Neuronal membrane remodelling during the oestrus cycle: a freeze-fracture study in the arcuate nucleus of the rat hypothalamus

Estrogen-Induced Hypothalamic Synaptic Plasticity and Pituitary Sensitization in the Control of the Estrogen-Induced Gonadotrophin Surge

Proper gonadal function requires coordinated (feedback) interactions between the gonads, adenohypophysis, and brain: the gonads elaborate sex steroids (progestins, androgens, and estrogens) and proteins (inhibin-activin family) during gamete development. In both sexes, the brain-pituitary gonadotrophin-regulating interaction is coordinated by estradiol through its opposing actions on pituitary gonadotrophs (sensitization of the response to gonadotrophin-releasing hormone [GnRH]) versus hypothalamic neurons (inhibition of GnRH secretion). This dynamic tension between the gonadotrophs and the GnRH cells in the brain regulates the circulating gonadotrophins and is termed reciprocal/negative feedback. In females, reciprocal/negative feedback dominates approximately 90% of the ovarian cycle. In a spectacular exception, the dynamic tension is broken during the surge of circulating estrogen that marks follicle and oocyte(s) maturation. The cause is an estradiol-induced disinhibition of the GnRH neurons that releases GnRH secretion to the highly sensitized pituitary gonadotrophs that in turn release the gonadotrophin surge (the estrogen-induced gonadotrophin surge [EIGS], also known as positive feedback). Studies during the past 4 decades have shown this disinhibition to result from estrogen-induced synaptic plasticity (EISP), including a reversible approximately 50% loss in arcuate nucleus synapses. The disinhibited GnRH secretion occurs during maximal gonadotroph sensitization and results in the EIGS. Specific immunoneutralization of estradiol blocks the EISP and EIGS. The EISP is accompanied by increases in insulinlike growth factor 1, polysialylated neural cell adhesion molecule, and ezrin, 3 proteins that the authors believe are the links between estrogen-induced astroglial extension and the EISP that releases GnRH secretion at the moment of maximal sensitization of the pituitary gonadotrophs. The result is the paradoxical surge of gonadotrophins at the peak of ovarian estrogen secretion and the triggering of ovulation. This enhanced understanding of the mechanics of gonadotrophin control clarifies elements of the involved feedback loops and opens the way to a better understanding of the neurobiology of reproduction.

Evidence for Changes in Numbers of Synaptic Inputs onto KNDy and GnRH Neurones during the Preovulatory LH Surge in the Ewe

Kisspeptin neurones located in the arcuate nucleus (ARC) and preoptic area (POA) are critical mediators of gonadal steroid feedback onto gonadotrophin-releasing hormone (GnRH) neurones. ARC kisspeptin cells that co-localise neurokinin B (NKB) and dynorphin (Dyn), are collectively referred to as KNDy (Kisspeptin/NKB/Dyn) neurones, and have been shown in mice to also co-express the vesicular glutamate transporter, vGlut2, an established glutamatergic marker. The ARC in rodents has long been known as a site of hormone-induced neuroplasticity, and changes in synaptic inputs to ARC neurones in rodents occur over the oestrous cycle. Based on this evidence, the the present study aimed to examine possible changes across the ovine oestrous cycle in synaptic inputs onto kisspeptin cells in the ARC (KNDy) and POA, and inputs onto GnRH neurones. Gonadal-intact breeding season ewes were perfused using 4% paraformaldehyde during either the luteal or follicular phase of the oestrous cycle, with the latter group killed at the time of the luteinising hormone (LH) surge. Hypothalamic sections were processed for triple-label immunodetection of kisspeptin/vGlut2/synaptophysin or kisspeptin/vGlut2/GnRH. The total numbers of synaptophysin- and vGlut2-positive inputs to ARC KNDy neurones were significantly increased at the time of the LH surge compared to the luteal phase; because these did not contain kisspeptin, they do not arise from KNDy neurones. By contrast to the ARC, the total number of synaptophysin-positive inputs onto POA kisspeptin neurones did not differ between luteal phase and surge animals. The total number of kisspeptin and vGlut2 inputs onto GnRH neurones in the mediobasal hypothalamus (MBH) was also increased during the LH surge, and could be attributed to an increase in the number of KNDy (double-labelled kisspeptin + vGlut2) inputs. Taken together, these results provide novel evidence of synaptic plasticity at the level of inputs onto KNDy and GnRH neurones during the ovine oestrous cycle. Such changes may contribute to the generation of the preovulatory GnRH/LH surge.

Second generation anti-epileptic drugs adversely affect reproductive functions in young non-epileptic female rats

Reproductive endocrine disturbances are a major health concern in women with epilepsy due to their long term use of antiepileptic drugs (AEDs). Second generation AEDs such as topiramate (TPM) and gabapentin are frequently used for the treatment of epilepsy as well as migraine, bipolar disorder etc. Despite the widespread clinical complications, however the definitive mechanism(s) mediating the side effects of TPM and gabapentin remain obscure. The present study was aimed to evaluate the long term effects of TPM and gabapentin on reproductive functions in young female Wistar rats. Estrous cyclicity, ovarian histology as well as estradiol, LH, leptin and insulin hormones level were studied to elucidate the long-term effect of these AEDs monotherapy on reproductive functions in non-epileptic animals. Further to explore the effects on gonadotropin releasing hormone (GnRH) neuroendocrine plasticity, the expression of GnRH, gamma-amino butyric acid (GABA), glutamic acid decarboxylase (GAD), glial fibrilliary acidic protein (GFAP) and polysialylated form of neural cell adhesion molecule (PSA-NCAM) was studied in median eminence (ME) region of these animals by immunohistochemistry, Western blot hybridization and RT-PCR. Our results demonstrate that TPM and gabapentin treatment for 8 weeks cause reproductive dysfunction as ascertained by disturbed hormonal levels and estrous cyclicity as well as alterations in GABAergic system and GnRH neuronal-glial plasticity. Our findings suggest that treatment with TPM and gabapentin disrupts the complete hypothalamo-hypophyseal-gonadal axis (HPG) through GnRH pulse generator in hypothalamus.

