**4. Distribution of kisspeptin neurons and effects on the HPG axis**

Kisspeptin gene neurons are involved in several actions including steroid hormone feedback, metabolic signalling, and photoperiodic information regulation. It is suggested that, compatibly with their mediator role in steroid hormone feedback mechanisms, Kiss1 neurons are involved in the expression of estrogen receptors (ERα and ERβ) and progesterone receptors [21].

Kiss1 neurons are located in the periventricular nucleus (PeN), arcuate nucleus (ARC) and anteroventral periventricular nucleus (AVPV) which are located in the preoptic area of the hypothalamus (POA) in the brain and which are the regions that regulate the secretion of the GnRH hormone in mouse [28]. Kisspeptins are secreted by these nuclei. However, AVPV displays sexual dimorphism. Kiss1 mRNA expression is higher in AVPV in females compared to males. This indicates that the role of kisspeptin neurons in AVPV varies by gender [29].

The different patterns of Kiss1 mRNA regulation in the forebrain nuclei are important in the emergence of the different physiological effects of Kiss1 on the HPG axis. ARC acts as the negative feedback regulation centre for the GnRH and gonadotropin secretion, while AVPV acts as the positive feedback regulation centre responsible for the LH surge in females. ERα, ERβ, and progesterone receptors are abundant in AVPV. When these receptors bind to their ligands; they increase LH secretion, resulting in the LH surge. Furthermore, Kiss1 neurons in AVPV synapse with GnRH neurons; Kiss1 mRNA expression in AVPV peaks simultaneously with the GnRH/LH release, and the estrogen-dependent Kiss1 mRNA induction in females is involved in the GnRH/LH surge during preovulation [30]. While gonadal steroids inhibit Kiss1 neurons in ARC (negative feedback), they stimulate Kiss1 neurons in AVPV (positive feedback) [23]. Signals arising from kisspeptin

**Figure 3.** *Kiss1 signalling in mouse brain [32].*

binding to GPR54 receptors of the hypothalamic GnRH neurons induce the release of GnRH to the pituitary circulation. GnRH induces the release of gonadotropins (FSH, LH) from the pituitary by binding to the GnRH receptors in the pituitary gland (**Figure 3**) [31, 32].

The molecular mechanisms underlying the different effects of estrogen on the Kiss1 expression in ARC and AVPV is yet to be known. However, progesterone receptors are also thought to be involved in this phenomenon. Kiss1 neurons are colocalized with or stand very close to progesterone neurons in mouse. Tyrosine hydroxylase and Kiss1 mRNAs are colocalized in AVPV but not in ARC. Consequently, it is thought that dopamine is also involved in the induction of estrogen-dependent Kiss1 expression in AVPV [33].

In mouse, ARC Kiss1 neurons synthesize Tachykinin (TAC), neurokinin-B (NKB), and dynorphin (Dyn) in variable quantities depending on species; therefore, they are also known as KNDy (Kisspeptin/Neurokinin/Dynorphin) neurons. NKB and Dyn have been shown to mutually act on kisspeptin neurosecretion [34].

#### **5. The effect of kisspeptin on puberty**

The detection of the role of the kisspeptin receptor mutation in leading to idiopathic hypogonadotropic hypogonadism paved the way for studies to further investigate the other roles played by the Kisspeptin, Kiss1, and Kiss1r systems in the field of reproductive endocrinology [10]. The direct/indirect effects of kisspeptin on GnRH neurons leave no doubt that kisspeptin plays a critical role in pubertal activation. Indeed, studies on various species have shown that the Kiss1 and/or Kiss1r expression increases significantly with the onset of puberty [21]. In a study on mice, it was shown that the Kiss1 mRNA expression significantly increased in AVPV during puberty [35]. Also, centrally or peripherally administered kisspeptin in juvenile female rats shortened the timing of vaginal opening by stimulating LH release and ovulation [36, 37]. Similarly, the increase in kisspeptin-54 signal frequency in primates occur at the beginning of puberty, supporting the findings associated with the pubertal increase in kisspeptin secretion [38]. Kiss1 mRNA

#### *Kisspeptin: Role in Female Infertility DOI: http://dx.doi.org/10.5772/intechopen.94925*

expression increases in ARC during puberty along with an associated increase in LH secretion in Rhesus monkeys [39].

Kisspeptin receptor damage impair normal sexual development and to result in the failure of starting puberty. Despite the synthesis of GnRH at normal levels in the hypothalamic GnRH neurons, LH/FSH secretions do not occur and pituitary gonadotropic cells remain unresponsive to externally administered GnRH in such cases [40]. The emergence of problems in the process of starting puberty in the presence of kisspeptin deficiencies has led to the idea that kisspeptin may be an important factor for the start of sexual maturation. This idea has been confirmed by significantly increased levels of released GnRH via kisspeptin injections in mammals and by the acceleration of pubertal start via repeated Kisspeptin injections in juvenile rats. Thus, it has been proven that Kisspeptin certainly participates in pubertal development [41].

