Hormone replacement therapy may alleviate sleep apnea in menopausal women: a pilot study

OBJECTIVE

The incidence of sleep apnea syndrome (SAS) in women increases after menopause. Progestins alone do not alleviate SAS in menopausal women. However, progestins may require concomitant estrogen administration and estrogen alone may stimulate breathing during sleep. To test these hypotheses, we studied the effects of estrogen alone and estrogen combined with progestin on SAS in menopausal women, using a prospective, cross-over, inception cohort study.

DESIGN

In this pilot study, five women who developed SAS after menopause underwent 2 nights of polysomnography to obtain a baseline, then returned for polysomnography after 3-4 weeks of taking micronized 17 beta-estradiol (E2) and after 10-12 days of taking E2 combined with medroxyprogesterone acetate (E2 + P). Sleep stages were scored according to Rechtshaffen and Kales, frequency and length of apneas were recorded for each subject each night, and the data were analyzed by Student's t test.

RESULTS

E2 and E2 + P both reduced the Respiratory Distress Index. E2 also raised the lowest oxygen desaturation associated with apneic episodes. Total minutes of rapid eye movement sleep increased, and the number of waking episodes decreased when the women were taking E2 and E2 + P, as previously reported.

CONCLUSIONS

Within 1 month after initiating E2 or E2 + P, SAS was reduced in all patients. The Respiratory Distress Index decreased by 25%, and the addition of progestin brought the SAS reduction to 50% in this pilot study. A randomized study in a large group of patients is justified by the findings of this study. Because SAS increases the risk of cardiovascular disease and fatal accidents, the amelioration of SAS by sex steroid hormones could have significant implications for the health of menopausal women.

Estrogen and microglia: A regulatory system that affects the brain

Sex hormones are involved in the physiological regulation of several aspects of behavior and neuroendocrine events. It has been accepted that such effects are mediated directly by steroid actions on neurons; however, new studies have shown that the glial cells are also affected by gonadal steroids. The microglia are one specialized brain glial cell type, which is a target for estrogen actions. In fact, we believe that many of the immune and nonimmune regulatory functions of microglia in the brain are influenced directly by estrogen via expression and secretion of cytokines, and growth factors by the microglia. The present review details only a section of the known aspects of microglial function, focusing mainly on nonimmune regulatory actions in the brain and their functional relationship with sex hormones. Moreover, we present evidence for the presence of estrogen receptor-beta (ERbeta) in rat microglial cells.

Role of astroglia in estrogen regulation of synaptic plasticity and brain repair

Astroglia are targets for estrogen and testosterone and are apparently involved in the action of sex steroids on the brain. Sex hormones induce changes in the expression of glial fibrillary acidic protein, the growth of astrocytic processes, and the degree of apposition of astroglial processes to neuronal membranes in the rat hypothalamus. These changes are linked to modifications in the number of synaptic inputs to hypothalamic neurons. These findings suggest that astrocytes may participate in the genesis of androgen-induced sex differences in synaptic connectivity and in estrogen-induced synaptic plasticity in the adult brain. Astrocytes and tanycytes may also participate in the cellular effects of sex steroids by releasing neuroactive substances and by regulating the local accumulation of specific growth factors, such as insulin-like growth factor-I, that are involved in estrogen-induced synaptic plasticity and estrogen-mediated neuroendocrine control. Astroglia may also be involved in regenerative and neuroprotective effects of sex steroids, since astroglia formation after brain injury or after peripheral nerve axotomy is regulated by sex hormones. Furthermore, the expression of aromatase, the enzyme that produces estrogen, is induced de novo in astrocytes in lesioned brain areas of adult male and female rodents. Since astroglia do not express aromatase under normal circumstances, the induction of this enzyme may be part of the program of glial activation to cope with the new conditions of the neural tissue after injury. Given the neuroprotective and growth-promoting effects of estrogen after injury, the local production of this steroid may be a relevant component of the reparative process.

FasL (CD95L, Apo1L) is expressed in the normal rat and human brain: evidence for the existence of an immunological brain barrier

Despite the mechanical blood-brain barrier, activated T-cells can cross brain vessels. Thus, the CNS is routinely surveyed by immune competent cells; yet the healthy brain is not a target of antigen-specific immune reactions. Therefore, mechanisms must exist to prevent brain-antigen-specific T-cells from inducing immune responses. Data indicate that activated T-cells entering the CNS may undergo apoptosis rather than leaving the brain to induce immune responses. Applying RT-PCR, Western-blots, and immunocytochemistry, we have demonstrated expression of the apoptosis-inducing protein Fas ligand on astrocytes and neurons of apparently normal rat and human brains. FasL-positive astrocytes were often situated in close vicinity to cerebral blood vessels in vivo and induced apoptosis of Fas expressing Jurkat cells in vitro. We propose that similar to other immune privileged organs FasL-induced apoptosis of activated T-cells in the brain protects the tissue from self damaging immune attacks by forming an immunological brain barrier.

