Neuroendocrine changes in the aging reproductive axis of female rhesus macaques (Macaca mulatta)

Comparison of assay methods for quantifying sex hormone concentrations across the menstrual cycle in rhesus macaques†

Immunoassays have been the preferred method for steroid hormone analysis for more than 50 years. Automated immunoassays (AIAs) offer high throughput, rapid data turnaround, and low cost for measuring steroid hormone concentrations. The application of liquid chromatography-tandem mass spectrometry (LC-MS/MS) for steroid quantification provides greater specificity and selectivity for individual steroids, the ability to simultaneously analyze multiple steroids, and high throughput and automation. We compared AIA and LC-MS/MS for analysis of 17beta-estradiol (E2) and progesterone (P4) over the course of several menstrual cycles in 12 rhesus macaques (Macaca mulatta). Serum samples were collected every 4 days across four menstrual cycles from each monkey. AIAs were performed on a Roche cobas e411 analyzer. LC-MS/MS analysis was performed on a Shimadzu-Nexera-LCMS-8060 instrument. Scatter plots with Passing-Bablok regression showed excellent agreement between AIA and LC-MS/MS for both E2 and P4. Bland-Altman plots revealed no bias for either method; however, AIA overestimated E2 at concentrations >140 pg/ml and underestimated P4 at concentrations >4 ng/ml compared to LC-MS/MS. A comparison of testosterone concentrations measured by AIA and LC-MS/MS in the same samples was also performed. In contrast to E2 and P4, AIA and LC-MS/MS yielded significantly different results for testosterone concentrations, with AIA consistently underestimating concentrations relative to those obtained by LC-MS/MS. Well-characterized automated immunoassays are an excellent tool for daily monitoring of monkey menstrual cycles or providing single data points requiring fast turnaround. In certain situations where AIAs may provide inaccurate estimations of E2 and P4 concentrations, LC-MS/MS assays are preferable.

Modulation of neural gene networks by estradiol in old rhesus macaque females

The postmenopausal decrease in circulating estradiol (E2) levels has been shown to contribute to several adverse physiological and psychiatric effects. To elucidate the molecular effects of E2 on the brain, we examined differential gene expression and DNA methylation (DNAm) patterns in the nonhuman primate brain following ovariectomy (Ov) and subsequent subcutaneous bioidentical E2 chronic treatment. We identified several dysregulated molecular networks, including MAPK signaling and dopaminergic synapse response, that are associated with ovariectomy and shared across two different brain areas, the occipital cortex (OC) and prefrontal cortex (PFC). The finding that hypomethylation (p = 1.6 × 10-51) and upregulation (p = 3.8 × 10-3) of UBE2M across both brain regions provide strong evidence for molecular differences in the brain induced by E2 depletion. Additionally, differential expression (p = 1.9 × 10-4; interaction p = 3.5 × 10-2) of LTBR in the PFC provides further support for the role E2 plays in the brain, by demonstrating that the regulation of some genes that are altered by ovariectomy may also be modulated by Ov followed by hormone replacement therapy (HRT). These results present real opportunities to understand the specific biological mechanisms that are altered with depleted E2. Given E2's potential role in cognitive decline and neuroinflammation, our findings could lead to the discovery of novel therapeutics to slow cognitive decline. Together, this work represents a major step toward understanding molecular changes in the brain that are caused by ovariectomy and how E2 treatment may revert or protect against the negative neuro-related consequences caused by a depletion in estrogen as women approach menopause.

Effect of hormone replacement therapy on amyloid beta (Aβ) plaque density in the rhesus macaque amygdala

Background

Amyloid beta (Aβ) plaque density was examined in the amygdala of rhesus macaques, to elucidate the influence of age, diet and hormonal environment.

Methods

Luminex technology was used to measure cerebrospinal fluid (CSF) concentrations of Aβ40 and Aβ42 across three decades, while immunohistochemistry was used to examine Aβ plaque density in the amygdala.

Results

Aβ40 was found to be the predominant isoform of Aβ in the CSF, but neither Aβ40 or Aβ42 concentrations showed an age-related change, and the ratio of Aβ42 to Aβ40 showed only a marginal increase. Significantly fewer Aβ plaques were detected in the amygdala of old ovariectomized animals if they received estradiol HRT (p < 0.001); similar results were obtained regardless of whether they had been maintained on a regular monkey chow for ∼48 months or on a high-fat, high-sugar, Western-style diet for ∼30 months.

Conclusion

The results demonstrate that HRT involving estrogen can reduce Aβ plaque load in a cognitive brain region of aged non-human primates. The results from this translational animal model may therefore have clinical relevance to the treatment of AD in post-menopausal women, whether used alone, or as a supplement to current pharmacological and monoclonal antibody-based interventions.

Pathological Markers of Alzheimer's Disease and Related Dementia in the Rhesus Macaque Amygdala

Rhesus macaques develop amyloid-β (Aβ) plaques during old age, but it is unclear how extensively they express other pathological hallmarks of dementia. Here we used immunohistochemistry to examine expression of phosphorylated tau (pTau) protein and cytoplasmic inclusions of TAR DNA binding protein 43 kDa (TDP-43) within the amygdala of young and old males, and also in old surgically-menopausal females that were maintained on regular or obesogenic diets. Only one animal, a 23-year-old female, showed pTau expression and none showed TDP-43 inclusions. What genetic and/or environmental factors protect macaques from expressing more severe human neuro-pathologies remains an interesting unresolved question.

Modulation of neural gene networks by estradiol in old rhesus macaque females

The postmenopausal decrease in circulating estradiol (E2) levels has been shown to contribute to several adverse physiological and psychiatric effects. To elucidate the molecular effects of E2 on the brain, we examined differential gene expression and DNA methylation (DNAm) patterns in the nonhuman primate brain following ovariectomy (Ov) and subsequent E2 treatment. We identified several dysregulated molecular networks, including MAPK signaling and dopaminergic synapse response, that are associated with ovariectomy and shared across two different brain areas, the occipital cortex (OC) and prefrontal cortex (PFC). The finding that hypomethylation (p=1.6×10-51) and upregulation (p=3.8×10-3) of UBE2M across both brain regions, provide strong evidence for molecular differences in the brain induced by E2 depletion. Additionally, differential expression (p=1.9×10-4; interaction p=3.5×10-2) of LTBR in the PFC, provides further support for the role E2 plays in the brain, by demonstrating that the regulation of some genes that are altered by ovariectomy may also be modulated by Ov followed by hormone replacement therapy (HRT). These results present real opportunities to understand the specific biological mechanisms that are altered with depleted E2. Given E2's potential role in cognitive decline and neuroinflammation, our findings could lead to the discovery of novel therapeutics to slow cognitive decline. Together, this work represents a major step towards understanding molecular changes in the brain that are caused by ovariectomy and how E2 treatment may revert or protect against the negative neuro-related consequences caused by a depletion in estrogen as women approach menopause.

Effect of estradiol replacement on hippocampal concentrations of estrogens in aged rhesus macaques maintained on an obesogenic diet

Replacement involving estrogens has proven efficacy at treating a wide range of disorders that develop with menopause or after surgical removal of the ovaries. Here, we tested whether an estradiol (E2) replacement paradigm that recapitulates physiological E2 levels in the circulation also recapitulates physiological E2 levels within the hippocampus. E2 was delivered continuously to old ovariectomized (OVX) rhesus macaques, maintained on a high-fat, high-sugar Western-style diet (WSD) for ∼30 months, via subcutaneous implants; this resulted in physiological concentrations of both estrone (E1) and E2 in the circulation (determined by LC-MS/MS). Surprisingly, however, hippocampal concentrations of E2 were markedly (P < 0.01) higher than in ovary-intact animals maintained on a regular chow diet. The data suggest that E2 replacement paradigms that appear to recapitulate physiological E2 concentrations in the circulation may produce hyper-physiological E2 levels within some brain areas, especially when individuals are maintained on a WSD.

