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F Naftolin

Publications and source records attributed to F Naftolin.

At least 19 recordsLinked to original sources
PubMed1999

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.

Adult↗
PubMed1999

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.

Animals↗
PubMed1998

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.

Animals↗
PubMed1998

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.

Animals↗
PubMed1998

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.

Antineoplastic Agents↗
PubMed1998

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.

Aromatase↗
PubMed1998

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.

Animals↗
PubMed1998

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.

Animals↗
PubMed1998

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.

Animals↗
PubMed1998

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.

Animals↗
PubMed1997

Heterogeneity in the neuropeptide Y-containing neurons of the rat arcuate nucleus: GABAergic and non-GABAergic subpopulations.

Neuropeptide Y, produced in the arcuate nucleus of the hypothalamus, plays a key role in the central regulation of anterior pituitary and appetitive functions. The pleiotropic nature of neuropeptide Y in these mechanisms indicates the existence of heterogeneity in the hypothalamic neuronal population producing neuropeptide Y. In this study, we report the coexistence of neuropeptide Y and the amino acid transmitter, gamma-aminobutyric acid (GABA), in neuronal perikarya of the arcuate nucleus. Fluorescent double immunolabeling for neuropeptide Y and glutamic acid decarboxylase was carried out on vibratome sections collected through the hypothalamic arcuate nuclei of animals that were pretreated with colchicine. It was found that about one third of the neuropeptide Y-producing arcuate nucleus perikarya co-expressed glutamic acid decarboxylase. This population of neuropeptide Y-containing GABAergic neurons were distributed longitudinally within the arcuate nucleus located predominantly in its dorsomedial aspects. These results show that there are at least two distinct populations of neuropeptide Y-producing neurons in the arcuate nucleus: a subset of neuropeptide Y and GABA-co-producing neurons located in the dorsomedial arcuate nucleus and a subset of non-GABAergic neuropeptide Y cells located in the ventral arcuate nucleus. This heterogeneity in the neuropeptide Y-producing perikarya of the hypothalamus may help explain adverse neuroendocrine and behavioral effects of arcuate nucleus neuropeptide Y.

Animals↗
PubMed1997

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.

Animals↗
PubMed1997

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.

Afferent Pathways↗
PubMed1997

Lack of gonadotropin-positive feedback in the male rat is associated with lack of estrogen-induced synaptic plasticity in the arcuate nucleus.

We have demonstrated that estrogen induces synaptic reorganization in the hypothalamic arcuate nucleus of female rats during the ovarian cycle and proposed that estrogen-induced synaptic retraction plays a role in the disinhibition of gonadotropin secretion that occurs during the afternoon of proestrus. This so-called positive feedback of gonadotropins is developmentally determined. It is present in female rats and absent in males. To confirm the role of the estrogen-induced synaptic retraction in positive feedback, we tested whether administration of estrogen to male rats also fails to induce synaptic remodeling of the arcuate nucleus. Male and female rats were gonadectomized and studied as follows. One month following gonadectomy, animals received either a single injection of estradiol (100 micrograms/animal in sesame oil; 12 males and 12 females) or vehicle (6 males and 6 females). Twenty-four hours following injections, all animals in the vehicle-injected group and 6 animals of each sex in the treatment groups were sacrificed, while the rest of the animals were killed 48 h following the hormone injections (6 per group). As expected, quantitative electron microscopic analysis of the female arcuate nuclei revealed that compared to oil-injected controls, estradiol induced drastic decreases in the overall synapse counts by 24 h (121 +/- 10 vs. 74 +/- 5 synapses/1,000 microns membrane; p < 0.05). Synaptic counts had recovered to control levels by 48 h. On the contrary, in males, estradiol treatment did not cause changes in the total synapse counts at either time. As a further control, the lack of an estrogen-induced gonadotropin surge in long-term castrate males was also confirmed. Our study confirmed that in males estradiol does not alter the net synaptology of the arcuate nucleus or cause gonadotropin positive feedback. This is in clear contradistinction to females which show both synaptic plasticity and gonadotropin-positive feedback upon receiving exogenous or endogenous estrogen. The lack of estrogen-induced synaptic plasticity may be an underlying mechanism in the abolishment of positive gonadotropin, feedback in developing males and the development of constant estrus in aging female rats.

Animals↗
PubMed1997

Gonadal steroids target AMPA glutamate receptor-containing neurons in the rat hypothalamus, septum and amygdala: a morphological and biochemical study.

