The role of the brain in female reproductive aging

(1)H NMR metabolic profiling analysis offers evaluation of Nilestriol treatment in ovariectomised rats

Nilestriol (NIL) has been applied to treat menopausal dysfunctions, yet its mechanism has remained unknown. To understand the relationship between the changes in homeostatic metabolites and ovarian oestrogen deficiency syndromes after NIL treatment, proton Nuclear Magnetic Resonance ((1)H NMR)-based metabonomic technologies were used to analyse a rat model of oestrogen deficiency. An orthogonal partial least-squares regression (OPLS) differentiation model was used on 12-week metabolic analyses of ovariectomised (OVX) rats treated or mock treated with NIL. Furthermore, data analysis using Chenomx software quantified results to identify the most significantly altered metabolite concentrations, allowing for metabolic explanations of the effects of NIL therapies. In this study, PLS results revealed that there are considerably distinct differences between treatment groups. Additionally, a total of 45 metabolites shown to have a high variation between groups were selected for target quantification. Using a one-way LSD ANOVA analysis, 32 metabolite concentrations were significantly altered in the OVX group. A total of 21 metabolites were altered significantly in the NIL-treatment group but later returned to normal. According to the OPLS VIP calculation, the metabolites most affected by NIL treatment were mostly involved in insulin resistance. In addition, abnormal concentration changes in lactate in the NIL-treatment group and 3-indoxylsulfate in the OVX group were observed. To our knowledge, this study is the first to address the molecular mechanism of NIL from a metabonomic perspective, and, more specifically, to establish a catalogue of endo-molecular changes effected by NIL in the regulation of oestrogen deficiency disorder.

Cumulative lead exposure and age at menopause in the Nurses' Health Study cohort

BACKGROUND

Early menopause has been associated with many adverse health outcomes, including increased risk of cardiovascular disease morbidity and mortality. Lead has been found to be adversely associated with female reproductive function, but whether exposures experienced by the general population are associated with altered age at menopause has not been explored.

OBJECTIVE

Our goal was to assess the association between cumulative lead exposure and age at natural menopause.

METHODS

Self-reported menopausal status and bone lead concentration measured with K-shell X-ray fluorescence-a biomarker of cumulative lead exposure-were obtained from 434 women participants in the Nurses' Health Study.

RESULTS

The mean (± SD) age at natural menopause was 50.8 ± 3.6 years. Higher tibia lead level was associated with younger age at menopause. In adjusted analyses, the average age of menopause for women in the highest tertile of tibia lead was 1.21 years younger (95% CI: -2.08, -0.35) than for women in the lowest tertile (p-trend = 0.006). Although the number of cases was small (n = 23), the odds ratio for early menopause (< 45 years of age) was 5.30 (95% CI: 1.42, 19.78) for women in the highest tertile of tibia lead compared with those in the lowest tertile (p-trend = 0.006). There was no association between patella or blood lead and age at menopause.

CONCLUSIONS

Our results support an association between low-level cumulative lead exposure and an earlier age at menopause. These data suggest that low-level lead exposure may contribute to menopause-related health outcomes in older women through effects on age at menopause.

CITATION

Eum KD, Weisskopf MG, Nie LH, Hu H, Korrick SA. 2014. Cumulative lead exposure and age at menopause in the Nurses' Health Study Cohort. Environ Health Perspect 122:229–234; http://dx.doi.org/10.1289/ehp.1206399

QTLs influencing IGF-1 levels in a LOU/CxFischer 344F2 rat population. Tracks towards the metabolic theory of Ageing

OBJECTIVE

Since a reduction of the insulin/IGF-1 signaling cascade extends life span in many species and IGF-1 signaling might partly mediate the effects of caloric restriction (CR), an experimental intervention for increasing longevity, the purpose of the present study was to use quantitative trait loci (QTL) analysis, an unbiased genetic approach, to identify particular regions of the genome influencing plasma IGF-1 levels in an F2 intercross between F344 and LOU/C rats; the latter being an inbred strain of Wistar origin, considered as a model of healthy aging since it resists to age (and diet)-induced obesity.

DESIGN

F1 hybrids were obtained by crossbreeding LOU/C with F344 rats, and then F1 were bred inter se to obtain the F2 population, of which 93 males and 94 females were studied. Total plasma IGF-1 levels were determined by radioimmunoassay. A genome scan of the F2 population was made with 100 microsatellite markers) selected for their polymorphism between LOU/C and F344 strains (and by covering evenly the whole genome.

RESULTS

By simple interval mapping sex-dependent QTLs were found on chromosome 17 in males and on chromosome 18 in females. By multiple interval mapping, additional QTLs were found on chromosomes 1, 4, 5, 6, 12, 15 and 19 in males and on chromosomes 3, 5, 6, 12 and 17 in females. Only the markers D1Rat196 and D12Mgh5 were found in both males and females. The majority of QTLs corresponded to metabolic syndrome (cardiac function: n = 45 (30%), obesity/diabetes: n = 22 (15%), inflammation: n = 19 (13%) and only a limited number to body weight: n = 13 (9%), proliferation (n = 10 (7%) or ossification: n = 7 (5%). Ninety-six candidate genes were located on the different QTLs. A significant proportion of these genes are connected to IGF-1 production and receptor pathways (n = 18) or metabolic syndrome (n = 11).

CONCLUSIONS

Subsequent studies are necessary to determine whether the genetic networks underscored are also involved in age-associated obesity, diabetes and inflammation as well as cardiovascular impairments.

Reduced responsiveness of kisspeptin neurons to estrogenic positive feedback associated with age-related disappearance of LH surge in middle-age female rats

Age-related disappearance of the LH surge is one of major biomarkers of reproductive aging in female rats. Kisspeptin neurons in the hypothalamic anteroventral periventricular nucleus (AVPV) are proposed as the critical regulator of the preovulatory LH surge in response to estrogenic positive feedback. Here we investigated the possible involvement of the AVPV kisspeptin neurons in the disappearance of the LH surge in middle-age rats. Middle-age rats exhibiting persistent estrus (M-PE) did not show an LH surge although neither Kiss1 mRNA nor peptide in the AVPV was differentially expressed when compared to young rats exhibiting normal estrous cycles (YN). M-PE released LH in response to exogenous kisspeptin in a similar dose-dependent manner as YN, suggesting that their GnRH neurons still maintained responsiveness to kisspeptin. To investigate the estrogenic positive feedback effect on kisspeptin neurons in the AVPV, rats were ovariectomized and supplemented with estradiol (OVX+E2). We performed in situ hybridization and immunohistochemistry for Kiss1 mRNA and cFos, respectively, and found that M-PE exhibited a significantly lower percentage of Kiss1 mRNA positive neurons with cFos immunoreactivity, although the total number of kisspeptin neurons was not different from that in cyclic rats. Furthermore, OVX+E2 M-PE did not show the surge-like LH release under high estradiol administration while YN did. Thus our current study suggests that the reduced responsiveness of the AVPV kisspeptin neurons to estrogenic positive feedback presumably results in the decrease in kisspeptin secretion from neurons and eventually causes the age-related disappearance of the LH surge in middle age female rats.