Detrusor underactivity is associated with metabolic syndrome in aged primates

Lower urinary tract (LUT) dysfunction is prevalent in the elderly population, and clinical manifestations include urinary retention, incontinence, and recurrent urinary tract infections. Age-associated LUT dysfunction is responsible for significant morbidity, compromised quality of life, and rising healthcare costs in older adults, but its pathophysiology is not well understood. We aimed to investigate the effects of aging on LUT function by urodynamic studies and metabolic markers in non-human primates. Adult (n = 27) and aged (n = 20) female rhesus macaques were evaluated by urodynamic and metabolic studies. Cystometry showed detrusor underactivity (DU) with increased bladder capacity and compliance in aged subjects. Metabolic syndrome indicators were present in the aged subjects, including increased weight, triglycerides, lactate dehydrogenase (LDH), alanine aminotransferase (ALT), and high sensitivity C-reactive protein (hsCRP), whereas aspartate aminotransferase (AST) was unaffected and the AST/ALT ratio reduced. Principal component analysis and paired correlations showed a strong association between DU and metabolic syndrome markers in aged primates with DU but not in aged primates without DU. The findings were unaffected by prior pregnancies, parity, and menopause. Our findings provide insights into possible mechanisms for age-associated DU and may guide new strategies to prevent and treat LUT dysfunction in older adults.

Estropause, Sex Hormones and Metal Homeostasis in the Mouse Brain

Alterations in brain metal ion homeostasis have been reported with aging and are implicated in the pathogenesis of neurodegenerative diseases. To assess whether age-related changes in hypothalamic-pituitary-gonadal (HPG) hormones might be involved in modulating brain metal ion homeostasis, we treated 7.5-month intact, sham-ovariecomized and ovariectomized C57B6SJL mice with vehicle or leuprolide acetate (for 9-months) to differentiate between whether sex steroids or gonadotropins might modulate brain metal ion concentrations. Unlike other aging mammals, there was no increase in plasma luteinizing hormone (LH) and follicle-stimulating hormone (FSH) concentrations following estropause in mice, suggesting there was sufficient residual production by the follicle depleted ovary, of sex steroids like estrogens and protein hormones like the inhibins, in order to suppress pituitary LH/FSH production. Castration on the other hand induced significant increases in circulating LH and FSH. Modulation of plasma sex steroid and gonadotropin levels did not significantly alter the concentrations of brain metals tested (Fe, Zn, Cu, Mn, Co, Ni, Al, Li), although there was a tendency for a decrease in all brain metals following ovariectomy (low estrogens and progesterone, high gonadotropins), a response that was reversed with leuprolide acetate treatment (low sex steroids, low gonadotropins). Brain Cu concentration was the only metal correlated with plasma LH (−0.37, n = 30, p < 0.05) and FSH (−0.42, n = 29, p < 0.01). This study demonstrates that sex hormones do not markedly alter brain metal ion homeostasis, unlike previously reported studies of circulating metal ion homeostasis. The role of gonadotropins in regulating metal ion homeostasis does however warrant further study.

Effects of estradiol supplementation on the brain transcriptome of old rhesus macaques maintained on an obesogenic diet

Obesity, the cessation of ovarian steroids with menopause, and age are risk factors for mood disorders, dementia, and Alzheimer's disease (AD). However, immediate hormone therapy (HT) after menopause may have beneficial effects in different brain regions involved in memory and cognition. To more closely replicate the age, endocrine, and metabolic environment of obese postmenopausal women, either on or off HT, middle-aged female rhesus macaques were ovariectomized/hysterectomized (OvH) and maintained on a high-fat, high-sugar, obesogenic Western-style diet (WSD) for 30 months; half of the animals received HT immediately after OvH and half served as placebo controls. RNAseq of the occipital (OC) and prefrontal cortex (PFC), hippocampus (HIP), and amygdala (AMG) identified 293, 379, 505, and 4993 differentially expressed genes (DEGs), respectively. Pathway enrichment analysis identified an activation of neuroinflammation in OC and HIP, but an inhibition in the AMG with HT. Synaptogenesis, circadian rhythm, mitochondrial dysfunction, mTOR, glutamate, serotonin, GABA, dopamine, epinephrine/norepinephrine, glucocorticoid receptor signaling, neuronal NOS, and amyloid processing were exclusively enriched in AMG. As compared to the placebo control group, most of these signaling pathways are downregulated after HT, suggesting a protective effect of HT in OvH females under a WSD. Overall, our results suggest that a chronic obesogenic diet may induce a wide range of alterations in multiple signaling pathways that are linked to age-associated brain pathology and dementia. In these individuals, HT seems to have a protective effect against neuroinflammation, amyloid beta depositions, and tau tangle formation.

The Rhesus Macaque as a Translational Model for Neurodegeneration and Alzheimer's Disease

A major obstacle to progress in understanding the etiology of normative and pathological human brain aging is the availability of suitable animal models for experimentation. The present article will highlight our current knowledge regarding human brain aging and neurodegeneration, specifically in the context of Alzheimer's disease (AD). Additionally, it will examine the use of the rhesus macaque monkey as a pragmatic translational animal model in which to study underlying causal mechanisms. Specifically, the discussion will focus on behavioral and protein-level brain changes that occur within the central nervous system (CNS) of aged monkeys, and compare them to the changes observed in humans during clinically normative aging and in AD.

Lack of effect of short-term DHEA supplementation on the perimenopausal ovary†

Dehydroepiandrosterone (DHEA) hormonal supplementation can improve oocyte quality in women with diminished ovarian function. However, it is unclear whether DHEA supplementation can also enhance ovarian function during the perimenopause (i.e., when the number of follicles in the ovary has undergone a marked reduction). To address this question, we examined the impact of 2.5-months of daily 5-mg oral DHEA supplementation on the number of ovarian follicles and the concentration of anti-Müllerian hormone (AMH) in perimenopausal rhesus macaques. Like women, these long-lived nonhuman primates have ~ 28-day menstrual cycles and eventually undergo menopause. They also show similar age-related neuroendocrine changes, including a marked decrease in circulating concentrations of DHEA and DHEA sulfate (DHEAS). Our experimental design involved the following three groups of animals (N = 6 per group): Young adult (mean age = 11.6 years), Old control (mean age = 23.1 years), and Old DHEA-treated (mean age = 23.5 years). Histological examination of the ovaries revealed a significant age-related decrease in the mean number of primordial follicles despite DHEA supplementation. Moreover, AMH concentrations within the ovaries and circulation, assessed by Western analysis and ELISA, respectively, showed significant age-related decreases that were not attenuated by DHEA supplementation. Taken together, these results fail to show a clear effect of short-term physiological DHEA supplementation on the perimenopausal ovary. However, they do not exclude the possibility that alternative DHEA supplementation paradigms (e.g., involving an earlier start date, longer duration and using pharmacological doses) may extend reproductive potential during aging.

