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Biol Reprod 2006, 10.1095/biolreprod.106.051839
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BIOLOGY OF REPRODUCTION 75, 539–546 (2006)
DOI: 10.1095/biolreprod.106.051839
© 2006 by the Society for the Study of Reproduction, Inc.

Research Article

Neuroendocrine Changes in the Aging Reproductive Axis of Female Rhesus Macaques (Macaca mulatta)1

Jodi L. Downs 34 , and Henryk F. Urbanski 2 345  

Division of Neuroscience,3 Oregon National Primate Research Center, Beaverton, Oregon 97006 Departments of Physiology & Pharmacology4 Behavioral Neuroscience,5 Oregon Health & Science University, Portland, Oregon 97239

ABSTRACT

Femalerhesus macaques show monthly menstrual cycles and eventually enter menopause at approximately 25 yr of age. To help identify early biomarkers of menopause in this nonhuman primate, we monitored reproductive hormones longitudinally from aged female macaques during the transitions from premenopause to perimenopause and postmenopause and found that, indeed, elevated plasma FSH was a better predictive factor of menopause onset than age. In a second experiment, we compared reproductive hormone profiles of young adult macaques (8–10 yr old) with those of regularly cycling old macaques (approximately 24 yr old). Indwelling vascular catheters were used for remote blood collection for at least 100 consecutive days, thereby covering three complete menstrual cycles in each macaque. Plasma levels of estradiol, progesterone, LH, FSH, follicular phase inhibin B, and anti-müllerian hormone (AMH) were determined during each menstrual cycle and were averaged for each animal; group mean differences were analyzed using one-way ANOVA. Old premenopausal macaques showed regular menstrual cycles that were qualitatively indistinguishable from those of young macaques; peak plasma levels of estradiol, progesterone, and LH were not significantly different. In marked contrast, peak plasma FSH concentrations were significantly higher, while inhibin B and AMH levels were generally lower, in the old premenopausal macaques compared with those in the young macaques. These data provide further evidence that rhesus macaques serve as an excellent model to study underlying mechanisms of human menopause. Furthermore, the data suggest that an age-related change in FSH, inhibin B, and AMH secretion may be the first endocrine manifestation of the transition into perimenopause, potentially having value in predicting the onset of the perimenopausal transition.

aging, follicle-stimulating hormone, inhibin, menstrual cycle, neuroendocrinology

INTRODUCTION

Although the mean life span of humans continues to increase, the mean age at which women begin to transition into menopause remains at 45.5–47.5 yr, a process that lasts for about 4 yr [1–5]. Therefore, women can expect to spend a great proportion of their lives in the postmenopausal state. The menopause transition is a naturally occurring and inevitable process, but the associated decrease in circulating concentrations of sex steroids may cause women to develop various physiological abnormalities, including decreased bone mineral density [6–9], cognitive decline [10–17], increased risk of cardiovascular disease [18–21], and attenuated immune function [22–24]. Moreover, because menopause has a variable age of onset and because it is clinically diagnosed only after 12 mo of amenorrhea, women may unknowingly be exposed to highly attenuated levels of circulating sex steroids for an extended period before deciding to seek treatment. The recent disclosure by the Women's Health Initiative (WHI) of increased health risks in women who began hormone replacement therapy (HRT) after already being postmenopausal further emphasizes the need for early detection of transition from the premenopausal condition to the perimenopausal condition [25–27].

