Effects of long-term treatment with 17 β-estradiol and medroxyprogesterone acetate on water maze performance in middle aged female rats

Although previous research has indicated that hormone replacement therapy benefits memory in menopausal women, several recent studies have shown either detrimental or no effects of treatment. These inconsistencies emphasize the need to evaluate the role of ovarian hormones in protecting against age-related cognitive decline in an animal model. The present study investigated the effects of long-term hormone treatment during aging on the Morris water maze. Female Long Evans hooded rats were ovariectomized at middle age (12-13 months) and were immediately placed in one of five groups: no replacement, chronic 17 β-estradiol only, chronic 17 β-estradiol and progesterone, chronic 17 β-estradiol and medroxyprogesterone acetate (MPA), or cyclic 17 β-estradiol only. 17 β-estradiol was administered in the drinking water in either a chronic or cyclic (3 out of 4 days) fashion. Progesterone and MPA were administered via subcutaneous pellets. Following 6 months of hormone treatment, animals were tested on the Morris water maze. Animals performed four trials a day for 4 days and after the final day of testing a subset of animals completed a probe trial. Across 4 days of testing, rats receiving 17 β-estradiol in combination with MPA performed significantly worse than all other groups receiving hormone replacement. In addition on the last day of testing, chronic 17 β-estradiol administration was more beneficial than cyclic administration and no replacement. Thus compared to other hormone-treated groups, long-term 17 β-estradiol treatment in combination with MPA results in impaired performance on the spatial Morris water maze.     

Shifts in preferred learning strategy across the estrous cycle in female rats

The current status of the effects of ovarian steroids on learning and memory remains somewhat unclear, despite a large undertaking to evaluate these effects. What is emerging from this literature is that estrogen, and perhaps progesterone, influences learning and memory, but does so in a task-dependent manner. Previously, we have shown that ovariectomized rats given acute treatments of estrogen acquire allocentric or "place" tasks more easily than do rats deprived of estrogen, but acquire egocentric or "response" learning tasks more slowly than do those deprived of hormone, suggesting that estrogen treatment may bias the strategy a rat is able to use to solve tasks. To determine if natural fluctuations in ovarian hormones influence cognitive strategy, we tested whether strategy use fluctuated across the estrous cycle in reproductively intact female rats. We found that in two tasks in which rats freely choose the strategy used to solve the task, rats were more likely to use place strategies at proestrous, that is, when ovarian steroids are high. Conversely, estrous rats were biased toward response strategies. The data suggest that natural fluctuations in ovarian steroids may bias the neural system used and thus the cognitive strategies chosen during learning and memory.     

Cardioprotection in female rats subjected to chronic volume overload: synergistic interaction of estrogen and phytoestrogens

Intact female rats fed a high-phytoestrogen diet are protected against adverse left ventricular (LV) remodeling induced by chronic volume overload. We hypothesized that both phytoestrogens and ovarian hormones, particularly estrogen, are necessary for this dietary-induced cardioprotection. To test this hypothesis, eight groups of female rats were studied; rats were fed either a high-phytoestrogen (+phyto) or phytoestrogen-free diet. Groups included sham-operated rats, intact rats with fistula (Fist), ovariectomized rats with fistula (Fist-OX), and Fist-OX rats treated with estrogen (EST). Myocardial function and remodeling were assessed after 8 wk of volume overload using a blood-perfused isolated heart apparatus. Fist-OX rats developed significant ventricular dilatation and increased compliance vs. intact Fist rats, which were associated with a significant decrease in contractility. Estrogen treatment prevented pulmonary edema and attenuated LV hypertrophy and dilatation but did not maintain contractility. However, dietary phytoestrogens completely prevented LV dilatation in both the Fist+phyto and Fist-OX+EST+phyto groups but had no effect on LV remodeling in the Fist-OX+phyto group. Contractility was significantly greater in the estrogen-treated rats fed the phytoestrogen diet than in those treated with estrogen alone. Dietary phytoestrogens did not affect LV or uterine mass, serum estrogen, LV estrogen receptor expression, or cardiac function in sham animals. These data indicate that estrogen is not solely responsible for the cardioprotection exhibited by intact females and that phytoestrogens can work synergistically with ovarian hormones to attenuate ventricular remodeling induced by chronic volume overload in female rats.     

