Distribution of orexin-A-like immunoreactivity (ORX-LI) in rat brains was investigated with the use of a rabbit polyclonal antibody against the full length peptide orexin A. Virtually all the ORX-LI cell bodies were observed in the lateral hypothalamus at the level of median eminence. The large majority of ORX-LI neurons appeared spherical or fusiform, 20-30 μm in diameter and issued two to five cell processes with few secondary branchings. Numerous ORX-LI fibers were observed in subregions of the hypothalamus. ORX- LI cell processes were sparsely distributed in the cortex, hippocampus and thalamus. Many varicose ORX-LI cell processes were situated close to the 3rd and lateral ventricles, some of which appeared to be protruding into the lumen. As a corollary, orexin A may be released into the ventricles and interact with neurons in distant targets, in addition to influencing the activity of neurons with which ORX-LI axons make synaptic contacts.

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Yes / Tanycytes play multiple roles in hypothalamic functions, including sensing peripheral nutrients and metabolic hormones, regulating neurosecretion and mediating seasonal cycles of reproduction and metabolic physiology. This last function reflects the expression of TSH receptors in tanycytes, which detect photoperiod-regulated changes in TSH secretion from the neighbouring pars tuberalis. The present overall aim was to determine the signal transduction pathway by which TSH signals in tanycytes. Expression of the TSH receptor in tanycytes of 10-day-old Sprague Dawley rats was observed by in situ hybridisation. Primary ependymal cell cultures prepared from 10-day-old rats were found by immunohistochemistry to express vimentin but not GFAP and by PCR to express mRNA for Dio2, Gpr50, Darpp-32 and Tsh receptors that are characteristic of tanycytes. Treatment of primary tanycyte/ependymal cultures with TSH (100 IU/l) increased cAMP as assessed by ELISA and induced a cAMP-independent increase in the phosphorylation of ERK1/2 as assessed by western blot analysis. Furthermore, TSH (100 IU/l) stimulated a 2.17-fold increase in Dio2 mRNA expression. We conclude that TSH signal transduction in cultured tanycytes signals via Gαs to increase cAMP and via an alternative G protein to increase phosphorylation of ERK1/2.

Menopause is a natural transition heralded by the cessation of menstrual cycles and ovulation, and it occurs in all women at an average of about 50 years of age. While not a disease, menopause is often accompanied by symptoms that interfere with the quality of life and these symptoms are due to the relatively abrupt deprivation of E2 and P4 experienced during reproductive aging. Reproductive aging consists of changes in the synthesis and release of hormones from the hypothalamus, pituitary and gonad, which make up the HPG axis. Because gonadal hormones play critical roles in many systems throughout the body and brain, not just reproduction, treatment of menopausal symptoms to date largely involves hormone replacement therapy (HRT) with E2, P4 or their combination. While not intended to treat other neurobiological symptoms beyond hot flushes, HRT has the potential to exert widespread actions due to the abundance of hormone receptors throughout the nervous system. Thus, a fuller understanding of the neurobiology of menopause is badly needed. Although much of the research into the mechanisms that underlie reproductive aging focuses on ovarian failure and follicular atresia (cell death), there is evidence that there are significant alterations in the function of the neuroendocrine levels - the hypothalamus and pituitary - that also contribute to this process. As the mean age of the population increases, the number of post-menopausal women continues to grow with broad economic, healthcare and social costs. It is increasingly important to understand the complex mechanisms underlying reproductive aging and the effects of HRT. In this dissertation, I focus on the question of how the female non-human primate hypothalamus changes both with aging and in response to steroid hormone treatments. / text

Muscarinic ligand binding sites in fragments of rat hypothalamus and pituitary were studied using [3H]quinuclidinyl benzilate (QNB). In the hypothalamus, the highest amount of specific QNB binding was to n. paraventricularis and n. dorsomedialis. Specific QNB binding in other hypothalamic regions varied within a relatively narrow range. Fragments of whole pituitary also bound QNB but to a much smaller degree than brain. Pituitary binding of QNB was blocked by atropine but not by hexamethonium or d-tubocurarine. Within the pituitary, specific QNB binding to posterior pituitary was three times greater than to anterior pituitary. These findings are consistent with the operation of cholinergic mechanisms in hypothalamic and pituitary function.

