Early treatment of young female rats with progesterone delays the aging-associated reproductive decline: a counteraction by estradiol

We have recently reported that successive treatments of young virgin rats with progesterone (P) implants produce elevated circulating P and consistently low estradiol (E2) concentrations, and subsequently delay the aging-associated reproductive decline. Inasmuch as E2 has been implicated in causing the loss of regular estrous cyclicity in aging rats, the present study examined if the concomitant presence of moderately increased circulating E2 levels could counteract the effects of P implants on reproductive aging. Starting at 3 1/2 mo and continuing to 8 mo of age, regularly cyclic, virgin rats received either s.c. Silastic implants of P (P-implanted), blank Silastic implants (virgin controls), or P + E2 implants (P + E2-implanted) for 3 wk, followed by implant removal for 1 wk. Each of these implant treatments was repeated in the same female rats 5 times. Blood samples were obtained on different days of the estrous cycle from the control group and on Day 11 of successive treatments with P or P + E2 implants for measurements of serum P and E2 values. At 8 1/2 and 10 mo of age, estrous cyclicity of these same virgin rats was again monitored, and 10-mo-old regularly cyclic females from each treatment group were mated with young fertile males to complete term pregnancies. While virgin controls showed cyclic increases in E2 and P secretion during the estrous cycle, P-implanted virgins exhibited consistently low serum E2 and moderately increased P levels during 5 successive treatments. The latter indicates a potent inhibition of ovarian E2 secretion by P implants.(ABSTRACT TRUNCATED AT 250 WORDS)     

The relation of ovarian steroid levels in young female rats to subsequent estrous cyclicity and reproductive function during aging

In multiparous rats, the incidence of regular estrous cyclicity and fertility decreases markedly at middle age. However, recent studies have shown that repeated pregnancies or progesterone (P) implants can subsequently cause retired breeder females to maintain regular cyclicity for an extended period of time; these results suggest a P-mediated deceleration of reproductive aging. In the present study, we examined the relation of ovarian steroid levels in young virgin females to their subsequent estrous cyclicity and reproductive function during aging as compared to multiparous females. Beginning at 4 mo of age and continuing to 6 mo of age, regularly cyclic virgin rats received either consecutive P implants (n = 41) or no implants (controls, n = 45) for 3 wk, followed by implant removal for 1 wk. Additional females (n = 72) were mated and allowed to undergo repeated pregnancies at 4, 6 1/2, and 8 mo of age. Blood samples were obtained throughout the estrous cycle (virgin females), during pregnancy (multiparous rats), and on Day 11 of successive treatments with P implants (virgins with P implants) for P, estradiol (E2), and testosterone (T) measurements. Subsequently, regularly cyclic females from all three groups were mated with fertile males to undergo term pregnancies at 10 and 12 mo of age. While the virgin controls showed cyclic increases in P, T, and E2 secretion during their estrous cycles, the P-implanted females had persistently low E2 and high P and T levels during treatment, which indicates an inhibition of ovarian E2 synthesis by P.(ABSTRACT TRUNCATED AT 250 WORDS)     

Influences of age and ovarian follicular reserve on estrous cycle patterns, ovulation, and hormone secretion in the Long-Evans rat

This study examined the influences of aging and reduced ovarian follicular reserve on estrous cyclicity, estradiol (E(2)) production, and gonadotropin secretion. Young virgin and middle-aged (MA) retired breeder female rats were unilaterally ovariectomized (ULO) or sham operated (control). Unilateral ovariectomy of young rats reduced the ovarian follicular reserve by one-half, to a level similar to that found in MA controls. Unilateral ovariectomy of MA females reduced the follicular pool further, to one half of MA controls. The incidence of regular cyclicity was significantly lower in MA ULO females than in young controls, with intermediate cycle frequency in young ULO and MA controls. Among cyclic rats, the magnitude of the proestrous LH surge was highest in young controls, intermediate in young ULO rats and MA controls, and lowest in MA ULO females. Similarly, ovulation rates were highest in young controls, intermediate in young ULO rats and MA controls, and lowest in MA ULO females. While young ULO rats exhibited augmented secondary FSH surges on estrous morning, middle-aged ULO females displayed secondary FSH levels comparable to young controls. The effects of age and reduced follicle number on estrous cyclicity and gonadotropin secretion were not due to altered E(2) secretion, as preovulatory E(2) levels were similar among all groups. Thus, experimental reduction in the follicular reserve exerts acute effects on the preovulatory LH surge, ovulation rate, and estrous cyclicity in both young and MA rats. However, decreased follicle number increases FSH levels only in young rats, indicating aging-related alterations in the feedback regulation of FSH.     

