DARPP-32, a dopamine- and cyclic AMP-regulated phosphoprotein in tanycytes of the mediobasal hypothalamus: distribution and relation to dopamine and luteinizing hormone-releasing hormone neurons and other glial elements

GFAP-expressing cells in the adult hypothalamus can generate multiple neural cell lineages in vitro

Adult neural stem cells (NSCs) located in the two canonical neurogenic niches, the subventricular zone (SVZ) and the subgranular zone (SGZ), express the glial fibrillary acidic protein (GFAP). Recently, proliferative activity has been described in the hypothalamus although the characterization of hypothalamic neural stem/progenitor cells (NSPCs) is still uncertain. We therefore investigated whether hypothalamic GFAP-positive cells, as in the SVZ and SGZ, also have neurogenic potential. We used a transgenic mouse line expressing green fluorescent protein (GFP) under the control of the GFAP promoter. GFAP-GFP expressing cells are localized in the ependymal layer as well as in the parenchyma of the mediobasal hypothalamus (MBH) and express Sox2, a marker for NSCs. Interestingly, no sexual dimorphism was observed in the numbers of GFP + and GFP-Sox2 + cells. After cells sorting, these cells were able to generate neurospheres in vitro and give rise to neurons, astrocytes and oligodendrocytes. Taken together, these results show that hypothalamic GFAP-expressing cells form a population of NSPCs.

The polygamous GnRH neuron: Astrocytic and tanycytic communication with a neuroendocrine neuronal population

To ensure the survival of the species, hypothalamic neuroendocrine circuits controlling fertility, which converge onto neurons producing gonadotropin-releasing hormone (GnRH), must respond to fluctuating physiological conditions by undergoing rapid and reversible structural and functional changes. However, GnRH neurons do not act alone, but through reciprocal interactions with multiple hypothalamic cell populations, including several glial and endothelial cell types. For instance, it has long been known that in the hypothalamic median eminence, where GnRH axons terminate and release their neurohormone into the pituitary portal blood circulation, morphological plasticity displayed by distal processes of tanycytes modifies their relationship with adjacent neurons as well as the spatial properties of the neurohemal junction. These alterations not only regulate the capacity of GnRH neurons to release their neurohormone, but also the activation of discrete non-neuronal pathways that mediate feedback by peripheral hormones onto the hypothalamus. Additionally, a recent breakthrough has demonstrated that GnRH neurons themselves orchestrate the establishment of their neuroendocrine circuitry during postnatal development by recruiting an entourage of newborn astrocytes that escort them into adulthood and, via signalling through gliotransmitters such as prostaglandin E2, modulate their activity and GnRH release. Intriguingly, several environmental and behavioural toxins perturb these neuron-glia interactions and consequently, reproductive maturation and fertility. Deciphering the communication between GnRH neurons and other neural cell types constituting hypothalamic neuroendocrine circuits is thus critical both to understanding physiological processes such as puberty, oestrous cyclicity and aging, and to developing novel therapeutic strategies for dysfunctions of these processes, including the effects of endocrine disruptors.

The Dopamine D4 Receptor Regulates Gonadotropin-Releasing Hormone Neuron Excitability in Male Mice

Gonadotropin-releasing hormone (GnRH)-secreting neurons control fertility. The release of GnRH peptide regulates the synthesis and release of both luteinizing hormone (LH) and Follicle stimulation hormone (FSH) from the anterior pituitary. While it is known that dopamine regulates GnRH neurons, the specific dopamine receptor subtype(s) involved remain unclear. Previous studies in adult rodents have reported juxtaposition of fibers containing tyrosine hydroxylase (TH), a marker of catecholaminergic cells, onto GnRH neurons and that exogenous dopamine inhibits GnRH neurons postsynaptically through dopamine D1-like and/or D2-like receptors. Our microarray data from GnRH neurons revealed a high level of Drd4 transcripts [i.e., dopamine D4 receptor (D4R)]. Single-cell RT-PCR and immunocytochemistry confirmed GnRH cells express the Drd4 transcript and protein, respectively. Calcium imaging identified changes in GnRH neuronal activity during application of subtype-specific dopamine receptor agonists and antagonists when GABAergic and glutamatergic transmission was blocked. Dopamine, dopamine with D1/5R-specific or D2/3R-specific antagonists or D4R-specific agonists decreased the frequency of calcium oscillations. In contrast, D1/5R-specific agonists increased the frequency of calcium oscillations. The D4R-mediated inhibition was dependent on Gαi/o protein coupling, while the D1/5R-mediated excitation required Gαs protein coupling. Together, these results indicate that D4R plays an important role in the dopaminergic inhibition of GnRH neurons.

Neurogenesis in the adult hypothalamus: A distinct form of structural plasticity involved in metabolic and circadian regulation, with potential relevance for human pathophysiology

The adult brain harbors specific niches where stem cells undergo substantial plasticity and, in some regions, generate new neurons throughout life. This phenomenon is well known in the subventricular zone of the lateral ventricles and the subgranular zone of the hippocampus and has recently also been described in the hypothalamus of several rodent and primate species. After a brief overview of preclinical studies illustrating the pathophysiologic significance of hypothalamic neurogenesis in the control of energy metabolism, reproduction, thermoregulation, sleep, and aging, we review current literature on the neurogenic niche of the human hypothalamus. A comparison of the organization of the niche between humans and rodents highlights some common features, but also substantial differences, e.g., in the distribution and extent of the hypothalamic neural stem cells. Exploring the full dynamics of hypothalamic neurogenesis in humans raises a formidable challenge however, given among others, inherent technical limitations. We close with discussing possible functional role(s) of the human hypothalamic niche, and how gaining more insights into this form of plasticity could be relevant for a better understanding of pathologies associated with disturbed hypothalamic function.

Tanycyte-Like Cells Derived From Mouse Embryonic Stem Culture Show Hypothalamic Neural Stem/Progenitor Cell Functions

Tanycytes have recently been accepted as neural stem/progenitor cells in the postnatal hypothalamus. Persistent retina and anterior neural fold homeobox (Rax) expression is characteristic of tanycytes in contrast to its transient expression of whole hypothalamic precursors. In this study, we found that Rax+ residual cells in the maturation phase of hypothalamic differentiation in mouse embryonic stem cell (mESC) cultures had similar characteristics to ventral tanycytes. They expressed typical neural stem/progenitor cell markers, including Sox2, vimentin, and nestin, and differentiated into mature neurons and glial cells. Quantitative RT-PCR analysis showed that Rax+ residual cells expressed Fgf-10, Fgf-18, and Lhx2, which are expressed by ventral tanycytes. They highly expressed tanycyte-specific genes Dio2 and Gpr50 compared with Rax+ early hypothalamic progenitor cells. Therefore, Rax+ residual cells in the maturation phase of hypothalamic differentiation were considered to be more differentiated and similar to late progenitor cells and tanycytes. They self-renewed and formed neurospheres when cultured with exogenous FGF-2. Additionally, these Rax+ neurospheres differentiated into three neuronal lineages (neurons, astrocytes, and oligodendrocytes), including neuropeptide Y+ neuron, that are reported to be differentiated from ventral tanycytes toward the arcuate nuclei. Thus, Rax+ residual cells were multipotent neural stem/progenitor cells. Rax+ neurospheres were stably passaged and retained high Sox2 expression even after multiple passages. These results suggest the successful induction of Rax+ tanycyte-like cells from mESCs [induced tanycyte-like (iTan) cells]. These hypothalamic neural stem/progenitor cells may have potential in regenerative medicine and as a research tool.

Tanycytes and the Control of Thyrotropin-Releasing Hormone Flux Into Portal Capillaries

Central and peripheral mechanisms that modulate energy intake, partition and expenditure determine energy homeostasis. Thyroid hormones (TH) regulate energy expenditure through the control of basal metabolic rate and thermogenesis; they also modulate food intake. TH concentrations are regulated by the hypothalamus-pituitary-thyroid (HPT) axis, and by transport and metabolism in blood and target tissues. In mammals, hypophysiotropic thyrotropin-releasing hormone (TRH) neurons of the paraventricular nucleus of the hypothalamus integrate energy-related information. They project to the external zone of the median eminence (ME), a brain circumventricular organ rich in neuron terminal varicosities and buttons, tanycytes, other glial cells and capillaries. These capillary vessels form a portal system that links the base of the hypothalamus with the anterior pituitary. Tanycytes of the medio-basal hypothalamus express a repertoire of proteins involved in transport, sensing, and metabolism of TH; among them is type 2 deiodinase, a source of 3,3',5-triiodo-L-thyronine necessary for negative feedback on TRH neurons. Tanycytes subtypes are distinguished by position and phenotype. The end-feet of β2-tanycytes intermingle with TRH varicosities and terminals in the external layer of the ME and terminate close to the ME capillaries. Besides type 2 deiodinase, β2-tanycytes express the TRH-degrading ectoenzyme (TRH-DE); this enzyme likely controls the amount of TRH entering portal vessels. TRH-DE is rapidly upregulated by TH, contributing to TH negative feedback on HPT axis. Alterations in energy balance also regulate the expression and activity of TRH-DE in the ME, making β2-tanycytes a hub for energy-related regulation of HPT axis activity. β2-tanycytes also express TRH-R1, which mediates positive effects of TRH on TRH-DE activity and the size of β2-tanycyte end-feet contacts with the basal lamina adjacent to ME capillaries. These end-feet associations with ME capillaries, and TRH-DE activity, appear to coordinately control HPT axis activity. Thus, down-stream of neuronal control of TRH release by action potentials arrival in the external layer of the median eminence, imbricated intercellular processes may coordinate the flux of TRH into the portal capillaries. In conclusion, β2-tanycytes appear as a critical cellular element for the somatic and post-secretory control of TRH flux into portal vessels, and HPT axis regulation in mammals.

