Stress regulation of kisspeptin in the modulation of reproductive function

Regulation of stress response on the hypothalamic-pituitary-gonadal axis via gonadotropin-inhibitory hormone

Under stressful condition, reproductive function is impaired due to the activation of various components of the hypothalamic-pituitaryadrenal (HPA) axis, which can suppress the activity of the hypothalamic-pituitary-gonadal (HPG) axis at multiple levels. A hypothalamic neuropeptide, gonadotropin-inhibitory hormone (GnIH) is a key negative regulator of reproduction that governs the HPG axis. Converging lines of evidence have suggested that different stress types and their duration, such as physical or psychological, and acute or chronic, can modulate the GnIH system. To clarify the sensitivity and reactivity of the GnIH system in response to stress, we summarize and critically review the available studies that investigated the effects of various stressors, such as restraint, nutritional/metabolic and social stress, on GnIH expression and/or its neuronal activity leading to altered HPG action. In this review, we focus on GnIH as the potential novel mediator responsible for stress-induced reproductive dysfunction.

Male subfertility effects of sub-chronic ethanol exposure during stress in a rat model

BACKGROUND

Stressful conditions increase alcohol consumption in men. Clinical studies link disruption of the neuroendocrine stress system with alcoholism, but the effect of alcohol in a stress condition on male fertility is still relatively poorly understood. This project was undertaken to evaluate the effect of sub-chronic alcohol in a stress condition on male fertility in a rat model.

METHODS

Male Sprague-Dawley rats were randomly divided into a control group, a stress group that was exposed to restraint stress, an ethanol group that was injected with ethanol daily, and a stress + ethanol group that was injected with ethanol daily and was exposed to restraint stress, simultaneously. Furthermore, testis tissue was evaluated histomorphometrically and immunohistochemically for apoptosis using a TUNEL assay after 12 days. Epididymis sperm analysis was done. Blood cortisol and testosterone were measured and expression of hypothalamic kisspeptin (Kiss1), RFRP-3, and MC4R mRNA were evaluated.

RESULTS

Ethanol exposure during restraint stress did not alter body weight. Ethanol exposure decreased the cellular diameter and area, and stress increased the cellular diameter and area, in comparison with the control group. In the stress group, in comparison with the other groups, the number of seminiferous tubules decreased and the numerical density of seminiferous tubules increased. In addition, ethanol exposure and/or stress reduced semen analysis parameters (sperm viability and motility), but did not change serum testosterone concentrations. Apoptosis increased in spermatogonia with ethanol exposure, but spermatocytes were not affected. Our data present the novel finding that ethanol and stress reduced hypothalamic Kiss1 mRNA expression, while ethanol exposure decreased hypothalamic RFRP-3 and MC4R mRNA expression.

CONCLUSIONS

Ethanol decreased cortisol hormone level during the restraint stress condition and attenuated hypothalamic reproductive-related gene expressions. Therefore, ethanol exposure may induce reduction of sperm viability, increased sperm mortality, and increased apoptosis, with long-term effects, and may induce permanent male subfertility.

Social hierarchy reveals thermoregulatory trade-offs in response to repeated stressors

Coping with stressors can require substantial energetic investment, and when resources are limited, such investment can preclude simultaneous expenditure on other biological processes. Among endotherms, energetic demands of thermoregulation can also be immense, yet our understanding of whether a stress response is sufficient to induce changes in thermoregulatory investment is limited. Using the black-capped chickadee as a model species, we tested a hypothesis that stress-induced changes in surface temperature (Ts), a well-documented phenomenon across vertebrates, stem from trade-offs between thermoregulation and stress responsiveness. Because social subordination is known to constrain access to resources in this species, we predicted that Ts and dry heat loss of social subordinates, but not social dominants, would fall under stress exposure at low ambient temperatures (Ta), and rise under stress exposure at high Ta, thus permitting a reduction in total energetic expenditure toward thermoregulation. To test our predictions, we exposed four social groups of chickadees to repeated stressors and control conditions across a Ta gradient (n=30 days/treatment/group), whilst remotely monitoring social interactions and Ts Supporting our hypothesis, we show that: (1) social subordinates (n=12), who fed less than social dominants and alone experienced stress-induced mass-loss, displayed significantly larger changes in Ts following stress exposure than social dominants (n=8), and (2) stress-induced changes in Ts significantly increased heat conservation at low Ta and heat dissipation at high Ta among social subordinates alone. These results suggest that chickadees adjust their thermoregulatory strategies during stress exposure when resources are limited by ecologically relevant processes.