Estradiol Replacement Timing and Obesogenic Diet Effects on Body Composition and Metabolism in Postmenopausal Macaques

Whether hormone replacement therapy has beneficial metabolic effects in postmenopausal women remains controversial because of between-study differences in menopausal duration, estrogen formulations, and diet. Additionally, animal studies have not reflected the typical human obesogenic, Western-style diet (WSD). In this study, we determined the effects of immediate 17β-estradiol (ImE) or delayed 17β-estradiol treatment on weight and metabolism parameters in old ovo-hysterectomized rhesus macaques consuming a WSD over a 30-month period. The placebo and ImE groups exhibited progressive gains in weight and fat mass, which ImE initially attenuated but did not prevent. Progression of insulin resistance (IR) was lessened by ImE compared with placebo under both fasting and IV glucose-stimulated conditions, plateauing in all groups between 24 and 30 months. Consequently, relative euglycemia was maintained through lower stimulated insulin levels with ImE than with placebo. Bone mineral density decreased in the placebo group but was maintained in the ImE group, whereas bone mineral content was unaffected by placebo and increased with ImE. Daily activity was reduced while macaques consumed a WSD and was not affected by ImE. Over time, total cholesterol, triglyceride, very-low-density cholesterol, high-density lipoprotein cholesterol (HDL-C), non-HDL-C, and IL-8 levels increased or trended upward in all animals, with only the change in HDL-C affected by ImE. Delayed estrogen treatment (months 24 to 30) had no significant impact on body composition or glucometabolic parameters. In summary, detrimental WSD-induced changes in body composition and metabolism were only temporarily ameliorated by ImE, with the important exception of glucose homeostasis, which benefited from E replacement even as body composition worsened.

Gene expression profiling of the SCN in young and old rhesus macaques

In mammals, the suprachiasmatic nucleus (SCN) is the location of a master circadian pacemaker. It receives photic signals from the environment via the retinal hypothalamic tract, which play a key role in synchronizing the body's endogenously generated circadian rhythms with the 24-h rhythm of the environment. Therefore, it is plausible that age-related changes within the SCN contribute to the etiology of perturbed activity-rest cycles that become prevalent in humans during aging. To test this hypothesis, we used gene arrays and quantitative RT-PCR to profile age-related gene expression changes within the SCN of male rhesus macaques - a pragmatic translational animal model of human aging, which similarly displays an age-related attenuation of daytime activity levels. As expected, the SCN showed high expression of arginine vasopressin, vasoactive intestinal polypeptide, calbindin and nuclear receptor subfamily 1, group D, member 1 (NR1D1) (also known as reverse strand of ERBA (REV-ERBα), both at the mRNA and protein level. However, no obvious difference was detected between the SCNs of young (7-12 years) and old animals (21-26 years), in terms of the expression of core clock genes or genes associated with SCN signaling and neurotransmission. These data demonstrate the resilience of the primate SCN to normal aging, at least at the transcriptional level and, at least in males, suggest that age-related disruption of activity-rest cycles in humans may instead stem from changes within other components of the circadian system, such as desynchronization of subordinate oscillators in other parts of the body.

Aged Black-and-Gold Howler Monkey Female (Alouatta caraya): A Sign of Reproductive Senescence?

Reproductive senescence patterns have been scarcely studied in Neotropical primates. The few studies available on the hormonal profiles of aging female monkeys indicate that the decline of ovarian function in nonhuman primates may resemble the hormonal events associated with the perimenopause in women. In this study, we explore a reproductive hormone profile of an aged black-and-gold howler monkey female (Alouatta caraya) from a wild population in northeastern Argentina and compare this profile with that of a cycling female in the same population. As part of a larger study, we recorded sociosexual behaviors in adult and subadult females belonging to two groups, and we collected urine (n = 877) to determine the sex hormone profile of each female. These samples were analyzed using enzyme immunoassays for estrone conjugates and pregnanediol-3-glucuronide (PdG). We found differences in mean values of PdG between the younger (cycling) and the older female. These hormone values were lower in the older female, and she did not show any signs of cyclicity for either reproductive hormone. Our results show that the aging female in this wild population shows signs of ovarian senescence, indicated by low, acyclic levels of progesterone metabolites.

Accelerated Episodic Luteinizing Hormone Release Accompanies Blunted Progesterone Regulation in PCOS-like Female Rhesus Monkeys (Macaca Mulatta) Exposed to Testosterone during Early-to-Mid Gestation

BACKGROUND/AIMS

Ovarian theca cell hyperandrogenism in women with polycystic ovary syndrome (PCOS) is compounded by androgen receptor-mediated impairment of estradiol and progesterone negative feedback regulation of episodic luteinizing hormone (LH) release. The resultant LH hypersecretion, likely the product of accelerated episodic release of gonadotropin-releasing hormone (GnRH) from the median eminence of the hypothalamus, hyperstimulates ovarian theca cell steroidogenesis, enabling testosterone (T) and androstenedione excess. Prenatally androgenized (PA) female monkeys exposed to fetal male levels of T during early-to-mid gestation, when adult, demonstrate PCOS-like traits, including high T and LH levels. This study tests the hypothesis that progesterone resistance-associated acceleration in episodic LH release contributes to PA monkey LH excess.

METHODS

A total of 4 PA and 3 regularly cycling, healthy control adult female rhesus monkeys of comparable age and body mass index underwent (1) a 10 h, frequent intravenous sampling assessment for LH episodic release, immediately followed by (2) IV infusion of exogenous GnRH to quantify continuing pituitary LH responsiveness, and subsequently (3) an SC injection of a progesterone receptor antagonist, mifepristone, to examine LH responses to blockade of progesterone-mediated action.

RESULTS

Compared to controls, the relatively hyperandrogenic PA females exhibited ~100% increase (p = 0.037) in LH pulse frequency, positive correlation of LH pulse amplitude (p = 0.017) with androstenedione, ~100% greater increase (p = 0.034) in acute (0-10 min) LH responses to exogenous GnRH, and an absence (p = 0.008) of modest LH elevation following acute progesterone receptor blockade suggestive of diminished progesterone negative feedback.

CONCLUSION

Such dysregulation of LH release in PCOS-like monkeys implicates impaired feedback control of episodic release of hypothalamic GnRH reminiscent of PCOS neuroendocrinopathy.

Age-associated gene expression changes in the arcuate nucleus of male rhesus macaques

The hypothalamic arcuate nucleus (ARC) represents a major component of the neuroendocrine reproductive axis and plays an important role in controlling the onset of puberty as well as age-associated reproductive senescence. Although significant gene expression changes have been observed in the ARC during sexual maturation, it is unclear what changes occur during aging, especially in males. Therefore, in the present study, we profiled the expression of reproduction-related genes in the ARC of young and old male rhesus macaques, as well as old males that had received 6 months of hormone supplementation (HS) in the form of daily testosterone and dehydroepiandrosterone; we also compared morning vs night ARC gene expression in the old males. Using Affymetrix gene microarrays, we found little evidence for age-associated expression changes for genes associated with the neuroendocrine reproductive axis, whereas using qRT-PCR, we detected a similar age-associated decrease in PGR (progesterone receptor) that we previously observed in postmenopausal females. We also detected a sex-steroid-dependent and age-associated decrease in androgen receptor (AR) expression, with highest AR levels being expressed at night (i.e., coinciding with the natural peak in daily testosterone secretion). Finally, unlike previous observations made in females, we did not find a significant age-associated increase in KISS1 (Kisspeptin) or TAC3 (Neurokinin B) expression in the ARC of males, most likely because the attenuation of circulating sex-steroid levels in the males was much less than that in postmenopausal females. Taken together, the data highlight some similarities and differences in ARC gene expression between aged male and female nonhuman primates.

