Potential neuronal mechanisms of estrogen actions in synaptogenesis and synaptic plasticity

Role of neuroestrogens in the regulation of social behaviors - From social recognition to mating

In this mini-review, we summarize the brain distribution of aromatase, the enzyme catalyzing the synthesis of estrogens from androgens, and the mechanisms responsible for regulating estrogen production within the brain. Understanding this local synthesis of estrogens by neurons is pivotal as it profoundly influences various facets of social behavior. Neuroestrogen action spans from the initial processing of socially pertinent sensory cues to integrating this information with an individual's internal state, ultimately resulting in the manifestation of either pro-affiliative or - aggressive behaviors. We focus here in particular on aggressive and sexual behavior as the result of correct individual recognition of intruders and potential mates. The data summarized in this review clearly point out the crucial role of locally synthesized estrogens in facilitating rapid adaptation to the social environment in rodents and birds of both sexes. These observations not only shed light on the evolutionary significance but also indicate the potential implications of these findings in the realm of human health, suggesting a compelling avenue for further investigation.

Estradiol in the Preoptic Area Regulates the Dopaminergic Response to Cocaine in the Nucleus Accumbens

The sex-steroid hormone estradiol (E2) enhances the psychoactive effects of cocaine, as evidenced by clinical and preclinical studies. The medial preoptic area (mPOA), a region in the hypothalamus, is a primary neural locus for neuroendocrine integration, containing one of the richest concentrations of estrogen receptors in the CNS and also has a key role in the regulation of naturally rewarding behaviors. However, whether estradiol enhances the neurochemical response to cocaine by acting in the mPOA is still unclear. Using neurotoxic lesions and microdialysis, we examined whether the mPOA modulates cocaine-induced neurochemical activity in the nucleus accumbens. Tract tracing and immunohistochemical staining were used to determine whether projections from the mPOA to the ventral tegmental area (VTA) are sensitive to estrogen signaling. Finally, estradiol microinjections followed by microdialysis were used to determine whether estrogenic signaling in the mPOA modulates cocaine-induced changes of dopamine in the nucleus accumbens. Results showed that lesions of the mPOA or microinjections of estradiol directly into the mPOA increased cocaine-induced release of dopamine in the nucleus accumbens. Immunohistochemical analyses revealed that the mPOA modulates cocaine responsiveness via projections to both dopaminergic and GABAergic neurons in the VTA, and that these projections are sensitive to estrogenic stimulation. Taken together, these findings point to a novel estradiol-dependent pathway that modulates cocaine-induced neurochemical activity in the mesolimbic system.

Estradiol Rapidly Attenuates ORL-1 Receptor-Mediated Inhibition of Proopiomelanocortin Neurons via Gq-Coupled, Membrane-Initiated Signaling

Estradiol rapidly regulates the activity of arcuate nucleus (ARH) proopiomelanocortin (POMC) neurons that project to the medial preoptic nucleus (MPN) to regulate lordosis. Orphanin FQ/nociceptin (OFQ/N) acts via opioid receptor-like (ORL)-1 receptors to inhibit these POMC neurons. Therefore, we tested the hypothesis that estradiol excites POMC neurons by rapidly attenuating inhibitory ORL-1 signaling in these cells. Hypothalamic slices through the ARH were prepared from ovariectomized rats injected with Fluorogold into the MPN. Electrophysiological recordings were generated in ARH neurons held at or near -60 mV, and neuronal phenotype was determined post hoc by immunohistofluorescence. OFQ/N application induced robust outward currents and hyperpolarizations via G protein-gated, inwardly rectifying K+ (GIRK) channels that were attenuated by pretreatment with either 17-β estradiol (E2) or E2 conjugated to bovine serum albumin. This was blocked by the estrogen receptor (ER) antagonist ICI 182,780 and mimicked by the Gq-coupled membrane ER (Gq-mER) ligand STX and the ERα agonist PPT. Inhibiting phosphatidylinositol-3-kinase (PI3K) blocked the estrogenic attenuation of ORL-1/GIRK currents. Antagonizing either phospholipase C (PLC), protein kinase C (PKC), protein kinase A (PKA) or neuronal nitric oxide synthase (nNOS) also abrogated E2 inhibition of ORL-1/GIRK currents, whereas activation of PKC, PKA, protein kinase B (Akt) and nNOS substrate L-arginine all attenuated the OFQ/N response. This was observed in 92 MPN-projecting, POMC-positive ARH neurons. Thus, ORL-1 receptor-mediated inhibition of POMC neurons is rapidly and negatively modulated by E2, an effect which is stereoselective and membrane initiated via Gq-mER and ERα activation that signals through PLC, PKC, PKA, PI3K and nNOS.