Kiss2 but not kiss1 is involved in the regulation of social stress on the gonad development in yellowtail clownfish, Amphiprion clarkii

The yellowtail clownfish (Amphiprion clarkii) is a hermaphrodite fish, whose sex differentiation and gonad development are closely related to its social status. The kisspeptin/KissR system is regarded as a key factor mediating social stress on reproductive regulation. In order to understand the effects of social rank stress on the yellowtail clownfish gonadal differentiation, full-length cDNAs of two paralogous genes encoding kisspeptin (kiss1 and kiss2) and KissR (kissr2 and kissr3) were cloned and characterized. The results of real-time PCR showed that kiss1 was primarily expressed in the hypothalamus, and kiss2/kissr2 were abundantly expressed in the liver, while kissr3 was almost exclusively concentrated in the cerebellum and pituitary. Moreover, both Kiss1-10 and Kiss2-10 peptides could initiate downstream signaling pathways by interacting with cognate receptors expressed in eukaryotic cells. Among the three social status groups, the mRNA levels of kiss2 in the hypothalamus and pituitary as well as kissr2 in the pituitary were significantly higher in subordinate individuals (nonbreeders) than dominate individuals (females and males); while the mRNA levels of kissr3 in the hypothalamus and gonad were low in subordinate individuals. Furthermore, the plasma estradiol (E₂) and testosterone (T) levels were higher in subordinate than dominate individuals. This study shows that kiss2 is involved in the regulation of social stress on the gonad development in the yellowtail clownfish, but not kiss1.

Kisspeptin and its Effect on Mammalian Spermatogensis

BACKGROUND

Kisspeptin and its receptor, GPR54, are regarded as key regulators of and catalysts for male puberty onset, and also fundamental gatekeepers of spermatogenesis in mammals. Consequently, the loss function of kisspeptin or GPR54 leads to a symptom of Hypogonadotropic Hypogonadism (HH) in human and HH accompanied by lower gonadotrophic hormone levels, smaller testes, impaired spermatogenesis and abnormal sexual maturation in mice. Besides its well-recognized functions in hypothalamus before and during puberty, accumulating data strongly support kisspeptin production in testis, and participation in somatic and germ cell development and sperm functions as well. This review aims to summarize recent findings regarding kisspeptin activity in the testes and sperm function.

METHODS

We undertook a keyword search of peer-reviewed research literature including data from in vivo and in vitro studies in humans and genetically modified animal models to identify the roles of kisspeptins in male reproduction.

RESULTS

A plethora of studies detail the role of kisspeptins and GPR54 in mammalian spermatogenesis in vivo and in vitro. This review identified recent findings regarding the kisspeptin system in male gonads, and regulation of kisspeptin in testicular physiology and male reproductive defects and disorders.

CONCLUSION

The findings of this review confirm the importance role of kisspeptins in male fertility. Understanding their biphasic roles in testis may help to consider kisspeptins as potential pharmacological targets for treating human infertility.

Deciphering the Contributions of CRH Receptors in the Brain and Pituitary to Stress-Induced Inhibition of the Reproductive Axis

Based on pharmacological studies, corticotropin-releasing hormone (CRH) and its receptors play a leading role in the inhibition of the hypothalamic–pituitary–gonadal (HPG) axis during acute stress. To further study the effects of CRH receptor signaling on the HPG axis, we generated and/or employed male mice lacking CRH receptor type 1 (CRHR1) or type 2 (CRHR2) in gonadotropin-releasing hormone neurons, GABAergic neurons, or in all central neurons and glia. The deletion of CRHRs revealed a preserved decrease of plasma luteinizing hormone (LH) in response to either psychophysical or immunological stress. However, under basal conditions, central infusion of CRH into mice lacking CRHR1 in all central neurons and glia, or application of CRH to pituitary cultures from mice lacking CRHR2, failed to suppress LH release, unlike in controls. Our results, taken together with those of the earlier pharmacological studies, suggest that inhibition of the male HPG axis during acute stress is mediated by other factors along with CRH, and that CRH suppresses the HPG axis at the central and pituitary levels via CRHR1 and CRHR2, respectively.

KNDy neurone activation prior to the LH surge of the ewe is disrupted by LPS

In the ewe, steroid hormones act on the hypothalamic arcuate nucleus (ARC) to initiate the GnRH/LH surge. Within the ARC, steroid signal transduction may be mediated by estrogen receptive dopamine-, β-endorphin- or neuropeptide Y (NPY)-expressing cells, as well as those co-localising kisspeptin, neurokinin B (NKB) and dynorphin (termed KNDy). We investigated the time during the follicular phase when these cells become activated (i.e., co-localise c-Fos) relative to the timing of the LH surge onset and may therefore be involved in the surge generating mechanism. Furthermore, we aimed to elucidate whether these activation patterns are altered after lipopolysaccharide (LPS) administration, which is known to inhibit the LH surge. Follicular phases of ewes were synchronised by progesterone withdrawal and blood samples were collected every 2 h. Hypothalamic tissue was retrieved at various times during the follicular phase with or without the administration of LPS (100 ng/kg). The percentage of activated dopamine cells decreased before the onset of sexual behaviour, whereas activation of β-endorphin decreased and NPY activation tended to increase during the LH surge. These patterns were not disturbed by LPS administration. Maximal co-expression of c-Fos in dynorphin immunoreactive neurons was observed earlier during the follicular phase, compared to kisspeptin and NKB, which were maximally activated during the surge. This indicates a distinct role for ARC dynorphin in the LH surge generation mechanism. Acute LPS decreased the percentage of activated dynorphin and kisspeptin immunoreactive cells. Thus, in the ovary-intact ewe, KNDy neurones are activated prior to the LH surge onset and this pattern is inhibited by the administration of LPS.

