Estradiol promotion of changes in the morphology of astroglia growing in culture depends on the expression of polysialic acid of neural membranes

Unusual suspects: Glial cells in fertility regulation and their suspected role in polycystic ovary syndrome

Gonadotropin-releasing-hormone (GnRH) neurons sitting within the hypothalamus control the production of gametes and sex steroids by the gonads, therefore ensuring survival of species. As orchestrators of reproductive function, GnRH neurons integrate information from external and internal cues. This occurs through an extensively studied neuronal network known as the "GnRH neuronal network." However, the brain is not simply composed of neurons. Evidence suggests a role for glial cells in controlling GnRH neuron activity, secretion and fertility outcomes, although numerous questions remain. Glial cells have historically been seen as support cells for neurons. This idea has been challenged by the discovery that some neurological diseases originate from glial dysfunction. The prevalence of infertility disorders is increasing worldwide, with one in four couples being affected; therefore, it remains essential to understand the mechanisms by which the brain controls fertility. The "GnRH glial network" could be a major player in infertility disorders and represent a potential therapeutic target. In polycystic ovary syndrome (PCOS), the most common infertility disorder of reproductive aged women worldwide, the brain is considered a prime suspect. Recent studies have demonstrated pathological neuronal wiring of the "GnRH neuronal network" in PCOS-like animal models. However, the role of the "GnRH glial network" remains to be elucidated. In this review, I aim to propose glial cells as unusual suspects in infertility disorders such as PCOS. In the first part, I state our current knowledge about the role of glia in the regulation of GnRH neurons and fertility. In the second part, based on our recent findings, I discuss how glial cells could be implicated in PCOS pathology.

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.

Role of astrocytes in the neuroprotective actions of 17β-estradiol and selective estrogen receptor modulators

Neuroprotective actions of 17β-estradiol (estradiol) are in part mediated by direct actions on neurons. Astrocytes, which play an essential role in the maintenance of the homeostasis of neural tissue, express estrogen receptors and are also involved in the neuroprotective actions of estradiol in the brain. Estradiol controls gliosis and regulates neuroinflammation, edema and glutamate transport acting on astrocytes. In addition, the hormone regulates the release of neurotrophic factors and other neuroprotective molecules by astrocytes. In addition, reactive astrocytes are a local source of neuroprotective estradiol for the injured brain. Since estradiol therapy is not free from peripheral risks, alternatives for the hormone have been explored. Some selective estrogen receptor modulators (SERMs), which are already in use in clinical practice for the treatment of breast cancer, osteoporosis or menopausal symptoms, exert similar actions to estradiol on astrocytes. Therefore, SERMs represent therapeutic alternatives to estradiol for the activation of astroglia-mediated neuroprotective mechanisms.

Endosialidases: Versatile Tools for the Study of Polysialic Acid

Polysialic acid is an α2,8-linked N-acetylneuraminic acid polymer found on the surface of both bacterial and eukaryotic cells. Endosialidases are bacteriophage-borne glycosyl hydrolases that specifically cleave polysialic acid. The crystal structure of an endosialidase reveals a trimeric mushroom-shaped molecule which, in addition to the active site, harbors two additional polysialic acid binding sites. Folding of the protein crucially depends on an intramolecular C-terminal chaperone domain that is proteolytically released in an intramolecular reaction. Based on structural data and previous considerations, an updated catalytic mechanism is discussed. Endosialidases degrade polysialic acid in a processive mode of action, and a model for its mechanism is suggested. The review summarizes the structural and biochemical elucidations of the last decade and the importance of endosialidases in biochemical and medical applications. Active endosialidases are important tools in studies on the biological roles of polysialic acid, such as the pathogenesis of septicemia and meningitis by polysialic acid-encapsulated bacteria, or its role as a modulator of the adhesion and interactions of neural and other cells. Endosialidase mutants that have lost their polysialic acid cleaving activity while retaining their polysialic acid binding capability have been fused to green fluorescent protein to provide an efficient tool for the specific detection of polysialic acid.