Causes and consequences of age-related steroid hormone changes: insights gained from nonhuman primates

Similar to humans, rhesus macaques (Macaca mulatta) are large, long-lived diurnal primates, and show similar age-related changes in the secretion of many steroid hormones, including oestradiol, testosterone, cortisol and dehydroepiandrosterone (DHEA). Consequently, they represent a pragmatic animal model in which to examine the mechanisms by which these steroidal changes contribute to perturbed sleep-wake cycles and cognitive decline in the elderly. Using remote serial blood sampling, we have found the circulating levels of DHEA sulphate, as well as oestradiol and testosterone, decline markedly in old monkeys. Furthermore, using the real-time polymerase chain reaction, we have shown that the genes for the enzymes associated with the conversion of DHEA to oestradiol and testosterone (3β-hydroxysteroid dehydrogenase, 17β-hydroxysteroid dehydrogenase, and aromatase) are highly expressed in brain areas associated with cognition and behaviour, including the hippocampus, prefrontal cortex and amygdala. Taken together, these findings suggest that the administration of supplementary DHEA in the elderly may have therapeutic potential for cognitive and behavioural disorders, although with fewer negative side effects outside of the central nervous system. To test this, we have developed a novel steroid supplementation paradigm for use in old animals; this involves the oral administration of DHEA and testosterone at physiologically relevant times of the day to mimic the circadian hormone patterns observed in young adults. We are currently evaluating the efficacy of this steroid supplementation paradigm with respect to reversing age-associated disorders, including perturbed sleep-wake cycles and cognitive decline, as well as an impaired immune response.

Aging-related changes in RP3V kisspeptin neurons predate the reduced activation of GnRH neurons during the early reproductive decline in female mice

Kisspeptin neurons in the rostral periventricular area of the third ventricle (RP3V) play a key role in relaying the positive feedback effects of estradiol that activate gonadotropin-releasing hormone (GnRH) neurons and drive a surge in the GnRH/luteinizing hormone (LH) level. However, the precise role of kisspeptin neurons during female reproductive senescence remains unclear. Focusing on middle-aged intact female mice with irregular estrous cycles, we found a parallel decline in c-Fos-positive kisspeptin neurons and c-Fos-positive GnRH neurons at the time of the GnRH/LH surge. Furthermore, in kisspeptin neurons, the expression of estrogen receptor α (ERα), but not progesterone receptor (PR), decreased with age. Interestingly, some kisspeptin neurons in the RP3V, but none of the GnRH neurons in the rostral preoptic area (rPOA), had a characteristic cellular senescence in middle-aged mice and old mice. These data suggest that, among the groups of neurons involved in reproductive control, the kisspeptin neurons in the RP3V are likely among the earliest to undergo aging processes and thus participate in initiating the early reproductive decline.

Increased vesicular γ-GABA transporter and decreased phosphorylation of synapsin I in the rostral preoptic area is associated with decreased gonadotrophin-releasing hormone and c-Fos coexpression in middle-aged female mice

Hypothalamic glutamate (Glu) and γ-GABA neurotransmission are involved in the ovarian hormone-induced gonadotrophin-releasing hormone (GnRH)/luteinising hormone (LH) surge in rodents. Studies have shown that reduced Glu and increased γ-GABA in the rostral preoptic area (rPOA) of the hypothalamus, where most activated GnRH neurones are located, play a key role in decreasing the reproductive function of female rats. However, the mechanism underlying the altered balance of these neurotransmitters is poorly understood. In the present study, we observed a decline in the function of GnRH neurones in the rPOA at the time of the GnRH/LH surge in middle-aged intact female mice with regular oestrous cycles. In young mice, there is an increase of vesicular Glu transporter 2 on the pro-oestrus afternoon, which is not observed in middle-aged mice. By contrast, vesicular γ-GABA transporter levels in young mice decrease at the time of the LH surge, whereas they increase in middle-aged mice. Of note, we found that, in middle-aged mice at the time of the GnRH/LH surge, the phosphorylation of synapsin I at Ser603 and Ca(2+) /calmodulin-dependent kinase IIα was significantly lower than in young mice. These data suggest that, in middle-aged mice, higher levels of presynaptic stores of GABA, a lack of increase of Glu and a decreased ability of synaptic vesicle mobilisation could account for the imbalance of Glu and GABA in the rPOA, which decreases the activation of GnRH neurones.

Sex differences in the physiology of eating

Hypothalamic-pituitary-gonadal (HPG) axis function fundamentally affects the physiology of eating. We review sex differences in the physiological and pathophysiological controls of amounts eaten in rats, mice, monkeys, and humans. These controls result from interactions among genetic effects, organizational effects of reproductive hormones (i.e., permanent early developmental effects), and activational effects of these hormones (i.e., effects dependent on hormone levels). Male-female sex differences in the physiology of eating involve both organizational and activational effects of androgens and estrogens. An activational effect of estrogens decreases eating 1) during the periovulatory period of the ovarian cycle in rats, mice, monkeys, and women and 2) tonically between puberty and reproductive senescence or ovariectomy in rats and monkeys, sometimes in mice, and possibly in women. Estrogens acting on estrogen receptor-α (ERα) in the caudal medial nucleus of the solitary tract appear to mediate these effects in rats. Androgens, prolactin, and other reproductive hormones also affect eating in rats. Sex differences in eating are mediated by alterations in orosensory capacity and hedonics, gastric mechanoreception, ghrelin, CCK, glucagon-like peptide-1 (GLP-1), glucagon, insulin, amylin, apolipoprotein A-IV, fatty-acid oxidation, and leptin. The control of eating by central neurochemical signaling via serotonin, MSH, neuropeptide Y, Agouti-related peptide (AgRP), melanin-concentrating hormone, and dopamine is modulated by HPG function. Finally, sex differences in the physiology of eating may contribute to human obesity, anorexia nervosa, and binge eating. The variety and physiological importance of what has been learned so far warrant intensifying basic, translational, and clinical research on sex differences in eating.

An update on the cognitive impact of clinically-used hormone therapies in the female rat: models, mazes, and mechanisms

In women, ovarian hormone loss associated with menopause has been related to cognitive decline. Hormone therapy (HT) may ameliorate some of these changes. Understanding the cognitive impact of female steroids, including estrogens, progestogens, and androgens, is key to discovering treatments that promote brain health in women. The preclinical literature has presented elegant and methodical experiments allowing a better understanding of parameters driving the cognitive consequences of ovarian hormone loss and HT. Animal models have been a valuable tool in this regard, and will be vital to future discoveries. Here, we provide an update on the literature evaluating the impact of female steroid hormones on cognition, and the putative mechanisms mediating these effects. We focus on preclinical work that was done with an eye toward clinical realities. Parameters that govern the cognitive efficacy of HT, from what we know thus far, include but are not limited to: type, dose, duration, and route of HT, age at HT initiation, timing of HT relative to ovarian hormone loss, memory type examined, menopause history, and hormone receptor status. Researchers have identified intricate relationships between some of these factors by studying their individual effects on cognition. As of late, there is increased focus on studying interactions between these variables as well as multiple hormone types when administered concomitantly. This is key to translating preclinical data to the clinic, wherein women typically have concurrent exposure to endogenous ovarian hormones as well as exogenous combination HTs, which include both estrogens and progestins. Gains in understanding the parameters of HT effects on cognition provide exciting novel avenues that can inform clinical treatments, eventually expanding the window of opportunity to optimally enhance cognition and brain health in aging women. This article is part of a Special Issue entitled Hormone Therapy.