Depletion of follicles from the ovaries and the associated decline in circulating estradiol concentrations are traditionally recognized as being the ultimate markers of menopause, but they have limited predictive value. Instead, there is a need for identification of more overt proximate biomarkers that can reliably predict the earliest stages of transition from premenopause to perimenopause. Having the capability to identify the onset of this transition would enable women to initiate HRT before they experience a significant loss of circulating estradiol concentrations and, perhaps, to increase the health benefit-to-risk ratio of HRT. There is also need for a better understanding of the neuroendocrine mechanisms (beyond follicular depletion alone) that contribute to the onset of the perimenopausal transition [28–36]. Although aging of the female reproductive system involves all three levels of the hypothalamic-pituitary-gonadal (HPG) axis, most research on the underlying mechanisms contributing to menopause has focused on the role of depleted follicular reserves. The ovary undoubtedly plays a major role in female age-related reproductive decline, but data from human, nonhuman primate, and rodent models suggest that the hypothalamus and pituitary may play important roles as well [28–36]. Although investigations in this area have primarily focused on rodents, there is evidence from nonhuman primates indicating changes in pulsatile GnRH release (mean levels and amplitude) with age [28]. An age-related change in GnRH secretion that favors FSH production and release over LH may result in the monotropic FSH rise that is the hallmark neuroendocrine change associated with the perimenopause transition promoting follicular depletion [34–36]. To a large extent, the lack of progress in both of these areas (identification of predictive biomarkers of reproductive decline and elucidation of potential extraovarian contributors to the onset of perimenopause) stems from the absence of availability of appropriate experimental animal models [33].

Like women, adult female rhesus macaques (Macaca mulatta) show monthly menstrual cycles and eventually enter menopause (at approximately 25 yr of age in the macaque). However, there may be subtle differences between the characteristic signs of menopause onset in macaques and humans. For example, the rise in circulating FSH concentrations that is characteristic of middle-aged women just before the onset of irregular cyclicity and the entry into perimenopause was not detected in the urine of old female rhesus macaques [37], although they showed the expected decline in sex steroid concentrations after the onset of menopause. In addition, the decrease in follicular phase inhibin B secretion from granulosa cells of the ovary that is typically associated with the monotropic FSH rise in older ovulatory women [38, 39] was not observed in intact old rhesus macaque females, even in cases of advanced postmenopause [37]. Consequently, it is unclear whether macaques represent ideal animal models for human menopause-related research [37, 40, 41]. In the present study, we examined in detail the circulating reproductive hormone profiles of female rhesus macaques during transitions from premenopause to perimenopause and finally to postmenopause. In addition, we measured circulating anti-müllerian hormone (AMH) as a possible predictor of the menopausal transition. AMH, which is produced by primary, secondary, and antral follicles, has recently been identified as a potential biomarker of reproductive decline and as an indicator of ovarian reserve [42–45]. Our objectives were to help resolve the issue of whether menopause in macaques closely resembles that in women and to identify a reliable predictive endocrine marker for the onset of menopausal transitions. In doing so, we extended the observations of menopause onset in rhesus macaques previously reported by Shideler et al. [37], Gilardi et al. [40], and Walker [41]. Experiment 1 focused on the menstrual and neuroendocrine changes that occur during the transitions from premenopause to perimenopause and eventually to postmenopause, while experiment 2 focused on age-related neuroendocrine changes that occur during the premenopausal state.

MATERIALS AND METHODS

Animals

Female rhesus macaques were housed in a temperature-controlled environment (24°C) under a 12L:12D photoperiod (lights on at 0700 h), with meal times at 0800 h and 1500 h (Purina High Protein Monkey Chow No. 5045, Purina Mills, Inc., St. Louis, MO). Routine animal husbandry was provided by the Oregon National Primate Research Center (ONPRC) in accord with the Guide for the Care and Use of Laboratory Animals [46]. In addition to health checks, gross assessment of menstrual cyclicity was performed on a daily basis and included close inspection of the animal's perineum and cage pan for signs of menstrual bleeding. Based on these daily menstruation records, the animals were assigned to one of three categories: premenopausal (regular monthly menstruation; mean cycle length, 25–35 days), perimenopausal (irregular menstruation within the past 6 mo, mean cycle length, 36–45 days), or postmenopausal (no episodes of menstrual bleeding for ≥12 mo). This study was approved by the Institutional Animal Care and Use Committee of the ONPRC.