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.

Sex steroid-induced neuroplasticity and behavioral activation in birds

The brain of adult homeothermic vertebrates exhibits a higher degree of morphological neuroplasticity than previously thought, and this plasticity is especially prominent in birds. In particular, incorporation of new neurons is widespread throughout the adult avian forebrain, and the volumes of specific nuclei vary seasonally in a prominent manner. We review here work on steroid-dependent plasticity in birds, based on two cases: the medial preoptic nucleus (POM) of Japanese quail in relation to male sexual behavior, and nucleus HVC in canaries, which regulates song behavior. In male quail, POM volume changes seasonally, and in castrated subjects testosterone almost doubles POM volume within 2 weeks. Significant volume increases are, however, already observable after 1 day. Steroid receptor coactivator-1 is part of the mechanism mediating these effects. Increases in POM volume reflect changes in cell size or spacing and dendritic branching, but are not associated with an increase in neuron number. In contrast, seasonal changes in HVC volume reflect incorporation of newborn neurons in addition to changes in cell size and spacing. These are induced by treatments with exogenous testosterone or its metabolites. Expression of doublecortin, a microtubule-associated protein, is increased by testosterone in the HVC but not in the adjacent nidopallium, suggesting that neuron production in the subventricular zone, the birthplace of newborn neurons, is not affected. Together, these data illustrate the high degree of plasticity that extends into adulthood and is characteristic of avian brain structures. Many questions still remain concerning the regulation and specific function of this plasticity.

Membrane estradiol signaling in the brain

While the physiology of membrane-initiated estradiol signaling in the nervous system has remained elusive, a great deal of progress has been made toward understanding the activation of cell signaling. Membrane-initiated estradiol signaling activates G proteins and their downstream cascades, but the identity of membrane receptors and the proximal signaling mechanism(s) have been more difficult to elucidate. Mounting evidence suggests that classical intracellular estrogen receptor-alpha (ERalpha) and ERbeta are trafficked to the membrane to mediate estradiol cell signaling. Moreover, an interaction of membrane ERalpha and ERbeta with metabotropic glutamate receptors has been identified that explains the pleomorphic actions of membrane-initiated estradiol signaling. This review focuses on the mechanism of actions initiated by membrane estradiol receptors and discusses the role of scaffold proteins and signaling cascades involved in the regulation of nociception, sexual receptivity and the synthesis of neuroprogesterone, an important component in the central nervous system signaling.

17beta-estradiol-mediated neuroprotection and ERK activation require a pertussis toxin-sensitive mechanism involving GRK2 and beta-arrestin-1

17-beta-Estradiol (E2) is a steroid hormone involved in numerous bodily functions, including several brain functions. In particular, E2 is neuroprotective against excitotoxicity and other forms of brain injuries, a property that requires the extracellular signal-regulated kinase (ERK) pathway and possibly that of other signaling molecules. The mechanism and identity of the receptor(s) involved remain unclear, although it has been suggested that E2 receptor alpha (ERalpha) and G proteins are involved. We, therefore, investigated whether E2-mediated neuroprotection and ERK activation were linked to pertussis toxin (PTX)-sensitive G-protein-coupled effector systems. Biochemical and image analysis of organotypic hippocampal slices and cortical neuronal cultures showed that E2-mediated neuroprotection as well as E2-induced ERK activation were sensitive to PTX. The sensitivity to PTX suggested a possible role of G-protein- and beta-arrestin-mediated mechanisms. Western immunoblots from E2-treated cortical neuronal cultures revealed an increase in phosphorylation of both G-protein-coupled receptor-kinase 2 and beta-arrestin-1, a G-protein-coupled receptor adaptor protein. Transfection of neurons with beta-arrestin-1 small interfering RNA prevented E2-induced ERK activation. Coimmunoprecipitation experiments indicated that E2 increased the recruitment of beta-arrestin-1 and c-Src to ERalpha. These findings suggested that ERalpha is regulated by a mechanism associated with receptor desensitization and downregulation. In support of this idea, we found that E2 treatment of cortical synaptoneurosomes resulted in internalization of ERalpha, whereas treatment of cortical neurons with the ER agonists E-6-BSA-FITC [beta-estradiol-6-(O-carboxymethyl)oxime-bovine serum albumin conjugated with fluorescein isothiocyanate] and E-6-biotin [1,3,5(10)-estratrien-3,17beta-diol-6-one-6-carboxymethloxime-NH-propyl-biotin] resulted in agonist internalization. These results demonstrate that E2-mediated neuroprotection and ERK activation involve ERalpha activation of G-protein- and beta-arrestin-mediated mechanisms.

Neuronal-glial and synaptic remodelling in the adult hypothalamus in response to physiological stimuli

Activation of certain neurosecretory systems of the mammalian hypothalamus induces remodelling of the conformation of their neurons and glial cells. During stimulation of the hypothalamo-neurohypophysial system, astrocytic coverage of oxytocinergic somata and dendrites diminishes and their surfaces become extensively juxtaposed. In the neurohypophysis and median eminence, stimulation evokes a retraction of glial processes and an increase in the contact area between neurosecretory terminals and the perivascular space. These changes are reversible and glial coverage returns to normal upon cessation of stimulation. Neuronal-astrocytic rearrangements also occur in the arcuate nucleus in response to changes in sex steroid levels. The significance of such modifications is a matter of speculation. In the hypothalamic nuclei they may permit synaptic remodelling that takes place concurrently; in the neurohaemal structures they may facilitate neuropeptide release. We know little about the cellular mechanisms involved but glia and neurons of these systems express certain molecules implicated in cell-cell interactions in the developing central nervous system, such as the polysialylated isoform of the neural cell adhesion molecule; this may allow them to manifest their capacity for morphological plasticity in adulthood. The factors inducing the changes vary in the different structures: while oxytocin, in synergy with steroids, appears essential to the induction of the changes in the oxytocinergic system, oestrogen alone is critical in the arcuate nucleus; in the neurohypophysis noradrenaline appears important.

