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

Glial Cell Modulation of Dendritic Spine Structure and Synaptic Function

Glia comprise a heterogeneous group of cells involved in the structure and function of the central and peripheral nervous system. Glial cells are found from invertebrates to humans with morphological specializations related to the neural circuits in which they are embedded. Glial cells modulate neuronal functions, brain wiring and myelination, and information processing. For example, astrocytes send processes to the synaptic cleft, actively participate in the metabolism of neurotransmitters, and release gliotransmitters, whose multiple effects depend on the targeting cells. Human astrocytes are larger and more complex than their mice and rats counterparts. Astrocytes and microglia participate in the development and plasticity of neural circuits by modulating dendritic spines. Spines enhance neuronal connectivity, integrate most postsynaptic excitatory potentials, and balance the strength of each input. Not all central synapses are engulfed by astrocytic processes. When that relationship occurs, a different pattern for thin and large spines reflects an activity-dependent remodeling of motile astrocytic processes around presynaptic and postsynaptic elements. Microglia are equally relevant for synaptic processing, and both glial cells modulate the switch of neuroendocrine secretion and behavioral display needed for reproduction. In this chapter, we provide an overview of the structure, function, and plasticity of glial cells and relate them to synaptic maturation and modulation, also involving neurotrophic factors. Together, neurons and glia coordinate synaptic transmission in both normal and abnormal conditions. Neglected over decades, this exciting research field can unravel the complexity of species-specific neural cytoarchitecture as well as the dynamic region-specific functional interactions between diverse neurons and glial subtypes.

Hormonal and nutritional regulation of postnatal hypothalamic development

The hypothalamus is a key centre for regulation of vital physiological functions, such as appetite, stress responsiveness and reproduction. Development of the different hypothalamic nuclei and its major neuronal populations begins prenatally in both altricial and precocial species, with the fine tuning of neuronal connectivity and attainment of adult function established postnatally and maintained throughout adult life. The perinatal period is highly susceptible to environmental insults that, by disrupting critical developmental processes, can set the tone for the establishment of adult functionality. Here, we review the most recent knowledge regarding the major postnatal milestones in the development of metabolic, stress and reproductive hypothalamic circuitries, in the rodent, with a particular focus on perinatal programming of these circuitries by hormonal and nutritional influences. We also review the evidence for the continuous development of the hypothalamus in the adult brain, through changes in neurogenesis, synaptogenesis and epigenetic modifications. This degree of plasticity has encouraging implications for the ability of the hypothalamus to at least partially reverse the effects of perinatal mal-programming.

IGF-1 in the Brain as a Regulator of Reproductive Neuroendocrine Function

Given the close relationship among neuroendocrine systems, it Is likely that there may be common signals that coordinate the acquisition of adult reproductive function with other homeo-static processes. In this review, we focus on central nervous system insulin-like growth factor-1 (IGF-1) as a signal controlling reproductive function, with possible links to somatic growth, particularly during puberty. In vertebrates, the appropriate neurosecretion of the decapeptide gonadotropin-releas-ing hormone (GnRH) plays a critical role in the progression of puberty. Gonadotropin-releasing hormone is released in pulses from neuroterminals in the median eminence (ME), and each GnRH pulse triggers the production of the gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH). These pituitary hormones in turn stimulate the synthesis and release of sex steroids by the gonads. Any factor that affects GnRH or gonadotropin pulsatility is important for puberty and reproductive function and, among these factors, the neurotrophic factor IGF-1 is a strong candidate. Although IGF-1 is most commonly studied as the tertiary peripheral hormone in the somatotropic axis via its synthesis in the liver, IGF-1 Is also synthesIzed in the brain, within neurons and glia. In neuroendocrine brain regions, central IGF-1 plays roles in the regulation of neuroendocrine functions, including direct actions on GnRH neurons. Moreover, GnRH neurons themselves co-express IGF-1 and the IGF-1 receptor, and this expression is developmentally regulated. Here, we examine the role of IGF-1 acting in the hypothalamus as a critical link between reproductive and other neuroendocrine functions.

A Concerted Action Of Estradiol And Insulin Like Growth Factor I Underlies Sex Differences In Mood Regulation By Exercise

Mood homeostasis present sexually dimorphic traits which may explain sex differences in the incidence of mood disorders. We explored whether diverse behavioral-setting components of mood may be differentially regulated in males and females by exercise, a known modulator of mood. We found that exercise decreases anxiety only in males. Conversely, exercise enhanced resilience to stress and physical arousal, two other important components of mood, only in females. Because exercise increases brain input of circulating insulin-like growth factor I (IGF-I), a potent modulator of mood, we explored whether sex-specific actions of exercise on mood homeostasis relate to changes in brain IGF-I input. We found that exercise increased hippocampal IGF-I levels only in cycling females. Underlying mechanism involved activation of estrogen (E₂) receptors in brain vessels that led to increased uptake of serum IGF-I as E₂ was found to stimulate IGF-I uptake in brain endothelial cells. Indeed, modulatory effects of exercise on mood were absent in female mice with low serum IGF-I levels or after either ovariectomy or administration of an E₂ receptor antagonist. These results suggest that sex-specific brain IGF-I responses to physiological stimuli such as exercise contribute to dimorphic mood homeostasis that may explain sex differences in affective disorders.

Insulin-like growth factor-I regulates LH release by modulation of kisspeptin and NMDA-mediated neurotransmission in young and middle-aged female rats

This study investigated potential mechanisms by which age and IGF-I receptor (IGF-Ir) signaling in the neuroendocrine hypothalamus affect estradiol-positive feedback effects on GnRH neuronal activation and on kisspeptin and N-methyl-D-aspartate (NMDA)-induced LH release and on the abundance of NMDA receptor subunits Nr1 and Nr2b and Kiss1r transcript and protein in the hypothalamus of young and middle-aged female rats. We infused vehicle, IGF-I, or JB-1, a selective antagonist of IGF-Ir, into the third ventricle of ovariectomized female rats primed with estradiol or vehicle and injected with vehicle, kisspeptin (3 or 30 nmol/kg), or NMDA (15 or 30 mg/kg). Regardless of dose, NMDA and kisspeptin resulted in significantly more LH release, GnRH/c-Fos colabeling, and c-Fos immunoreative cells in young than in middle-aged females. Estradiol priming significantly increased Kiss1r, Nr1, and Nr2b receptor transcript and protein abundance in young but not middle-aged female hypothalamus. JB-1 attenuated kisspeptin and NMDA-induced LH release, numbers of GnRH/c-Fos and c-Fos cells, and Kiss1r, Nr1, and Nr2b transcript and protein abundance in young females to levels observed in middle-aged females. IGF-I significantly enhanced NMDA and kisspeptin-induced LH release in middle-aged females without increasing numbers of GnRH/c-Fos or c-Fos immunoreactive cells. IGF-I infusion in middle-aged females also increased Kiss1r, Nr1, and Nr2b protein and transcript to levels that were equivalent to young estradiol-primed females. These findings indicate that age-related changes in estradiol-regulated responsiveness to excitatory input from glutamate and kisspeptin reflect reduced IGF-Ir signaling.

Exercise and obesity in fibromyalgia: beneficial roles of IGF-1 and resistin?

Introduction

Severe fatigue is a major health problem in fibromyalgia (FM). Obesity is common in FM, but the influence of adipokines and growth factors is not clear. The aim was to examine effects of exercise on fatigue, in lean, overweight and obese FM patients.

Methods

In a longitudinal study, 48 FM patients (median 52 years) exercised for 15 weeks. Nine patients were lean (body mass index, BMI 18.5 to 24.9), 26 overweight (BMI 25 to 29.9) and 13 obese. Fatigue was rated on a 0 to 100 mm scale (fibromyalgia impact questionnaire [FIQ] fatigue) and multidimensional fatigue inventory (MFI-20) general fatigue (MFIGF). Higher levels in FIQ fatigue and MFIGF indicate greater degree of fatigue. Free and total IGF-1, neuropeptides, adipokines were determined in serum and cerebrospinal fluid (CSF).

Results

Baseline FIQ fatigue correlated negatively with serum leptin (r = -0.345; P = 0.016) and nerve growth factor (NGF; r = -0.412; P = 0.037). In lean patients, baseline MFIGF associated negatively with serum resistin (r = -0.694; P = 0.038). FIQ Fatigue associated negatively with CSF resistin (r = -0.365; P = 0.073). Similarly, FIQ fatigue (r = -0.444; P = 0.026) and MFIGF correlated negatively with CSF adiponectin (r = -0.508; P = 0.01). In lean patients, FIQ fatigue (P = 0.046) decreased after 15 weeks. After 30 weeks, MFIGF decreased significantly in lean (MFIGF: P = 0.017), overweight (MFIGF: P = 0.001), and obese patients (MFIGF: P = 0.016). After 15 weeks, total IGF-1 increased in lean (P = 0.043) patients. ∆Total IGF-1 differed significantly between lean and obese patients (P = 0.010). ∆Total IGF-1 related negatively with ∆MFIGF after 15 weeks (r = -0.329; P = 0.050). After 30 weeks, ∆FIQ fatigue negatively correlated with ∆NGF (r = -0.463; P = 0.034) and positively with ∆neuropeptide Y (NPY) (r = 0.469; P = 0.032). Resistin increased after 30 weeks (P = 0.034). ∆MFIGF correlated negatively with ∆resistin (r = -0.346; P = 0.031), being strongest in obese patients (r = -0.815; P = 0.007). In obese patients, ∆FIQ fatigue after 30 weeks correlated negatively with ∆free IGF-1 (r = -0.711; P = 0.032).