Interactions between glutamate and gonadal steroids are involved in the regulation of limbic and hypothalamic functions. We hypothesized that hormonal signals affect excitatory neurotransmission by regulating the expression of glutamate receptors (GluR) in limbic and hypothalamic regions. To test this hypothesis, first, the coexpression of dl-alpha-amino-3-hydroxy-5-methyl-4-isoxazone-propionate (AMPA) GluR1, GluR2/3, and androgen receptors or estrogen receptors was revealed in the same cells of septal, amygdaloid, and hypothalamic areas by double immunocytochemistry. The highest incidence of co-localization was detected in hypothalamic regions. To demonstrate a regulatory role of testosterone or estradiol on AMPA receptor expression, the hormonal milieu of male and female rats was manipulated by gonadectomy and hormonal treatment. GluR1 and GluR2/3 expression was assessed by Western blots. Statistical analysis demonstrated that testosterone and estradiol have a stimulatory influence on the expression of AMPA receptors in the hypothalamus. The regulatory effect of estradiol on AMPA receptors was found to be site and gender specific: after estradiol treatment, samples taken from the hypothalamus contained increased levels of GluR1 and GluR2/3, whereas in the septum, bed nucleus and amygdala, no changes could be detected. Furthermore, the increase in hypothalamic GluR 2/3 levels was two times higher in females, compared with males, whereas the changes in hypothalamic GluR 1 levels showed no sex differences. Our results support the hypothesis that the interaction between gonadal steroids and glutamate involves hormone regulation of GluR. This mechanism seems to be gender and site specific, suggesting that excitatory neurotransmission and related physiological mechanisms also may be distinctly different in males and females.

Amygdala↗
PubMed1997

Evidence showing that beta-endorphin regulates cyclic guanosine 3',5'-monophosphate (cGMP) efflux: anatomical and functional support for an interaction between opiates and nitric oxide.

Nitric oxide (NO) is now recognized as a diffusible messenger molecule that normally augments intercellular communication in the central nervous system, but is neurotoxic if released in excessive amounts. NO is synthesized from L-arginine by the Ca2+/calmodulin-dependent neuronal isoform NO synthase (NOS) localized in sub-populations of neurons throughout the brain, including the hypothalamus. In the hypothalamus, NO stimulates the release of GnRH, the primary neurohormone governing reproduction in mammals. Although the excitatory amino acid, glutamate, acting through the N-methyl-D-aspartate (NMDA) receptor is believed to be responsible for stimulation of NO release, the neuronal system(s) that inhibits NO efflux is unknown. As the endogenous opioids, primarily beta-endorphin (betaEND), exert a tonic restraint on GnRH secretion, we sought evidence for a possible functional link between betaEND and NOS pathways in the hypothalamus. We observed that restraining the opioid influence with the opiate receptor antagonist, naloxone, in intact, but not in castrated, rats rapidly augmented extracellular cGMP/NO efflux in the medial preoptic area, where GnRH, NOS, and betaEND immunoreactive pathways are coextensive. Pituitary LH secretion increased in conjunction with this augmented cGMP/NO response and pretreatment with the mu opiate receptor agonist, morphine, suppressed these naloxone-induced responses. Further, visualization of hypothalamic sections immunostained for both betaEND and NOS revealed betaEND-immunoreactive axon terminals in close proximity to NOS-positive cell bodies and dendrites in a number of hypothalamic subdivisions, including the medial preoptic area. These close appositions represented conventional synapses between betaEND nerve terminals and NOS-positive perikarya and dendrites under the electron microscope. Clearly, the experimental data, corroborated by morphological evidence, point to a direct inhibitory control of betaEND on NOS-immunoreactive neurons in monitoring cGMP/NO release. These findings together with the previous observations that the glutamate neurotransmitter acting through NMDA receptors located on NOS-immunopositive cells stimulates cGMP/NO efflux and plasma LH selectively in intact rats document the existence of a dual control comprised of the excitatory NMDA and the inhibitory mu opiate receptors in modulating cGMP/NO release, a response also directed by gonadal steroids. This new knowledge of an inhibitory opioid influence on cGMP/NO release is probably extremely important both in the generation of periodicities in GnRH secretion that underlie hypothalamic control of reproduction and in protecting against neurotoxic overstimulation of NO release by excitatory amino acids.

Animals↗
PubMed1997

Control of gonadotropin feedback: the possible role of estrogen-induced hypothalamic synaptic plasticity.

The preovulatory gonadotropin surge is a critical event in reproduction. Although many explanations have been given for the inhibition--disinhibition cycle referred to as positive feedback, none are completely satisfactory. Recent evidence indicates that the preovulatory surge of gonadotropin is induced, in part, by the disengagement of inhibitory synaptic connections in the hypothalamic arcuate nucleus. This disinhibition of gonadotropin-releasing hormone secretion results in a massive release by the estrogen-sensitized pituitary gonadotrophs. Investigations are presently under way to determine whether other areas of the hypothalamus are involved in this estrogen-induced synaptic plasticity.

Animals↗