[Structural plasticity of the adult central nervous system: insights from the neuroendocrine hypothalamus]

Accumulating evidence renders the dogma obsolete according to which the structural organization of the brain would remain essentially stable in adulthood, changing only in response to a need for compensatory processes during increasing age and degeneration. It has indeed become clear from investigations on various models that the adult nervous system can adapt to physiological demands by altering reversibly its synaptic circuits. This potential for structural and functional modifications results not only from the plastic properties of neurons but also from the inherent capacity of the glial cellular components to undergo remodeling as well. This is currently known for astrocytes, the major glial cells in brain which are well-recognized as dynamic partners in the mechanisms of synaptic transmission, and for the tanycytes and pituicytes which contribute to the regulation of neurosecretory processes in neurohemal regions of the hypothalamus. Studies on the neuroendocrine hypothalamus, whose role is central in homeostatic regulations, have gained good insights into the spectacular neuronal-glial rearrangements that may subserve functional plasticity in the adult brain. Following pioneering works on the morphological reorganizations taking place in the hypothalamo-neurohypophyseal system under certain physiological conditions such as dehydration and lactation, studies on the gonadotropic system that orchestrates reproductive functions have re-emphasized the dynamic interplay between neurons and glia in brain structural plasticity processes. This review summarizes the major contributions provided by these researches in the field and also addresses the question of the morphological rearrangements that occur on a 24-h basis in the central component of the circadian clock responsible for the temporal aspects of endocrine regulations. Taken together, the reviewed data highlight the close cooperation between neurons and glia in developing strategies for functional adaptation of the brain to the changing conditions of the internal and external environment.

Pregnancy and maternal behavior induce changes in glia, glutamate and its metabolism within the cingulate cortex

An upregulation of the astrocytic proteins GFAP and bFGF within area 2 of the cingulate cortex (Cg2) occurs within 3 hours of parturition in rats. These changes are the result of an interaction between hormonal state and maternal experience and are associated with increased dendritic spine density in this area. Here, we examined whether this upregulation of astrocytic proteins generalized to other glial markers and, in particular those associated with glutamate metabolism. We chose glial markers commonly used to reflect different aspects of glial function: vimentin, like GFAP, is a marker of intermediate filaments; glutamine synthetase (GS), and S-100beta, are used as markers for mature astrocytes and GS has also been used as a specific marker for glutamatergic enzymatic activity. In addition, we examined levels of proteins associated with glutamine synthetase, glutamate, glutamine and two excitatory amino acid transporters found in astrocytes, glt-1 and glast. S100beta immunoreactivity did not vary with reproductive state in either Cg2 or MPOA suggesting no change in the number of mature astrocytes across these conditions. Vimentin-ir did not differ across groups in Cg2, but expression of this protein decreased from Day 1 postpartum onwards in the MPOA. By contrast, GS-ir was increased within 24 h postpartum in Cg2 but not MPOA and similarly to GFAP and bFGF this upregulation of GS resulted from an interaction between hormonal state and maternal experience. Within Cg2, upregulation of GS was not accompanied by changes in the astrocytic glutamatergic transporters, glt-1 and glast, however, an increase in both glutamate and glutamine proteins were observed within the Cg2 of postpartum animals. Together, these changes suggest postpartum upregulation of glutamatergic activity and metabolism within Cg2 that is stimulated by pregnancy hormones and maternal experience.

Functional role of the interaction between polysialic acid and extracellular histone H1

Polysialic acid (PSA) is a large and highly negatively charged glycan that plays crucial roles in nervous system development and function in the adult. It has been suggested to facilitate cell migration, neurite outgrowth, and synaptic plasticity because its hydration volume could enhance flexibility of cell interactions. Evidence for receptors of PSA has so far been elusive. We now identified histone H1 as binding partner of PSA via a single-chain variable fragment antibody using an anti-idiotypic approach. Histone H1 directly binds to PSA as shown by ELISA. Surface biotinylation of cultured cerebellar neurons indicated an extracellular localization of histone H1. Immunostaining of live cerebellar neurons and Schwann cells confirmed that an extracellular pool of histone H1 colocalizes with PSA at the cell surface. Histone H1 was also detected in detergent-insoluble synaptosomal membrane subfractions and postsynaptic densities. When applied in vitro, histone H1 stimulated neuritogenesis, process formation and proliferation of Schwann cells, and migration of neural precursor cells via a PSA-dependent mechanism, further indicating that histone H1 is active extracellularly. These in vitro observations suggested an important functional role for the interaction between histone H1 and PSA not only for nervous system development but also for regeneration in the adult. Indeed, histone H1 improved functional recovery, axon regrowth, and precision of reinnervation of the motor branch in adult mice with femoral nerve injury. Our findings encourage investigations on the therapeutic potential of histone H1 in humans.

