Mechanisms of glial retraction in the hypothalamo-neurohypophysial system of the rat

Aquaporins: Multiple Roles in the Central Nervous System

Aquaporins (AQPs) represent a diverse family of membrane proteins found in prokaryotes and eukaryotes. The primary aquaporins expressed in the mammalian brain are AQP1, which is densely packed in choroid plexus cells lining the ventricles, and AQP4, which is abundant in astrocytes and concentrated especially in the end-feet structures that surround capillaries throughout the brain and are present in glia limitans structures, notably in osmosensory areas such the supraoptic nucleus. Water movement in brain tissues is carefully regulated from the micro- to macroscopic levels, with aquaporins serving key roles as multifunctional elements of complex signaling assemblies. Intriguing possibilities suggest links for AQP1 in Alzheimer's disease, AQP4 as a target for therapy in brain edema, and a possible contribution of AQP9 in Parkinson's disease. For all the aquaporins, new contributions to physiological functions are likely to continue to be discovered with ongoing work in this rapidly expanding field of research. NEUROSCIENTIST 13(5):470—485, 2007.

Morphological plasticity of astroglia: Understanding synaptic microenvironment

Memory formation in the brain is thought to rely on the remodeling of synaptic connections which eventually results in neural network rewiring. This remodeling is likely to involve ultrathin astroglial protrusions which often occur in the immediate vicinity of excitatory synapses. The phenomenology, cellular mechanisms, and causal relationships of such astroglial restructuring remain, however, poorly understood. This is in large part because monitoring and probing of the underpinning molecular machinery on the scale of nanoscopic astroglial compartments remains a challenge. Here we briefly summarize the current knowledge regarding the cellular organisation of astroglia in the synaptic microenvironment and discuss molecular mechanisms potentially involved in use-dependent astroglial morphogenesis. We also discuss recent observations concerning morphological astroglial plasticity, the respective monitoring methods, and some of the newly emerging techniques that might help with conceptual advances in the area.

Astrocyte-neuron interplay in maladaptive plasticity

The complexity of neuronal networks cannot only be explained by neuronal activity so neurobiological research in the last decade has focused on different components of the central nervous system: the glia. Glial cells are fundamental elements for development and maintenance of physiological brain work. New data confirm that glia significantly influences neuronal communication through specific molecules, named "gliotransmitters", and their related receptors. This new approach to the traditional model of the way synapses work is also supported by changes occurring in pathological conditions, such as neurodegenerative diseases or toxic/traumatic injury to nervous system. Experimental models have revealed that glial cells are the starting point of damage progression that subsequently involves neurons. The "bedside to bench" approach has demonstrated that clinical phenotypes are strictly related to neuronal death, however it is conceivable that the disease begins earlier, years before clinical onset. This temporal gap is necessary to determine complex changes in the neuro-glial network organization and produce a "maladaptive plasticity". We review the function of glial cells in health and disease, pointing the putative mechanisms of maladaptive plasticity, suggesting that glial cells may represent a fascinating therapeutic target to prevent irreversible neuronal cell death.

Astrocyte-synapse structural plasticity

The function and efficacy of synaptic transmission are determined not only by the composition and activity of pre- and postsynaptic components but also by the environment in which a synapse is embedded. Glial cells constitute an important part of this environment and participate in several aspects of synaptic functions. Among the glial cell family, the roles played by astrocytes at the synaptic level are particularly important, ranging from the trophic support to the fine-tuning of transmission. Astrocytic structures are frequently observed in close association with glutamatergic synapses, providing a morphological entity for bidirectional interactions with synapses. Experimental evidence indicates that astrocytes sense neuronal activity by elevating their intracellular calcium in response to neurotransmitters and may communicate with neurons. The precise role of astrocytes in regulating synaptic properties, function, and plasticity remains however a subject of intense debate and many aspects of their interactions with neurons remain to be investigated. A particularly intriguing aspect is their ability to rapidly restructure their processes and modify their coverage of the synaptic elements. The present review summarizes some of these findings with a particular focus on the mechanisms driving this form of structural plasticity and its possible impact on synaptic structure and function.