Age-related alterations in follicular development and hormonal profiles in rats with 4-day estrous cycles

Two experiments were conducted to examine the hypothesis that an alteration in follicular development is associated with advancing maternal age in the absence of prolonged estrous cycles. In Experiment 1, serum and four follicles (from one ovary per rat) were collected from young and middle-aged, 4-day cycling rats on estrus or metestrus. Number and diameter of nonatretic antral follicles greater than 200 microns in diameter were determined from serial sections of the other ovary from each rat. In Experiment 2, serum and follicles (12 +/- 2) from both ovaries were collected from young and middle-aged rats on each day of a 4-day estrous cycle. All microdissected follicles were assayed for estradiol-17 beta (E2) and all sera were assayed for E2, follicle-stimulating hormone, and luteinizing hormone by radioimmunoassay. Numbers of follicles greater than 400 microns in diameter did not differ, while numbers of follicles 200-400 microns in diameter were reduced in middle-aged rats compared to young rats (Experiment 1). The mean diameter of follicles greater than 400 microns in diameter and the follicular content of E2 was greater in middle-aged than in young rats. In Experiment 2, a greater proportion of large follicles were observed in middle-aged rats than in young rats on all days, and a greater proportion of follicles with high concentrations of E2 were observed on diestrus. We interpreted these data as indicative of an early age-related change in the control of follicular recruitment, growth, and maturation.     

Physiological pineal effects on female reproductive function of laboratory rats: prenatal development of pups, litter size and estrous cycle in middle age

The present study investigates whether and how the pineal or its hormone melatonin influences female reproductive functions, namely the litter size, prenatal development of offsprings, and estrous cyclicity, especially its age-related cessation in a non-seasonal breeder, the laboratory rat. Wistar rats were maintained under a 24 h light-dark (12Lratio12D) cycle. Female rats were divided into 3 groups: non-operated (NO), sham-operated (SX), and pinealectomized (PX). Surgeries were performed in 35-40 day-old females. Starting at an age between 70 days and 7 months, female rats of all 3 groups were repeatedly mated with intact males. PX mothers more frequently delivered pups with malformations (e.g., taillessness, hydronephrosis, 7 out of 1263 pups) than control rats (0/1323; p<0.007). In the first delivery at 3 months of age, but not at later ages, PX mothers delivered more pups of lower body weight than control animals (p<0.001). Examination of vaginal smears showed that almost all female rats of the NO, SX, and PX groups had 4-day estrous cyclicity when they were young-between 60 days and 5 months of age. At an age of 17 to 18 months, most female rats of the NO and SX groups showed irregular, continuously diestrous or pseudopregnancy-like patterns, and 4-day estrous cyclicity was found in only 10% of the NO or SX animals. In contrast, about 50% of the PX rats showed 4-day estrous cyclicity at this older age (p< 0.001). Melatonin, when added to drinking water (0.4 mg/L) for 16 days during the dark phase increased the frequency of diestrous phase, except in continuously diestrous rats and very few others. This melatonin effect was strong in PX rats but relatively weak in SX rats. In conclusion, the pineal hormone appears to influence various reproductive functions and developmental processes, especially pregnancy and the timing of reproductive aging in rats. The effects of pinealectomy are more prominent at an age of 60 to 80 days (i.e., shortly after puberty) and at the beginning of the cessation of cycles in middle-aged females.     

