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THE ROLE OF PROSTAGLANDINS IN LIVESTOCK PRODUCTION

Prostaglandins belong to the family of lipid. Soluble unsaturated hydroxyl acid containing twenty carbon (c) atoms and based on the prostanoic acid skeleton. There are two main types of Prostaglandins (PGs), the E and F series each having 3 members E1, E2, E3 and F1σ, F2σ, F3σ. The other PGs are known as secondary PGs and are products of enzymic or chemical dehydrations of PGEs e.g PG+2, PGA2, PGD2 and PGB. Prostaglandins are probably the most important regulators of female productive functions (ovulation, uterine receptivity, Implantation and parturition) and associated with pathologies (pain, fever, and inflammation), apart from sex steroids. Prostaglandins are not stored in tissues but are synthesized and released in response to a given stimulus. Prostaglandins are produced by all nucleated cells of the body and act locally in a paracrine (locally active) or autocrine (acting on the same cell from which it is in a synthesized) fashion. Prostaglandins are therefore regarded as essential mediators of female reproductive processes, hence, this paper seeks to review the role of Prostaglandins which is exploited in livestock production especially oestrus synchronization and induced parturition.


INTRODUCTION
Prostaglandins were first discovered and isolated from human semen in the 1930s by Ulf Von Euler of Sweden. Thinking they had come from the prostrate gland, he named them Prostaglandins. Prostaglandins are like hormones in that they act as chemical messengers, but do not move to other sites, but work right within the cells where they are synthesized (Ophardt, 2003). Von Euler (1937) also stated that biologically active lipids in human seminal plasma were first detected and called Prostaglandins. Despite their name, Prostaglandins are synthesized and stored in the seminal vesicle prior to ejaculation with testosterone being the stimulus for Prostaglandins synthesis.
Prostaglandins is a major lipid mediator synthesized from arachidonic acid via the catalytic activities of cyclooxygenase (cox) and Karim (1976) with Olson (2003) pointed out that, there are different types of Prostaglandins ranging from PGE, PGF, PGI 2 , PDG 2 , PGH 2 . The PGEs and PGFs are known as primary Prostaglandins with each having three members E 1 , E 2 , E 3 and F 1 σ, F 2 σ and F 3 σ. Others are secondary Prostaglandins formed as a result of PGEs. These biologically active substances are said to occur in all mammalian tissues in the body with lungs, liver, kidney and placenta being the major organs of metabolism.
Although it is clear that Prostaglandins play an important role in a number of reproductive processes; Clark and Myatt (2008) also added that Prostaglandins produced in the female are necessary for several reproductive processes and are involved from the earliest events (i.e Luteinizing hormone release) through ovulation and up to the final event which leads to successful parturition.
This review work is to outline the role of Prostaglandins which is exploited in livestock reproduction especially in oestus synchronization and indeed parturition.

Reproductive Effects
Prostaglandins specifically PGF 2 σ stimulate activity of the uterus during pregnancy. This has found practical application in the induction or expulsion of the products of conceptus viz the fetus and palcenta. According to Karim (1976), only small quantities of the PGs are required, either intravenously or by introduction directly into the uterine cavity, to produce abortion. In addition, both the in-vitro and in-vivo PGF 2 σ produce leutolysis of a functional Corpus luteum and can facilitate interruption of pregnancy by the reduction in blood level progesterone. Blatchley et al., (1975) pointed out that a significant increase in the output of oestrodiol from the ovary at oestus stimulates the secretion of the luteinizing hormone from the anterior pituitary gland. The luteinizing hormone (LH) which causes ovulation and the transformation of the ovulated follicle into a Corpus luteum, is also involved in the secretion of progesterone in-vivo. An exception occurs in rats where a fully functional release of prolactin occurs as a result of coitus. Regression of the Corpus luteum terminates progresterone production and a new cycle begins. Carlson et al., (1977), administering PGF 2 σ to sheep during the luteal phase of the cycle obtained an increase in the plasma concentrations of LH. The release of LH caused by oestradiol in anoestrus sheep is prevented by indomethacin (an inhibitor of PG sysnthesis). In sheep also, an increase in PGF 2 σ output from the brain is associated with induced oestradiol increase in LH release. This output is pulsatile like the outputs of luteinizing hormone releasing hormone (LHRH) and LH.

Luteinizing Hormone (LH) Release
Conversely, Hams et al., (1974) noted that PGE 2 stimulates the release of LH when injected into the third ventricle of the brain at the anterior pituitary gland in pre-vestrous and ovariectomised rats treated with oestradiol. PGE 2 actually stimulates LH release by causing the release of LHRH and not by acting directly on the pituitary gland.

Ovulation
Ovulation has three major phases in which Prostaglandins may be involved.
(1) Follicular maturation (2) Rupture of the follicle (3) Formation of the Corpus luteum La Maire and associates showed that isolated follicles could be stimulated with human chorionic ganadotropin (HCG) to increase their PGE and PGF 2 σ levels (Le Marie et al., 1973). This increase only occurred in those follicles destined to be ovulated. Pretreatment of animals with Prostaglandin synthesis inhibitors blocks ovulation (Zanagno et al., 1999).
Apparently, PGs cause ovulation by increasing collagenolysis through activating collagenase. This increase in collagenolysis causes "thining" of the follicular wall at the apex of the follicle resulting in eventual rupture of the follicle and release of ovum (Downs and Longe, 1983).