Effect of an obesogenic diet on circadian activity and serum hormones in old monkeys

Like women, old female rhesus macaques undergo menopause and show many of the same age-associated changes, including perturbed activity/rest cycles and altered circulating levels of many hormones. Previous studies showed that administration of an estrogen agonist increased activity in female monkeys, that hormone therapy (HT) increased activity in postmenopausal women and that obesity decreased activity in women. The present study sought to determine if postmenopausal activity and circulating hormone levels also respond to HT when monkeys are fed a high-fat, high-sugar Western style diet (WSD). Old female rhesus macaques were ovo-hysterectomized (OvH) to induce surgical menopause and fed a WSD for 2 years. Half of the animals received estradiol-17β (E), beginning immediately after OvH, while the other half received placebo. Animals in both groups showed an increase in body weight and a decrease in overall activity levels. These changes were associated with a rise in both daytime and nocturnal serum leptin concentrations, but there was no change in serum concentrations of either cortisol or dehydroepiandrosterone sulfate (DHEAS). These data suggest that 2 years of HT has little or no effect on locomotor activity or circadian hormone patterns in menopausal macaques fed an obesogenic diet.

Effect of short-term DHEA supplementation on serum and hippocampal estrogen concentrations in perimenopausal female rhesus macaques

The hippocampus of rhesus macaques expresses genes that encode key enzymes involved in the intracrine conversion of dehydroepiandrosterone (DHEA) to estradiol. Therefore, it is plausible that supplementary DHEA may enhance hippocampal estradiol concentrations and help to compensate for the marked postmenopausal attenuation of circulating estrogen levels. To test this hypothesis, we used LC-MS/MS to measure estradiol and estrone concentrations in the serum and hippocampus of young and old perimenopausal female rhesus macaques, as well as old perimenopausal females that received daily DHEA (5 mg) oral supplementation for 1 week. Despite lower concentrations of these estrogens in the serum of the older animals, their concentrations in the hippocampus did not show any obvious differences due to age or to DHEA supplementation. The results suggest that de novo estrogen synthesis in the brain may compensate for the perimenopausal loss of estrogens in the circulation even without supplemental DHEA.

Progesterone increased β-endorphin innervation of the locus coeruleus, but ovarian steroids had no effect on noradrenergic neurodegeneration

With the decline of ovarian steroids levels at menopause, many women experience an increase in anxiety and stress sensitivity. The locus coeruleus (LC), a central source of noradrenaline (NE), is activated by stress and is inhibited by β-endorphin. Moreover, increased NE has been implicated in pathological anxiety syndromes. Hormone replacement therapy (HRT) in menopause appears to decrease anxiety and vulnerability to stress. Therefore, we questioned the effect of HRT on the inhibitory β-endorphin innervation of the LC. In addition, we found that progesterone protects serotoninergic neurons in monkeys, leading us to question whether ovarian steroids are also neuroprotective in LC neurons in monkeys. Adult Rhesus monkeys (Macaca mulatta) were ovariectomized, and either treated with Silastic capsules that contained estradiol, estradiol+progesterone, progesterone alone or that were empty (ovariectomized; control). After 1month, the LC was obtained and processed for immunohistochemistry for β-endorphin and terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labeling (TUNEL). The density of β-endorphin axons was determined with image analysis using ImageJ. The TUNEL-positive neurons were counted in the entire LC. Progesterone-alone significantly increased the density of the β-endorphin axons in the LC (p<0.01). No significant differences between groups in the number of TUNEL-positive cells in the LC were found. In conclusion, we found that HRT increases the inhibitory influence of β-endorphin in the LC, which could, in turn, contribute to reduce anxiety and increase stress resilience. In addition, we did not find compelling evidence of neurodegeneration or neuroprotection by HRT in the LC of Rhesus monkeys.

Effect of androgen supplementation on 24-hour activity-rest patterns of aged male rhesus macaques

Like elderly men, old male rhesus macaques show attenuated circulating levels of testosterone and dehydroepiandrosterone sulfate, and many of them also show reduced levels of daytime activity. It is unclear, however, if this age-associated behavioral change is causally related to the underlying decrease in circulating androgen levels. To test this possibility, old male rhesus macaques were given daily supplements of testosterone and DHEA for 6 months, designed to mimic the mean 24-hour circulating hormone patterns of young adults. Compared with the young adults, the old controls showed attenuated daytime activity levels. However, there was no difference between the androgen-supplemented old animals and the aged-matched controls, even after 6 months of treatment. The data suggest that age-associated decreases in circulating androgen levels are unlikely to be a primary reason for altered activity-rest patterns in elderly men, and that androgen supplementation paradigms might not provide any obvious therapeutic benefit.

Does the degree of endocrine dyscrasia post-reproduction dictate post-reproductive lifespan? Lessons from semelparous and iteroparous species

Post-reproductive lifespan varies greatly among species; human post-reproductive lifespan comprises ~30-50% of their total longevity, while semelparous salmon and dasyurid marsupials post-reproductive lifespan comprises <4% of their total longevity. To examine if the magnitude of hypothalamic-pituitary-gonadal (HPG) axis dyscrasia at the time of reproductive senescence determines post-reproductive lifespan, we examined the difference between pre- and post-reproductive (1) circulating sex hormones and (2) the ratio of sex steroids to gonadotropins (e.g., 17β-estradiol/follicle-stimulating hormone (FSH)), an index of the dysregulation of the HPG axis and the level of dyotic (death) signaling post-reproduction. Animals with a shorter post-reproductive lifespan (<4% total longevity) had a more marked decline in circulating sex steroids and corresponding elevation in gonadotropins compared to animals with a longer post-reproductive lifespan (30-60% total longevity). In semelparous female salmon of short post-reproductive lifespan (1%), these divergent changes in circulating hormone concentration post-reproduction equated to a 711-fold decrease in the ratio of 17β-estradiol/FSH between the reproductive and post-reproductive periods. In contrast, the decrease in the ratio of 17β-estradiol/FSH in iteroparous female mammals with long post-reproductive lifespan was significantly less (1.7-34-fold) post-reproduction. Likewise, in male semelparous salmon, the decrease in the ratio of testosterone/FSH (82-fold) was considerably larger than for iteroparous species (1.3-11-fold). These results suggest that (1) organisms with greater reproductive endocrine dyscrasia more rapidly undergo senescence and die, and (2) the contribution post-reproduction by non-gonadal (and perhaps gonadal) tissues to circulating sex hormones dictates post-reproductive tissue health and longevity. In this way, reproduction and longevity are coupled, with the degree of non-gonadal tissue hormone production dictating the rate of somatic tissue demise post-reproduction and the differences in post-reproductive lifespans between species.

Circulating anti-Müllerian hormone concentrations in relation to age and season in male and female beluga (Delphinapterus leucas)

The present study aimed to describe serum anti-Müllerian hormone (AMH) patterns of ex situ male and female beluga to examine the influence of age (divided into 5-year categories) or sexual maturation and reproductive season. In males aged 5–9 years, AMH concentrations were significantly (P < 0.05) higher than those in all age categories exceeding 15 years and were not influenced by season (P = 0.57). AMH concentrations in females peaked in the 5–9-year age category during the breeding season and decreased (P < 0.05) after 9 years of age. Aged females displayed lower (P < 0.05) AMH concentrations than immature and mature animals and immature females secreted higher concentrations than mature animals (P = 0.03). For mature females, seasonal differences (P = 0.02) in AMH concentrations were detected, with females in the breeding season displaying higher AMH concentrations than in the non-breeding season. This is the first time AMH has been characterised in a cetacean species and the first potential hormonal evidence of reproductive senescence in beluga. Further research is required to determine if this hormone can be used as a predictor of fertility for the species.

