Leptin - Endocrine Signal Regulating the Onset of Post-weaning Estrus in Pigs

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1 Leptin - Endocrine Signal Regulating the Onset of Post-weaning Estrus in Pigs Christa M. Cook 1, Robert A. Easter 2, Janice M. Bahr 3 1 University of Illinois, Ph.D. Candidate; 2 University of Illinois Dept. of Animal Sciences, Chairman; 3 University of Illinois Dept. of Animal Sciences, Professor. Introduction Profitability in the swine industry is based on prolificacy. One measurement of prolificacy is the number of piglets produced per year. To optimize prolificacy and profits it is essential that sows are re-bred as soon as possible after weaning. An extended post-weaning anestrus decreases overall productivity and increases costs of production. A large number of sows undergo prolonged postpartum anovulatory periods. Furthermore a significant number of sows, after having their first litter never return to estrus and are removed from the herd. Many factors influence the duration of the weaning-to-estrous interval including: parity, season, duration of lactation, breed and nutritional status (Kirkwood et al.,1984; Aherne and Kirkwood, 1985; Clark et al., 1986; Dial et al., 1987). Energy Balance One of the most defined causes of prolonged weaning-to-estrous intervals is negative energy balance (Pettigrew and Tokach, 1993; Jones and Stahley, 1995). Negative energy balance is defined as the use of more energy metabolically than consumption of energy contained in the diet. Animals on a long term energy restricted diet typically have depleted fat stores, very low conception rates and often enter into a period of anovulation (Imakawa et al., 1983; Foster et. al., 1985; Armstrong and Britt, 1987). Similarly during periods of heavy milk production, lactation rather than reproduction takes priority in the partitioning of nutrients. Insufficient feed intake, inadequate energy and heavy milk production cause a large mobilization of energy stores within the animal (Kirkwood et al. 1987a; Kirkwood et al., 1987b). The majority of these molecules are transported to and utilized in the mammary gland for synthesis of milk components. This creates a precarious position for the producer who needs to maintain adequate milk production to sustain the current litter but who also needs to have sows return to estrus promptly after weaning to optimize profitability. Numerous diets and management schemes have been examined in an attempt to reduce the weaning-to-estrous interval. These schemes have improved piglet survivability but prolonged post-partum anestrus still persists. Nutritional Status and Reproductive Function Many investigators have examined the relationship between nutritional status, body fat reserves and reproductive function. As a result it has been hypothesized that there is a direct signal or hormone from adipose tissue to the brain or ovary to mediate reproductive function. Adequate nutrition and reserves of the metabolic fuel, fat, are required for ovulation, conception and maintenance of the developing embryo. Animals in a starved or metabolically compromised state are not good reproductive candidates. Logically, there is an evolutionary mechanism which enables the animal to determine if there is enough metabolic fuel or adequate energy storage to support the energy demands of pregnancy and lactation. Identification of this signal will enhance our understanding of the mechanisms which control post-partum reproductive function. Better understanding of these