Conclusions

Exercise reduced fatigue in all FM patients, this effect was achieved earlier in lean patients. Baseline levels of resistin in both serum and CSF associated negatively with fatigue. Resistin was increased after the exercise period which correlated with decreased fatigue. Changes in IGF-1 indicate similar long-term effects in obese patients. This study shows reduced fatigue after moderate exercise in FM and indicates the involvement of IGF-1 and resistin in these beneficial effects.

Trial registration

ClinicalTrials.gov: NCT00643006

Rapid control of male typical behaviors by brain-derived estrogens

Beside their genomic mode of action, estrogens also activate a variety of cellular signaling pathways through non-genomic mechanisms. Until recently, little was known regarding the functional significance of such actions in males and the mechanisms that control local estrogen concentration with a spatial and time resolution compatible with these non-genomic actions had rarely been examined. Here, we review evidence that estrogens rapidly modulate a variety of behaviors in male vertebrates. Then, we present in vitro work supporting the existence of a control mechanism of local brain estrogen synthesis by aromatase along with in vivo evidence that rapid changes in aromatase activity also occur in a region-specific manner in response to changes in the social or environmental context. Finally, we suggest that the brain estrogen provision may also play a significant role in females. Together these data bolster the hypothesis that brain-derived estrogens should be considered as neuromodulators.

Glycogen synthase kinase-3β/β-catenin signaling in the rat hypothalamus during the estrous cycle

During the estrous cycle, a remodeling of synapses on somas and dendritic spines occurs in the rat hypothalamic arcuate nucleus. The synaptic remodeling is known to be induced by estradiol, but the molecular mechanisms involved still have not been fully clarified. β-catenin is known to regulate synaptic plasticity, so we have assessed possible modifications of β-catenin in the rat mediobasal hypothalamus during the estrous cycle. Our findings indicate that β-catenin expression is increased during proestrus and estrus in comparison with diestrus day. This increase was accompanied by an enhanced phosphorylation of Akt in Ser473 and of glycogen synthase kinase-3β (GSK3β) in Ser9. Also, the association of β-catenin with the synaptic protein PSD95 was increased during these same stages of the estrous cycle, whereas the levels of synapsin I were significantly decreased in proestrus. These findings suggest that Akt/GSK3β/β-catenin signaling is involved in the synaptic modifications that occur in the basal hypothalamus during the estrous cycle.

Differential effects of hypothalamic IGF-I on gonadotropin releasing hormone neuronal activation during steroid-induced LH surges in young and middle-aged female rats

Interactions between brain IGF-I receptors and estrogen receptors regulate female reproductive physiology and behavior. The present study investigated potential mechanisms by which IGF-I receptors in the neuroendocrine hypothalamus regulate GnRH neuronal activation and LH release in young and middle-aged female rats under estradiol (E2) positive feedback conditions. We infused vehicle, IGF-I, or JB-1, a selective antagonist of IGF-I receptors, into the third ventricle of ovariectomized female rats primed with E2 and progesterone or vehicle. In young females, blockade of IGF-I receptors attenuated the steroid hormone-induced LH surge, reduced the percent of GnRH neurons expressing c-fos on the day of the LH surge, and decreased the total number of neurons expressing c-fos in the preoptic area. Middle-aged females had fewer GnRH neurons expressing c-fos during the LH surge than young females, and the LH surge amplitude was attenuated. Infusion of an IGF-I dose previously shown to increase LH surge amplitude did not increase the percent of GnRH neurons expressing c-fos in middle-aged females. Brain IGF-I receptor blockade did not modify E2 induction of progestin receptor-immunoreactive neurons in the preoptic area, arcuate, or ventromedial hypothalamus of young rats. These findings indicate that brain IGF-I receptors are required for E2 activation of GnRH neurons in young rats and for robust GnRH release from axon terminals in middle-aged females. IGF-I likely exerts its effects by actions on E2-sensitive neurons that are upstream of GnRH neurons and terminals.

Manganese induces IGF-1 and cyclooxygenase-2 gene expressions in the basal hypothalamus during prepubertal female development

Precocious puberty is a significant child health problem, especially in girls, because 95% of cases are idiopathic. Our earlier studies demonstrated that low-dose levels of manganese (Mn) caused precocious puberty via stimulating the secretion of luteinizing hormone-releasing hormone (LHRH). Because glial-neuronal communications are important for the activation of LHRH secretion at puberty, we investigated the effects of prepubertal Mn exposure on specific glial-derived puberty-related genes known to affect neuronal LHRH release. Animals were supplemented with MnCl(2) (10 mg/kg) or saline by gastric gavage from day 12 until day 22 or day 29, then decapitated, and brains removed. The site of LHRH release is the medial basal hypothalamus (MBH), and tissues from this area were analyzed by real-time PCR for transforming growth factor α (TGFα), insulin-like growth factor-1 (IGF-1), and cyclooxygenase-2 (COX-2) messenger RNA levels. Protein levels for IGF-1 receptor (IGF-1R) were measured by Western blot analysis. LHRH gene expression was measured in the preoptic area/anteroventral periventricular (POA/AVPV) region. In the MBH, at 22 days, IGF-1 gene expression was increased (p < 0.05) with a concomitant increase (p < 0.05) in IGF-1R protein expression. Mn also increased (p < 0.01) COX-2 gene expression. At 29 days, the upregulation of IGF-1 (p < 0.05) and COX-2 (p < 0.05) continued in the MBH. At this time, we observed increased (p < 0.05) LHRH gene expression in the POA/AVPV. Additionally, Mn stimulated prostaglandin E(2) and LHRH release from 29-day-old median eminences incubated in vitro. These results demonstrate that Mn, through the upregulation of IGF-1 and COX-2, may promote maturational events and glial-neuronal communications facilitating the increased neurosecretory activity, including that of LHRH, resulting in precocious pubertal development.

Membrane-initiated estradiol signaling regulating sexual receptivity

Estradiol has profound actions on the structure and function of the nervous system. In addition to nuclear actions that directly modulate gene expression, the idea that estradiol can rapidly activate cell signaling by binding to membrane estrogen receptors (mERs) has emerged. Even the regulation of sexual receptivity, an action previously thought to be completely regulated by nuclear ERs, has been shown to have a membrane-initiated estradiol signaling (MIES) component. This highlighted the question of the nature of mERs. Several candidates have been proposed, ERα, ERβ, ER-X, GPR30 (G protein coupled estrogen receptor), and a receptor activated by a diphenylacrylamide compound, STX. Although each of these receptors has been shown to be active in specific assays, we present evidence for and against their participation in sexual receptivity by acting in the lordosis-regulating circuit. The initial MIES that activates the circuit is in the arcuate nucleus of the hypothalamus (ARH). Using both activation of μ-opioid receptors (MOR) in the medial preoptic nucleus and lordosis behavior, we document that both ERα and the STX-receptor participate in the required MIES. ERα and the STX-receptor activation of cell signaling are dependent on the transactivation of type 1 metabotropic glutamate receptors (mGluR1a) that augment progesterone synthesis in astrocytes and protein kinase C (PKC) in ARH neurons. While estradiol-induced sexual receptivity does not depend on neuroprogesterone, proceptive behaviors do. Moreover, the ERα and the STX-receptor activation of medial preoptic MORs and augmentation of lordosis were sensitive to mGluR1a blockade. These observations suggest a common mechanism through which mERs are coupled to intracellular signaling cascades, not just in regulating reproduction, but in actions throughout the neuraxis including the cortex, hippocampus, striatum, and dorsal root ganglias.

PI3K: An Attractive Candidate for the Central Integration of Metabolism and Reproduction

In neurons, as in a variety of other cell types, the enzyme phosphatidylinositol-3-kinase (PI3K) is a key intermediate that is common to the signaling pathways of a number of peripheral metabolic cues, including insulin and leptin, which are well known to regulate both metabolic and reproductive functions. This review article will explore the possibility that PI3K is a key integrator of metabolic and neural signals regulating gonadotropin releasing hormone (GnRH)/luteinizing hormone (LH) release and explore the hypothesis that this enzyme is pivotal in many disorders where gonadotropin release is at risk. Although the mechanisms mediating the influence of metabolism and nutrition on fertility are currently unclear, the strong association between metabolic disorders and infertility is undeniable. For example, women suffering from anorectic disorders experience amenorrhea as a consequence of malnutrition-induced impairment of LH release, and at the other extreme, obesity is also commonly co-morbid with menstrual dysfunction and infertility. Impaired hypothalamic insulin and leptin receptor signaling is thought to be at the core of reproductive disorders associated with metabolic dysfunction. While low levels of leptin and insulin characterize states of negative energy balance, prolonged nutrient excess is associated with insulin and leptin resistance. Metabolic models known to alter GnRH/LH release such as diabetes, diet-induced obesity, and caloric restriction are also accompanied by impairment of PI3K signaling in insulin and leptin sensitive tissues including the hypothalamus. However, a clear link between this signaling pathway and the control of GnRH release by peripheral metabolic cues has not been established. Investigating the role of the signaling pathways shared by metabolic cues that are critical for a normal reproductive state can help identify possible targets in the treatment of metabolic and reproductive disorders such as polycystic ovarian syndrome.