60 YEARS OF NEUROENDOCRINOLOGY: MEMOIR: Geoffrey Harris and my brush with his unit

Geoffrey Harris is chiefly known for his demonstration of the control of the pituitary gland by the portal vessels coming from the hypothalamus. This does not do justice to his extraordinary contribution to biology. Harris' life's work was central in demonstrating the brain/body interactions by which animals and humans adapt to their environment, and above all the control of that most crucial and proximate of all evolutionary events - reproduction. In this brief review, I have tried to put Geoffrey Harris' work in the context of the scientific thinking at the time when he began his work, and above all, the contribution of his mentor, FHA Marshall, on whose towering shoulders Harris rose. But this is mainly my personal story, in which I have tried to show the debt that my work owed to Harris and especially to my dear friend, the late Keith Brown-Grant in Harris' team. I myself was never an endocrinologist, but over a short period in the early 1970s, under the influence of such inspirational mentors, and using purely anatomical methods, I was able to demonstrate sexual dimorphism and hormone-dependent sexual differentiation in the connections of the preoptic area, regeneration of the median eminence, the ultrastructure of apoptosis, the requirement for the suprachiasmatic nuclei in reproductive rhythms, the existence of non-rod or cone photoreceptors in the albino rat retina and, later, the expression of vasopressin by solitary (one in 600) magnocellular neurons in the polydipsic di/di Brattleboro mutant rat; this phenomenon was subsequently shown to be due to a+1 reading frameshift. I end this brief overview by mentioning some of the abiding and fascinating mysteries of the endocrine memory of the brain that arise from Harris' work on the control of the endocrines, and by pointing out how the current interest in chronobiology emphasises what a Cinderella the endocrine mechanisms have become in current brain imaging studies.

Traumatized and inflamed--but resilient: glial aromatization and the avian brain

Steroids like estrogens have potent effects on the vertebrate brain, and are provided to neural targets from peripheral and central sources. Estradiol synthesized within the vertebrate CNS modulates neural structure and function, including the pathways involved in neuroprotection, and perhaps, neural repair. Specifically, aromatase; the enzyme responsible for the conversion of testosterone to estradiol, is upregulated in the avian and mammalian brain following disruption of the neuropil by multiple forms of perturbation including mechanical injury, ischemia and excitotoxicity. This injury induced aromatase expression is somewhat unique in that it occurs in astroglia rather than neurons, and is stimulated in response to factors associated with brain damage. In this review, we focus on the induction, expression and consequences of glial aromatization in the songbird brain. We begin with a review of the anatomical consequences of glial estrogen provision followed by a discussion of the cellular mechanisms whereby glial aromatization may affect injury-induced neuroplasticity. We then present the current status of our understanding regarding the inductive role of inflammatory processes in the transcription and translation of astrocytic aromatase. We consider the functional aspects of glial aromatization before concluding with unanswered questions and suggestions for future studies. Birds have long informed us about fundamental questions in endocrinology, immunology, and neuroplasticity; and their unique anatomical and physiological characteristics continue to provide an excellent system in which to learn about brain trauma, inflammation, and neuroprotection.

Reproductive hormonal treatments for mood disorders in women

There has been a century-long view in medicine that reproductive function in both men and women is intimately involved with mood regulation. The 19th century witnessed a proliferation of medical reports documenting beneficial effects on mood and behavior after medical or surgical manipulations of women's reproductive functíon. More recently, the results of several studies suggest that gonadal steroids do regulate mood in some women. Thus, there is considerable interest in the potential role of reproductive therapies in the management of depressive illness, including both classical and reproductive endocrine-related mood disorders. Future studies need to determine the predictors of response to hormonal therapies compared with traditional antidepressant agents, and to characterize the long-term safety and benefits of these therapies.

A comparative, developmental, and clinical perspective of neurobehavioral sexual dimorphisms

Women and men differ in a wide variety of behavioral traits and in their vulnerability to developing certain mental disorders. This review endeavors to explore how recent preclinical and clinical research findings have enhanced our understanding of the factors that underlie these disparities. We start with a brief overview of some of the important genetic, molecular, and hormonal determinants that contribute to the process of sexual differentiation. We then discuss the importance of animal models in studying the mechanisms responsible for sex differences in neuropsychiatric disorders (e.g., drug dependence) - with a special emphasis on experimental models based on the neurodevelopmental and "three hits" hypotheses. Next, we describe the most common brain phenotypes observed in vivo with magnetic resonance imaging. We discuss the challenges in interpreting these phenotypes vis-à-vis the underlying neurobiology and revisit the known sex differences in brain structure from birth, through adolescence, and into adulthood. This is followed by a presentation of pertinent clinical and epidemiological data that point to important sex differences in the prevalence, course, and expression of psychopathologies such as schizophrenia, and mood disorders including major depression and posttraumatic stress disorder. Recent evidence implies that mood disorders and psychosis share some common genetic predispositions and neurobiological bases. Therefore, modern research is emphasizing dimensional representation of mental disorders and conceptualization of schizophrenia and major depression as a continuum of cognitive deficits and neurobiological abnormalities. Herein, we examine available evidence on cerebral sexual dimorphism to verify if sex differences vary quantitatively and/or qualitatively along the psychoses-depression continuum. Finally, sex differences in the prevalence of posttraumatic disorder and drug abuse have been described, and we consider the genomic and molecular data supporting these differences.

Framework for sex differences in adolescent neurobiology: a focus on cannabinoids

This review highlights the salient findings that have furthered our understanding of how sex differences are initiated during development and maintained throughout life. First we discuss how gonadal steroid hormones organize the framework for sex differences within critical periods of development-namely, during those exposures which occur in utero and post-partum, as well as those which occur during puberty. Given the extensive precedence of sex differences in cannabinoid-regulated biology, we then focus on the disparities within the endogenous cannabinoid system, as well as those observed with exogenously administered cannabinoids. We start with how the expression of cannabinoid CB(1) receptors is regulated throughout development. This is followed by a discussion of differential vulnerability to the pathological sequelae stemming from cannabinoid exposure during adolescence. Next we talk about sex differences in the interactions between cannabinoids and other drugs of abuse, followed by the organizational and activational roles of gonadal steroids in establishing and maintaining the sex dependence in the biological actions of cannabinoids. Finally, we discuss ways to utilize this knowledge to strategically target critical developmental windows of vulnerability/susceptibility and thereby implement more effective therapeutic interventions for afflictions that may be more prevalent in one sex vs. the other.