Estrogen replacement enhances acquisition of a spatial memory task and reduces deficits associated with hippocampal muscarinic receptor inhibition

A delayed matching-to-position (DMP) T-maze task was used to examine the effects of estrogen replacement on spatial learning and memory, as well as the ability of estrogen replacement to reduce performance deficits produced by acute systemic and intrahippocampal muscarinic cholinergic inhibition. Two experiments were performed. In Experiment 1, ovariectomized animals were trained to criterion on the DMP task and then tested with increased intertrial delays and following systemic scopolamine administration. The animals then received either continuous estrogen replacement or sham surgery and were retested beginning 10 days later. In Experiment 2, ovariectomized animals received guide cannulae implanted bilaterally into the hippocampus. Half of these animals also began receiving continuous estrogen replacement. Two months later, the animals were trained on the DMP task and then tested with increased intertrial delays and following systemic as well as intrahippocampal scopolamine administration. Animals received the same test battery 8 months later and were then immediately trained on a reversal task. The results indicate that estrogen-treated animals acquired the DMP task at a significantly faster rate than the ovariectomized, non-estrogen-treated controls. In addition, estrogen replacement significantly reduced deficits in DMP performance produced by intrahippocampal, but not systemic, scopolamine administration. This occurred when animals were tested after 3.5 months, as well as after 12 months, of continuous estrogen replacement. No evidence for an effect of estrogen replacement on spatial working memory or reversal learning was detected. These findings demonstrate that estrogen replacement can enhance acquisition of a spatial memory task and reduce performance deficits associated with hippocampal cholinergic impairment.     

Sexual Dimorphism in Development of Kidney Damage in Aging Fischer-344 Rats

BackgroundAging kidneys exhibit slowly developing injury and women are usually protected compared with men, in association with maintained renal nitric oxide.ObjectivesOur purpose was to test 2 hypotheses: (1) that aging intact Fischer-344 (F344) female rats exhibit less glomerular damage than similarly aged males, and (2) that loss of female ovarian hormones would lead to greater structural injury and dysregulation of the nitric oxide synthase (NOS) system in aging F344 rat kidneys.MethodsWe compared renal injury in F344 rats in intact, ovariectomized, and ovariectomized with estrogen replaced young (6 month) and old (24 month) female rats with young and old intact male rats and measured renal protein abundance of NOS isoforms and oxidative stress.ResultsThere was no difference in age-dependent glomerular damage between young or old intact male and female F344 rats, and neither ovariectomy nor estrogen replacement affected renal injury; however, tubulointerstitial injury was greater in old males than in old females. These data suggest that ovarian hormones do not influence these aspects of kidney aging in F344 rats and that the greater tubulointerstitial injury is caused by male sex. Old males had greater kidney cortex NOS3 abundance than females, and NOS1 abundance (alpha and beta isoforms) was increased in old males compared with both young males and old females. NOS abundance was preserved with age in intact females, ovariectomy did not reduce NOS1 or NOS3 protein abundance, and estrogen replacement did not uniformly elevate NOS proteins, suggesting that estrogens are not primary regulators of renal NOS abundance in this strain. Nicotinamide adenine dinucleotide phosphate oxidase-dependent superoxide production and nitrotyrosine immunoreactivity were increased in aging male rat kidneys compared with females, which could compromise renal nitric oxide production and/or bioavailability.ConclusionsThe kidney damage expressed in aging F344 rats is fairly mild and is not related to loss of renal cortex NOS3 or NOS1 alpha.     

Variation in the responsiveness of female rats to ovarian hormones as a function of preceding hormonal deprivation

Ovariectomized female rats were tested for the display of lordosis behavior 30 days after gonadectomy. They were then tested 7, 14, 21 and 81 days later following estrogen and progesterone treatment. Finally, on Day 88 of the experiment the animals were tested after either estrogen and progesterone treatment or after progesterone alone. The response to estrogen and progesterone treatment was found to be limited on the first test and on the fifth test which occurred after 2 mo without hormone treatment. When hormone treatment was repeated at seven day intervals (Tests 2–4) the tendency to show lordosis increased markedly. On the final test the animals given both hormones showed lordosis, while those which received only progesterone did not. The data suggest that the response to estrogen decreases after estrogen deprivation.     