An overview of nonhuman primates in aging research

A graying human population and the rising costs of healthcare have fueled the growing need for a sophisticated translational model of aging. Nonhuman primates (NHPs) experience aging processes similar to humans and, as a result, provide an excellent opportunity to study a closely related species. Rhesus monkeys share >92% homology and are the most commonly studied NHP. However, their substantial size, long lifespan, and the associated expense are prohibitive factors. Marmosets are rapidly becoming the preferred NHP for biomedical testing due to their small size, low zoonotic risk, reproductive efficiency, and relatively low-cost. Both species experience age-related pathology similar to humans, such as cancer, diabetes, arthritis, cardiovascular disease, and neurological decline. As a result, their use in aging research is paving the way to improved human health through a better understanding of the mechanisms of aging.

Cognition in aged rhesus monkeys: effect of DHEA and correlation with steroidogenic gene expression

Estradiol supplementation has been shown to enhance cognitive performance in old ovariectomized rhesus macaques (Macaca mulatta). To determine if similar benefits could be achieved in perimenopausal animals using alternative hormonal supplements, we administered dehydroepiandrosterone (DHEA) to old ovary-intact female rhesus macaques for ∼2.5 months. Using computerized touch screen memory tasks, including delayed response (DR) and delayed matching-to-sample (DMS), we observed improved performance with time in all of the animals but failed to detect a significant effect of DHEA. On the other hand, gene expression profiling disclosed a significant correlation between cognitive performance and the expression of several steroidogenic and steroid-responsive genes. The DR performance was positively correlated with hippocampal expression of AKR1C3 and STAR and negatively correlated with the expression of SDRD5A1. A positive correlation was also found between DMS performance and prefrontal cortical expression of AKR1C3 and a negative correlation with STAR, as well as a negative correlation with the hippocampal expression of HSD11B1 and NR3C1. Taken together, the results suggest that steroidogenic gene regulation within the brain may help to maintain cognitive function during the perimenopausal transition period, despite a decline in sex-steroid levels in the circulation.

Effect of Ovarian Hormone Therapy on Cognition in the Aged Female Rhesus Macaque

Studies of the effect of hormone therapy on cognitive function in menopausal women have been equivocal, in part due to differences in the type and timing of hormone treatment. Here we cognitively tested aged female rhesus macaques on (1) the delayed response task of spatial working memory, (2) a visuospatial attention task that measured spatially and temporally cued reaction times, and (3) a simple reaction time task as a control for motor speed. After task acquisition, animals were ovariectomized (OVX). Their performance was compared with intact controls for 2 months, at which time no group differences were found. The OVX animals were then assigned to treatment with either a subcutaneous sham implant (OVX), 17-β estradiol (E) implant (OVX+E) or E implant plus cyclic oral progesterone (OVX+EP). All groups were then tested repeatedly over 12 months. The OVX+E animals performed significantly better on the delayed response task than all of the other groups for much of the 12 month testing period. The OVX+EP animals also showed improved performance in the delayed response task, but only at 30 s delays and with performance levels below that of OVX+E animals. The OVX+E animals also performed significantly better in the visuospatial attention task, particularly in the most challenging invalid cue condition; this difference also was maintained across the 12 month testing period. Simple reaction time was not affected by hormonal manipulation. These data demonstrate that chronic, continuous administration of E can exert multiple beneficial cognitive effects in aged, OVX rhesus macaque females.

SIGNIFICANCE STATEMENT

Hormone therapy after menopause is controversial. We tested the effects of hormone replacement in aged rhesus macaques, soon after surgically-induced menopause [ovariectomy (OVX)], on tests of memory and attention. Untreated ovarian-intact and OVX animals were compared with OVX animals receiving estradiol (E) alone or E with progesterone (P). E was administered in a continuous fashion via subcutaneous implant, whereas P was administered orally in a cyclic fashion. On both tests, E-treated animals performed better than the other 3 experimental groups across 1 year of treatment. Thus, in this monkey model, chronic E administered soon after the loss of ovarian hormones had long-term benefits for cognitive function.

Endocrine assessment of ovarian cycle activity in wild female mountain gorillas (Gorilla beringei beringei)

Variability of fertility (i.e. number of births per female per year) has been reported in females of many primate species but only a few studies have explored the associated physiological and behavioral patterns. To investigate the proximate mechanisms of variability in fertility of wild female mountain gorillas (Gorilla beringei beringei), we quantified the occurrence of ovulation, matings, and successful pregnancies among females. We examined the profiles of immunoreactive pregnanediol-3-glucuronide (iPdG) for sixteen females (seven nulliparous and nine parous females, including one geriatric female; average sampling period for fecal sample collection and behavioral observations per female=175 days; SD=94 days, range=66-358 days) monitored by the staff of the Dian Fossey Gorilla Fund's Karisoke Research Center in Parc National des Volcans, Rwanda. We quantified ovarian cycles from iPdG profiles using an algorithm that we developed by adjusting the method of Kassam et al. (1996) to the characteristics of ovarian cycle profiles based on fecal hormone measurements. The mean length of ovarian cycles was 29±4 days (median: 28 days, N=13 cycles), similar to ovarian cycle lengths of other great apes and humans. As expected, we found that female mountain gorillas exhibit longer follicular phases (mean±SD: 21±3 days, N=13 cycles) than luteal phases (mean±SD: 8±3 days, N=13 cycles). We also found that the frequency of ovarian cycles was greater in parous females (i.e. 20 ovarian cycles across 44 periods of 28 days; 45.5%) than in nulliparous females (i.e. two ovarian cycles across 34 periods of 28 days; 6%). However, the frequency of days on which matings were observed did not differ significantly between parous and nulliparous females, nor between pregnant and non-pregnant females. Five pregnancies were detected with iPdG levels, but only three resulted in live births, indicating miscarriages of the other two. In sum, this study provides information on the underlying endocrine patterns of variation in fertility depending on parity, mating behavior, and pregnancy success in a critically endangered great ape.

Ovarian steroids regulate gene expression in the dorsal raphe of old female macaques

With extended life spans in modern humans, menopause has become a significant risk factor for depression, anxiety, loss of cognitive functions, weight gain, metabolic disease, osteoporosis, cardiovascular disease, and neurodegenerative diseases. Clinical studies have found beneficial neural effects of ovarian steroid hormone therapy (HT) during the menopausal transition and data are emerging that it can be continued long term. To further understand molecular underpinnings of the clinical studies, we used quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR) to examine gene expression in the serotonergic dorsal raphe of old (>18 years) rhesus macaques, focusing on genes related to depression, cellular resilience, and neurodegenerative diseases. The animals were ovariectomized (Ovx, surgically menopausal) and subjected to either estradiol or estradiol plus progesterone HT, or to placebo, starting 2 months after Ovx and continuing for ∼ 4 years. Significant changes were observed in 36 of 48 genes examined that encode proteins supporting serotonin neurotransmission, synapse assembly, glutamate neurotransmission, DNA repair, chaperones, ubiquinases and transport motors, as well as genes encoding proteins that have potential to delay the onset of neuropathologies. The data reveal important gene targets for chronic HT that contribute to neural health. Alternatively, the loss of ovarian steroids may lead to loss of functions at the gene level that contribute to many of the observable neural deficits after menopause.

Characterization of the Genital Microenvironment of Female Rhesus Macaques Prior to and After SIV Infection

PROBLEM

HIV infection among women is frequently modeled in female rhesus macaques. Longitudinal studies on genital compartment and hormonal factors that can influence susceptibility to SIV infection are lacking in this animal model.

METHOD OF STUDY

Genital specimens and menstruation of indoor-housed female rhesus macaques were analyzed prior to and after SIV infection.