Hypothalamic insulin-like growth factor-I receptors are necessary for hormone-dependent luteinizing hormone surges: implications for female reproductive aging

Brain IGF-I receptors are required for maintenance of estrous cycles in young adult female rats. Circulating and hypothalamic IGF-I levels decrease with aging, suggesting a role for IGF-I in the onset of reproductive senescence. Therefore, the present study investigated potential mechanisms of action of brain IGF-I receptors in the regulation of LH surges in young adult and middle-aged rats. We continuously infused IGF-I, the selective IGF-I receptor antagonist JB-1, or vehicle into the third ventricle of ovariectomized young adult and middle-aged female rats primed with estradiol and progesterone. Pharmacological blockade of IGF-I receptors attenuated and delayed the LH surge in young adult rats, reminiscent of the LH surge pattern that heralds the onset of reproductive senescence in middle-aged female rats. Infusion of IGF-I alone had no effect on the LH surge but reversed JB-1 attenuation of the surge in young females. In middle-aged rats, infusion of low doses of IGF-I partially restored LH surge amplitude, and infusion of JB-1 completely obliterated the surge. Intraventricular infusion of IGF-I or JB-1 did not modify pituitary sensitivity to exogenous GnRH or GnRH peptide content in the anterior or mediobasal hypothalamus in either young or middle-aged rats. These findings support the hypothesis that brain IGF-I receptor signaling is necessary for GnRH neuron activation under estrogen-positive feedback conditions and that decreased brain IGF-I signaling in middle-aged females contributes, in part, to LH surge dysfunction by disrupting estradiol-sensitive processes that affect GnRH neuron activation and/or GnRH release.

Interactions of estradiol and insulin-like growth factor-I signalling in the nervous system: new advances

Estradiol and insulin-like growth factor-I (IGF-I) interact in the brain to regulate a variety of developmental and neuroplastic events. Some of these interactions are involved in the control of hormonal homeostasis and reproduction. However, the interactions may also potentially impact on affection and cognition by the regulation of adult neurogenesis in the hippocampus and by promoting neuroprotection under neurodegenerative conditions. Recent studies suggest that the interaction of estradiol and IGF-I is also relevant for the control of cholesterol homeostasis in neural cells. The molecular mechanisms involved in the interaction of estradiol and IGF-I include the cross-regulation of the expression of estrogen and IGF-I receptors, the regulation of estrogen receptor-mediated transcription by IGF-I and the regulation of IGF-I receptor signalling by estradiol. Current investigations are evidencing the role exerted by key signalling molecules, such as glycogen synthase kinase 3 and beta-catenin, in the cross-talk of estrogen receptors and IGF-I receptors in neural cells.

Role of estrogen receptor alpha in membrane-initiated signaling in neural cells: interaction with IGF-1 receptor

The mechanisms of action of estradiol in the nervous system involve nuclear-initiated steroid signaling and membrane-initiated steroid signaling. Estrogen receptors (ERs) are involved in both mechanisms. ERalpha interacts with the signaling of IGF-1 receptor in neural cells: ERalpha transcriptional activity is regulated by IGF-1 receptor signaling and estradiol regulates IGF-1 receptor signaling. The interaction between ERalpha and the IGF-1 receptor in the brain may occur at the plasma membrane of neurons and glial cells. Caveolin-1 may provide the scaffolding for the interaction of different membrane-associated molecules, including voltage-dependent anion channel, ERalpha and IGF-I receptor.

Insulin-like growth factor-I activates KiSS-1 gene expression in the brain of the prepubertal female rat

KiSS-1 gene expression has been shown to increase as puberty approaches, and its peptide products, kisspeptins, are involved in LHRH secretion at puberty. Factors contributing to increased KiSS-1 expression, however, have not been identified; thus, the purpose of this study was to assess whether IGF-I could induce transcription of this gene in prepubertal female rats. IGF-I or saline was centrally administered to immature rats that were killed 2, 4, and 6 h later. Real-time PCR revealed that IGF-I induced (P < 0.01) KiSS-1 gene expression at 6 h in a tissue fragment that contained both the anteroventral periventricular (AVPV) and arcuate (ARC) nuclei. Subsequently, the AVPV and ARC nuclei were separated to assess whether region-specific effects could be identified. IGF-I stimulated (P < 0.01) KiSS-1 gene expression in the AVPV nucleus at 6 h after injection, with no change observed in the ARC nucleus. Serum estradiol (E2) levels were not altered at any time point after IGF-I, demonstrating that the increased KiSS-1 expression observed was not caused by an elevation in E2. Additionally, the IGF-I action to induce KiSS-1 gene expression in the AVPV nucleus was further demonstrated when the IGF-I was administered systemically. E2 appears to play an important permissive role because 1-d ovariectomized rats responded to IGF-I with increased (P < 0.01) KiSS-1 expression, whereas, 20 d after ovariectomy, when the E2 levels had fallen below assay sensitivity, the IGF-I was unable to induce KiSS-1 expression. The IGF-I effect was further demonstrated by showing that the IGF-I receptor antagonist, JB-1, blocked the IGF-I-induced increase in KiSS-1 expression. Collectively, these data indicate that IGF-I is an activator of the KiSS-1 gene in the prepubertal female rat.

The role of glia in the hypothalamus: implications for gonadal steroid feedback and reproductive neuroendocrine output

Neuron-to-glia, glia-to-neuron, and glia-to-glia communication are implicated in the modulation of neuronal activity and synaptic transmission relevant to reproduction. Glial cells play an important role in neuroendocrine regulation and participate in the sexual differentiation of neuronal connectivity of brain regions involved in the control of reproductive neuroendocrine output. During puberty, modifications in the morphology and chemistry of astrocytes and tanycytes in the hypothalamus and median eminence influence the maturation of the neuronal circuits controlling the secretion of GnRH. During adult reproductive life, the glial cells participate in the transient remodeling of neuronal connectivity in the preoptic area, the arcuate nucleus, the median eminence, and other brain regions involved in the control of reproduction. Gonadal hormones regulate glial plasticity by direct and indirect effects and regulate various other endocrine signals, local soluble factors and adhesion molecules that also affect glial function and glia-to-neuron communication. The glial cells, therefore, are central to the coordination of endocrine and local inputs that bring about neural plasticity and adapt reproductive capacity to homeostatic signals.

Glial-gonadotrophin hormone (GnRH) neurone interactions in the median eminence and the control of GnRH secretion

A wealth of information now exists showing that glial cells are actively involved in the cell-cell communication process generating and disseminating information within the central nervous system. In the hypothalamus, two types of glial cells, astrocytes and ependymal cells lining the latero-ventral portion of the third ventricle (known as tanycytes), regulate the secretory activity of neuroendocrine neurones. This function, initially described for astrocytes apposing magnocellular neurones, has been more recently characterised for neurones secreting gonadotrophin hormone-releasing hormone (GnRH). The available evidence suggests that glial cells of the median eminence regulate GnRH secretion via two related mechanisms. One involves the production of growth factors acting via receptors with tyrosine kinase activity. The other involves plastic rearrangements of glia-GnRH neurone adhesiveness. GnRH axons reach the median eminence, at least in part, directed by basic fibroblast growth factor. Their secretory activity is facilitated by insulin-like growth factor 1 and members of the epidermal growth factor family. A structural complement to these soluble molecules is provided by at least three cell-cell adhesion systems endowed with signalling capabilities. One of them uses the neuronal cell adhesion molecule (NCAM), another employs the synaptic cell adhesion molecule (SynCAM), and the third one consists of neuronal contactin interacting with glial receptor-like protein tyrosine phosphatase-beta. It is envisioned that, within the median eminence, soluble factors and adhesion molecules work coordinately to control delivery of GnRH to the portal vasculature.

Central insulin-like growth factor 1 receptors play distinct roles in the control of reproduction, food intake, and body weight in female rats

Estradiol and progesterone induction of the LH surge in ovariectomized female rats requires concurrent activation of brain insulin-like growth factor 1 (IGF1) receptors. The present study determined whether brain IGF1 receptor signaling is required for estrous cyclicity in gonadally intact female rats. A selective IGF1 receptor antagonist (JB-1) or vehicle was continuously administered into the third ventricle by osmotic minipumps. Following surgical placement of the minipumps, all rats temporarily reduced food intake, lost weight, and suspended estrous cycles. Control rats resumed cycles within a few days and exhibited compensatory hyperphagia until they returned to presurgical body weight. Animals receiving JB-1 had severely delayed or absent estrous cycles, failed to show rebound feeding, and regained body weight more slowly. Vehicle-infused animals pair fed to JB-1-treated rats had even lower body weights but resumed estrous cycles sooner than those given drug alone. Chronic infusion of IGF1 alone had no effect on any of these parameters, but coinfusion of IGF1 with the antagonist completely reversed JB-1 effects on food intake and estrous cyclicity and partially reversed the effects on body weight. There were no significant differences in the expression of galanin-like peptide (Galp) or Kiss1 mRNA in the arcuate or periventricular hypothalamic area of control and JB-1-treated animals at a time point when food intake and estrous cycles were different between controls and JB-1-treated rats. These data suggest that brain IGF1 signaling is necessary for normal estrous cycles as well as compensatory hyperphagia and that IGF1 modulation of the reproductive axis is not secondary to reduced food intake.