A cerebral antibody binds and inactivates kisspeptin in the female rats, impairing or even stopping reproductive functions. Also, the administration of kisspeptin in fasted rats sustains the release of GnRH and that mammals suspend their reproductive functions during states of long-term hunger in order to spend energy only enough to maintain physiological requirements. Nonetheless, the administration of kisspeptin reverses this natural process and restarts reproductive functions [42].

Nitric oxide (NO) is another potential mediator that can affect the onset of puberty since it is a mediator involved in several vital functions such as gonadotropin release, steroidogenesis, folliculogenesis, ovulation, luteal development, luteolysis, and pregnancy [43, 44]. Neuronal nitric oxide synthase (nNOS) is one of the three forms of an enzyme that oxidizes L-arginine to L-citrulline and NO. nNOS neurons in mice and rats contain high densities of ERα [45]. In rats, NO stimulates LH and GnRH release. In mice, deletion of nNOS causes infertility and hypogonadism [46]. Furthermore, recent evidence suggests that kisspeptin may directly act on the release of NO. In adult female mice, kisspeptin close-contacts to nNOS neurons have been observed in the ARC and preoptic region; however, Kiss1r is expressed only from nNOS neurons in the preoptic region [47].

### **6. Kisspeptin in female infertility**

Kisspeptin and neurokinin-B agonists can be used to stimulate the HPG axis in conditions associated with infertility due to central nervous system causes if the system of GnRH neurons is intact. Kisspeptin and neurokinin-B may offer a novel therapeutic approach to treat failures associated with increased/reduced gonadotropin pulsatile secretion. Kisspeptins may be associated with a lower risk of ovarian hyperstimulation syndrome (OHSS) compared to human chorionic gonadotropin (hCG) injections [48]. In polycystic ovary syndrome (PCOS), kisspeptin antagonists can help normalize LH hypersecretion along with ovulation and follicular development [49].

In a study on patients with unexplained infertility (UI), PCOS, and male factor infertility (MFI); In PCOS group kisspeptin levels were measured and found to be significantly higher compared to the MFI and UI groups. The investigators suggested that IU can be treated with kisspeptin injections and that high kisspeptin levels can be a reliable indicator to estimate the antral follicle count (AFC) and to diagnose PCOS [50].

In women with functional hypothalamic amenorrhea (HA) due to low body weight, the administration of kisspeptin-54 acutely stimulates secretion of gonadotropin and that the effect on gonadotropin secretion is significant after the first injection but diminishes considerably (tachyphylaxis) after injections for two

weeks [51]. Nevertheless, the frequency of injections was changed from twice a day to twice a week in another study, and gonadotropin response was sustained [52]. Eight-hour intravenous infusion of kisspeptin-54 has been shown to temporarily increase LH pulse frequency and amplitude in women with hypothalamic amenorrhea [53].

Hyperprolactinemia is one of the leading causes of female infertility as it causes hypogonadotropic anovulation [54]. Since kisspeptin neurons have recently been shown to express prolactin receptors, kisspeptin has been identified as the key mediator involved in this system [55]. In mice, hyperprolactinemia causes anovulation through decreased gonadotropin and GnRH secretion and low levels of kisspeptin expression. Kisspeptin administration in these mice repairs ovarian cycle and gonadotropin secretion. It suggests that kisspeptin neurons have a role in hyperprolactinemic anovulation [56].

Endometriosis is a disease that causes infertility in women [57]. A study conducted in 2012 about endometriosis reported that Kiss1 expression could not be detected in any sample taken from endometriosis patients [58]. In one study, researchers found that the level of Kiss1 expression was statistically significantly higher in endometriosis lesions compared to the level determined in eutopic glandular endometrium and concluded that Kiss1 might have a possible role in the pathogenesis of endometriosis [59]. Another study reported that Kiss1r mRNA levels were statistically significantly higher in the cumulus cells of endometriosis patients compared to healthy oocyte donors. In consequence, researchers argue that the increased Kiss1r expression may be one of the many factors involved in the root cause of endometriosis and related infertility [60].

A study in women suffering from infertility, investigating the genetic association between the neurokinin (TAC3/TAC3R) systems and kisspeptin (Kiss1/Kiss1r) and investigating the expression of these systems, found that the expression of Kiss1, TAC3, and TAC3R was downregulated in the cumulus cells. Similarly, these three genes have been reported to be downregulated in older women with agerelated infertility [60]. In that study, these findings could only be attributed to age because infertile patients were significantly older than healthy donors and because the endometriosis patients were younger and showed just an opposite expression profile compared to all other patients, including patients with age-related infertility and low responders [61].