Experiment 1 (Reproductive Hormone Rhythms During Premenopause, Perimenopause, and Postmenopause)

Serial blood samples (1.5 ml) were collected via femoral venipuncture from young premenopausal (n = 5, mean age, 10 yr), old perimenopausal (n = 3, mean age, 25 yr), and old postmenopausal (n = 3; mean age, 24 yr) female macaques; the samples were collected on alternating days and spanned at least 70 days. The blood was allowed to clot at 4°C and was then centrifuged at 4°C; the serum supernatant was stored at –20°C until assay for estradiol, progesterone, LH, FSH, inhibin B, and AMH.

Experiment 2 (Effect of Age on Premenopausal Reproductive Hormone Rhythms)

Adult female macaques were each surgically fitted with an indwelling subclavian vein catheter, as previously described [47], which enabled remote serial blood samples to be collected from an adjacent room without disturbing the animals. Daily 1-ml blood samples were collected from young adult (n = 3; mean age, 9 yr) and from old premenopausal (n = 3; mean age, 25 yr) animals for at least 100 consecutive days. Blood samples were placed in EDTA-coated glass tubes and centrifuged at 4°C; the plasma supernatant was stored at –20°C until assay for estradiol, progesterone, LH, FSH, inhibin B, and AMH.

Hormone Assays

The concentrations of estradiol and progesterone in the plasma and serum samples were measured by electrochemiluminescence using the Elecsys 2010 System (Roche Diagnostics, Indianapolis, IN). FSH was measured by radioimmunoassay, as previously described [47], using an antirecombinant monkey FSH (National Institute of Diabetes and Digestive and Kidney Diseases [NIDDK] lot no. AFP782594) antibody; the results are expressed in terms of the recombinant monkey FSH-research preparation 1 (RP1) (NIDDK lot no. AFP6940A) standard. LH was measured using a previously described mouse Leydig cell bioassay that involves radioimmunoassay for testosterone [48]; the results are expressed in terms of a cynomologus LH-RP1 standard. The inter-assay and intra-assay coefficients of variation for each assay were less than 10%. Circulating inhibin B and AMH were measured from samples collected during days 3–5 of the follicular phase of cycling females and from an equivalent number of samples from postmenopausal females using commercially available inhibin B and AMH ELISA kits (Diagnostic Systems Laboratories, Inc., Webster, TX). All inhibin B and AMH measurements were made in duplicate, each within the same hormone-specific assay. Inhibin B and AMH values that were determined to fall below the limit of detection (<10 pg/ml and <0.1 ng/ml, respectively) were assigned values of 9 pg/ml and 0.09 ng/ml, respectively. The intra-assay coefficients of variation for each assay were less than 10%.

Statistical Analysis

Individual macaques' peak concentrations of follicular phase preovulatory estradiol, luteal phase progesterone, and follicular phase FSH were identified for each cycle by taking the mean of the peak and the two consecutive time points surrounding the peak. Then, to determine the overall mean "peaks" across multiple cycles for an individual macaque, the means of the individual cycle peaks were calculated (including values from ≥2 consecutive cycles per animal). Next, the overall group mean values were determined by taking the mean of the overall mean peaks. Preovulatory LH and FSH surges were based on taking the mean of consecutive single midcycle time points across multiple cycles. Menstrual cycle days 3–5, spanning 2–3 consecutive cycles, were assayed for FSH, inhibin B, and AMH. Group differences were analyzed using one-way repeated-measures ANOVA, followed by the Newman-Keuls test. A simple logarithmic correlation analysis was performed between inhibin B and FSH values obtained from the same samples. P < 0.05 was considered statistically significant.