Polycystic ovary syndrome: etiology and pathogenesis

OBJECTIVE

To provide a review of the pathogenesis of polycystic ovary syndrome.

DESIGN

Literature survey.

RESULT(S)

Three major pathophysiologic hypotheses have been proposed to explain the clinical findings of polycystic ovary syndrome (PCOS) related to three major laboratory findings: the LH hypothesis, the insulin hypothesis and the ovarian hypothesis. Although the presence of many small follicles with a high androgen to estrogen ratio was first thought to represent a high rate of follicular atresia in polycystic ovaries, recent studies have demonstrated that the granulosa cells are viable and able to respond to FSH stimulation with normal increases in estradiol production. Thus, a new hypothesis has arisen that FSH activity is somehow blocked at the ovarian level.

CONCLUSION(S)

PCOS is a syndrome involving defects in primary cellular control mechanisms that result in the expression of chronic anovulation and hyperandrogenism. In this syndrome, the relation between the various parameters is of particular interest. These relations constitute the cornerstone of the pathogenesis of PCOS. The fact that the pathogenesis of PCOS has not yet been clarified, despite the plethora of relative information, may be the result of a general way of thinking in the interpretation of several scientific data, and especially those that refer to biochemical phenomena. The use of the various models of the theory of chaos, that permits a concrete approach for the interpretation of data, may constitute an optional procedure for the future understanding of the association of different parameters and their disturbances in the pathogenesis of the polycystic ovary syndrome.

Effects of gonadal hormones on central nitric oxide producing systems

Nitric oxide-containing neurons are widely distributed within the CNS, including regions involved in the control of reproduction and sexual behavior. The expression of neuronal nitric oxide synthase is influenced by testosterone in male rat, and by estrogens in female. Moreover, nitric oxide synthase may co-localize with gonadal hormones' receptors. Gonadal hormones may influence nitric oxide synthase expression in adulthood as well as during the development. In fact, in mice knockout for estrogen receptor alpha, the nitric oxide synthase-expressing population is deeply reduced in specific regions. In physiological conditions, the female in mammalian species is exposed to short-term changes of gonadal hormones levels (estrous cycle). Our recent studies, performed in the rat vomeronasal system and in mouse hypothalamic and limbic systems reveal that, in rodents, the expression of nitric oxide synthase-producing elements within regions relevant for the control of sexual behavior is under the control of gonadal hormones. The expression of nitric oxide synthase may vary according to the rapid variations of hormonal levels that take place during the estrous cycle. This seems in accordance with the hypothesis that gonadal hormone activation of nitric oxide-cyclic guanosine-monophosphate pathway is important for lordosis behavior, as well as that this system is activated during mating behavior. Finally, comparative data available for other vertebrates suggest that class-specific and species-specific differences occur in the nitric oxide synthase system of hypothalamus and limbic structures. Therefore, particular caution is needed to generalize data obtained from studies in rodents.

Synaptic remodeling induced by gonadal hormones: neuronal plasticity as a mediator of neuroendocrine and behavioral responses to steroids

During recent decades, it has become a generally accepted view that structural neuroplasticity is remarkably involved in the functional adaptation of the CNS. Thus, cellular morphology in the brain is in continuous transition throughout the life span, as a response to environmental stimuli. The effects of the environment on neuroplasticity are mediated by, to some extent, the changing levels of circulating gonadal steroid hormones. Today, it is clear that the function of gonadal steroids in the brain extends beyond simply regulating reproductive and/or neuroendocrine events. In addition, or even more importantly, gonadal steroids participate in the shaping of the developing brain, while their actions during adult life are implicated in higher brain functions such as cognition, mood and memory. A large body of evidence indicates that gonadal steroid-induced functional changes are accompanied by alterations in neuron and synapse numbers, as well as in dendritic and synaptic morphology. These structural modifications are believed to serve as a morphological basis for changes in behavior and cellular activity. Due to their growing functional and clinical significance, the specificity, timeframe, as well as the molecular and cellular mechanisms of hormone-induced neuroplasticity have become the focus of many studies. In this review, we briefly summarize current knowledge and the most significant recent discoveries from our laboratories on estrogen- and dehydroepiandrosterone-induced synaptic remodeling in the hypothalamus and hippocampus, two important brain areas heavily involved in autonomic and cognitive operations, respectively.

Neuroanatomical organization of gonadotropin-releasing hormone neurons during the oestrus cycle in the ewe

BACKGROUND

During the preovulatory surge of gonadotropin-releasing hormone (GnRH), a very large amount of the peptide is released in the hypothalamo-hypophyseal portal blood for 24-36H00. To study whether this release is linked to a modification of the morphological organization of the GnRH-containing neurons, i.e. morphological plasticity, we conducted experiments in intact ewes at 4 different times of the oestrous cycle (before the expected LH surge, during the LH surge, and on day 8 and day 15 of the subsequent luteal phase). The cycle stage was verified by determination of progesterone and LH concentrations in the peripheral blood samples collected prior to euthanasia.