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

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

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.

Steroids and glial cell function

Hormonal and locally produced steroids act in the nervous system as neuroendocrine regulators, as trophic factors and as neuromodulators and have a major impact on neural development and function. Glial cells play a prominent role in the local production of steroids and in the mediation of steroid effects on neurons and other glial cells. In this review, we examine the role of glia in the synthesis and metabolism of steroids and the functional implications of glial steroidogenesis. We analyze the mechanisms of steroid signaling on glia, including the role of nuclear receptors and the mechanisms of membrane and cytoplasmic signaling mediated by changes in intracellular calcium levels and activation of signaling kinases. Effects of steroids on functional parameters of glia, such as proliferation, myelin formation, metabolism, cytoskeletal reorganization, and gliosis are also reviewed, as well as the implications of steroid actions on glia for the regulation of synaptic function and connectivity, the regulation of neuroendocrine events, and the response of neural tissue to injury.

Astrocytes in 17beta-estradiol treated mixed hippocampal cultures show attenuated calcium response to neuronal activity

Glial cells in the brain are capable of responding to hormonal signals. The ovarian steroid hormone 17beta-estradiol, in addition to its actions on neurons, can directly affect glial cells. Estrogen receptors have been described on both neurons and astrocytes, suggesting a complex interplay between these two in mediating the effects of the hormone. Astrocytes sense and respond to neuronal activity with a rise in intracellular calcium concentration ([Ca(2+)](i)). Using simultaneous electrophysiology and calcium imaging techniques, we monitored neuronal activity evoked astrocyte ([Ca(2+)](i)) changes in mixed hippocampal cultures loaded with fluo-3 AM. Action potential firing in neurons, elicited by injecting depolarizing current pulses, was associated with ([Ca(2+)](i)) elevations in astrocytes, which could be blocked by 200 microM MCPG and also 1 microM TTX. We compared astrocytic ([Ca(2+)](i)) transients in control and 24-hour estradiol treated cultures. The amplitude of the ([Ca(2+)](i)) transient, the number of responsive astrocytes, and the ([Ca(2+)](i)) wave velocity were all significantly reduced in estradiol treated cultures. ([Ca(2+)](i)) rise in astrocytes in response to local application of the metabotropic glutamate receptor (mGluR) agonist t-ACPD was attenuated in estradiol treated cultures, suggesting functional changes in the astrocyte mGluR following 24-h treatment with estradiol. Since astrocytes can modulate synaptic transmission by release of glutamate, the attenuated ([Ca(2+)](i)) response seen following estradiol treatment could have functional consequences on astrocyte-neuron signaling.

Estradiol modulation of astrocytes and the establishment of sex differences in the brain

The role of steroid hormones as a conduit for reciprocal glial-neuronal communication is an emerging but relatively unexplored concept. Research in our laboratory has discovered that the relationship between astrocytic and neuronal morphology during development is distinct for different brain regions and provides a fundamental basis for region-specific sexual differentiation. The functional significance of estradiol-induced differentiation of astrocytes and the cross-talk of these cells with neurons includes permanent changes in synaptic patterning and control of adult reproductive behaviors. The cellular mechanisms as currently understood for each region are discussed and unanswered questions as well as other areas for future research are reviewed.