Macrophage derived growth factors modulate Fas ligand expression in cultured endometrial stromal cells: a role in endometriosis

Fas-Fas ligand (FasL) interactions play a significant role in the immune privilege status of certain cell populations, and several cytokines and growth factors can modulate their expression. When a FasL-expressing cell binds a Fas-bearing immune cell, it triggers its death by apoptosis. In this study, we demonstrate that normal human endometrial epithelial but not stromal cells express FasL. Moreover, we showed that macrophage-conditioned media induced FasL expression by endometrial stromal cells in a dose-dependent manner. To elucidate which macrophage product was responsible for the up-regulation of FasL, endometrial stromal cell cultures were treated with the macrophage products platelet-derived growth factor (PDGF), transforming growth factor (TGF)-beta1, and basic fibroblast growth factor (bFGF). The first two (which are known to be elevated in the peritoneal fluid of women with endometriosis) induced a dose-dependent up-regulation of FasL expression, which was specifically inhibited by the antibody. Interestingly, bFGF (which is not elevated in peritoneal fluid of women with endometriosis) did not induce any response. These results suggest that the pro-inflammatory nature of the peritoneal fluid of women with endometriosis induces the FasL expression by regurgitated endometrial cells, and signals Fas-mediated cell death of activated immune cells. This could be a mechanism for endometrial cells to escape immune surveillance, implant and grow.

Induction of apoptosis and suppression of clonogenicity of ovarian carcinoma cells with estrogen mustard

BACKGROUND

This study was conducted to evaluate whether estramustine (estrogen mustard [EM]) is a promising alternative in the treatment of patients with epithelial ovarian carcinoma (OVCA). EM is a microtubule-associated protein (MAP) specific antimicrotubule agent with low toxicity.

METHODS

The authors investigated the ability of EM to induce apoptosis and suppress colony formation of OVCA cells. Paclitaxel was used as a positive control. DNA electrophoresis and terminal deoxynucleotidyl dUTP-X nick end labeling (TUNEL) assays were used to detect internucleosomal DNA fragmentation. Flow cytometry was used to identify apoptotic cells and disturbance of the cell cycle of EM-treated OVCA cells further. Quantitation of detached cultured cells also provided a relative estimate of the apoptotic response of OVCA cells to treatment with EM. The colony formation assay was used to evaluate the effects of EM on clonogenicity.

RESULTS

The effects of EM on four OVCA cell lines in culture were highly similar to those of paclitaxel in causing apoptosis and inhibiting clonogenicity. DNA electrophoresis and TUNEL assays showed that EM induced internucleosomal DNA fragmentation in OVCA cells. Flow cytometry showed changes typical of apoptotic changes and cell cycle block and synchronization at the G2/M-phase. Counting of detached cells showed a log-dose response to EM treatment. The colony formation assay also showed a log-dose response suppression of OVCA cell clonogenicity after treatment with EM.

CONCLUSIONS

EM may be a promising candidate in the clinical treatment of patients with OVCA. The lower toxicity and MAP specific action of EM is a novel chemotherapy for OVCA.

The potential of aromatase inhibitors in breast cancer prevention

Substantial evidence supports the concept that estrogens cause breast cancer in animals and in women but the precise mechanism is unknown. The most commonly held theory is that estrogens stimulate proliferation of breast cells and thus statistically increase the chances for genetic mutations which could result in cancer. Another theory is that estrogen metabolism generates oxygen-free radicals and quinones which produce both stable and unstable DNA adducts. Both result in genetic mutations which accumulate and could ultimately cause cancer. A major criticism of the latter hypothesis is that breast tissue contains insufficient concentrations of estrogen for accumulation of genotoxic metabolites. Our hypothesis is that breast tissue estrogen levels, as a result of in situ synthesis, are much higher than previously thought. We and others have shown that estrogen can be made in the breast itself through conversion of androgens to estrogens, a process catalyzed by the enzyme aromatase. The levels of estrogen in the breast increase when aromatase is overexpressed. With sufficient amounts of aromatase in breast tissue, enough estradiol as substrate should be available to allow formation of substantial amounts of genotoxic metabolites. We postulate that aromatase overexpression may in this way cause breast cancer. As evidence supporting this concept, four animal models of aromatase overexpression and either breast cancer or premalignant lesions have been described. We have provided evidence that normal breast tissue can make estrogen and that certain stimulatory compounds can increase aromatase activity in the breast by nearly 10,000-fold. If our concepts are correct, it might be possible to prevent breast cancer by blocking the aromatase enzyme. Drugs are currently available to inhibit aromatase nearly completely without causing significant side-effects. Aromatase inhibitors might be more effective than antiestrogens in preventing breast cancer because of their dual role to block both initiation and promotion of breast cancer. To inhibit the initiation process, these inhibitors would reduce levels of the genotoxic metabolites of estradiol by lowering estradiol concentrations in tissue. At the same time, aromatase inhibitors would inhibit the process of tumor promotion by lowering tissue levels of estradiol and thus blocking cell proliferation. These concepts provide a strong rationale for studies of aromatase inhibitors to prevent breast cancer.