Neurons that co-localize aromatase- and kisspeptin-like immunoreactivity may regulate the HPG axis of the Mallard drake (Anas platyrhynchos)

Kisspeptin is a potent regulator of the hypothalamo-pituitary-gonadal axis. The activation of several vernal and pubertal behaviors involves the action of locally synthesized estradiol by hypothalamic aromatase-expressing neurons. Little is known about kisspeptin in non-mammalian systems, and its interaction with aromatase remains unexamined. The Mallard drake is a seasonal breeder and an excellent model for studying the neural mechanisms that regulate the HPG. The goals of these studies were to determine (a) if and how kisspeptin regulates the drake HPG, (b) if kisspeptin and aromatase are expressed in the Mallard brain, and (c) if kisspeptin is co-localized or in apposition with, aromatase- and gonadotropin hormone releasing hormone (GnRH) positive neurons. Central kisspeptin administration increased plasma luteinizing hormone, an effect blocked by pretreatment with the GnRH antagonist, acyline, suggesting a conservation of kisspeptin function and mechanism of action in birds and mammals. The distribution of kisspeptin in the mallard brain was examined with immunocytochemistry (ICC). Neurons that express kisspeptin-like immunoreactive (ir) protein were observed in the medial preoptic nucleus (POM) and in ir fibers throughout the drake brain. Virtually all POM kisspeptin-ir soma also expressed aromatase-ir, suggesting that autocrine mechanisms may predominate in the interaction between steroid provision and kisspeptin expression. No co-localization was observed between KP-ir and GnRH-ir, although both were easily detected in close-proximity in the tuberoinfundibular area. Taken together, these data suggest that in the drake, estradiol synthesized by aromatase and kisspeptin co-expressing POM neurons may regulate the HPG via an effect on GnRH secretion.

Estrogens regulate posttranslational modification of neural cell adhesion molecule during the estrogen-induced gonadotropin surge

Estrogen-induced synaptic plasticity (EISP) in the periventricular area (PVA) of the hypothalamus is necessary for the preovulatory gonadotropin surge. Because in situ enzymatic desialization of hypothalamic polysialylated (PSA) neural cell adhesion molecule (NCAM) blocked EISP, we examined the presence and amount of NCAM isotopes, PSA-NCAM, and sialylation enzymes in microdissected mouse hypothalamus tissues from proestrous afternoon [peak of estrogens and nadir of arcuate nucleus (AN) synapses] and metestrous morning (nadir of estrogens and highest AN synapses). Immunohistochemistry confirmed immunoreactive (ir) PSA-NCAM staining in the perineural spaces of the PVA. The extent of staining was cycle dependent, with more dense and complete profiles of individual neurons limned by the ir-PSA-NCAM staining on proestrus and less on metestrus. Western blots showed that high levels of ir-PSA-NCAM on proestrus are accompanied by diminished ir-NCAM-140 and -180 but not ir-NCAM-120 and the reverse on metestrus (P < 0.05). To evaluate the increase of sialylated NCAM at the expense of desialylated protein, expression of the responsible polysialyltransferase enzymes polysialyltransferase (ST8Sia IV) and sialyltransferase (ST8Sia II) mRNA levels were measured using RT-PCR. Both polysialyltransferase and sialyltransferase mRNA are more abundant on proestrus than metestrus (P < 0.05), indicating that these enzymes are regulated by estrogens. These results support estrogen-regulated formation and extrusion of hydrophilic PSA-NCAM into perineural spaces in the PVA as part of the mechanism of EISP.

Complex actions of sex steroids in adipose tissue, the cardiovascular system, and brain: Insights from basic science and clinical studies

Recent publications describing the results of the Women's Health Initiative (WHI) and other studies reporting the impact of hormone therapy on aging women have spurred reexamination of the broad use of estrogens and progestins during the postmenopausal years. Here, we review the complex pharmacology of these hormones, the diverse and sometimes opposite effects that result from the use of different estrogenic and progestinic compounds, given via different delivery routes in different concentrations and treatment sequence, and to women of different ages and health status. We examine our new and growing appreciation of the role of estrogens in the immune system and the inflammatory response, and we pose the concept that estrogen's interface with this system may be at the core of some of the effects on multiple physiological systems, such as the adipose/metabolic system, the cardiovascular system, and the central nervous system. We compare and contrast clinical and basic science studies as we focus on the actions of estrogens in these systems because the untoward effects of hormone therapy reported in the WHI were not expected. The broad interpretation and publicity of the results of the WHI have resulted in a general condemnation of all hormone replacement in postmenopausal women. In fact, careful review of the extensive literature suggests that data resulting from the WHI and other recent studies should be interpreted within the narrow context of the study design. We argue that these results should encourage us to perform new studies that take advantage of a dialogue between basic scientists and clinician scientists to ensure appropriate design, incorporation of current knowledge, and proper interpretation of results. Only then will we have a better understanding of what hormonal compounds should be used in which populations of women and at what stages of menopausal/postmenopausal life.