Age increase of estrogen receptor-α (ERα) in cortical astrocytes impairs neurotrophic support in male and female rats

Rodent models show decreased neuronal responses to estradiol (E2) during aging (E2-desensitization) in association with reduced neuronal estrogen receptor (ER)-α, but little is known about age changes of E2-dependent astrocytic neurotrophic support. Because elevated expression of astrocyte glial fibrillary acidic protein (GFAP) is associated with impaired neurotrophic activity and because the GFAP promoter responds to ERα, we investigated the role of astrocytic ERα and ERβ in impaired astrocyte neurotrophic activity during aging. In vivo and in vitro, ERα was increased greater than 50% with age in astrocytes from the cerebral cortex of male rats (24 vs 3 months), whereas ERβ did not change. In astrocytes from 3-month-old males, experimentally increasing the ERα to ERβ ratio induced the aging phenotype of elevated GFAP and impaired E2-dependent neurite outgrowth. In 24-month-old male astrocytes, lowering ERα reversed the age elevation of GFAP and partially restored E2-dependent neurite outgrowth. Mixed glia (astrocytes to microglia, 3:1) of both sexes also showed these age changes. In a model of perimenopause, mixed glia from 9- to 15-month rats showed E2 desensitization: 9-month regular cyclers retained young-like ERα to ERβ ratios and neurotrophic activity, whereas 9-month noncyclers had elevated ERα and GFAP but low E2-dependent neurotrophic activity. In vivo, ERα levels in cortical astrocytes were also elevated. The persisting effects of ovarian acyclicity in vitro are hypothesized to arise from steroidal perturbations during ovarian senescence. These findings suggest that increased astrocyte ERα expression during aging contributes to the E2 desensitization of the neuronal responses in both sexes.

Ovarian hormones promote recovery from sleep deprivation by increasing sleep intensity in middle-aged ovariectomized rats

Sleep disturbances are commonly associated with menopause. Hormone replacement therapy is often used to treat various menopausal symptoms, but its efficacy for improving sleep is a matter of debate. We addressed this question by using a rodent model of ovarian hormone loss and replacement in midlife. Middle-aged female rats were ovariectomized and implanted with capsules containing estradiol with or without progesterone, or oil. After two weeks, sleep/wake states were recorded polygraphically during a 24-h baseline period, followed by 6h of sleep deprivation in the second half of the light phase, and a 24-h recovery period. During the baseline dark phase, hormone treatments increased wakefulness, and decreased non-rapid eye movement sleep (NREMS) by shortening NREMS episodes; however, NREMS EEG delta power or energy (cumulative power) was unaffected by combined hormones. Following sleep deprivation, all the groups showed NREMS and rapid eye movement sleep (REMS) rebounds, with similar relative increases from respective baseline levels. The increases in NREMS EEG delta power/energy during recovery were enhanced by combined hormones. These results from middle-aged ovariectomized rats indicate that replacement with estrogen with or without progesterone reduces baseline NREMS without affecting sleep intensity, particularly during the dark (active) phase, whereas following sleep deprivation the same hormone treatments do not affect the ability to increase NREMS or REMS, but treatment with both hormones, in particular, enhances the intensity of recovery sleep. These results support the usefulness of ovariectomized middle-aged rats as a model system to study the biological effects of hormone replacement on sleep regulation.

Changes in pituitary function with ageing and implications for patient care

The pituitary gland has a role in puberty, reproduction, stress-adaptive responses, sodium and water balance, uterine contractions, lactation, thyroid function, growth, body composition and skin pigmentation. Ageing is marked by initially subtle erosion of physiological signalling mechanisms, resulting in lower incremental secretory-burst amplitude, more disorderly patterns of pituitary hormone release and blunted 24 h rhythmic secretion. Almost all pituitary hormones are altered by ageing in humans, often in a manner dependent on sex, body composition, stress, comorbidity, intercurrent illness, medication use, physical frailty, caloric intake, immune status, level of exercise, and neurocognitive decline. The aim of this article is to critically discuss the mechanisms mediating clinical facets of changes in the hypothalamic-pituitary axis during ageing, and the extent to which confounding factors operate to obscure ageing-related effects.

Effect of a derived herbal recipe from an ancient Chinese formula, Danggui Buxue Tang, on ovariectomized rats

ETHNOPHARMACOLOGICAL RELEVANCE

Radix Astragali (root of Astragalus membranaceus (Fisch.) Bge. var. mongholicus (Bge.) Hsiao, RA), Radix Angelicae sinensis (root of Angelica sinensis (Oliv.) Diels, RAS) and Folium Epimedii (leaves of Epimedium brevicomum Maxim., FE) are three of the extensively applied herbal remedies among traditional Chinese medicines for gynecological disorders and osteoporosis. A derivative herbal recipe-RRF, composed of the three medicines with a weight ratio of 5:1:5, is derived from a famous Chinese herbal formula-Danggui Buxue Tang (DBT). RRF has shown noteworthy protective effect in ovariectomized rats, which might represent a promising candidate for the treatment of perimenopausal disorders. The aim of this study was to investigate the herbal recipe RRF for its efficacy on perimenopausal disorders and the underlying mechanisms via ovariectomy (OVX) models.

MATERIALS AND METHODS

An experimental model of OVX female rats was applied. Vehicle (Sham and OVX group), RRF (564, 282 and 141 mg/kg/d) and conjugated equine estrogens (CEE, 0.1mg/kg/d, reference drug) were all administrated orally once daily for 16 weeks post operation. After the treatment, radioimmunoassay for estradiol (E(2)), lutenizing hormone (LH), follicle stimulating hormone (FSH) and β-endorphin (β--EP), neurotransmitter determination by high-performance liquid chromatography with electrochemical detector (HPLC-ECD) for norepinephrine (NE), dopamine (DA), 5-hydroxytryptamine (5-HT) and 5-HIAA (5-hydroxyindoleacetic acid), bone mineral density (BMD) assay as well as lipid peroxidation assessment, were carried out to probe into the effectiveness of RRF.

RESULTS

(1) RRF treatment enhanced E(2) synthese while diminished the elevated serum FSH and LH levels; in terms of neurotransmitter, β-EP syntheses rallied whereas the hypothalamic NE, DA and 5-HT release experienced varying mitigation in OVX female rats. (2) Repeated administration of RRF was able to attenuate osteoporosis by elevating the BMD levels of total body, and arrest the bone trabeculae degradation. (3) RRF exposure decreased serum levels of constituent MDA and increased endogenous SOD activity.

CONCLUSIONS

Results of the current studies revealed that RRF was capable of acting at multiple targets which presumably underlay its potential protective effect in OVX rats mimicking symptoms as observed in perimenopausal women. Hence, RRF might represent a promising candidate in the treatment of perimenopausal disorders in midlife women.

Sex and gonadal hormones in mouse models of Alzheimer's disease: what is relevant to the human condition?

Biologic sex and gonadal hormones matter in human aging and diseases of aging such as Alzheimer's - and the importance of studying their influences relates directly to human health. The goal of this article is to review the literature to date on sex and hormones in mouse models of Alzheimer's disease (AD) with an exclusive focus on interpreting the relevance of findings to the human condition. To this end, we highlight advances in AD and in sex and hormone biology, discuss what these advances mean for merging the two fields, review the current mouse model literature, raise major unresolved questions, and offer a research framework that incorporates human reproductive aging for future studies aimed at translational discoveries in this important area. Unraveling human relevant pathways in sex and hormone-based biology may ultimately pave the way to novel and urgently needed treatments for AD and other neurodegenerative diseases.

