Age-related changes in the regulation of luteinizing hormone secretion by estrogen in women

[Peripheral blood EMR3 gene methylation level is correlated with breast cancer in Chinese women]

OBJECTIVE

To explore the association of methylation levels of C19orf57, MAP9, EMR3, NEK6 and PCOLCE2 genes in peripheral blood with breast cancer (BC) in Chinese women.

METHODS

We collected peripheral blood samples from 258 early-stage BC patients and 272 healthy women. Agena matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) was utilized to quantitatively measure the methylation levels of CpG sites in the genes. The association between DNA methylation and BC was analyzed using a logistic regression model adjusted for covariants. Spearman's correlation analysis was performed to analyze the association between the gene methylation levels and age. The methylation levels of the genes in the BC patients with different clinical characteristics were investigated using non-parametric tests.

RESULTS

In stead of EMR3 gene hypermethylation as found in BC patients as found in the Caucasian population, EMR3 gene hypomethylation was found to correlate with BC in Chinese women, but this correlation was significant only in women beyond the age of 50 years (for every 10% reduction of the methylation level, EMR3_CpG_1: OR=1.40; EMR3_CpG_2: OR=2.31; EMR3_CpG_3: OR=2.76, P < 0.05). EMR3 methylation was not or was only weakly correlated with tumor stage, size, lymphatic metastasis, ER, PR, HER2, or Ki67. Our data did not show a correlation between C19orf57 methylation and BC.

CONCLUSION

Peripheral blood EMR3 gene hypomethylation is associated with BC in Chinese women, especially in those at an old age and in postmenopausal women.

Mapping changes in women's visual functions during the menstrual cycle: narrative review

BACKGROUND

This article systematically updates the literature on changes in visual functions during the phases of the normal menstrual cycle in women.

OBJECTIVES

To update Guttridge's 1994 review of visual structures and functions associated with the menstrual cycle and broaden the search through psychophysical, neuroimaging and neurobehavioral measurements covering 1994-2020.

DESIGN AND SETTING

Narrative review conducted in a neurosciences and behavior laboratory in Brazil.

METHODS

The PubMed, Cochrane Clinical Answers and Google Scholar databases were searched. After screening and applying the eligibility criteria, 32 articles were examined. Through this analysis, the following information was extracted: (1) geographical distribution of the study; (2) sample size (according to age and phase of the menstrual cycle); (3) type of measurements according to psychophysical, neuroimaging and neurobehavioral instruments; (4) vision testing model; (5) visual subcategory evaluated; (6) categories of processed visual stimuli; and (7) main findings.

RESULTS

The menstrual phases give rise to significant changes in visual functions, including in relation to orientation and spatial attention, visual campimetry and visual sensitivity. These relate specifically to the follicular and luteal phases.

CONCLUSIONS

These findings theoretically expand the effects of menstrual cycles on visual functions found by Guttridge (1994). Despite some inconsistencies in the studies analyzed, it was found that visual processing during the follicular and luteal phases of the normal menstrual cycle of healthy women can explain physiological, cognitive, behavioral and social modulations.

How to study the menstrual cycle: Practical tools and recommendations

Despite decades of research on the physiological and psychological effects of the menstrual cycle, studies have not sufficiently adopted consistent methods for operationalizing the menstrual cycle. This has resulted in substantial confusion in the literature and limited possibilities to conduct systematic reviews and meta-analyses. In order to facilitate more rapid accumulation of knowledge on cycle effects, the present paper offers a set of integrative guidelines and standardized tools for studying the menstrual cycle as an independent variable. We begin with (1) an overview of the menstrual cycle and (2) premenstrual disorders, followed by (3) recommendations and tools regarding data collection in cycle studies. These recommendations address selecting the appropriate study design and sampling strategy, managing demand characteristics, identifying a sample of naturally-cycling individuals, and measuring menstrual bleeding dates, ovarian hormones, and ovulation. We proceed with suggestions for (4) data preparation and coding of cycle day and phases, as well as (5) data visualization, statistical modeling, and interpretation of menstrual cycle associations. We also provide (6) recommendations for using menses start day and ovulation testing to schedule visits in laboratory studies and end with a (7) comprehensive summary and conclusion. Regardless of whether the influence of the menstrual cycle is of central interest in a study or should be controlled to accurately assess the effects of another variable, the use of these recommendations and tools will help make study results more meaningful and replicable.