RESULTS

Menstrual Cyclicity

Thirteen indoor-housed females were monitored daily in a longitudinal manner for 6–22 yr for signs of menstrual bleeding (Fig. 1A); based on these records, they were categorized as being young premenopausal (n = 5; mean age, 10 yr), old premenopausal (n = 3; mean age, 25 yr), perimenopausal (n = 3; mean age, 25 yr), or postmenopausal (n = 2; mean age, 24 yr) at the time of the study. The animals showed approximately 13 menstrual periods per year from the ages of 10–20 yr, with no obvious hiatus during the summer months. After age 20 yr, the number of observed menstrual cycles per year decreased in many of the animals, coinciding with a lengthening of each cycle from a mean ± SEM of 28.3 ± 0.4 days to 30.1 ± 0.7 days in animals 20–23 yr old and to 35.4 ± 1.9 days in animals 24 yr and older (Fig. 1B). The three aged animals that showed irregular menstrual cycles (missing ≥2 cycles during 12 consecutive months) and lengthened menstrual cycles (36–45 days) were considered to be oligomenorrheic (Fig. 1C) and perimenopausal, while the two animals that showed complete amenorrhea for 12 or more consecutive months were considered to be postmenopausal. All females in the premenopausal groups were eumenorrheic (regular menstrual cycles; mean cycle length, 25–35 days) at the time of sampling.


Figure 1
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  		FIG. 1. Longitudinal assessment of menstrual cyclicity in rhesus macaques maintained under 12L:12D photoperiods. A) Age-related change in the annual number of menstrual cycles is shown for 12 animals. B) Age-related change in the menstrual cycle length. Each plot represents the mean menstrual cycle length per year from an individual animal. C) Numbers of females at each age class that were identified as eumenorrheic (regular cycles, 25–35 days), oligomenorrheic (irregular cycles, 36–45 days), or amenorrheic (no menstrual cycles within the past 6 mo or longer). A total of 13 females are represented in a longitudinal manner, so that each animal is depicted more than once as it progresses to the next age class

Experiment 1 (Reproductive Hormone Profiles of Premenopausal, Perimenopausal, and Postmenopausal Rhesus Macaques)

To examine the reproductive hormone changes during the transitions from premenopause to perimenopause to postmenopause, circulating estradiol, progesterone, LH, and FSH concentrations were determined every 2 days for at least 70 consecutive days from young premenopausal, old perimenopausal, and old postmenopausal rhesus macaques. Representative reproductive hormone profiles from each of these groups are shown in Figure 2. As expected, the old postmenopausal animals showed very low circulating estradiol and progesterone concentrations and very high circulating LH and FSH concentrations (Figs. 2 and 3). The old perimenopausal animals' peak circulating FSH concentrations were significantly elevated (Fig. 3) compared with those of the young premenopausal animals during the follicular phase (P < 0.001) and during the midcycle preovulatory surge (P < 0.001). These elevated FSH concentrations were similar in magnitude to the sustained FSH concentrations observed in the postmenopausal animals. Serum concentrations of peak follicular phase estradiol, peak luteal phase progesterone, and peak midcycle preovulatory LH of the old perimenopausal animals were not significantly different from those of young premenopausal animals, suggesting uncompromised ovulation and corpus luteum formation, despite the advanced age of the old perimenopausal animals.


Figure 2
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  		FIG. 2. Representative reproductive serum hormone profiles of premenopausal, perimenopausal, and postmenopausal rhesus macaques. Plasma estradiol, progesterone, LH, and FSH values were determined every 2 days for at least 70 consecutive days. Shaded vertical bars represent days of menstruation


Figure 3
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  		FIG. 3. Analysis of reproductive serum hormone peaks during 70 days in premenopausal (open bars), perimenopausal (dashed bars), and postmenopausal (shaded bars) rhesus macaques; each bar represents the mean (± SEM) value from five, three, and two animals, respectively, and is based on the mean determination from each animal. Mean peak serum FSH concentrations were significantly higher in the perimenopausal animals compared with the premenopausal animals during the follicular phase and around the midcycle surge; overall, they were similar in magnitude to the elevated mean serum FSH concentrations of the postmenopausal animals. In contrast, preovulatory peak serum estradiol, midcycle LH, and luteal phase progesterone concentrations were not significantly different between the old perimenopausal and young premenopausal groups. *P < 0.001

Experiment 2 (Reproductive Hormone Profiles of Young and Old Premenopausal Rhesus Macaques)

