Constant Vaginal Estrus Research Articles

Estrogen, synaptic plasticity and hypothalamic reproductive aging

Unlike primates who undergo ovarian failure and loss of sex steroids at the end of reproduction, aging rodents undergo constant vaginal estrus followed by constant diestrus and finally anestrus, which indicates the absence of responsive ovarian follicles. The latter state is analogous to menopause in women. The timing of the appearance of constant estrus is determined by many factors including estrogen exposure in the brain during development and the number of times that the animal gets pregnant. The chief site of this reproductive aging in rat brains is the arcuate nucleus of the hypothalamus. The transition from normal cycles to constant estrus parallels the females' gradually decreased ability to respond to administered estradiol with a cycle of inhibition followed by disinhibition of gonadotrophin-releasing hormone. Evidence has accumulated indicating this to be due to a loss of the rat's ability to respond to markedly elevated estradiol with the usual arcuate nucleus neuro-glial plasticity that supports the estrogen-induced gonadotrophin surge (EIGS). Just as male rats are not capable of an EIGS, aged females loose this ability through repeated EIGS. Experiments indicate that in male rats the hypothalamic synaptology that develops as a result of exposure to testicular androgens in the perinatal period (brain sexual differentiation) is a result of conversion of testosterone from the testes to estrogen in the brain and is therefore due to early estrogen exposure. Aging females appear to reach a synaptology similar to males and constant estrus as a result of repeated exposure to ovarian estrogens during their reproductive careers. The relative role of aging and hormonal factors remains unclear. Morphological evidence is presented that indicates the above effects of estrogen involve changes in hypothalamic arcuate nucleus neurons and glia, including changes in the organization of perikaryal membranes as well as arcuate nucleus synaptology and the load of peroxidase in the astroglia. A possible role for free radicals (reactive oxygen species) in hypothalamic reproductive aging has been proposed. Such a mechanism is supported by evidence that the anti-oxidant vitamin E delays the onset of constant estrus and the accumulation of glial peroxidase in aging female rats. However, since the synaptology and peroxidase load in constant estrus females is independent of the age at which the constant estrus occurs, it appears that the role of (repeated) estradiol exposure is more deterministic of hypothalamic failure than is aging, per se.

Prepubertal testosterone treatment of female rats: Defeminization of behavioral and endocrine function in adulthood

This study assesed the capacity of testosterone (T) administered well after the neonatal “critical” period to permanently sexually differentiate reproductive function. Females received T filled or empty Silastic capsules during days 15–30 of age and vaginal cyclicity, ovarian weight and. appearance, lordosis and proceptive behaviors, mounting behavior, and the gonadotropin response to estrogen and progesterone were measured in adulthood. T-treated females (plasma levels of 0.66 ng T/ml) showed constant vaginal estrus from the day of vaginal opening and small, polyfollicular ovaries. Proceptive behaviors were dramatically reduced whether or not the ovaries were present after day 15 of age, but lordosis behavior was not affected. Exposure to T for 5–6 h was ineffective. Compared to controls, T-treated females had dramatically reduced plasma FSH and LH surges. No effects were observed on mounting behavior, phallus size, or body weights. These results suggest that androgen at approximately male levels can act on neural substrates well beyond the neonatal period to permanently defeminize endocrine and behavioral function in the female rat.

Effects ofN-Methyl-D,L-Aspartic Acid on Luteinizing Hormone Secretion in Normal Mice and in Hypogonadal Mice with Fetal Preoptic Area Implants*