Neurosteroid regulation of oxytocin and vasopressin release from the rat supraoptic nucleus

In adult rats somato-dendritic release of oxytocin and vasopressin from magnocellular neurones in the supraoptic nucleus of the hypothalamus has important autoregulatory actions on the neuronal electrical activity, and in neonatal rats it plays a role in the development of dendritic arborisation. In the adult, oxytocin effects are modulated by allopregnanolone via an interaction with inhibitory GABAA receptors. This study examined the effects of allopregnanolone, progesterone and 17beta-oestradiol on oxytocin and vasopressin release from intact isolated supraoptic nuclei and from the neurophypophyses in rats of differing ages. In supraoptic nuclei from rats of 3-4 weeks old or less, all three neurosteroids induced oxytocin release from the isolated supraoptic nucleus, but only allopregnanolone induced significant release of vasopressin. Surprisingly, in these very young rats, allopregnanolone-induced oxytocin release was inhibited by GABAA receptor antagonists as well as by an oxytocin receptor antagonist. By contrast, in supraoptic nuclei from adult rats allopregnanolone-induced oxytocin release was much smaller, and was enhanced in the presence of bicuculline. The GABAA receptor agonist muscimol also induced oxytocin release from supraoptic nuclei in young rats, but had no effect in adult rats. Oxytocin cells isolated from young rats showed an increase in [Ca2+]i in response to both allopregnanolone and muscimol. Allopregnanolone had no effect on [Ca2+]i or on the release of oxytocin or vasopressin from neurohypophysial axon terminals in either young or old rats. We conclude that, in very young rats, (i) neurosteroids induce oxytocin release from the supraoptic nucleus by a mechanism that partly depends on the presence of GABA, which in young rats is depolarising to oxytocin cells, and which also partly depends upon endogenous oxytocin, and (ii) the effect of allopregnanolone upon oxytocin release changes with age, as the functional activity of GABAA receptors changes from excitation to inhibition of oxytocin cells.

Ontogeny of sexually dimorphic astrocytes in the neonatal rat arcuate

The arcuate nucleus of the hypothalamus is one of several sexually dimorphic hypothalamic nuclei. We have previously demonstrated that astrocytes in the neonatal arcuate nucleus exhibit a marked sexually dimorphic morphology as a result of differential exposure to gonadal steroids by postnatal day (PN) 2, with males having complex stellate cells compared to the simple bipolar ones found in females. Here, we present data demonstrating that arcuate astrocytes are sexually dimorphic by the day of birth and continue as such throughout postnatal development (PN0-PN15), and persist into adulthood. These data suggest that early gonadal steroid exposure permanently organizes arcuate astrocyte morphology. The male versus female difference in astrocyte morphology may contribute to the sexually dimorphic regulation of neuroendrocrine secretions from the pituitary in the adult.

Transneuronal induction of the highly sialylated isoform of the neural cell adhesion molecule following nerve injury

The polysialylated, embryonic form of the neuronal cell adhesion molecule (PSA-NCAM) is known to participate in a whole series of synaptic rearrangements even in adult animals. The possible role of this molecule in neuroplastic changes of the adult rat somatosensory cortex induced by unilateral transection of the infraorbital branch of the trigeminal nerve was studied with PSA-NCAM immunostaining at light microscopic level. Two- and three-month-old CFY albino rats were sacrificied on days 1, 4, 6, 14 and 21 following operation and PSA-NCAM immunoreaction was examined at three levels of the vibrissa-cortex neuraxis, namely, in the principal nucleus of the trigeminal nerve, in the ventral posteromedial nucleus of the thalamus and in the somatosensory cortex. The lower levels of the neuraxis remained free of PSA-NCAM labeling, similarly to control, intact animals. However, a large number of scattered small neurons became PSA-NCAM immunoreactive in layers IV-VI on both ipsi- and contralateral sides of the somatosensory cortex from day 6 onwards, suggesting a possible transynaptic regulation of NCAM sialylation state.