Estrogen differentially modulates the cannabinoid- induced presynaptic inhibition of amino acid neurotransmission in proopiomelanocortin neurons of the arcuate nucleus

The present study sought to determine whether cannabinoids inhibit glutamatergic and GABAergic synaptic input onto neurons of the hypothalamic arcuate nucleus (ARC), and whether estrogen modulates this process. Whole-cell patch clamp recordings were performed in hypothalamic slices prepared from ovariectomized female guinea pigs. CB1 receptor activation reduced the amplitude of excitatory postsynaptic currents (EPSCs) evoked by electrical stimulation that were sensitive to ionotropic glutamate receptor antagonists. The CB1 receptor antagonist AM251 increased evoked EPSC (eEPSC) amplitude, and reversed the agonist-induced decrease. CB1 receptor activation similarly decreased the amplitude of evoked inhibitory postsynaptic currents (eIPSCs). The cannabinoid-induced reduction in eEPSC and eIPSC amplitude correlated with a decrease in the frequency of miniature EPSCs (mEPSCs) and IPSCs (mIPSCs) that were abolished by ionotropic glutamate and GABA(A) receptor antagonists, respectively. AM251 increased mEPSC frequency, and antagonized the agonist-induced decrease. Compared to neurons obtained from vehicle-treated controls, estradiol benzoate (25 mug; s.c.) given 24 h prior to experimentation increased mEPSC frequency, and markedly decreased the potency of CB1 receptor agonists to decrease mEPSC frequency. Conversely, the steroid potentiated the cannabinoid-induced decrease in mIPSC frequency. These effects were observed in neurons subsequently identified as proopiomelanocortin (POMC) neurons. These data reveal that ARC neurons, including POMC neurons, receive glutamatergic and GABAergic synaptic inputs that are presynaptically inhibited by cannabinoids, and differentially modulated by estrogen. These opposing effects of estrogen on the cannabinoid regulation of amino acid neurotransmission excite POMC neurons, and lend additional insight into the mechanisms underlying estrogen-induced anorexia and negative feedback of the reproductive axis.

Estradiol Prevents Neural Tau Hyperphosphorylation Characteristic of Alzheimer's Disease

Alzheimer's disease (AD) is three times more prevalent in women than men, and epidemiological studies have shown that estrogen replacement in aging women forestalls the onset of AD. Hyperphosphorylation of the tau protein that forms the neurofibrillary tangles found in AD brains might be responsible for the breakdown of microtubules in affected neurons. The mechanisms by which tau protein is phosphorylated in the AD brain are not fully understood. Using a human neuroblastoma cell line (SH-SY5Y) and primary cultures of newborn male or female rat cerebral cortical neurons, we investigated the effect of 17beta-estradiol on tau protein expression and phosphorylation. We found that estradiol increased total tau and induced dephosphorylation at the proline-directed site of the molecule. Further, estradiol prevented okadaic acid-induced hyperphosphorylation of tau in both proline- and non-proline-directed sites, and antiestrogens blocked this effect. To our knowledge, this is the first report of an effect of estradiol on naturally occurring and induced tau phosphorylation. This assumes special significance because the estrogen action was found to be sexually dimorphic in rat cortical neurons and differentiation-sensitive in human neuroblastoma cells.

Amygdala and hippocampal volumes in Turner syndrome: a high-resolution MRI study of X-monosomy

Turner syndrome (TS) results from partial or complete X-monosomy and is characterized by deficits in visuospatial functioning as well as social cognition and memory. Neuroimaging studies have demonstrated volumetric differences in the parietal region of females with TS compared to controls. The present study examined amygdala and hippocampus morphology in an attempt to further understand the neural correlates of psychosocial and memory functioning in TS. Thirty females with TS age 7.6-33.3 years (mean = 14.7 +/- 6.4) and 29 age-matched controls (mean age = 14.8 +/- 5.9; range = 6.4-32.7) were scanned using high resolution MRI. Volumetric analyses of the MRI scans included whole brain segmentation and manual delineation of the amygdala and hippocampus. Compared to controls, participants with TS demonstrated significantly larger left amygdala gray matter volumes, irrespective of total cerebral tissue and age. Participants with TS also showed disproportionately reduced right hippocampal volumes, involving both gray and white matter. Amygdala and hippocampal volumes appear to be impacted by X-monosomy. Aberrant morphology in these regions may be related to the social cognition and memory deficits often experienced by individuals with TS. Further investigations of changes in medial temporal morphology associated with TS are warranted.

Membrane receptors for oestrogen in the brain

Oestrogen is important for the development of neuroendocrine centres and other neural networks including limbic and motor systems. Later in adulthood, oestrogen regulates the functional performance of different neural systems and is presumably implicated in the modulation of cognitive efficiency. Although still a matter of controversial discussion, clinical and experimental studies point at a potential neuroprotective role of oestrogen. Concerning the concept of cellular oestrogen action, it is undisputed that it comprises the binding and activation of nuclear receptors. The last decades have, however, immensely broadened the spectrum of steroid signalling within a cell. Novel steroid-activated intracellular signalling mechanisms were described which are usually termed 'non-classical' or 'non-genomic'. The brain appears to be a rich source of this new mode of oestrogen action. Studies from the past years have pinpointed non-classical oestrogen effects in many CNS regions. All available data support the view that non-classical oestrogen action requires interactions with putative membrane binding sites/receptors. In this article, we aim at compiling the most recent findings on the nature and identity of membrane oestrogen receptors with respect to the brain. We also attempt to turn readers attention to the coupling of these 'novel' receptors to distinct intracellular signalling pathways.