Changes in pituitary oxytocin and vasopressin during the estrous cycle and after ovarian hormones: evidence for mediation by norepinephrine

Pituitary levels of oxytocin and vasopressin were maximal on the morning of proestrus, declined during estrus, and were lowest on metestrus in cycling female rats. Norepinephrine levels in the paraventricular nucleus were decreased on proestrus and estrus when compared with metestrus-diestrus. Norepinephrine did not vary in the supraoptic nucleus. Administration of estradiol benzoate to ovariectomized rats elevated oxytocin in the pituitary 54 hr later. This elevation was not affected by a subsequent injection of estrogen or progesterone. Estrogen priming did not affect vasopressin levels in the pituitary, but a second injection of estrogen or of progesterone 48 hr later increased vasopressin in the pituitary when measured 6 hr after the second injection. Vasopressin was decreased 30 hr after a second injection of estrogen. The ovarian hormone treatments that elevated pituitary vasopressin decreased steady state levels of norepinephrine in the paraventricular nucleus and reduced the depletion of norepinephrine after administration of the catecholamine synthesis inhibitor α-methyltyrosine, suggesting a decrease in turnover. Ovarian hormones did not affect norepinephrine in the supraoptic nucleus. The present results suggest a role for posterior pituitary hormones in reproductive processes and a role for noradrenergic mechanisms in the paraventricular nucleus in mediating the effects of ovarian steroids on pituitary vasopressin.     

Evaluation of the role of calcitonin deficiency in ovariectomy-induced osteopenia

Studies were carried out in rats to examine the role of calcitonin deficiency in the pathogenesis of ovariectomy-induced osteopenia. The parathyroid glands of 80 female Wistar rats were autotransplanted to their thigh muscle and the animals divided into 4 groups. Group 1 rats were sham ovariectomized, and thyroidectomized to make them calcitonin deficient; Group 2 rats were thyroidectomized, and ovariectomized to make them deficient in ovarian hormones as well; Group 3 rats were sham thyroidectomized and sham ovariectomized, and Group 4 rats were sham thyroidectomized and ovariectomized. A fifth group of rats were unoperated upon and served as controls. Thyroidectomized animals were maintained on thyroxine replacement and 11 months after ovariectomy all the animals were bled, killed and their femurs dissected out. In both the thyroid intact and thyroidectomized animals, ovariectomy decreased femur density significantly (P less than 0.01). Similarly, ovariectomy resulted in a decrease in femur calcium (P less than 0.01) in both groups of animals, and in a significant decrease in serum calcitonin (P less than 0.05) in the thyroid intact animals. We conclude from these findings that ovarian hormone deficiency can cause bone loss independently of lowering circulating calcitonin levels.     

Sex steroid hormones enhance immune function in male and female Siberian hamsters

Immune function is better in females than in males of many vertebrate species, and this dimorphism has been attributed to the presence of immunosuppressive androgens in males. We investigated the influence of sex steroid hormones on immune function in male and female Siberian hamsters. Previous studies indicated that immune function was impaired in male and female hamsters housed under short-day photoperiods when androgen and estrogen concentrations were virtually undetectable. In experiment 1, animals were gonadally intact, gonadectomized (gx), or gx with hormone replacement. Females exhibited the expected increase in antibody production over males, independent of hormone treatment condition, whereas male and female gx animals exhibited decreased lymphocyte proliferation to the T cell mitogen, phytohemagglutinin (PHA) compared with intact animals, and this effect was reversed in gx hamsters following testosterone and estradiol treatment, respectively. In experiment 2, testosterone, dihydrotestosterone, and estradiol all enhanced cell-mediated immunity in vitro, suggesting that sex steroid hormones may be enhancing immune function through direct actions on immune cells. In experiment 3, an acute mitogen challenge of lipopolysaccharide significantly suppressed lymphocyte proliferation to PHA in intact males but not females, suggesting that males may be less reactive to a subsequent mitogenic challenge than females. Contrary to evidence in many species such as rats, mice, and humans, these data suggest that sex steroid hormones enhance immunity in both male and female Siberian hamsters.     