Early postpartum discharges. Impact on distress and outpatient problems

OBJECTIVE

To determine the impact of shortened postpartum hospital stays on common clinical phenomena in a sociodemographically diverse, unselected group of general maternity patients.

DESIGN

Observational cohort study in which the preapproved hospital stay duration of either 1 or 2 nights was set by third-party payers before each mother's admission.

SETTING

Yale-New Haven Hospital, New Haven, Conn, from June 19 through August 10, 1995.

PATIENTS

Two hundred forty-four volunteers from among 400 eligible deliveries.

MAIN OUTCOME MEASURES

Readmission within 1 month of hospital discharge, report of outpatient morbidity and use of outpatient health services within 1 week of discharge, status of breast-feeding during the first post-discharge week, and patient satisfaction.

RESULTS

At discharge from the hospital, the hospital stay was regarded as "too short" by 80 (47%) of 171 mothers and 19 (26%) of 73 mothers in the 1- and 2-night groups, respectively (P = .002). Although readmission rates were similar (5% vs 3%, P = .48), the 1-night group reported significantly more morbidity in the newborns (31% vs 16%, P = .03) and averaged more pediatric visits (96 vs 54 per 100 newborns, P = .002). Mothers in the 1-night group also reported more fatigue (49% vs 29%, P = .001) and more worries about their newborns' health (24% vs 11%, P = .02). They were less likely to start breast-feeding (64% vs 77%, P = .06), and, if they started, were somewhat more likely to stop prematurely (14% vs 8%, P = .43). A series of disturbing events was reported only in the 1-night group.

CONCLUSIONS

In a relatively unselected group, mothers who stayed 1 night after routine vaginal delivery reported more distress and more pediatric problems and had greater use of outpatient health services than mothers who stayed 2 nights.

Effect of estrogen on brain activation patterns in postmenopausal women during working memory tasks

CONTEXT

Preclinical studies suggest that estrogen affects neural structure and function in mature animals; clinical studies are less conclusive with many, but not all, studies showing a positive influence of estrogen on verbal memory in postmenopausal women.

OBJECTIVE

To investigate the effects of estrogen on brain activation patterns in postmenopausal women as they performed verbal and nonverbal working memory tasks.

DESIGN

Randomized, double-blind, placebo-controlled, crossover trial from 1996 through 1998.

SETTING

Community volunteers tested in a hospital setting.

PATIENTS

Forty-six postmenopausal women aged 33 to 61 years (mean [SD] age, 50.8 [4.7] years).

INTERVENTION

Twenty-one-day treatment with conjugated equine estrogens, 1.25 mg/d, randomly crossed over with identical placebo and a 14-day washout between treatments.

MAIN OUTCOME MEASURES

Brain activation patterns measured using functional magnetic resonance imaging during tasks involving verbal and nonverbal working memory.

RESULTS

Treatment with estrogen increased activation in the inferior parietal lobule during storage of verbal material and decreased activation in the inferior parietal lobule during storage of nonverbal material. Estrogen also increased activation in the right superior frontal gyrus during retrieval tasks, accompanied by greater left-hemisphere activation during encoding. The latter pattern represents a sharpening of the hemisphere encoding/retrieval asymmetry (HERA) effect. Estrogen did not affect actual performance of the verbal and nonverbal memory tasks.

CONCLUSIONS

Estrogen in a therapeutic dosage alters brain activation patterns in postmenopausal women in specific brain regions during the performance of the sorts of memory function that are called upon frequently during any given day. These results suggest that estrogen affects brain organization for memory in postmenopausal women.

Aromatase and estrogen receptor immunoreactivity in the coronary arteries of monkeys and human subjects

OBJECTIVE

The objective of this study was to determine whether estrogen could be formed locally in the coronary arteries.

DESIGN

Coronary arteries were examined from monkeys (Macaca fascicularis, one male and one female) and human subjects (one premenopausal woman, one postmenopausal woman, and one man) by immunocytochemistry, using purified antisera against human placental estrogen synthetase (aromatase) and ER alpha. The arteries were graded for the amount of atherosclerosis.

RESULTS

There was clear immunopositivity for both aromatase and estrogen receptors in all arteries studied. Although all endothelial cells (CD31 positive) stained for both antigens, the staining in macrophages, fibroblasts, and smooth muscle cells was irregular.