Dose- and sex-dependent effects of the neurotoxin 6-hydroxydopamine on the nigrostriatal dopaminergic pathway of adult rats: differential actions of estrogen in males and females

Epidemiological and clinical studies provide growing evidence for marked sex differences in the incidence of certain neurological disorders that are largely attributed to the neuroprotective effects of estrogen. Thus there is a keen interest in the clinical potential of estrogen-related compounds to act as novel therapeutic agents in conditions of neuronal injury and neurodegeneration such as Parkinson's disease. Studies employing animal models of neurodegeneration in ovariectomised female rats treated with estrogen support this hypothesis, yet experimental evidence for sex differences in the CNS response to direct neurotoxic insult is limited and, as yet, few studies have addressed the role played by endogenously produced hormones in neuroprotection. Therefore, in this study we aimed to determine (1) whether the prevailing levels of sex steroid hormones in the intact rat provide a degree of protection against neuronal assault in females compared with males and (2) whether sex differences depend solely on male/female differences in circulating estrogen levels or whether androgens could also play a role. Using the selective, centrally administered neurotoxin 6-hydroxydopamine, which induces a lesion in the nigrostriatal dopaminergic pathway similar to that seen in Parkinson's disease, we have demonstrated a sexually dimorphic (male-dominant), dose-dependent susceptibility in rats. Furthermore, following gonadectomy, dopamine depletion resulting from a submaximal dose of 6-hydroxydopamine (1 microg) was reduced in male rats, whereas in females, ovariectomy enhanced dopamine depletion. Administration of the nonaromatizable androgen dihydrotestosterone to gonadectomized animals had no significant effect on 6-hydroxydopamine toxicity in either males or females, whereas treatment of gonadectomized males and females with physiological levels of estrogen restored the extent of striatal dopamine loss to that seen in intact rats, viz, estrogen therapy reduced lesion size in females but increased it in males. Taken together, our findings strongly suggest that there are sex differences in the mechanisms whereby nigrostriatal dopaminergic neurones respond to injury. They also reveal that the reported clinically beneficial effects of estrogen in females may not be universally adopted for males. While the reasons for this gender-determined difference in response to the activational action of estrogen are unknown, we hypothesize that they may well be related to the early organizational events mediated by sex steroid hormones, which ultimately result in the sexual differentiation of the brain.

gamma-MSH increases intracellular cAMP accumulation and GnRH release in vitro and LH release in vivo

The roles of the melanocortin 3 receptor (MC3-R) and its agonist, gamma(2)-melanocyte-stimulating hormone (gamma(2)-MSH) in the regulation of the hypothalamo-pituitary-gonadal (HPG) axis are poorly understood. Here we show gamma(2)-MSH stimulated intracellular cAMP accumulation and gonadotrophin-releasing hormone (GnRH) secretion in the immortalised GnRH cell line GT(1)-7. The MC3/4-R antagonist Agrp blocked these actions. Reverse transcriptase polymerase chain reaction demonstrated GT(1)-7 cells express MC3-R mRNA. gamma(2)-MSH also stimulated GnRH release from hypothalamic explants. In vivo, gamma(2)-MSH administration into the medial preoptic area significantly increased plasma luteinising hormone. MC3-R and gamma(2)-MSH may modulate the HPG axis.

Estrogens: protective or risk factors in brain function?

Over the past century, the average lifespan of women has increased from 50 to over 80 years, but the age of the menopause has remained fixed at 51 years. This "change of life" is marked by a dramatic and permanent decrease in circulating levels of ovarian estrogens. Therefore, more women will live a greater proportion of their lives in a chronic hypoestrogenic state. Ovarian steroid hormones are pleiotropic and have multiple, diverse, and possibly opposing actions in different contexts. In light of recent reports of the possible health risks of hormone replacement therapy (HRT) on several different physiological systems, the question of whether estrogens are protective or risk factors must be carefully re-evaluated.

Retinoic acid signaling at sites of plasticity in the mature central nervous system

We used transgenic reporter mice to determine whether brain regions that respond to retinoic acid (RA) during development do so in maturity. We focused on two prominent sites of embryonic RA signaling: the dorsal spinal cord and the olfactory bulb. In the mature dorsal spinal cord, expression of a direct repeat 5 RA response element (DR5-RARE) transgene is seen in interneurons in laminae I and II, as well as in ependymal cells around the central canal. In the olfactory bulb, DR5-RARE transgene-expressing neurons are seen in the mature granule cell and periglomerular layers, as well as in cells in the subventricular zone of the forebrain-the established source for newly generated granule and periglomerular neurons. In addition, there are transgene-labeled neurons in a small number of other brain regions. These include the spinal trigeminal nucleus, area postrema, habenula, amygdala, and the cerebral cortex. Thus, a distinct type of RA-mediated gene expression, detected with the DR5-RARE reporter transgene, defines neurons, subependymal, or ependymal cells in discrete locations throughout the neuraxis. Some of these cells--particularly those in the spinal cord and olfactory bulb--are found in central nervous system regions that receive local RA signals early in development, and retain a significant amount of functional or structural plasticity in the adult.