Prss56 expression in the rodent hypothalamus: Inverse correlation with pro-opiomelanocortin suggests oscillatory gene expression in adult rat tanycytes

We recently reported that the number of hypothalamic tanycytes expressing pro-opiomelanocortin (Pomc) is highly variable among brains of adult rats. While its cause and significance remain unknown, identifying other variably expressed genes in tanycytes may help understand this curious phenomenon. In this in situ hybridization study, we report that the Prss56 gene, which encodes a trypsin-like serine protease and is expressed in neural stem/progenitor cells, shows a similarly variable mRNA expression in tanycytes of adult rats and correlates inversely with tanycyte Pomc mRNA. Prss56 was expressed in α1, β1, subsets of α2, and some median eminence γ tanycytes, but virtually absent from β2 tanycytes. Prss56 was also expressed in vimentin positive tanycyte-like cells in the parenchyma of the ventromedial and arcuate nuclei, and in thyrotropin beta subunit-expressing cells of the pars tuberalis of the pituitary. In contrast to adults, Prss56 expression was uniformly high in tanycytes in adolescent rats. In mice, Prss56-expressing tanycytes and parenchymal cells were also observed but fewer in number and without significant variations. The results identify Prss56 as a second gene that is expressed variably in tanycytes of adult rats. We propose that the variable, inversely correlating expression of Prss56 and Pomc reflect periodically oscillating gene expression in tanycytes rather than stable expression levels that vary between individual rats. A possible functional link between Prss56 and POMC, and Prss56 as a potential marker for migrating tanycytes are discussed.

The Versatile Tanycyte: A Hypothalamic Integrator of Reproduction and Energy Metabolism

The fertility and survival of an individual rely on the ability of the periphery to promptly, effectively, and reproducibly communicate with brain neural networks that control reproduction, food intake, and energy homeostasis. Tanycytes, a specialized glial cell type lining the wall of the third ventricle in the median eminence of the hypothalamus, appear to act as the linchpin of these processes by dynamically controlling the secretion of neuropeptides into the portal vasculature by hypothalamic neurons and regulating blood-brain and blood-cerebrospinal fluid exchanges, both processes that depend on the ability of these cells to adapt their morphology to the physiological state of the individual. In addition to their barrier properties, tanycytes possess the ability to sense blood glucose levels, and play a fundamental and active role in shuttling circulating metabolic signals to hypothalamic neurons that control food intake. Moreover, accumulating data suggest that, in keeping with their putative descent from radial glial cells, tanycytes are endowed with neural stem cell properties and may respond to dietary or reproductive cues by modulating hypothalamic neurogenesis. Tanycytes could thus constitute the missing link in the loop connecting behavior, hormonal changes, signal transduction, central neuronal activation and, finally, behavior again. In this article, we will examine these recent advances in the understanding of tanycytic plasticity and function in the hypothalamus and the underlying molecular mechanisms. We will also discuss the putative involvement and therapeutic potential of hypothalamic tanycytes in metabolic and fertility disorders.

The special relationship: glia-neuron interactions in the neuroendocrine hypothalamus

Natural fluctuations in physiological conditions require adaptive responses involving rapid and reversible structural and functional changes in the hypothalamic neuroendocrine circuits that control homeostasis. Here, we discuss the data that implicate hypothalamic glia in the control of hypothalamic neuroendocrine circuits, specifically neuron-glia interactions in the regulation of neurosecretion as well as neuronal excitability. Mechanistically, the morphological plasticity displayed by distal processes of astrocytes, pituicytes and tanycytes modifies the geometry and diffusion properties of the extracellular space. These changes alter the relationship between glial cells of the hypothalamus and adjacent neuronal elements, especially at specialized intersections such as synapses and neurohaemal junctions. The structural alterations in turn lead to functional plasticity that alters the release and spread of neurotransmitters, neuromodulators and gliotransmitters, as well as the activity of discrete glial signalling pathways that mediate feedback by peripheral signals to the hypothalamus. An understanding of the contributions of these and other non-neuronal cell types to hypothalamic neuroendocrine function is thus critical both to understand physiological processes such as puberty, the maintenance of bodily homeostasis and ageing and to develop novel therapeutic strategies for dysfunctions of these processes, such as infertility and metabolic disorders.

Functional interleukin-6 receptor-α is located in tanycytes at the base of the third ventricle

Interleukin (IL)-6^- /^- mice develop mature onset obesity, whereas i.c.v. injection of IL-6 decreases obesity in rodents. Moreover, levels of IL-6 in cerebrospinal fluid (CSF) were reported to be inversely correlated with obesity in humans. Tanycytes lining the base of the third ventricle (3V) in the hypothalamus have recently been reported to be of importance for metabolism. In the present study, we investigated whether tanycytes could respond to IL-6 in the CSF. With immunohistochemistry using a well characterised antibody directed against the ligand binding receptor for IL-6, IL-6 receptor α (IL-6Rα), it was found that tanycytes, identified by the two markers, vimentin and dopamine- and cAMP-regulated phosphoprotein of 32 kDa, contained IL-6Rα. There were fewer IL-6Rα on another type of ventricle-lining cells, ependymal cells, as identified by the marker glucose transporter-1. To demonstrate that the immunoreactive IL-6Rα were responsive to IL-6, we injected IL-6 i.c.v. This treatment increased immunoreactive phosphorylated signal transducer and activator of transcription-3 (pSTAT3) in tanycytes after 5 minutes and in cells in the medial part of the arcuate nucleus after 5 and 15 minutes. Intracerebroventricular injection of leptin exerted similar effects. As expected, i.p. injection of leptin also induced pSTAT3 staining in the hypothalamus, whereas i.p. IL-6 injection had little effect on this parameter. Intracerebroventricular or i.p. injection of vehicle only had no effect on pSTAT3-immunoreactivity. In summary, there are functional IL-6Rα on tanycytes at the bottom of the 3V, in agreement with the possibility that ventricular administration of IL-6 decreases obesity in mice via an effect on this cell type.

Glial Endozepines Inhibit Feeding-Related Autonomic Functions by Acting at the Brainstem Level

Endozepines are endogenous ligands for the benzodiazepine receptors and also target a still unidentified GPCR. The endozepine octadecaneuropeptide (ODN), an endoproteolytic processing product of the diazepam-binding inhibitor (DBI) was recently shown to be involved in food intake control as an anorexigenic factor through ODN-GPCR signaling and mobilization of the melanocortinergic signaling pathway. Within the hypothalamus, the DBI gene is mainly expressed by non-neuronal cells such as ependymocytes, tanycytes, and protoplasmic astrocytes, at levels depending on the nutritional status. Administration of ODN C-terminal octapeptide (OP) in the arcuate nucleus strongly reduces food intake. Up to now, the relevance of extrahypothalamic targets for endozepine signaling-mediated anorexia has been largely ignored. We focused our study on the dorsal vagal complex located in the caudal brainstem. This structure is strongly involved in the homeostatic control of food intake and comprises structural similarities with the hypothalamus. In particular, a circumventricular organ, the area postrema (AP) and a tanycyte-like cells forming barrier between the AP and the adjacent nucleus tractus solitarius (NTS) are present. We show here that DBI is highly expressed by ependymocytes lining the fourth ventricle, tanycytes-like cells, as well as by proteoplasmic astrocytes located in the vicinity of AP/NTS interface. ODN staining observed at the electron microscopic level reveals that ODN-expressing tanycyte-like cells and protoplasmic astrocytes are sometimes found in close apposition to neuronal elements such as dendritic profiles or axon terminals. Intracerebroventricular injection of ODN or OP in the fourth ventricle triggers c-Fos activation in the dorsal vagal complex and strongly reduces food intake. We also show that, similarly to leptin, ODN inhibits the swallowing reflex when microinjected into the swallowing pattern generator located in the NTS. In conclusion, we hypothesized that ODN expressing cells located at the AP/NTS interface could release ODN and modify excitability of NTS neurocircuitries involved in food intake control.