Metastin/kisspeptin and control of estrous cycle in rats

Estrous cyclicity is controlled by a cascade of neuroendocrine events, involving the activation of the hypothalamo-pituitary-gonadal axis. Two modes of gonadotropin-releasing hormone (GnRH) are well established to regulate the estrous cycle: one is a tonic or pulse mode of secretion which is responsible for the stimulation of follicular development and steroidogenesis; the other is a surge mode, which is solely responsible for the induction of luteinizing hormone (LH) surges, eventually leading to ovulation. Metastin/kisspeptin-GPR54 signaling has been suggested to control ovarian cyclicity through regulating the two modes of GnRH release. A population of metastin/kisspeptin neurons located in the anteroventral periventricular nucleus (AVPV) is considered to trigger GnRH surge and thus to mediate the estrogen positive feedback action on GnRH release. The other hypothalamic population of metastin/kisspeptin neurons is located in the arcuate nucleus (ARC) and could be involved in generating GnRH pulses and mediating negative feedback action of estrogen on GnRH release. GnRH neurons express mRNA for GPR54, a metastin/kisspeptin receptor, and have a close association with metastin/kisspeptin neurons at the cell body and terminal level, but the precise mechanism by which this peptide regulates the two modes of GnRH release needs to be determined. Metastin/kisspeptin, therefore, is a key hypothalamic neuropeptide, which is placed immediately upstream of GnRH neurons and relays the peripheral steroidal information to GnRH neurons to control estrous cyclicity.

Cross-talk between estrogen receptors and insulin-like growth factor-I receptor in the brain: cellular and molecular mechanisms

Accumulating evidence suggests that insulin-like growth factor-I (IGF-I) and estradiol interact to regulate neural function. In this review, we focus on the cellular and molecular mechanisms involved in this interaction. The expression of estrogen receptors (ERs) and IGF-I receptor is cross-regulated in the central nervous system and many neurons and astrocytes coexpress both receptors. Furthermore, estradiol activates IGF-I receptor and its intracellular signaling. This effect may involve classical ERs since recent findings suggest that ERalpha may affect IGF-I actions in the brain by a direct interaction with some of the components of IGF-I signaling. In turn, IGF-I may regulate ER transcriptional activity in neuronal cells. In conclusion, ERs appear to be part of the signaling mechanism of IGF-I, and IGF-I receptor part of the mechanism of estradiol signaling in the nervous system.

The role of insulin-like growth factor-I and growth factor-associated signal transduction pathways in estradiol and progesterone facilitation of female reproductive behaviors

We are examining the role of insulin-like growth factor-I (IGF-I) and downstream signal transduction pathways associated with growth factors (e.g., mitogen-activated protein kinase, MAPK) in estradiol and progesterone facilitation of female reproductive behavior in rats. Brain IGF-I receptor activity is required for the long-term, priming actions of estradiol on the female reproductive axis. Infusions of an IGF-I receptor antagonist during estradiol priming blocks induction of hypothalamic alpha(1B)-adrenergic receptors and luteinizing hormone surges, and attenuates lordosis behavior. Infusion of MAPK and phosphatidylinositol-3-kinase inhibitors inhibitors during estradiol priming completely blocks hormone-facilitated lordosis. Because progestin receptors (PRs) can be phosphorylated and activated by MAPKs, growth factor signaling pathways may also participate in progesterone facilitation of reproductive behaviors. Infusion of a MAPK inhibitor in estradiol-primed rats blocks progestin facilitation and sequential inhibition of lordosis and proceptive behaviors. Interference with MAPK signaling also inhibits behavioral responses to cGMP and a delta-opioid agonist, both of which can activate MAPK in some cells. Thus MAPK is involved in the facilitation of lordosis and proceptive behaviors, perhaps by phosphorylation of hypothalamic PRs.

Central and opposing effects of IGF-I and IGF-binding protein-3 on systemic insulin action

IGF-I is recognized as an insulin sensitizer at the liver and muscle, while recent evidence suggests that IGF-binding protein (IGFBP)-3 acts as an insulin antagonist. As there is a paucity of IGF-I receptors in the liver and as the IGF-IGFBP system in the central nervous system is emerging as physiologically relevant, we examined whether the effects of IGF-I and IGFBP-3 on insulin action are mediated through central mechanisms. Intracerebroventricular (ICV) infusion of IGF-I during the insulin clamp (3 mU x kg(-1) x min(-1)) resulted in significant improvement in hepatic insulin action (50%, P < 0.05). In contrast, ICV infusion of IGFBP-3 significantly impaired insulin action at the liver (45% increase in hepatic glucose production, P < 0.01). While IGF-I marginally increased peripheral glucose uptake, IGFBP-3 significantly decreased peripheral glucose uptake (approximately 30%, P < 0.01). As the nuclear localization signal mutant IGFBP-3, which has a normal affinity to IGFs but binds other IGFBP-3 partners poorly and fails to normally internalize, has reduced central activity on metabolism, we conclude that the effects of IGFBP-3 on the hypothalamus involve activity mediated by interfacing with other molecules in addition to IGFs. Marked, opposing, and independent physiological effects of IGF-I and IGFBP-3 through central mechanisms may have implications on potential strategies in specific modulation of peripheral insulin action.

From clinical evidence to molecular mechanisms underlying neuroprotection afforded by estrogens

Recent studies have highlighted that female sex hormones represent potential neuroprotective agents against damage produced by acute and chronic injuries in the adult brain. Clinical reports have documented the effectiveness of estrogens to attenuate symptoms associated with Parkinson's disease, and to reduce the risk of Alzheimer's disease and cerebrovascular stroke. This evidence is corroborated by numerous experimental studies documenting the protective role of female sex hormones both in vitro and in vivo. Accordingly, estrogens have been shown to promote survival and differentiation of several neuronal populations maintained in culture, and to reduce cell death associated with excitotoxicity, oxidative stress, serum deprivation or exposure to beta-amyloid. The neuroprotective effects of estrogens have been widely documented in animal models of neurological disorders, such as Alzheimer's and Parkinson's diseases, as well as cerebral ischemia. Although estrogens are known to exert several direct effects on neurones, the cellular and molecular mechanisms implicated in their protective actions on the brain are not completely understood. Thus, on the basis of clinical and experimental evidence, in this review, we discuss recent findings concerning the neuronal effects of estrogens that may contribute to their neuroprotective actions. Both estrogen receptor-dependent and -independent mechanisms will be described. These include modulation of cell death regulators, such as Bcl-2, Akt and calpain, as well as interaction with growth factors, such as BDNF, NGF, IGF-I and their receptors. The anti-inflammatory effects of estrogens will also be described, namely their ability to reduce brain levels of inflammatory mediators, cytokines and chemokines. Finally, a brief overview about receptor-independent mechanisms of neuroprotection will aim at describing the antioxidant effects of estrogens, as well as their ability to modulate neurotransmission.

Insulin-like growth factor-I receptor and estrogen receptor crosstalk mediates hormone-induced neurite outgrowth in PC12 cells

Estradiol (E(2)) and insulin-like growth factor-I (IGF-I) can act independently or in concert to promote neurite outgrowth in vivo and in cultured neurons. This study examined the role of crosstalk between estrogen receptor (ER)alpha and the IGF-I receptor as a critical mediator of hormone- and growth factor-dependent neurite outgrowth in a homogenous cell system. We used control PC12 cells and PC12 cells stably transfected with ER alpha, both of which express IGF-I receptor. Cells were treated for 1 week with vehicle, 1 nM E(2) or 100 ng/ml IGF-I alone or with E(2) or IGF-I in the presence of either the IGF-I receptor antagonist JB1 or the ER antagonist ICI 182,780. IGF-I significantly increased neurite outgrowth, as measured by the percentage of process-bearing cells, and absolute neurite length per cell in both control and ER alpha-transfected PC12 cells. In contrast, E(2) increased process formation and extension only in PC12 cells that were stably transfected with ER alpha. ICI 182,780 and JB1 blocked the IGF-I-induced increases in neurite length in both cell types. The efficacy of ICI 182,780 in control PC12 cells may have been due to the upregulation of ER alpha in these cells by the 7-day treatment with IGF-I. The ER and IGF-I receptor antagonists similarly blocked the E(2)-induced increase in neurite lengths in ER alpha-transfected cells. Immunofluorescent analysis of the cellular distribution of an axonal marker, phospho-neurofilament, verified that the processes extended by PC12 cells were neurites. These data suggest that receptor crosstalk between IGF-I receptors and ER alpha has an important role in neurite formation and extension even in a single-cell system.