RESULTS

The distribution of GnRH-containing neurons throughout the preoptic area around the vascular organ of the lamina terminalis was studied following visualisation using immunohistochemistry. No difference was observed in the staining intensity for GnRH between the different groups. Clusters of GnRH-containing neurons (defined as 2 or more neurons being observed in close contact) were more numerous during the late follicular phase (43 +/- 7) than during the luteal phase (25 +/- 6), and the percentage of clusters was higher during the beginning of the follicular phase than during the luteal phase. There was no difference in the number of labelled neurons in each group.

CONCLUSIONS

These results indicate that the morphological organization of the GnRH-containing neurons in ewes is modified during the follicular phase. This transitory re-organization may contribute to the putative synchronization of these neurons during the surge. The molecular signal inducing this plasticity has not yet been identified, but oestradiol might play an important role, since in sheep it is the only signal which initiates the GnRH preovulatory surge.

The role of estrogen replacement therapy in Alzheimer's disease

Multiple factors appear to contribute to the expression of Alzheimer's disease (AD). About 30% of cases of dementia of the Alzheimer's type (DAT) can be attributed to genetic factors. These observations raise the possibility of identifying multiple interventions that may modify the disease process and, therefore, the clinical expression of the dementia. Prominent among factors that may contribute to dementia and, specifically, to dementia of the Alzheimer's type is cerebral vascular disease. Estrogen is a potent factor that not only prevents vascular disease but also improves blood flow in diseased vessels, including blood flow in regions of the brain affected by AD. Estrogen also has direct effects on neuronal function that may play an important role not only in the preservation of neurons but in the repair of neurons damaged by the disease process. These effects of estrogen on the CNS suggest that the hormone may be effective not only in the prevention of dementia but also in its treatment. Given the distressingly high prevalence of AD among older women and the exorbitant social and economic costs associated with this disorder, a true risk reduction on the order of one-third to one-half, as suggested by several recent analytical studies, would be of tremendous public health importance.

Steroid-induced developmental plasticity in hypothalamic astrocytes: implications for synaptic patterning

We have previously demonstrated that astrocytes in the developing arcuate nucleus of the rat hypothalamus exhibit a sexually dimorphic morphology as a result of differential exposure to gonadal steroids. Testosterone via its aromatized byproduct, estrogen, induces arcuate astrocytes to undergo differentiation during the first few days of life. These differentiated astrocytes exhibit a stellate morphology. Coincident with the steroid-induced increase in astrocyte differentiation is a reduction of dendritic spines on arcuate neurons. As a result, the arcuate nucleus of males has fewer axodendritic spine synapses than females and this dimorphism is retained throughout life. In the immediately adjacent ventromedial nucleus, neonatal astrocytes are immature and unresponsive to steroids. Neurons in this region show no change in dendritic spines in the first few days of life but do exhibit increased dendritic branching as a result of testosterone exposure. These findings illustrate the importance of distinct populations of astrocytes in restricted brain regions and their potential importance to the establishment of regionally specific synaptic patterning. Conflicting reports leave the site of steroid-mediated astrocyte responsiveness in the arcuate nucleus unresolved: Are gonadal steroids acting directly on astrocytes or are steroid-concentrating neurons mediating astrocytic responsiveness? In this review, we discuss the current understanding of astrocyte-neuron interactions and the possible mechanisms for steroid-mediated, astrocyte-directed synaptic patterning in the developing hypothalamus.

Neuroplastic changes in the hypothalamic arcuate nucleus: the estradiol effect is accompanied by increased exoendocytotic activity of neuronal membranes

1. In the rat hypothalamic arcuate nucleus, estradiol induces coordinated changes in the number of axosomatic synapses, the amount of glial ensheathing, and the ultrastructure of the membrane of neuronal somas. In the present study we used conventional electron microscopy and freeze-fracture to examine cellular mechanisms responsible for the estradiol-induced changes at the membrane level. 2. In freeze-fracture replicas taken 10-60 min and 24 hr after injection of 17 beta-estradiol to adult ovariectomized females, it was found that there was a rapid increase in the number of exoendocytotic images that reached a plateau by 30 min. 3. In thin sections from animals injected 24 hr earlier we demonstrated a significant increase in coated vesicles in the periphery of the neurons and coated pits in the perikaryal membranes and decreased axosomatic synapses. 4. We conclude that these morphological alterations are signaling estrogen-induced transport and/or turnover of perikaryal membrane constituents and extracellular components which may affect interneuronal and neuroglial interactions.

Regulation of arcuate nucleus synaptology by estrogen

Estrogen modulates the synaptology of the hypothalamic arcuate nucleus during sexual differentiation of the rat brain in both males and females. In males, testosterone of gonadal origin is converted to estrogen in the brain by an enzyme, aromatase, which is also present in females. The exposure of the male's hypothalamus to relatively high levels of estrogen (following a perinatal testosterone surge) leads to the development of a pattern of synaptogenesis which does not support an estrogen-induced gonadotrophin surge in the adult. In female rats, hypothalamic development occurs with permissively low levels of estrogen, enabling a midcycle estrogen-induced gonadotrophin surge and ovulation in adulthood. During adult reproductive life in female rats, circulating estrogen modulates the synaptology of the arcuate nucleus. The most physiological example of this is the 30-50% loss of axosomatic synapses following the preovulatory estrogen surge on diestrus-proestrus. Studies on post-synaptic membranes of the arcuate nucleus reveal sex differences in membrane organization and protein content which are estrogen-dependent. Estrogen apparently stimulates endocytosis of areas of post-synaptic membrane that are dense with small intramembranous protein particles, resulting in a reduction in the number of small intramembranous particles. This also appears to be the physiologic mechanism of neuronal changes in females during the estrus cycle. Repeated exposure to preovulatory levels of estrogen may lead to an age-related decline in reproductive capacity in female rats. Aging females lose the estrogen-induced gonadotrophin surge responsible for ovulation. This loss of function may result from a cumulative estrogen effect during the repeated ovarian cycles which results in a reorganization of the synaptology on which regulates the estrogen-induced gonadotrophin surge. The membrane organization of the senescent constant estrus aged female appears indistinguishable from the males. The hypothalamic circuits modulated by estrogen have yet to be delineated. However, recent research has shown that GABA, the monoamines, and several neuropeptides are participants in the estrogen-sensitive network which regulates GNRH secretion. In this regard, present work shows estrogen-induced changes in GABA and dopamine synapses in the arcuate nucleus.

