Different critical perinatal periods and hypothalamic sites of oestradiol action in the defeminisation of luteinising hormone surge and lordosis capacity in the rat

Neonatal Aromatase Inhibition Blocked Defeminization of AVPV Kiss1 Neurons and LH Surge-Generating System in Male Rats

The neuroendocrine system that controls the preovulatory surge of gonadotropin-releasing hormone (GnRH)/luteinizing hormone (LH), which triggers ovulation in female mammals, is sexually differentiated in rodents. A transient increase in circulating testosterone levels in male rats within a few hours of birth is primarily responsible for the defeminization of anteroventral periventricular nucleus (AVPV) kisspeptin neurons, which are critical regulators of the GnRH/LH surge. The present study aimed to determine whether neonatal estradiol-17β (E2) converted from testosterone by aromatase primarily causes the defeminization of AVPV kisspeptin neurons and the surge of GnRH/LH in male rodents. The results of the present study showed that the neonatal administration of letrozole (LET), a nonsteroidal aromatase inhibitor, within 2 hours of birth rescued AVPV Kiss1 expression and the LH surge in adult male rats, while the neonatal administration of testosterone propionate (TP) irreversibly attenuated AVPV Kiss1 expression and the LH surge in adult female rats. Furthermore, the neonatal LET-treated Kiss1-Cre-activated tdTomato reporter males exhibited a comparable number of AVPV Kiss1-Cre-activated tdTomato-expressing cells to that of vehicle-treated female rats, while neonatal TP-treated females showed fewer AVPV Kiss1-Cre-activated tdTomato-expressing cells than vehicle-treated females. Moreover, neonatal TP administration significantly decreased the number of arcuate Kiss1-expressing and Kiss1-Cre-activated tdTomato-positive cells and suppressed LH pulses in adult gonadectomized female rats; however, neonatal LET administration failed to affect them. These results suggest that E2 converted from neonatal testosterone is primarily responsible for the defeminization of AVPV kisspeptin neurons and the subsequent GnRH/LH surge generation in male rats.

Kisspeptin Neurons and Estrogen-Estrogen Receptor α Signaling: Unraveling the Mystery of Steroid Feedback System Regulating Mammalian Reproduction

Estrogen produced by ovarian follicles plays a key role in the central mechanisms controlling reproduction via regulation of gonadotropin-releasing hormone (GnRH) release by its negative and positive feedback actions in female mammals. It has been well accepted that estrogen receptor α (ERα) mediates both estrogen feedback actions, but precise targets had remained as a mystery for decades. Ever since the discovery of kisspeptin neurons as afferent ERα-expressing neurons to govern GnRH neurons, the mechanisms mediating estrogen feedback are gradually being unraveled. The present article overviews the role of kisspeptin neurons in the arcuate nucleus (ARC), which are considered to drive pulsatile GnRH/gonadotropin release and folliculogenesis, in mediating the estrogen negative feedback action, and the role of kisspeptin neurons located in the anteroventral periventricular nucleus-periventricular nucleus (AVPV-PeN), which are thought to drive GnRH/luteinizing hormone (LH) surge and consequent ovulation, in mediating the estrogen positive feedback action. This implication has been confirmed by the studies showing that estrogen-bound ERα down- and up-regulates kisspeptin gene (Kiss1) expression in the ARC and AVPV-PeN kisspeptin neurons, respectively. The article also provides the molecular and epigenetic mechanisms regulating Kiss1 expression in kisspeptin neurons by estrogen. Further, afferent ERα-expressing neurons that may regulate kisspeptin release are discussed.