Influence of estradiol on insulin-like growth factor-1-induced luteinizing hormone secretion

Several studies suggest an interrelationship between estradiol (E2) and insulin-like growth factor-1 (IGF-1) at the hypothalamic level. The present study was designed to discern if the capability of IGF-1 to release LH and influence the timing of female puberty is influenced by E2. Twenty-eight-day-old female rats were ovariectomized (OVEX), then implanted with a third ventricular (3V) cannula. Two weeks later, these animals received subcutaneous (s.c.) injection of oil, or either one or two injections of E2 in the form of estradiol benzoate (1 microg). Forty-eight hours later, four basal blood samples were drawn then the animals received IGF-1 (200 ng) or saline via the 3V and four more blood samples were taken. Results indicated that E2 replacement lowered basal LH levels and IGF-1 induced a significant LH release in only animals that had E2 levels above 20 pg/ml. These levels of E2 were also associated with increases (p<0.05) in the expression of both IGF-1 receptor (IGF-1R) mRNA and protein. In order to further support the hypothesis that the action of IGF-1 at the time of puberty is influenced by E2, 24-day-old intact female rats received s.c. injection of sesame oil or 0.1 microg of E2. The next day, the E2-treated animals also received twice daily s.c. injections of either IGF-1 (500 ng) or saline until vaginal opening (VO) occurred. The animals that received E2 plus IGF-1 showed VO at 31.1 days, which was 2.5 days earlier (p<0.01) than E2-treated animals and 4 days earlier (p<0.001) than IGF-1-treated and saline control animals. Taken together, these results indicate that the hypothalamic action of IGF-1 to stimulate LH release and advance female pubertal development is dependent upon the influence of E2.

Estradiol inhibits GSK3 and regulates interaction of estrogen receptors, GSK3, and beta-catenin in the hippocampus

Estrogens regulate a wide set of neuronal functions such as gene expression, survival and differentiation in a manner not very different from that exerted by neurotrophins or by growth factors. The best-studied hormonal action is the transcriptional activation mediated by estrogen receptors. However, the direct effects of estrogen on growth factor signaling have not been well clarified. The present data show that estradiol, in vivo, induces a transient activation of GSK3 in the adult female rat hippocampus, followed by a more sustained inhibition, as inferred from phosphorylation levels of Tau. Similar data was obtained from cultured hippocampal neurons when treated with the hormone. The transient activation was confirmed by direct measure of GSK3 kinase activity. In addition, our results show a novel complex of estrogen receptor alpha, GSK3, and beta-catenin. The presence of the hormone removes beta-catenin from this complex. There is a second complex, also affected by estradiol, in which Tau is associated with GSK3, beta-catenin, and elements of the PI3 kinase complex. Considering the role of GSK3 in neurodegeneration, our data suggest that part of the neuroprotective effects of estrogen may be due to the control of GSK3.

Estrogen interacts with the IGF-1 system to protect nigrostriatal dopamine and maintain motoric behavior after 6-hydroxdopamine lesions

The most prominent neurochemical hallmark of Parkinson's disease (PD) is the loss of nigrostriatal dopamine (DA). Animal models of PD have concentrated on depleting DA and therapies have focused on maintaining or restoring DA. Within this context estrogen protects against 6-hydroxdopamine (6-OHDA) and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) lesions of the nigrostriatal DA pathway. Present studies tested the hypothesis that neuroprotective estrogen actions involve activation of the insulin-like growth factor-1 (IGF-1) system. Ovariectomized rats were treated with either a single subcutaneous injection of 17beta-estradiol benzoate or centrally or peripherally IGF-1. All rats were infused unilaterally with 6-OHDA into the medial forebrain bundle (MFB) to lesion the nigrostriatal DA pathway. Tyrosine hydroxylase (TH) immunocytochemistry confirmed that rats injected with 6-OHDA had a massive loss of TH immunoreactivity in both the ipsilateral substantia nigra compacta (60% loss) and the striatum (>95% loss) compared to the contralateral side. Loss of TH immunoreactivity was correlated with loss of asymmetric forelimb movements, a behavioral assay for motor deficits. Pretreatment with estrogen or IGF-1 significantly prevented 6-OHDA-induced loss of substantia nigra compacta neurons (20% loss) and TH immunoreactivity in DA fibers in the striatum (<20% loss) and prevented the loss of asymmetric forelimb use. Blockage of IGF-1 receptors by intracerebroventricular JB-1, an IGF-1 receptor antagonist, attenuated both estrogen and IGF-1 neuroprotection of nigrostriatal DA neurons and motor behavior. These findings suggest that IGF-1 and estrogen acting through the IGF-1 system may be critical for neuroprotective effects of estrogen on nigrostriatal DA neurons in this model of PD.

Differential expression of p120 catenin in glial cells of the adult rat brain

p120 catenin (p120ctn) is involved in the regulation of cadherin-mediated adhesion and the dynamic organization of the actin cytoskeleton by modulating RhoGTPase activity. We have previously described the distribution of p120ctn during rat brain development and provided substantial evidence for the potential involvement of p120ctn in morphogenetic events and plasticity in the central nervous system. Here, we analyzed the cellular and ultrastructural distribution of p120ctn in glial cells of the adult rat forebrain. The highest intensity of immunostaining for p120ctn was found in cells of the choroid plexus and ependyma and was mainly restricted to the plasma membrane. However, p120ctn was almost absent from astrocytes. In contrast, in tanycytes, a particular glial cell exhibiting remarkable morphological plasticity, p120ctn, was localized at the plasma membrane and also in the cytoplasm. We show that a large subpopulation of oligodendrocytes expressed multiple isoforms, whereas other neural cells predominantly expressed isoform 1, and that p120ctn immunoreactivity was distributed through the cytoplasm and at certain portions of the plasma membrane. Finally, p120ctn was expressed by a small population of cortical NG2-expressing cells, whereas it was expressed by a large population of these cells in the white matter. However, in both regions, proliferating NG2-positive cells consistently expressed p120ctn. The expression of p120ctn by cells of the oligodendrocyte lineage suggests that p120ctn may participate in oligodendrogenesis and myelination. Moreover, the expression of p120ctn by various cell types and its differential subcellular distribution strongly suggest that p120ctn may serve multiple functions in the central nervous system.

Cell death in the nervous system: lessons from insulin and insulin-like growth factors

Programmed cell death is an essential process for proper neural development. Cell death, with its similar regulatory and executory mechanisms, also contributes to the origin or progression of many or even all neurodegenerative diseases. An understanding of the mechanisms that regulate cell death during neural development may provide new targets and tools to prevent neurodegeneration. Many studies that have focused mainly on insulin-like growth factor-I (IGF-I), have shown that insulin-related growth factors are widely expressed in the developing and adult nervous system, and positively modulate a number of processes during neural development, as well as in adult neuronal and glial physiology. These factors also show neuroprotective effects following neural damage. Although some specific actions have been demonstrated to be anti-apoptotic, we propose that a broad neuroprotective role is the foundation for many of the observed functions of the insulin-related growth factors, whose therapeutical potential for nervous system disorders may be greater than currently accepted.

Ovarian Steroid and Growth Factor Regulation of Female Reproductive Function Involves Modification of Hypothalamic α1-Adrenoceptor Signaling

The ovarian steroids estradiol (E(2)) and progesterone (P) act on target neurons in the hypothalamus and preoptic area to coordinate the expression of female reproductive behaviors with the timing of the preovulatory luteinizing hormone (LH) surge. This chapter will summarize evidence that E(2) and P facilitation of the receptive component of female reproductive behavior, lordosis, involves changes in both the expression of and intracellular signal transduction pathways engaged by alpha(1)-adrenergic receptors in the hypothalamus and preoptic area. The alpha(1)-adrenoceptors are thought to mediate the facilitatory effects of the catecholamine neurotransmitter norepinephrine on both lordosis behavior and LH release. E(2) first induces the expression of the alpha(1B)-adrenergic receptor subtype in the hypothalamus and preoptic area. P then acts in an E(2)-dependent manner to promote linkage of hypothalamic alpha(1)-adrenoceptors to an intracellular signaling pathway involving nitric oxide and cyclic GMP. This chapter will also describe recent findings that implicate brain insulin-like growth factor-I (IGF-I) receptors as obligatory co-mediators of hormonal regulation of hypothalamic alpha(1)-adrenoceptors and female neuroendocrine function. Additional studies suggest that E(2) and IGF-I facilitate lordosis behavior by activating kinases traditionally associated with growth factor signal transduction (mitogen-activated protein kinases and phosphatidlyinositol-3-kinases). These molecular events are proposed to help coordinate the timing of ovulation with the expression of sexual receptivity, thereby maximizing reproductive success.

An antagonist of estrogen receptors blocks the induction of adult neurogenesis by insulin-like growth factor-I in the dentate gyrus of adult female rat

Interdependence between estradiol and insulin-like growth factor-I has been documented for different neural events, including neuronal differentiation, synaptic plasticity, neuroendocrine regulation and neuroprotection. In the present study we have assessed whether both factors interact in the regulation of neurogenesis in the adult rat dentate gyrus. Wistar albino female rats were bilaterally ovariectomized and treated with estradiol, insulin-like growth factor-I and/or the estrogen receptor antagonist ICI 182,780. Estradiol was administered in a subcutaneous silastic capsule. Insulin-like growth factor-I and ICI 182,780 were delivered in the lateral cerebral ventricle. Animals received six daily injections of 5-bromo-2-deoxyuridine and were killed 24 h after the last injection. The total number of 5-bromo-2-deoxyuridine-positive neurons was significantly increased in animals treated with insulin-like growth factor-I, compared with rats treated with vehicles, while rats treated with both insulin-like growth factor-I and estradiol showed a higher number of 5-bromo-2-deoxyuridine-positive neurons than rats treated with insulin-like growth factor-I or estradiol alone. The antiestrogen ICI 182,780 blocked the effect of insulin-like growth factor-I on the number of 5-bromo-2-deoxyuridine neurons with independence of whether the animals were treated or not with estradiol. These findings suggest that estrogen receptors are involved in the induction of adult neurogenesis by insulin-like growth factor-I in the dentate gyrus, and that estradiol and insulin-like growth factor-I have a cooperative interaction to promote neurogenesis. The interaction between insulin-like growth factor-I and estradiol may participate in changes in the rate of neurogenesis during different endocrine and physiological conditions, and may be related to the decline in neurogenesis with ageing.