Ovariectomy ameliorates dextromethorphan--induced memory impairment in young female rats

We have previously found that dextromethorphan (DM), over-the-counter cough suppressant, impairs memory retention in water maze task, when it is repeatedly administrated to adolescent female rats at high doses. In this study we examined first if ovariectomy ameliorates the DM-induced memory impairment in female rats, and then whether or not the DM effect is revived by estrogen replacement in ovariectomized female rats. Female rat pups received bilateral ovariectomy or sham operation on postnatal day (PND) 21, and then intraperitoneal DM (40 mg/kg) daily during PND 28-37. Rats were subjected to the Morris water maze task from PND 38, approximately 24 h after the last DM injection. In probe trial, goal quadrant dwell time was significantly reduced by DM in the sham operated group, however, the reduction by DM did not occur in the ovariectomy group. When 17beta-estradiol was supplied to ovariectomized females during DM treatment, the goal quadrant dwell time was significantly decreased, compared to the vehicle control group. Furthermore, a major effect of estrogen replacement was found in the escape latency during the last 3 days of initial learning trials. These results suggest that ovariectomy may ameliorate the adverse effect of DM treatment on memory retention in young female rats, and that estrogen replacement may revive it, i.e. estrogen may take a major role in DM-induced memory impairment in female rats.     

Cardiac norepinephrine release: modulation by ovariectomy and estrogen

Previously, we have demonstrated that in contrast to male rats, female rats do not show an age-related reduction of depolarization-elicited norepinephrine (NE) release from cardiac synaptosomes (resealed nerve terminals). These results suggest that sex hormones such as estrogen may modulate NE release from cardiac synaptosomes prepared from female rats. The present study was designed to test the hypotheses that long-term estrogen depletion, resulting from ovariectomy, and estrogen replacement alters depolarization-elicited NE release from cardiac synaptosomes. Female F344 rats were divided into two groups, one of which underwent bilateral ovariectomy, whereas the other underwent a sham operation. Three ovariectomized subgroups received daily injections of conjugated equine estrogens, delta8,9-dehydroestrone or 17 alpha-dihydroequilenin. Another ovariectomized control subgroup and the sham-operated animals received daily injections of vehicle. After 90 or 270 days of treatment, the animals were sacrificed. Cardiac synaptosomes were prepared from each heart, incubated with [(3)H]-NE, and used to evaluate NE release capacity by exposure to 50 mM K(+). The effectiveness of the ovariectomy and the estrogenic actions of the test compounds was confirmed by evaluating vaginal smears, determining uterine weights, and measuring serum luteinizing hormone (LH) concentrations. Ovariectomy (after both 90 and 270 days) significantly increased depolarization-induced NE release compared with sham-operated rats. Treatment with all three estrogenic preparations reduced NE release in ovariectomized rats to values similar to those observed in sham-operated animals. Interestingly, NE release rates from rats treated with conjugated estrogens for 270 but not 90 days were significantly below that observed in age-matched sham animals. These results demonstrate that estrogen modulates depolarization-elicited NE release from cardiac nerve terminals. Such modulation may represent a protective action by estrogen at the cardiac synapse.     

The mammary tumor response in triazine-treated female rats: a threshold-mediated interaction with strain and species-specific reproductive senescence.