RESULTS

Median menstrual cycle length averaged 27 days, although highly variable cycle lengths and frequent periods of amenorrhea were observed during summer months. The vaginal microbiota, characterized by adapted Nugent scoring, showed predominance of small Gram-variable rods and Gram-positive cocci. Highly variable vaginal cytokine levels were observed pre- and post-SIV infection. Vaginal viral loads correlated with plasma viral loads, but were not associated with progesterone levels.

CONCLUSION

These results provide an integrated characterization of important factors in the vaginal microenvironment that are relevant to the experimental design of HIV prevention and transmission studies in female rhesus macaques.

Neurocognitive effects of estrogens across the adult lifespan in nonhuman primates: State of knowledge and new perspectives

This article is part of a Special Issue "Estradiol and cognition". This review discusses the unique contribution of nonhuman primate research to our understanding of the neurocognitive effects of estrogens throughout the adult lifespan in females. Mounting evidence indicates that estrogens affect many aspects of hippocampal, prefrontal and cholinergic function in the primate brain and the underlying mechanisms are beginning to be elucidated. In addition, estrogens may also influence cognitive function indirectly, via the modulation of other systems that impact cognition. We will focus on the effects of estrogens on sleep and emphasize the need for primate models to better understand these complex interactions. Continued research with nonhuman primates is essential for the development of therapies that are optimal for the maintenance of women's cognitive health throughout the lifespan.

Effects of Age and Estradiol on Gene Expression in the Rhesus Macaque Hypothalamus

BACKGROUND

The hypothalamus plays a key role in mediating the effects of estrogen on many physiological functions, including reproduction, metabolism, and thermoregulation. We have previously observed marked estrogen-dependent gene expression changes within the hypothalamus of rhesus macaques during aging, especially in the KNDy neurons of the arcuate-median eminence (ARC-ME) that produce kisspeptin, neurokinin B, and dynorphin A. Little is known, however, about the mechanisms involved in mediating the feedback from estrogen onto these neurons.

METHODS

We used quantitative real-time PCR to profile age- and estrogen-dependent gene expression changes in the rhesus macaque hypothalamus. Our focus was on genes that encode steroid receptors (ESR1, ESR2, PGR, and AR) and on enzymes that contribute to the local synthesis of 17β-estradiol (E2; STS, HSD3B1/2, HSD17B5, and CYP19A). In addition, we used RT(2) Profiler™ PCR Arrays to profile a larger set of genes that are integral to hypothalamic function.

RESULTS

KISS1, KISS1R, TAC3, and NPY2R mRNA levels increased in surgically menopausal (ovariectomized) old females relative to age-matched ovariectomized animals that received E2 hormone therapy. In contrast, PGR, HSD17B, GNRH2, SLC6A3, KISS1, TAC3, and NPY2R mRNA levels increased after E2 supplementation.

CONCLUSION

The rhesus macaque ARC-ME expresses many genes that are responsive to changes in circulating estrogen levels, even during old age, and these may contribute to causing the normal and pathophysiological changes that occur during menopause.

Why primate models matter

Research involving nonhuman primates (NHPs) has played a vital role in many of the medical and scientific advances of the past century. NHPs are used because of their similarity to humans in physiology, neuroanatomy, reproduction, development, cognition, and social complexity-yet it is these very similarities that make the use of NHPs in biomedical research a considered decision. As primate researchers, we feel an obligation and responsibility to present the facts concerning why primates are used in various areas of biomedical research. Recent decisions in the United States, including the phasing out of chimpanzees in research by the National Institutes of Health and the pending closure of the New England Primate Research Center, illustrate to us the critical importance of conveying why continued research with primates is needed. Here, we review key areas in biomedicine where primate models have been, and continue to be, essential for advancing fundamental knowledge in biomedical and biological research.

Androgen supplementation during aging: development of a physiologically appropriate protocol

Men show an age-related decline in the circulating levels of testosterone (T) and dehydroepiandrosterone sulfate (DHEAS). Consequently, there is interest in developing androgen supplementation paradigms for old men that replicate the hormone profiles of young adults. In the present study, we used old (21-26 years old) male rhesus monkeys as a model to examine the efficacy of an androgen supplementation paradigm that comprised oral T administration (12 mg/kg body weight, dissolved in sesame oil/chocolate) in the evening, and two oral DHEA administrations, 3 hr apart (0.04 mg/kg body weight, dissolved in sesame oil/chocolate) in the morning. After 5 days of repeated hormone supplementation, serial blood samples were remotely collected from each animal hourly across the 24-hr day, and assayed for cortisol, DHEAS, T, 5α-dihydrotestosterone (DHT), estrone (E1), and 17β-estradiol (E2). Following androgen supplementation, T levels were significantly elevated and this was associated with a more sustained nocturnal elevation of T's primary bioactive metabolites, DHT and E1 and E2. Plasma DHEAS levels were also significantly elevated after androgen supplementation; DHEAS levels rose in the early morning and gradually declined during the course of the day, closely mimicking the profiles observed in young adults (7-12 years old); in contrast, cortisol levels were unaltered by the supplementation. Together the data demonstrate a non-invasive androgen supplementation paradigm that restores youthful circulating androgen levels in old male primates. Because this paradigm preserves the natural circulating circadian hormone patterns, we predict that it will produce fewer adverse side effects, such as perturbed sleep or cognitive impairment.

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.

The relevance of age in female human reproduction--current situation in Switzerland and pathophysiological background from a comparative perspective

During recent years women tend to postpone childbirth to ages when fertility declines. Consequently, an increasing number of women experiences reproductive difficulties and seeks help by assisted reproductive techniques (ART). To investigate the dynamics of age-related fertility in Switzerland we evaluated data from the nationwide FIVNAT-CH statistics on ART as well as from a subsample receiving ART at the Division of Reproductive Endocrinology, University Hospital Zurich. Since 2000 the average age of women receiving ART increased from 34.9 to 36.3 years in Switzerland and since 2006 numbers of annual ART cycles augmented steadily. The percentage of women ≥40 increased from 17.2% in 2007 to 19.6% in 2011. In the Zurich cohort AMH, the number of oocytes retrieved, the number of fertilized oocytes with two pronuclei, the number of embryos with an adequate cell number, clinical pregnancy rates as well as life birth/ongoing pregnancy rates were lower in the age group ≥40 years, especially when compared to 33 year-old women. In the nationwide sample pregnancy rates decreased from about 45% at the age of 30 to less than 3% at the age of 45; delivery rates declined from about 38% to nearly 0%. In the Zurich cohort percentages of clinical pregnancies declined from 46% in women ≤34 years to 21% in women ≥40 years. In the national sample as well as in the Zurich cohort the percentage of miscarriages increased dramatically from 15.4% and 22% in women ≤34 years to 38.6% and 33% in women ≥40 years, respectively. Even in a country with high health standards such as Switzerland fertility is declining with age and ART does not succeed to improve reduced fertility. Rodent and primate models enrich our knowledge on the pathophysiological mechanisms underlying reproductive senescence. As non-infertility specialist physicians as well as the general public are not sufficiently aware of the dramatic reduction of chances for life births in women ≥40 years, medical counseling as well as schools and media should support the distribution of information future parents need for successful family planning.

Premenopausal antimüllerian hormone concentration is associated with subsequent atherosclerosis

OBJECTIVE

The aim of this study was to determine if premenopausal ovarian reserve is associated with susceptibility for atherosclerosis.

METHODS

Female cynomolgus macaques (n = 66, women's equivalent age = 45 y) consumed an atherogenic diet for ∼5 months before the measurement of a marker of ovarian reserve (antimüllerian hormone [AMH]), plasma lipids, follicular phase estradiol, and body weight (BW). Monkeys were then ovariectomized (OVX; n = 17), remained premenopausal (n = 20), or were induced to have reduced ovarian reserve (ROR, n = 29). After 26 additional months consuming the diet, atherosclerosis measurements and risk variables were reassessed.