A simple strategy for culturing morphologically-conserved rat hypothalamic tanycytes

Hypothalamic tanycytes are specialized bipolar ependymal cells that line the floor of the third ventricle. Given their strategic location, tanycytes are believed to play several key functions including being a selective barrier and controlling the amount of hypothalamic-derived factors reaching the anterior pituitary. The in vitro culture of these cells has proved to be difficult. Here, we report an improved method for the generation of primary cultures of rat hypothalamic tanycytes. Ependymal cultures were derived from tissue dissected out of the median eminence region of 10-day-old rats and cultured in a chemically defined medium containing DMEM:F12, serum albumin, insulin, transferrin and the antibiotic gentamycin. After 7 days in vitro, ∼30% of the cultured cells exhibited morphological features of tanycytes as observed by phase contrast or scanning electron microscopy. Tanycyte-like cells were strongly immuno-reactive for vimentin and dopamine-cAMP-regulated phospho-protein (DARPP-32) and weakly immune-reactive for glial fibrillary acidic protein. Tanycyte-like cells displayed a stable negative resting plasma membrane potential and failed to show spiking properties in response to current injections. When exposed to fluorescent beads in the culture medium, tanycyte-like cells exhibited a robust endocytosis. Thus, the present method effectively yields cultures containing tanycyte-like cells that resemble in vivo tanycytes in terms of morphologic features and molecular markers as well as electrical and endocytic activity. To our knowledge, this is the first protocol that allows the culturing of tanycyte-like cells that can be individually identified and that conserve the morphology of tanycytes in their natural physiological environment.

Variable proopiomelanocortin expression in tanycytes of the adult rat hypothalamus and pituitary stalk

It is generally believed that proopiomelanocortin (POMC) is expressed exclusively by neurons in the adult rodent brain. Unbeknownst to most researchers, however, Pomc in situ hybridization studies in the rat show specific labeling in the ventral wall of the hypothalamic third ventricle, which is formed by specialized ependymal cells, called tanycytes. Here we characterized this non-neuronal POMC expression in detail using in situ hybridization and immunohistochemical techniques, and report two unique characteristics. First, POMC mRNA and precursor protein expression in non-neuronal cells varies to a great degree as to the extent and abundance of expression. In brains with low-level expression, POMC mRNA and protein was largely confined to a population of tanycytes within the infundibular stalk/caudal median eminence, termed here γ tanycytes, and a subset of closely located β and α2 tanycytes. In brains with high-level expression, POMC mRNA and protein was observed in the vast majority of α2, β, and γ tanycytes. This variability was observed in both adult males and females; of 41 rats between 8 and 15 weeks of age, 17 had low-, 9 intermediate-, and 15 high-level POMC expression in tanycytes. Second, unlike other known POMC-expressing cells, tanycytes rarely contained detectable levels of adrenocorticotropin or α-melanocyte-stimulating hormone. The results indicate either a dynamic spatiotemporal pattern whereby low and high POMC syntheses in tanycytes occur periodically in each brain, or marked interindividual differences that may persist throughout adulthood. Future studies are required to examine these possibilities and elucidate the physiologic importance of POMC in tanycytes. J. Comp. Neurol. 525:411-441, 2017. © 2016 Wiley Periodicals, Inc.

Thyroid hormone activation of retinoic acid synthesis in hypothalamic tanycytes

Thyroid hormone (TH) is essential for adult brain function and its actions include several key roles in the hypothalamus. Although TH controls gene expression via specific TH receptors of the nuclear receptor class, surprisingly few genes have been demonstrated to be directly regulated by TH in the hypothalamus, or the adult brain as a whole. This study explored the rapid induction by TH of retinaldehyde dehydrogenase 1 (Raldh1), encoding a retinoic acid (RA)-synthesizing enzyme, as a gene specifically expressed in hypothalamic tanycytes, cells that mediate a number of actions of TH in the hypothalamus. The resulting increase in RA may then regulate gene expression via the RA receptors, also of the nuclear receptor class. In vivo exposure of the rat to TH led to a significant and rapid increase in hypothalamic Raldh1 within 4 hours. That this may lead to an in vivo increase in RA is suggested by the later induction by TH of the RA-responsive gene Cyp26b1. To explore the actions of RA in the hypothalamus as a potential mediator of TH control of gene regulation, an ex vivo hypothalamic rat slice culture method was developed in which the Raldh1-expressing tanycytes were maintained. These slice cultures confirmed that TH did not act on genes regulating energy balance but could induce Raldh1. RA has the potential to upregulate expression of genes involved in growth and appetite, Ghrh and Agrp. This regulation is acutely sensitive to epigenetic changes, as has been shown for TH action in vivo. These results indicate that sequential triggering of two nuclear receptor signalling systems has the capability to mediate some of the functions of TH in the hypothalamus.

Endocannabinoids and prostaglandins both contribute to GnRH neuron-GABAergic afferent local feedback circuits

Gonadotropin-releasing hormone (GnRH) neurons form the final common pathway for central control of fertility. Regulation of GnRH neurons by long-loop gonadal steroid feedback through steroid receptor-expressing afferents such as GABAergic neurons is well studied. Recently, local central feedback circuits regulating GnRH neurons were identified. GnRH neuronal depolarization induces short-term inhibition of their GABAergic afferents via a mechanism dependent on metabotropic glutamate receptor (mGluR) activation. GnRH neurons are enveloped in astrocytes, which express mGluRs. GnRH neurons also produce endocannabinoids, which can be induced by mGluR activation. We hypothesized the local GnRH-GABA circuit utilizes glia-derived and/or cannabinoid mechanisms and is altered by steroid milieu. Whole cell voltage-clamp was used to record GABAergic postsynaptic currents (PSCs) from GnRH neurons before and after action potential-like depolarizations were mimicked. In GnRH neurons from ovariectomized (OVX) mice, this depolarization reduced PSC frequency. This suppression was blocked by inhibition of prostaglandin synthesis with indomethacin, by a prostaglandin receptor antagonist, or by a specific glial metabolic poison, together suggesting the postulate that prostaglandins, potentially glia-derived, play a role in this circuit. This circuit was also inhibited by a CB1 receptor antagonist or by blockade of endocannabinoid synthesis in GnRH neurons, suggesting an endocannabinoid element, as well. In females, local circuit inhibition persisted in androgen-treated mice but not in estradiol-treated mice or young ovary-intact mice. In contrast, local circuit inhibition was present in gonad-intact males. These data suggest GnRH neurons interact with their afferent neurons using multiple mechanisms and that these local circuits can be modified by both sex and steroid feedback.

Gliotransmission by prostaglandin e(2): a prerequisite for GnRH neuronal function?

Over the past four decades it has become clear that prostaglandin E(2) (PGE(2)), a phospholipid-derived signaling molecule, plays a fundamental role in modulating the gonadotropin-releasing hormone (GnRH) neuroendocrine system and in shaping the hypothalamus. In this review, after a brief historical overview, we highlight studies revealing that PGE(2) released by glial cells such as astrocytes and tanycytes is intimately involved in the active control of GnRH neuronal activity and neurosecretion. Recent evidence suggests that hypothalamic astrocytes surrounding GnRH neuronal cell bodies may respond to neuronal activity with an activation of the erbB receptor tyrosine kinase signaling, triggering the release of PGE(2) as a chemical transmitter from the glia themselves, and, in turn, leading to the feedback regulation of GnRH neuronal activity. At the GnRH neurohemal junction, in the median eminence of the hypothalamus, PGE(2) is released by tanycytes in response to cell-cell signaling initiated by glial cells and vascular endothelial cells. Upon its release, PGE(2) causes the retraction of the tanycyte end-feet enwrapping the GnRH nerve terminals, enabling them to approach the adjacent pericapillary space and thus likely facilitating neurohormone diffusion from these nerve terminals into the pituitary portal blood. In view of these new insights, we suggest that synaptically associated astrocytes and perijunctional tanycytes are integral modulatory elements of GnRH neuronal function at the cell soma/dendrite and nerve terminal levels, respectively.

Nitric oxide as key mediator of neuron-to-neuron and endothelia-to-glia communication involved in the neuroendocrine control of reproduction

Nitric oxide (NO) is a peculiar chemical transmitter that freely diffuses through aqueous and lipid environments and plays a role in major aspects of brain function. Within the hypothalamus, NO exerts critical effects upon the gonadotropin-releasing hormone (GnRH) network to maintain fertility. Here, we review recent evidence that NO regulates major aspects of the GnRH neuron physiology. Far more active than once thought, NO powerfully controls GnRH neuronal activity, GnRH release and structural plasticity at the neurohemal junction. In the preoptic region, neuronal nitric oxide synthase (nNOS) activity is tightly regulated by estrogens and is found to be maximal at the proestrus stage. Natural fluctuations of estrogens control both the differential coupling of this Ca²+-activated enzyme to glutamate N-methyl-D-aspartic acid receptor channels and phosphorylation-mediated nNOS activation. Furthermore, NO endogenously produced by neurons expressing nNOS acutely and directly suppresses spontaneous firing in GnRH neurons, which suggests that neuronal NO may serve as a synchronizing switch within the preoptic region. At the median eminence, NO is spontaneously released from an endothelial source and follows a pulsatile and cyclic pattern of secretion. Importantly, GnRH release appears to be causally related to endothelial NO release. NO is also highly involved in mediating the dialogue set in motion between vascular endothelial cells and tanycytes that control the direct access of GnRH neurons to the pituitary portal blood during the estrous cycle. Altogether, these data raise the intriguing possibility that the neuroendocrine brain uses NO to coordinate both GnRH neuronal activity and GnRH release at key stages of reproductive physiology.