To better understand the timing of the age-associated monotropic rise in FSH that we observed in experiment 1, and to determine if circulating FSH concentrations could predict the entry into perimenopause, we examined in detail the reproductive hormone profiles of six adult rhesus macaques while they were still premenopausal (three of which were also examined in experiment 1). Daily circulating estradiol, progesterone, LH, and FSH concentrations were determined for at least 100 consecutive days from young and old premenopausal rhesus macaques. Representative hormone profiles from each group are shown in Figure 4. Although the menstrual cycles from both groups were qualitatively similar (Fig. 4), the quantitative analysis of the reproductive hormones in Figure 5 shows that peak plasma FSH concentrations were significantly higher in the old premenopausal animals during the follicular phase (P < 0.05) and during the midcycle preovulatory surge (P < 0.01). In contrast, peak plasma estradiol, progesterone, and LH concentrations were not significantly different between the young and old premenopausal groups.


Figure 4
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  		FIG. 4. Representative reproductive plasma hormone profiles of young premenopausal and old premenopausal rhesus macaques. Daily plasma estradiol, progesterone, LH, and FSH values were determined for at least 100 consecutive days. Shaded vertical bars represent days of menstruation


Figure 5
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  		FIG. 5. Analysis of reproductive plasma hormone peaks across the menstrual cycle of young premenopausal (open bars) and old premenopausal (shaded bars) rhesus macaques. Each bar represents the mean (± SEM) value from three animals and is based on the mean determination from three consecutive time points spanning three consecutive menstrual cycles. *P < 0.05, **P < 0.01

FSH, Inhibin B, and AMH in Young and Old Female Rhesus Macaques

To examine the relationship between the observed elevation in FSH concentrations and the circulating inhibin B and AMH levels in old female macaques in experiment 1, we compared FSH, inhibin B, and AMH values between young premenopausal, old perimenopausal, and old postmenopausal females (Fig. 6). Although there was an age-related negative trend in inhibin B levels, only old postmenopausal females had a significant decline in inhibin B compared with young premenopausal females (P < 0.01). FSH concentrations for the same samples revealed a significant elevation (P < 0.01) in old perimenopausal and postmenopausal females compared with young premenopausal females. Moreover, a significant elevation (P < 0.01) in plasma FSH was detected in old postmenopausal females compared with old perimenopausal females during their early follicular phase. In addition, there was a significant decline (P < 0.01) in circulating AMH in old perimenopausal and postmenopausal females compared with young premenopausal females.


Figure 6
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  		FIG. 6. Analysis of plasma FSH, inhibin B, and AMH from young and old female rhesus macaques at different stages of reproductive aging. Experiment 1: FSH, inhibin B, and AMH values correspond to the mean values obtained from menstrual cycle days 3–5 in premenopausal (open bars) and perimenopausal (dashed bars) females spanning three menstrual cycles, or the equivalent number of samples spanning several weeks in postmenopausal females (shaded bars). Experiment 2: FSH, inhibin B, and AMH values correspond to the mean values obtained from menstrual cycle days 3–5 in young premenopausal (open bars) and old premenopausal (shaded bars) females spanning two menstrual cycles. Each bar represents the mean (± SEM) value and is based on the mean determination from three consecutive menstrual cycles (Experiment 1) or two consecutive menstrual cycles (Experiment 2). An inhibin B value of 9 pg/ml and an AMH value of 0.09 ng/ml were assigned to samples that fell below the detectable limits of the assays (<10 pg/ml and <0.1 ng/ml, respectively). *P < 0.02, **P < 0.01, ***P < 0.001

To examine the relationship between the observed elevation in FSH concentrations and the circulating inhibin B and AMH levels in old female macaques in experiment 2, we compared FSH, inhibin B, and AMH values between young and old premenopausal females (Fig. 6). Plasma inhibin B was significantly lower (P < 0.02) while plasma FSH was significantly higher (P < 0.001) in old premenopausal females compared with young premenopausal females. Circulating AMH concentrations for the same samples disclosed a significant decline (P < 0.01) in old premenopausal females.

