Gonadotropins in Lactating Sows Exposed to Long or Short Days during Pregnancy and Lactation: Serum Concentrations and Ovarian Receptors

BIOLOGY OF REPRODUCTION 53, 1095-1102 (1995) Gonadotropins in Lactating Sows Exposed to Long or Short Days during Pregnancy and Lactation: Serum Concentrations and Ovarian Receptors' Anne-Yvonne Kermabon, 2 '3 Armelle Prunier, 4 Jean Djiane, 3 and Roland Salesse 2' 3 Laboratoirede Biologie Cellulaireet Molkculaire,3 Institut National de la Recherche Agronomique 78352Jouy-en-Josas Cedex, France Station de Recherches Porcines,4 InstitutNational de la Recherche Agronomique, 35590 St. Gilles, France The objective of this study was to determine the influence of day length on gonadotropin profiles and the expression of their ovarian receptors in lactating sows. Primiparous Large White gilts were exposed to either agradual increase (from 12 to 16 h/day, LONG treatment, n = 13) or decrease (from 12 to 8 h/day, SHORT treatment, n = 12) in photoperiod during gestation. Weaning occurred at Day 21 of lactation. All 4 sows that were submitted to the SHORT light duration and checked for postpartum estrus demonstrated an estrus by 10 days postwearing in contrast to 2 of 5 sows submitted to the LONG light duration (p < 0.05). In the remaining 16 sows, day length had no significant effect on the number of LH pulses or on mean or basal concentrations of gonadotropins (FSH and LH) or estradiol-17p measured at Day 20 of lactation. Ovarian receptors for gonadotropins and prolactin (PRL) and their mRNA were measured through use of receptor-binding and slot-blotanalyses, respectively, at Day21 of lactation inthese 16 sows. A LONG photoperiod duration had no influence on receptor number, binding, or affinity, but it significantly increased LH receptor mRNA levels (p < 0.05). However, FSH receptor mRNA levels were similar in the two groups of sows. Plasma LH concentration was positively related to LH and FSH receptor content but not to their cognate mRNA levels. Plasma concentration of FSH was positively related to the level of its own receptor mRNA as well as to that of the PRL receptor mRNA. Although the LONG day length may have delayed the return to estrus, there was no effect on gonadotropin secretion. Our results show an effect of photoperiod only on the level of LH receptor mRNA. We suggest that not all transcripts of the LH receptor are translated and that nontranslatable mRNAs accumulate in ovaries of sows exhibiting delayed estrus. INTRODUCTION European wild boars are seasonal breeders, and the females exhibit anestrus during summer [1]. This phenomenon has waned in domestic pigs, even though a tendency for summer infertility exists that has considerable economic significance [2]. Sows usually exhibit estrus 4-8 days after weaning, but this interval is often prolonged in summer, particularly in primiparous sows [3-6]. High ambient temperatures and long light duration in summer may explain this delay [2, 7, 8]. Actually, high ambient temperatures during lactation prolong the weaning-to-estrus interval, and this effect could be mediated, at least in part, through a decrease in food intake [7, 9]. Increasing the light duration progressively during gestation in order to simulate the natural day length transition also increases the weaning-to-estrus interval [8, 10]. The endocrine mechanisms underlying the influence of photoperiod on the return to estrus in sows after weaning are poorly understood and are probably of multifactorial origin. The 4-8-day time lapse between weaning and estrus is similar to the length of the follicular phase of the estrous cycle in gilts; this suggests that follicular recruitment for preovulatory growth normally occurs immediately after weaning. A drop Accepted June 12, 1995. Received December 5, 1994. 'This study was supported by grants from the Institut National de la Recherche Agronomique, France. 2Correspondence: A.Y. Kermabon/Roland Salesse, Laboratoire de Biologie Cellulaire et Moleculaire, Institut National de la Recherche Agronomique, Batiment des Biotechnologies, 78352 Jouy-en-Josas Cedex, France. FAX: 33 (1)34.65.22.73. in plasma prolactin (PRL) concentrations and an increase in LH pulsatility and FSH levels are generally observed at weaning and probably allow and/or stimulate follicular recruitment [11, 12]. Thus, delayed estrus after weaning in summer may result from an inadequate responsiveness of the follicles at the time of weaning, which itself may result from insufficient gonadotropin or excessive PRL secretions during lactation. Some investigators have compared LH secretion in female pigs at different seasons, but they were unable to distinguish between the influences of photoperiod, temperature, and nutrition [6, 13, 14] on females submitted abruptly to 8 or 16 h of light per day [151. LH pulsatility did not change with season or light duration in intact lactating sows, intact sows after weaning, or ovariectomized females. No influence of light duration on mean plasma LH concentration was observed in intact lactating or ovariectomized females [15]. However, LH concentration is lower in sows shortly after weaning in summer as compared to winter [6], and the influence of season in ovariectomized females is controversial (higher LH concentration in summer [131; no effect [14]). The effect of photoperiod on FSH secretion has been less well studied. Mean plasma FSH levels do not seem to be influenced by photoperiod in lactating sows [8] and are higher in summer than in winter in sows shortly after weaning, probably in relation to lower ovarian negative feedback tone [6]. Plasma concentrations of PRL are higher in summer than in winter in ovariectomized gilts, but the difference is less pronounced for cyclic gilts [16]. Abrupt variation in light duration has no influence on PRL levels in intact lactating or ovariectomized females [17]. 1095 ABSTRACT 1096 KERMABON ET AL. 18 . 16 0 LONG .............. 14 Parturition Weaning 12 10 03 8 SHORT 6 0 9 12 15 18 21 23 27 30 33 100 105 110 115 120 125 130 DAYS AFTER INSEMINATION FIG. 1. Experimental design. Sows were bred at 10-13 mo of age and exposed to one of two light regimens that mimic variations occurring during spring (group LONG, n = 13) and autumn (group SHORT, n = 12). Weaning occurred at Day 21 of lactation. To our knowledge, the influence of season and photoperiod on follicular growth and responsiveness to gonadotropin stimulation has not been described in cyclic and postpartum female pigs. The objective of the present study was to determine the influence of day length on the return to estrus after weaning and on the hormonal profiles of FSH and LH as well as on the expression of the ovarian receptors for FSH, LH, and PRL at the mRNA and protein levels in lactating sows. The fluctuation of these receptors and of their cognate mRNAs could provide insight into the ability of the ovary to respond to pituitary gonadotropins after weaning. MATERIALS AND METHODS Animals and ExperimentalDesign From 240 days of age, 25 pure Large White gilts were reared under controlled l (...truncated)