Gonadotrophin-releasing hormone nerve terminals, tanycytes and neurohaemal junction remodelling in the adult median eminence: functional consequences for reproduction and dynamic role of vascular endothelial cells

Although coordinated actions of several areas within the hypothalamus are involved in the secretion of gonadotrophin-releasing hormone (GnRH), the median eminence of the hypothalamus, where the nerve terminals are located, plays a particularly critical role in the release of GnRH. In adult females, prior to the preovulatory surge of GnRH, the retraction of specialised ependymoglial cells lining the floor of the third ventricle named tanycytes allows for the juxtaposition of GnRH nerve terminals with the adjacent pericapillary space of the pituitary portal vasculature, thus forming direct neurohaemal junctions. These morphological changes occur within a few hours and are reversible. Such remodelling may promote physiological conditions to enhance the central release of GnRH and potentiate oestrogen-activated GnRH release. This plasticity involves dynamic cell interactions that bring into play tanycytes, astrocytes, vascular endothelial cells and GnRH neurones themselves. The underlying signalling pathways responsible for these structural changes are comprised of highly diffusible gaseous molecules, such as endothelial nitric oxide, and paracrine communication processes involving receptors of the erbB tyrosine kinase family, transforming growth factor beta 1 and eicosanoids, such as prostaglandin E(2). Some of these molecules, as a result of their ability to diffuse within the median eminence, may also serve as synchronizing cues allowing for the occurrence of functionally meaningful episodes of GnRH secretion by coordinating GnRH release from the GnRH neuroendocrine terminals.

Function-related structural plasticity of the GnRH system: a role for neuronal-glial-endothelial interactions

As the final common pathway for the central control of gonadotropin secretion, GnRH neurons are subjected to numerous regulatory homeostatic and external factors to achieve levels of fertility appropriate to the organism. The GnRH system thus provides an excellent model in which to investigate the complex relationships between neurosecretion, morphological plasticity and the expression of a physiological function. Throughout the reproductive cycle beginning from postnatal sexual development and the onset of puberty to reproductive senescence, and even within the ovarian cycle itself, all levels of the GnRH system undergo morphological plasticity. This structural plasticity within the GnRH system appears crucial to the timely control of reproductive competence within the individual, and as such must have coordinated actions of multiple signals secreted from glial cells, endothelial cells, and GnRH neurons. Thus, the GnRH system must be viewed as a complete neuro-glial-vascular unit that works in concert to maintain the reproductive axis.

Role of estradiol in the dynamic control of tanycyte plasticity mediated by vascular endothelial cells in the median eminence

In the ever-changing physiological context of the neuroendocrine brain, the mechanisms by which cellular events involving neurons, astroglia, and vascular cells are coordinated to bring forth the appropriate neuronal signaling is not yet known but is amenable to examination. In the median eminence of the hypothalamus, endothelial cells are key players in the plasticity of tanycytes (specialized astroglia) and neuroendocrine synapse efficacy. Here we report that estradiol acts on both purified endothelial cells and isolated tanycytes to trigger endothelial-to-glial communication that leads to a sudden and massive retraction of tanycyte processes. The blockade of endothelial nitric oxide synthase by in vitro adenoviral-mediated gene transfer of a dominant-negative form of endothelial nitric oxide synthase abrogates the estradiol-induced tanycyte plasticity mediated by endothelial cells. In parallel, increases in prostaglandin-E(2) (PGE(2)) due to changes in cyclooxygenase (COX)-1 and COX-2 expression induced by the exposure of tanycytes to estradiol promote acute tanycyte plasticity. We also demonstrate by electron microscopy that the administration of PGE(2) to median eminence explants induces rapid neuroglial plasticity at the neurovascular junction of neurons that release GnRH (the neuropeptide controlling reproduction). Conversely, preventing local PGE(2) synthesis in the median eminence of adult female rats with the COX inhibitor indomethacin impairs the ovarian cycle, a process that requires a pulsatile, coordinated delivery of GnRH into the hypothalamo-hypophyseal portal system. Taken together, our findings show that estradiol controls the dialog between endothelial cells and astroglia to regulate neuroglial plasticity in the neuroendocrine brain.

Immunochemical and molecular characterization of a novel cell line derived from the brain of Trachinotus blochii (Teleostei, Perciformes): A fish cell line with oligodendrocyte progenitor cell and tanycyte characteristics

Ependymal radial glial cells, also called tanycytes, are the predominant glial fibrillary acidic protein (GFAP)- and vimentin (VIM)-expressing cells in fish ependyma. Radial glial cells have been proposed to be neural stem cells but their molecular expression is not well understood. Previous studies revealed that fish neural progenitor and neural stem cells have A2B5, a marker for oligodendrocyte progenitor cells (OPCs). In this study, an A2B5(+) cell line, SPB, was isolated from the brain of the teleost Trachinotus blochii and characterized. SPB cells usually grew as polygonal epithelial cells, but at high density, long processes were commonly observed. Using immunocytochemistry, SPB cells were shown to exhibit oligodendrocyte markers such as galactocerebroside and Olig2, and radial glial cell markers such as brain lipid-binding protein, GFAP, Sox2, and VIM. SPB cells were also observed to have DARPP-32, a marker for tanycytes in mammals, and primary cilia. RT-PCR additionally revealed expression of bone morphogenetic protein 4, connexin35, Noggin2, and proteolipid protein in SPB cells. Results of this study suggest that SPB cells are OPCs that can display tanycyte characteristics. Fish tanycytes can be neural stem cells suggesting that SPB cells are neural stem cells. SPB is the first fish cell line showing primary cilia and markers for both OPCs and tanycytes.

Protein components of the blood-brain barrier (BBB) in the mediobasal hypothalamus

The blood-brain barrier (BBB) plays an important role in controlling the access of substances to the brain. Of the circumventricular organs (CVO), i.e. areas that lack a BBB, the median eminence and its close relationship with the hypothalamic arcuate nucleus plays an important role in controlling the entry of blood-borne substances to neurons of the mediobasal hypothalamus. In order to clarify the nature of the BBB in the median eminence-arcuate nucleus complex, we have used immunohistochemistry and antisera to protein components of the BBB-(1) tight junctions, claudin-5 and zona occludens-1 (ZO-1); (2) endothelial cells: (a) all endothelial cells: rat endothelial cell antigen-1 (RECA-1), (b) endothelial cells at BBB: endothelial barrier antigen (EBA), glucose transporter 1 (GLUT1) and transferrin receptor (TfR), and (c) endothelial cells at CVOs: dysferlin; (3) basal lamina: laminin; (4) vascular smooth muscle cells: smooth muscle actin (SMA); (5) pericytes: chondroitin sulfate proteoglycan (NG2); (6) glial cells: (a) astrocytes: glial fibrillary acidic protein (GFAP), (b) tanycytes: dopamine- and cAMP-regulated phosphoprotein of 32kDA (DARPP-32), (c) microglia: CD11b. Neuronal cell bodies located in the ventromedial aspect of the arcuate nucleus were visualized by antiserum to agouti-related protein (AgRP). The study provides a detailed analysis on the cellular localization of BBB components in the mediobasal hypothalamus. Some vessels in the ventromedial aspect of the arcuate nucleus lacked the BBB markers EBA and TfR, suggesting an absence of an intact BBB. These vessels may represent a route of entry for circulating substances to a subpopulation of arcuate nucleus neurons.

Morphological evidence for direct interaction between gonadotrophin-releasing hormone neurones and astroglial cells in the human hypothalamus

In rodents, there is compelling evidence indicating that dynamic cell-to-cell communications involving cross talk between astroglial cells (such as astrocytes and specialised ependymoglial cells known as tanycytes) and neurones are important in regulating the secretion of gonadotrophin-releasing hormone (GnRH), the neurohormone that controls both sexual maturation and adult reproductive function. However, whether such astroglial cell-GnRH neurone interactions occur in the human brain is not known. In the present study, we used immunofluorescence to examine the anatomical relationship between GnRH neurones and glial cells within the hypothalamus of five women. Double-staining experiments demonstrated the ensheathment of GnRH neurone perikarya by glial fibrillary acidic protein (GFAP)-immunoreactive astrocyte processes in the periventricular zone of the tuberal region of the hypothalamus. GFAP immunoreactivity did not overlap that of GnRH at the GnRH neurone's projection site (i.e. the median eminence of the hypothalamus). Rather, human GnRH neuroendocrine fibres were found to be closely associated with vimentin or nestin-immunopositive radial glial processes likely belonging to tanycytes. In line with these light microscopy data, ultrastructural examination of GnRH-immunoreactive neurones showed numerous glial cells in direct apposition to pre-embedding-labelled GnRH cell bodies and/or dendrites in the infundibular nucleus, whereas postembedding immunogold-labelled GnRH nerve terminals were often seen to be enwrapped by glial cell processes in the median eminence. GnRH nerve button were sometimes visualised in close proximity to fenestrated pituitary portal blood capillaries and/or evaginations of the basal lamina that delineate the pericapillary space. In summary, these data demonstrate that GnRH neurones morphologically interact with astrocytes and tanycytes in the human brain and provide evidence that glial cells may contribute physiologically to the process by which the neuroendocrine brain controls the function of GnRH neurones in humans.