Steroid modulation of astrocytes in the neonatal brain: implications for adult reproductive function

There is a growing appreciation for the importance of astrocytes, a type of nonneuronal glial cell, to overall brain functioning. The ability of astrocytes to respond to gonadal steroid hormones with changes in morphology has been well documented in the adult brain. It is also apparent that astrocytes of the developing brain are permanently differentiated by the neonatal hormonal milieu, in particular by estradiol, resulting in sexually dimorphic cell morphology, synaptic patterning, and density in males and females. The mechanisms of hormonally mediated astrocyte differentiation are likely to be region specific. In the arcuate nucleus of the hypothalamus, neuron-to-astrocyte signaling appears to play a critical role in estradiol-induced astrocyte differentiation during the first few days of life. Gamma aminobutyric acid (GABA) is an amino acid neurotransmitter that is synthesized and released exclusively by neurons. The levels of GABA are increased in the arcuate nucleus of neonatal males versus females. Preventing the increase in males or mimicking GABA action in females modulates astrocytes accordingly. Speculation about and evidence in support of the functional significance of this dimorphism to adult reproductive functioning is the topic of this review.

Glial expression of estrogen and androgen receptors after rat brain injury

Estrogens and androgens can protect neurons from death caused by injury to the central nervous system. Astrocytes and microglia are major players in events triggered by neural lesions. To determine whether glia are direct targets of estrogens or androgens after neural insults, steroid receptor expression in glial cells was assessed in two different lesion models. An excitotoxic injury to the hippocampus or a stab wound to the parietal cortex and hippocampus was performed in male rats, and the resultant expression of steroid receptors in glial cells was assessed using double-label immunohistochemistry. Both lesions induced the expression of estrogen receptors (ERs) and androgen receptors (ARs) in glial cells. ERalpha was expressed in astrocytes immunoreactive (ERalpha-ir) for glial fibrillary acidic protein or vimentin. AR immunoreactivity colocalized with microglial markers, such as Griffonia simplicifolia lectin-1 or OX-6. The time course of ER and AR expression in glia was studied in the stab wound model. ERalpha-ir astrocytes and AR-ir microglia were observed 3 days after lesion. The number of ERalpha-ir and AR-ir glial cells reached a maximum 7 days after lesion and returned to low levels by 28 days postinjury. The studies of ERbeta expression in glia were inconclusive; different results were obtained with different antibodies. In sum, these results suggest that reactive astrocytes and reactive microglia are a direct target for estrogens and androgens, respectively.

Regional, laminar, and cellular distribution of immunoreactivity for ER alpha and ER beta in the cerebral cortex of hormonally intact, adult male and female rats

Behavioral, biochemical and anatomical studies suggest that estrogen stimulates structure and/or function in the adult cerebral cortex. The studies presented here used immunocytochemistry to map the alpha and beta isoforms of intracellular estrogen receptors (ER alpha, ER beta) in major subdivisions of adult rat cortex to identify potential sites for relevant receptor-mediated hormone actions. These studies revealed that immunoreactivity for ER alpha (ER alpha-IR) and ER beta (ER beta-IR) was present in most cortical areas, was associated exclusively with neurons, and was similar in males and females. Each receptor isoform also had its own unique distribution with respect to cortical regions, layers, and cells. In sensorimotor areas, for example, ER beta-IR was more prominent than ER alpha-IR, and was concentrated in layer V neurons that were immunoreactive for parvalbumin. In contrast, ER alpha-IR was scattered among parvalbumin-immunonegative cells in layers II/III and V/VI. Likewise, in entorhinal cortex, ER beta-IR was present in calbindin-containing cells in layers III-VI, while ER alpha-IR was restricted to small numbers of calbindin-negative neurons in infragranular layers. In sum, ER beta-IR and ER alpha-IR were differentially distributed both with respect to cortical compartments and with respect to each other. Accordingly, estrogen activation at these two sites may be anticipated to impact disparate sets of cortical circuits, cells, and functions.