Ovarian and behavioral cyclicity of the laboratory maintained cat

This study examined the sexual behavior-ovarian activity relationship of the laboratory cat maintained under consistent environmental conditions. Mean (±SEM) lengths of estrus for nonovulating, mated ovulating, and human chorionic gonadotrophin (HCG)-induced ovulating queens were 5.8 ± 0.17, 6.5 ± 0.75, and 10.0 ± 0.65 days, respectively. The duration of estrus was significantly (P < 0.01) longer in HCG-treated queens in comparison to the nonovulating or mated ovulating groups. No differences were observed in the number of behavioral estrous periods detected monthly. Duration of estrus was affected by season with significantly shorter periods of estrus observed in June (P < 0.05), September (P < 0.05), October (P < 0.05), and November (P < 0.01) compared to the March, April, May, August, or December averages. Length of estrous cycle (from Day 1 of estrus to Day 1 of next estrus) was variable (range: 6–120 days); however, 49.8% of the estrous cycles were 12 to 21 days in length. Although the ovaries of queens displaying sexual receptivity always contained developing or mature foilicles, ovaries of cats showing no estrous behavior also showed patterns of follicular development and regression at 14- to 19-day intervals. These results suggest that the estrous cycle of the laboratory maintained cat varies from 2 to 3 weeks in duration and that reproductive behavioral patterns alone are not always an accurate indication of ovarian activity or duration of the reproductive cycle.     

Prolongation and cessation of estrous cycles in aging C57BL/6J mice are differentially regulated events

The relative contributions of ovarian failure and hypothalamic-pituitary dysfunction to the prolongation and cessation of estrous cycles were assessed by measuring the ability of acutely ovariectomized (OVX) middle-aged (12 mo) mice to cycle after receiving grafts (under the renal capsule) of ovaries from young (2 mo) mice. The potentially disruptive effect of the acyclic state on the cycling response to grafted, young ovaries was avoided restricting grafting to middle-aged hosts that were still cycling. The effect of chronic exposure to ovarian secretions before the cessation of cyclicity on age-related hypothalamic-pituitary dysfunction was also assessed. The cycling ability of long-term OVX middle-aged mice (i.e., OVX at 3 mo) bearing grafts of young ovaries was compared to that of age-matched acutely OVX controls. Grafted young ovaries extended the cycling lifespan of acutely OVX middle-aged hosts by 60%. The length of this extended cycling lifespan, however, was only 80% of that achieved by young hosts bearing grafts of young ovaries. Young ovaries in middle-aged mice markedly lowered the incidence of long cycles (greater than 5 days), shifting the modal cycle length to 5 days. However, young ovaries in middle-aged mice failed to increase the incidence of 4-day cycles, the modal cycle of young controls. Middle-aged ovaries grafted into young hosts lengthened their cycles and shortened their cycling lifespan to middle-aged values. Long-term ovariectomy failed to increase the cycling lifespan of middle-aged hosts bearing grafts of young ovaries beyond that achieved in acutely OVX mice. Long-term ovariectomy did shorten the modal cycle length of middle-aged mice to 4 days, although the duration of 4-day cycling was only one-third (2 mo) that of young controls. These results indicate that the relative contributions of ovarian and neuroendocrine factors to three major events of reproductive aging vary with each event. Whereas the hypothalamic-pituitary unit appears to play an important role in the initial shift from 4- to 5-day cycles, the aging ovary plays the major role in the subsequent shift to longer cycles and in the ultimate cessation of cyclicity. Although chronic exposure to ovarian secretions during the period of cyclicity does not play a major role in the cessation of cyclicity, it appears to contribute to the hypothalamic-pituitary changes responsible for the initial shift from 4- to 5-day cycles.     