RESULTS

No differences in baseline AMH, plasma lipids, BW, and estradiol or postdiet lipids and BW were observed among the groups subsequently assigned to the OVX, premenopausal control, or reduced ovarian reserve conditions. Postdiet measurements of atherosclerosis extent did not differ among the groups. However, analysis of plaque size by tertile of baseline AMH revealed that plaques were largest in monkeys that began the experiment with the lowest baseline AMH, followed by those in the middle and high tertiles (plaque extent: low AMH, 0.76 ± 0.12 mm; mid AMH, 0.46 ± 0.1 mm; high AMH, 0.34 ± 0.08 mm; P = 0.02). Baseline AMH and plaque size were also correlated negatively (r = -0.31, P = 0.01). Plasma lipids were also correlated significantly with plaque extent (all P < 0.01) but not with AMH.

CONCLUSIONS

We report for the first time an inverse relationship between a marker of ovarian reserve (AMH) and subsequent atherosclerosis risk.

Age-related changes in neuroendocrine rhythmic function in the rhesus macaque

Many environmental conditions show rhythmic changes across the 24-h day; these include changes in light intensity, ambient temperature, food availability, and presence or absence of predators. Consequently, many organisms have developed corresponding adaptations, which ensure that specific physiological and behavioral events occur at an appropriate time of the day. In mammals, the underlying mechanism responsible for synchronizing internal biochemical processes with circadian environmental cues has been well studied and is thought to comprise three major components: (1) photoreception by the retina and transmission of neural signals along the retinohypothalamic tract, (2) integration of photoperiodic information with an internal reference circadian pacemaker located in the suprachiasmatic nucleus, and (3) dissemination of circadian information to target organs, via the autonomic nervous system and through humoral pathways. Given the importance that neuroendocrine rhythms play in coordinating normal circadian physiology and behavior, it is plausible that their perturbation during aging contributes to the etiology of age-related pathologies. This mini-review highlights some of the most dramatic rhythmic neuroendocrine changes that occur in primates during aging, focusing primarily on data from the male rhesus macaques (Macaca mulatta). In addition to the age-associated attenuation of hormone levels and reduction of humoral circadian signaling, there are also significant age-related changes in intracrine processing enzymes and hormone receptors which may further affect the functional efficacy of these hormones. Rhesus macaques, like humans, are large diurnal primates and show many of the same physiological and behavioral circadian changes during aging. Consequently, they represent an ideal translational animal model in which to study the causes and consequences of age-associated internal circadian disruption and in which to evaluate novel therapies.

Hormone supplementation during aging: how much and when?

Circulating levels of dehydroepiandrosterone, a major adrenal steroid, show a marked age-related decrease in both humans and nonhuman primates. Because this decrease has been implicated in age-related cognitive decline, we administered supplementary dehydroepiandrosterone to perimenopausal rhesus macaques (Macaca mulatta) to test for cognitive benefits. Although recognition memory improved, there was no benefit to spatial working memory. To address the limitations of this study we developed a hormone supplementation regimen in aged male macaques that more accurately replicates the 24-hr androgen profiles of young animals. We hypothesize that this more comprehensive physiological hormone replacement paradigm will enhance cognitive function in the elderly.

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.

Neuroendocrine control of the transition to reproductive senescence: lessons learned from the female rodent model

The natural transition to reproductive senescence is an important physiological process that occurs with aging, resulting in menopause in women and diminished or lost fertility in most mammalian species. This review focuses on how rodent models have informed our knowledge of age-related changes in gonadotropin-releasing hormone (GnRH) neurosecretory function and the subsequent loss of reproductive capacity. Studies in rats and mice have shown molecular, morphological and functional changes in GnRH cells. Furthermore, during reproductive aging altered sex steroid feedback to the hypothalamus contributes to a decrease of stimulatory signaling and increase in inhibitory tone onto GnRH neurons. At the site of the GnRH terminals where the peptide is released into the portal vasculature, the cytoarchitecture of the median eminence becomes disorganized with aging, and mechanisms of glial-GnRH neuronal communication may be disrupted. These changes can result in the dysregulation of GnRH secretion with reproductive decline. Interestingly, reproductive aging effects on the GnRH circuitry are observed in middle age even prior to any obvious physiological changes in cyclicity. We speculate that the hypothalamus may play a critical role in this mid-life transition. Because there are substantial species differences in these aging processes, we also compare and contrast rodent aging to that in primates. Work discussed herein shows that in order to understand neuroendocrine mechanisms of reproductive senescence, further research needs to be conducted in ovarian-intact models.

Differential roles of GnRH-I and GnRH-ii neurons in the control of the primate reproductive axis

In vertebrates, gonadotropin-releasing hormone (GnRH) represents the primary neuroendocrine link between the brain and the reproductive axis, and in some species up to three different forms of GnRH have been detected. Until recently, it had been assumed that humans and non-human primates only express one form (GnRH-I), but it is now clear they also express a second form (GnRH-II). GnRH-II, like GnRH-I, is highly effective at stimulating gonadotropin release, both in vitro and in vivo, but the neurons that produce GnRH-II are completely distinct from those producing GnRH-I. Moreover, GnRH-II and GnRH-I producing neurons respond very differently to estradiol; specifically, estradiol stimulates GnRH-II gene expression in the former and inhibit GnRH-I gene expression in the latter. Consequently, the negative feedback action of estradiol may be mediated exclusively by the subpopulation of GnRH neurons that express GnRH-I, while the positive feedback action may be mediated exclusively by the subpopulation that expresses GnRH-II. Taken together, these findings raise the possibility that two completely different GnRH neuronal systems participate in the control of primate reproductive physiology. The primary role of GnRH-I neurons is likely to be focused on the maintenance and modulation of tonic pulsatile LH release, whereas the primary role of GnRH-II neurons is likely to be focused on the generation of the preovulatory LH surge. This functional segregation of the primate neuroendocrine reproductive axis lends itself for novel targeted approaches to fertility control and for treatment of human reproductive disorders.

Accelerated immune senescence and reduced response to vaccination in ovariectomized female rhesus macaques

Aging is associated with a general dysregulation in immune function, commonly referred to as "immune senescence". Several studies have shown that female sex steroids can modulate the immune response. However, the impact of menopause-associated loss of estrogen and progestins on immune senescence remains poorly understood. To help answer this question, we examined the effect of ovariectomy on T-cell homeostasis and function in adult and aged female rhesus macaques. Our data show that in adult female rhesus macaques, ovariectomy increased the frequency of naïve CD4 T cells. In contrast, ovariectomized (ovx) aged female rhesus macaques had increased frequency of terminally differentiated CD4 effector memory T cells and inflammatory cytokine-secreting memory T cells. Moreover, ovariectomy reduced the immune response (T-cell cytokine and IgG production) following vaccination with modified vaccinia ankara in both adult and aged female rhesus macaques compared to ovary-intact age-matched controls. Interestingly, hormone therapy (estradiol alone or in conjunction with progesterone) partially improved the T-cell response to vaccination in aged ovariectomized female rhesus macaques. These data suggest that the loss of ovarian steroids, notably estradiol and progesterone, may contribute to reduced immune function in post-menopausal women and that hormone therapy may improve immune response to vaccination in this growing segment of the population.