Gene regulation in the magnocellular hypothalamo-neurohypophysial system

The hypothalamo-neurohypophysial system (HNS) is the major peptidergic neurosecretory system through which the brain controls peripheral physiology. The hormones vasopressin and oxytocin released from the HNS at the neurohypophysis serve homeostatic functions of water balance and reproduction. From a physiological viewpoint, the core question on the HNS has always been, "How is the rate of hormone production controlled?" Despite a clear description of the physiology, anatomy, cell biology, and biochemistry of the HNS gained over the last 100 years, this question has remained largely unanswered. However, recently, significant progress has been made through studies of gene identity and gene expression in the magnocellular neurons (MCNs) that constitute the HNS. These are keys to mechanisms and events that exist in the HNS. This review is an inventory of what we know about genes expressed in the HNS, about the regulation of their expression in response to physiological stimuli, and about their function. Genes relevant to the central question include receptors and signal transduction components that receive and process the message that the organism is in demand of a neurohypophysial hormone. The key players in gene regulatory events, the transcription factors, deserve special attention. They do not only control rates of hormone production at the level of the gene, but also determine the molecular make-up of the cell essential for appropriate development and physiological functioning. Finally, the HNS neurons are equipped with a machinery to produce and secrete hormones in a regulated manner. With the availability of several gene transfer approaches applicable to the HNS, it is anticipated that new insights will be obtained on how the HNS is able to respond to the physiological demands for its hormones.

The highly sialylated isoform of the neural cell adhesion molecule is required for estradiol-induced morphological synaptic plasticity in the adult arcuate nucleus

The large quantities of polysialic acid (PSA) characterizing highly sialylated isoform of the neural cell adhesion molecule (PSA-NCAM), greatly reduce cell adhesion and render this particular cell surface adhesion molecule a likely candidate to intervene in dynamic neuronal phenomena, such as synaptic plasticity. The hypothalamic arcuate nucleus expresses high levels of PSA-NCAM and maintains a high capacity for neuroplastic changes in the adult. Thus, in the arcuate nucleus of female rats, varying circulating levels of estrogen give rise to a reversible reduction in the number of axo-somatic GABA synapses, together with a changing ensheathing of neuronal somata by astrocytes. To examine the role of PSA in such changes, we perturbed its expression, either by blockade with antibodies raised against this carbohydrate moiety (delivered intracerebroventricularly), or by its enzymatic cleavage after microinjection of endoneuraminidase N over the arcuate nucleus. Either procedure was performed in ovariectomized adult rats that received concurrent treatment with 17 beta-estradiol. Morphological synaptic plasticity was analysed using the unbiased disector method to assess synaptic densities in ultrathin sections of the arcuate nucleus immunogold-labelled for GABA. As expected, 17 beta-estradiol induced a significant reduction in the number of GABAergic axo-somatic synapses, a reduction which did not occur after infusion of anti-PSA antibodies or in vivo enzymatic removal of PSA from NCAM. Taken together, our results provide strong evidence that the presence of large quantities of the PSA moiety on NCAM is a necessary prerequisite for estrogen-induced phasic remodelling of synapses in the adult female arcuate nucleus.

Sex steroids and the brain: lessons from animal studies

Gonadal steroid hormones have multiple effects throughout development on steroid responsive tissues in the brain. The belief that the cellular morphology of the adult brain cannot be modulated or that the synaptic connectivity is "hard-wired" is being rapidly refuted by abundant and growing evidence. Indeed, the brain is capable of undergoing many morphological changes throughout life and gonadal steroids play an important role in many of these processes. Gonadal steroids are implicated in the development of sexually dimorphic structures in the brain, in the control of physiological behaviors and functions and the brain's response to physiological or harmful substances. The effect of sex steroids on neuroprotection and neuroregeneration is an important and expanding area of investigation. Astroglia are targets for estrogen and testosterone and are apparently involved in the actions of sex steroids on the central nervous system. Sex hormones induce changes in the expression of glial fibrillary acidic protein, the growth of astrocytic processes and the extent to which neuronal membranes are covered by astroglial processes. These changes are linked to modifications in the number of synaptic inputs to neurons and suggest that astrocytes may participate in the genesis of gonadal steroid-induced sex differences in synaptic connectivity and synaptic plasticity in the adult brain. Astrocytes and tanycytes may also participate in the cellular effects of sex steroids by releasing neuroactive substances and by regulating the local accumulation of specific growth factors, such as insulin-like growth factor-I, that are involved in estrogen-induced synaptic plasticity and estrogen-mediated neuroendocrine control. Astroglia may also be involved in the regenerative and neuroprotective effects of sex steroids since astroglial activation after brain injury or after peripheral nerve axotomy is regulated by sex hormones.