Architecture of the hypothalamo-posthypophyseal complex is controlled by monoamines

The hypothalamo-neurohypophyseal system displays significant plasticity when subjected to physiological stimuli, such as dehydration, parturition, or lactation. This plasticity arises at the neurochemical and electrophysiological levels but also at a structural level. Several studies have demonstrated the role of monoaminergic afferents in controlling neurochemical and electrophysiological plasticity of the supraoptic nucleus (SON) and of the neurohypophysis (NH), but little is known about how the changes in structural plasticity are triggered. We used Tg8 mice, disrupted for the monoamine oxidase A gene, to study monamine involvement in the architecture of the SON and of the NH. SON astrocytes in Tg8 mice displayed an active status, characterized by an increase in S100β expression and a significant decrease in vimentin expression, with no modification in glial fibrillary acidic protein (GFAP) levels. Astrocytes showed a decrease in glutamate dehydrogenase (GDH) levels, whereas glutamine synthetase (GS) levels remained constant, suggesting a reduction in astrocyte glutamate catabolism. Tenascin C and polysialic acid-neural cell adhesion molecule (PSA-NCAM) expressions were also elevated in the SON of Tg8 mice, suggesting an increased capacity for structural remodelling in the SON. In the NH, similar date were obtained with a stability in GFAP expression and an increase in PSA-NCAM immunostaining. These results establish monoamine (serotonin and noradrenaline) involvement in SON and NH structural arrangement. Monoamines therefore appear to be crucial for the coordination of the neurochemical and structural aspects of neuroendocrine plasticity, allowing the hypothalamo-neurohypopyseal system to respond appropriately when stimulated.

Effects of vagotomy, splanchnic nerve lesion, and fluorocitrate on the transmission of acute hyperosmotic stress signals to the supraoptic nucleus

The response to hyperosmotic stresses in the abdominal cavity is regulated, in part, by vasopressin (VP)-secreting neurons in the supraoptic nucleus (SON). How osmotic stress signals are transmitted to the brain is incompletely understood, and whether the transmission routes for osmotic stress signals differ between acute and chronic stresses is unknown. Here we investigated the role of the vagus, splanchnic nerves, and astrocytes in the SON in transducing acute hyperosmotic-stress signals from the abdominal cavity. We found that acute administration of hyperosmotic saline triggered the activation of neurons as well as astrocytes in the SON and the adjoining ventral glia limitans (SON-VGL). Severing the subdiaphragmatic vagal nerve (SDV) prevented the normal response of cells in the SON to HS treatment and attenuated the release of VP into the bloodstream. Lesioning the splanchnic nerves (SNL) diminished HS-induced release of VP, but to a much lesser extent than SDV. Furthermore, SNL did not significantly affect the up-regulation of Fos in SON neurons or the up-regulation of Fos and GFAP in SON and SON-VGL astrocytes that normally occurred in response to HS and did not affect HS-induced expansion of the SON-VGL. Inhibiting astrocytes with fluorocitrate (FCA) prevented the response of the SON to HS and attenuated the release of VP, similarly to SDV surgery. These results suggest that the vagus is the principle route for the transmission of hyperosmotic signals to the brain and that astrocytes in the SON region are necessary for the activation of SON neurons and the release of VP into the bloodstream.

Acute hyperosmotic stimulus-induced Fos expression in neurons depends on activation of astrocytes in the supraoptic nucleus of rats

Acute hyperosmolarity induced a time-dependent expression of Fos protein in both neurons and astrocytes of the rat supraoptic nucleus, with peak Fos expression occurring at 45 min in astrocytes and at 90 min in neurons after hypertonic stimulation in vivo. To determine whether the two cell types were activated separately or in an integrated manner, animals were pretreated with fluorocitrate, a glial metabolic blocker or carbenoxolone, a gap junction blocker followed by an acute hypertonic stimulation similar to that of the controls. Antibodies against glial fibrillary acidic protein, connexin 43, vasopressin, and oxytocin were used in serial sections to identify the cellular elements of the supraoptic nucleus. It was found that interruption of astrocyte metabolism with fluorocitrate significantly reduced Fos protein expression in both astrocytes and neurons, whereas blockage of gap junctions with carbenoxolone clearly reduced Fos protein expression in neurons, but not in astrocytes. These results indicate that both neurons and astrocytes in the rat supraoptic nucleus are involved in regulating osmolarity. Astrocytes are activated first, whereas connexin 43 functional hemichannels in SON astrocytes are required for the subsequent activation of the neurons.

