Neuronal/glial plasticity in the supraoptic dendritic zone in response to acute and chronic dehydration

Astroglial Regulation of Magnocellular Neuroendocrine Cell Activities in the Supraoptic Nucleus

Studies on the interactions between astrocytes and neurons in the hypothalamo-neurohypophysial system have significantly facilitated our understanding of the regulation of neural activities. This has been exemplified in the interactions between astrocytes and magnocellular neuroendocrine cells (MNCs) in the supraoptic nucleus (SON), specifically during osmotic stimulation and lactation. In response to changes in neurochemical environment in the SON, astrocytic morphology and functions change significantly, which further modulates MNC activity and the secretion of vasopressin and oxytocin. In osmotic regulation, short-term dehydration or water overload causes transient retraction or expansion of astrocytic processes, which increases or decreases the activity of SON neurons, respectively. Prolonged osmotic stimulation causes adaptive change in astrocytic plasticity in the SON, which allows osmosensory neurons to reserve osmosensitivity at new levels. During lactation, changes in neurochemical environment cause retraction of astrocytic processes around oxytocin neurons, which increases MNC's ability to secrete oxytocin. During suckling by a baby/pup, astrocytic processes in the mother/dams exhibit alternative retraction and expansion around oxytocin neurons, which mirrors intermittently synchronized activation of oxytocin neurons and the post-excitation inhibition, respectively. The morphological and functional plasticities of astrocytes depend on a series of cellular events involving glial fibrillary acidic protein, aquaporin 4, volume regulated anion channels, transporters and other astrocytic functional molecules. This review further explores mechanisms underlying astroglial regulation of the neuroendocrine neuronal activities in acute processes based on the knowledge from studies on the SON.

Advances in the neurophysiology of magnocellular neuroendocrine cells

Hypothalamic magnocellular neuroendocrine cells have unique electrical properties and a remarkable capacity for morphological and synaptic plasticity. Their large somatic size, their relatively uniform and dense clustering in the supraoptic and paraventricular nuclei, and their large axon terminals in the neurohypophysis make them an attractive target for direct electrophysiological interrogation. Here, we provide a brief review of significant recent findings in the neuroplasticity and neurophysiological properties of these neurones that were presented at the symposium "Electrophysiology of Magnocellular Neurons" during the 13th World Congress on Neurohypophysial Hormones in Ein Gedi, Israel in April 2019. Magnocellular vasopressin (VP) neurones respond directly to hypertonic stimulation with membrane depolarisation, which is triggered by cell shrinkage-induced opening of an N-terminal-truncated variant of transient receptor potential vanilloid type-1 (TRPV1) channels. New findings indicate that this mechanotransduction depends on actin and microtubule cytoskeletal networks, and that direct coupling of the TRPV1 channels to microtubules is responsible for mechanical gating of the channels. Vasopressin neurones also respond to osmostimulation by activation of epithelial Na⁺ channels (ENaC). It was shown recently that changes in ENaC activity modulate magnocellular neurone basal firing by generating tonic changes in membrane potential. Both oxytocin and VP neurones also undergo robust excitatory synapse plasticity during chronic osmotic stimulation. Recent findings indicate that new glutamate synapses induced during chronic salt loading express highly labile Ca²⁺ -permeable GluA1 receptors requiring continuous dendritic protein synthesis for synapse maintenance. Finally, recordings from the uniquely tractable neurohypophysial terminals recently revealed an unexpected property of activity-dependent neuropeptide release. A significant fraction of the voltage-dependent neurohypophysial neurosecretion was found to be independent of Ca²⁺ influx through voltage-gated Ca²⁺ channels. Together, these findings provide a snapshot of significant new advances in the electrophysiological signalling mechanisms and neuroplasticity of the hypothalamic-neurohypophysial system, a system that continues to make important contributions to the field of neurophysiology.