Effects of growth hormone administration on luteinizing hormone secretion in healthy older men and women

The known interactions between the somatotropic and hypothalamic-pituitary-gonadal (HPG) axes have not been well delineated in older individuals. Aging-associated decline in insulin like growth factor-1 (IGF-1) levels has been proposed to play a role in reproductive senescence in animals. However, the effects of GH on LH secretion are unknown in older individuals. Our objective was to determine whether GH modulates LH secretion or levels of sex steroids (SS) in healthy older (ages 65-88 years) men (n = 24) and women (n = 24) with low-normal plasma IGF-1 levels. In a double-masked, placebo-controlled (n = 24), randomized study, we evaluated the effects of GH (n = 24, 20 μg/kg sc 3×/week) for 26 weeks on nocturnal LH secretory dynamics [(8 pm to 8 am, Q20) min sampling and analyzed by multiparameter deconvolution algorithm]. Indices of LH secretion [frequency, mass per burst, pulsatile production rate, and approximate entropy (ApEn)] and fasting serum IGF-1, SHBG, and SS (TT, fT, or E2) were measured. At baseline, all indices of LH secretion (frequency, mass per burst, pulsatile production rate) were inversely (P < 0.05) related to IGF-1, but not to mean nocturnal serum GH concentrations. GH administration for 26 weeks increased serum IGF-1, but exerted no significant effects on LH secretory dynamics, or concentrations of SSs (TT, fT, or E2) or SHBG in older women or men. These data suggest that GH-mediated increases in IGF-1 do not modulate the HPG axis in older individuals.

Age and Long-Term Hormone Treatment Effects on the Ultrastructural Morphology of the Median Eminence of Female Rhesus Macaques

The median eminence (ME) of the hypothalamus comprises the hypothalamic nerve terminals, glia (especially tanycytes) and the portal capillary vasculature that transports hypothalamic neurohormones to the anterior pituitary gland. The ultrastructure of the ME is dynamically regulated by hormones and undergoes organizational changes during development and reproductive cycles in adult females, but relatively little is known about the ME during aging, especially in nonhuman primates. Therefore, we used a novel transmission scanning electron microscopy technique to examine the cytoarchitecture of the ME of young and aged female rhesus macaques in a preclinical monkey model of menopausal hormone treatments. Rhesus macaques were ovariectomized and treated for 2 years with vehicle, estradiol (E2), or estradiol + progesterone (E2 + P4). While the overall cytoarchitecture of the ME underwent relatively few changes with age and hormones, changes to some features of neural and glial components near the portal capillaries were observed. Specifically, large neuroterminal size was greater in aged compared to young adult animals, an effect that was mitigated or reversed by E2 alone but not by E2 + P4 treatment. Overall glial size and the density and tissue fraction of the largest subset of glia were greater in aged monkeys, and in some cases reversed by E2 treatment. Mitochondrial size was decreased by E2, but not E2 + P4, only in aged macaques. These results contrast substantially with work in rodents, suggesting that the ME of aging macaques is less vulnerable to age-related disorganization, and that the effects of E2 on monkeys' ME are age specific.

Age-related decline in ovarian follicle stocks differ between chimpanzees (Pan troglodytes) and humans

Similarity in oldest parturitions in humans and great apes suggests that we maintain ancestral rates of ovarian aging. Consistent with that hypothesis, previous counts of primordial follicles in postmortem ovarian sections from chimpanzees (Pan troglodytes) showed follicle stock decline at the same rate that human stocks decline across the same ages. Here, we correct that finding with a chimpanzee sample more than three times larger than the previous one, which also allows comparison into older ages. Analyses show depletion rates similar until about age 35, but after 35, the human counts continue to fall with age, while the change is much less steep in chimpanzees. This difference implicates likely effects on ovarian dynamics from other physiological systems that are senescing at different rates, and, potentially, different perimenopausal experience for chimpanzees and humans.