Gonadal hormones as promoters of structural synaptic plasticity: cellular mechanisms

It is now obvious that the CNS is capable of undergoing a variety of plastic changes at all stages of development. Although the magnitude and distribution of these changes may be more dramatic in the immature animal, the adult brain retains a remarkable capacity for undergoing morphological and functional modifications. Throughout development, as well as in the postpubertal animal, gonadal steroids exert an important influence over the architecture of specific sex steroid-responsive areas, resulting in sexual dimorphisms at both morphological and physiological levels. We are only now beginning to gain insight into the mechanisms involved in gonadal steroid-induced synaptic changes. The number of synaptic inputs to specific neuronal populations is sexually dimorphic and this can be modulated by changes in the sex steroid environment. These modifications can be correlated with other morphological changes, such as glial cell activation, that are occurring simultaneously in the same anatomical area. Indeed, the close physical relationship between glial cells and neuronal synaptic contacts makes them an ideal candidate for participating in this process. Interestingly, not only can the morphology and immunoreactivity of glial cells be modulated by gonadal steroids, but a close negative correlation between the number of synapses and the amount of glial ensheathing of a neuron has been demonstrated, suggesting an active participation of these cells in this process. Glia have sex steroid receptors, are capable of producing and metabolizing steroids, and can produce other neuronal trophic factors in response to sex steroids. Hence, their role in gonadal steroid-induced synaptic plasticity is becoming more apparent. In addition, there is recent evidence that this process may involve certain cell surface molecules, such as the N-CAMs, since a specific isoform of this molecule, previously referred to as the embryonic form, is found in those areas of the brain which maintain the capacity to undergo synaptic remodelling. However, there is much work to be done in order to fully understand this phenomenon and before bringing it into a clinical setting in hopes of treating neurodegenerative diseases or injuries to the nervous system.

Gonadal steroids as promoters of neuro-glial plasticity

Estradiol induces coordinated modifications in the extension of glial and neuronal processes in the arcuate nucleus of the hypothalamus of adult female rats. This hormonal effect results in natural fluctuations in the ensheathing of arcuate neurons by glial processes and these glial changes are linked to a remodelling of inhibitory GABAergic synapses during the estrous cycle. Hormonally induced glial and synaptic changes appear to be dependent on specific recognition or adhesion molecules on the neuronal and/or glial membranes.

The role of estradiol and progesterone in phased synaptic remodelling of the rat arcuate nucleus

During the estrous cycle there is a phasic synaptic remodelling in the hypothalamic arcuate nucleus, consisting in a loss and regain of axo-somatic synapses during the 48 h period between the morning of proestrus and the morning of metestrus. Synaptic changes are accompanied by cyclic modifications in the number of intramembrane particles in the plasma membrane of arcuate neuronal somas. To test the effect of the ovarian steroids on arcuate axo-somatic synapses we treated castrated females either with oil vehicle, 17 beta-estradiol, progesterone, or a combination of estradiol and progesterone, and observed them for 48 h. The number of axo-somatic synaptic profiles showed a 33% fall by 24 h after estradiol treatment and returned to control levels by 48 h. The effect of estradiol on axo-somatic synapses was accompanied by a marked and reversible modification of the number of intramembrane particles in the plasma membrane of arcuate neuronal somas. Progesterone alone did not affect the number of axo-somatic synaptic profiles nor the number of intramembrane particles, but when administered together with estradiol, blocked the effects of estradiol on neuronal membrane and synapses.

Cycloheximide mimics effects of oestradiol that are linked to synaptic plasticity of hypothalamic neurons

The synaptic connectivity of the rat arcuate nucleus, a hypothalamic area rich in oestradiol receptors, is rapidly affected by physiological modifications of hormonal levels. A rise of oestradiol in plasma elicits a coordinated neuronal-glial response that begins with a rapid fall in the number of small (< 10 nm) intramembrane particles and a rapid increase in the number of large (> 10 nm) intramembrane particles in neuronal membranes, followed by a modification in the branching of astrocytic processes and finally results in decreased number of axo-somatic synapses and increased glial wrapping of the neuronal somas. In the course of a series of studies aimed to test possible non-genomic effects of oestradiol on neuronal membranes we analyzed the effect of the systemic administration of the protein synthesis inhibitor cycloheximide on the ultrastructure of arcuate neurons and granule cells of the cerebellar cortex, an area of the brain with low levels of estrogen receptors. Cycloheximide resulted in a significant inhibition of protein synthesis in hypothalmus and cerebellum of ovariectomized rats. Under these circumstances, the number of small intramembrane particles was reduced in hypothalamic and cerebellar neuronal membranes while the number of large intramembrane particles showed a decrease in cerebellar membranes and a transient increase in arcuate neuronal somas. Furthermore, cycloheximide resulted in an increased glial wrapping of arcuate neuronal somas but not of cerebellar granule cells. The ensheathing of arcuate neurons by glial was associated with a 41% decrease in the number of axo-somatic synapses. These results indicate that the protein synthesis inhibitor cycloheximide may elicit the integrated neuronal-glial response that is associated with the hormonally induced remodelling of synaptic contacts on arcuate neurons.