Glucocorticoid Regulation of Reproduction

It is well accepted that stress, measured by increased glucocorticoid secretion, leads to profound reproductive dysfunction. In times of stress, glucocorticoids activate many parts of the fight or flight response, mobilizing energy and enhancing survival, while inhibiting metabolic processes that are not necessary for survival in the moment. This includes reproduction, an energetically costly procedure that is very finely regulated. In the short term, this is meant to be beneficial, so that the organism does not waste precious energy needed for survival. However, long-term inhibition can lead to persistent reproductive dysfunction, even if no longer stressed. This response is mediated by the increased levels of circulating glucocorticoids, which orchestrate complex inhibition of the entire reproductive axis. Stress and glucocorticoids exhibits both central and peripheral inhibition of the reproductive hormonal axis. While this has long been recognized as an issue, understanding the complex signaling mechanism behind this inhibition remains somewhat of a mystery. What makes this especially difficult is attempting to differentiate the many parts of both of these hormonal axes, and new neuropeptide discoveries in the last decade in the reproductive field have added even more complexity to an already complicated system. Glucocorticoids (GCs) and other hormones within the hypothalamic-pituitary-adrenal (HPA) axis (as well as contributors in the sympathetic system) can modulate the hypothalamic-pituitary-gonadal (HPG) axis at all levels-GCs can inhibit release of GnRH from the hypothalamus, inhibit gonadotropin synthesis and release in the pituitary, and inhibit testosterone synthesis and release from the gonads, while also influencing gametogenesis and sexual behavior. This chapter is not an exhaustive review of all the known literature, however is aimed at giving a brief look at both the central and peripheral effects of glucocorticoids on the reproductive function.

A role for neurokinin B in pulsatile GnRH secretion in the ewe

The recent description of infertility in humans with loss-of-function mutations in genes for neurokinin B (NKB) or its receptor (NK3R) has focused attention on the importance of this tachykinin in the control of GnRH secretion. In a number of species, NKB neurons in the arcuate nucleus also produce two other neuropeptides implicated in the control of GnRH secretion: (1) kisspeptin, which is also essential for fertility in humans, and (2) dynorphin, an inhibitory endogenous opioid peptide. A number of characteristics of this neuronal population led to the hypothesis that they may be responsible for driving synchronous release of GnRH during episodic secretion of this hormone, and there is now considerable evidence to support this hypothesis in sheep and goats. In this article, we briefly review the history of work on the NKB system in sheep and then review the anatomy of NKB signaling in the ewe. We next describe evidence from a number of species that led to development of a model for the role of these neurons in episodic GnRH secretion. Finally, we discuss recent experiments in sheep and goats that tested this hypothesis and led to a modified version of the model, and then broaden our focus to briefly consider the possible roles of NKB in other species and systems.

A role for glucocorticoids in stress-impaired reproduction: beyond the hypothalamus and pituitary

In addition to the well-characterized role of the sex steroid receptors in regulating fertility and reproduction, reproductive events are also mediated by the hypothalamic-pituitary-adrenal axis in response to an individual's environment. Glucocorticoid secretion in response to stress contributes to the well-characterized suppression of the hypothalamic-pituitary-gonadal axis through central actions in the hypothalamus and pituitary. However, both animal and in vitro studies indicate that other components of the reproductive system are also regulated by glucocorticoids. Furthermore, in the absence of stress, it appears that homeostatic glucocorticoid signaling plays a significant role in reproduction and fertility in all tissues comprising the hypothalamic-pituitary-gonadal axis. Indeed, as central regulators of the immune response, glucocorticoids are uniquely poised to integrate an individual's infectious, inflammatory, stress, nutritional, and metabolic status through glucocorticoid receptor signaling in target tissues. Endocrine signaling between tissues regulating the immune and stress response and those determining reproductive status provides an evolutionary advantage, facilitating the trade-off between reproductive investment and offspring fitness. This review focuses on the actions of glucocorticoids in tissues important for fertility and reproduction, highlighting recent studies that show glucocorticoid signaling plays a significant role throughout the hypothalamic-pituitary-gonadal axis and characterizing these effects as permissive or inhibitory in terms of facilitating reproductive success.