Estrogen receptor alpha forms estrogen-dependent multimolecular complexes with insulin-like growth factor receptor and phosphatidylinositol 3-kinase in the adult rat brain

Estradiol and insulin-like growth factor-I (IGF-I) have numerous functional interactions in the brain, including the regulation of neuroendocrine events, the control of reproductive behavior and the promotion of synaptic plasticity and neuronal survival. To explore the mechanisms involved in these interdependent actions of estradiol and IGF-I in the adult brain, the potential interactions of estrogen receptors with components of the IGF-I signaling system were assessed in this study. Systemic estradiol administration resulted in a transient immunocoprecipitation of the IGF-I receptor with the estrogen receptor alpha and in a transient increase in tyrosine phosphorylation of the IGF-I receptor in the hypothalamus of adult ovariectomized Wistar rats. Both effects were coincident in time, with a peak between 1 and 3 h after systemic estradiol administration. Three hours after estradiol treatment, there was an enhanced immunocoprecipitation of estrogen receptor alpha with p85 subunit of phosphatidylinositol 3-kinase, as well as an enhanced immunocoprecipitation of p85 with insulin receptor substrate-1. The interaction with the IGF-I receptor was specific for the alpha form of the estrogen receptor and was also induced by intracerebroventricular injection of IGF-I. These hormonal actions may be part of the mechanism by which estradiol activates IGF-I receptor signaling pathways in the brain and may explain the interdependence of estrogen receptors and the IGF-I receptor in synaptic plasticity, neuroprotection and other neural events.

Astrocytes and brain function: implications for reproduction

Recent evidence suggests that astrocytes have important neuroregulatory functions in addition to their classic functions of support and segregation of neurons. These newly revealed functions include regulation of neuron communication, neurosecretion, and synaptic plasticity. Although these actions occur throughout the brain, this review will focus on astrocyte-neuron interactions in the hypothalamus, particularly with respect to their potential contribution to the regulation of gonadotropin-releasing hormone (GnRH) secretion and reproduction. Hypothalamic astrocytes have been documented to release a variety of neuroactive factors, including transforming growth factors-alpha and -beta, insulin-like growth factor-1, prostaglandin E2, and the neurosteroid, 3 alpha-hydroxy-5 alpha-pregnane-20-one. Each of these factors has been shown to stimulate GnRH release, and receptors for each factor have been documented on GnRH neurons. Astrocytes have also been implicated in the regulation of synaptic plasticity in key areas of the hypothalamus that control GnRH release, an effect achieved by extension and retraction of glial processes (i.e., glial ensheathment). Through this mechanism, the number of synapses on GnRH neurons and GnRH regulatory neurons can potentially be modulated, thereby influencing the activation state of GnRH neurons. The steroid hormone 17beta-estradiol, which triggers the GnRH and luteinizing hormone surge, has been shown to induce the astrocyte-regulated changes in hypothalamic synaptic plasticity, as well as enhance formation and release of the astrocyte neuroactive factors, thereby providing another potential mechanistic layer for astrocyte regulation of GnRH release. As a whole, these studies provide new insights into the diversity of astrocytes and their potential role in reproductive neuroendocrine function.

Axonal sprouting of a brainstem-spinal pathway after estrogen administration in the adult female rhesus monkey

The nucleus retroambiguus (NRA) is located in the caudal medulla oblongata and contains premotor neurons that project to motoneuronal cell groups in the brainstem and spinal cord. NRA projections to the lumbosacral cord are species specific and might be involved in mating behavior. In the female cat, this behavior is estrogen dependent, and estrogen induces axonal sprouting in the NRA-lumbosacral pathway. Because female receptive behavior in primates is not fully dependent on estrogen, the question arises as to whether the capacity of estrogen-induced sprouting is preserved in primates. The effect of estrogen was studied on the NRA-lumbosacral projection with the use of wheat germ agglutinin conjugated to horseradish peroxidase as a tracer in six adult ovariectomized rhesus monkeys with or without estrogen priming (three controls and three treated with 20 microg/day of estradiol benzoate subcutaneously for 14 days). Light microscopy showed that the density of arborizing labeled NRA axons in the lumbosacral cord was greater in estrogen-treated than in control animals. Ultrastructurally, labeled NRA terminal profiles were quantified in motoneuron pools that supply muscles of the abdominal wall, axial, and pelvic floor. After estrogen treatment, the average number of labeled terminal profiles per area of the abdominal wall, axial, and pelvic floor motoneuron pool increased 1.5-, 3.3-, and 2.8-fold, respectively. In the estrogen-treated cases, 8.9% of labeled terminal profiles showed characteristics of growth cones. In controls, such profiles were rarely observed. The results showed that estrogen induces axonal sprouting in a brainstem-spinal pathway in the adult female rhesus monkey. These findings supported the concept that the NRA-lumbosacral pathway may be involved in sexual behavior. Moreover, they demonstrated that a long descending brainstem-spinal tract in adult nonhuman primates retains the capacity for axonal sprouting.

Interactions of estrogen and insulin-like growth factor-I in the brain: molecular mechanisms and functional implications

In the brain, as in other tissues, estradiol interacts with growth factors. One of the growth factors that is involved in the neural actions of estradiol is insulin-like growth factor-I (IGF-I). Estradiol and IGF-I cooperate in the central nervous system to regulate neuronal development, neural plasticity, neuroendocrine events and the response of neural tissue to injury. The precise molecular mechanisms involved in these interactions are still not well understood. In the central nervous system there is abundant co-expression of estrogen receptors (ERs) and IGF-I receptors (IGF-IRs) in the same cells. Furthermore, the expression of estrogen receptors and IGF-I receptors in the brain is cross-regulated. In addition, using specific antibodies for the phosphorylated forms of extracellular-signal regulated kinase (ERK) 1 and ERK2 and Akt/protein kinase B (Akt/PKB) it has been shown that estradiol affects IGF-I signaling pathways in the brain. Estradiol treatment results in a dose-dependent increase in the phosphorylation of ERK and Akt/PKB in the brain of adult ovariectomized rats. In addition, estradiol and IGF-I have a synergistic effects on the activation of Akt/PKB in the adult rat brain. These findings suggest that estrogen effects in the brain may be mediated in part by the activation of the signaling pathways of the IGF-I receptor.

Synergistic interaction of estradiol and insulin-like growth factor-I in the activation of PI3K/Akt signaling in the adult rat hypothalamus

Estradiol and insulin-like growth factor-I (IGF-I) interact in the hypothalamus to regulate neuronal function, synaptic plasticity and neuroendocrine events. However, the molecular mechanisms involved in these interactions are still unknown. In the present study, the effect of estradiol on the signaling pathways of IGF-I receptor has been assessed in the hypothalamus of young adult ovariectomized rats, using specific antibodies for the phosphorylated forms of extracellular-signal regulated kinase (ERK) 1 and ERK2 and Akt/protein kinase B (Akt/PKB). Estradiol treatment resulted, between 6 and 24 h after systemic administration, in dose-dependent effects on the phosphorylation of ERK and Akt/PKB. Estradiol did not modify the level of ERK phosphorylation induced by intracerebroventricular administration of IGF-I. However, both hormones had a synergistic effect on the phosphorylation of Akt/PKB. These findings suggest that estrogen effects in the hypothalamus may be mediated in part by the activation of the signaling pathways of the IGF-I receptor.

Growth factors and steroid hormones: a complex interplay in the hypothalamic control of reproductive functions

The mechanisms through which LHRH-secreting neurons are controlled still represent a crucial and debated field of research in the neuroendocrine control of reproduction. In the present review, we have specifically considered two potential signals reaching these hypothalamic neurons: steroid hormones and growth factors. Examples of the relevant physiological role of the interactions between these two families of biologically acting molecules have been provided. In many cases, these interactions occur at the level of hypothalamic astrocytes, which are presently accepted as functional partners of the LHRH-secreting neurons. On the basis of the observations here summarized, we have formulated the hypothesis that a functional co-operation of steroid hormones and growth factors occurring in the hypothalamic astrocytic compartment represents a key factor in the neuroendocrine control of reproductive functions.