Control of Long-Term Plasticity by Glutamate Transporters

Activity-dependent long-term changes in synaptic strength constitute key elements for learning and memory formation. Long-term plasticity can be induced in vivo and ex vivo by various physiologically relevant activity patterns. Depending on their temporal statistics, such patterns can induce long-lasting changes in the synaptic weight by potentiating or depressing synaptic transmission. At excitatory synapses, glutamate uptake operated by excitatory amino acid transporters (EAATs) has a critical role in regulating the strength and the extent of receptor activation by afferent activity. EAATs tightly control synaptic transmission and glutamate spillover. EAATs activity can, therefore, determine the polarity and magnitude of long-term plasticity by regulating the spatiotemporal profile of the glutamate transients and thus, the glutamate access to pre- and postsynaptic receptors. Here, we summarize compelling evidence that EAATs regulate various forms of long-term synaptic plasticity and the consequences of such regulation for behavioral output. We speculate that experience-dependent plasticity of EAATs levels can determine the sensitivity of synapses to frequency- or time-dependent plasticity paradigms. We propose that EAATs contribute to the gating of relevant inputs eligible to induce long-term plasticity and thereby select the operating learning rules that match the physiological function of the synapse adapted to the behavioral context.

NMDA receptors potentiate activity-dependent dendritic release of neuropeptides from hypothalamic neurons

KEY POINTS

Using 'sniffer' cell biosensors, we evaluated the effects of specific firing patterns and frequencies on activity-dependent somatodendritic release of vasopressin from paraventricular nucleus neurones. Somatodendritic release of vasopressin was rarely observed during continuous firing but was strengthened by clustered activity. Moreover, release evoked at any given frequency was robustly potentiated by NMDA receptor (NMDAR)-mediated firing. Differently from axonal release, NMDAR activation was necessary for somatodendritic release to occur at physiological firing frequencies, acting thus as a gating mechanism by which activity-dependent release from these two neuronal compartments could be independently regulated. The NMDAR-mediated potentiation was independent of a specific firing pattern and was not accompanied by increased spike broadening, but correlated with higher dendritic Ca²⁺ levels. Our studies provide fundamental novel information regarding stimulus-secretion coupling at somatodendritic compartments, and shed light into mechanisms by which activity-dependent release of neuronal signals from axonal terminals and dendrites could be regulated in a spatially compartmentalized manner.

ABSTRACT

Dendrites are now recognized to be active transmitting neuronal compartments subserving complex brain functions, including motor behaviours and homeostatic neurohumoral responses. Still, the precise mechanisms underlying activity-dependent release of dendritic signals, and how dendritic release is regulated independently from axonal release, remains largely unknown. We used 'sniffer' biosensor cells to enable the measurement and study of activity-dependent dendritic release of vasopressin (VP) from hypothalamic neurones in brain slices. Sniffer_VP responses were dose-dependent, with a threshold detection level of 0.5 nM for VP, being thus a highly sensitive tool to detect endogenous physiological levels of the neuropeptide. Somatodendritic release of VP was rarely observed in response to a burst of action potentials fired in continuous mode, but was strengthened by clustered firing activity. Moreover, release evoked at any given frequency was robustly potentiated when firing was triggered by NMDA receptor (NMDAR) activation. Differently from axonal release, NMDAR activation was necessary for dendritic release to occur at physiological firing frequencies. Thus, we propose that NMDARs may act as a gating mechanism by which activity-dependent release from these two neuronal compartments can be independently regulated. The NMDAR-mediated potentiation of dendritic release was independent of a particular action potential waveform, firing pattern evoked, or a more pronounced spiked broadening, but correlated with higher dendritic Ca²⁺ levels. Overall, our studies provide fundamental novel information regarding stimulus-secretion coupling at neuronal dendrites, and shed light into mechanisms by which activity-dependent release of neuronal signals from axonal terminals and dendrites can be regulated in a spatially compartmentalized manner.