Potassium signalling in the brain: its role in behaviour

This paper examines evidence that glial cells respond to changes in extracellular potassium ([K+]e) in ways that contribute to modulation of neuronal activity and thereby behaviour. Glial cells spatially (and probably directionally) redistribute potassium from regions of increasing concentration to those with a lesser concentration. This redistribution is largely responsible for slow potential shifts associated with behavioural responses of animals. These slow shifts are related in amplitude to the level of 'arousal' of an animal, and its motivational state. In addition, glia, especially astrocytes, respond to changes in [K+]e, the presence of transmitters like nor-adrenaline and glutamate and at least some hormones with changes in their metabolism and/or the morphological characteristics of the cell. The ionic, metabolic and morphological responses of glia to changes in extracellular potassium after neuronal activity have been associated with at least some forms of learning, including habituation, one trial passive avoidance learning and changes associated with enriched environments. The implication of these effects of potassium signalling in the brain is that there is considerable involvement of glia in a number of processes crucial to neuronal activity. Glia may also form another route for information distribution in the brain that is at least bi-directional, though less specific than its neuronal counterparts. It is evident that the Neuroscience of the future will have to incorporate much more study of neuron-glial interactions than hitherto.

Consequences of neural cell adhesion molecule deficiency on cell migration in the rostral migratory stream of the mouse

In vertebrates, interneurons of the olfactory bulb (OB) are generated postnatally and throughout life at the subventricular zone of the forebrain. The neuronal precursors migrate tangentially through the forebrain using a well defined pathway, the rostral migratory stream (RMS), and a particular mode of migration in a chain-like organization. A severe size reduction of the OB represents the most striking morphological phenotype in neural cell adhesion molecule (NCAM)-deficient mice. This defect has been traced back to a migration deficit of the precursors in the RMS and linked to the lack of the polysialylated form of NCAM. In this study we investigate the morphological alterations and functional properties of the RMS in mice totally devoid of all isoforms of NCAM and polysialic acid (PSA). We show that a morphologically altered, but defined and continuous pathway exists in mutants, and we present in vivo and in vitro evidence that PSA-NCAM in the RMS is not essential for the formation and migration of chains. Instead, we find a massive gliosis associated with the formation of membrane specializations in a heterotypic manner, linking precursors to astrocytes. This finding and the over-representation and defasciculation of axons in the pathway suggest that important interactions between migrating cells and their stationary environment are perturbed in the mutants. Finally, we used transplantation experiments to demonstrate that lack of PSA-NCAM leads to a decrease but not a total blockade of migration and demonstrate that the mutant RMS is functional in transporting normal neuronal precursors to the OB.

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.

Expression of type 2 iodothyronine deiodinase in hypothyroid rat brain indicates an important role of thyroid hormone in the development of specific primary sensory systems

Thyroid hormone is an important epigenetic factor in brain development, acting by modulating rates of gene expression. The active form of thyroid hormone, 3,5,3'-triiodothyronine (T3) is produced in part by the thyroid gland but also after 5'-deiodination of thyroxine (T4) in target tissues. In brain, approximately 80% of T3 is formed locally from T4 through the activity of the 5'-deiodinase type 2 (D2), an enzyme that is expressed mostly by glial cells, tanycytes in the third ventricle, and astrocytes throughout the brain. D2 activity is an important point of control of thyroid hormone action because it increases in situations of low T4, thus preserving brain T3 concentrations. In this work, we have studied the expression of D2 by quantitative in situ hybridization in hypothyroid animals during postnatal development. Our hypothesis was that those regions that are most dependent on thyroid hormone should present selective increases of D2 as a protection against hypothyroidism. D2 mRNA concentration was increased severalfold over normal levels in relay nuclei and cortical targets of the primary somatosensory and auditory pathways. The results suggest that these pathways are specifically protected against thyroid failure and that T3 has a role in the development of these structures. At the cellular level, expression was observed mainly in glial cells, although some interneurons of the cerebral cortex were also labeled. Therefore, the T3 target cells, mostly neurons, are dependent on local astrocytes for T3 supply.