Postnatal androgenization induces premature aging of rat ovaries

In the present paper, we report that ovaries of adult rats treated with testosterone propionate (TP) on a critical postnatal Day 5 exhibit histologic and immunohistochemical findings which resemble those of the anovulatory ovaries in middle-aged female rats. The sterile rat model has been long known whereas ovarian failure seems to be a reason for anovulation with normal hypothalamo-pituitary-gonadotropin background. Appropriate function of ovarian steroidogenic cells is also regulated by mesenchymal cells. To characterize the ovarian failure, we studied the histology, luteinizing hormone receptor (LHR) expression, and characterized changes of vascular pericytes, T cells, and dendritic cells in ovarian steroidogenic compartments consisting of interstitial cells (ISC) of ovarian interstitial glands, and granulosa and theca interna cells of ovarian follicles. Normal adult ovaries contained 63% of mature interstitial glands. The mature ISC exhibited moderate cytoplasmic and strong surface LHR expression and fine (<5 micrometer) cytoplasmic vacuoles (ISC of 'luteal type'). They originated from young ISC of 'thecal type,' which exhibited strong cytoplasmic LHR expression. Remaining 37% were aged interstitial glands, which consisted of aged ISC (increased cytoplasmic vacuolization, nuclear pyknosis, and reduced surface LHR expression) and regressing ISC (weak cytoplasmic and no surface LHR expression). However, no mature ISC of 'luteal type' were detected in anovulatory ovaries of adult rats (45- and 60-day-old) injected with TP (100 or 500 microgram) on postnatal Day 5 (TP rats). Their ovaries contained 96% of aged interstitial glands with aged and regressing ISC. Remaining 4% were abnormal interstitial glands with direct transition of young ISC of 'thecal type' into aged ISC (young/aged glands). Lack of mature ISC, and similar amount of aged (96%) and young/aged interstitial glands (4%) was also detected in anovulatory ovaries of untreated persistently estrous middle-aged (10-month-old) females (aging PE rats). The aging process in TP and aging PE rats was accompanied by regression of vascular pericytes, T cells, and dendritic cells within the interstitial glands. In addition, anovulatory ovaries of TP rats and aging PE females contained mature follicles exhibiting LHR overexpression by granulosa cells, and aged (cystic) follicles with reduced layers of granulosa cells lacking LHR expression. In contrast, when the rats were injected with 500 microgram of TP later, on postnatal Day 10, the adult females exhibited estrous cycles and normal ovaries with corpora lutea. These results show that injection of TP during the critical postnatal period causes a lack of mature and preponderance of aged ISC in adult ovaries, accompanied by degeneration of mesenchymal cells. We suggest that mesenchymal cells regulate qualitative aspects of tissue-specific cells, and this function of mesenchymal cells is programmed during the critical period of development.     

Reproductive characteristics of cloned heifers derived from adult somatic cells.

This study examined the onset of puberty, follicular dynamics, reproductive hormone profiles, and ability to maintain pregnancy in cloned heifers produced by somatic cell nuclear transfer. Four adult somatic cell-cloned heifers, derived from a 13-yr-old Holstein cow, were compared to 4 individual age- and weight-matched heifers produced by artificial insemination (AI). From 7 to 9 mo of age, jugular venous blood samples were collected twice weekly, and from 10 to 11 or 12 mo of age, blood sampling was carried out every other day. After the heifers reached puberty (defined as the first of 3 consecutive blood samples with peripheral plasma progesterone concentrations of >1 ng/ml), ultrasound examination of ovaries and jugular plasma sample collection were carried out daily for 1 estrous cycle. Cloned heifers reached puberty later than controls (mean +/- SEM, 314.7 +/- 9.6 vs. 272 +/- 4.4 days and 336.7 +/- 13 vs. 302.8 +/- 4.5 kg for clones and controls, respectively; P < 0.05). However, cloned and control heifers were not different in estrous cycle length, ovulatory follicle diameter, number of follicular waves, or profiles of hormonal changes (LH, FSH, estradiol, and progesterone). Three of the 4 clones and all 4 control heifers became pregnant after AI. These results demonstrate that clones from an aged adult have normal reproductive development.     

Orthotopic ovarian transplantations in young and aged C57BL/6J mice

Orthotopic ovarian transplantations were done between young (6-wk-old) and aged (17-mo-old) C57BL/6J mice. The percentages of mice mating following surgery from the four possible ovarian transfer combinations were as follows: young into young, 83%; young into aged, 46%; aged into young, 83%; and aged into aged, 36%. The percentages of these mice that were pregnant 10 days following the presence of a vaginal sperm plug were as follows: young into young, 58%; young into aged, 9%; aged into young, 50%; and aged into aged, 0%. Some of the fetuses derived from matings of the above mice were dissociated and their cells prepared for chromosomal smears. No evidence of aneuploidy or mosaicism was found in fetuses derived from ovaries of young or aged mice. Aged ovaries, transferred to either young or aged recipients, were found to have fewer developing follicles and lower weight, which was most apparent in recipients that failed to mate or to get pregnant. Concentrations of luteinizing hormone, follicle-stimulating hormone (FSH), and prolactin in plasma from each of the pregnant recipients were analyzed by radioimmunoassay. The only statistical differences found between the transfer groups occurred in FSH concentrations. Plasma FSH was markedly elevated (P less than 0.005) in young recipients with ovaries transplanted from aged donors, in comparison to young recipients with ovaries from young donors. These data indicate that the aging ovary and uterus play a secondary role in reproductive failure and that the aging hypothalamic-hypophyseal complex is primarily responsible for the loss of fecundity in older female C57BL/6J mice.     