Localization of gonadotropin-releasing hormone (GnRH), gonadotropin-inhibitory hormone (GnIH), kisspeptin and GnRH receptor and their possible roles in testicular activities from birth to senescence in mice

The changes in distribution and concentration of neuropeptides, gonadotropin-releasing hormone (GnRH), gonadotropin-inhibitory hormone (GnIH), kisspeptin, and gonadotropin-releasing hormone receptor (GnRH-R) were evaluated and compared with reproductive parameters, such as cytochrome P450 side-chain cleavage (P450 SCC) enzyme activity, androgen receptors (AR) in the testis and serum testosterone levels, from birth to senescence in mice. The results showed the localization of these molecules mainly in the interstitial and germ cells as well as showed significant variations in immunostatining from birth to senescence. It was found that increased staining of testicular GnRH-R coincided with increased steroidogenic activity during pubertal and adult stages, whereas decreased staining coincides with decreased steroidogenic activity during senescence. Similar changes in immunostaining were confirmed by Western/slot blot analysis. Thus, these results suggest a putative role of GnRH during testicular pubertal development and senescence. Treatment with a GnRH agonist ([DTrp(6), Pro(9)-NEt] GnRH) to mice from prepubertal to pubertal period showed a significant increase in steroidogenic activity of the mouse testis and provided further support to the role of GnRH in testicular pubertal maturation. The significant decline in GnRH-R during senescence may be due to a significant increase in GnIH synthesis during senescence causing the decrease in GnRH-R expression. It is considered that significant changes in the levels of GnRH-R may be responsible for changes in steroidogenesis that causes either pubertal activation or senescence in testis of mice. Furthermore, changes in the levels of GnRH-R may be modulated by interactions among GnRH, GnIH, and kisspeptin in the testis.

Early menopause and severity of rheumatoid arthritis in women older than 45 years

Introduction

We aimed to investigate whether recognized hormonal predictors of rheumatoid arthritis (RA) also influence the severity of RA.

Methods

One hundred thirty-four incident RA cases identified by four different local and national registers, who had participated in a community-based health survey between 1991 and 1996, were included. By a retrospective structured review of the medical records, information on the use of disease-modifying antirheumatic drugs (DMARDs), erosions on radiographs, rheumatoid factor (RF) status, and disability measured by using the health assessment questionnaire (HAQ) were collected. The variables were added to the SPSS TwoStep Cluster Analysis to reveal natural groupings of RA severity. Known hormonal predictors analyzed were breastfeeding history, history of oral contraceptive (OC) use, and menopausal age.

Results

The mean age at RA diagnosis was 63.4 years; 72% were RF positive, and 28% had received biological treatment. Three clusters were identified, one with severe RA, one with mild/moderate RF-positive RA, and one with mild/moderate RF-negative RA. A significant difference (P = 0.005) was found in the distribution of clusters between patients with a history of early menopause compared with those with menopause after 45 years, with a higher proportion with mild/moderate RF-negative RA in the early-menopause subset. No major difference in severity of the disease was noted depending on OC use or history of breastfeeding.

Conclusions

Early menopause was associated with a milder form of RA. Hormonal changes may influence pathways that are distinct from those leading to severe, progressive disease.

Minireview: translational animal models of human menopause: challenges and emerging opportunities

Increasing importance is placed on the translational validity of animal models of human menopause to discern risk vs. benefit for prediction of outcomes after therapeutic interventions and to develop new therapeutic strategies to promote health. Basic discovery research conducted over many decades has built an extensive body of knowledge regarding reproductive senescence across mammalian species upon which to advance animal models of human menopause. Modifications to existing animal models could rapidly address translational gaps relevant to clinical issues in human menopausal health, which include the impact of 1) chronic ovarian hormone deprivation and hormone therapy, 2) clinically relevant hormone therapy regimens (cyclic vs. continuous combined), 3) clinically relevant hormone therapy formulations, and 4) windows of opportunity and optimal duration of interventions. Modifications in existing animal models to more accurately represent human menopause and clinical interventions could rapidly provide preclinical translational data to predict outcomes regarding unresolved clinical issues relevant to women's menopausal health. Development of the next generation of animal models of human menopause could leverage advances in identifying genotypic variations in estrogen and progesterone receptors to develop personalized menopausal care and to predict outcomes of interventions for protection against or vulnerability to disease. Key to the success of these models is the close coupling between the translational target and the range of predictive validity. Preclinical translational animal models of human menopause need to keep pace with changes in clinical practice. With focus on predictive validity and strategic use of advances in genetic and epigenetic science, new animal models of human menopause have the opportunity to set new directions for menopausal clinical care for women worldwide.

Middle-aged female rats retain sensitivity to the anorexigenic effect of exogenous estradiol

It is well established that estradiol (E2) decreases food intake and body weight in young female rats. However, it is not clear if female rats retain responsiveness to the anorexigenic effect of E2 during middle age. Because middle-aged females exhibit reduced responsiveness to E2, manifesting as a delayed and attenuated luteinizing hormone surge, it is plausible that middle-aged rats are less responsive to the anorexigenic effect of E2. To test this we monitored food intake in ovariohysterectomized young and middle-aged rats following E2 treatment. E2 decreased food intake and body weight to a similar degree in both young and middle-aged rats. Next, we investigated whether genes that mediate the estrogenic inhibition of food intake are similarly responsive to E2 by measuring gene expression of the anorexigenic genes corticotropin-releasing hormone (CRH), proopiomelanocortin (POMC), the long form of the leptin receptor (Lepr) and serotonin 2C receptors (5HT2CR) and the orexigenic genes agouti-related peptide (AgRP), neuropeptide Y (NPY), prepromelanin-concentrating hormone (pMCH) and orexin in the hypothalamus of young and middle-aged OVX rats treated with E2. As expected, E2 increased expression of all anorexigenic genes while decreasing expression of all orexigenic genes in young rats. Although CRH, 5HT2CR, Lepr, AgRP, NPY and orexin were also sensitive to E2 treatment in middle-aged rats, POMC and pMCH expression were not influenced by E2 in middle-aged rats. These data demonstrate that young and middle-aged rats are similarly sensitive to the anorexigenic effect of E2 and that most, but not all feeding-related genes retain sensitivity to E2.