2 mechanisms will enable researchers to develop management strategies, nutritional regimens and/or drug therapies which will improve prolifically and profitability in the swine industry. Therefore our overall goal is to identify and characterize the metabolic signal which regulates reproductive function based on the metabolic state of the individual. Leptin Leptin, a newly discovered hormone is the focus of our research. Leptin is hypothesized to be a regulator of satiety, metabolic activity and reproductive function. Adipose tissue synthesizes and secretes leptin into the blood. In feed restricted or malnourished individuals, fat reserves are minimal and concentrations of leptin in the plasma are low. Refeeding or improvement in metabolic status replenishes fat reserves and increases concentrations of leptin in plasma. Similarly, mice that do not produce endogenous leptin are sterile. Injections of leptin reverse this sterility. Several studies have documented the action of leptin on the hypothalamus and its role in regulating reproductive neurohormones and satiety centers (Campfield et al., 1995; Considine et al., 1996; Erickson et al., 1996). However, the ovary appears to also be a target for leptin. Investigators have found that leptin treatment increases the total number of follicles, and synthesis of estrogen (Barash et al., 1996) but decreases synthesis of progesterone in the ovary of rodents in vitro (Spicer and Francisco, 1997). The ovarian leptin receptor has been identified in some species (Cioffi et al., 1997; Zamorano et al., 1997) including pigs (T. Ramsey personal communication, 1997); however its overall function is not known. At this time we are presenting our research objectives. We do not have current data because the procedures needed to measure leptin in plasma and mrna for the leptin receptor are being developed. The first specific objective for our research is to determine if concentrations of leptin in plasma of pigs are correlated with body fat composition, stage of lactation and concentrations of protein or energy in the diet. The amount of body fat changes throughout lactation. During peak lactation body fat composition is at its lowest. If adequate dietary energy is not provided during lactation, fat is mobilized from adipose tissue and breakdown of body protein in skeletal muscle occurs (Pettigrew and Tokach, 1993; Jones and Stahley, 1995). These products are then used to provide the necessary energy needed to maintain lactation. Typically fat and protein stores are broken down in a 1:1 ratio (Burlacu, 1983). However, diets containing adequate protein but low energy cause depletion of fat stores but no breakdown of body proteins (Garlic et al., 1980). Since adipose tissue synthesizes leptin and secretes it into the circulation, any physiological process affecting the amount of adipose tissue could affect concentrations of leptin in plasma. Blood was collected from 72 primiparious sows on days 1, 7,14,21 and 28 post partum with 8 sows per day. These sows were fed one of four diets containing either: 1) Control (HE-HP), 2) high protein, low energy (HP-LE); 3) low protein, high energy (LP-HE); or 4) low protein, low energy (LP-LE) diets during lactation. Concentrations of leptin in these samples will be determined and compared statistically with whole body fat compositions, diet and stage of lactation. We expect that body composition will be related positively to diet and that 2

3 concentrations of leptin in plasma will be positively correlated with body fat. As fat within the carcass increases, concentrations of leptin in the plasma increases. We also expect that as lactation progresses, body fat composition will decrease which results in decreased concentrations of leptin in the plasma. Animals being fed HE-HP (control) diets should be adequately fed and should be mobilizing very little body stores, particularly during late lactation. These animals then should have relatively high concentrations of leptin in plasma. Animals being fed HE-LP diets should be mobilizing protein stores rather than fat stores. Therefore, these animals should have similar body fat composition and concentrations of leptin in plasma as the animals on the HE-HP(control) diet. Animals being fed LE-HP diets will be mobilizing large amounts of body lipids (fats) but very little protein stores. Leptin concentrations should be lower in these animals because of the large decrease in body fat composition. Similarly, animals being fed LE-LP will be mobilizing large energy reserves including both protein and fat. As fat stores are utilized, concentrations of leptin should decrease. 2) Our second objective is to determine if loss of body fat stores during lactation affects expression of the leptin receptor in the ovary. Approximately ten years ago we developed a nutritional model which causes significant fat loss during lactation and extended postweaning anestrus. These physiological changes are similar to those observed in many pigs during lactation, especially those having their first litters. Loss of fat stores equates to a loss of adiposity which should result in decreased concentrations of leptin in plasma. As mentioned before, sows with minimal body fat stores are reproductive quiescent. Leptin may exerts its effects on either the brain, ovary or both. If different concentrations of leptin in plasma affect the ovary directly, it is anticipated that leptin receptor expression in the ovary would change with varying concentrations of leptin in plasma. Twenty primiparious pigs will be randomly assigned to either a control or low energy diet at parturition (n=10/group). Litters will be standardized to 8 per sow. Blood will be collected on day 0 (parturition), day 7, day 14, day 21 and day 23 post-partum and analyzed for concentrations of leptin. Litters will be weaned on day 21. Sows will be sacrificed on day 23 and ovaries will collected and frozen rapidly. Total RNA will be isolated within a day of collection. Expression of the leptin receptor will be analyzed using Northern Blot analysis. Blots will be standardized using 28S ribosomal RNA, scanned and density of hybridization signal expressed as a % of control. Results between treatments will be statistically examined using a one-way analysis of variance with leptin as a co-variate (SAS, 1985). Our studies are a step toward a better understanding of the controls of post-partum anestrus in sows. Leptin may be the metabolic signal that shuts off reproductive function in response to poor nutritional status. Further investigation needs to be conducted involving the direct effects of leptin in the ovary and its role as a whole in reproduction. 3

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