CONCLUSION

The present results provide the first evidence for the local formation of estrogen in the coronary arteries. In addition to complementing the evidence of a cardioprotective effect of estrogen on the coronary circulation, our results highlight the potential importance of local regulation of estrogen formation and the role of available precursor androgens in maintaining the cardiovascular system.

The Fas/Fas-ligand system: a mechanism for immune evasion in human breast carcinomas

Breast tumors are frequently associated with a predominantly lymphocytic infiltrate, which constitutes an immune response against the tumor. In spite of this massive infiltrate, the immune response appears to be inefficient and the tumor is able to evade it. We propose that in breast cancer, tumor escape from immunological surveillance results from the induction of apoptosis of Fas-bearing activated lymphocytes by FasL-bearing breast cancer cells. To test this proposal we studied the expression of FasL by human breast carcinomas and the MCF-7 breast cancer cell line by RT-PCR, immunohistochemistry, and Western Blot. Moreover, we describe the presence of apoptosis and Fas expression in the lymphocytic population surrounding the tumor. Strong membranous and cytoplasmic staining was detected in ductal carcinomas and hyperplastic breast tissue, but it was absent from normal breast tissue. No staining was found in normal glands in the non-tumor quadrants; however, the normal appearing ducts surrounding the carcinoma (tumor quadrant) showed intense immunoreactivity. Apoptosis was found predominantly among the lymphocytic population, as well as in the blood vessels and fibro-fatty tissue close to the tumor. Further characterization of apoptotic cells demonstrated that they were CD3+ cells. Our results suggest the breast tumors may elude immunological surveillance by inducing, via the Fas/FasL system, the apoptosis of activated lymphocytes. Recent data have demonstrated FasL RNA in other tumor types. Upregulation of FasL expression in hyperplastic and normal breast ducts close to the tumor also suggests a possible role in early neoplastic transformation and proliferation.

Phasic synaptic remodeling of the rat arcuate nucleus during the estrous cycle depends on insulin-like growth factor-I receptor activation

Insulin-like growth factor-I (IGF-I) has trophic and plastic effects on neurons and glial cells and modulates neuroendocrine events by acting at the level of the hypothalamus. IGF-I and estrogen signaling interact to regulate in vitro hypothalamic neuronal survival and differentiation. In vivo, IGF-I levels fluctuate in the rat hypothalamic arcuate nucleus during the estrous cycle in parallel with a phasic remodeling of synaptic contacts and glial cell processes. Both the fluctuation of IGF-I levels and the synaptic and glial changes are induced by estrogen. The possible role of IGF-I in the regulation of arcuate nucleus synaptic plasticity has been assessed in the present study by intracerebroventricular administration to cycling female rats of a specific IGF-I receptor antagonist. In agreement with previous findings, the number of synaptic inputs to arcuate neuronal somas in control rats showed a significant decrease between the morning of proestrus and the morning of estrus. This decline in synaptic inputs and the accompanying increase in glial ensheathing of neuronal somas were blocked by the IGF-I receptor antagonist. In contrast, the IGF-I receptor antagonist did not affect the basal number of synapses or the morphology of synaptic terminals or length of the synaptic contacts. These findings indicate that IGF-I receptor activation may be involved in the phasic remodeling of arcuate nucleus synapses during the estrous cycle. Res.

Raloxifene induces neurite outgrowth in estrogen receptor positive PC12 cells

OBJECTIVE

It is well established that gonadal steroids have direct in vivo and in vitro effects on neurons. To further study these effects, we used rat PC 12 cells to examine the effects of estrogen receptor (ER) ligands on neuronal morphology.

DESIGN

PC 12 cells constitutively express ER beta, but only strongly express ER alpha after long-term priming with nerve growth factor (NGF). We therefore primed PC12 cells with NGF for 14 days before testing them for estradiol (10(-9)M)- and/or raloxifene (10(-7) M)-induced neurite growth. Neurite growth was assessed by quantitative light microscopy. As control, ER status of the PC12 cells was assessed by reverse transcription-polymerase chain reaction (RT-PCR).

RESULTS

In this study, both estradiol and raloxifene induced the outgrowth of neurites in NGF-treated PC 12 cells (p < 0.05). The combination of estradiol- and raloxifene-induced neurite growth was statistically greater than the effects of either agent alone. RT-PCR confirms that NGF-treated PC 12 cells express both ERalpha and ERbeta.

CONCLUSIONS

This report is the first on the neurotrophic effect of raloxifene. At 10(-7) M, raloxifene's effect equaled that of estradiol; moreover, raloxifene did not block the neurite growth of simultaneously estradiol-treated PC 12 cells, despite its functional antiestrogenic effects in vivo. We conclude that raloxifene is estrogen agonistic in this animal model and therefore studies are warranted to delineate the relationship between steroidal estrogen and raloxifene.