Estrogenic regulation of the GnRH neuron

Reproductive function is regulated by the secretion of luteinizing hormone (LH) and follicle-stimulating hormone from the pituitary and the steroid hormones from the gonads. The dynamic changes in the levels of the reproductive hormones regulate secondary sex characteristics, gametogenesis, cellular function, and behavior. Hypothalamic GnRH neurons, with cell bodies located in the basal hypothalamus, represent the final common pathway for neuronally derived signals to the pituitary. As such, they serve as integrators of a dizzying array of signals including sensory inputs mediating information about circadian, seasonal, behavioral, pheromonal, and emotional cues. Additionally, information about peripheral physiological function may also be included in the integrative signal to the GnRH neuron. These signals may communicate information about metabolic status, disease, or infection. Gonadal steroid hormones arguably exert the most important effects on GnRH neuronal function. In both males and females, the gonadal steroid hormones exert negative feedback regulation on axis activity at both the level of the pituitary and the hypothalamus. These negative feedback loops regulate homeostasis of steroid hormone levels. In females, a cyclic reversal of estrogen feedback produces a positive feedback loop at both the hypothalamic and pituitary levels. Central positive feedback results in a dramatic increase in GnRH secretion (Moenter et al., 1992; Xia et al., 1992; Clarke, 1993; Sisk et al., 2001). This is coupled with an increase in pituitary sensitivity to GnRH (Savoy-Moore et al., 1980; Turzillo et al., 1995), which produces the massive surge in secretion of LH that triggers ovulation. While feedback regulation of the axis in males is in part mediated by estrogen receptors (ER), there is not a clear consensus as to the relative role of ER versus AR signaling in males (Lindzey et al., 1998; Wersinger et al., 1999). Therefore, this review will focus on estrogenic signaling in the female.

Chinese herb mix Tiáo-Gēng-Tāng possesses antiaging and antioxidative effects and upregulates expression of estrogen receptors alpha and beta in ovariectomized rats

Background

Herb mixtures are widely used as an alternative to hormonal therapy in China for treatment of the menopausal syndrome. However, composition of these herb mixtures are complex and their working mechanism is often unknown. This study investigated the effect of Tiáo-Gēng-Tāng (TG-decoction), a Chinese herbal mixture extract, in balancing female hormones, regulating expression of estrogen receptors (ERs), and preventing aging-related tissue damage.

Methods

Ovariectomized 5-month-old female rats were used to model menopause and treated with either TG-decoction or conjugated estrogen for 8 weeks. Estradiol (E₂), luteinizing hormone (LH) and follicle-stimulating hormone (FSH) were measured in serum and in the hypothalamus. Hypothalamic expression of estrogen receptor (ER) alpha and beta were studied by real-time PCR and western blotting. Total antioxidant capacity (T-AOC), oxidation indicator superoxide dismutase (SOD) activity and tissue damage parameter malondialdehyde (MDA) were measured using standard assays. Aging-related ultrastructural alterations in mitochondria were studied in all animals by transmission electron microscopy.

Results

TG-decoction-treatment elevated E₂ and lowered FSH in serum of ovariectomized rats. The potency and efficacy of TG-decoction on the hypothalamus was generally weaker than that of conjugated estrogens. However, TG-decoction was superior in upregulating expression of ERα and β. TG-decoction increased hypothalamic SOD and T-AOC levels and decreased MDAlevels and mitochondrial damage in hypothalamic neurons.

Conclusions

TG-decoction balances female hormones similarly to conjugated estrogens but less effectively. However, it is superior in up regulating ERα and β and exhibits antioxidative antiaging activities. Whilst it shares similar effects with estrogen, TG-decoction also seems to have distinctive and more complex functions and activities.

Differential effects of hypothalamic IGF-I on gonadotropin releasing hormone neuronal activation during steroid-induced LH surges in young and middle-aged female rats

Interactions between brain IGF-I receptors and estrogen receptors regulate female reproductive physiology and behavior. The present study investigated potential mechanisms by which IGF-I receptors in the neuroendocrine hypothalamus regulate GnRH neuronal activation and LH release in young and middle-aged female rats under estradiol (E2) positive feedback conditions. We infused vehicle, IGF-I, or JB-1, a selective antagonist of IGF-I receptors, into the third ventricle of ovariectomized female rats primed with E2 and progesterone or vehicle. In young females, blockade of IGF-I receptors attenuated the steroid hormone-induced LH surge, reduced the percent of GnRH neurons expressing c-fos on the day of the LH surge, and decreased the total number of neurons expressing c-fos in the preoptic area. Middle-aged females had fewer GnRH neurons expressing c-fos during the LH surge than young females, and the LH surge amplitude was attenuated. Infusion of an IGF-I dose previously shown to increase LH surge amplitude did not increase the percent of GnRH neurons expressing c-fos in middle-aged females. Brain IGF-I receptor blockade did not modify E2 induction of progestin receptor-immunoreactive neurons in the preoptic area, arcuate, or ventromedial hypothalamus of young rats. These findings indicate that brain IGF-I receptors are required for E2 activation of GnRH neurons in young rats and for robust GnRH release from axon terminals in middle-aged females. IGF-I likely exerts its effects by actions on E2-sensitive neurons that are upstream of GnRH neurons and terminals.

Pregnancy-induced changes in breast cancer risk

Breast cancer is the malignant disease most frequently diagnosed in women of all races and nationalities. Since the 1970s the worldwide incidence of this disease has increased 30-40% in postmenopausal women, in whom, paradoxically, the risk of developing breast cancer is significantly reduced by an early first full term pregnancy (FTP) as compared to nulliparous and late parous women. Although the cause of breast cancer is not known, the mechanisms mediating the protection conferred by an early FTP have been identified to reside in the breast itself, and to be modulated by endogenous and environmental exposures that might negatively affect this organ during specific windows in its development that extend from prenatal life until the first pregnancy. Soon after conception the embryo initiates the production of human chorionic gonadotropin (hCG), the glycoprotein hormone that is diagnostic of pregnancy. HCG in conjunction with ovarian steroid hormones primes the hypothalamic neuroendocrine system for maintaining the pregnancy. Higher levels of hCG during the first trimester of pregnancy have been associated with a reduction in maternal breast cancer incidence after age 50. In preclinical studies it has been demonstrated that both FTP and hCG treatment of virgin rats prevent the development of chemically-induced mammary tumors, a phenomenon mediated by the differentiation of the mammary gland epithelial cells prior to carcinogen exposure. Complete differentiation proceeds through complex morphological, physiological and molecular changes that occur during pregnancy and lactation, that ultimately result in increased DNA repair capabilities of the mammary epithelium, activation of genes controlling differentiation and programmed cell death and imprinting in the breast epithelium a specific and permanent genomic signature of pregnancy. This signature is indicative of a reduced breast cancer risk and serves as a molecular biomarker of differentiation for evaluating the potential use of chemopreventive agents.

Differential effects of aging on estrogen negative and positive feedback

Recent studies have demonstrated an age-related decline in gonadotropins and a decrease in pituitary responsiveness to GnRH, indicating that aging influences the neuroendocrine components of the female reproductive axis independently of changes in ovarian function. To determine whether aging might also affect the luteinizing hormone (LH) negative and positive feedback responses to gonadal steroids, we administered a controlled, graded sex steroid infusion to 11 younger (45-56 yr) and nine older (70-80 yr) postmenopausal women (PMW) in whom endogenous ovarian steroids and peptides are uniformly low. The doses of estradiol (E(2)) and progesterone (P) were chosen to mimic levels across the normal follicular phase and have been shown previously to induce negative followed by positive feedback on LH. Similar E(2) and P levels were achieved in younger and older PMW (P = 0.4 and 0.3, respectively) and produced a biphasic LH response in all subjects. The early decline in LH to 53% of baseline was not different in older vs. younger PMW. However, the positive feedback effect was attenuated in older compared with younger PMW (peak LH 144.4 ± 19.5 vs. 226.8 ± 22.3 IU/l, respectively, P = 0.01). In conclusion, these studies in PMW demonstrate preservation of short-term steroid negative and positive feedback in response to exogenous E(2) and P with aging. Attenuation of positive feedback in older compared with younger PMW is consistent with previous reports of declining GnRH responsiveness with aging.