Role of astroglia in diet-induced central neuroplasticity

Obesity, characterized by increased adiposity that develops when energy intake outweighs expenditure, is rapidly becoming a serious health crisis that affects millions of people worldwide and is associated with severe comorbid disorders including hypertension, cardiovascular disease, and type II diabetes. Obesity is also associated with the dysregulation of central neurocircuits involved in the control of autonomic, metabolic, and cognitive functions. Systemic inflammation associated with diet-induced obesity (DIO) has been proposed to be responsible for the development of these comorbidities as well as the dysregulation of central neurocircuits. A growing body of evidence suggests, however, that exposure to a high-fat diet (HFD) may cause neuroinflammation and astroglial activation even before systemic inflammation develops, which may be sufficient to cause dysregulation of central neurocircuits involved in energy homeostasis before the development of obesity. The purpose of this review is to summarize the current literature exploring astroglial-dependent modulation of central circuits following exposure to HFD and DIO, including not only dysregulation of neurocircuits involved in energy homeostasis and feeding behavior, but also the dysregulation of learning, memory, mood, and reward pathways.

Effects of Salt Loading on the Morphology of Astrocytes in the Ventral Glia Limitans of the Rat Supraoptic Nucleus

In the ventral glial limitans (VGL) of the supraoptic nucleus (SON) of the rat, a unique astrocyte type is found with an ability to undergo striking morphological plasticity in response to a wide range of physiological stimulations such as chronic hypernatraemia. This includes a thinning of the VGL, which contains the somata and proximal processes of these astrocytes, as well as an almost complete withdrawal of their vertically-oriented distal processes. Currently, there is little information available on the types of astrocytes that reside in the SON-VGL and which of these exhibit state-dependent structural plasticity. To address this, we enabled the visualisation of single SON-VGL glia using two novel cell labelling techniques with fluorescence microscopy. First, we used an inducible genetic reporter mouse line that allowed the specific labelling of a low density of astrocytes expressing glutamate and aspartate transporter (GLAST)/excitatory amino acid transporter 1. This approach revealed a high degree of variability in the morphology of mouse SON-VGL astrocytes, in contrast to what has been reported for cortical astrocytes. Next, we used the DiOlistlic labelling approach to label single glial cells with DiI in the SON-VGL of rats. Astrocytes observed using this approach shared the morphological features of GLAST-expressing astrocytes in the mouse SON-VGL. Specific structural aspects of these cells were modified by chronic hypernatraemia achieved by 7-day salt loading. Notably, the average area of cells exhibiting protoplasmic features was significantly reduced in the horizontal plane, and the size of varicosities present on fibrous projections was significantly enlarged. These observations indicate that novel cell labelling methods can significantly advance our understanding of SON-VGL cells and reveal specific forms of morphological plasticity that can be driven by chronic hypernatraemia.

The role of astrocytes in the hypothalamic response and adaptation to metabolic signals

The hypothalamus is crucial in the regulation of homeostatic functions in mammals, with the disruption of hypothalamic circuits contributing to chronic conditions such as obesity, diabetes mellitus, hypertension, and infertility. Metabolic signals and hormonal inputs drive functional and morphological changes in the hypothalamus in attempt to maintain metabolic homeostasis. However, the dramatic increase in the incidence of obesity and its secondary complications, such as type 2 diabetes, have evidenced the need to better understand how this system functions and how it can go awry. Growing evidence points to a critical role of astrocytes in orchestrating the hypothalamic response to metabolic cues by participating in processes of synaptic transmission, synaptic plasticity and nutrient sensing. These glial cells express receptors for important metabolic signals, such as the anorexigenic hormone leptin, and determine the type and quantity of nutrients reaching their neighboring neurons. Understanding the mechanisms by which astrocytes participate in hypothalamic adaptations to changes in dietary and metabolic signals is fundamental for understanding the neuroendocrine control of metabolism and key in the search for adequate treatments of metabolic diseases.

