Apparent absence of negative feedback in middle-aged persistent-estrous rats following luteinizing hormone-releasing hormone agonist treatment: relation to plasma inhibin and 17 beta-estradiol

Maternal undernutrition induces premature reproductive senescence in adult female rat offspring

OBJECTIVE

To determine the effects of maternal undernutrition (MUN) on the reproductive axis of aging offspring.

DESIGN

Animal (rat) study.

SETTING

Research laboratory.

ANIMAL(S)

Female Sprague-Dawley rats.

INTERVENTION(S)

Food restriction during the second half of pregnancy in rats.

MAIN OUTCOME MEASURE(S)

Circulating gonadotropins, antimüllerian hormone (AMH), ovarian morphology, estrous cyclicity, and gene expression studies in the hypothalamus and ovary in 1-day-old (P1) and aging adult offspring.

RESULT(S)

Offspring of MUN dams had low birth weight (LBW) and by adult age developed obesity. In addition, 80% of adult LBW offspring had disruption of estrous cycle by 8 months of age, with the majority of animals in persistent estrous. Ovarian morphology was consistent with acyclicity, with ovaries exhibiting large cystic structures and reduced corpora lutea. There was an elevation in circulating T, increased ovarian expression of enzymes involved in androgen synthesis, an increase in plasma LH/FSH levels, a reduction in E2 levels, and no changes in AMH in adult LBW offspring compared with in control offspring. Hypothalamic expression of leptin receptor (ObRb), estrogen receptor-α (ER-α), and GnRH protein was altered in an age-dependent manner with increased ObRb and ER-α expression in P1 LBW hypothalami and a reversal of this expression pattern in adult LBW hypothalami.

CONCLUSION(S)

Our data indicate that the maternal nutritional environment programs the reproductive potential of the offspring through alteration of the hypothalamic-pituitary-gonadal axis. The premature reproductive senescence in LBW offspring could be secondary to the development of obesity and hyperleptinemia in these animals in adult life.

Hypothalamic molecular changes underlying natural reproductive senescence in the female rat

The role of the hypothalamus in female reproductive senescence is unclear. Here we identified novel molecular neuroendocrine changes during the natural progression from regular reproductive cycles to acyclicity in middle-aged female rats, comparable with the perimenopausal progression in women. Expression of 48 neuroendocrine genes was quantified within three hypothalamic regions: the anteroventral periventricular nucleus, the site of steroid positive feedback onto GnRH neurons; the arcuate nucleus (ARC), the site of negative feedback and pulsatile GnRH release; and the median eminence (ME), the site of GnRH secretion. Surprisingly, the majority of changes occurred in the ARC and ME, with few effects in anteroventral periventricular nucleus. The overall pattern was increased mRNA levels with chronological age and decreases with reproductive cycle status in middle-aged rats. Affected genes included transcription factors (Stat5b, Arnt, Ahr), sex steroid hormone receptors (Esr1, Esr2, Pgr, Ar), steroidogenic enzymes (Sts, Hsd17b8), growth factors (Igf1, Tgfa), and neuropeptides (Kiss1, Tac2, Gnrh1). Bionetwork analysis revealed region-specific correlations between genes and hormones. Immunohistochemical analyses of kisspeptin and estrogen receptor-α in the ARC demonstrated age-related decreases in kisspeptin cell numbers as well as kisspeptin-estrogen receptor-α dual-labeled cells. Taken together, these results identify unexpectedly strong roles for the ME and ARC during reproductive decline and highlight fundamental differences between middle-aged rats with regular cycles and all other groups. Our data provide evidence of decreased excitatory stimulation and altered hormone feedback with aging and suggest novel neuroendocrine pathways that warrant future study. Furthermore, these changes may impact other neuroendocrine systems that undergo functional declines with age.

Increasing blunting of inhibin responses to dynamic ovarian challenge is associated with reproductive aging in the rat

Reproductive aging results in declines in female fertility and in slight declines in baseline serum inhibins in humans. The authors tested the hypothesis that exogenous ovarian stimulation as a noninvasive dynamic test, by amplifying inhibin levels, can define more accurately than baseline serum inhibin levels the ovarian age of female rats. Female rats--young (immature, 26 days old), adult (65-75 days old), and reproductively aged (8- to 9-month-old retired breeders)--were administered pregnant mare's serum gonadotropin to stimulate the ovaries and the serum inhibin A and inhibin B, and ovarian inhibins were measured. The young and adult females had at least a 4- to 10-fold increase in serum inhibin A and inhibin B after ovarian stimulation, but the reproductively aged females did not. Adult female serum inhibin measurements after stimulation at every stage of the estrous cycle revealed a consistent 4- to 5-fold increase in all stages, whereas the reproductively aged females did not. Western blot analyses of ovarian lysates were consistent with the serum inhibin level results and revealed that ovarian inhibin alpha, inhibin beta A, and inhibin beta B subunit levels in adult ovaries were elevated after ovarian stimulation but not in the reproductively aged ovaries. The data presented here demonstrate that young, adult, and reproductively aged female inhibin responses after exogenous ovarian stimulation were different. Inhibin measurement in serum after ovarian challenge in rats, therefore, is a noninvasive method that could be used to dissect ovarian function in aging in more detail.