A simple logarithmic correlation analysis of inhibin B and FSH concentrations (Fig. 7) revealed a significant negative logarithmic correlation (r = –0.75, P < 0.0001). Three inconsistent data points from old premenopausal and perimenopausal females maintained elevated concentrations of FSH, although the corresponding inhibin B values were more similar to those of young females. These outlier points from old premenopausal and perimenopausal females are shown in Figure 7 but were omitted from the analysis, so as to avoid skewing the outcome of the regression line in favor of a falsely linear relationship.


Figure 7
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  		FIG. 7. A significant negative logarithmic correlation between plasma inhibin B and FSH in young premenopausal (open circles), old premenopausal (shaded circles), old perimenopausal (open squares), and old postmenopausal (shaded squares) females. Data points represent individual samples obtained during menstrual cycle days 3–5 or individual samples obtained during several weeks in amenorrheic females

DISCUSSION

Controversies surrounding the findings of the WHI and the use of HRT by postmenopausal women emphasize the need for a better understanding of the mechanisms underlying the onset of menopause. In particular, there is a need for the identification of reliable predictive markers of menopause so that HRT could be applied at the most effective time and with reduced health risks [25–27]. In the present study, we addressed these issues by examining the reproductive hormone profiles of young and old female rhesus macaques. These nonhuman primates have long been regarded as pragmatic animal models in investigations pertaining to human reproductive function. Like women, adult female rhesus macaques show monthly menstrual cycles and eventually enter menopause (during the third decade of life in the macaque). Therefore, our detailed longitudinal characterization of menstrual cyclicity and reproductive hormone profile changes through the premenopausal, perimenopausal, and postmenopausal stages in the rhesus macaque provides the basis for an improved understanding of the underlying neuroendocrine mechanisms that may contribute to the onset of menopause in women [1–5, 33–36, 49].

The premenopausal animals utilized in the present study showed regular menstrual bleeding, with a mean of 13 menses per year. As expected, circulating estradiol concentrations increased gradually after each bleeding episode and peaked during the middle of the cycle in association with the LH and FSH surges. Also as expected, circulating progesterone concentrations increased markedly after the midcycle gonadotropin surge and then remained elevated until a few days before menses. Old perimenopausal animals experienced irregular menstrual cycles and a lengthening of each cycle, which is consistent with observations from previous human menopausal studies [1–5, 49]. The reproductive hormone profiles of old perimenopausal animals appeared to be a fusion of those belonging to premenopausal and postmenopausal animals. Before ovulation in the perimenopausal animal, a prolonged elevation of FSH closely resembled what we observed in the postmenopausal animals. In contrast, after the perimenopausal animals experienced a midcycle gonadotropin surge, their hormone profiles closely resembled the premenopausal hormone profiles qualitatively and quantitatively. It is likely that the observed hyperelevated FSH is necessary for the perimenopausal animal to sufficiently recruit its few remaining follicles to eventually produce a gonadotropin surge. Although, we did not observe a significant decline in serum estradiol in old perimenopausal females compared with young premenopausal females, it is likely that the large variability in preovulatory peak estradiol of young females is why we did not see a significant difference. This notion was supported by power calculations that indicated that 11 animals in the young premenopausal group would be required to reveal a significant difference in estradiol levels compared with the old perimenopausal group.