Plasma kisspeptin levels are elevated in cord blood and present sexual dimorphism in the adult population: relation with leptin, gonadotropins and anthropometrical data

Kisspeptin, the product of the hypothalamic KISS1 gene, is a main regulator of the hypothalamic-pituitary-gonadal axis and could be a link between metabolism and reproduction through its interaction with leptin. Kisspeptin could be involved in gonadotropin regulation and responsive to leptin levels from the first stages of life, exhibiting, as does leptin, sexual dimorphism. To test our hypothesis, we have analyzed plasma kisspeptin levels and their possible relationship with gonadotropins and leptin in a cohort composed of newborns (n = 86) and adults (n = 55). Plasma kisspeptin, gonadotropin and leptin levels were measured by RIA and multiplexed bead immunoassays, respectively. We have built a multivariate linear regression model (analyzing kisspeptin and LH separately as dependent variables) by stepwise analysis, incorporating the variables that had shown significant correlation in the univariate analysis. Cord blood samples exhibited high kisspeptin levels 127.01(113-141.02 pmol/l), but these were not sexually dimorphic. The adult population exhibited sexual dimorphism (3.72(2.95-4.49) vs. 1.77(1.23-2.31)pmol/l women vs. men, p<0.05). Leptin levels showed sexual dimorphism in cord blood samples and also in the adult population. Furthermore, there was a significant interaction between LH and kisspeptin levels and kisspeptin was negatively correlated with age. The high kisspeptin levels observed in cord blood, with no sexual dimorphism, suggest a placental source. The sexual dimorphism exhibited in adulthood supports the notion that there are different sources and/or differential kisspeptin regulation between men and women.

Longitudinal, epidemiologic studies of female reproductive aging

Human female reproductive aging consists of multiple processes and interacts with other physiological systems in unique ways. Here we discuss eight recent longitudinal, epidemiologic studies of female reproductive aging that include endocrine data to highlight their contributions to our understanding of these various aging processes and their interactions. Specifically, we review data on ovarian and nonovarian reproductive aging processes and reproductive staging. We consider these data in the context of longitudinal research design and research goals, identify limitations of the studies but also ways in which existing longitudinal data can further our understanding of aging processes, and make recommendations for future studies of female reproductive aging.

Ovarian aging in developmental and evolutionary contexts

Evolutionary theory predicts that aging-related fertility declines result from tradeoffs between reproduction and somatic maintenance. Developmental programs for oogenesis also contribute to variation in aging-related reproductive declines among female vertebrates. Documented reproductive aging patterns in female vertebrates, including humans, are consistent with canonical aging patterns determined developmentally and require no special adaptive explanation. Here we discuss patterns of aging-related ovarian decline in diverse female vertebrates, and place human ovarian aging in comparative context. Depletion of finite oocyte stores accompanied by fertility loss occurs in a variety of nonhuman mammals and vertebrates, including short-lived rodents, birds, and some fishes; moreover, postreproductive lifespans of considerable length clearly are not limited to long-lived, social species with well-developed kin networks. We argue for a more rigorous comparative approach for understanding the evolutionary and developmental bases of ovarian aging in vertebrates with a wider range of aging patterns and social structures.

Neuroscientists as cartographers: mapping the crossroads of gonadal hormones, memory and age using animal models

Cognitive function is multidimensional and complex, and research in multiple species indicates it is considerably impacted by age and gonadal hormone milieu. One domain of cognitive function particularly susceptible to age-related decrements is spatial memory. Gonadal hormones can alter spatial memory, and they are potent modulators of brain microstructure and function in many of the same brain areas affected by aging. In this paper, we review decades of animal and human literature to support a tertiary model representing interactions between gonadal hormones, spatial cognition and age given that: 1) gonadal hormones change with age, 2) age impacts spatial learning and memory, and 3) gonadal hormones impact spatial learning and memory. While much has been discovered regarding these individual tenets, the compass for future aging research points toward clarifying the interactions that exist between these three points, and understanding mediating variables. Indeed, identifying and aligning the various components of the complex interactions between these tenets, including evaluations using basic science, systems, and clinical perspectives, is the optimal approach to attempt to converge the many findings that may currently appear contradictory. In fact, as discoveries are being made it is becoming clear that the findings across studies that appear contradictory are not contradictory at all. Rather, there are mediating variables that are influencing outcome and affecting the extent, and even the direction, of the effects that gonadal hormones have on cognition during aging. These mediating variables are just starting to be understood. By aligning basic scientific discoveries with clinical interpretations, we can maximize the opportunities for discoveries and subsequent interventions to allow individuals to "optimize their aging" and find their own map to cognitive health as aging ensues.

Influence of age and 17beta-estradiol on kisspeptin, neurokinin B, and prodynorphin gene expression in the arcuate-median eminence of female rhesus macaques

The neuropeptides kisspeptin, neurokinin B, and dynorphin A (collectively abbreviated as KNDy) are, respectively, encoded by KiSS-1, NKB, and PDYN and are coexpressed by neurons of the hypothalamic arcuate nucleus (ARC). Here, using quantitative real-time PCR, we examined age-related changes in the expression of genes encoding KNDy and associated receptors G protein-coupled receptor 54 (encoded by GPR54), neurokinin 3 receptor (encoded by NK3), and kappa-opioid receptor (encoded by KOR), in the female rhesus macaque ARC-median eminence (ARC-ME). Expression of KiSS-1 and NKB was highly elevated in old perimenopausal compared with young or middle-aged premenopausal animals. To test whether these age-related changes could be attributed to perimenopausal loss of sex steroids, we then examined KNDy, GPR54, NK3, and KOR expression changes in response to ovariectomy (OVX) and exposure to 17beta-estradiol (E(2)). Short-term (7 months) OVX (with or without 1 month of estrogen replacement) failed to modulate the expression of any of the KNDy-related genes. In contrast, long-term ( approximately 4 yr) OVX significantly increased KiSS-1 and NKB expression, and this was reversed by E(2) administration. Finally, we examined the expression of KNDy-related genes in young adult females during the early follicular, late follicular, or midluteal phases of their menstrual cycle but found no difference. Together, the results suggest that short-term alterations in circulating E(2) levels, such as those occurring during the menstrual cycle, may have little effect on the ARC-ME expression of KNDy and associated receptors. Nevertheless, they clearly demonstrate that loss of ovarian steroid negative feedback that occurs during perimenopause plays a major role in modulating the activity of KNDy circuits of the aging primate ARC-ME.

Region-specific changes in the immunoreactivity of vasoactive intestinal peptide and pituitary adenylate cyclase-activating polypeptide receptors (VPAC2, and PAC1 receptor) in the aged rat brains

Pituitary adenylate cyclase-activating peptide (PACAP) and vasoactive intestinal peptide (VIP) have been implicated in a large array of physiological and patho-physiological processes through their receptors (VPAC(1), VPAC(2), and PAC(1) receptor) in the central nervous system. Previously, we demonstrated age-related decreases in VPAC(1) receptor expression in the rat brain providing a possible basis of several age-induced functional changes in the aged brain. In the current study, we also examined age-related changes in PAC(1) and VPAC(2) receptors in aged rat brains using an immunohistochemical approach. We found that PAC1 immunoreactivity was significantly increased in the hippocampal formation, hypothalamus, thalamus, midbrain septal nuclei, and white matter of aged rats compared with young control rats although its distribution pattern was not altered. In contrast, both distribution pattern and immunoreactivity of VPAC(2) receptor remained unchanged in aged rat brains. These results suggest that the PACAP/VIP receptors exhibit specific expressional changes in the aged brain and that these specific changes could underlie age-associated memory and cognitive functional declines as well as several other age-induced functional changes in the brain. However, the exact regulatory mechanism and its functional significance require further elucidation.