Salt-induced sympathoexcitation involves vasopressin V1a receptor activation in the paraventricular nucleus of the hypothalamus

A high-salt diet can lead to hydromineral imbalance and increases in plasma sodium and osmolality. It is recognized as one of the major contributing factors for cardiovascular diseases such as hypertension. The paraventricular nucleus (PVN) plays a pivotal role in osmotically driven sympathoexcitation and high blood pressure, the precise mechanisms of which are not fully understood. Recent evidence indicates that AVP released from magnocellular neurons might be involved in this process. Using a combination of in vivo and in situ studies, we sought to investigate whether AVP, acting on PVN neurons, can change mean arterial pressure (MAP) and sympathetic nerve activity (SNA) in euhydrated male rats. Furthermore, we wanted to determine whether V1a receptors on PVN neurons would be involved in salt-induced sympathoexcitation and hypertension. In rats, 4 days of salt loading (NaCl 2%) elicited a significant increase in plasma osmolality (39 ± 7 mosmol/kgH2O), an increase in MAP (26 ± 2 mmHg, P < 0.001), and sympathoexcitation compared with euhydrated rats. Microinjection of AVP into the PVN of conscious euhydrated animals (100 nl, 3 μM) elicited a pressor response (14 ± 2 mmHg) and a significant increase in lumbar SNA (100 nl, 1 mM) (19 ± 5%). Pretreatment with a V1a receptor antagonist, microinjected bilaterally into the PVN of salt-loaded animals, elicited a decrease in lumbar SNA (-14 ± 5%) and MAP (-19 ± 5 mmHg), when compared with the euhydrated group. Our findings show that AVP plays an important role in modulating the salt-induced sympathoexcitation and high blood pressure, via V1a receptors, within the PVN of male rats. As such, V1a receptors in the PVN might contribute to neurogenic hypertension in individuals consuming a high-salt diet.

Neuronal-derived nitric oxide and somatodendritically released vasopressin regulate neurovascular coupling in the rat hypothalamic supraoptic nucleus

The classical model of neurovascular coupling (NVC) implies that activity-dependent axonal glutamate release at synapses evokes the production and release of vasoactive signals from both neurons and astrocytes, which dilate arterioles, increasing in turn cerebral blood flow (CBF) to areas with increased metabolic needs. However, whether this model is applicable to brain areas that also use less conventional neurotransmitters, such as neuropeptides, is currently unknown. To this end, we studied NVC in the rat hypothalamic magnocellular neurosecretory system (MNS) of the supraoptic nucleus (SON), in which dendritic release of neuropeptides, including vasopressin (VP), constitutes a key signaling modality influencing neuronal and network activity. Using a multidisciplinary approach, we investigated vasopressin-mediated vascular responses in SON arterioles of hypothalamic brain slices of Wistar or VP-eGFP Wistar rats. Bath-applied VP significantly constricted SON arterioles (Δ-41 ± 7%) via activation of the V1a receptor subtype. Vasoconstrictions were also observed in response to single VP neuronal stimulation (Δ-18 ± 2%), an effect prevented by V1a receptor blockade (V2255), supporting local dendritic VP release as the key signal mediating activity-dependent vasoconstrictions. Conversely, osmotically driven magnocellular neurosecretory neuronal population activity leads to a predominant nitric oxide-mediated vasodilation (Δ19 ± 2%). Activity-dependent vasodilations were followed by a VP-mediated vasoconstriction, which acted to limit the magnitude of the vasodilation and served to reset vascular tone following activity-dependent vasodilation. Together, our results unveiled a unique and complex form of NVC in the MNS, supporting a competitive balance between nitric oxide and activity-dependent dendritic released VP, in the generation of proper NVC responses.

Glial regulation of extrasynaptic NMDA receptor-mediated excitation of supraoptic nucleus neurones during dehydration