Yes / Hypothalamic tanycytes in median eminence (ME) are emerging as a crucial cell population that regulates endocrine output, energy balance and the diffusion of blood-born molecules. Tanycytes have recently been considered as potential somatic stem cells in the adult mammalian brain, but their regenerative and tumorigenic capacities are largely unknown. Here we found that Rax+ tanycytes in ME of mice are largely quiescent but quickly enter the cell cycle upon neural injury for self-renewal and regeneration. Mechanistically, Igf1r signaling in tanycytes is required for tissue repair under injury conditions. Furthermore, Braf oncogenic activation is sufficient to transform Rax+ tanycytes into actively dividing tumor cells that eventually develop into a papillary craniopharyngioma-like tumor. Together, these findings uncover the regenerative and tumorigenic potential of tanycytes. Our study offers insights into the properties of tanycytes, which may help to manipulate tanycyte biology for regulating hypothalamic function and investigate the pathogenesis of clinically relevant tumors.

Neuropeptide W-23 (NPW23) and neuropeptide W-30 (NPW30) are 23- and 30-amino acid peptides recently isolated from the porcine hypothalamus. Immunohistochemical studies using a rabbit polyclonal antiserum against the rat NPW23 peptide revealed a limited distribution in the rat brain. NPW23-immunoreactive (irNPW) cells were detected in the paraventricular nucleus (PVH), mainly in the parvocellular division, supraoptic nucleus (SO), accessory neurosecretory nuclei, dorsal and lateral hypothalamic areas, perifornical nucleus, arcuate nucleus, and anterior and posterior pituitary; whereas, irNPW fibers were noted in the PVH and SO, retrochiasmatic nucleus, dorsal and lateral hypothalamic areas, median eminence, amygdala, and posterior pituitary. The pattern of distribution of irNPW in the hypothalamus corroborates a possible role of NPW on prolactin release and feeding behavior reported by others.

Distribution of the novel peptide beacon in the hypothalamus of Sprague-Dawley rats was examined by immunohistochemical methods. Beacon-immunoreactive (irBC) neurons were found in the paraventricular, supraoptic, and accessory neurosecretory nuclei, and intensely labeled fibers in the median eminence and infundibulo-pituitary stalk. Scattered cells and/or fibers were noted in the suprachiasmatic nucleus, arcuate nucleus, retrochiasmatic area, lateral and medial preoptic area, as well as anterior and lateral hypothalamic area. The wide distribution of irBC in the hypothalamus of Sprague-Dawley rats suggests that the peptide may influence, in addition to a proposed role in feeding, a multitude of biological activities associated with the hypothalamic-pituitary axis.

Female reproductive senescence is widespread among mammalian species, but menopause is limited to species with menstrual cycles. While hormonal changes at menopause have profound impacts in the lives of women at middle age, the complex mechanisms underlying this process remain obscure. All three levels of the hypothalamic-pituitary-gonadal (HPG) axis are involved in reproductive aging, and evidence highlights a critical role for the dysregulation of gonadotropin-releasing hormone (GnRH) neurons, the hypothalamic cells that drive reproductive function. To investigate neuroendocrine mechanisms that may initiate and perpetuate reproductive decline at each step in the transition to acyclicity, I utilized an ovarian-intact middle-aged female rat model of natural reproductive senescence. These studies focused on three hypothalamic nuclei that are known to control GnRH activity: the anteroventral periventricular nucleus (AVPV), the site of positive hormone feedback onto GnRH neurons; the arcuate nucleus (ARC), the site of negative feedback; and the median eminence (ME), the site of GnRH release, with the following specific aims: 1) Characterize neuroendocrine gene and protein expression in female rats throughout the natural transition to acyclicity; 2) Determine the effects of chronic N-methyl-D-asparate receptor subunit 2b (NMDAR-NR2b) inhibition in acyclic females; and 3) Examine neuroendocrine gene expression during premature reproductive senescence after perturbation of the HPG axis. The results of these studies identified novel molecular and cellular changes with age and reproductive cycle status in the ARC and ME, two regions that are underappreciated for their roles in reproductive senescence. Surprisingly, few molecular targets were identified in the AVPV, a region that is much better-studied in this context. In the ME and ARC, I found changes in transcription factors and evidence of altered hormone feedback via changes in sex steroid hormone receptors and enzyme expression with reproductive aging. I also discovered decreased expression of genes for the excitatory neuropeptides, kisspeptin and neurokinin B, as well as decreased percentage of kisspeptin immunoreactive cells and their co-expression with estrogen receptor alpha in the ARC. And finally, in the ME, neurotrophic factor expression was changed with age, and the presence and phosphorylation state of the NR2b subunit of the NMDA receptor contributes to a greater inhibitory tone with acyclicity. Together these studies have identified novel pathways, especially in the ARC and ME, that are related to reproductive decline. Furthermore, changes in the hypothalamic neural and glial network of neurotransmitters, neuropeptides, hormone receptors and other transcription factors are likely contributing to altered responses to hormonal feedback and decreased excitatory drive for GnRH release. / text