CD38/cyclic ADP-ribose system: a new player for oxytocin secretion and regulation of social behaviour

Oxytocin is important for regulating a number of physiological processes. Disruption of the secretion, metabolism or action of oxytocin results in an impairment of reproductive function, social and sexual behaviours, and stress responses. This review discusses current views on the regulation and autoregulation of oxytocin release in the hypothalamic-neurohypophysial system, with special focus on the activity of the CD38/cADP-ribose system as a new component in this regulation. Data from our laboratories indicate that an impairment of this system results in alterations of oxytocin secretion and abnormal social behaviour, thus suggesting new clues that help in our understanding of the pathogenesis of neurodevelopmental disorders.

Neonatal separation stress reduces glial fibrillary acidic protein- and S100beta-immunoreactive astrocytes in the rat medial precentral cortex

The interactions between the mother/parents and their offspring provides socioemotional input, which is essential for the establishment and maintenance of synaptic networks in prefrontal and limbic brain regions. Since glial cells are known to play an important role in developmental and experience-driven synaptic plasticity, the effect of an early adverse emotional experience induced by maternal separation for 1 or 6 h on the expression of the glia specific proteins S100beta and glial fibrillary acidic protein (GFAP) was quantitatively analyzed in anterior cingulate cortex, hippocampus, and precentral medial cortex. Three animal groups were analyzed at postnatal day 14: (i) separated for 1 h; (ii) separated for 6 h; (iii) undisturbed (control). Twenty-four hours after stress exposure, the stressed brains showed significantly reduced numbers of S100beta-immunoreactive (ir) cells in the anterior cingulate cortex (6-h stress) and in the precentral medial cortex (1- and 6-h stress). Significantly reduced numbers of GFAP-ir cells were observed only in the medial precentral cortex (1- and 6-h stress); no significant changes were observed in the anterior cingulate cortex. No significant changes of the two glial markers were observed in the hippocampus. Double-labeling experiments with GFAP and pCREB revealed pCREB labeling only in the hippocampus, where the stressed brains (1 and 6 h) displayed significantly reduced numbers of GFAP/pCREB-ir glial cells. The observed downregulation of glia-specific marker proteins is in line with our hypothesis that emotional experience can alter glia cell activation in the juvenile limbic system.

Divergent glial fibrillary acidic protein and its mRNA in the activated supraoptic nucleus

Previous studies have shown decreased immunoreactive glial fibrillary acidic protein (GFAP) in the supraoptic nucleus (SON) when magnocellular neuroendocrine cells (MNCs) are activated by lactation or dehydration. This is thought to underlie structural plasticity of glial processes that occurs during these times. Here, we investigated how this apparent reduction in protein relates to GFAP mRNA expression in the dehydrated rat as visualized by in situ hybridization. Densitometry of silver grains in the SON revealed low levels of mRNA expression in control, 2-day dehydrated and 21-day rehydrated (R21) animals. Conversely, the SON from 7-day dehydrated (D7) subjects displayed significantly more silver grains. Thus, the pattern of GFAP mRNA expression is the inverse of what we previously observed for GFAP immunoreactivity in tissue sections of the SON. No differences in mRNA levels due to hydration state were seen in the lateral hypothalamic area, suggesting that increases in GFAP mRNA at D7 were specifically related to MNC activation. These data indicate a divergence in GFAP mRNA and protein expression in the SON.