Functional interactions between estrogen and insulin-like growth factor-I in the regulation of alpha 1B-adrenoceptors and female reproductive function

The ovarian hormone estradiol (E(2)) and insulin-like growth factor-I (IGF-I) interact in the CNS to regulate neuroendocrine function and synaptic remodeling. Previously, our laboratory showed that 2 d E(2) treatment induces alpha(1B)-adrenoceptor expression and promotes IGF-I enhancement of alpha(1)-adrenoceptor potentiation of cAMP accumulation in the preoptic area (POA) and hypothalamus (HYP). This study examined the hypothesis that E(2)-dependent aspects of female reproductive function, including alpha(1B)-adrenoceptor expression and function in the POA and HYP, are mediated by brain IGF-I receptors (IGF-IRs) in female rats. Ovariohysterectomized rats were implanted with a guide cannula aimed at the third ventricle and treated in vivo with vehicle or E(2) daily for 2 d before experimentation. Intracerebroventricular infusions of JB-1, a selective IGF-IR antagonist, were administered every 12 hr beginning 1 hr before the first E(2) injection. Administration of JB-1 during E(2) priming completely blocks hormone-induced luteinizing hormone release and partially inhibits hormone-dependent reproductive behavior. Reproductive behavior is restored by intracerebroventricular infusion of 8-bromo-cGMP, the second messenger implicated in alpha(1)-adrenergic facilitation of lordosis. In addition, blockade of IGF-IRs during E(2) priming prevents E(2)-induced increases in alpha(1B)-adenoceptor binding density and abolishes acute IGF-I enhancement of NE-stimulated cAMP accumulation in HYP and POA slices. These data document the existence of a novel mechanism by which IGF-I participates in the remodeling of noradrenergic receptor signaling in the HYP and POA after E(2) treatment. These events may help coordinate the timing of ovulation with the expression of sexual receptivity.

Interactions of estrogens and insulin-like growth factor-I in the brain: implications for neuroprotection

Data from epidemiological studies suggest that the decline in estrogen following menopause could increase the risk of neurodegenerative diseases. Furthermore, experimental studies on different animal models have shown that estrogen is neuroprotective. The mechanisms involved in the neuroprotective effects of estrogen are still unclear. Anti-oxidant effects, activation of different membrane-associated intracellular signaling pathways, and activation of classical nuclear estrogen receptors (ERs) could contribute to neuroprotection. Interactions with neurotrophins and other growth factors may also be important for the neuroprotective effects of estradiol. In this review we focus on the interaction between insulin-like growth factor-I (IGF-I) and estrogen signaling in the brain and on the implications of this interaction for neuroprotection. During the development of the nervous system, IGF-I promotes the differentiation and survival of specific neuronal populations. In the adult brain, IGF-I is a neuromodulator, regulates synaptic plasticity, is involved in the response of neural tissue to injury and protects neurons against different neurodegenerative stimuli. As an endocrine signal, IGF-I represents a link between the growth and reproductive axes and the interaction between estradiol and IGF-I is of particular physiological relevance for the regulation of growth, sexual maturation and adult neuroendocrine function. There are several potential points of convergence between estradiol and IGF-I receptor (IGF-IR) signaling in the brain. Estrogen activates the mitogen-activated protein kinase (MAPK) pathway and has a synergistic effect with IGF-I on the activation of Akt, a kinase downstream of phosphoinositol-3 kinase. In addition, IGF-IR is necessary for the estradiol induced expression of the anti-apoptotic molecule Bcl-2 in hypothalamic neurons. The interaction of ERs and IGF-IR in the brain may depend on interactions between neural cells expressing ERs with neural cells expressing IGF-IR, or on direct interactions of the signaling pathways of alpha and beta ERs and IGF-IR in the same cell, since most neurons expressing IGF-IR also express at least one of the ER subtypes. In addition, studies on adult ovariectomized rats given intracerebroventricular (i.c.v.) infusions with antagonists for ERs or IGF-IR or with IGF-I have shown that there is a cross-regulation of the expression of ERs and IGF-IR in the brain. The interaction of estradiol and IGF-I and their receptors may be involved in different neural events. In the developing brain, ERs and IGF-IR are interdependent in the promotion of neuronal differentiation. In the adult, ERs and IGF-IR interact in the induction of synaptic plasticity. Furthermore, both in vitro and in vivo studies have shown that there is an interaction between ERs and IGF-IR in the promotion of neuronal survival and in the response of neural tissue to injury, suggesting that a parallel activation or co-activation of ERs and IGF-IR mediates neuroprotection.

Interactions between growth factors and steroids in the control of LHRH-secreting neurons

How the gene expression and the release of luteinizing hormone releasing hormone (LHRH) are controlled in LHRH-secreting neurons is a very crucial and still debated topic of the neuroendocrinology. Several observations present in literature have recently indicated that glial cells may influence the activity of hypothalamic LHRH-secreting neurons, via the release of growth factors. The present review will summarize data obtained in our laboratory indicating that: (a) type 1 astrocytes, a kind of glial cells, are able to release in vitro growth factors belonging to the transforming growth factors beta (TGFbeta) family (i.e. TGFbeta1 and TGFbeta2) which influence the gene expression and the release of the decapeptide in immortalized LHRH-secreting neurons; (b) glial cells are also able to influence the steroid metabolism occurring in these neurons and in some cases this effect is exerted by TGFbeta1; (c) the mRNA levels of TGFbeta1 and of basic fibroblast growth factor (bFGF), another growth factor involved in the control of LHRH-secreting neurons, are modified in the rat hypothalamus during the different phases of the estrous cycle; (d) steroid hormones are able to modulate the gene expression of TGFbeta1 and bFGF both in vivo (i.e. in the whole hypothalamus of ovariectomized rats) and in vitro (cultures of type 1 astrocytes). On the basis of these results a possible functional correlation in the control of LHRH-secreting neurons between growth factors and gonadal steroids will be discussed and proposed.

Mechanisms of ovarian steroid regulation of norepinephrine receptor-mediated signal transduction in the hypothalamus: implications for female reproductive physiology

In many mammalian species, the ovarian steroid hormones estradiol (E(2)) and progesterone (P) act in the hypothalamus and preoptic area to coordinate the timing of female sexual receptivity with ovulation. We study lordosis behavior, an important component of sexual receptivity in rats, and its regulation by E(2) and P as a model system for understanding how hormonal modulation of synaptic neurotransmission influences reproductive physiology and behavior. Our findings suggest that E(2) and P extensively regulate synaptic communication involving the catecholamine norepinephrine (NE) in the hypothalamus. Estrogen priming shifts the balance of postsynaptic NE receptor signaling in the hypothalamus and preoptic area away from beta-adrenergic activation of cAMP synthesis toward alpha(1)-adrenergic signaling pathways. Attenuation of beta-adrenergic signal transduction is achieved by receptor-G-protein uncoupling, apparently due to stable receptor phosphorylation. E(2) modification of alpha(1)-adrenergic signaling includes both increased expression of the alpha(1B)-adrenoceptor subtype and a dramatic, P-induced reconfiguration of the biochemical responses initiated by agonist activation of alpha(1)-adrenoceptors. Among these is the emergence of alpha(1)-adrenergic receptor coupling to cGMP synthesis. We also present evidence that estrogen promotes novel, functional interactions between insulin-like growth factor-1 (IGF-1) and alpha(1)-adrenergic receptor signaling in the hypothalamus and preoptic area. Thus, estrogen amplification of signaling mediated by alpha(1)-adrenoceptors is multifaceted, involving changes in gene expression (of the alpha(1B)-adrenoceptor), switching of receptor linkage to previously inactive intracellular pathways, and the promotion of cross talk between IGF-1 and NE receptors. We propose that this hormone-dependent remodeling of hypothalamic responses to NE maximizes reproductive success by coordinating the timing of the preovulatory release of gonadotropins with the period of behavioral receptivity in female rodents.

Effect of 17-beta estradiol and epidermal growth factor on DNA and RNA labeling in astroglial cells during development, maturation and differentiation in culture

Growth factors stimulate astroglial and neuronal proliferation and differentiation in culture. Estrogens markedly influence astroglia, and are key factors participating in neurodegeneration. The aim of the present study was to investigate interactions between estradiol (E2) and epidermal growth factor (EGF) during astroglia development, maturation and differentiation in culture. DNA or RNA labeling in 16 or 40 or 60 days in vitro (DIV) astrocyte cultures treated for 24 or 48 h with EGF and/or E2 was evaluated. A significant increase in DNA labeling in 16 DIV astrocyte cultures treated for 24 h with EGF (5 ng/ml) and E2 (1 nM) was found. EGF (5 or 10 ng/ml) addition in the last 24 h in 48 h E2 (1 or 5 nM)-treated astrocyte cultures at 16 DIV caused a slight, but significant increase in DNA labeling. No differences in RNA labeling were observed in 16 DIV astrocyte cultures treated for 24 or 48 h with EGF (5 or 10 ng/ml) in the presence of E(2) (1 or 5 nM). A significant stimulation in DNA labeling was shown in 40 DIV astrocyte cultures treated for 48 h with E2 (1 or 5 nM) in the presence of EGF (5 or 10 ng/ml) added in the last 24 h. In well differentiated astroglial cell cultures (60 DIV), DNA labeling was remarkably increased after 24 h treatment with 1 nM E2 or 5 ng/ml EGF. Co-addition of 1 nM E2 and 5 ng/ml EGF for 24 h reduced [methyl-(3)H]thymidine incorporation, when data are compared to E2- or EGF-treated cultures. Addition of EGF in the presence of E2 for 48 h or only in the last 24 h caused a significant decrease of [methyl-(3)H]thymidine incorporation in comparison with EGF-treated cultures at 60 DIV or with untreated cultures. Treatment of cultures for 24 h with EGF (5 or 10 ng/ml) alone or in combination with E2 (1 or 5 nM) induced a strong increase of RNA labeling in 60 DIV astrocyte cultures. Addition for 48 h of E2 (1 or 5 nM) or EGF (5 or 10 ng/ml) alone or in association stimulated significantly RNA labeling in astrocyte cultures at 60 DIV. When 60 DIV astrocyte cultures were treated for 48 h with E2 (1 or 5 nM) in the presence of EGF (5 or 10 ng/ml) added only in the last 24 h, a potentiating effect of RNA labeling was observed. The above results suggest that interaction between growth factors and estrogens may contribute to regulate astroglia development, maturation and differentiation.