Magnocellular neurosecretory cells (MNCs) of the supraoptic nucleus (SON) project to the posterior pituitary gland where they release the hormones, vasopressin and oxytocin into the circulation to maintain plasma osmolality. Hormone release is proportionate to SON MNC action potential (spike) firing rate. When activated by ambient extracellular glutamate, extrasynaptic NMDA receptors (eNMDARs) mediate a tonic (persistent) depolarisation to increase the probability of action potential firing. In the present study, in vivo single-unit electrophysiological recordings were made from urethane-anaesthetised female Sprague-Dawley rats to investigate the impact of tonic eNMDAR activation on MNC activity. Water deprivation (for up to 48 h) caused an increase in the firing rate of SON MNCs that was associated with a general increase in post-spike excitability. To determine whether eNMDAR activation contributes to the increased MNC excitability during water deprivation, memantine, which preferentially blocks eNMDARs, was administered locally into the SON by microdialysis. Memantine significantly decreased the firing rate of MNCs recorded from 48-h water-deprived rats but had no effect on MNCs recorded from euhydrated rats. In the presence of the glial glutamate transporter-1 (GLT-1) blocker, dihydrokainate, memantine also reduced the MNC firing rate in euhydrated rats. Taken together, these observations suggest that GLT-1 clears extracellular glutamate to prevent the activation of eNDMARs under basal conditions and that, during dehydration, eNMDAR activation contributes to the increased firing rate of MNCs.

State-dependent plasticity in vasopressin neurones: dehydration-induced changes in activity patterning

Moderate dehydration impairs concentration and co-ordination, whereas severe dehydration can cause seizures, brain damage or death. To slow the progression of dehydration until body fluids can be replenished by drinking, the increased body fluid osmolality associated with dehydration increases vasopressin (antidiuretic hormone) secretion from the posterior pituitary gland. Increased vasopressin secretion reduces water loss in the urine by promoting water reabsorption in the collecting ducts of the kidney. Vasopressin secretion is largely determined by action potential discharge in vasopressin neurones, and depends on both the rate and pattern of discharge. Vasopressin neurone activity depends on intrinsic and extrinsic mechanisms. We review recent advances in our understanding of the physiological regulation of vasopressin neurone activity patterning and the mechanisms by which this is altered to cope with the increased secretory demands of dehydration.

Regulation of radial glial motility by visual experience

Radial glia in the developing optic tectum express the key guidance molecules responsible for topographic targeting of retinal axons. However, the extent to which the radial glia are themselves influenced by retinal inputs and visual experience remains unknown. Using multiphoton live imaging of radial glia in the optic tectum of intact Xenopus laevis tadpoles in conjunction with manipulations of neural activity and sensory stimuli, radial glia were observed to exhibit spontaneous calcium transients that were modulated by visual stimulation. Structurally, radial glia extended and retracted many filopodial processes within the tectal neuropil over minutes. These processes interacted with retinotectal synapses and their motility was modulated by nitric oxide (NO) signaling downstream of neuronal NMDA receptor (NMDAR) activation and visual stimulation. These findings provide the first in vivo demonstration that radial glia actively respond both structurally and functionally to neural activity, via NMDAR-dependent NO release during the period of retinal axon ingrowth.

Chronic vs. acute interactions between supraoptic oxytocin neurons and astrocytes during lactation: role of glial fibrillary acidic protein plasticity

In this article, we review studies of astrocytic-neuronal interactions and their effects on the activity of oxytocin (OXT) neurons within the magnocellular hypothalamo-neurohypophysial system. Previous work over several decades has shown that withdrawal of astrocyte processes increases OXT neuron excitability in the hypothalamic supraoptic nucleus (SON) during lactation. However, chronically disabling astrocyte withdrawal does not significantly affect the functioning of OXT neurons during suckling. Nevertheless, acute changes in a cytoskeletal element of astrocytes, glial fibrillary acidic protein (GFAP), occur in concert with changes in OXT neuronal activity during suckling. Here, we compare these changes in GFAP and related proteins with chronic changes that persist throughout lactation. During lactation, a decrease in GFAP levels accompanies retraction of astrocyte processes surrounding OXT neurons in the SON, resulting from high extracellular levels of OXT. During the initial stage of suckling, acute increases in OXT levels further strengthen this GFAP reduction and facilitate the retraction of astrocyte processes. This change, in turn, facilitates burst discharges of OXT neurons and leads to a transient increase in excitatory neurochemicals. This transient neurochemical surge acts to reverse GFAP expression and results in postburst inhibition of OXT neurons. The acute changes in astrocyte GFAP levels seen during suckling likely recur periodically, accompanied by rhythmic changes in glutamate metabolism, water transport, gliotransmitter release, and spatial relationships between astrocytes and OXT neurons. In the neurohypophysis, astrocyte retraction and reversal with accompanying GFAP plasticity also likely occur during lactation and suckling, which facilitates OXT release coordinated with its action in the SON. These studies of the dynamic interactions that occur between astrocytes and OXT neurons mediated by GFAP extend our understanding of astrocyte functions within the central nervous system.