Many aspects of reproductive function are corrected in hypogonadal mice with preoptic area grafts (HPG/POA). Gonadotropin release and gonadal development are dependent on the presence of GnRH cells within the grafts and GnRH innervation of the median eminence. This study examined the effect of a known modulator of GnRH secretion, N-methyl-D,L-aspartic acid (NMA), in adult normal and HPG/POA male and female mice. All HPG/POA males had significant testicular development after graft surgery, and most HPG/POA females were in constant vaginal estrus and showed ovarian and uterine development; a few also demonstrated ovulatory cyclicity after pregnancies initiated by reflex ovulation. Groups of normal and HPG/POA males that were intact (INT) or castrated (CX) 7 days before testing were challenged with saline, NMA (20 mg/kg), and GnRH (100 ng/0.1 ml). Sequential blood samples from awake animals were obtained via intracardiac catheters for evaluation of plasma LH. There were significant increases in plasma LH after NMA challenge in normal INT [n = 15; 0 min, 0.26 +/- 0.02 (mean +/- SE); 10 min, 1.20 +/- 0.10 ng/ml; P less than 0.05] and normal CX (n = 13; 0 min, 0.36 +/- 0.06, 10 min, 3.25 +/- 0.27). Plasma LH secretion in response to NMA was significantly correlated (r = 0.786; P less than 0.001) with plasma LH release after the GnRH challenge in normal males. In contrast, only 3 of 17 HPG/POA (1 INT and 2 CX) showed increased circulating LH after NMA challenge, despite heightened pituitary sensitivity to GnRH. Normal and HPG/POA female mice were ovariectomized (OX) or OX and estrogen primed (OXE2) 7 days before testing. Intact cycling normal and cycling HPG/POA mice were tested in estrus (EST). There was a greater response to NMA in normal OX (n = 8; 0 min, 0.39 +/- 0.02; 10 min, 1.44 +/- 0.28) than in OXE2 (n = 13; 0 min, 0.29 +/- 0.01; 10 min, 0.52 +/- 0.07) despite similar gonadotroph sensitivity to GnRH. There was also a significant increase in plasma LH in response to NMA in HPG/POA-OX (n = 7; 0 min, 0.50 +/- 0.10; 10 min, 1.62 +/- 0.22) and HPG/POA-OXE2 (n = 12; 0 min, 0.39 +/- 0.04; 10 min, 1.31 +/- 0.26). Plasma LH levels after NMA treatment were significantly correlated with responses to GnRH in female HPG/POA (r = 0.58; P less than 0.03), but not in normal females. Neither normal-EST nor HPG/POA-EST had increased LH release after NMA challenge, perhaps due to the low gonadotroph sensitivity in this state.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of neonatal androgenization on positive feedback in female mice.

Exposure of female mice to androgens within 5 days of birth impairs fertility. Such treatment in rats results in a post-pubertal acyclic state of persistent vaginal cornification and in an inability, when ovariectomized, to show normal positive feedback on luteinizing hormone (LH) release in response to steroid challenge. In the present study, we explored whether neonatally androgenized mice demonstrate positive feedback. Female mice were administered 100 micrograms of testosterone propionate (TP) on either Day 1 (TP1) or Day 5 (TP5) after birth, or vehicle on Day 1 (SO1). Androgen-treated mice had a statistically significant advance in onset of vaginal opening as compared with vehicle-treated mice. All mice that received TP entered constant vaginal estrus, whereas those given vehicle showed variable cytology. All mice were ovariectomized at 7 wk of age and received Silastic capsules containing a priming dose of 17 beta-estradiol. When all mice were challenged 1 wk later with sequential administration of estradiol benzoate and progesterone, a significant increase in plasma LH level was present only in the vehicle-treated mice. We conclude that neonatal androgenization defeminizes the neuroendocrine mechanisms controlling gonadotropin release.

Estrogen-induced synaptic remodelling in adult rat brain is accompanied by the reorganization of neuronal membranes

Adult cycling female rats were injected with estradiol valerate (2 mg/100 g body wt.), a treatment which has previously been shown to result in synaptic remodelling in the arcuate nucleus and constant vaginal estrus. During the 32 weeks following estrogen treatment, arcuate nucleus neuronal plasma membranes were quantitatively assessed for intermembrane (IMP) number and size using freeze-fracture techniques. Neuronal membranes from untreated cycling females, females injected with oil and untreated males were also studied. Untreated rats had dimorphic sexual phenotypes in membrane organization; female rats had more IMP than males, mainly due to greater numbers of small ( < 10 nm) particles. These sex differences were observed in perikarya and dendritic shafts, but not in dendritic spines. Following estrogen treatment the density of IMP in membranes from females decreased. The IMP changes were found only in neuronal perikarya and dendritic shafts, not in dendritic spines, and were mainly due to a massive decrease in the number of small ( < 10 nm) IMP which was only partially offset by an increase in the number of large ( ⪖ 10 nm) IMP. Thus, by 32 weeks after estradiol valerate treatment, the number and size of IMP in neuronal membranes from females were not different from those seen in normal males. These results strengthen the idea that estradiol may affect the turnover of certain neuronal membrane components in sex-steroid sensitive areas of the brain.