L’hypothalamus médiobasal contient de nombreux noyaux régulant l’homéostasie énergétique en réponse aux variations des signaux métaboliques périphériques, telles que les nutriments et les hormones, l’informant de l’état énergétique de l’individu. Parmi ces noyaux, le noyau arqué hypothalamique (NA) est considéré comme le noyau clé de la régulation de la prise alimentaire. En effet, il est capable de recevoir et d’intégrer les informations métaboliques périphériques, pour ensuite les relayer vers les autres noyaux hypothalamiques régulant la prise alimentaire. Dans ce contexte, l’accès des molécules périphériques au NA est une étape importante dans la régulation de la prise alimentaire. L’organisation des interfaces sang/cerveau à ce niveau est d’ailleurs très particulière, suggérant une régulation spécifique de l’accès des molécules périphériques vers le NA. En effet, deux types de vaisseaux sont retrouvés dans cette région cérébrale : 1- les vaisseaux de la barrière hématoencéphalique (BHE) dans le NA et 2- les vaisseaux fenêtrés dans l’éminence médiane (EM), un organe circumventriculaire (OCV) adjacent au NA. Alors que les vaisseaux de la BHE présentent des propriétés de barrière et régulent les échanges sang/NA, les vaisseaux de l’EM possèdent de nombreuses fenestrations facilitant les échanges sang/EM. Ces deux types de vaisseaux ont la particularité d’être contactés par des cellules épendymaires hautement spécialisées formant le bas du 3ème ventricule. Ces cellules, appelées « tanycytes », expriment des protéines de jonctions serrées suggérant leur participation à la régulation des échanges sang/cerveau dans cette région cérébrale. En effet, des études menées au sein du laboratoire ont montré que les tanycytes de l’EM, contactant les vaisseaux fenêtrés, expriment des protéines de jonctions serrées (JS) organisées en ceinture continue autour de leur pôle apical. Ces JS créent ainsi un épendyme étanche qui limite les échanges EM/LCR. A l’inverse, les tanycytes du NA, contactant les vaisseaux de la BHE, expriment des protéines de JS non organisées en leur pôle apical. L’épendyme du NA est ainsi perméable et favorise les échanges LCR/NA. Le but de mon travail de thèse a donc été de comprendre, en prenant en compte tous ces éléments -c’est-à-dire la présence de vaisseaux fenêtrés, de vaisseaux de la BHE et des tanycytes -, comment est organisé l’accès des signaux métaboliques périphériques vers le NA et si cet accès pouvait être modulé afin de contrôler l’homéostasie énergétique. Nos expériences ont montré que, chez la souris mâle adulte, une glucopénie induite par le jeûne ou par le 2-desoxyglucose induisait une réorganisation structurale des vaisseaux et de l’épendyme au niveau de l’EM et du NA, modifiant ainsi les échanges sang/cerveau. En effet, chez ces souris, nous avons observé une augmentation du nombre de vaisseaux fenêtrés au niveau de l’EM et du NA, ainsi qu’une réorganisation fonctionnelle des protéines de JS au niveau du ventricule : les tanycytes du NA, contactant des vaisseaux fenêtrés à présent, réorganisent leurs protéines de jonctions serrées (JS) afin d’assurer l’homéostasie cérébrale. Ces réorganisations induisent alors un meilleur accès des molécules périphériques vers le NA. De plus, nos résultats ont montré que cette plasticité est induite par le VEGF-A, produit localement par les tanycytes. En effet, la neutralisation du VEGF-A bloque la plasticité de l’EM/NA induite par l’hypoglycémie et perturbent la réponse physiologique hyperphagique lors de la réalimentation. Enfin, nos données supplémentaires indiquent que cette plasticité de l’EM/NA est conservée dans différents modèles alimentaires et se produit également au cours de la journée, suggérant son