What is hidden in the pannexin treasure trove: the sneak peek and the guesswork

Connexins had been considered to be the only class of the vertebrate proteins capable of gap junction formation; however, new candidates for this function with no homology to connexins, termed pannexins were discovered. So far three pannexins were described in rodent and human genomes: Panx1, Panx2 and Panx3. Expressions of pannexins can be detected in numerous brain structures, and now found both in neuronal and glial cells. Hypothetical roles of pannexins in the nervous system include participating in sensory processing, hippocampal plasticity, synchronization between hippocampus and cortex, and propagation of the calcium waves supported by glial cells, which help maintain and modulate neuronal metabolism. Pannexin also may participate in pathological reactions of the neural cells, including their damage after ischemia and subsequent cell death. Recent study revealed non-gap junction function of Panx1 hemichannels in erythrocytes, where they serve as the conduits for the ATP release in response to the osmotic stress. High-throughput studies produced some evidences of the pannexin involvement in the process of tumorigenesis. According to brain cancer gene expression database REMBRANDT, PANX2 expression levels can predict post diagnosis survival for patients with glial tumors. Further investigations are needed to verify or reject hypotheses listed.

Response of interleukin-1beta in the magnocellular system to salt-loading

Drinking 2% NaCl decreases interleukin (IL)-1beta in the neural lobe and enhances IL-1 Type 1 receptor expression in magnocellular neurones and pituicytes. To quantify cytokine depletion from the neural lobe during progressive salt loading and determine whether the changes are reversible and correspond with stores of vasopressin (VP) or oxytocin (OT), rats were given water on day 0 and then 2% NaCl to drink for 2, 5, 8 or 5 days followed by 5 days of water (rehydration). Control rats drinking only water were pair-fed amounts eaten by 5-day salt-loaded animals. Animals were decapitated on day 8, the neural lobe frozen and plasma hormones analysed by radioimmunoassay (OT, VP) or enzyme-linked immunosorbent assay (IL-1beta). IL-1beta, VP and OT in homogenates of the neural lobe were quantified by immunocapillary electrophoresis with laser-induced fluorescence detection. Differences were determined by ANOVA, Tukey's t-test, Dunnett's procedure, Fisher's least significant difference and linear regression analysis. In response to salt-loading, rats lost body weight similar to pair-fed controls, drank progressively more 2% NaCl and excreted greater urine volumes. Plasma VP increased at days 2 and 8 of salt-loading, whereas osmolality, OT and cytokine were enhanced after 8 days with IL-1beta remaining elevated after rehydration. In the neural lobe, all three peptides decreased progressively with increasing duration of salt-loading (IL-1beta, r2 = 0.98; OT, r2 = 0.94; VP, r2 = 0.93), beginning on day 2 (IL-1beta; VP) or 5 (OT), with only VP replenished by rehydration. IL-1beta declined more closely (P < 0.0001; ANOVA interaction analysis) with OT (r2 = 0.96) than VP (r2 = 0.86), indicative of corelease from the neural lobe during chronic dehydration. Local effects of IL-1beta on magnocellular terminals, pituicytes and microglia in the neural lobe with activation of forebrain osmoregulatory structures by circulating cytokine may sustain neurosecretion of OT and VP during prolonged salt-loading.