Estradiol induces plasticity of gabaergic synapses in the hypothalamus

The number of axosomatic synapses on arcuate neurons of the adult rat hypothalamus fluctuates following the sequence of increasing circulatory estradiol during the ovarian cycle. To determine whether estrogen is affecting GABAergic synaptic contacts we studied the number of GABA-immunoreactive axosomatic synapses in adult ovariectomized rats injected either with 17 beta estradiol (100 micrograms/100 g body weight) or with sesame oil vehicle. The number of immunoreactive axosomatic synapses was significantly reduced in estradiol-treated rats (77 +/- 8 vs 56 +/- 6 synapses per 1000 microns of perikaryal membrane in control and estradiol-treated rats, respectively) while the number of non-immunoreactive synapses was not significantly affected by the hormonal treatment (44 +/- 6 vs 35 +/- 5 synapses per 1000 microns of perikaryal membrane in control and estradiol-treated rats, respectively). Estradiol administration also resulted in a significant decrease in the percentage of perikaryal membrane covered by immunoreactive synapses. These results suggest that physiological levels of estradiol may induce a remodeling of GABAergic inhibitory inputs on arcuate neurons.

Changes of rat striatal neuronal membrane morphology and steroid content during the estrous cycle

It is well documented that sex steroids affect striatal dopamine systems. However, the mechanism(s) of these hormonal effects in the striatum is still not well understood. We now report that gonadal steroid hormones during the estrous cycle affect the morphology and steroid hormone content of the rat striatum. Rats displaying at least two consecutive estrous cycles were included in this study as well as a group of female rats ovariectomized two weeks before being killed. The striatum was dissected from one half of each brain and used for morphological studies. From the other half of each brain, the striatum was dissected and steroid hormone concentrations in striatum and the remainder of the brain were determined. Tissues and serum concentrations of 17 beta-estradiol, progesterone and prolactin were measured by specific radioimmunoassays. Serum 17 beta-estradiol and prolactin concentrations peaked in proestrus, while progesterone was high in diestrus and proestrus. 17 beta-Estradiol levels were higher in the striatum than in the rest of the brain; both were also shown to fluctuate during the estrous cycle and with a pattern similar to that observed in serum. Progesterone serum levels showed a similar pattern of changes during the estrous cycle to progesterone concentrations in the striatum and the rest of the brain. The ultrastructure of the striatal dendritic membranes was studied by freeze-fracture. A significant difference in the content of intramembranous particles in dendritic shafts, which are mainly contacted by dopaminergic synapses, was found during the estrous cycle. The numerical density of large (greater than 10 nm) intramembranous particles was increased in diestrus I and II and in the afternoon of proestrus compared to estrus, the morning of proestrus and ovariectomized rats. In contrast, the numerical density of small (less than 10 nm) intramembranous particles was decreased in cycling animals compared to ovariectomized rats and fell in the afternoon of proestrus and then progressively increased in the following days to peak in the morning of proestrus. A negative correlation between steroid concentrations and small intramembranous particle density was observed, while the correlation was positive for large particles. No changes were observed in the membranes of dendritic spines, the main postsynaptic target for cortical afferents. In summary, this is the first report that concentrations of 17 beta-estradiol and progesterone in the striatum fluctuate during the estrous cycle. This is associated with estrous cycle-dependent changes of intramembranous particle density of striatal dendritic membranes. Our data therefore indicate that the striatum is a brain region hormonally modulated under physiological conditions.

Estrogen-like effects of the mammary carcinogen 7,12-dimethylbenz(alpha)anthracene on hypothalamic neuronal membranes

Previous studies have shown that in Sprague-Dawley female rats, but not in Wistar females, the mammary carcinogen dimethylbenz(alpha)anthracene (DMBA) results in extended preovulatory prolactin and estradiol surges, associated with inhibition of preovulatory gonadotropin surges, and in the induction of mammary tumors. Because earlier studies of similar endocrine states have shown this to be linked to hypothalamic arcuate nucleus neuronal membrane organization, in this study freeze-fracture methodology was used to determine whether DMBA may affect the ultrastructure of the neuronal membrane in the arcuate nucleus. The effects of estradiol valerate and DMBA were studied on 55- to 60-day-old cycling females, in Sprague-Dawley and Wistar rats, 8 weeks after the treatment. DMBA alone (15 mg/rat by gastric intubation) resulted in a significant decrease in the numerical density of intramembrane protein particles (IMP) in Sprague-Dawley rats but not in Wistar rats. The SC injection of estradiol valerate (1 mg/rat) resulted in a significant decrease of IMP numbers in both strains of rats. Although the subcutaneous injection of DMBA alone (1 mg/rat) did not affect IMP numerical density in either strain, the same potentiated the effect of estradiol valerate (1 mg/rat) on IMP's in Sprague-Dawley but not in Wistar females. These results indicate that DMBA affects the organization of neuronal plasma membrane in the hypothalamus of Sprague-Dawley rats. Wistar females are insensitive to both the endocrine and neuronal membrane effects of DMBA. Estradiol affected neuronal membranes in both strains and potentiated DMBA's effect. It appears that the estrogen-sensitive mechanism of DMBA activation may be lacking in Wistar rats.