Luteinizing hormone and human chorionic gonadotropin: distinguishing unique physiologic roles

Luteinizing hormone (LH) and human chorionic gonadotropin (hCG) are integral components of the hypothalamic-pituitary-gonadal axis, which controls sexual maturation and functionality. In the absence of signaling through their shared receptor, fetal sexual differentiation and post-natal development cannot proceed normally. Although they share a high degree of homology, the physiologic roles of these hormones are unique, governed by differences in expression pattern, biopotency and regulation. Whereas LH is a key regulator of gonadal steroidogenesis and ovulation, hCG is predominantly active in pregnancy and fetal development. Emerging evidence has revealed endogenous functions not previously ascribed to hCG, including participation in ovulation and fertilization, implantation, placentation and other activities in support of successful pregnancy. Spontaneous and induced mutations in LH, hCG and their mutual receptor have contributed substantially to our understanding of reproductive development and function. The lack of naturally occurring, functionally significant mutations in the β-subunit of hCG reinforce its putative role in establishment of pregnancy. Rescue of reproductive abnormalities resulting from aberrant gonadotropin signaling is possible in certain clinical contexts, depending on the nature of the underlying defect. By understanding the physiologic roles of LH and hCG in normal and pathologic states, we may better harness their diagnostic, prognostic and therapeutic potential.

Changes in serum leptin during phases of menstrual cycle of fertile women: relationship to age groups and fertility

BACKGROUND

It is established that serum level of leptin is affected by transitional phases of reproduction. It is also reported that the puberty is triggered when body fat and circulating levels of leptin exceed the critical thresholds, butthere is less focus on the serum level of leptin and its relationship with different phases of menstrual cycle and the fertility.

OBJECTIVES

The present study try to determines the serum concentration of leptin and fertility hormonesin the various phases of normal menstrual cycle of fertile women and compare any difference in serum concentration between age groups of 18-30 years and 31-41 years It is a well known fact that fertility start to decrease from age 31 years.

PATIENTS AND METHODS

A total of 118 healthy fertile women with normal menstrual cycle aged between 18-40, were divided into two age groups (n = 65) 18-30 years and (n = 53) 31-40 years. Serum concentrations of leptin, estradiol, progesterone, luteinising hormone (LH) and follicle-stimulating hormone (FSH) were measured on day1 (menstrual phase), day7 (proliferative/follicular phase), day14 (ovulatory phase), day21 (luteal phase) and day 28(secretory phase) of the menstrual cycle.

RESULTS

There was a significant increase (P < 0.05) in leptin levels on day 14 (12.75 + 5.8 ng/mL) and day 21 (12.91 + 3.2 ng/mL) for age group 18-30 years compared to day 14 (11.60 + 3.2 ng/mL) and day 21 (11.60 + 3.2 ng/mL) for age group 31-40 years. Leptin was positively correlated with FSH on day 14, with LH on day 7 and day 21; likewise, with progesterone on day 21 and day 28 and with estradiol on day 7 and day 14 for both age groups.

CONCLUSIONS

The serum leptin level was at the lowest level during the menstrual and secretory phase and the highest level was around the luteal phase. The significant increase ofleptin in the younger age group raise this question whether circulating leptin has any role to play in the age of pregnancy and fertility. Data in this study shows that leptin level was affected with increase in age; therefore changes in leptin level will affect fertility in this study suggest that there may be a relation between leptin levels and fertility.

Reproductive aging: insights from model organisms

Aging was once thought to be the result of a general deterioration of tissues as opposed to their being under regulatory control. However, investigations in a number of model organisms have illustrated that aspects of aging are controlled by genetic mechanisms and are potentially manipulable, suggesting the possibility of treatment for age-related disorders. Reproductive decline is one aspect of aging. In model organisms and humans of both sexes, increasing age is associated with both a decline in the number of progeny and an increased incidence of defects. The cellular mechanisms of reproductive aging are not well understood, although a number of factors, both intrinsic and extrinsic to an organism's germline, may contribute to aging phenotypes. Recent work in a variety of organisms suggests that nuclear organization and nuclear envelope proteins may play a role in these processes.

Evaluation of melatonin on the precocious puberty: a pilot study

OBJECTIVE

To evaluate the effect of 6-sulphatoxymelatonin on the sleep quality of girls with precocious puberty.