LHRH-induced LH and FSH responses in the aged female rat.

LHRH-induced LH and FSH concentrations were measured in 8 to 12-month-old rats which had just begun to exhibit constant vaginal estrus (CE) or prolonged vaginal diestrus (PD) and in young rats on the day of proestrus (PE). All animals were anesthetized with sodium pentobarbital (31 mg/kg). They received (1) i.v. infusion of LHRH (Beckman 100 ng/ml at a rate of 0.5 ml/hr) for 3 hrs or (2) 2 i.v. injections of LHRH (50 ng/100 g body wt) spaced 120 min apart. LHRH stimulation resulted in a comparable gonadotropin response in young rats and old CE rats; however, old PD rats exhibited a markedly attenuated LH and FSH response to LHRH infusion. These data suggest that pituitary responsiveness to LHRH does not change in 8 to 12-month-old CE rats and therefore does not play a major role in the transition to acyclicity. In contrast, pituitary responsiveness is decreased in old PD rats early during the aging process and therefore may be an important factor in the transition to their acyclic vaginal smear pattern.

Reduction of elevated prolactin levels in neonatally androgenized female rats by corticomedial amygdaloid lesions.

The effect of corticomedial amygdaloid lesions on serum PRL levels was investigated in adult female rats neonatally androgenized (NA) on day 3 of life with 1.25 mg testosterone propionate. NA animals exhibiting constant vaginal estrus were studied at 4 and 6 months of age in a series of three experiments. Basal (1100 h) PRL levels were elevated in adult NA females compared to levels in control females that were ovariectomized and estrogen primed. Four-month-old NA rats, decapitated at 1100 and 1700 h, had statistically significant diurnal fluctuations in serum PRL levels (118.1 ± 14.8 and 207.8 ± 13.7 ng/ml, respectively). Ablation of the corticomedial amygdala 1 month before sacrifice significantly reduced these elevated levels. A second group of NA females, decapitated at 1100 h at 6 months of age to determine if the elevated basal PRL levels in NA rats were affected by increasing age, had high serum PRL levels, similar to those of younger NA rats, which were significantly reduced by corticomedial amygd...

Neonatal ablation of the suprachiasmatic nucleus. Effects on the development of the pituitary-gonadal axis in the female rat.

Ablation of the suprachiasmatic nucleus (SCN) at 2 days of age, prior to the formation of the retinohypothalamic projection, produces a permanent state of constant vaginal estrus in the postpubertal female rat. Although such lesions do not alter the onset of puberty in sighted rats, they do compensate for the delay in vaginal opening induced by neonatal binding. The ovaries of sighted and blinded SCN-lesion rats are small and polyfollicular and the pituitaries of blinded SCN-lesion rats are abnormally large. Sampling of plasma in the morning and afternoon for up to 12 consecutive days in sighted SCN-lesion rats reveals continuously low luteinizing hormone levels. This constellation of endocrine alterations does not correlate with damage to any structures outside the SCN. Since the organization of the rodent estrous cycle is circadian, these results emphasize further the importance of the SCN in circadian rhythm generation. The necessity of an intact SCN for the development of normal, cyclic reproductive function implies that sparing or recovery of function does not occur.

Neonatal suprachiasmatic nucleus ablation: absence of functional and morphological plasticity.

Neonatal ablation of the suprachiasmatic nucleus in the rat has two important consequences. First, the direct projection from the retina to the suprachiasmatic nucleus fails to develop and no other retinal projection to any hypothalamic nucleus is formed. Second, circadian rhythms in drinking and spontaneous locomotor activity to not appear in these rats when they are tested as adults, and the females exhibit constant vaginal estrus. These observations indicate that the central neural mechanisms responsible for the generation and entrainment of circadian rhythmicity in the rat are not capable of either the functional or morphological plasticity characteristic of other developing neural systems.

Masculine-liky hypothalamic-pituitary axis in the androgen-insensitive genetically male rat pseudohermaphrodite.