Estradiol promotes cell shape changes and glial fibrillary acidic protein redistribution in hypothalamic astrocytes in vitro: a neuronal-mediated effect

We have previously shown that in hypothalamic mixed neuronal-glial cultures both astrocytic shape and distribution of glial fibrillary acidic protein (GFAP) are modified by estradiol. In the present study, we have investigated whether or not the presence of neurons is necessary for these hormonal effects. In mixed neuronal-glial hypothalamic cultures the proportion of process-bearing GFAP-immunoreactive cells was significantly increased after treatment for 30 min with 10(-12) M 17 beta estradiol. This effect was present for at least 1 day and was reverted by incubating the cells in estradiol-free medium. Estradiol incubation resulted in a progressive differentiation of GFAP-immunoreactive cells from a flattened epithelioid morphology to bipolar, radial, and stellate shapes. This effect was not observed in pure hypothalamic glial cultures. Furthermore, incubation of hypothalamic glial cells with medium conditioned by estradiol-treated mixed hypothalamic cultures did not affect the shape of GFAP-immunoreactive astrocytes. In contrast, addition of hypothalamic neurons, but not cerebellar neurons or fibroblasts, to established hypothalamic glial cultures affected the development of estradiol sensitivity in astrocytes. These results indicate that estradiol induction of shape changes in hypothalamic astrocytes is not only dependent on the presence of hypothalamic neurons, but that physical contact between astrocytes and neurons is necessary for the manifestation of the effect of this hormone.

Loss of sexual dimorphism in rat arcuate nucleus neuronal membranes with reproductive aging

Arcuate neurons of the rat hypothalamus have a sexual dimorphic membrane phenotype: quantitative analysis of freeze-fracture replicas has revealed that a population of intramembrane protein particles (IMP) of small size (less than 10 nm) is enriched in the plasma membrane of perikarya and dendritic shafts of cycling females compared to males, whereas a population of large IMPs (greater than 10 nm) is enriched in the membrane of dendritic shafts of males. This different membrane organization is associated with a sex dimorphic synaptic connectivity. To determine whether sex differences in neuronal membrane are affected by reproductive senescence, IMPs were assessed in freeze-fracture replicas of arcuate neuronal plasma membranes of male and female Sprague-Dawley rats aged 3, 15, and 18 months. Three-month-old cycling females were studied on the morning of estrus. Senescent females were in constant estrus (15 months old) or in constant diestrus (18 months old). Young females had more IMPs with diameters under 10 nm in the inner and outer leaflets of the plasma membrane of the perikarya and dendritic shafts compared to males of the same age. In addition, young males showed an increased number of large (greater than 10 nm) IMPs in the outer membrane leaflet of dendritic shafts. No sex differences were detected in the membrane of dendritic spines. In senescent females the number of small IMPs was decreased in the perikarya and dendritic shafts compared to young females while the number of large particles was increased in the outer leaflet of the membrane of dendritic shafts, reaching values similar to those observed in males. IMP counts were not modified with aging in males and in dendritic spines of females. These results indicate that reproductive aging in female rats is associated with a remodeling of neuronal plasma membranes in arcuate neurons.

The pre- and postnatal influence of hormones and neurotransmitters on sexual differentiation of the mammalian hypothalamus

A number of brain structures and a great number of brain functions have been shown to be sexually dimorphic. It has also been shown that development and differentiation of these structures and functions proceeds during a critical pre- and postnatal period of increased susceptibility, and is controlled by gonadal steroids and neurotransmitter substances. The brain of male and female mammals seems to be still undifferentiated before the period of increased susceptibility to gonadal steroids and neurotransmitters starts. Feminization of brain structure and functions, e.g., establishment of the cyclic LH-surge mechanism and the expression of lordosis behavior, seems to depend on the moderate interaction of estrogens with the developing nervous system. Defeminization and masculinization of brain functions seem to be established during interaction of the developing nervous system with androgens, which have to be converted, at least in part, into estrogens. Structural differentiation of the male brain, e.g., the sexually dimorphic nucleus of the preoptic area (SDN-POA), seems to be exclusively estrogen-dependent, during differentiation of male brain functions, however, estrogens may be supportive, rather than directive, to the primary action of androgens. The molecular mechanisms of sexual differentiation of the brain are not yet fully understood. It seems, however, that the priming action of gonadal steroids during the period of increased susceptibility is either mediated by neurotransmitters, or neurotransmitters modulate the priming action of gonadal steroids. In particular, the adrenergic, the serotoninergic, the cholinergic, and possibly the dopaminergic system were shown to have strong influences on sexual differentiation of brain structure and functions. In contrast to the great number of available studies on the influence of gonadal steroids on sexual differentiation of the brain, there are rather few studies available concerning the influence of neurotransmitter systems. The available results are partly contradictory, so that an interpretation must be done with caution and will leave plenty of room for speculation. Postnatal application of compounds which stimulate or inhibit adrenergic activity mainly affected the neural control of gonadotropin secretion, and had only minor influences on differentiation of behavior patterns. It seems, however, that adrenergic participation in the differentiation of the center for cyclic gonadotropin release is very complex and stimulatory and inhibitory components may operate simultaneously. Activation or inhibition of beta-adrenergic receptors during postnatal development was shown to impair the responsiveness of the center for cyclic gonadotropin release to gonadal steroids, and impairs the expression of ejaculatory behavior in male rats.(ABSTRACT TRUNCATED AT 400 WORDS)

African green monkeys have sexually dimorphic and estrogen-sensitive hypothalamic neuronal membranes

Previous studies have shown sex differences in intramembrane particle content in the arcuate neurons of the rat hypothalamus. In this study, freeze-fracture replicas were prepared from the infundibular hypothalamus of adult African green monkeys (Cercopithecus aethiops) in order to determine whether primates also have sexual dimorphism in neuronal membranes. Intramembrane particles (IMP) were quantitatively assessed in the perikaryal plasma membranes of infundibular neurons. Four groups of monkeys were studied: intact males, intact females, ovariectomized females injected with 20 mg of estradiol valerate over 10 days and ovariectomized females injected with vehicle (castor oil). Membranes from females showed an increased numerical density of IMPs when compared to males. Ovariectomy of females did not affect IMP content, while estrogen administration resulted in a significant decrease in IMP numerical density to reach male values. These findings indicate a sex difference in neuronal membranes in the hypothalamus of monkeys and suggest that as in rodents, neuronal plasma membrane organization in higher primates may be modulated by gonadal steroids.