Sexual dimorphism in the organization of the rat hypothalamic infundibular area

The hypothalamic infundibular area is located outside the blood-brain barrier and includes, the ventromedial arcuate nucleus (vmARC) sensing circulating substances, and the median eminence (ME) where neurohormones are released into the hypothalamo-hypophysial vasculature. This integrated functional unit, pivotal in endocrine control, adjusts neuroendocrine output to feedback information. Despite a differing physiology in males and females, this functional unit has not appeared differently organized between sexes. Using immunocytochemistry, we describe here for the first time in adult rats, a conspicuous sex-difference in its axonal wiring by intrinsic glutamatergic neurons containing the neuropeptides neurokinin B (NKB) and dynorphin. In the male, NKB neurons send axons to capillary vessels of the vmARC and of the ME (only where gonadotropin-releasing hormone (GnRH) axons terminate). Electron microscopy revealed that NKB axons target the barrier of tanycytes around fenestrated capillary vessels (in addition to GnRH axons), suggesting a control of regional bidirectional permeability. In the female, NKB neurons send axons to the neuropile of the vmARC, suggesting a direct control of its sensor neurons. The other projections of NKB neurons, studied by surgical isolation of the ARC-ME complex and confocal microscopy, are not sexually dimorphic and target both integrative and neuroendocrine centers controlling reproduction and metabolism, suggesting a broad influence over endocrine function. These observations demonstrate that the mechanisms subserving hypothalamic permeability and sensitivity to feedback information are sexually dimorphic, making the infundibular area a privileged site of generation of the male-to-female differences in the adult pattern of pulsatile hormonal secretions.

Neuroendocrine control of reproductive aging: roles of GnRH neurons

The process of reproductive senescence in many female mammals, including humans, is characterized by a gradual transition from regular reproductive cycles to irregular cycles to eventual acyclicity, and ultimately a loss of fertility. In the present review, the role of the hypothalamic gonadotropin-releasing hormone (GnRH) neurons is considered in this context. GnRH neurons provide the primary driving force upon the other levels of the reproductive axis. With respect to aging, GnRH cells undergo changes in biosynthesis, processing and release of the GnRH decapeptide. GnRH neurons also exhibit morphologic and ultrastructural alterations that appear to underlie these biosynthetic properties. Thus, functional and morphologic changes in the GnRH neurosecretory system may play causal roles in the transition to acyclicity. In addition, GnRH neurons are regulated by numerous inputs from neurotransmitters, neuromodulators and glia. The relationship among GnRH cells and their inputs at the cell body (thereby affecting GnRH biosynthesis) and the neuroterminal (thereby affecting GnRH neurosecretion) is crucial to the function of the GnRH system, with age-related changes in these relationships contributing to the reproductive senescent process. Therefore, the aging hypothalamus is characterized by changes intrinsic to the GnRH cell, as well as its regulatory inputs, which summate to contribute to a loss of reproductive competence in aging females.

DARPP-32 genomic fragments drive Cre expression in postnatal striatum

To direct Cre-mediated recombination to differentiated medium-size spiny neurons (MSNs) of the striatum, we generated transgenic mice that express Cre recombinase under the regulation of DARPP-32 genomic fragments. In this reported line, recombination of an R26R reporter allele occurred postnatally in the majority of medium-size spiny neurons of the dorsal and ventral striatum (caudate nucleus and nucleus accumbens/olfactory tubercle), as well as in the piriform cortex and choroid plexus. Although regulatory fragments were selected to target MSNs, low levels of Cre-recombinase expression, as detected by beta-galactosidase activity from the R26R reporter gene, were also apparent in widely dispersed areas or cells of the forebrain and hindbrain. These included the primary and secondary motor cortex, and association cortex, as well as in the olfactory bulb and cerebellar Purkinje cells. Notably, expression in these regions was well below that of endogenous DARPP-32. Analysis of colocalization of beta-galactosidase, as detected either by histochemistry or immunocytochemistry, and DARPP-32 revealed double-labeling in almost all DARPP-32-expressing MSNs in the postnatal striatum, but not in extrastriatal regions. The DARPP-32Cre transgenic mouse line thus provides a useful tool to specifically express and/or inactivate genes in mature MSNs of the striatum.

Role of thyroid hormone deiodination in the hypothalamus

Iodothyronine deiodinases (D1, D2, and D3) comprise a family of selenoproteins that are involved in the conversion of thyroxine (T(4)) to active triiodothyronine (T(3)), and also the inactivation of both thyroid hormones. The deiodinase enzymes are of critical importance for the normal development and function of the central nervous system. D1 is absent from the human brain, suggesting that D2 and D3 are the two main enzymes involved in the maintenance of thyroid hormone homeostasis in the central nervous system, D2 as the primary T(3)-producing enzyme, and D3 as the primary inactivating enzyme. While the coordinated action of D2 and D3 maintain constant T(3) levels in the cortex independently from the circulating thyroid hormone levels, the role of deiodinases in the hypothalamus may be more complex, as suggested by the regulation of D2 activity in the hypothalamus by infection, fasting and changes in photoperiod. Tanycytes, the primary source of D2 activity in the hypothalamus, integrate hormonal and probably neuronal signals, and under specific conditions, may influence neuroendocrine functions by altering local T(3) tissue concentrations. This function may be of particular importance in the regulation of the hypothalamic-pituitary-thyroid axis during fasting and infection, and in the regulation of appetite and reproductive function. Transient expression of D3 in the preoptic region during a critical time of development suggests a special role for this deiodinase in sexual differentiation of the brain.

Vascular endothelial cells promote acute plasticity in ependymoglial cells of the neuroendocrine brain

Glial and endothelial cells interact throughout the brain to define specific functional domains. Whether endothelial cells convey signals to glia in the mature brain is unknown but is amenable to examination in circumventricular organs. Here we report that purified endothelial cells of one of these organs, the median eminence of the hypothalamus, induce acute actin cytoskeleton remodeling in isolated ependymoglial cells and show that this plasticity is mediated by nitric oxide (NO), a diffusible factor. We found that both soluble guanylyl cyclase and cyclooxygenase products are involved in this endothelial-mediated control of ependymoglia cytoarchitecture. We also demonstrate by electron microscopy that activation of endogenous NO release in the median eminence induces rapid structural changes, allowing a direct access of neurosecretory axons containing gonadotropin-releasing hormone (GnRH) (the neuropeptide controlling reproductive function) to the portal vasculature. Local in vivo inhibition of NO synthesis disrupts reproductive cyclicity, a process that requires a pulsatile, coordinated delivery of GnRH into the hypothalamic-adenohypophyseal portal system. Our results identify a previously unknown function for endothelial cells in inducing neuroglial plasticity and raise the intriguing possibility that endothelial cells throughout the brain may use a similar signaling mechanism to regulate glial-neuronal interactions.

Activation of erbB-1 signaling in tanycytes of the median eminence stimulates transforming growth factor beta1 release via prostaglandin E2 production and induces cell plasticity

The activation of transforming growth factor alpha (TGFalpha)-erbB-1 and neuregulin-erbB-4 signaling pathways in hypothalamic astrocytes has been shown to play a key role in the process by which the neuroendocrine brain controls luteinizing hormone-releasing hormone (LHRH) secretion. Earlier studies suggested that tanycytes, an ependymoglial cell type of the median eminence, regulate LHRH release during the estrous cycle by undergoing plastic changes that alternatively allow or prevent direct access of the LHRH nerve terminals to the portal vasculature. Neither the molecules responsible for these plastic changes nor the underlying controlling mechanisms have been identified. Here we show that cultured tanycytes express erbB-1 and erbB-2, two of the four members of the erbB receptor family, and respond to TGFalpha with receptor phosphorylation, release of prostaglandin E2 (PGE2), and a PGE2-dependent increase in the release of TGFbeta1, a growth factor previously implicated in the glial control of LHRH secretion. Blockade of either erbB-1 receptor signal transduction or prostaglandin synthesis prevented the stimulatory effect of TGFalpha on both PGE2 and TGFbeta1 release. Time-lapse studies revealed that TGFalpha and TGFbeta1 have dramatically opposite effects on tanycyte plasticity. Whereas TGFalpha promotes tanycytic outgrowth, TGFbeta1 elicits retraction of tanycytic processes. Blockade of metalloproteinase activity abolished the effect of TGFbeta1, suggesting that TGFbeta1 induces tanycytic retraction by facilitating dissolution of the extracellular matrix. Prolonged (>12 hr) exposure of tanycytes to TGFalpha resulted in focal tanycytic retraction, an effect that was abolished by immunoneutralization of TGFbeta1 action, indicating that the retraction was attributable to TGFalpha-induced TGFbeta1 formation. These in vitro results identify tanycytes as targets of TGFalpha action and demonstrate that activation of erbB-1-mediated signaling in these cells results in plastic changes that, involving PGE2 and TGFbeta1 as downstream effectors, mimic the morphological plasticity displayed by tanycytes during the hours encompassing the preovulatory surge of LHRH.