Differential changes in the mechanisms controlling luteinizing hormone and prolactin secretion in middle-aged female rats

Serum luteinizing hormone (LH) and prolactin (PRL) concentrations were measured in young (3-4 month old) and middle-aged (10-12 month old) intact female rats on proestrus, in ovariectomized rats after two estrogen injections (estradiol benzoate; EB, 10 micrograms/100 g body weight, s.c.) or after preoptic stimulation in EB-primed ovariectomized rats. Only animals showing regular 4-day estrous cycles were selected for the experiment. The magnitude of proestrous LH surge was significantly smaller in middle-aged than in young rats. Two BE injections, at noon on Days 0 and 3, in ovariectomized middle-aged rats failed to induce surges in LH secretion on Day 4 whereas the same treatment produced LH surges in ovariectomized young rats. The preoptic electrochemical stimulation (50 microA for 60 sec) produced a prompt rise in serum LH levels in ovariectomized EB-primed young but not in middle aged rats. The preoptic stimulation with a larger current (200 microA) induced LH secretin in middle-aged rats. In none of these situations serum PRL concentrations were different between young and middle-age rats. These results suggest differential aging rates in the preoptic mechanisms governing LH and PRL secretion in the rat. The function of the preoptic ovulatory center in responding to the estrogen positive feedback action and inducing LH secretion may become impaired and independent of the PRL control mechanism, even before the regular estrous cycle terminates.     

Effect of reduced ovarian tissue on cyclicity, basal hormonal levels and follicular development in old rats

Reduction of the number of growing follicles was proposed to contribute to the decline in reproductive performance with aging (Butcher and Page, 1981). To investigate the effects of a reduced number of follicles, rats which maintained regular estrous cycles at greater than 1 yr of age had either unilateral ovariectomy (ULO) or control surgery. Irregular estrous cycles and periods of constant estrus were more frequent during a period of 90 days after ULO than in controls. Follicle-stimulating hormone (FSH) concentration in plasma collected at 0900-1100 h of the metestrus nearest to 20, 50, and 90 days after surgery was increased by ULO; in both treatment groups, FSH increased between 20 and 90 days. Compensation in ovarian weight and number of corpora lutea had occurred by 90 days after ULO. Estradiol, estrone and luteinizing hormone (LH) concentrations did not change with time or treatment. Numbers of small, medium and large antral follicles per ovary at metestrus were increased by ULO, while the number of follicles per rat was decreased. It was concluded that the reduction in ovarian tissue (which decreased the number of growing follicles) resulted in an elevation of basal FSH followed by irregularity in estrous cycles.     

Changes of estrous cycles with aging in female F344/n rats.

Changes of estrous cycles with aging of F344/N rats between 1 and 30 months of age (M) were monitored by vaginal smear cytology. The vaginal opening and first cornified cell phase were identified at 1.3 +/- 0.1 M and 1.5 +/- 0.2 M, respectively. Thereafter, estrous cycles showed about 5-day intervals, and ceased at 16.4 +/- 1.2 M. Thereafter irregular appearance of single cornified cell phases without the preceding of nucleated cell phases interspersed with a predominant leukocyte phase was seen in vaginal smears until 26.9 +/- 0.5 M. Growing and mature follicles as well as corpora lutea persisted until at least 30 M, and characterized the post reproductive aging of F344/N females. The F344/N rats seem to resemble humans in that the cessation of estrous cycles occurs at approximately half their entire lifespan. However, other aging characteristics are unknown in postmenopausal women. Therefore, we must be careful when extrapolating the aging changes of reproduction in F344/N rats to human beings.     

The peripubertal golden hamster and the transition between daily and estrous cycle hormone rhythms

Transitional patterns of LH, FSH, and progesterone (P4) in the circulation were studied in peripubertal female golden hamsters. A daily rhythm, with afternoon surges of these hormones, is typical of the immature female, whereas 4-day rhythms characterize the estrous cycle of the adult. Blood samples were collected repeatedly from maturing individuals at either 1400 or 1700 h. Each animal was examined daily for the appearance of regular vaginal estrous cycles as indicated by a mucous exudate on the morning of ovulation. Between Days -10 and -5 relative to first vaginal estrus (FVE), afternoon surges of LH, FSH, and P4 were often observed. From Days -5 to -1 relative to FVE, afternoon surges of LH and FSH were less frequent, but P4 retained the daily rhythmicity until Day -2. A 4-day pattern of LH secretion, but not of FSH or P4, was established prior to FVE. To determine whether or not ovulations were occurring prior to the appearance of external vaginal estrous cycles, reproductive tracts were collected from 26-34 days of age and examined for evidence of ovulation. Of 124 females, concordance between the record of daily vaginal examinations and the examinations of the ovaries and oviducts was found in 103 cases (83%). The development of ovarian follicles was correlated with FVE in peripubertal hamsters by unilateral ovariectomy. Antral follicles were found only in the last 3 days prior to vaginal estrus.     