Macrophages, estrogen and the microenvironment of breast cancer

Estrogen is a major mitogenic stimulus to established breast cancer. Estrogen sources include ovarian, extraglandular sites and breast tissue. Which source primarily maintains benign and breast cancer tissue estrogen concentrations remains unclear. While macrophages may comprise up to 50% of the mass of breast carcinomas, previous studies neglected to study them as possible sources of estrogen. We present evidence that breast macrophages constitute an in situ source of estradiol and that the amount produced is sufficient to mediate cellular proliferation. We utilized immunohistochemistry and RT-PCR to study cell-specific aromatase expression in (i) 29 breast biopsies, (ii) human monocytes/macrophages and (iii) a myeloid cell line (THP-1) capable of differentiating into macrophages. Use of a breast cancer cell line (MCF-7) provided biologic confirmation of the role of aromatization in cell proliferation. We demonstrated considerable amounts of immunoreactive-aromatase (irARO) in breast tissue macrophages and a positive correlation between the proportion of irARO present in macrophages and lesion severity. Using in vitro techniques, we demonstrated that monocytes and THP-1 cells require differentiation into macrophages to produce aromatase in amounts approaching placental levels. The amount of estrogen produced by THP-1 cells stimulated MCF-7 cells to proliferate, an effect blocked by aromatase inhibitors. Estrogen production by macrophages in breast tissue appears sufficient to stimulate the proliferation of adjacent epithelial cells and to autoregulate cytokine production. These findings represent a new dimension of cellular regulation in breast tissue with major biologic implications, amenable to pharmacological manipulation.

Reproductive actions of phytoestrogens

This chapter reviews the reproductive actions of phytoestrogens, comparing mechanisms of action, dose-response relationships, and human exposures. Although a wide range of biochemical actions have been reported for phytoestrogens, in vitro tests suggest that phytoestrogens may be more likely to act through receptor-mediated mechanisms than through enzyme inhibition. Epithelial cell proliferation in the reproductive tract and anestrus are well-documented actions of isoflavonoids in experimental studies of animals. However, thus far, soy-based diets have generally failed to produce epithelial proliferation in ovariectomized rats and monkeys or menopausal women, and clinical studies have produced mixed evidence for effects of soy isoflavones on the human menstrual cycle or post-menopausal gonadotropin secretion. There has been considerable interest in the use of phytoestrogens as oestrogen replacement therapy in menopausal women. Reported results of initial clinical trials have been mixed, and it is unclear whether isoflavones in presently advised doses can substantially reduce menopausal symptoms. Some recent trials with oral isoflavone supplements report reductions in hot flushes, vaginal dryness, and breast pain. There is also limited clinical evidence for protective actions of isoflavones in mammary cancer. Like other oestrogenic substances, the isoflavonoids are effective differentiating agents in rodent models of development. The consequences of these actions for humans is of interest due to the high concentrations of isoflavonoids in some infant formulae. Thus, it is likely that some humans may experience greater exposure to phytoestrogens in infancy than in any other lifestage. At the time of writing, no ill effects of such exposure have been reported.

Demonstration of aromatase activity and its regulation in breast tumor and benign breast fibroblasts

Breast tumors from post-menopausal women contain higher amounts of estradiol than would be predicted from levels circulating in plasma. This observation raised the hypothesis that tumors may synthesize estradiol in situ and increase their tissue estradiol levels via this mechanism. The key enzyme involved in tissue estrogen synthesis, aromatase, is present in breast tumors but, according to some investigators, not in sufficient concentration to be biologically meaningful. We postulated that foci of cells in breast tumors might contain high amounts of aromatase and this locally produced estrogen might act in a paracrine or autocrine fashion. To test this hypothesis, we utilized immunohistochemistry to localize the aromatase enzyme, an histological scoring system to quantitate it, and culture of isolated breast cells to demonstrate its potential regulation. In 26 archival breast tumors, 16 (62%) contained aromatase by radiometric assay. With the immunohistochemical method, we detected areas with staining in the stroma as well as tumor epithelial cells. Staining ranged from the intensity approaching that seen in placenta to levels just distinguishable from background. We adopted an histological scoring system (H-score) from that used to quantitate progesterone receptor levels in tissue and used it to quantitate aromatase activity. A higher histologic score was found in stromal spindle cells (13) than in tumor epithelial cells (4.8). The biochemical aromatase results correlated with the H-score of stromal but not epithelial cells. To further study stromal cells from tumors, we isolated stromal cells from breast tumors and the benign areas of breast distal to the tumor and grew them in culture. Addition of dexamethasone, phorbol esters, and cyclic AMP analogues stimulated aromatase enzyme and messenger RNA levels substantially. Use of aromatase enzyme inhibitors such as letrozole blocked estrogen production but did not alter aromatase message levels. Epithelial cells, whether nonmalignant or cancer derived, exhibited no regulation by dexamethasone, phorbol esters, or cAMP analogues. These data, taken together, suggest that stromal cells may be more important than epithelial cancer cells for estrogen production in breast tumors. The ability to stimulate aromatase activity substantially with various enhancers of aromatase provides further credence for an important biologic role of estrogen production in tumor tissue.