Hypothalamic ependymal-glial cells express the glucose transporter GLUT2, a protein involved in glucose sensing

The GLUT2 glucose transporter and the K-ATP-sensitive potassium channels have been implicated as an integral part of the glucose-sensing mechanism in the pancreatic islet beta cells. The expression of GLUT2 and K-ATP channels in the hypothalamic region suggest that they are also involved in a sensing mechanism in this area. The hypothalamic glial cells, known as tanycytes alpha and beta, are specialized ependymal cells that bridge the cerebrospinal fluid and the portal blood of the median eminence. We used immunocytochemistry, in situ hybridization and transport analyses to demonstrate the glucose transporters expressed in tanycytes. Confocal microscopy using specific antibodies against GLUT1 and GLUT2 indicated that both transporters are expressed in alpha and beta tanycytes. In addition, primary cultures of mouse hypothalamic tanycytes were found to express both GLUT1 and GLUT2 transporters. Transport studies, including 2-deoxy-glucose and fructose uptake in the presence or absence of inhibitors, indicated that these transporters are functional in cultured tanycytes. Finally, our analyses indicated that tanycytes express the K-ATP channel subunit Kir6.1 in vitro. As the expression of GLUT2 and K-ATP channel is linked to glucose-sensing mechanisms in pancreatic beta cells, we postulate that tanycytes may be responsible, at least in part, for a mechanism that allows the hypothalamus to detect changes in glucose concentrations.

TTF-1, a homeodomain-containing transcription factor, participates in the control of body fluid homeostasis by regulating angiotensinogen gene transcription in the rat subfornical organ

In recent years, it has become increasingly evident that angiotensins synthesized in the brain contribute to regulating body fluid homeostasis. Although angiotensinogen, the unique angiotensin precursor, is produced in the brain, the factors that regulate its gene expression remain unknown. We recently found that TTF-1, a homeodomain-containing transcription factor essential for the development of the fetal diencephalon, is postnatally expressed in discrete areas of the hypothalamus. We now report that the subfornical organ, an important site of angiotensinogen synthesis, is an extra-hypothalamic site of TTF-1 expression. Double in situ hybridization histochemistry demonstrated the presence of TTF-1 mRNA in angiotensinogen-producing cells of the rat subfornical organ. RNase protection assays showed that TTF-1 and angiotensinogen mRNA levels are simultaneously increased in the subfornical organ by water deprivation. The angiotensinogen promoter contains seven presumptive TTF-1 binding motifs, four of which are recognized by the TTF-1 homeodomain. In the C6 glioma cell line, TTF-1 transactivates the angiotensinogen promoter in a dose-dependent manner. This transactivation is abolished by deletion of the TTF-1 binding motif at -125. Intracranial administration of an antisense TTF-1 oligodeoxynucleotide decreased angiotensinogen mRNA in the subfornical organ and dramatically reduced the animal's water intake while increasing urine excretion. Moreover, plasma arginine vasopressin content was decreased by the same treatment. These results demonstrate a novel role for TTF-1 in the regulation of body fluid homeostasis, exerted via the transactivational control of angiotensinogen synthesis in the subfornical organ.

DARPP-32 and CREB are present in type 2 iodothyronine deiodinase-producing tanycytes: implications for the regulation of type 2 deiodinase activity

Type 2 iodothyronine deiodinase, an enzyme involved in the conversion of thyroxin to the biologically active 3,5, 3'-triiodothyronine, is highly concentrated in a group of specialized ependymal cells, tanycytes, lining the wall and floor of the third ventricle. As this distribution is highly reminiscent of the distribution of cells containing the phosphatase inhibitor, DARPP-32, we raised the possibility that these two proteins may coexist in tanycytes and that DARPP-32 may modulate type 2 deiodinase activity by regulating the phosphorylation state of the cAMP regulatory factor, CREB. To address this question, double-labeling histochemical studies were performed for type 2 deiodinase mRNA and DARPP-32 immunoreactivity (IR), or DARPP-32- and CREB-IR in the same tissue sections. Type 2 deiodinase mRNA was found in the cell bodies of all DARPP-32-immunolabeled tanycytes. Both type 2 deiodinase mRNA and DARPP-32-IR also extended into tanycyte processes that ramified in the arcuate nucleus and median eminence, in close association with blood vessels and portal capillaries. In contrast, type 2 deiodinase mRNA was not present in the same cells that contained DARPP-32-IR in the pituitary gland. All tanycytes containing DARPP-32-IR also contained CREB-IR in their nucleus. Since type 2 deiodinase activity can be induced by substances that increase cAMP, we hypothesize that DARPP-32 may regulate the activity of type 2 deiodinase by prolonging the activation of CREB. Selectivity for the colocalization of these factors to tanycytes but not the pituitary gland, may explain the heterogeneous response of type 2 deiodinase activity in these two loci in response to specific stimuli such as fasting.

Inhibition of Na,K-ATPase activity by cGMP is isoform-specific in brain endothelial cells

cGMP has been shown to either activate or inhibit Na,K-ATPase activity. Using mouse brain endothelial cells which express both ouabain-resistant alpha1 and ouabain-sensitive alpha2 and alpha3 isoforms, we show that cGMP reduces total Na,KATPase activity to about 58%. The inhibition is prevented by the protein kinase G (PKG)-specific inhibitor KT5823, indicating that cGMP-mediated activation of PKG leads to inhibition of the pump. A similar extent of inhibition is obtained with nitric oxide. cGMP-induced inhibition acts mainly on alpha1 isoforms but hardly affects alpha2/alpha3 isoforms. These data suggest that inhibition of Na,K-ATPase activity by cGMP occurs in an isoform-selective manner in brain endothelial cells.

Specificity of interleukin-1beta-induced changes in monoamine concentrations in hypothalamic nuclei: blockade by interleukin-1 receptor antagonist

The purpose of this study was to examine specificity in the effects of interleukin-1beta (IL-1beta) on monoamines in various areas of the hypothalamus. Adult male rats were injected i.p. with saline or 2.5 or 5.0 microg of IL-1beta or were pretreated with 500 microg of IL-1 receptor antagonist (IL-1ra) followed 5 min later by 5 microg of IL-1beta. The paraventricular nucleus (PVN), arcuate nucleus (AN), median eminence (ME), and medial preoptic area (MPA) were microdissected and analyzed for neurotransmitter concentrations by high-performance liquid chromatography with electrochemical detection (HPLC-EC). In the PVN, IL treatment produced significant increases in the concentrations of norepinephrine (NE), dopamine (DA), DA metabolite dihydroxyphenylacetic acid (DOPAC), serotonin (5-HT), and its metabolite 5-hydroxyindoleacetic acid (5-HIAA). IL-1 treatment increased the concentrations of NE and DA in the AN but only of NE in the ME, and it was without any effect in the MPA. Pretreatment with IL-1ra completely blocked the IL-1 effects. It is concluded that IL-1 induces highly specific changes in monoamine metabolism in the hypothalamus, and the nature of these changes depends on specific hypothalamic nuclei.

Developmental study of Müller cells in the rat retina using a new monoclonal antibody, RT10F7

We produced the monoclonal antibody RT10F7, characterized its antigenic specificity and expression in the adult and developing retina, in cultured retinal cells and in other parts of the central nervous system. In metabolically-labelled retinal cultures RT10F7 immunoprecipitated a protein of approximately 36,000 mol. wt. In the adult, RT10F7 stained endfeet of Müller cells in the ganglion cell layer, four horizontal bands in the inner plexiform layer, and radial fibres in the outer plexiform layer which terminated at the outer limiting membrane. In the inner nuclear layer, most somata were underlined by Müller processes that wrapped around them, but some cell bodies were immunoreactive for RT10F7 in the cytoplasm. During development, postnatal day 21 was the first age at which the adult pattern of immunoreactivity was present, although a fourth band in the inner plexiform layer was less clear than for the adult. By 14 and eight days after birth, the pattern of RT10F7 immunoreactivity approximated that of the adult; however, only three bands and one band were present, respectively, in the inner plexiform layer. At earlier ages, postnatal days 4, 1 and embryonic ages 19 and 15, the monoclonal antibody stained Müller cell endfeet and radial fibres, from the inner plexiform layer through the neuroblastic layer to the outer limiting membrane. At these ages, the immunoreactivity was more prominent at the level of Müller cell endfeet. The monoclonal antibody stained glia in preparations of dissociated retinal cells maintained in culture but not astrocytes or oligodendrocytes from optic nerve cultures. In brain sections, tanycytes exhibited RT10F7 immunoreactivity. The monoclonal antibody RT10F7 recognized a specific cell type in the retina, the Müller cell. In the adult and developing retina, RT10F7 recognized an antigen that is present primarily in Müller cell processes. This feature allowed us to follow the maturation of the Müller cell and correlate it with developmental events in the retina. RT10F7 is a specific marker for Müller cells in vivo and in vitro and may be useful for studies of function of Müller cells after ablation or after injuries that are known to activate Müller cells.