Aging alters norepinephrine release in the medial preoptic area in response to steroid priming in ovariectomized rats

Changes in luteinizing hormone (LH) secretion that are observed in aging animals have been attributed to a reduction in hypothalamic norepinephrine (NE). The reason for the reduction in NE levels with aging is unclear. We hypothesized that the responsiveness of noradrenergic neurons to ovarian steroids is altered during aging. To test this, regularly cycling female Sprague-Dawley rats (young: 4-5 months old and middle age: 8-11 months old) were implanted with a push-pull cannula in the medial preoptic area (MPA) and ovariectomized bilaterally. On the 8th day after ovariectomy, they were injected with estrogen (30 microg/100 microl corn oil, s.c.) at 1000 h and on the 9th day they were implanted with a jugular catheter. On the 10th day they were injected with progesterone (2 mg/100 microl corn oil, s.c.) at 1000 h and subjected to push-pull perfusion. Perfusate samples from the MPA were collected at the rate of 10 microl/min every 30 min from 1300 to 1800 h and blood samples (0.3 ml) were collected hourly. The perfusate samples were analyzed for NE and dopamine (DA) concentrations using high performance liquid chromatography with electrochemical detection and serum LH levels were determined by RIA. In young animals, NE release (mean+/-S.E., pg/min) was 4.0+/-1.1 pg/min at 1300 h and increased significantly (p<0.05) to 10.4+/-4.3 pg/min at 1500 h and remained elevated until 1600 h and then declined to 6.8+/-2.5 at 1730 h. In contrast, the increase in NE release occurred briefly in middle-aged animals and was delayed by an hour. LH patterns in both age groups followed the pattern in NE release. There was no change in the release of DA in both young and middle-aged animals. It is concluded that the altered responsiveness of noradrenergic neurons to steroid priming in middle-aged rats probably plays a critical role in the alterations seen in LH secretion in older animals.

Alterations in plasma and ovarian immunoreactive inhibin during reproductive aging in the female rat

Previously, we showed that ovarian inhibin α- and β;(A)-subunit mRNAs are elevated in middle-aged and old persistent-estrous (PE) female rats. To determine whether higher inhibin subunit mRNA expressions result in increased circulating inhibins during reproductive aging, plasma immunoreactive inhibin α (ir-inh α) and gonadotropins were measured in young, middle-aged and PE rats. Plasma LH profiles were distinctly divergent in the middle-aged rats with some showing LH surges indistinguishable from young rats and others showing significantly attenuated LH surges. Plasma ir-inh α in middle-aged rats with LH surges were similar to those of young rats. However, animals with attenuated LH surges had higher peak ir-inh α levels than young and middle-aged animals with LH surges. Immunohistochemistry revealed increased levels of ovarian inhibin α staining in those animals with attenuated LH surges. Overall, the highest plasma and ovarian inhibin α were found in PE rats which lack LH surges. However, significant decreases of plasma and ovarian inhibin α were seen after reinstatement of estrous cyclicity with progesterone implant treatment. Thus, increases in both plasma and ovarian inhibin α appear to be closely associated with attenuation or loss of the preovulatory gonadotropin surge that occurs during aging.

Sterility in neonatally androgenized female rats and the decidual cell reaction

In female rats, administration of androgen during early postnatal life results in anovulatory sterility characterized by polyfollicular ovaries and persistent vaginal cornification in adulthood. In these androgen-sterilized rats, the acyclic male pattern of gonadotropin secretion results from permanent damage to hypothalamic centers normally responsive to steroid feedback. The capacity of the endometrium to differentiate into deciduoma in response to endometrial stimulation is markedly reduced in the uterus of the androgenized rats receiving an appropriate regimen of progesterone and estrogen injections. The hypothesis is presented that the lowered uterine responsiveness in the androgenized rats is largely ascribable to the effect of androgen given neonatally rather than to the influence of continued exposure to ovarian estrogen. This review deals with the nature of the uterine response to ovarian hormones and the deciduogenic stimulus in androgenized rats in order to verify the mechanism involved in androgen action on the neonatal uterus.