Physiological Pineal Effects on Female Reproductive Function of Laboratory Rats: Prenatal Development of Pups,Litter Size and Estrous Cycle in Middle Age

The present study investigates whether and how the pineal or its hormone melatonin influences female reproductive functions, namely the litter size, prenatal development of offsprings, and estrous cyclicity, especially its age‐related cessation in a non‐seasonal breeder, the laboratory rat. Wistar rats were maintained under a 24 h light‐dark (12L∶12D) cycle. Female rats were divided into 3 groups: non‐operated (NO), sham‐operated (SX), and pinealectomized (PX). Surgeries were performed in 35–40 day‐old females. Starting at an age between 70 days and 7 months, female rats of all 3 groups were repeatedly mated with intact males. PX mothers more frequently delivered pups with malformations (e.g., taillessness, hydronephrosis, 7 out of 1263 pups) than control rats (0/1323; p<0.007). In the first delivery at 3 months of age, but not at later ages, PX mothers delivered more pups of lower body weight than control animals (p<0.001). Examination of vaginal smears showed that almost all female rats of the NO, SX, and PX groups had 4‐day estrous cyclicity when they were young–between 60 days and 5 months of age. At an age of 17 to 18 months, most female rats of the NO and SX groups showed irregular, continuously diestrous or pseudopregnancy‐like patterns, and 4‐day estrous cyclicity was found in only 10% of the NO or SX animals. In contrast, about 50% of the PX rats showed 4‐day estrous cyclicity at this older age (p< 0.001). Melatonin, when added to drinking water (0.4 mg/L) for 16 days during the dark phase increased the frequency of diestrous phase, except in continuously diestrous rats and very few others. This melatonin effect was strong in PX rats but relatively weak in SX rats. In conclusion, the pineal hormone appears to influence various reproductive functions and developmental processes, especially pregnancy and the timing of reproductive aging in rats. The effects of pinealectomy are more prominent at an age of 60 to 80 days (i.e., shortly after puberty) and at the beginning of the cessation of cycles in middle‐aged females.     

Meloxicam and Buprenorphine Treatment after Ovarian Transplantation Does Not Affect Estrous Cyclicity and Follicular Integrity in Aged CBA/J Mice

Angiogenesis, the formation of new blood vessels, is important for the survival of ovarian transplants and the restoration of ovarian functions. Without angiogenesis, transplanted ovarian tissue becomes more susceptible to tissue damage and necrosis. Administration of analgesics for pain management has been shown to decrease angiogenesis, which can influence transplant success especially in aged animals. Aging and the effects of hypoxia after transplantation decrease reproductive viability of the ovarian transplant; therefore, it is important to understand the additional effects of analgesics on aged animal models. The present study investigated the effects of two analgesics, buprenorphine, an opiate, and meloxicam, a non-steroidal anti-inflammatory drug (NSAID), on the reproductive indicators related to estrous cyclicity and follicular integrity after ovarian transplantation of young ovaries into aged CBA/J mice. These aged females did not show any different reproductive responses when treated with either buprenorphine or meloxicam. No significant differences were observed in estrous cycle length, the onset of estrous cycling, the regularity of estrous cycles, and the proportion of viable follicles and total number of follicles per ovarian sample across treatment groups.     