The neuroendocrine physiology of female reproductive aging: An update

The transition into menopause is a complex process that affects fertility and increases the risk for a number of health problems in aging women that include, but are not limited to osteoporosis, heart disease, diabetes mellitus and cognitive dysfunction. Improved nutrition and enhanced access to medical care have increased the average lifespan for women in developed countries, and many will spend more than one-third of their life in a post-menopausal state. Epidemiological studies indicate that a delayed natural menopause confers longevity and decelerates the appearance of much age-related morbidity, suggesting that developing treatments to delay menopause would significantly improve quality of life for women. Although menopause is ultimately defined by ovarian follicular exhaustion, several lines of scientific evidence in humans and animals now suggest that dysregulation of estradiol feedback mechanisms and hypothalamic-pituitary dysfunction contributes to the onset and progression of reproductive senescence, independent of ovarian failure. This article provides a brief update on our current understanding of the role of the hypothalamic-pituitary axis in the onset of and transition into female reproductive senescence.

Characterization of an animal model of postmenopausal cardiac hypertrophy and novel mechanisms responsible for cardiac decompensation using ovariectomized pressure-overloaded rats

OBJECTIVE

The development of animal models of cardiovascular disease are critical to define pathophysiological mechanisms and to advance diagnosis and therapy. The lack of a suitable animal model represents a failure to define the mechanisms responsible for postmenopausal myocardial hypertrophy in hypertension and adverse cardiac remodeling.

METHODS

In this review, we presented a rat model of postmenopausal myocardial hypertrophy, with particular focus on the similarities between the animal model and postmenopausal women regarding myocardial function as well as molecular and subcellular mechanisms. To elucidate the molecular mechanism of left ventricular (LV) hypertrophy and remodeling in postmenopausal women, we analyzed myocardial hypertrophy as well as cardiac function and hypertrophy-related protein expression in ovariectomized (OVX) and pressure overloaded (PO) rats.

RESULTS

The model is characterized by depletion of serum estrogen and increased heart-to-body weight and lung-to-body weight ratios. Moreover, the OVX-PO rats also show increased mean arterial blood pressure, LV end-diastolic pressure, LV developed pressure, and maximal rates of LV contraction and relaxation compared with the OVX group. Importantly, Akt activity was largely attenuated, and both endothelial nitric oxide synthase expression and activity were markedly reduced in the OVX-PO group. Finally, significant increased mortality was observed in the OVX-PO group after chronic isoproterenol administration.

CONCLUSIONS

Our results demonstrate that rats subject to OVX are unable to compensate for hypertrophy partly due to impaired Akt-endothelial nitric oxide synthase signaling along with deteriorated heart function and demonstrated increased mortality. In this review, we discussed the mechanisms of cardiac injury, which could play a critical role in postmenopausal hypertrophy, as well as the characteristics of the OVX-PO female rats as a model to test cardioprotective drugs in postmenopausal women.

Hypothalamic insulin-like growth factor-I receptors are necessary for hormone-dependent luteinizing hormone surges: implications for female reproductive aging

Brain IGF-I receptors are required for maintenance of estrous cycles in young adult female rats. Circulating and hypothalamic IGF-I levels decrease with aging, suggesting a role for IGF-I in the onset of reproductive senescence. Therefore, the present study investigated potential mechanisms of action of brain IGF-I receptors in the regulation of LH surges in young adult and middle-aged rats. We continuously infused IGF-I, the selective IGF-I receptor antagonist JB-1, or vehicle into the third ventricle of ovariectomized young adult and middle-aged female rats primed with estradiol and progesterone. Pharmacological blockade of IGF-I receptors attenuated and delayed the LH surge in young adult rats, reminiscent of the LH surge pattern that heralds the onset of reproductive senescence in middle-aged female rats. Infusion of IGF-I alone had no effect on the LH surge but reversed JB-1 attenuation of the surge in young females. In middle-aged rats, infusion of low doses of IGF-I partially restored LH surge amplitude, and infusion of JB-1 completely obliterated the surge. Intraventricular infusion of IGF-I or JB-1 did not modify pituitary sensitivity to exogenous GnRH or GnRH peptide content in the anterior or mediobasal hypothalamus in either young or middle-aged rats. These findings support the hypothesis that brain IGF-I receptor signaling is necessary for GnRH neuron activation under estrogen-positive feedback conditions and that decreased brain IGF-I signaling in middle-aged females contributes, in part, to LH surge dysfunction by disrupting estradiol-sensitive processes that affect GnRH neuron activation and/or GnRH release.

Formalin-induced nociceptive behavior and c-Fos expression in middle-aged female rats

The impact of the estrous cycle on the nociceptive response in middle-aged female rats was assessed using the formalin test and c-Fos immunoreactivity as a marker of neural activation. Young (2-month-old) and middle-aged (11-month-old) rats were examined, dividing the middle-aged rats into two groups based on their estrous cycle: regular 4-day estrous cycle and irregular estrous cycle. The right hind paw was subcutaneously injected with 50microl of 2% formalin or saline as a control. Behavioral changes were observed for 1h. Cycling rats were used during proestrus. Middle-aged female rats had a significantly higher score for nociceptive behavior compared to young rats, irrespective of estrous cyclicity, which suggests that aging, not the ability to maintain estrous cyclicity, causes hypersensitivity to the formalin injection. Immunohistochemical analysis found that the brain response to formalin injection was also more sensitive in middle-aged rats than young rats; a significant increase in the number of c-Fos immunoreactive cells was found in the ventral portion of the lateral septum of middle-aged rats injected with formalin compared to young and middle-aged rats injected with saline, irrespective of estrous cyclicity. Based on these results, we conclude that the sensitivity to painful stimuli in middle-aged female rats, which are in a neuroendocrine state similar to pre- and peri-menopausal women, is associated with age and not affected by reproductive ability.

Proximate mechanisms driving circadian control of neuroendocrine function: Lessons from the young and old

Circadian rhythms impact a variety of behavioral and physiological functions contributing to longevity and successful reproduction. In their natural environments, individuals of a species are faced with a multitude of challenges and the coordination of internal processes and behavior with external pressures has been hypothesized to be an important target of natural selection. Several lines of evidence from cyanobacteria, Drosophila, and plants provide strong support for an important role of the circadian clock in survival and reproductive success. Similarly in mammals, disruptions in circadian function markedly impact reproduction and lifespan. The present review discusses research outlining the proximate and ultimate mechanisms responsible for the central and peripheral control of the reproductive axis. Because precise temporal coordination of the endocrine system is particularly crucial for reproduction by females, the present overview focuses on the role of circadian timing in this sex.