In a previous study by Shideler et al. [37], a monotropic rise in peak FSH concentrations was observed together with very low estradiol and progesterone concentrations in perimenopausal rhesus macaques, and very high and sustained LH and FSH concentrations were observed in postmenopausal animals. Although our results generally confirm these previous findings, they also disclose novel early biomarkers of the rhesus macaque premenopause to perimenopause transition. In contrast to previous studies that relied primarily on urine hormone measurements [37, 40], we measured hormone concentrations in serum or plasma, collected by venipuncture or by a remote venous catheter. The significance of our findings is that we were able to demonstrate a marked elevation in follicular phase and midcycle FSH concentrations and a decline in AMH and inhibin B levels during the follicular phase, well before the animals showed any menstrual irregularity or alterations in their estradiol, progesterone, or LH concentrations. The exact reason why we so readily detected this elevation of FSH concentrations, while the study by Shideler et al. [37] did not, is unclear but may involve several factors. For example, premenopausal animals in our study may have been of a more advanced age than those examined by Shideler et al. Also, our hormonal observations may underscore the value of frequent blood sampling, rather than urine sampling, in characterizing the late premenopause to perimenopause transition in the rhesus macaque. It might also be pertinent that none of the animals in our study showed any obvious signs of retaining seasonality. Rhesus macaques are short-day seasonal breeders when maintained outdoors [50, 51] and show seasonal amenorrhea during the summer, which often persists for many summers even after the animals are moved indoors and are exposed to fixed photoperiods. However, in our study, all of the animals had been maintained indoors under fixed 12L:12D photoperiods for 6–22 yr with no obvious seasonal cues, and in no case did any of them show persistent amenorrhea selectively during the summer months.

The possible neuroendocrine mechanisms that contribute to the onset of menopause are poorly understood. Therefore, our findings that circulating FSH concentrations are highly elevated and that inhibin B and AMH levels are reduced in old premenopausal rhesus macaques are highly pertinent. Unpublished observations presented at the Annual Society for the Study of Reproduction meeting in 2004 by Wu et al. indicate that an age-related decline in circulating inhibin B concentrations in rhesus macaques may be at least partially responsible for the rise in FSH concentrations. On the other hand, one cannot exclude the possibility that other nonovarian factors are involved, such as an age-related change in the pattern or amount of GnRH secretion [28, 34–36]. Unlike the study by Shideler et al. [37], we observed a significant decline in circulating inhibin B in old premenopausal and postmenopausal females that is very similar to what is observed in older premenopausal and postmenopausal women [38, 39]. However, although we observed a trend, we did not observe a significant decline in early follicular phase inhibin B in old perimenopausal females. Overall, the measurement of inhibin B as a tool to predict menopause onset appeared to be less consistent than FSH and AMH measurements.

Predicting the age of perimenopause in women and in nonhuman primates is challenging because the age at onset covers a wide range [1–5, 33, 37, 40, 41]. The observed elevation of FSH concentrations (and possibly the decline in inhibin B and AMH concentrations) in old rhesus macaques before manifestation of irregular menstrual cycles and attenuation of ovarian sex steroid output may serve as an early predictive factor for the initiation of the perimenopause transition. Indeed, these endocrine changes were much more reliable in predicting the transition to the perimenopausal state than age alone, as the perimenopausal females were older, on average, than the postmenopausal females.

In summary, our results support the view that an increase in circulating FSH concentrations and a decline in inhibin B and AMH levels may be the first endocrine events associated with the onset of perimenopause in the rhesus macaque and, most important, may be predictive of the ensuing decline in sex steroid concentrations. Our data also represent a refined characterization of the neuroendocrine changes that occur in the rhesus macaque during the menopausal transitions and confirm the value of this animal model for human menopausal research [33, 37, 40, 41, 49]. With this valuable predictive knowledge, postmortem studies utilizing tissue from old female macaques can be used to advance the exploration of molecular and histological changes at all levels of the aging HPG axis that are associated with reproductive senescence, which would be impossible to carry out in the human.

ACKNOWLEDGMENTS

We wish to thank the ONPRC Division of Animal Resources for help with the care and psychological enrichment of the animals, Dr. Dick R. Yeoman for his technical assistance, and Vasilios T. Garyfallou and Laura L. James for help with maintenance of catheter patency.

FOOTNOTES

1 Supported by grants AG-023477, AG-16935, AG-19100, AG-19914, HD-29186, and RR-00163 from the National Institutes of Health. Back

2 Correspondence: Henryk F. Urbanski, Division of Neuroscience, Oregon National Primate Research Center, 505 N.W. 185th Ave., Beaverton, OR 97006. FAX: 503 690 5384; urbanski{at}ohsu.edu Back

Received: 22 February 2006.

First decision: 20 March 2006.

Accepted: 6 July 2006.

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