Tanycytes present in the adult rat mediobasal hypothalamus support the regeneration of monoaminergic axons

We have recently shown that tanycytes present in the median eminence (ME) constitute a preferential support for the regeneration of lesioned neurohypophysial oxytocinergic and vasopressinergic axons. However, although tanycytes are particularly abundant in the ME, they are also present along the third ventricle wall. This study was thus undertaken to determine whether tanycytes present in the mediobasal hypothalamus overlying the ME were also able to support the regeneration of the numerous monoaminergic axons innervating this region. Using confocal laser scanning microscopy combined with double or triple fluorescence immunostaining, we have compared the relationships occurring between glial cells and lesioned catecholaminergic and serotonergic axons at the levels of surgical cuts placed in the dorsomedial hypothalamus devoid of tanycytes or in the ventromedial hypothalamus containing numerous tanycyte processes. In dorsal lesions, catecholaminergic and serotonergic transected fibers were found to abut onto the scar formed along the surgical cut and composed of closely inderdigitating astrocyte processes strongly immunoreactive for both glial fibrillary acidic protein (GFAP) and vimentin (VIM). In ventral lesions, the lesional scar was composed of GFAP-immunoreactive (IR) and VIM-IR astrocyte processes and of VIM-IR but GFAP-negative processes that were identified as tanycytic processes. In all the ventral lesions examined, numerous catecholaminergic and serotonergic fibers were found to regenerate into the surgical cut in association with the VIM-IR, GFAP-negative tanycyte processes. On the other hand, such regenerating fibers were never found in scar portions containing only GFAP-IR astrocytic structures. These data indicate that, like in the ME, tanycytes present in the mediobasal hypothalamus of adult rat provide a substrate that favors the regeneration of lesioned axons.

Ependymal and choroidal cells in culture: characterization and functional differentiation

During the past 10 years, our teams developed long-term primary cultures of ependymal cells derived from ventricular walls of telencephalon and hypothalamus or choroidal cells (modified ependymal cells) derived from plexuses dissected out of fetal or newborn mouse or rat brains. Cultures were established in serum-supplemented or chemically defined media after seeding on serum-, fibronectin-, or collagen-laminin-coated plastic dishes or semipermeable inserts. To identify and characterize cell types growing in our cultures, we used morphological features provided by phase contrast, scanning, and transmission electron microscopy. We used antibodies against intermediate filament proteins (vimentin, glial fibrillary acidic protein, cytokeratin, desmin, neurofilament proteins), actin, myosin, ciliary rootlets, laminin, and fibronectin in single or double immunostaining, and monoclonal antibodies against epitopes of ependymal or endothelial cells, to recognize ventricular wall cell types with immunological criteria. Ciliated or nonciliated ependymal cells in telencephalic cultures, tanycytes and ciliated and nonciliated ependymal cells in hypothalamic cultures always exceeded 75% of the cultured cells under the conditions used. These cells were characterized by their cell shape and epithelial organization, by their apical differentiations observed by scanning and transmission electron microscopy, and by specific markers (e.g., glial fibrillary acidic protein, ciliary rootlet proteins, DARPP 32) detected by immunofluorescence. All these cultured ependymal cell types remarkably resembled in vivo ependymocytes in terms of molecular markers and ultrastructural features. Choroidal cells were also maintained for several weeks in culture, and abundantly expressed markers were detected in both choroidal tissue and culture (Na+-K+-dependent ATPase, DARPP 32, G proteins, ANP receptors). In this review, the culture models we developed (defined in terms of biological material, media, substrates, duration, and subculturing) are also compared with those developed by other investigators during the last 10 years. Focusing on morphological and functional approaches, we have shown that these culture models were suitable to investigate and provide new insights on (1) the gap junctional communication of ependymal, choroidal, and astroglial cells in long-term primary cultures by freeze-fracture or dye transfer of Lucifer Yellow CH after intracellular microinjection; (2) some ionic channels; (3) the hormone receptors to tri-iodothyronine or atrial natriuretic peptides; (4) the regulatory effect of tri-iodothyronine on glutamine synthetase expression; (5) the endocytosis and transcytosis of proteins; and (6) the morphogenetic effects of galactosyl-ceramide. We also discuss new insights provided by recent results reported on in vitro ependymal and choroidal expressions of neuropeptide-processing enzymes and neurosecretory proteins or choroidal expression of transferrin regulated through serotoninergic activation.

Ependymal development, proliferation, and functions: a review

A survey of the literature shows that proliferation of ependyma occurs largely during the embryonic and early postnatal periods of development in most species. Differentiation of these cells proceeds along particular regional and temporal gradients as does the expression of various cytoskeletal (vimentin, cytokeratins, glial fibrillary acidic protein) and secretory proteins (S-100). Turnover declines significantly postnatally, and only low levels of residual activity persist into adulthood under normal conditions. Although the reported response of ependyma to injury is somewhat equivocal, only limited regenerative capacity appears to exist and to varying degrees in different regions of the neuraxis. Proliferation has been most often observed in response to spinal cord injury. Indeed, the ependyma plays a significant role in the initiation and maintenance of the regenerative processes in the spinal cord of inframammalian vertebrates. In the human, however, ependyma appears never to regenerate at any age nor re-express cytoskeletal proteins characteristic of immature cells. The functions of ependyma including tanycytes, a specialized form of ependymal cell that persists into adulthood within circumscribed regions of the nervous system, are still largely speculative. Fetal unlike mature ependyma is believed to be secretory and is believed to play a role in neurogenesis, neuronal differentiation/axonal guidance, transport, and support. In the adult brain, mature ependyma is not merely an inert lining but may regulate the transport of ions, small molecules, and water between the cerebrospinal fluid and neuropil and serve an important barrier function that protects neural tissue from potentially harmful substances by mechanisms that are still incompletely understood.

Evidence for a role for calcium ions in the dephosphorylation of glial fibrillary acidic protein (GFAP) in immature hippocampal slices and in astrocyte cultures from the rat

Evidence was sought for a role for Ca2+ in the dephosphorylation of the astrocyte marker glial fibrillary acidic protein (GFAP) in immature hippocampal slices. Although previous work showed that the main phosphatase dephosphorylating GFAP in this preparation is a Ca2+-independent type 1 enzyme, a role for Ca2+ was suggested by the observation that the incorporation of [32P]phosphate into GFAP in immature slices is inhibited by external Ca2+. This inhibition is strikingly different to the situation in mature slices where GFAP phosphorylation is completely dependent on Ca2+. Pure astrocyte cultures were probed by immunoblotting for the presence of the Ca2+-dependent phosphatase calcineurin. An enzyme content, amounting to about 2% of that found in fresh hippocampal tissue, was detected for both the catalytic (alpha) and regulatory (beta) subunits. The direct or indirect association of calcineurin with GFAP was suggested by observations showing that FK506, a specific inhibitor of calcineurin, increased the phosphorylation state of GFAP in immature slices and of GFAP and vimentin in astrocyte cultures.

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.

Regional distribution of type 2 thyroxine deiodinase messenger ribonucleic acid in rat hypothalamus and pituitary and its regulation by thyroid hormone

To identify the specific locations of type 2 deiodinase (D2) messenger RNA (mRNA) in the hypothalamus and pituitary gland and determine its regulation by thyroid hormone, we performed in situ hybridization histochemistry, Northern analysis, and quantitative RT-PCR in euthyroid, hypothyroid, and hyperthyroid rats. By in situ hybridization histochemistry, silver grains were concentrated over ependymal cells lining the floor and infralateral walls of the third ventricle extending from the rostral tip of the median eminence (ME) to the infundibular recess, surrounding blood vessels in the arcuate nucleus (ARC), and in the ME adjacent to the portal vessels and overlying the tuberoinfundibular sulci. Silver grains also accumulated over distinct cells in the midportion of the anterior pituitary. In hypothyroid animals, an increase in signal intensity was observed in the caudal hypothalamus, and a marked increase in the number of positive cells occurred in the anterior pituitary. Microdissection of the hypothalamus for Northern and PCR analysis established the authenticity of D2 mRNA in the caudal hypothalamus, and confirmed that the majority of D2 mRNA is concentrated in this region. The distribution of D2 mRNA suggests its expression in specialized ependymal cells, termed tanycytes, originating from the third ventricle. Thus, the tanycyte is the source of the high D2 activity previously found in the ARC-ME region of the hypothalamus. The results indicate that tanycytes may have a previously unrecognized integral role in feedback regulation of TSH secretion by T4.