Kisspeptin signaling in the brain

Kisspeptin (a product of the Kiss1 gene) and its receptor (GPR54 or Kiss1r) have emerged as key players in the regulation of reproduction. Mutations in humans or genetically targeted deletions in mice of either Kiss1 or Kiss1r cause profound hypogonadotropic hypogonadism. Neurons that express Kiss1/kisspeptin are found in discrete nuclei in the hypothalamus, as well as other brain regions in many vertebrates, and their distribution, regulation, and function varies widely across species. Kisspeptin neurons directly innervate and stimulate GnRH neurons, which are the final common pathway through which the brain regulates reproduction. Kisspeptin neurons are sexually differentiated with respect to cell number and transcriptional activity in certain brain nuclei, and some kisspeptin neurons express other cotransmitters, including dynorphin and neurokinin B (whose physiological significance is unknown). Kisspeptin neurons express the estrogen receptor and the androgen receptor, and these cells are direct targets for the action of gonadal steroids in both male and female animals. Kisspeptin signaling in the brain has been implicated in mediating the negative feedback action of sex steroids on gonadotropin secretion, generating the preovulatory GnRH/LH surge, triggering and guiding the tempo of sexual maturation at puberty, controlling seasonal reproduction, and restraining reproductive activity during lactation. Kisspeptin signaling may also serve diverse functions outside of the classical realm of reproductive neuroendocrinology, including the regulation of metastasis in certain cancers, vascular dynamics, placental physiology, and perhaps even higher-order brain function.

Transitional versus surgical menopause in a rodent model: etiology of ovarian hormone loss impacts memory and the acetylcholine system

Clinical research suggests that type of ovarian hormone loss at menopause influences cognition. Until recently ovariectomy (OVX) has been the primary rodent model to examine effects of ovarian hormone loss on cognition. This model limits evaluations to abrupt and complete ovarian hormone loss, modeling less than 13% of women who receive surgical menopause. The majority of women do not have their ovaries surgically removed and undergo transitional hormone loss via ovarian follicular depletion. 4-Vinylcyclohexene-diepoxide (VCD) produces gradual ovarian follicular depletion in the rodent, with hormone profiles more similar to naturally menopausal women vs. OVX. We directly compared VCD and OVX models to examine whether type of hormone loss (transitional vs. surgical) impacted cognition as assessed on a maze battery as well as the cholinergic system tested via scopolamine mnemonic challenge and brain acetylcholinesterase activity. Middle-aged rats received either sham surgery, OVX surgery, VCD, or VCD then OVX to assess effects of removal of residual ovarian output after transitional menopause and follicular depletion. VCD-induced transitional menopause impaired learning of a spatial recent memory task; surgical removal of residual ovarian hormones by OVX abolished this negative effect of transitional menopause. Furthermore, transitional menopause before OVX was better for memory than an abrupt loss of hormones via OVX only. Surgical ovarian hormone loss, regardless of menopause history, increased hippocampal acetylcholinesterase activity. Circulating gonadotropin and androstenedione levels were related to cognitive competence. Collectively, findings suggest that in the rat, initiation of transitional menopause before surgical ovary removal can benefit mnemonic function and could obviate some negative cognitive consequences of surgical menopause alone.

The excitatory peptide kisspeptin restores the luteinizing hormone surge and modulates amino acid neurotransmission in the medial preoptic area of middle-aged rats

Reproductive success depends on a robust and appropriately timed preovulatory LH surge. The LH surge, in turn, requires ovarian steroid modulation of GnRH neuron activation by the neuropeptide kisspeptin and glutamate and gamma-aminobutyric acid (GABA) neurotransmission in the medial preoptic area (mPOA). Middle-aged females exhibit reduced excitation of GnRH neurons and attenuated LH surges under estrogen-positive feedback conditions, in part, due to increased GABA and decreased glutamate neurotransmission in the mPOA. This study tested the hypothesis that altered kisspeptin regulation by ovarian steroids plays a role in age-related LH surge dysfunction. We demonstrate that middle-aged rats exhibiting delayed and attenuated LH surges have reduced levels of Kiss1 mRNA in the anterior hypothalamus under estrogen-positive feedback conditions. Kisspeptin application directly into the mPOA rescues total LH release and the LH surge amplitude in middle-aged rats and increases glutamate and decreases GABA release to levels seen in the mPOA of young females. Moreover, the N-methyl-D-aspartate receptor antagonist MK801 blocks kisspeptin reinstatement of the LH surge. These observations suggest that age-related LH surge dysfunction results, in part, from reduced kisspeptin drive under estrogen-positive feedback conditions and that kisspeptin regulates GnRH/LH release, in part, through modulation of mPOA glutamate and GABA release.

Age-related LH surge dysfunction correlates with reduced responsiveness of hypothalamic anteroventral periventricular nucleus kisspeptin neurons to estradiol positive feedback in middle-aged rats

Female reproductive aging in rats is characterized by reduced gonadotropin releasing hormone (GnRH) neuronal activation under estradiol positive feedback conditions and a delayed and attenuated luteinizing hormone (LH) surge. The newly identified excitatory neuropeptide kisspeptin is proposed to be a critical mediator of the pubertal transition and the ovarian steroid-induced LH surge. We previously showed that estradiol induces less kisspeptin mRNA expression in the anterior hypothalamus [anatomical location of anteroventral periventricular nucleus (AVPV)] in middle-aged than in young rats and intrahypothalamic infusion of kisspeptin restores LH surge amplitude in middle-aged females. Thus, reduced kisspeptin neurotransmission may contribute to age-related LH surge abnormalities. This study tested the hypothesis that middle-aged females will also exhibit reduced numbers of kisspeptin immunopositive neurons in the AVPV under estradiol positive feedback conditions. Using immunohistochemistry, we demonstrate that middle-aged females primed with ovarian steroids have fewer AVPV kisspeptin immunopositive neurons than young females. Age did not affect kisspeptin mRNA expression in the pituitary, numbers of kisspeptin immunopositive neurons in the arcuate nucleus, or estradiol-dependent reductions in kisspeptin mRNA expression in the posterior hypothalamus (containing the arcuate nucleus). These data strongly suggest that age-related LH surge dysfunction results, in part, from a reduced sensitivity of AVPV kisspeptin neurons to estradiol and hence decreased availability of AVPV kisspeptin neurons to activate GnRH neurons under positive feedback conditions.

Estrogens and age-related memory decline in rodents: what have we learned and where do we go from here?

The question of whether ovarian hormone therapy can prevent or reduce age-related memory decline in menopausal women has been the subject of much recent debate. Although numerous studies have demonstrated a beneficial effect of estrogen and/or progestin therapy for certain types of memory in menopausal women, recent clinical trials suggest that such therapy actually increases the risk of cognitive decline and dementia. Because rodent models have been frequently used to examine the effects of age and/or ovarian hormone deficiency on mnemonic function, rodent models of age-related hormone and memory decline may be useful in helping to resolve this issue. This review will focus on evidence suggesting that estradiol modulates memory, particularly hippocampal-dependent memory, in young and aging female rats and mice. Various factors affecting the mnemonic response to estradiol in aging females will be highlighted to illustrate the complications inherent to studies of estrogen therapy in aging females. Avenues for future development of estradiol-based therapies will also be discussed, and it is argued that an approach to drug development based on identifying the molecular mechanisms underlying estrogenic modulation of memory may lead to promising future treatments for reducing age-related mnemonic decline.