Deciphering the mechanisms of homeostatic plasticity in the hypothalamo-neurohypophyseal system—genomic and gene transfer strategies

The hypothalamo-neurohypophyseal system (HNS) is the specialised brain neurosecretory apparatus responsible for the production of a peptide hormone, vasopressin, that maintains water balance by promoting water conservation at the level of the kidney. Dehydration evokes a massive increase in the regulated release of hormone from the HNS, and this is accompanied by a plethora of changes in morphology, electrical properties and biosynthetic and secretory activity, all of which are thought to facilitate hormone production and delivery, and hence the survival of the organism. We have adopted a functional genomic strategy to understand the activity dependent plasticity of the HNS in terms of the co-ordinated action of cellular and genetic networks. Firstly, using microarray gene-profiling technologies, we are elucidating which genes are expressed in the HNS, and how the pattern of expression changes following physiological challenge. The next step is to use transgenic rats to probe the functions of these genes in the context of the physiological integrity of the whole organism.

Increased morphological diversity of microglia in the activated hypothalamic supraoptic nucleus

Microglia are the immune cells of the CNS. In the normal adult mammalian brain, the majority of these cells is quiescent and exhibits a ramified morphology. Microglia are perhaps best known for their swift transformation to an activated ameboid morphology in response to pathological insults. Here we have observed the responsiveness of these cells to events surrounding the normal activation of neurosecretory neurons in the hypothalamic supraoptic nucleus (SON), a well studied model of structural plasticity in the CNS. Neurons in the SON were activated by substituting 2% saline for drinking water. Brain sections were collected from four experimental groups [controls (C), 2 d-dehydrated (2D), 7 d-dehydrated (D7), and 7 d-dehydrated/21 d-rehydrated animals (R21)] and stained with Isolectin-B4-HRP to visualize microglial cells. Based on morphological criteria, we quantified ramified, hypertrophied, and ameboid microglia using unbiased stereological techniques. Statistical analyses showed significant increases in the number of hypertrophied microglia in the D2 and D7 groups. Moreover, there was a significant increase in the number of ameboid microglia in the D7 group. No changes were seen across conditions in the number of ramified cells, nor did we observe any significant phenotypic changes in a control area of the cingulate gyrus. Hence, increased morphological diversity of microglia was found specifically in the SON and was reversible with the cessation of stimulation. These results indicate that phenotypic plasticity of microglia may be a feature of the normal structural remodeling that accompanies neuronal activation in addition to the activation that accompanies brain pathology.

The effects of nitric oxide on magnocellular neurons could involve multiple indirect cyclic GMP-dependent pathways

Nitric oxide (NO) is known to regulate the release of arginine-vasopressin (AVP) and oxytocin (OT) by the paraventricular nucleus (PVN) and the supraoptic nucleus (SON). The aim of the current study was to identify in these nuclei the NO-producing neurons and the NO-receptive cells in mice. The determination of NO-synthesizing neurons was performed by double immunohistochemistry for the neuronal form of NO synthase (NOS), and AVP or OT. Besides, we visualized the NO-receptive cells by detecting cyclic GMP (cGMP), the major second messenger for NO, by immunohistochemistry on hypothalamus slices. Neuronal NOS was exclusively colocalized with OT in the PVN and the SON, suggesting that NO is mainly synthesized by oxytocinergic neurons in mice. By contrast, cGMP was not observed in magnocellular neurons, but in GABA-, tyrosine hydroxylase- and glutamate-positive fibers, as well as in GFAP-stained cells. The cGMP-immunostaining was abolished by incubating brain slices with a NOS inhibitor (L-NAME). Consequently, we provide the first evidence that NO could regulate the release of AVP and OT indirectly by modulating the activity of the main afferents to magnocellular neurons rather than by acting directly on magnocellular neurons. Moreover, both the NADPH-diaphorase activity and the mean intensity of cGMP-immunofluorescence were increased in monoamine oxidase A knock-out mice (Tg8) compared to control mice (C3H) in both nuclei. This suggests that monoamines could enhance the production of NO, contributing by this way to the fine regulation of AVP and OT release and synthesis.

A brain adaptation view of plasticity: is synaptic plasticity an overly limited concept?