Aged median eminence glial cell cultures promote survival and neurite outgrowth of cocultured neurons

We have recently shown that tanycytes, a particular type of glial cell that has morphological and biochemical similarities with radial glial cells, constitute a preferential support for the regeneration of lesioned neurohypophysial axons. The present study was designed to explore the possible neurotrophic role of tanycytes in vitro. Glial cells derived from the median eminence or from the cerebral cortex of 10-day-old rats were cultured for 4-7 weeks. At these times the majority of the cells identified in the median eminence cultures exhibited immunostaining patterns of tanycytes, as detected in the mediobasal hypothalamus of 10-day-old and adult rats, i.e., they were immunoreactive to vimentin (VIM), to DARPP-32 (a dopamine- and adenosine 3':5'-monophosphate-regulated phosphoprotein), and to a lesser extent to glial fibrillary acidic protein (GFAP) antibodies. On the other hand, the majority of cells in cortex cultures showed immunostaining patterns of astrocytes, i.e., they were intensely immunoreactive to GFAP and VIM antibodies but negative to DARPP-32. Cells obtained from the dissociation of 3-day-old rat mesencephalon, cortex, and hypothalamus were cocultured on these glial monolayers, and the number of surviving neurons and their neurite length were quantified after 8 days. Our data showed that, when compared with astrocytes, tanycytes greatly improved both survival (six-to ten-fold higher) and neurite outgrowth (two- to five-fold longer) of cocultured neurons whatever their origin. Experiments performed by coculturing neurons on millicell inserts placed above the glial monolayers showed that diffusible factors from median eminence glial cells slightly increased survival (1.7-fold higher) of cocultured neurons but had no significant effect on neurite outgrowth. These observations indicate: 1) that aged tanycytes have a capacity to support survival and neurite outgrowth for a variety of postnatal neurons; and 2) that this neurotrophic effect is exerted mainly by means of specific molecules bound to the tanycytic plasmalemma limiting membrane and/or to the extracellular matrix.

Short-photoperiod exposure reduces L-aromatic-amino-acid decarboxylase immunostaining in the arcuate nucleus and median eminence of male Syrian hamsters

In male hamsters, exposure to short photoperiod (SD) results in a decrease in median eminence and posterior pituitary dopamine (DA) concentrations. To determine if the SD-induced decline in DA is due to a decrease in the number of neurons synthesizing DA, immunocytochemistry was used to identify cells containing tyrosine hydroxylase (TH) and L-aromatic amino acid decarboxylase (AADC) in animals housed under long photoperiod (LD) or SD. Immunopositive cells were counted in the arcuate and caudal periventricular nuclei, regions where the DA neurons that project to the median eminence and posterior pituitary are located. AADC immunopositive (AADC +) cells were also counted in the median eminence. Photoperiod did not affect the number of TH immunopositive (TH+) or AADC + cells in the caudal periventricular nuclei. In the arcuate nuclei, SD exposure did not affect the number of TH+ neurons, but produced a significant decline in the number of AADC + cells. The number of AADC + cells was also reduced in the median eminence of SD-exposed animals. This decline in AADC+ cells may play a role in SD-induced changes in hypothalamic regulation of anterior pituitary hormone release, including the decline in median eminence DA concentrations.

Age-related alterations in tanycytes of the mediobasal hypothalamus of the male rat

By means of semiquantitative immunocytochemistry, possible age-related changes in dopamine and cyclic AMP-regulated phosphoprotein mr 32 (DARPP-32) and glial fibrillary acidic protein (GFAP) immunoreactivities (IR) were investigated in tanycytes of the arcuate nucleus. These two markers showed opposite changes during aging. DARPP-32 IR decreased by around 70%, whereas GFAP IR increased by around 300% in 24-month-old vs. 3-month-old rats. These changes were accompanied by a progressive loss in the number of tanycytes, measured by counting of their long processes in the arcuate nucleus. No significant age-related change was observed either in GFAP IR in astrocytic populations of the mediobasal hypothalamus or in tyrosine hydroxylase IR in dopaminergic neurons of the dorsal arcuate nucleus. These observations indicate that the tanycytic population of the arcuate nucleus undergoes important modifications during aging, which include cell loss, impairment in the intracellular signalling cascade linked to DARPP-32, and hypertrophy. These changes may be related to the alterations in the neuroendocrine systems known to occur during aging.

Dopamine- and adenosine-3',5'-monophosphate (cAMP)-regulated phosphoprotein of 32 kDa (DARPP-32) in the adrenal gland: immunohistochemical localization

The cellular localization of a dopamine- and adenosine-3',5'-monophosphate (cAMP)-regulated phosphoprotein of an apparent molecular weight of 32,000 (DARPP-32) was investigated in mouse, rat, rabbit, guinea pig, cat, monkey (Macaca fascicularis and Marmoset) and human adrenal gland by means of indirect immunofluorescence histochemistry. DARPP-32-like immunoreactivity (-LI) was demonstrated in chromaffin cells in the adrenal medulla of rabbit, guinea-pig, cat, monkey and human, but not in mouse or rat. In the Marmoset monkey, DARPP-32-LI was also observed in the zona glomerulosa of the adrenal cortex. It has been shown that dopamine and dopaminergic agonists inhibit catecholamine release from chromaffin cells and aldosterone secretion from cells in the adrenal cortex. The present results suggest that DARPP-32, an intracellular third messenger for dopamine, may be part of the signal transduction mechanism for dopamine acting on the adrenal gland.

Identification of a dopamine- and 3'5'-cyclic adenosine monophosphate-regulated phosphoprotein of 32 kD (DARPP-32) in parathyroid hormone-producing cells of the human parathyroid gland

The cellular localization of DARPP-32, a dopamine- and cyclic AMP-regulated phosphoprotein of 32 kD, enriched in dopamine-innervated brain regions, was investigated in the human parathyroid gland using indirect immunofluorescence histochemistry. Monoclonal antibodies were used to demonstrate DARPP-32-like immunoreactivity (LI) in chief cells of the normal human parathyroid gland and in cells of human parathyroid adenoma. Direct double-labelling revealed coexistence of DARPP-32-LI with parathyroid hormone (PTH)-LI. It has previously been demonstrated that dopamine D1-receptors are present in the parathyroid gland and that dopamine and D1-agonists stimulate the release of PTH. The present results suggest that DARPP-32 may play a role in the cellular mechanisms leading to dopamine-induced PTH secretion.

Immunocytochemical localization of phosphatase inhibitor-1 in rat brain

The localization of phosphatase inhibitor-1 was investigated in rat brain by use of immunocytochemistry. Studies were performed with an affinity purified IgG raised against purified rabbit skeletal muscle inhibitor-1. In rat brain tissue homogenates, this antibody reacted only with a 29 kDa protein corresponding to inhibitor-1. Immunocytochemical studies with this antibody revealed numerous immunoreactive cell bodies and fibers. The highest concentration of immunoreactive perikarya was observed in the caudate-putamen and nucleus accumbens, and these appeared to be exclusively medium-sized neurons. Other areas containing substantial populations of immunoreactive neurons included the suprachiasmatic nucleus of the hypothalamus, lateral hypothalamus, horizontal limb of the diagonal band of Broca, dentate gyrus of the hippocampal formation, habenula, superior colliculus, claustrum, endopiriform nuclei, and neocortex. The distribution of terminals containing inhibitor-1 coincided with the distribution of terminal fields known to originate from the above regions. Thus, plexuses of immunoreactive axons were seen in the globus pallidus, substantia nigra pars reticulata, paraventricular hypothalamus, dorsal thalamus, CA3 region of the hippocampus, and interpeduncular nucleus. These results demonstrate that phosphatase inhibitor-1, a cyclic AMP-regulated inhibitor of phosphatase-1, is differentially distributed in the rat CNS. Given the widespread role of protein phosphorylation and dephosphorylation in intracellular signal transduction, these results suggest that neurons containing high levels of inhibitor-1 may share common, hitherto unrecognized, properties in terms of neurotransmitter regulation and/or responsiveness.

Localization of immunoreactive lipocortin-1 in the brain and pituitary gland of the rat. Effects of adrenalectomy, dexamethasone and colchicine treatment

The presence and localization of endogenous lipocortin-1 (LC-1, a protein which has been proposed to mediate the anti-inflammatory actions of the glucocorticoids) was studied by immunohistochemical techniques in rat brain and pituitary. A polyclonal antiserum specific for a fragment of lipocortin-1 (alpha alpha 1-188) was used to visualize immunoreactive LC-1 (iLC-1) in both neuronal and non-neuronal cell structures. Neuronal staining, which was independent of microtubular axonal transport mechanisms (in that it was not affected by blockade of axonal transport), was found in varicose nerve fibres in various regions of the brain. In addition, iLC-1 was found in the cytoplasm of neuronal cells throughout the brain. Of all brain regions which showed iLC-1, only the hippocampal neurons showed a reduced staining intensity after adrenalectomy. However, iLC-1 was not affected by dexamethasone treatment. Non-neuronal iLC-1 was found in ependymocytes lining the cerebral ventricles and aqueduct. In addition, iLC-1 was found in tanycytes in all circumventricular organs studied and in the ventral walls of the third ventricle, where some of the branching tail processes appeared to envelop local capillaries and neuronal cell bodies. A tancycyte-mediated release of LC-1 from varicose nerve fibres into the portal vasculature is proposed.