STUDY DESIGN

Ninety-nine girls were divided into three groups: GI, precocious puberty; GII, normal prepubescent; GIII, normal puberty. Questionnaires containing demographic and clinical data were applied. Blood was collected for hormonal evaluation for 6-sulphatoxymelatonin. The modified Rush Sleep Diary was used.

RESULTS

The levels of 6-sulphatoxymelatonin were highest in the group without pubertal development (75.23 ± 10.84 ng/ml), second highest in the group with normal puberty (45.66 ± 3.87 ng/ml, p < 0.001) and lowest in the group with true precocious puberty (37.04 ± 5.47 ng/ml). The amount of day sleep was greater in the group without pubertal development compared to other groups.

CONCLUSION

Despite the sleep data, melatonin may be involved in the precocious puberty process.

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.

Carbohydrate complexity and proportion of serum FSH isoforms reflect pituitary-ovarian activity in perimenopausal women and depot medroxyprogesterone acetate users

BACKGROUND

FSH is synthesized and secreted in multiple glycosylation variants with different oligosaccharide structures; the endocrine milieu regulates the composition of FSH carbohydrate moiety.

OBJECTIVES

To characterize serum FSH isoforms according to their sialic acid content and oligosaccharide complexity in regularly menstruating women and in depot medroxyprogesterone acetate (DMPA) users during the menopausal transition. Subjects and methods Ten regularly menstruating perimenopausal women aged 45-52, with mid-follicular phase FSH levels < or =10 IU/l and 10 regularly menstruating women, aged 20-39, were included. Blood samples were collected on the ninth day of the menstrual cycle. Twenty DMPA users were divided into two groups (n = 10) according to age: DMPA(1), age range 20-39 and DMPA(2), age range 45-52. Blood samples were collected 90 +/- 5 days after the last injection of DMPA. Oestradiol (E(2)), inhibin B (Inh B), Pro-alphaC levels and the relative abundance of FSH isoforms on the basis of charge (preparative isoelectric focusing) and carbohydrate complexity (Concanavalin A chromatography) were determined.

RESULTS

Decreased Inh B and moderately elevated E(2) levels were observed in perimenopausal women associated with an increase in FSH sialylation and a decrease in its oligosaccharide complexity. DMPA induced changes in the hormonal profile and FSH molecular microheterogeneity; the secreted hormone was more heterogeneous and its oligosaccharides were less complex under this condition.

CONCLUSION

Serum FSH glycoforms with increased sialylation and decreased oligosaccharide complexity reflect the decline of the gonadal activity induced either by age or by the use of a DMPA as a contraceptive.

Total and unopposed estrogen exposure across stages of the transition to menopause

Detailed characterization of estrogen dynamics during the transition to menopause is an important step toward understanding its potential implications for reproductive cancers developing in the transition years. We conducted a 5-year prospective study of endogenous levels of total and unopposed estrogen. Participants (n=108; ages 25-58 years) collected daily urine specimens for 6 months in each of 5 consecutive years. Specimens were assayed for estrone-3-glucuronide (E1G) and pregnanediol-3-glucuronide. Linear mixed-effects models were used to estimate exposure to total and unopposed estrogen by age and reproductive stage. Reproductive stage was estimated using menstrual cycle length variance. E1G mean area under the curve and mean E1G 5th and 95th percentiles represented total estrogen exposure. An algorithm identifying days of above-baseline E1G that coincided with the days of baseline pregnanediol-3-glucuronide was used to identify days of unopposed estrogen. Mean E1G area under the curve increased with age in the pretransition and early transition and decreased in the late transition. Ninety-fifth percentile E1G levels did not decline until after menopause, whereas 5th percentile levels declined from the early transition to the postmenopause. The number of days of unopposed estrogen was significantly higher during the transition compared with the pretransition. Given the length of time women spend in the transition, they are exposed to more total and unopposed estrogen than has been previously appreciated. Coupled with epidemiologic evidence on lifetime exposure to estrogen, these results suggest that variation in the amount of time spent in the transition may be an important risk factor for reproductive cancers.