Estrogen modulation of K(+) channel activity in hypothalamic neurons involved in the control of the reproductive axis

Here we report on the progress we have made in elucidating the mechanisms through which estrogen alters synaptic responses in hypothalamic neurons. We examined the modulation by estrogen of the coupling of various receptor systems to inwardly rectifying and small conductance, Ca(2+)-activated K(+) (SK) channels. We used intracellular sharp-electrode and whole-cell recordings in hypothalamic slices from ovariectomized female guinea pigs. Estrogen rapidly uncouples mu-opioid receptors from G protein-gated inwardly rectifying K(+) (GIRK) channels in beta-endorphin neurons, manifest by a reduction in the potency of mu-opioid receptor agonists to hyperpolarize these cells. This effect is blocked by inhibitors of protein kinase A and protein kinase C. Estrogen also uncouples gamma-aminobutyric acid (GABA)(B) receptors from the same population of GIRK channels coupled to mu-opioid receptors. At 24 h after steroid administration, the GABA(B)/GIRK channel uncoupling observed in GABAergic neurons of the preoptic area (POA) is associated with reduced agonist efficacy. Conversely, estrogen enhances the efficacy of alpha(1)-adrenergic receptor agonists to inhibit apamin-sensitive SK currents in these POA GABAergic neurons, and does so in both a rapid and sustained fashion. Finally, we observed a direct, steroid-induced hyperpolarization of both arcuate and POA neurons, among which gonadotropin-releasing hormone (GnRH) neurons are particularly sensitive. These findings indicate a richly complex yet coordinated steroid modulation of K(+) channel activity that serves to control the excitability of hypothalamic neurons involved in regulating the reproductive axis.

The role of mitogen-activated protein (MAP) kinase in breast cancer

Mitogen-activated protein kinase (MAP kinase) cascades transmit and amplify signals involved in cell proliferation as well as cell death. These signal transduction pathways serve as an indicators of the intensity of trafficking induced by various growth factor, steroid hormone, and G protein receptor mediated ligands. Three major MAP kinase pathways exist in human tissues, but the one involving ERK-1 and -2 is most relevant to breast cancer. Peptide growth factors acting through tyrosine kinase containing receptors are the major regulators of ERK-1 and -2. Estradiol, progesterone, and testosterone can act non-genomically via membrane associated receptors to activate MAP kinase as can various other ligands acting through heterotrimeric G protein receptors. Recent studies demonstrate that breast cancers frequently contain an increased proportion of cells with the activated form of MAP kinase. In estrogen receptor positive breast tumors, MAP kinase pathways can exert "cross talk" effects at the level of ER induced transcription as well as at the level of the cell cycle. Estradiol stimulates cell proliferation by mechanisms which involve activation of MAP kinase, either through rapid, non-transcription effects or by increasing growth factor production and consequently MAP kinase. Progesterone and androgens also stimulate MAP kinase through both of these two mechanisms. Strategies used to treat hormone dependent breast cancer appear to result in upregulation of MAP kinase activation. Direct experimental data demonstrate that the pressure of estradiol deprivation results in the upregulation of MAP kinase in breast cancer cells growing in tissue culture and as xenografts. A number of investigators have now studied the expression of activated MAP kinase in human breast cancer tissues by enzymatic assay and by immunohistochemical techniques. Approximately half of breast tumors express more activated MAP kinase than does the surrounding benign tissue. Studies show a trend toward higher MAP kinase activity in primary tumors of node positive than in node negative patients. However, larger numbers of patients must be studied for these results to achieve statistical significance. The up-regulation of MAP kinase activity does not represent mutations of Ras, but appears to result from enhancement of growth factor pathway activation. No data are yet available on the relationship between MAP kinase activation and apoptosis. Additional studies are now needed to determine the precise relationship between MAP kinase activation and tumor proliferation, apoptosis, and degree of invasiveness as well as on disease free and overall survival.

Estrogen biphasically modifies hypothalamic GABAergic function concomitantly with negative and positive control of luteinizing hormone release

The principal role of estrogen is its control of the female ovulatory cycle via negative and positive feedback on gonadotropin secretion. However, a detailed, cohesive picture of how the steroid specifically regulates the excitability of hypothalamic neurons involved in the central control of gonadotropin secretion is still emerging. Here, we used an ovariectomized female guinea pig model to test the hypothesis that estrogen acts on GABAergic neurons in the preoptic area (POA) to elicit a biphasic profile of luteinizing hormone (LH) secretion. Intracellular electrophysiological recordings revealed that estradiol benzoate (EB; 25 microgram, s.c.) decreased the hyperpolarizing response of GABAergic neurons to the GABA(B) receptor agonist baclofen 24 hr after treatment. This effect of GABA(B) receptor stimulation in unidentified POA neurons was still depressed 42 hr after EB administration. By the use of a ribonuclease protection assay, however, EB reduced glutamic acid decarboxylase mRNA expression 42 hr but not 24 hr after its administration. Thus, estrogen attenuated the autoinhibition of GABAergic POA neurons during the initial LH suppressive (i.e., negative feedback) phase and subsequently reduced GABAergic function during the LH surge (i.e., positive feedback). These studies demonstrate that the effects of estrogen on hypothalamic GABAergic neurons coincide with the inhibitory and stimulatory actions, respectively, of the steroid on LH secretion. Furthermore, the data provide novel insights into the mechanism by which estrogen regulates hypothalamic GABAergic neurons, which are critical for the biphasic modulation of LH release observed over the course of the female ovulatory cycle.