Insulin-like growth factor I receptors and estrogen receptors colocalize in female rat brain

Several findings indicate that there is a close interaction between estrogen and insulin-like growth factor I in different brain regions. In adult brain, both estrogen and insulin-like growth factor I have co-ordinated effects in the regulation of neuroendocrine events, synaptic plasticity and neural response to injury. In this study we have qualitatively assessed whether estrogen receptors and insulin-like growth factor I receptor are colocalized in the same cells in the preoptic area, hypothalamus, hippocampus, cerebral cortex and cerebellum of female rat brain using confocal microscopy. Immunoreactivity for estrogen receptors alpha and beta was colocalized with immunoreactivity for insulin-like growth factor I receptor in many neurons from the preoptic area, hypothalamus, hippocampus and cerebral cortex. Furthermore, estrogen receptor beta and insulin-like growth factor I receptor immunoreactivities were colocalized in the Purkinje cells of the cerebellum. Colocalization of estrogen receptor beta and insulin-like growth factor I receptor was also detected in cells with the morphology of astrocytes in all regions assessed. The co-expression of estrogen receptors and insulin-like growth factor I receptor in the same neurons may allow a cross-coupling of their signaling pathways. Furthermore, the colocalization of immunoreactivity for estrogen receptor beta and insulin-like growth factor I receptor in glial cells suggests that glia may also play a role in the interactions of insulin-like growth factor I and estrogen in the rat brain. In conclusion, the co-expression of estrogen receptors and insulin-like growth factor I receptors in the same neural cells suggests that the co-ordinated actions of estrogen and insulin-like growth factor I in the brain may be integrated at the cellular level.

The impact of the GH-IGF-I axis on gonadotropin secretion: inferences from animal models

Given the tight, temporal coupling between growth and reproductive development, the idea that a common signal may regulate both adolescent growth and the initiation of puberty has been the focus of much research. Since the rate-limiting step for the onset of puberty is the appropriate hypothalamic secretion of gonadotropin-releasing hormone (GnRH), any factor important for the initiation of puberty must affect GnRH pulsatility. This review examines the hypothesis that GH and/or IGF-I are growth-related signals that regulate the release of GnRH, initiating puberty. By extension, this review also addresses the hypothesis that the GH axis also impacts GnRH and gonadotropin secretion in post-pubertal individuals and, thus, affects the maintenance of fertility in adults. The review examines data from a range of animal models employing a number of different strategies which directly manipulate the activity of either GH or IGF-I. The success of these strategies for producing the desired effects on the GH-IGF-I axis is somewhat variable. Although IGF-I may only play a permissive role in the maintenance of adult fertility, acting at the level of the gonad to increase sensitivity to gonadotropin stimulation, the data indicate that IGF-I is essential for reproductive maturation. However, in addition to its well-documented effects on the gonad, the specific mode of action of IGF-I on the neuroendocrine hypothalamus and GnRH pulsatility remains to be determined. Available evidence suggests that such action by IGF-I may be mediated through neurotransmitter effects on GnRH neurons, changing the availability of metabolic substrates for neuronal activity, or remodeling of synaptic input into GnRH neurons.

Neuroprotection by estradiol

This review highlights recent evidence from clinical and basic science studies supporting a role for estrogen in neuroprotection. Accumulated clinical evidence suggests that estrogen exposure decreases the risk and delays the onset and progression of Alzheimer's disease and schizophrenia, and may also enhance recovery from traumatic neurological injury such as stroke. Recent basic science studies show that not only does exogenous estradiol decrease the response to various forms of insult, but the brain itself upregulates both estrogen synthesis and estrogen receptor expression at sites of injury. Thus, our view of the role of estrogen in neural function must be broadened to include not only its function in neuroendocrine regulation and reproductive behaviors, but also to include a direct protective role in response to degenerative disease or injury. Estrogen may play this protective role through several routes. Key among these are estrogen dependent alterations in cell survival, axonal sprouting, regenerative responses, enhanced synaptic transmission and enhanced neurogenesis. Some of the mechanisms underlying these effects are independent of the classically defined nuclear estrogen receptors and involve unidentified membrane receptors, direct modulation of neurotransmitter receptor function, or the known anti-oxidant activities of estrogen. Other neuroprotective effects of estrogen do depend on the classical nuclear estrogen receptor, through which estrogen alters expression of estrogen responsive genes that play a role in apoptosis, axonal regeneration, or general trophic support. Yet another possibility is that estrogen receptors in the membrane or cytoplasm alter phosphorylation cascades through direct interactions with protein kinases or that estrogen receptor signaling may converge with signaling by other trophic molecules to confer resistance to injury. Although there is clear evidence that estradiol exposure can be deleterious to some neuronal populations, the potential clinical benefits of estrogen treatment for enhancing cognitive function may outweigh the associated central and peripheral risks. Exciting and important avenues for future investigation into the protective effects of estrogen include the optimal ligand and doses that can be used clinically to confer benefit without undue risk, modulation of neurotrophin and neurotrophin receptor expression, interaction of estrogen with regulated cofactors and coactivators that couple estrogen receptors to basal transcriptional machinery, interactions of estrogen with other survival and regeneration promoting factors, potential estrogenic effects on neuronal replenishment, and modulation of phenotypic choices by neural stem cells.

Estradiol and progesterone regulate the expression of insulin-like growth factor-I receptor and insulin-like growth factor binding protein-2 in the hypothalamus of adult female rats

Gonadal hormones interact with insulin-like growthfactor-I (IGF-I) to regulate synaptic plasticity during the estrous cycle in the rat mediobasal hypothalamus. It has been proposed that tanycytes, specialized glial cells lining the ventral region of the third ventricle, may regulate the availability of IGF-I to hypothalamic neurons. IGF-I levels in tanycytes fluctuate during the estrous cycle. Furthermore, estrogen administration to ovariectomized rats increases IGF-I levels in tanycytes, while progesterone, injected simultaneously with estrogen, blocks the estrogen-induced increase of IGF-I levels in tanycytes. To test whether hormonal regulation of IGF-I receptor (IGF-IR) and IGF binding protein-2 (IGFBP-2) may be involved in the accumulation of IGF-I in tanycytes, we assessed the effect of ovarian hormones on the levels of these molecules in the mediobasal hypothalamus of adult female rats. Ovariectomized animals were treated with either oil, estrogen, progesterone, or estrogen and progesterone simultaneously and then killed 6 or 24 h later. Some neurons, some astrocytes, and many tanycytes in the mediobasal hypothalamus were found by confocal microscopy to be immunoreactive for IGF-IR. IGFBP-2 immunoreactivity was restricted almost exclusively to tanycytes and ependymal cells and was colocalized with IGF-IR immunoreactivity in tanycytes. By electron microscope immunocytochemistry using colloidal gold labeling, IGF-IR and IGFBP-2 immunoreactivities were observed in the microvilli of tanycytes in the lumen of the third ventricle. IGF-IR and IGFBP-2 immunoreactive levels on the apical surface of tanycytes were significantly decreased by the administration of progesterone, either alone or in the presence of estradiol. IGF-IR levels in the mediobasal hypothalamus, measured by Western blotting, were not significantly affected by the separate administration of estradiol or progesterone to ovariectomized rats. However, the simultaneous administration of both hormones resulted in a marked decrease in IGF-IR protein levels. Estradiol administration to ovariectomized rats increased IGFBP-2 immunoreactive levels in the hypothalamus. While progesterone did not significantly affect IGFBP-2 expression, the simultaneous injection of estradiol and progesterone resulted in a marked decrease in IGFBP-2 protein levels. The effect of estradiol on IGFBP-2 was observed both in protein and mRNA levels, suggesting a transcriptional regulation. However, the simultaneous administration of progesterone and estradiol had different effects on IGF-IR protein and IGF-IR mRNA levels, as well as on IGFBP-2 protein and IGFBP-2 mRNA levels, suggesting a postranscriptional action. These findings indicate that estradiol and progesterone regulate the expression of IGF-IR and IGFBP-2 in the mediobasal hypothalamus of adult female rats. Regulation of the hypothalamic IGF-I system by ovarian hormones may be physiologically relevant for neuroendocrine regulation and for synaptic plasticity during the estrous cycle. These results do not support the hypothesis that estrogen-induced accumulation of IGF-I by tanycytes is mediated by the hormonal regulation of IGF-IR. However, estrogen-induced up-regulation of IGFBP-2 and progesterone-induced down-regulation of IGF-IR and IGFBP-2 levels in the apical plasma membrane of tanycytes may be involved in the fluctuation of IGF-I levels in the mediobasal hypothalamus during the estrous cycle.