Deceleration of age-associated changes in the preovulatory but not secondary follicle-stimulating hormone surge by progesterone

Recently, it has been reported that mating can delay the age-associated decline in reproductive function of female rats. Since circulating progesterone (P) levels are elevated for a 2- to 3-wk interval during pregnancy, the following study was conducted to determine whether intermittent elevation in P levels can alter the rate of reproductive aging in female rats. Beginning at 2 mo of age, 4-day-cycling, virgin rats were divided into two groups. In one group, 3 Silastic capsules containing crystalline P were inserted s.c. into each rat while rats in another group each received 1 empty capsule. After 2 wk, the capsules were removed for 2 wk. Thereafter, implantation and removal of capsules was repeated 5 additional times. Rats receiving P capsules became acyclic 3-4 days after exposure to P and resumed cyclicity 4-7 days after removal of P-capsules. One month after the last series of capsules was removed (rats approximately 8-mo-old), rats exhibiting consecutive 4-day cycles were inserted with indwelling atrial cannulae and bled at 4-h intervals from 1400 h on proestrus (Pr) to 1000 h on estrus (E). At 1600 h E, rats were killed and trunk blood was collected. For comparison, a group of 3-mo-old (young) rats was bled on Pr and E. In 8-mo-old rats that received empty capsules, 27% exhibited 4-day cycles compared to 66% of the young rats. However, in contrast to rats that received empty capsules, 63.1% of P-treated rats exhibited 4-day cycles. Surges of preovulatory luteinizing hormone (LH) and follicle-stimulating hormone (FSH) surges were attenuated in 8-mo-old rats given empty capsules compared to young rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Puberty and ovulatory release of gonadotropins in spontaneously hypertensive rats

The age at vaginal opening, estrous cyclicity, serum levels of luteinizing hormone (LH), follicle stimulating hormone (FSH) and prolactin (PRL) on the day of proestrus, and number of ova and ovarian weight as measured on the day of estrus in spontaneously hypertensive (SH) and genetically matched normotensive Wistar Kyoto (WKY) female rats were compared. In SH rats, there was a significant delay in the vaginal opening, but the regular 4-day estrous cycle followed afterwards. No significant changes were observed in the afternoon increase in serum LH, FSH and PRL on the day of proestrus in SH and WKY rats, although the basal levels of LH and PRL in the morning (11:00 h) were lower in SH rats than in WKY rats. The mean number of ova in SH rats was also less than in WKY rats, whereas the ovarian relative weight was similar in both species of rats. It can be said that SH rats undergo certain, but not critical, endocrine and/or neuroendocrine changes related to reproduction.     

Compensatory ovarian hypertrophy in the long-term hemicastrate rat: size distribution of growing and atretic follicles

In the intact rat, on estrus, the follicle-stimulating hormone (FSH) surge recruits nearly twice the correct number of follicles for ovulation, then, on metestrus, the excess follicles undergo atresia. In contrast, in the long-term hemicastrate rat, the FSH surge recruits fewer antral follicles on estrus, but there is little atresia on metestrus. To determine if fewer follicles are recruited by the FSH surge of long-term hemicastrates because the pool of follicles capable of responding to the FSH is smaller than in intact rats, preantral, antral, atretic, and healthy follicles were counted in ovaries of rats killed on each day of the estrous cycle. In general, there were only half as many healthy preantral follicles per rat in hemicastrates compared with intacts. There were an equal number of large antral follicles per rat in hemicastrates compared with intacts. Thus, compensatory hypertrophy did not extend to preantral follicles but was evident in large preovulatory follicles. These results suggest that fewer follicles are recruited on estrus in hemicastrate rats because fewer follicles are at the appropriate stage of development to respond to the FSH surge.     

Patterns of follicular growth during the four-day estrous cycle of the rat

Female Sprague-Dawley rats underwent laporatomy during metestrus at 70 to 75 days of age or remained untreated to study the effects of surgical stress on follicular growth. Groups of rats were killed on each day of a 4-day estrous cycle, serial sections of the ovaries were prepared histologically and the number and size of follicles with one or more complete layers of cuboidal granulosa cells were determined. Since no differences due to surgery were found, the data were pooled by day of the estrous cycle (17 or 18 rats/day of cycle) for characterization and comparison of size distribution of follicles on different days of the estrous cycle. Follicles were classified as atretic or healthy and divided into groups by increments of 20 micron of diameter for graphing. Data were analyzed by analysis of variance and least squares means. Significant differences were found in the distribution of both healthy and atretic follicles among days of the estrous cycle. At least 21 follicles/ovary were recruited from less than 260 micron into greater than 260 micron in diameter between proestrus and estrus, and the follicles for ovulation were selected by diestrus. A greater number of growing follicles of 70 to 100 micron in diameter were present at diestrus. From the disappearance of follicles greater than 260 micron between estrus and proestrus, it appears that atresia is a very rapid process.