Mating-induced increase in Kiss1 mRNA expression in the anteroventral periventricular nucleus prior to an increase in LH and testosterone release in male rats

Kisspeptin has an indispensable role in gonadotropin-releasing hormone/gonadotropin secretion in mammals. In rodents, kisspeptin neurons are located in distinct brain regions, namely the anteroventral periventricular nucleus-periventricular nucleus continuum (AVPV/PeN), arcuate nucleus (ARC), and medial amygdala (MeA). Among them, the physiological role of AVPV/PeN kisspeptin neurons in males has not been clarified yet. The present study aims to investigate the acute effects of the olfactory and/or mating stimulus with a female rat on hypothalamic and MeA Kiss1 mRNA expression, plasma luteinizing hormone (LH) and testosterone levels in male rats. Intact male rats were exposed to the following stimuli: exposure to clean bedding; exposure to female-soiled bedding as a female-olfactory stimulus; exposure to female-soiled bedding and mating stimulus with a female rat. The mating stimulus significantly increased the number of the AVPV/PeN Kiss1 mRNA-expressing cells in males within 5 minutes after the exposure, and significantly increased LH and testosterone levels, followed by an increase in male sexual behavior. Whereas, the males exposed to female-soiled bedding showed a moderate increase in LH levels and no significant change in testosterone levels and the number of the AVPV/PeN Kiss1 mRNA-expressing cells. Importantly, none of the stimuli affected the number of Kiss1 mRNA-expressing cells in the ARC and MeA. These results suggest that the mating-induced increase in AVPV/PeN Kiss1 mRNA expression may be, at least partly, involved in stimulating LH and testosterone release, and might consequently ensure male mating behavior. This study would be the first report suggesting that the AVPV/PeN kisspeptin neurons in males may play a physiological role in ensuring male reproductive performance.

Neonatal Estrogen Causes Irreversible Male Infertility via Specific Suppressive Action on Hypothalamic Kiss1 Neurons

Aberrant exposure to estrogen-like compounds during the critical developmental period may cause improper hypothalamic programming, thus resulting in reproductive dysfunction in adulthood in male mammals. Kisspeptin-neurokinin B-dynorphin A (KNDy) neurons in the arcuate nucleus (ARC) have been suggested to govern tonic GnRH/gonadotropin release to control reproduction in male mammals. In this study, we report that chronic exposure to supraphysiological levels of estrogen during the neonatal period caused an irreversible suppression of KNDy genes in the ARC, resulting in reproductive dysfunction in male rats. Daily estradiol benzoate (EB) administration from days 0 to 10 postpartum caused smaller seminiferous tubules, abnormal spermatogenesis, and a decrease in plasma testosterone in adult male rats. The neonatal EB treatment profoundly suppressed LH pulse and ARC KNDy gene expression at adulthood, but it failed to affect the number of GnRH gene-expressing cells in male rats. The EB treatment failed to affect gene expression of other neuropeptides, such as GHRH, proopiomelanocortin, and agouti-related protein in the ARC, suggesting that ARC KNDy neurons would be a specific target of neonatal estrogen to cause male reproductive dysfunction. Because LH secretory responses to kisspeptin challenge and GnRH expression were spared in male rats with the EB treatment, LH pulse suppression is most probably due to ARC KNDy deficiency. Taken together, the current study indicates that chronic exposure to estrogenic chemicals in the developing brain causes a defect of ARC KNDy neurons, resulting in an inhibition of pulsatile GnRH/LH release and the failure of spermatogenesis and steroidogenesis.

Long-Term Neonatal Estrogen Exposure Causes Irreversible Inhibition of LH Pulses by Suppressing Arcuate Kisspeptin Expression via Estrogen Receptors α and β in Female Rodents

Exposure to estrogen during the developmental period causes reproductive dysfunction in mammals, because the developing brain is highly sensitive to estrogens. In the present study, we report that long-term exposure to supraphysiological doses of estrogen during the neonatal critical period causes irreversible suppression of Kiss1/kisspeptin expression in the arcuate nucleus (ARC) via estrogen receptor-alpha (ERα) and ERβ, resulting in reproductive dysfunction in female rats. Daily estradiol-benzoate (EB) administration from days 0 to 10 postpartum caused persistent vaginal diestrus in female rats. The female rats showed profound suppression of pulsatile luteinizing hormone (LH) release and ARC Kiss1/kisspeptin expression even after ovariectomy at adulthood. In contrast, female rats treated with a single EB injection at day 5 postpartum exhibited persistent vaginal estrus and showed comparable LH pulses and numbers of ARC Kiss1-expressing cells to vehicle-treated controls after ovariectomy at adulthood. Because the LH secretory response to exogenous kisspeptin was spared in female rats with neonatal long-term estrogen exposure, the LH pulse suppression was most probably due to ARC kisspeptin deficiency. Furthermore, neonatal estrogen might act through both ERα and ERβ, because EB exposure significantly reduced the number of ARC Kiss1-expressing cells in wild-type mice but not in ERα or ERβ knockout mice. Taken together, long-term exposure to supraphysiological doses of estrogen in the developing brain might cause defects in ARC kisspeptin neurons via ERα and ERβ, resulting in inhibition of pulsatile LH release and lack of estrous cyclicity.