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

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

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

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

Cell biology of polysialic acid

The unusual carbohydrate polysialic acid (PSA), attached uniquely to neural cell adhesion molecule (NCAM) through a developmentally regulated process, modulates neural cell interactions. Major advances in the past two years have increased our understanding of PSA biosynthesis and regulation. Of particular interest is the cloning of the genes encoding polysialyltransferases (PSTs) and the finding that a single enzyme is able to confer polysialylation to NCAM. The electrical activity of neurons and transmembrane signalling are probably major players in controlling both PSA biosynthesis and its expression at the cell surface. A direct causal relationship between PSA expression and activity-induced synaptic plasticity has been reported.

Interaction of insulin-like growth factor-I and estradiol signaling pathways on hypothalamic neuronal differentiation

Neurotrophic effects of estradiol and insulin-like growth factor-I were assessed in primary cultures from fetal rat hypothalamus. Cultured neurons were immunostained with an antibody for the microtubule-associated protein-2. While both estradiol and insulin-like growth factor-I increased the number of microtubule-associated protein-2-immunoreactive neurons and the extension of immunoreactive processes, the effect of these two factors was not additive. The estradiol-induced increases in neuronal numbers and extension of neuronal processes were blocked by either the estrogen receptor antagonist ICI 182,780 or by an anti-sense oligonucleotide to the estrogen receptor. Furthermore, incubation of the cultures with an anti-sense oligonucleotide directed against the insulin-like growth factor-I messenger RNA also blocked the effect of estradiol. In turn, the effects of insulin-like growth factor-I were blocked by the estrogen receptor antagonist ICI 182,780 and by the anti-sense oligonucleotide to the estrogen receptor. These findings suggest that estradiol-induced activation of the estrogen receptor in developing hypothalamic cells requires the presence of insulin-like growth factor-I, and that both estradiol and insulin-like growth factor-I use the estrogen receptor as a mediator of their trophic effects on hypothalamic neurons.

Potential neuronal mechanisms of estrogen actions in synaptogenesis and synaptic plasticity

1. Studies conducted on the rat arcuate nucleus, an area involved in the development and control of LH and FSH secretion, have shown the existence of hormonally regulated developmental sex differences in synaptic patterns and estrogen-induced synaptic plasticity during adult life. Several questions raised by these findings are examined in this review: 2. The mechanisms of estrogen-regulated developmental synaptogenesis. These include the role of glycocalyx glycoproteins in neuronal membranes, neural cell adhesion molecules, and insulin-like growth factor I. 3. The relationship among circulating estrogen, gonadotropin levels, and hypothalamic synaptic plasticity. Recent evidence for the role of GABAergic and dopaminergic synaptic inputs and POMC projections from the arcuate nucleus to the GnRH cells is discussed. 4. The synaptologic basis of age-related failure of positive feedback. The role of the cumulative effect of repeated preovulatory synaptic retraction and reapplication cycles on sensescent constant estrus is analyzed.

Gonadal steroids and astroglial plasticity

1. Recent evidence indicates that astroglia participate in the metabolism of gonadal hormones, in the synthesis of neurosteroids, and in the plastic responses of neurons to gonadal steroids. The role of astroglia on plastic responses of neural tissue to gonadal hormones and neurosteroids is examined in this review. 2. Gonadal steroids and neurosteroids promote astroglia plasticity in several areas of the central nervous system, including the hypothalamus, the striatum, and the hippocampus. 3. Gonadal steroids and neurosteroids modulate astroglia proliferation and the formation of reactive astroglia after brain injury. 4. Astroglia is a source of trophic factors that may mediate effects of gonadal steroids on neural tissue. 5. Astroglia is involved in the promotion of synaptic plastic changes by gonadal hormones. 6. The effect of gonadal hormones on astroglial plasticity is dependent on specific membrane interactions with neurons and on the expression of the embryonic highly polysialylated isoform of the neural cell adhesion molecule on neuronal membranes. 7. In conclusion, coordinated responses of neurons and astroglia appear to be involved in the modulation of neural function and response to injury by gonadal hormones and neurosteroids.