Stimulation of Progesterone secretion
Progesterone secretion by Corpus luteum in the ovary in many species is stimulated by PGE 2 and PGI 2 . The actions of these PGs is mediated by cyclic AMP. Consequently in this respect, these PGs act as LH mimics.
A detailed study carried out in sheep has shown that the intra-follicular injection of indomethacin prevents ovulation but does not prevent luteinization of the follicle into a structure which secretes normal amounts of progesterone (Merdock and Dunn, 1983).
Although PGs mediate ovulation induced by LH, they are not involved in luteinization and progesterone secretion which are also induced by LH (see figure 1 below). This points out that an increase in plasma progesterone concentrations does not necessarily mean that ovulation has taken place (Poyser, 1992).

Luteolysis
Progesterone which initially is produced in the Corpus luteum is necessary to prepare the uterus for implantation of a fertilized ovum and to monitor the pregnancy until the placenta can begin progesterone production. Interruption of progesterone production by the Corpus luteum then leads to termination of pregnancy. In 1969, it was shown that PGF 2 σ caused luteolysis in rats and in guinea pigs. The uterus was shown to be the source of luteolysin identified as PGF 2 σ (Blatchley, 1969). The luteolytic action of PGF 2 σ may be directly through blood flow away from the Corpus luteum (Clark and Myatt, 2008).

Early pregnancy
It is evident that if an animal becomes pregnant, the luteolytic effect of the uterus has to be prevented, since continued progesterone output from the ovary is necessary for the whole gestation in some species e.g in cattle, pig, and goat or the first third of the gestation e.g sheep and guinea pig.
In species like sheep and goat, the embryo LH Adenyl cyclase ATP ATP CAMP Prostaglandin production steroidegenesis ovulation Progesterone secretion secretes a protein called ovine trophoblast protein-1 (OTP-1) and caprine trophoblast protein-1 respectively and they are synthesized by the embryo from the 12 th and 22 nd day of pregnancy for the sheep and goat on the 15 th and 21 st day of pregnancy. These proteins inhibit endometrial PGF 2 σ synthesis (Imakawa et al., 1988 andGnatek et al,. 1989).
However in pigs the conceptus does not inhibit uterine PGF 2 σ secretion in-vivo (Harney and Bazer, 1989) and also Leckie and Poyser (1990) detected in guinea pig that interferon-σ does not inhibit endometrial PGF 2 σ synthesis.
Interestingly, in those species whose embryos secrete a trophoblast protein-1 are the species in which ovarian oxytocin form part of the physiological stimulus in increasing endometrial PGF 2 σ synthesis at the end of the oestrus cycle. While in pigs, endometrial PGF 2 σ synthesis is not inhibited during early pregnancy. The direction of secretion of PGF 2 σ produced by the uterus is changed from the uterine venous drainage into the uterine lumen, where the PGF 2 σ is secreted (Bazer and Thatcher, 1977). This means that the Corpora Iutea in the early pregnant pig is protected from the luteolytic influence of PGF 2 σ.

Parturition
Parturition occurs not only through withdrawal of inhibitory factors but also activation of stimulatory signals or hormones. Glucorcorticoids not only stimulate fetal tissue differentiation and maturation, but in many species they also initiate parturition by acting in a series of positive feed toward loops to alter placental steroidogenesis (Jenkin and Young, 2004). Glucocorticoids also activate immune and inflammatory pathways, unregulated PGHS-2, attenuate PGDH leading to prostaglandin synthesis and stimulate further glucocorticoid production (Whittle et al., 2006). It is clearly recognized in pregnant animals and women that labour, is associated with an inflammatory response that culminates in release of prostaglandin (Christiaens et al., 2008).

Placental Delivery
Once delivery is completed and the allantochorion begins to detach from the uterus, maternal prostaglandin concentrations decline (Haluska and Currie, 1998). Generally, prostaglandin concentrations are low within one hour of spontaneous foaling associated with placentary delivery (Vivrette et al., 1995). In other species, reduced maternal prostaglandin concentrations at birth are associated with retained fetal membranes (Olson, 2003). This relationship was not observed in one study of heavy draught mares (Ishii et al., 2008) but when prostaglandins were blocked by administration of the PGHS-2 inhibitor, flunixin meglumine, prior to labour, placental delivery was delayed (Vivrette et al., 1995).

Prostaglandins and Oestrus synchronization
Three products, lutalyse, estrumate and bovilene, have been approved for use in beef cows and heifers for estrous synchronization. All are prostaglandins and work similar, but differ slightly in their chemical makeup with different half lives and dosage levels.
When injected at the recommended dosage, these products act by rapidly regressing the Corpus luteum (CL) on the ovaries of cycling females that are in days 6-16 of their estrous cycles. In other words, the injection decreases the function of the CL; which allows these females to return to estrus within 2-5 days and synchronizing their estrous cycles. Females in days 17-20 will be estrous normally within 1-4 days and will also be synchronized. Females in days 1-5 of the cycle and non-cycling females that do not have a mature CL will not respond to the injection. Therefore with one injection, only about 75 percent of the cows cycling in a herd can be synchronized. If all cycling females are to be synchronized, two injections are needed (Deutscher, 1999).

Myometrial activity
It is well documented that PGF 2 σ and PGE 2 contract smooth muscles but PGF 2 σ infused into rats near their tail to induce uterine contractions and parturition (Liggius et al., 1972). However, PGF 2 σ administration to sheep during parturition does not have a stimulant effect on the uterus, so PGs may aid but not initiate expulsion of the fetus (Mitchell et al., 1976b).

CONCLUSION
In conclusion, prostaglandins are involved in several reproductive processes, namely: LH release, ovulation, luteolysis and parturition. Due to their luteolytic action, PGF 2 σ or lutalyse can be useful for oestrus synchronization and induced parturition in animals.