Triazine herbicides are among the most heavily used agricultural pesticides. Although they possess a very low acute toxicity in animals, a mammary tumor response has been consistently observed in Sprague-Dawley (SD) female rats following chronic oral dosing of atrazine and simazine at and above maximum tolerated doses. However, a substantial collection of detailed research has clearly shown that triazines are not genotoxic or mutagenic, nor do they possess estrogenic agonist activity that might promote mammary tumor growth. Examination of estrous cycling records of atrazine-treated SD rats revealed a premature appearance of persistent estrous episodes, beyond the prevalent occurrence normally seen in untreated, aging SD rats. A significant correlation has been found between early or severe estrous cycle disruption of atrazine-treated rats and the early appearance of mammary tumors. In studies using SD female rats fed atrazine for 6 months, then ovariectomized and administered an estrogen-containing silastic s.c. implant, a deficient luteinizing hormone surge was observed at a 400 parts per million (ppm) dose, but not at 25 or 50 ppm. Because SD rats exhibiting persistent estrus also have a prolonged elevation of estrogen secretion, it is proposed that the triazine-associated mammary tumor response is promoted by the test animal's own estrogen from ovarian follicles that fail to ovulate because gonadotropin surge sufficiency is blocked by the high dose of herbicide. It is further proposed that, because reproductive senescence in SD rats is fundamentally different from menopause in women, the animal response to dosing, as well as the enormous requisite dosing level, establishes a safety margin of very low risk to human health from this mode of action.     

Estrogen influences the effect of immobilization stress on immunoreactive beta-endorphin levels in the female rat pituitary

Immunoreactive beta-endorphin (IR-BE) levels in the plasma, anterior pituitary (AP), the neurointermediate lobe of the pituitary (NIL), and the hypothalamus were determined in castrated female rats and castrated female rats treated with estradiol benzoate (estrogen), after exposure to acute (once for 45 min) or chronic (45 min each day for 15 consecutive days) immobilization stress. Acute and chronic stress increased plasma levels of IR-BE to the same extent in castrated female rats and castrated female rats treated with estrogen. In castrated female rats, acute stress produced an increase in the concentration of IR-BE in the AP, which was attenuated by the administration of estrogen. Although IR-BE in the NIL was not influenced by acute stress in castrated animals, exposure to acute stress resulted in an elevation in IR-BE levels in the NIL of rats given estrogen. Chronic stress did not affect the concentration of IR-BE in the AP of castrated females or castrated females treated with estrogen. Chronic stress did, however, increase the concentration of IR-BE in the NIL of castrated animals. This affect of stress on IR-BE levels in the NIL was potentiated by estrogen administration. IR-BE levels in the hypothalamus were reduced by estrogen and were not affected by acute or chronic stress, regardless of the gonadal steroid environment. As determined by column chromatography, administration of estrogen, as well as subjection to chronic stress, promoted the processing of the proopiomelanocortin precursor to form beta-lipotropin rather than beta-endorphin in the AP. By these methods, the only immunoreactivity detected in the NIL and the hypothalamus was beta-endorphin. These data indicate that IR-BE levels in the plasma, the AP, and the NIL of female rats are affected by immobilization stress and that estrogen modulates the effects of acute immobilization stress on IR-BE levels in the AP and the NIL and the effects of chronic immobilization stress on the levels of IR-BE in the NIL.     

Gonadotropin secretion during prolonged hyperprolactinemia: basal secretion and the stimulatory feedback effect of estrogen

Inoculation of cyclic female rats with the prolactin (Prl)/growth hormone-secreting pituitary tumor, MtT.W15, resulted in a cessation of estrous cyclicity within 5--10 days. Associated with this acyclicity was a persistently low serum concentration of estradiol and marked increases in both circulating Prl and progesterone. At Day 26 of acyclicity, basal serum luteinizing hormone (LH) values measured in samples taken every 20 min from 0900--1100 h were significantly reduced when compared to cyclic, nontumor animals on diestrus Day 2. There was no difference in basal follicle-stimulating hormone (FSH) concentrations. In a separate group of acyclic, tumor-bearing females 42--56 days after transplantation, a single s.c. injection of 20 micrograms estradiol benzoate (EB) at 1030 h elicited significant increases in both serum LH and FSH values between 1700 and 1830 h on the next day. The magnitude of the LH surge was reduced and that of FSH was increased in tumor-bearing animals when compared to cyclic, nontumor females given a similar EB injection on diestrus Day 1. These results demonstrate that chronic hyperprolactinemia is associated with inhibition of basal LH secretion and ovarian estrogen production and an increase in circulating progesterone concentrations. Nevertheless, the stimulatory feedback effects of estrogen on LH and FSH release are still present and functioning in acyclic female rats under chronically hyperprolactinemic conditions. These data suggest that the cessation of regular ovulatory cycles associated with hyperprolactinemia may be due to a deficiency of LH and/or estrogen secretion, but not to a lack of central nervous system response to the stimulatory feedback action of estrogen.     

