Endocrine Feedback Loops: Kisspeptin-10 Control of the HPG Axis

 

Core Thesis: Kisspeptin-10 (Kp-10) is one of the most potent activators of GnRH secretion in the hypothalamic-pituitary-gonadal (HPG) axis. By acting on KISS1R receptors located on GnRH neurones, it drives pulsatile GnRH release, which in turn regulates LH/FSH secretion and gonadal function. Kp-10 serves as a central hub in both the initiation of puberty and the negative/positive feedback regulation by sex steroids.

1. Basic Architecture and Feedback Logic of the HPG Axis

The HPG axis is the core regulatory system of reproductive endocrinology, composed of three tiers: the hypothalamus, the pituitary, and the gonads. KNDy neurones (co-expressing kisspeptin, neurokinin B, and dynorphin A) in the hypothalamic arcuate nucleus (ARC) project to the median eminence and release kisspeptin to stimulate GnRH neurones. GnRH travels via the hypophyseal portal system to the anterior pituitary, promoting the synthesis and release of gonadotropins (LH and FSH), which in turn regulate gonadal steroidogenesis and gametogenesis. Sex steroids secreted by the gonads (oestrogen, testosterone, and progesterone) exert negative feedback to suppress excessive GnRH/gonadotropin secretion, maintaining reproductive axis homeostasis. Periovulatory, however, oestrogen switches to positive feedback, triggering the LH surge.

2. Kp-10 Signalling Pathway and GnRH Regulation

Kp-10 is the C-terminal decapeptide encoded by the Kiss1 gene, the natural ligand for the KISS1R (GPR54) receptor, and the core bioactive fragment. Upon binding KISS1R on GnRH neurones, Kp-10 activates the phospholipase C (PLC) pathway, hydrolysing PIP₂ into IP₃ and DAG. IP₃ triggers intracellular calcium release, while DAG activates protein kinase C (PKC), ultimately leading to depolarisation and increased firing of GnRH neurones.

This signalling has been directly validated in vivo. In ovariectomised goats, peripheral administration of Kp-10 caused an immediate increase in GnRH concentration in hypophyseal portal blood, and the rise in LH concentration was temporally consistent with increased GnRH release. This demonstrates that Kp-10's stimulatory effect on LH secretion is mediated through GnRH neurones rather than acting directly on the pituitary.

The effect of Kp-10 on GnRH neurones is dose-dependent and pulsatile in nature. Fluorescent calcium imaging studies show that Kp-10 induces a three-phase calcium signal change in GT1-7 GnRH neurones, a pattern believed to be associated with the mechanism generating pulsatile GnRH secretion.

3. The Role of Kp-10 in Feedback Regulation

3.1 Mediation of Negative Feedback

Experiments with kisspeptin antagonists provide key evidence for Kp-10's role in sex steroid negative feedback. In sheep, rats, and mice, kisspeptin antagonists can block the post-castration rise in LH, indicating that kisspeptin neurones are an important mediator through which sex steroids exert negative feedback on the GnRH system. After gonadectomy, Kiss1 mRNA expression increases, and sex steroid replacement therapy reverses this effect, further supporting kisspeptin neurones as targets of negative feedback.

3.2 Positive Feedback and the LH Surge

In rodents, kisspeptin neurone expression in the anteroventral periventricular nucleus (AVPV) is positively regulated by oestrogen: it decreases after ovariectomy and increases with oestrogen supplementation; AVPV kisspeptin mRNA levels rise significantly during the LH surge. Immunoneutralisation of central kisspeptin or blockade of its action with KISS1R antagonists completely blocks the preovulatory LH surge in cycling rats. This indicates that AVPV kisspeptin neurones mediate the positive feedback regulation of GnRH/LH by sex steroids.

4. Direct Effects of Kp-10 on the Pituitary and Gonads

Beyond central actions, Kp-10 also exerts direct effects on anterior pituitary cells. In bovine pituitary cell cultures, Kp-10 significantly stimulates LH secretion from cells pretreated with oestradiol or testosterone but has no significant effect on FSH secretion. In pituitary cells from pubescent ewes, Kp-10, together with NKB and Dyn A, participates in the regulation of gonadotropin secretion.

At the gonadal level, Kp-10 also shows direct regulatory effects. In rat testes, Kp-10 treatment affected the levels of endocannabinoid system-related proteins CB1 and FAAH, suggesting that the kisspeptin system and the endocannabinoid system interact at the gonadal level.

5. Research Tools and Translational Significance

The development of kisspeptin antagonists has provided powerful tools for dissecting HPG axis feedback mechanisms. These antagonists block Kp-10-induced KISS1R activation in vitro, inhibit GnRH neurone firing and reduce gonadotropin secretion in vivo, and have validated the necessity of Kisspeptin in puberty onset and negative feedback across multiple species.

From a translational perspective, kisspeptin and its analogues or antagonists hold potential therapeutic value in treating precocious puberty, endometriosis, hormone-dependent cancers (such as prostate and breast cancer), and assisted reproduction.

Summary: Kp-10 directly activates GnRH neurones through the KISS1R-PLC-Ca²⁺-PKC signalling pathway and serves as a key node in HPG axis feedback regulation—acting as a mediator of sex steroid negative feedback to maintain basal secretory homeostasis, while also mediating positive feedback in the AVPV region to trigger the LH surge, with additional direct regulatory actions at the pituitary and gonadal levels.