Testosterone modulates the dendritic architecture of arcuate neuroendocrine neurons in adult male rats

Recent studies have demonstrated that gonadectomy of adult male rats induces dendritic growth of neuroendocrine neurons in the arcuate nucleus. We have hypothesized that these changes are secondary to the loss of testosterone negative feedback. In the present study, we examined the effects of testosterone replacement on the dendritic morphology of arcuate neuroendocrine neurons in castrated rats. Rats were orchidectomized and implanted with silastic capsules designed to produce physiological levels of plasma testosterone (n=9) or empty silastic capsules (n=9) for 2 months. Retrograde labeling with systemically injected Fluoro-Gold, followed by intracellular injection of labeled neurons in a fixed slice preparation, were used to visualize arcuate neuroendocrine neurons. Quantitative analysis of dendritic morphology was performed using three-dimensional computer reconstruction. Serum levels of LH (luteinizing hormone) and testosterone were measured by radioimmunoassay. Treatment of castrated rats with physiological levels of testosterone significantly reduced dendritic length, volume and terminal branch number relative to the castrated rats receiving empty silastic capsules. Dendritic spine density was also greater in the testosterone-treated animals, although the total numbers of spines per dendrite was not significantly different between the two groups. In addition, testosterone replacement was effective in reducing serum LH to levels found in intact rats. These studies demonstrate that testosterone replacement suppresses the dendritic outgrowth of arcuate neuroendocrine neurons that occurs in response to castration. The parallel changes in dendritic arbor and serum LH after castration and hormone replacement suggests that the suppressive effects of testosterone are related to steroid negative feedback.

Estrogens: trophic and protective factors in the adult brain

Our appreciation that estrogens are important neurotrophic and neuroprotective factors has grown rapidly. Although a thorough understanding of the molecular and cellular mechanisms that underlie this effect requires further investigation, significant progress has been made due to the availability of animal models in which we can test potential candidates. It appears that estradiol can act via mechanisms that require classical intracellular receptors (estrogen receptor alpha or beta) that affect transcription, via mechanisms that include cross-talk between estrogen receptors and second messenger pathways, and/or via mechanisms that may involve membrane receptors or channels. This area of research demands attention since estradiol may be an important therapeutic agent in the maintenance of normal neural function during aging and after injury.

Estrogen-regulated developmental neuronal apoptosis is determined by estrogen receptor subtype and the Fas/Fas ligand system

Adult sexual dimorphism in neuronal cell number is controlled by estrogen exposure during a tightly defined period of rat brain development. The mechanisms of estrogen's effect are unknown; one possibility is regulation of programmed cell death (apoptosis). In this study we have shown that estradiol can function as a neuroprotective agent or an inducer of apoptosis, depending on the estrogen receptor-subtype present in the cell. Thus, ERalpha has a neuroprotective effect, while ERbeta mediates the induction of apoptosis in neuronal cells. Moreover, we show that estrogen-induced apoptosis through ER-beta requires the expression of Fas- and Fas ligand (FasL) proteins, since the absence of FasL in neurons prevents this effect. Furthermore, we demonstrate that microglia-secreted products induce the expression of FasL necessary to mediate estradiol-ERbeta apoptotic effect. These findings may explain the dichotomous effect of fetal estradiol on the adult neuronal number.

Brain aging and midlife tofu consumption

OBJECTIVE

To examine associations of midlife tofu consumption with brain function and structural changes in late life.

METHODS

The design utilized surviving participants of a longitudinal study established in 1965 for research on heart disease, stroke, and cancer. Information on consumption of selected foods was available from standardized interviews conducted 1965-1967 and 1971-1974. A 4-level composite intake index defined "low-low" consumption as fewer than two servings of tofu per week in 1965 and no tofu in the prior week in 1971. Men who reported two or more servings per week at both interviews were defined as "high-high" consumers. Intermediate or less consistent "low" and "high" consumption levels were also defined. Cognitive functioning was tested at the 1991-1993 examination, when participants were aged 71 to 93 years (n = 3734). Brain atrophy was assessed using neuroimage (n = 574) and autopsy (n = 290) information. Cognitive function data were also analyzed for wives of a sample of study participants (n = 502) who had been living with the participants at the time of their dietary interviews.

RESULTS

Poor cognitive test performance, enlargement of ventricles and low brain weight were each significantly and independently associated with higher midlife tofu consumption. A similar association of midlife tofu intake with poor late life cognitive test scores was also observed among wives of cohort members, using the husband's answers to food frequency questions as proxy for the wife's consumption. Statistically significant associations were consistently demonstrated in linear and logistic multivariate regression models. Odds ratios comparing endpoints among "high-high" with "low-low" consumers were mostly in the range of 1.6 to 2.0.

CONCLUSIONS

In this population, higher midlife tofu consumption was independently associated with indicators of cognitive impairment and brain atrophy in late life.

Evidence that IGF-I acts as an autocrine/paracrine growth factor in the magnocellular neurosecretory system: neuronal synthesis and induction of axonal sprouting