Oxytocin secretion induced by osmotic stimulation in rats during the estrous cycle and after ovariectomy and hormone replacement therapy

Hyperosmolality is a potent stimulus for the secretion of oxytocin. Oxytocinergic neurons are modulated by estrogen and oxytocin secretion in rats varies according to the phase of the estrous cycle, with higher activity during proestrus. We investigated the oxytocin secretion induced by an osmotic stimulus (0.5 M NaCl) in female rats. Plasma oxytocin and the oxytocin contents in the neurohypophysis and the paraventricular and supraoptic nuclei were determined during the morning (8-9 h) and afternoon (17-18 h) of the estrous cycle and after ovariectomy followed or not by hormone replacement. Plasma oxytocin peaked in control animals during proestrus. Oxytocin content decreased in the paraventricular and supraoptic nuclei during proestrus and estrus compared to diestrus and increased in the neurohypophysis during proestrus morning. No significant difference was observed in the oxytocin content of the neurohypophysis, nuclei or plasma between ovariectomized animals and ovariectomized animals treated with estrogen or estrogen plus progesterone. Therefore, any ovarian factor other than estrogen or progesterone seems to play a direct or indirect role in the increase in oxytocin secretion. The osmotic stimulus caused an increase in plasma oxytocin throughout the estrous cycle. A reduction in oxytocin content during diestrus and an increase during proestrus were observed in the paraventricular nuclei. In ovariectomized animals, the treatment with estrogen potentiated the response of oxytocin to the osmotic stimulus, with the response being even stronger in the case of estrogen plus progesterone. In conclusion, the ovarian steroids estrogen plus progesterone could modulate the osmoreceptor mechanisms related to oxytocin secretion.     

Multihormonal regulation of hepatic sinusoidal Ntcp gene expression

Bile acids are efficiently removed from sinusoidal blood by a number of transporters including the Na+-taurocholate-cotransporting polypeptide (Ntcp). Na+-dependent bile salt uptake, as well as Ntcp, are expressed twofold higher in male compared with female rat livers. Also, estrogen administration to male rats decreases Ntcp expression. The aims of this study were to determine the hormonal mechanism(s) responsible for this sexually dimorphic expression of Ntcp. We examined castrated and hypophysectomized rats of both sexes. Sex steroid hormones, growth hormone, thyroid, and glucocorticoids were administered, and livers were examined for changes in Ntcp messenger RNA (mRNA). Ntcp mRNA and protein content were selectively increased in males. Estradiol selectively decreased Ntcp expression in males, whereas ovariectomy increased Ntcp in females, confirming the importance of estrogens in regulating Ntcp. Hypophysectomy decreased Ntcp mRNA levels in males and prevented estrogen administration from decreasing Ntcp, indicating the importance of pituitary hormones. Although constant infusion of growth hormone to intact males reduced Ntcp, its replacement alone after hypophysectomy did not restore the sex differences. In contrast, thyroid hormone and corticosterone increased Ntcp mRNA in hypophysectomized rats. Sex differences in Ntcp mRNA levels were produced only when the female pattern of growth hormone was administered to animals also receiving thyroid and corticosterone. Thyroid and dexamethasone also increased Ntcp mRNA in isolated rat hepatocytes, whereas growth hormone decreased Ntcp. These findings demonstrate the essential role that pituitary hormones play in the sexually dimorphic control of Ntcp expression in adult rat liver and in the mediation of estrogen effects.     