A comparison of prepulse inhibition in pre- and postmenopausal women and age-matched men

Prepulse inhibition (PPI) of the startle response is sensitive to sex with women showing less PPI compared with age-matched men and varies according to the menstrual cycle in women. Relatively less is known about sex differences in prepulse facilitation (PPF). To examine further the roles of sex and circulating sex hormones, pre- (n=20) and postmenopausal women (n=20) were compared with men of similar ages (n=17, 18-40 years; n=18, 55-69 years). All participants were assessed on PPI and PPF, and provided saliva samples for measurement of 17beta-estradiol (estrogen) and testosterone. Premenopausal women showed less PPI compared with age-matched men, with no significant difference in PPF. Postmenopausal women did not differ in PPI but showed more PPF than age-matched men. There was less PPI and PPF in older, relative to young, men; pre- and postmenopausal women did not differ significantly. PPI showed no association with the levels of sex hormones. PPF showed small positive associations with both the levels of estrogen and testosterone, especially in young men. The present findings extend recent observations in mice showing less PPI in premenopausal, but not postmenopausal, female compared with male mice of similar ages (Ison and Allen, Behav Brain Res, 2007) to humans. There appear to be no substantial relationships between individual differences in endogenous levels of sex hormones and PPI; fluctuations within an individual may have a stronger role.

Restoration of the luteinizing hormone surge in middle-aged female rats by altering the balance of GABA and glutamate transmission in the medial preoptic area

Hypothalamic glutamate and gamma-aminobutyric acid (GABA) neurotransmission are involved in the ovarian hormone-induced GnRH-LH surge in rodents. We previously reported that middle-aged rats have significantly less glutamate release in the medial preoptic area than young rats on the day of the LH surge. The present study tested the hypothesis that the delayed and attenuated LH surge in ovariohysterectomized middle-aged rats primed with ovarian steroids results from reduced hypothalamic glutamate and increased GABA(A) neurotransmission. Microdialysis results show that middle-aged rats with attenuated LH surges had reduced extracellular glutamate and increased extracellular GABA levels in the medial preoptic area compared with young rats. Blocking GABA(A) receptors with bicuculline or inhibiting synaptic glutamate reuptake with L-trans-pyrrolidine-2,4-dicarboxylic acid increased extracellular Glu in the medial preoptic area and partially restored LH surge amplitude in middle-aged rats without altering LH surge onset. Complete recovery of LH surge amplitude was observed in middle-aged rats treated with the combination of bicuculline and L-trans-pyrrolidine-2,4-dicarboxylic acid. This treatment also restored the extracellular glutamate:GABA ratio in the medial preoptic area of middle-aged rats to the level of young rats. Immunoblot analysis revealed that estradiol and progesterone treatment reduced SLC32A1(formerly known as vesicular GABA transporter) levels and increased SLC17A6 (formerly known as vesicular glutamate transporter 2) levels in the anterior hypothalamus of ovariohysterectomized young but not middle-aged rats. These data suggest that both reduced availability of glutamate and increased activation of GABA(A) receptors under estrogen-positive feedback conditions contribute to the age-related delay in onset and attenuated amplitude of the LH surge.

Human chorionic gonadotropin (hCG) up-regulates wnt5b and wnt7b in the mammary gland, and hCGbeta transgenic female mice present with mammary Gland tumors exhibiting characteristics of the Wnt/beta-catenin pathway activation

Transgenic (TG) mice expressing human chorionic gonadotropin (hCG) beta-subunit under the ubiquitin C promoter, presenting with a moderately elevated level of LH/hCG bioactivity develop multiple neoplasms secondary to the endocrine abnormalities, including mammary gland tumors after the age of 9 months. The increased levels of circulating estradiol, progesterone, and prolactin of the TG females after puberty boost the lobuloalveolar development in the mammary gland resulting ultimately in the formation of estrogen and progesterone receptor-negative, malignant tumors. These tumors have a similar histopathology with those observed in TG mice with activated wnt/beta-catenin pathway, showing increased expression of beta-catenin, also a common finding in human breast tumors. Transdifferentiation is observed in mammary tumors of the hCGbeta TG mice, accompanied by abnormal expression of the Wnt genes in the tumorous and nontumorous mammary gland tissue. Specifically we found increased expression of Wnt5b in the TG mammary glands at the age of 3 months and up-regulation of Wnt7b and -5b in the subsequently appearing tumors. Importantly, hCG was found to up-regulate these wnt ligands in mouse mammary gland, independent of the changes in ovarian steroidogenesis. Thus, the hCGbeta-overexpressing TG mice represent a novel model that links enhanced hCG action to dysregulated wnt signaling in the mammary gland, resulting in beta-catenin-stabilizing mammary tumorigenesis. The novel finding of hCG up-regulating wnt7b and wnt5b could contribute to pregnancy-induced breast cancer in humans.