Monosynaptic pathway between the arcuate nucleus expressing glial type II iodothyronine 5'-deiodinase mRNA and the median eminence-projective TRH cells of the rat paraventricular nucleus

Recent evidence suggests that the thyroid regulation of thyrotropin-releasing hormone (TRH)-containing neurons in the paraventricular nucleus of the hypothalamus involves the activation of other hypothalamic neural circuits. For example, the arcuate nucleus and not the paraventricular nucleus contains the highest enzyme activity of 5'-deiodinase type II, an enzyme that is pivotal for the local synthesis of T3. This experiment was undertaken to demonstrate whether a monosynaptic pathway exists between the arcuate nucleus and those TRH cells of the paraventricular nucleus that are neuroendocrine, i.e. project to the external layer of the median eminence. A specific cRNA probe derived from the coding region of deiodinase type II was used for the in situ hybridization histochemistry which was combined with immunocytochemistry for a specific marker of glial cells, glial fibrillary acidic protein (GFAP). The hybridization signals were present within the hypothalamus in the arcuate nucleus-median eminence region and in the periventricular area. The periventricular labeling was localized to the ependymal layer of the third ventricle and no hybridization product was detected in the paraventricular nucleus and other hypothalamic nuclei adjacent to the third ventricle. Within the median eminence, numerous cells containing the hybridization product were located in the internal layer adjacent to the floor of the third ventricle and in the external layer adjacent to the surface of the brain. In the dorso- and ventromedial regions of the arcuate nucleus, deiodinase type II mRNA-containing cells were also detected. Numerous type II deiodinase mRNA-containing cells in the median eminence and arcuate nucleus were also found to be immunopositive for GFAP. The abundance of arcuate cells expressing the hybridization product was lower than those in the periventricular region or in the median eminence. The anterograde tracer, Phaseolus vulgaris leucoagglutinin, was injected into the medial parts of the arcuate nucleus where the in situ hybridization experiment detected deiodinase type II mRNA. Simultaneously with the anterograde tracing, the retrograde tracer, Fluoro-Gold, was injected into either the median eminence or the general circulation. Light and electron microscopic double and triple immunolabeling experiments on vibratome sections of colchicine-pretreated animals revealed that arcuate fibers innervate TRH cells within the parvicellular region of the paraventricular nucleus. Populations of these TRH cells receiving afferents from the arcuate nucleus were also retrogradely labelled from either the median eminence or the general circulation indicating their direct role in the regulation of thyrotropin secretion from the anterior pituitary. The majority of arcuate nucleus efferents on TRH cells were found to establish symmetrical synaptic connections. The present results provided direct evidence of a monosynaptic pathway between the hypothalamic site of local thyroid hormone production, the arcuate nucleus, and neuroendocrine TRH cells in the paraventricular nucleus. This signalling modality may play an important role in thyroid feedback on TRH cells. Since the arcuate nucleus is involved in the regulation of central mechanisms controlling diverse homeostatic functions, including reproduction and feeding, the pathway described in this study may also carry integrated signals related to reproduction and ingestion to TRH-producing cells.

Segregation of the intra- and extrahypothalamic neuropeptide Y and catecholaminergic inputs on paraventricular neurons, including those producing thyrotropin-releasing hormone