Perimenopausal regulation of steroidogenesis in the nonhuman primate

Human aging is characterized by a marked decrease in circulating levels of dehydroepiandrosterone (DHEA) and DHEA-sulfate (DHEAS), hormonal changes associated with cognitive decline. Despite beneficial effects of DHEA supplementation in rodents, studies in elderly humans have generally failed to show cognitive improvement after treatment. In the present study we evaluate the effects of age and estradiol supplementation on expression of genes involved in the de novo synthesis of DHEA and its conversion to estradiol in the rhesus macaque hippocampus. Using reverse transcription polymerase chain reaction (RT-PCR) we demonstrate the expression of genes associated with this synthesis in several areas of the rhesus brain. Furthermore, real-time PCR reveals an age-related attenuation of hippocampal expression level of the genes CYP17A1, STS, and 3BHSD1/2. Additionally, short-term administration of estradiol is associated with decreased expression of CYP17A1, STS, SULT2B1, and AROMATASE, consistent with a downregulation not only of estrogen synthesis from circulating DHEA, but also of de novo DHEA synthesis within the hippocampus. These findings suggest a decline in neurosteroidogenesis may account for the inefficacy of DHEA supplementation in elderly humans, and that central steroidogenesis may be a function of circulating hormones and menopausal status.

Role of circadian neuroendocrine rhythms in the control of behavior and physiology

Hormones play a major role in regulating behavior and physiology, and their efficacy is often dependent on the temporal pattern in which they are secreted. Significant insights into the mechanisms underlying rhythmic hormone secretion have been gained from transgenic rodent models, suggesting that many of the body's rhythmic functions are regulated by a coordinated network of central and peripheral circadian pacemakers. Some neuroendocrine rhythms are driven by transcriptional-posttranslational feedback circuits comprising 'core clock genes', while others represent a cyclic cascade of neuroendocrine events. This review focuses on recent data from the rhesus macaque, a non-human primate model with high clinical translation potential. With primary emphasis on adrenal and gonadal steroids, it illustrates the rhythmic nature of hormone secretion, and discusses the impact that fluctuating hormone levels have on the accuracy of clinical diagnoses and on the design of effective hormone replacement therapies in the elderly. In addition, this minireview raises awareness of the rhythmic expression patterns shown by many genes, and discusses how this could impact interpretation of data obtained from gene profiling studies, especially from nocturnal rodents.

The development of small primate models for aging research

Nonhuman primate (NHP) aging research has traditionally relied mainly on the rhesus macaque. But the long lifespan, low reproductive rate, and relatively large body size of macaques and related Old World monkeys make them less than ideal models for aging research. Manifold advantages would attend the use of smaller, more rapidly developing, shorter-lived NHP species in aging studies, not the least of which are lower cost and the ability to do shorter research projects. Arbitrarily defining "small" primates as those weighing less than 500 g, we assess small, relatively short-lived species among the prosimians and callitrichids for suitability as models for human aging research. Using the criteria of availability, knowledge about (and ease of) maintenance, the possibility of genetic manipulation (a hallmark of 21st century biology), and similarities to humans in the physiology of age-related changes, we suggest three species--two prosimians (Microcebus murinus and Galago senegalensis) and one New World monkey (Callithrix jacchus)--that deserve scrutiny for development as major NHP models for aging studies. We discuss one other New World monkey group, Cebus spp., that might also be an effective NHP model of aging as these species are longer-lived for their body size than any primate except humans.

The impact of age-related ovarian hormone loss on cognitive and neural function

On average, women now live one-third of their lives after menopause. Because menopause has been associated with an elevated risk of dementia, an increasing body of research has studied the effects of reproductive senescence on cognitive function. Compelling evidence from humans, nonhuman primates, and rodents suggests that ovarian sex-steroid hormones can have rapid and profound effects on memory, attention, and executive function, and on regions of the brain that mediate these processes, such as the hippocampus and prefrontal cortex. This chapter will provide an overview of studies in humans, nonhuman primates, and rodents that examine the effects of ovarian hormone loss and hormone replacement on cognitive functions mediated by the hippocampus and prefrontal cortex. For humans and each animal model, we outline the effects of aging on reproductive function, describe how ovarian hormones (primarily estrogens) modulate hippocampal and prefrontal physiology, and discuss the effects of both reproductive aging and hormone treatment on cognitive function. Although this review will show that much has been learned about the effects of reproductive senescence on cognition, many critical questions remain for future investigation.

The hypothalamic median eminence and its role in reproductive aging

The median eminence at the base of the hypothalamus serves as an interface between the neural and peripheral endocrine systems. It releases hypothalamic-releasing hormones into the portal capillary bed for transport to the anterior pituitary, which provides further signals to target endocrine systems. Of specific relevance to reproduction, a group of about 1000 neurons in mammals release the gonadotropin-releasing hormone (GnRH) peptide from neuroterminals in the median eminence. During the life cycle, there are dramatic changes in reproductive demands, and we focus this review on how GnRH terminals in the median eminence change during reproductive senescence. We discuss morphological and functional properties of the median eminence, and how relationships among GnRH terminals and their microenvironment of nerve terminals, glial cells, and the portal capillary vasculature determine the ability of GnRH peptide to be secreted and to reach its target in the anterior pituitary gland.

Mechanisms of reproductive aging: conserved mechanisms and environmental factors

The interplay of neuroendocrine processes and gonadal function is exquisitely expressed during aging. In females, loss of ovarian function results in decreased circulating estradiol. As a result, estrogen-dependent endocrine and behavioral responses decline, including impaired cognitive function reflecting the impact of declining estrogen on the hippocampus circuits, and decreased metabolic endocrine function. Concurrently, age-related changes in neuroendocrine response also contribute to the declining reproductive function. Our session considered key mechanisms in reproductive aging including the roles of ovarian function (Finch and Holmes) and the hypothalamic median eminence (Yin and Gore) with an associated age-related cognitive decline that accompanies estrogen loss (Morrison and colleagues). Effects of smoking, obesity, and insulin resistance (Sowers and colleagues) impact the timing of the perimenopause transition in women. Animal models provide excellent insights into conserved mechanisms and key overarching events that bring about endocrine and behavioral aging. Environmental factors are key triggers in timing endocrine aging with implications for eventual disease. Session presentations will be considered in the context of the broader topic of indices and predictors of aging-related change.

Reproductive aging and risk for chronic disease: Insights from studies of nonhuman primates

Reproductive aging and ovarian senescence have considerable public health relevance because they are associated with increased risk for coronary heart disease (CHD), osteoporosis and other degenerative conditions including cognitive decline and potentially the metabolic syndrome. It has been suggested that the hormonal dysregulation that occurs during the perimenopausal transition may play a role in the initiation of pathobiological changes (e.g., adverse lipid profiles, atherosclerotic plaques) that will increase risk for chronic disease (e.g., CHD) during the postmenopausal years. Moreover, these early changes are suspected to establish a trajectory of disease progression that may be difficult to alter if interventions are not begun until after menopause. Even a slight increase in the rate of disease progression during the pre- or perimenopausal years could have substantial consequences for health and quality of life over the postmenopausal lifespan. Thus, the years leading up to menopause may offer a "critical window" for interventions aimed at reducing the postmenopausal disease burden. The relationship between perimenopausal hormonal dysregulation and the risk for chronic disease is poorly understood due, in large part, to the lack of appropriate animal models of the perimenopausal transition and natural menopause. In this review we assesses studies of nonhuman primates (NHPs) evaluated in various reproductive stages (naturally pre-, peri- and postmenopausal, surgically menopausal) and their contribution to our understanding about risk factors for chronic disease. Finally, because large numbers of naturally perimenopausal and menopausal NHPs are not available for research at present, experimental approaches that have the potential to hasten the onset of the perimenopausal transition will be described.

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.

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.