The role of aromatase inhibitors in ameliorating deleterious effects of ovarian stimulation on outcome of infertility treatment

Clinical utilization of ovulation stimulation to facilitate the ability of a couple to conceive has not only provided a valuable therapeutic approach, but has also yielded extensive information on the physiology of ovarian follicular recruitment, endometrial receptivity and early embryo competency. One of the consequences of the use of fertility enhancing agents for ovarian stimulation has been the creation of a hyperestrogenic state, which may influence each of these parameters. Use of aromatase inhibitors reduces hyperestrogenism inevitably attained during ovarian stimulation. In addition, the adjunct use of aromatase inhibitors during ovarian stimulation reduces amount of gonadotropins required for optimum stimulation. The unique approach of reducing hyperestrogenism, as well as lowering amount of gonadotropins without affecting the number of mature ovarian follicles is an exciting strategy that could result in improvement in the treatment outcome by ameliorating the deleterious effects of the ovarian stimulation on follicular development, endometrial receptivity, as well as oocyte and embryo quality.

Ovarian aging and the perimenopausal transition: the paradox of endogenous ovarian hyperstimulation

The purpose of this review is to put into a useful clinical context the changing over time of basic ovarian-pituitary-hypothalamic relationships during perimenopause. "Perimenopause" means changes in ovarian hormones, feedback relationships, and clinical experiences beginning in women ages 35-50 with regular flow and ending 1 yr after the final menstrual flow. A key observation must be explained--estradiol levels are increased in perimenopause. Inhibin B levels are lower and activin may be higher in midlife, menstruating women. These changes probably cause higher follicular phase FSH levels--"endogenous ovarian hyperstimulation" results. The positive estradiol feedback on LH is also disturbed--midcycle LH peaks and mid-luteal slow-frequency, high-amplitude LH pulses are less frequent. In addition to higher levels, estradiol receptors may increase in tissues of symptomatic women. Despite hyperstimulation of follicles, progesterone levels and luteal phase lengths are paradoxically decreased--reasons probably include LH peak disruptions and estrogen-stimulated greater corticotrophin-mediated reproductive suppression. In summary, disturbed feedback relationships causing higher and unpredictable estrogen and lower progesterone levels occur throughout perimenopause, especially during regular cycles. Prospective, population-based research is needed to systematically relate these feedback hormonal changes to clinical characteristics and to allow a diagnosis of perimenopause in regularly cycling midlife women.

The aging reproductive neuroendocrine axis

It is well known that the reproductive system is one of the first biological systems to show age-related decline. While depletion of ovarian follicles clearly relates to the end of reproductive function in females, evidence is accumulating that a hypothalamic defect is critical in the transition from cyclicity to acyclicity. This minireview attempts to present a concise review on aging of the female reproductive neuroendocrine axis and provide thought-provoking analysis and insights into potential future directions for this field. Evidence will be reviewed, which shows that a defect in pulsatile and surge gonadotropin hormone-releasing hormone (GnRH) secretion exists in normal cycling middle-aged female rats, which is thought to explain the significantly attenuated pulsatile and surge luteinizing hormone (LH) secretion at middle-age. Evidence is also presented, which supports the age-related defect in GnRH secretion as being due to a reduced activation of GnRH neurons. Along these lines, stimulation of GnRH secretion by the major excitatory transmitter glutamate is shown to be significantly attenuated in middle-aged proestrous rats. Corresponding age-related defects in other major excitatory regulatory factors, such as catecholamines, neuropeptide Y, and astrocytes, have also been demonstrated. Age-related changes in hypothalamic concentrations of neurotransmitter receptors, steroid receptors, and circulating steroid hormone levels are also reviewed, and discussion is presented on the complex interrelationships of the hypothalamus-pituitary-ovarian (HPO) axis during aging, with attention to how a defect in one level of the axis can induce defects in other levels, and thereby potentiate the dysfunction of the entire HPO axis.