In fasting, declining circulating thyroid hormone levels coincide with suppressed thyrotropin-releasing hormone (TRH) mRNA and peptide levels and elevated NPY release and binding in the parvicellular paraventricular nucleus (PVN). It is suggested that NPY, in parallel with triggering feeding behavior, interrupts normal thyroid feedback in food deprivation. To gain further insights into the involvement of NPY in the regulation of TRH cells, this study sought to elucidate the source of the NPY innervation of TRH neurons. The median forebrain bundle (MFB) that carries the ascending NPY fibers from the brain stem catecholaminergic nuclei was unilaterally transected. Animals were sacrificed 2 and 5 days after surgery and double immunocytochemistry for NPY and TRH or tyrosine hydroxylase (TH) and TRH was performed on sections from the PVN. Two days after the surgery, light microscopic examination revealed no changes in the numbers of NPY boutons making putative contacts with TRH cell bodies and proximal dendrites. On the other hand, under the electron microscope, NPY- and TH-immunoreactive fibers containing autophagous cytolysosomes, an early sign of catecholaminergic fiber degeneration, were found to establish asymmetric synapses on distal dendrites and dendritic spines of TRH-immunoreactive cells. However, the same electron microscopic analysis did not reveal any degenerating NPY-immunolabeled fibers in synaptic contact with TRH cell bodies and proximal dendrites. Five days after the surgery, when NPY and TH immunoreactivities were no longer detected in the ipsilateral MFB, no decrease in the numbers of NPY and TH boutons on TRH cell bodies and proximal dendrites could be detected, when compared to the contralateral side. Electron microscopy revealed fibers with Wallerian degeneration establishing asymmetric synapses exclusively on the distal dendrites and spines of TRH neurons. In conclusion, this study demonstrated that the NPY and catecholaminergic input on PVN TRH cells are of mixed origin. The cell bodies and proximal dendrites of TRH neurons receive a robust, putative inhibitory NPY input from the hypothalamus. The distal dendrites and dendritic spines of the TRH cells also receive a putative stimulatory NPY input from the brain stem catecholaminergic neurons. It is suggested that because of its proximal location and abundance, NPY of hypothalamic origin exerts a tonic inhibition on PVN TRH cells that interrupts negative thyroid feedback during food deprivation. Furthermore, it is likely that a general inhibition and not stimulation of parvicellular PVN activity may underlie the triggering of feeding behavior by hypothalamic NPY.

Fasting-induced increase in type II iodothyronine deiodinase activity and messenger ribonucleic acid levels is not reversed by thyroxine in the rat hypothalamus

The importance of local formation of T3 in the feedback effect of the thyroid gland on hypothalamic TRH-producing cells has been established. Primary failure of the thyroid gland results in a fall in circulating T4 and T3 levels, leading to an elevation in the production and release of TRH in the hypothalamic paraventricular nucleus. In contrast, during short term fasting, declining plasma levels of thyroid hormones coincide with suppressed TRH production and release. In the brain, the prevalent enzyme that converts T4 to T3 is type II iodothyronine deiodinase (DII). The present study was undertaken to determine whether a differential hypothalamic expression of type II deiodinase may exist in fasted rats and in animals that are hypothyroid due to the failure of the thyroid gland. Using in situ hybridization, we assessed type II deiodinase messenger RNA (mRNA) levels in the hypothalamus of rats that were control euthyroid, hyperthyroid (T4), hypothyroid induced by propylthiouracil (PTU), and fasted. A group of fasted rats also received exogenous T4. DII mRNA was detected around the third ventricle, including the ependymal layer and adjacent periventricular regions as well as in the arcuate nucleus and the external layer of the median eminence. Quantitative in situ hybridization analysis demonstrated that PTU treatment and short term fasting resulted in significant elevations in DII messenger levels compared with those in euthyroid controls. Three weeks of PTU administration induced a consistent decline in circulating T3 and undetectable T4 levels, whereas 3 days of fasting resulted in only a 50% fall in the concentration of serum thyroid hormones. Interestingly, however, the expression of the DII mRNA was more than 2-fold higher in fasted animals compared with the values in PTU-treated rats. Furthermore, although T4 administration repressed DII mRNA expression in euthyroid animals, the same treatment had no effect on the fasting-induced elevations of DII message. To assess whether DII enzymatic activity is also affected during food deprivation, hypothalami were dissected out, and DII activity was measured in control euthyroid, fasted, and fasted plus T4-treated rats. To determine whether comparable changes in plasma thyroid hormone levels induced by fasting and PTU treatment could have affected DII enzymatic activity in a similar manner, animals were injected ip with PTU for 5 days to decrease plasma thyroid hormones to levels similar to those caused by fasting. DII enzymatic assay showed a significant increase in DII activity in fasted and fasted plus T4-treated animals compared with those in euthyroid controls and PTU-treated rats. No significant changes were found in PTU-treated rats compared with euthyroid animals. These data indicate that during short term fasting, a signal of nonthyroid origin underlies the robust elevation of DII production and activity in the hypothalamus. Thus, we propose that during the initial phase of food deprivation, an increased negative thyroid feedback exists on the hypothalamus due to locally formed T3. This local hyperthyroidism may, in turn, induce the suppression of TRH under these conditions.

Kainate glutamate receptors (GluR5-7) in the rat arcuate nucleus: relationship to tanycytes, astrocytes, neurons and gonadal steroid receptors

Glutamate action, through its ionotropic, kainate receptors, has been implicated in gonadal steroid-dependent mechanisms of the arcuate nucleus. The objective of the present study was to determine the expression of kainate glutamate receptors in neural and glial elements of this area and their potential relationship to gonadal steroid receptors. Single and double label, light and electron microscopic immunocytochemistry for kainate glutamate receptors and estrogen or androgen receptors revealed the existence of glutamate (GluR) 5-7 kainate receptors in tanycytes, astrocytes and neurons of the arcuate nucleus. In the arcuate nucleus, subsets of GluR5-7-containing neurons were also immunopositive for estrogen (20%) and/or androgen receptors (23%). Glial elements, however, lacked labeling for gonadal steroid receptors. The coexistence of gonadal and kainate receptors in the same perikarya of arcuate nucleus cell populations suggests hormone regulation of excitatory neurotransmission through ionotropic glutamate receptors in these regions. It is also indicated that a kainate receptor-mediated glutamate action may participate in neuro-glial interaction in the arcuate nucleus that, in turn, may underlie the morphological synaptic plasticity induced by gonadal steroids.