The hypothalamic-pituitary sex of the androgen insensitive, genetically male rat pseudohermaphrodite was studied by examining its vaginal cytology, response to ovarian transplants and urinary steroidal excretion patterns. More than half the pseudohermaphrodites studied were in constant vaginal estrus, while the remaining rats displayed either persistent diestrus or irregular cyclicity tending towards lengthened estrus. Following gonadectomy and ovarian transplantation, normal females displayed regular 14-day cycles while pseudohermaphrodites remained in constant vaginal estrus. In pseudohermaphrodites with ovarian transplants, only C19 steroids were detected in the urine while females excreted both C21 and C19 steroids. Indicative of the urinary findings, transplants in females had corpora lutea and maturing follicles while grafts from pseudohermaphrodites and males contained follicular cysts and luteinized theca. In addition, distribution and activity of histochemical 3beta-hydroxy-delta5 steroid oxidoreductase were similar in the grafts from pseudohermaphrodites and males, but unlike the females. Although previous reports have shown that much of the sex-dependent differentiation of the genetic male rat pseudohermaphrodite is phenotypically female, our results suggest that the phenotype of the hypothalamic-pituitary axis of this animal is, at least in certain respects, male.

Serum estradiol and prolactin concentrations in neonatal female rats treated with human chorionic gonadotropin (hCG).

The induction of a syndrome characterized by lack of ovulation or corpora lutea formation, and with constant vaginal estrus, following treatment of neonatal (beginning on day 6) female rats with human chorionic gonadotropin (hCG) was confirmed. Ten daily subcutaneous injections of 10 IU (days 6–15) produced this syndrome in all of the females by 60 days of age. Serum estradiol concentrations and uterine weights were increased on day 11 after 5 injections of hCG, but neither ovarian weights nor serum prolactin levels were different from control values. After 10 injections of hCG (day 16) ovarian and uterine weights were three times control values. Serum estradiol concentrations reached peak values of 434 pg⁄ml in these animals and prolactin levels were twice those of controls. Between day 16 and day 21 ovarian weights increased nearly 3-fold in both control and hCGtreated females, but only the latter animals had increased uterine weights. Serum estradiol was lower on day 21 than on day 16, but prolactin ha...

Gonadotrophin content of pituitaries of rats in constant estrus induced by continuous illumination.

LH and total gonadotrophin of the anterior pituitaries of cycling rats under normal lighting and rats in constant vaginal estrus (CVE) induced by continuous illumination were compared. The ovarian ascorbic acid depletion method and the uterine weight increase in immature mice were employed for LH and total gonadotrophin assays, respectively. Most of the rats placed in constant light developed continuous vaginal cornification within 30 days. There was an increase in pituitary weight and the ovaries were smaller than those of the controls. The pituitary LH content and concentration were markedly decreased in rats in CVE after 50 and 100 days of illumination. Administration of progesterone, SC-13664 (an antiestrogenic compound), or clomiphene or oophorectomy caused an increase in LH. The pituitary total gonadotrophin of rats in CVE was similar to that of normal rats and increased under treatment with SC-13664 or clomiphene and after oophorectomy. Progesterone caused no change.(Endocrinology77: 529,1965)

Effects of hypothalamic lesions on the secretion and storage of hypophysial luteinizing hormone.

Hypothalamic lesions in the median eminence of the tuber cincrcum produced a syndrome of constant vaginal diestrus (CD), accompanied by atrophic ovaries and uteri and a significant decrease in weight of the pars distalis. The ovaries of these rats were characterized by failure of follicular development and the persistence of large corpora lutea. Lesions in the rostral hypothalamus, on the other hand, produced a syndrome of constant vaginal estrus (CE), accompanied by atrophic ovaries, and enlargement of uteri and pars distalis. The ovaries of these rats were filled with large follicles, and there was a scarcity of corpora lutea or their remnants. Using the ovarian ascorbic acid depletion method, LH was measured in plasma and pars distalis before and after ovariectomy in normal rats and those with hypothalamic lesions. No detectable LH was found in plasma from normal rats, but detectable quantities of the hormone were found one to 16-weeks postovariectomy. Hypothalamic lesions producing CD or CE prevented ...