A view that is emerging is that the brain has multiple forms of plasticity that must be governed, at least in part, by independent mechanisms. This view is illustrated by: (1) the apparent separate governance of some non-neural changes by activity, in contrast to synaptic changes driven by learning; (2) the apparent independence of different kinds of synaptic changes that occur in response to the learning aspects of training; (3) the occurrence of separate patterns of synaptic plasticity in the same system in response to different task demands; and (4) apparent dissociations between behaviorally induced synaptogenesis and LTP. The historical focus of research and theory in areas ranging from learning and memory to experiential modulation of brain development has been heavily upon synaptic plasticity since shortly after the discovery of the synapse. Based upon available data, it could be argued that: (1) synaptic, and even neuronal, plasticity is but a small fraction of the range of changes that occur in response to experience; and (2) we are just beginning to understand the importance of these other forms of brain plasticity. Appreciation of this aspect of the brain's adaptive process may allow us to better understand the capacity of the brain to tailor a particular set of changes to the demands of the specific experiences that generated them.

Detection of a novel pattern of connexin 43 immunoreactivity responsive to dehydration in rat hypothalamic magnocellular nuclei

Immunocytochemical expression of Connexin 43 (Cx 43) in the rat Supraoptic Nucleus was analyzed following dehydration, using sequence-specific anti-Cx 43 antibodies (designated 13-8300, 71-0700, and sc-9059) that exhibit differential recognition of Cx 43. Punctate and longitudinally arranged immunostaining patterns of Cx 43 labeling, as evidenced by antibody sc-9059, was detected overlaying the nucleus of magnocellular neuroendocrine cells. This novel form of longitudinally arranged Cx 43 immunoreactivity was modified by dehydration and halothane exposure, but not lactation.

Expression of high levels of tubulin and microtubule-associated protein 2d in the neurohypophysial astrocytes of adult rat

The hypothalamo-neurohypophysial system, containing arginine vasopressin and oxytocin, is well known to show reversible morphological reorganization for both neurons and glial cells during chronic physiological stimulation. To determine the molecular background for these morphological changes, we investigated the expression of tubulin and microtubule-associated protein (MAP) 2d in the neurohypophysial astrocytes, pituicytes of adult rats by using reverse transcription-polymerase chain reaction, western blot, and immunohistochemistry. The mRNA of MAP2d was expressed at higher levels than that of MAP2c in the neurohypophysis, cerebral cortex, and cerebellum. In contrast, predominant expression of mRNA of MAP2c was detected in the olfactory bulb. Western blot analysis showed the presence of MAP2d in the neurohypophysis, however the amount was below the detection level in the cerebral cortex and cerebellum. A double labeling study using a confocal laser scanning microscope showed intense tubulin immunoreactivity in the glial fibrillary acidic protein (GFAP)-positive pituicytes of the intact neurohypophysis. Almost no tubulin immunoreactivity was observed in the astrocytes of the intact cerebral cortex, cerebellum, and supraoptic nucleus, in contrast to strong tubulin immunoreactivity in neuronal dendrites and somata. Interestingly, intense tubulin immunoreactivity was also observed in the GFAP-positive reactive astrocytes in the immediate vicinity of the artificial lesion of the cerebral cortex. Electron microscopic observation further demonstrated the presence of a lot of microtubules in the pituicytes of intact rats.The present results demonstrate that pituicytes in the adult rat neurohypophysis expresses high levels of tubulin and MAP2d compared with normal brain astrocytes, and suggest that the ability of astrocytic morphological alteration may be at least partly ascribed to this high expression of microtubule proteins.

Integration of sodium and osmosensory signals in vasopressin neurons

Vasopressin (antidiuretic hormone) release has been thought to be controlled by interacting osmoreceptors and Na(+)-detectors for > 20 years. Only recently, however, have molecular and cellular advances revealed how changes in the external concentration of Na+ and osmolality are detected during acute and chronic osmotic perturbations. In rat vasopressin-containing neurons, local osmosensitivity is conferred by intrinsic stretch-inactivated cation channels and by taurine release from surrounding glia. Na+ detection is accomplished by acute regulation of the permeability of stretch-inactivated channels and by changes in Na+ channel gene expression. These features provide a first glimpse of the integrative processes at work in a central osmoregulatory reflex.