Enhancement of the luteinising hormone surge by male olfactory signals is associated with anteroventral periventricular Kiss1 cell activation in female rats

Olfactory stimuli play an important role in regulating reproductive functions in mammals. The present study investigated the effect of olfactory signals derived from male rats on kisspeptin neuronal activity and luteinising hormone (LH) secretion in female rats. Wistar-Imamichi strain female rats were ovariectomised (OVX) and implanted with preovulatory levels of 17β-oestradiol (E₂ ). OVX+E₂ rats were killed 1 hour after exposure to either: clean bedding, female-soiled bedding or male-soiled bedding. Dual staining for Kiss1 mRNA in situ hybridisation and c-Fos immunohistochemistry revealed that the numbers of Kiss1-expressing cells and c-Fos-immunopositive Kiss1-expressing cells in the anteroventral periventricular nucleus (AVPV) were significantly higher in OVX+E₂ rats exposed to male-soiled bedding than those of the other groups. No significant difference was found with respect to the number of c-Fos-immunopositive Kiss1-expressing cells in the arcuate nucleus and c-Fos-immunopositive Gnrh1-expressing cells between the groups. The number of c-Fos-immunopositive cells was also significantly higher in the limbic system consisting of several nuclei, such as the bed nucleus of the stria terminalis, the cortical amygdala and the medial amygdala, in OVX+E₂ rats exposed to male-soiled bedding than the other groups. OVX+E₂ rats exposed to male-soiled bedding showed apparent LH surges, and the peak of the LH surge and area under the curve of LH concentrations in the OVX+E₂ group were significantly higher than those of the other two groups. These results suggest that olfactory signals derived from male rats activate AVPV kisspeptin neurones, likely via the limbic system, resulting in enhancement of the peak of the LH surge in female rats. Taken together, the results of the present study suggests that AVPV kisspeptin neurones are a target of olfactory signals to modulate LH release in female rats.

Neonatal Kisspeptin is Steroid-Independently Required for Defeminisation and Peripubertal Kisspeptin-Induced Testosterone is Required for Masculinisation of the Brain: A Behavioural Study Using Kiss1 Knockout Rats

Rodents show apparent sex differences in their sexual behaviours. The present study used Kiss1 knockout (KO) rats to evaluate the role of kisspeptin in the defeminisation/masculinisation of the brain mechanism that controls sexual behaviours. Castrated adult Kiss1 KO males treated with testosterone showed no male sexual behaviours but demonstrated the oestrogen-induced lordosis behaviours found in wild-type females. The sizes of some of the sexual dimorphic nuclei of Kiss1 KO male rats are similar to those of females. Plasma testosterone levels at embryonic day 18 and postnatal day 0 (PND0) in Kiss1 KO males were high, similar to wild-type males, indicating that perinatal testosterone is secreted in a kisspeptin-independent manner. Long-term exposure to testosterone from peripubertal to adult periods restored mounts and intromissions in KO males, suggesting that kisspeptin-dependent peripubertal testosterone secretion is required to masculinise the brain mechanism. This long-term testosterone treatment failed to abolish lordosis behaviours in KO males, whereas kisspeptin replacement at PND0 reduced lordosis quotients in Kiss1 KO males but not in KO females. These results suggest that kisspeptin itself is required to defeminise behaviour in the perinatal period, in cooperation with testosterone. Oestradiol benzoate treatment at PND0 suppressed lordosis quotients in Kiss1 KO rats, indicating that the mechanisms downstream of oestradiol work properly in the absence of kisspeptin. There was no significant difference in aromatase gene expression in the whole hypothalamus between Kiss1 KO and wild-type male rats at PND0. Taken together, the present study demonstrates that both perinatal kisspeptin and kisspeptin-independent testosterone are required for defeminisation of the brain, whereas kisspeptin-dependent testosterone during peripuberty to adulthood is needed for masculinisation of the brain in male rats.