Effects of thyroid hormones on the receptor level in estrogen target organs

The influence of thyroid hormones on the turnover of cytoplasmic estrogen receptors in the liver, kidney and uterus of intact and ovariectomized female rats was studied under in vivo conditions. Thyroidectomy had no significant effect on the receptor level in the uterus but caused a substantial reduction of the receptor content in the liver and kidney. In livers of intact and ovariectomized animals receptor values were reduced with 70 and 80%, respectively, 30 days after thyroidectomy. Substitution with triiodothyronine (T3) restored the hepatic estrogen receptor concentration in thyroidectomized rats to the preoperative level. If rats that had been both ovariectomized and thyroidectomized were substituted with thyroid hormone for the same time period, the receptor level was increased but did not reach the level seen in animals that had been ovariectomized only. The effects of thyroid hormone substitution was found to be dose dependent and paradoxical. Thus, a high dose of 50 micrograms/day of triiodothyronine given to intact animals for nine days caused a 30% reduction in the hepatic receptor content. The same level of reduction was seen in the ovariectomized rat given a hormone dose of only 1 micrograms/day. When this type of rats was treated with the higher dose of triiodothyronine the reduction in hepatic estrogen receptors was 50%. These results are discussed in relation to existing information concerning the multihormonal regulation of estrogen receptor concentration in the rat liver.     

Divergent regulation of gonadotropin subunit mRNA levels by androgens in the female rat.

To examine the effects of gonadal steroids on the pretranslational regulation of the gonadotropin subunits in the female, adult female rats, beginning 7 or 28 days after ovariectomy, received daily injections of testosterone propionate (T), dihydrotestosterone propionate (D), or estradiol benzoate (E) for 7 days. Intact cycling females and ovariectomized rats that received vehicle served as controls. Serum was obtained for LH and FSH levels to assess changes in gonadotropin secretion. Total RNA from individual rats was recovered and analyzed by blot hybridization with specific radiolabeled cDNA probes for the alpha, LH beta, and FSH beta subunits. Autoradiographic bands were quantitated and standardized to mRNA levels in the intact animals. Ovariectomy resulted in a rise in serum gonadotropin levels and all three gonadotropin subunit mRNA levels. Estrogen replacement resulted in suppression of alpha, LH beta, and FSH beta mRNAs whether given at 7 or 28 days after ovariectomy. In contrast, whereas androgen replacement decreased alpha and LH beta mRNAs, D or T did not consistently suppress FSH beta mRNAs. We conclude that chronic estrogen administration to the castrated female rat uniformly suppresses all three gonadotropin subunit mRNA levels. In female rats, as in male rats, chronic androgen administration fails to negatively regulate FSH beta mRNAs.     

Effects of septal lesions on the lordotic behavior of weanling male and female rats

Twenty-four-day-old weanling male and female rats were either lesioned in the septal area, gonadectomized, lesioned and gonadectomized, or not treated. Tests for lordotic behavior were carried out at 27 and 28 days of age after priming with estradiol alone and with estradiol plus progesterone. A second set of tests for lordotic behavior was carried out at 47 and 48 days of age. In the interim period, some of the animals were given chronic estrogen treatment. In tests given at 27 days of age, septal lesions facilitated lordotic responding after estrogen priming; no differences were observed between male and female animals. At 47 and 48 days of age, however, unless male rats had been exposed to chronic estrogens following septal lesioning, no facilitation of estrogen-induced lordotic behavior occurred. In addition, it was found that female rats gonadectomized at 24 days of age and given no exposure to estrogens between the tests at 27 and 28 days and those at 47 and 48 days of age showed reduced sensitivity to estrogens, as compared to normal or estrogen-treated females, whether lesioned or not.     

Gonadal Steroids do not Affect Apolipoprotein E Expression in Aging Mouse Cerebral Cortex

The allelic variant of apolipoprotein (Apo) E4 is a known risk factor for the development of most common late onset form of Alzheimer’s disease (AD). As aging is associated with reduced circulating level of gonadal steroid hormones, hormone replacement therapies have been used for the possible treatment of AD. Both estrogen and testosterone have beneficial effects on brain due to interaction with apoE, but the underlying mechanism is still not clear. In this article, we report the effects of gonadectomy and hormone supplementation on apoE protein level in male and female mouse cerebral cortex during normal aging. We could not get any effect of gonadectomy and estradiol or testosterone treatment in adult and old mice of either sex. This suggests that during normal aging apoE protein level is not affected due to steroid hormone withdrawal or supplementation in the mouse cerebral cortex.