implication dans le contrôle circadien du comportement alimentaire. / The mediobasal hypothalamus contains numerous nuclei regulating energy homeostasis in response to peripheral metabolic signals. Among these nuclei, the arcuate nucleus of the hypothalamus (ARH) is considered to be a critical component of the neural circuits regulating energy balance. Indeed, the ARH is able to integrate the metabolic information carried by nutrients and peripheral hormones, and to transmit their message to secondary nuclei regulating food intake. Therefore, the delivery of peripheral molecules conveying metabolic information to the ARH is a critical step in the regulation of food intake. At the level of the ARH, the organization of blood-brain interface is indeed peculiar. Both the vessels of the blood brain barrier (BBB) and the fenestrated vessels of the median eminence (ME), a circumventricular organ adjacent to the ARH, represent two pathways by which peripheral molecules may reach the ARH. While BBB vessels possess barrier properties restricting the access of peripheral molecules to the ARH, the vessels in the ME possess numerous fenestrations facilitating blood-brain exchanges. These two types of vessels are contacted by specialized ependymal cells lining the floor of the 3rd ventricle called tanycytes. Tanycytes express tight junction (TJ) proteins providing a putative role in the regulation of blood/brain exchanges. While ME tanycytes contact the fenestrated vessels which lie in the ME, they express TJ proteins organized as a continuous belt around their apical pole, creating a tight ependyma limiting ME/cerebrospinal fluid exchanges. In contrast, ARH tanycytes contact BBB vessels and express TJ proteins in a disorganized pattern thus creating a permeable ependyma allowing CSF/ARH exchanges. In the following studies we wished to examine how peripheral signals may reach the ARH through this peculiar blood-brain interface. Moreover, we wished to determine whether the blood-brain interface undergoes dynamic remodeling according to the energetic status of individual. To these aims the plasticity of the BBB vessels, fenestrated vessels of the ME, and tanycytes were analyzed under various metabolic challenges. Our studies show that both fasting- or 2-deoxyglucose-induced glucopenia induce vascular and ependymal reorganization in the ME and the ARH. This reorganization is characterized by an increase in the number of fenestrated vessels in the ME and in the ARH, and a reorganization of TJ proteins in ARH tanycytes, and led to improved access of peripheral molecules to the ARH. Furthermore, our results reveal that VEGF-A expression in tanycytes modulates the plasticity at the blood/brain interface. Indeed, the neutralization of VEGF signaling blocks fasting-induced barrier remodeling and significantly impairs the physiological response to refeeding. Finally, our supplementary results show that the ME-ARH reorganization is also observed in mice under different diet, as well as implicated in the circadian regulation of food intake. Strikingly, the ME-ARH organization is disturbed in diet-induced obese mice, which could be the origin of hormonal resistances observed in these mice. Altogether, our results suggest a new concept in the regulation of the food intake: peripheral glucose modulates blood/brain interface in the ME through a VEGF-dependent mechanism to improve the access of the metabolic signals towards the ARH. As other circumventricular organs possess a similar organization, characterized by fenestrated vessels and a tanycyte barrier, these exciting results pave the way to for future studies examining the remodeling of the blood-brain interface and its role in other neuroendocrine processes.