Estradiol upregulates Bcl-2 expression in adult brain neurons

Bcl-2, a protein which negatively modulates apoptosis, is up-regulated by estrogen in several tissues. To determine the effect of estradiol on Bcl-2 in the adult brain, its immunoreactive distribution was examined in the hypothalamic arcuate nucleus of female rats under different endocrine conditions. The number of Bcl-2-immunoreactive neurons was significantly increased (p < 0.001) on the day of estrus compared with proestrus, diestrus and metestrus, was decreased by ovariectomy and showed a dose-response increase after estradiol administration to ovariectomized rats. Progesterone, when injected simultaneously with estradiol, reduced the effect of estradiol. These findings indicate that ovarian hormones regulate Bcl-2 in hypothalamic neurons and suggest that this protein may be involved in the neuroprotective effects of estrogen.

Role of astroglia and insulin-like growth factor-I in gonadal hormone-dependent synaptic plasticity

Gonadal hormones exert a critical influence over the architecture of specific brain areas affecting the formation of neuronal contacts. Cellular mechanisms mediating gonadal hormone actions on synapses have been studied extensively in the rat arcuate nucleus, a hypothalamic center involved in the feed-back regulation of gonadotropins. Gonadal steroids exert organizational and activational effects on arcuate nucleus synaptic connectivity. Perinatal testosterone induces a sexual dimorphic pattern of synaptic contacts. Furthermore, during the preovulatory and ovulatory phases of the estrous cycle there is a transient disconnection of inhibitory synaptic inputs to the somas of arcuate neurons. This synaptic remodeling is induced by estradiol, blocked by progesterone, and begins with the onset of puberty in females. Astroglia appear to play a significant role in the organizational and the activational hormone effects on neuronal connectivity by regulating the amount of neuronal membrane available for the formation of synaptic contacts and by releasing soluble factors, such as insulin-like growth factor I (IGF-I), which promote the differentiation of neural processes. Recent evidence indicates that gonadal steroids and IGF-I may interact in their trophic effects on the neuroendocrine hypothalamus. Estradiol and IGF-I promote the survival and morphological differentiation of rat hypothalamic neurons in primary cultures. The effect of estradiol depends on IGF-I, while the effects of both estradiol and IGF-I depend on estrogen receptors. Furthermore, estrogen activation of astroglia in hypothalamic tissue fragments depends on IGF-I receptors. These findings indicate that IGF-I may mediate some of the developmental and activational effects of gonadal steroids on the brain and suggest that IGF-I may activate the estrogen receptor to induce its neurotrophic effects on hypothalamic cells. In addition, IGF-I levels in the neuroendocrine hypothalamus are regulated by gonadal steroids. IGF-I levels in tanycytes, a specific astroglia cell type present in the arcuate nucleus and median eminence, increase at puberty, are affected by neonatal androgen levels, show sex differences, and fluctuate in accordance to the natural variations in plasma levels of ovarian steroids that are associated with the estrous cycle. These changes appear to be mediated by hormonal regulation of IGF-I uptake from blood or cerebrospinal fluid by tanycytes. These results suggest that tanycytes may be involved in the regulation of neuroendocrine events in adult rats by regulating the availability of IGF-I to hypothalamic neurons. In summary, IGF-I and different forms of neuron-astroglia communication are involved in the effects of estradiol on synaptic plasticity in the hypothalamic arcuate nucleus.

The interactive language of the hypothalamus for the gonadotropin releasing hormone (GNRH) system

The enormous diversity in neurochemical signals employed within the network of afferents to GnRH neurons is well-documented. An examination of novel and accumulating knowledge on the operation of these messengers indicates the presence of an interactive language governing GnRH secretion. The basic operational structures identified to date to affirm this interactive form of communication summarized in this review are the following: (i) the demonstration of interconnections within various components of the afferent network; (ii) coexistence and possible co-release of excitatory and inhibitory neurotransmitters/neuromodulators; (iii) co-action of various messengers at synaptic targets, and (4) modulation by gonadal steroids of the synthesis and release of signals and their receptors, and induction of synaptic plasticity for the timely relay of signals for GnRH secretion. Unraveling the molecular sequelae that promote this interactive communication to elicit periodic GnRH secretion is now a new challenge.