The ability of mature oxytocinergic (OT) and vasopressinergic (VP) neurons of the magnocellular neurosecretory system (MNS) to undergo axonal growth implies that one or more growth factors may be active in the adult MNS, yet little is known regarding their possible identity. One such potential factor is insulin-like growth factor I (IGF-I). We have examined the expression of IGF-I mRNA and IGF-I-immunoreactivity (IGF-I-ir) in the mature MNS and have also determined the in vivo response of OT and VP neurons to hypothalamic implants of IGF-I. In situ hybridization revealed moderate labeling of IGF-I mRNA in both the supraoptic (SON) and the paraventricular (PVN) nuclei of adult male rats. RT-PCR analysis confirmed the presence of authentic IGF-I mRNA in extracts of the basal hypothalamus. Faint IGF-I-ir was detected in scattered magnocellular neurons within both the PVN and the SON of normal rats, but IGF-I-ir was much more intense and the majority of MNS neurons including those in the accessory nuclei were immunoreactive in sections from rats given colchicine, as were some parvocellular neurons in the PVN. Confocal microscopy revealed that IGF-I-ir was present in both OT and VP neurons, but VP neurons contained the most intense IGF-I-ir. Finally, a dramatic growth response of OT but not of VP fibers was observed following implantation of polymer rods containing IGF-I into the hypothalamus. A dense OT fiber plexus grew along the cannula track and OT fibers invaded the leptomeninges ventral to the SON and encircled the rostral cerebral artery. To our knowledge this is the first demonstration of axonal sprouting by mature OT neurons in response to an identified growth factor and the first direct demonstration of sprouting in response to exogenous IGF-I in the adult CNS. These findings suggest that IGF-I is synthesized and transported by adult MNS neurons where it may act as an autocrine and/or paracrine growth factor.

Effects of gonadal steroids on the GABA and glutamate contents of the early developing tilapia brain

The effects of gonadal steroids on the gamma-aminobutyric acid (GABA) and glutamate (Glu) contents of the early developing brain were investigated. Seven-day-old (7 days post-hatch) tilapia were divided into three groups which were continuously treated with 100 mg/kg diet 17beta-estradiol (E2), 100 mg/kg diet methyltestosterone (MT), and a normal diet, respectively. Until 10, 20, and 30 days old, the GABA and Glu contents of the brains were detected by HPLC-ECD. The brain GABA and Glu contents, before 30 days old, significantly increased with age. These results demonstrate that before 30 days old is a developing period of both GABA and Glu systems in the tilapia brain. During this period, both E2 and MT have a facilitative effect on the GABAergic and Gluergic system during a restricted effective period.

Synaptic density in the arcuate nucleus of female rats approaching middle age

Synaptic number and synapses/neuronal cell membrane were evaluated from ultrastructural micrographs of the arcuate nucleus from 90-, 120-, 180-, and 240-day female rats grouped into ovary-intact or ovariectomized animals treated with peanut oil vehicle or estradiol benzoate (10 microg/100 g body weight) for 3 days. In ovary-intact rats, synaptic density was significantly less in middle aged 240 day animals than in 90-, 120-, or 180-day animals with greatest decrease occurring between 180- and 240-day animals. Ovary-intact and ovariectomized animals treated with estradiol benzoate had significantly higher sera estradiol levels, but the estradiol was ineffective in increasing synaptic density in the middle aged animals. Logistical regression confirmed a correlation between a decrease in synapses and increasing age but not estradiol treatment.

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.

Estrogen facilitates induction of long term potentiation in the hippocampus of awake rats

In order to test the hypothesis that circulating levels of estrogen modulate synaptic plasticity in the hippocampus, we have studied the induction of long term potentiation (LTP) in awake rats. Ovariectomized animals, chronically implanted with a recording electrode in the cell body layer of CA1 and a stimulating electrode in stratum radiatum, were used to record evoked field potentials (population spike (PS) and summed EPSP) daily for at least 4 days before injection of sesame oil or 100 microg of estradiol benzoate per kg b.w. (E2). Basal levels of response to single square pulses (0.01 ms pulse width) delivered at 0.05 Hz through the stimulating electrode were recorded daily for 2 days after injection. To induce LTP a high-frequency 'theta pattern' stimulation was administered. Basal recordings at low-frequency stimulation did not change after injection. After high-frequency stimulation all (7/7) E2 injected animals showed LTP whereas only 1/6 oil injected controls did so; the mean increase in amplitude of the PS and slope of the EPSP after high-frequency stimulation were significantly greater in E2 treated rats. Input/output curves did not change significantly after E2 administration. These results show that at low-frequency stimulation, transynaptic responses of pyramidal neurones in CA1 are not affected by changes in levels of circulating estrogen, while synaptic plasticity -- which is at the basis of proposed hebbian associative memory -- is facilitated by estrogen treatment.

Responsiveness to depolarization of hypothalamic neurons secreting somatostatin under stress and estrous cycle conditions: involvement of GABAergic and steroidal interactions

We studied the sensitivity to a depolarizing stimulus of hypothalamic fragments dissected from cycling female donor rats exposed or not to 30-min stress at 4 degrees C. The neuronal response was estimated in terms of the ability of tissue to release somatostatin when stimulated with 40 mM K+. The data showed no differences in response to K+, regardless of the ovarian cycle of the female donors, whereas tissues dissected from ovariectomized or pregnant rats responded significantly to K+. However, when donors underwent previous cold stress, significant differences were noted at all stages of the cycle, except diestrus-1, compared with control rats. We tested whether GABA and/or neuroactive steroids could be involved in this phenomenon and observed no GABA inhibition of somatostatin release in vitro, but inhibition occurred in the presence of a neuroactive steroid, THDOC. The effect of GABA in vivo on somatostatin release was estrogen dependent because bicuculline modified the total amount of somatostatin secreted in estrus but not in diestrus II. Finally, in hypothalamic primary cultures, GABA inhibition of somatostatin release was only detected when steroids were present in the media throughout culture. Our results suggest that steroid-GABA-somatostatin interactions could explain the different responses of neurons to depolarization.