Sexual differentiation of synaptic connectivity and neuronal plasma membrane in the arcuate nucleus of the rat hypothalamus

Plasma membranes of the hypothalamic arcuate neurons of the rat show a sexually dimorphic phenotype: the numerical density of intramembrane protein particles is greater in females. Male and female Sprague-Dawley rats, 10, 20 and 100 days old, were studied in order to determine whether sexual differentiation of the neuronal plasma membrane in the soma of arcuate neurons is associated with the establishment of sex differences in the pattern of axo-somatic synaptic contacts. Axo-somatic synapses were counted in thin sections of the arcuate nucleus and intramembrane particles were assessed in freeze-fracture replicas of the neuronal membrane. The number of synapses per length of perikaryal membrane increased from day 10 to day 20 in both sexes, reaching by 20 days values similar to those found on day 100. A sex difference in the number of synapses was observed only in 20-day-old and 100-day-old rats: neurons from females showed a greater number of presynaptic inputs than males (P less than 0.05). This sex difference was abolished by administration of testosterone propionate to 5-day-old females. Quantitative evaluation of freeze-fracture replicas of the arcuate neuronal perikarya revealed sex differences in the numerical density of intramembrane particles at all time points studied: neurons from females contained significantly more particles in their plasma membranes than neurons from males or androgenized females of the same age (P less than 0.001). These results indicate that sexual differentiation of the plasma membrane in neuronal somas precedes the establishment of sex differences in axo-somatic synapses. The results are compatible with a possible role of neuronal membranes in the sexual differentiation of synaptic connectivity.

Gap junctions in the hypothalamic arcuate neurons of ovariectomized and estradiol-treated rats

Freeze-fracture methodology was used to study the organization of the neuronal plasma membrane in the rat arcuate nucleus, an estrogen sensitive area of the hypothalamus. Freeze-fracture replicas were prepared from 6 adult ovariectomized rats injected with a single dose of 17 beta-estradiol and from 6 ovariectomized littermates injected with vehicle. Rats were sacrificed 2 days after the injection. Occasional gap junctions were observed in freeze-fractured neuronal membranes from both groups of animals and their incidence was increased (P less than 0.01) in estradiol treated rats. This study demonstrates gap junctions in arcuate neurons and suggests that these structures may be affected by gonadal hormones.

Sex differences in plasma membrane concanavalin A binding in the rat arcuate neurons

Previous studies have shown that synaptic connections and organization of neuronal membranes are sexually dimorphic in the arcuate nucleus of developing and adult rats. These sex differences can be abolished by the perinatal androgenization of females. In this study the label-fracture method of Pinto da Silva and Kan was used in order to determine whether membrane sex differences are related to the glycoconjugates in neuronal plasma membranes. Six Sprague-Dawley female rats treated with testosterone on the day of birth, six control females injected with vehicle and six intact males were studied when they were 100 days old. The arcuate nucleus was dissected and incubated for 2 hours in a solution of 0.25 mg/ml concanavalin A, washed in buffer and incubated for 3 hours in a suspension of horseradish peroxidase-coated colloidal gold. Then, freeze-fracture replicas of the arcuate nucleus were prepared. Colloidal gold labeling was observed to be codistributed with intramembrane particles in the outer membrane face of the neuronal perikaryal plasma membrane. The numerical density of small (less than 10 nm) intramembrane particles and colloidal gold particles was significantly greater in control female membranes when compared to males or to androgenized females. The labeling was significantly reduced when the arcuate nucleus was incubated with concanavalin A in presence of 0.5 M methyl-alpha-manopyranoside. These findings indicate a sex difference in the density and distribution of glycoconjugates and intramembranous particles in the neuronal plasma membrane that is dependent on the perinatal levels of sex steroids and is concordant with, and could be the cause of, sex differences in the pattern of synaptic contacts.

Synaptic remodeling in the rat arcuate nucleus during the estrous cycle

Adult female rats showing regular vaginal cycles were studied in order to determine the number of axosomatic synapses in thin sections of the arcuate nucleus. The number of synapses per length of perikaryal membrane was significantly decreased in estrus, compared to other days of the estrous cycle (P less than 0.05). The reduction in the number of synapses in estrus was accompanied by a decrease in the percentage of the average length of perikaryal membrane covered by presynaptic terminals and by an increase in the percentage of membrane in close apposition of glial processes. Since the average perikaryal perimeter was not significantly changed during the estrous cycle, these results indicate a net decrease in the number of arcuate nucleus axosomatic synapses between proestrus and estrus, with a reinnervation of arcuate neurons between estrus and metestrus. These results suggest that there is a physiological synaptic turnover in the arcuate nucleus of the rat during the estrous cycle.

Sexual differentiation of the neuronal plasma membrane: neonatal levels of sex steroids modulate the number of exo-endocytotic images in the developing rat arcuate neurons

Exo-endocytotic images and intramembrane protein particles (IMP) were quantitatively assessed in freeze-fracture replicas from the plasma membrane of arcuate neurons of rats aged 0 (newborns), 10, 20 and 100 days postpartum. Membranes contained significantly (P less than 0.02) more IMPs in females than in males. Exo-endocytotic images were increased in newborn and 10-day-old males when compared to adult males or to developing females (48 +/- 6 vs 6 +/- 1 images/100 micron 2 in 10-day-old male and female rats, respectively). Androgenization of females with a single injection of testosterone propionate on the day of birth resulted in an increased number of exo-endocytotic images in developing animals (75 +/- 9 images/100 micron 2, 10-day-old rats) and in the abolishment of the sex differences in the number of IMPs.