The roles of kisspeptin revisited: inside and outside the hypothalamus

Kisspeptin, encoded by KISS1/Kiss1 gene, is now considered a master regulator of reproductive functions in mammals owing to its involvement in the direct activation of gonadotropin-releasing hormone (GnRH) neurons after binding to its cognate receptor, GPR54. Ever since the discovery of kisspeptin, intensive studies on hypothalamic expression of KISS1/Kiss1 and on physiological roles of hypothalamic kisspeptin neurons have provided clues as to how the brain controls sexual maturation at the onset of puberty and subsequent reproductive performance in mammals. Additionally, emerging evidence indicates the potential involvement of extra-hypothalamic kisspeptin in reproductive functions. Here, we summarize data regarding kisspeptin inside and outside the hypothalamus and revisit the physiological roles of central and peripheral kisspeptins in the reproductive functions of mammals.

Molecular and Epigenetic Mechanism Regulating Hypothalamic Kiss1 Gene Expression in Mammals

After the discovery of hypothalamic kisspeptin encoded by the Kiss1 gene, the central mechanism regulating gonadotropin-releasing hormone (GnRH) secretion, and hence gonadotropin secretion, is gradually being unraveled. This has increased our understanding of the central mechanism regulating puberty and subsequent reproductive performance in mammals. Recently, emerging evidence has indicated the molecular and epigenetic mechanism regulating hypothalamic Kiss1 gene expression. Here we compile data regarding DNA and histone modifications in the Kiss1 promoter region and provide a hypothetic scheme of the molecular and epigenetic mechanism regulating Kiss1 gene expression in two populations of hypothalamic kisspeptin neurons, which govern puberty and subsequent reproductive performance via GnRH/gonadotropin secretion.

Microarray analysis of perinatal-estrogen-induced changes in gene expression related to brain sexual differentiation in mice

Sexual dimorphism of the behaviors or physiological functions in mammals is mainly due to the sex difference of the brain. A number of studies have suggested that the brain is masculinized or defeminized by estradiol converted from testicular androgens in perinatal period in rodents. However, the mechanisms of estrogen action resulting in masculinization/defeminization of the brain have not been clarified yet. The large-scale analysis with microarray in the present study is an attempt to obtain the candidate gene(s) mediating the perinatal estrogen effect causing the brain sexual differentiation. Female mice were injected with estradiol benzoate (EB) or vehicle on the day of birth, and the hypothalamus was collected at either 1, 3, 6, 12, or 24 h after the EB injection. More than one hundred genes down-regulated by the EB treatment in a biphasic manner peaked at 3 h and 12-24 h after the EB treatment, while forty to seventy genes were constantly up-regulated after it. Twelve genes, including Ptgds, Hcrt, Tmed2, Klc1, and Nedd4, whose mRNA expressions were down-regulated by the neonatal EB treatment, were chosen for further examination by semiquantitative RT-PCR in the hypothalamus of perinatal intact male and female mice. We selected the genes based on the known profiles of their potential roles in brain development. mRNA expression levels of Ptgds, Hcrt, Tmed2, and Nedd4 were significantly lower in male mice than females at the day of birth, suggesting that the genes are down-regulated by estrogen converted from testicular androgen in perinatal male mice. Some genes, such as Ptgds encoding prostaglandin D2 production enzyme and Hcrt encording orexin, have been reported to have a role in neuroprotection. Thus, Ptgds and Hcrt could be possible candidate genes, which may mediate the effect of perinatal estrogen responsible for brain sexual differentiation.