated during the second or third week (maxima of 0.38 and 0.32 ng/ml, respectively; pooled SEM = 0.07; p less than 0.05). Concentrations of LH in sows that exhibited estrus were greater both before and after treatment than in sows that did not exhibit estrus after EB (p less than 0.05). By 2 wk after weaning, 8 sows had ovulated (6 of these exhibited estrus), and there were no effects of stage of lactation on these responses. We concluded that the behavioral responsiveness to EB increased as lactation progressed. The increased LH in sows treated during the fourth week indicated a partial recovery of the positive feedback response to EB. These data suggested that separate mechanisms caused behavioral and gonadotropin responses to EB in lactating sows.

Oestrus and LH reponses to oestradiol during lactational anoestrus in Chinese Meishan and Large White sows

“…The well-known negative feedback of oestradiol observed after injection of oestradiol in non-lactating sows (Elsaesser and Parvizi, 1979;Stevenson et al, 1981) was not apparent in this study or in other studies (Cox et al, 1988 (Ellendorff et al, 1988) (Barb et al, 1986, Mattioli et al, 1986) and since morphine, an endogenous opioid agonist, is able to shorten the duration of oestrus (Esbenshade and Huff, 1989). In the Mei-shan, the duration of OB-induced oestrus was similar to that described by Ellendorff et al (1988) …”

Endocrine bases of lactational anoestrus in the sow

“…Between the third and fourth week of lactation, there is a partial recovery of the positive feedback response to OB (Cox et al, 1988;Elsaesser et al, 1992;Sesti and Britt, 1993). However, even with a massive dose of OB (60 pg/kg body weight), the amplitude of the LH peak remains much lower than that observed in cyclic or OB-treated prepubertal females (Elsaesser and Parvizi, 1980).…”

“…In contrast, ovariectomy has no effect on pituitary LH content (Crighton and Lamming, 1969) or on basal or GnRH-induced secretion of LH in lactating sows (Parvizi et al, 1976 (Elsaesser and Parvizi, 1980;Cox et al, 1988). Between the third and fourth week of lactation, there is a partial recovery of the positive feedback response to OB (Cox et al, 1988;Elsaesser et al, 1992;Sesti and Britt, 1993).…”

“…Failure to activate positive feedback in early lactation could be due to a low pituitary pool of releasable LH. Lack of responsiveness of the sow to a low dose of OB (6 pg/kg body weight) in the first weeks after parturition may also be due to a high metabolic clearance rate of oestrogens preventing oestradioi-17p from reaching the threshold level (Cox et al, 1988). Finally, in lactating sows, the hormones and neurotransmitters associated with suckling could block the discharge of GnRH and that of LH in response to high levels of oestrogens.…”

“…The number of large follicles increases during the first 2 months of pregnancy and declines thereafter, whereas the number of small follicles tends to increase (Rexroad and Casida, 1975 (Elliot et al, 1980) and a high incidence of cystic follicles (Peters et al, 1969 (Ryan and Raeside, 1991). A dysfunction in the oestrogen-induced positive feedback in lactating sows shortly after farrowing (until 14-16 6 days pp) has also been described (Elsaesser and Parvizi, 1980;Cox et al, 1988;Sesti and Britt, 1993 Uterine involution could also play a role in the long-term effects of pregnancy. In lactating sows, uterine involution is achieved within 21-28 d pp (Palmer et al, 1965a,b (Baldwin and Plucinski, 1977 (Salmon Legagneur, 1956;Jensen and Recén, 1989).…”