Cellular sources, targets and actions of constitutive nitric oxide in the magnocellular neurosecretory system of the rat

Nitric oxide (NO) is a key activity-dependent modulator of the magnocellular neurosecretory system (MNS) during conditions of high hormonal demand. In addition, recent studies support the presence of a functional constitutive NO tone. The aim of this study was to identify the cellular sources, targets, signalling mechanisms and functional relevance of constitutive NO production within the supraoptic nucleus (SON). Direct visualization of intracellular NO, along with neuronal nitric oxide synthase (nNOS) and cGMP immunohistochemistry, was used to study the cellular sources and targets of NO within the SON, respectively. Our results support the presence of a strong NO basal tone within the SON, and indicate that vasopressin (VP) neurones constitute the major neuronal source and target of basal NO. NO induced-fluorescence and cGMP immunoreactivity (cGMPir) were also found in the glia and microvasculature of the SON, suggesting that they contribute as sources/targets of NO within the SON. cGMPir was also found in association with glutamic acid decarboxylase 67 (GAD67)- and vesicular glutamate transporter 2 (VGLUT2)-positive terminals. Glutamate, acting on NMDA and possibly AMPA receptors, was found to be an important neurotransmitter driving basal NO production within the SON. Finally, electrophysiological recordings obtained from SON neurones in a slice preparation indicated that constitutive NO efficiently restrains ongoing firing activity of these neurones. Furthermore, phasically active (putative VP) and continuously firing neurones appeared to be influenced by NO originating from different sources. The potential roles for basal NO as an autocrine signalling molecule, and one that bridges neuronal-glial-vascular interactions within the MNS are discussed.

Dynamic neuronal-glial interactions: an overview 20 years later

After commenting on some perceived reasons why our review may have been relatively frequently cited, a brief overview is presented that first summarizes what we knew 25 years ago about the dynamic neuronal-astroglial interactions that occur in response to changes in the physiological state of the animal. The brain system in which these dynamic interactions were studied was the magnocellular hypothalamo-neurohypophysial system (mHNS) of the rat. The mHNS developed as and continues to be the model system yielding the most coherent picture of dynamic morphological changes and insights into their functional consequences. Many other brain areas, however, have more recently come under scrutiny in the search for glial-neuronal dynamisms. Outlined next are some of the questions concerning this phenomenon that led to the research efforts immediately following the initial discoveries, along with the answers, both complete and incomplete, obtained to those research questions. The basis for this first wave of follow-up research can be characterized by the phrase "what we knew we didn't know at that time." The final section is an update and brief overview of highlights of both "what we know now" and "what we now know that we don't know" about dynamic neuronal-astroglial interactions in the mHNS.

Cardiovascular regulation of supraoptic neurons in the rat: synaptic inputs and cellular signals

The supraoptic nucleus of the hypothalamus contains a population of neurons that project to the posterior pituitary where they release peptides into systemic circulation. The system has two main secretory products--vasopressin and oxytocin. The main systemic affects of vasopressin are related to body fluid homeostasis while circulating oxytocin is involved in parturition and lactation. The circulating levels of both hormones are, to a large part, determined by the electrical activity of the supraoptic neurons and other neurosecretory cells, which is in turn determined by synaptic inputs. More recent work suggests that there may be other dimensions to the cellular response of supraoptic neurons to these synaptic inputs. For example, it has been demonstrated that supraoptic neurons alter their synthesis of vasopressin and oxytocin in response to prolonged stimulation and that the morphology of cells in the supraoptic nucleus and its number of synaptic inputs change with the physiological conditions of the animal. These responses would appear to require some type of activity-dependent set of cellular signals. Candidates for such signals include members of the AP-1 transcription factor family whose expression in neurons has been linked to synaptic stimulation. This review will describe the effects of cardiovascular-related stimuli on the expression of different members of the AP-1 family in the supraoptic nucleus.

Dendrite-associated opioid-binding cell adhesion molecule localizes at neurosecretory granules in the hypothalamic magnocellular neurons

Opioid-binding cell adhesion molecule (OBCAM) is a member of the immunoglobulin superfamily containing limbic system-associated membrane protein (IgLON) subgroup of glycosylphosphatidylinositol-anchored immunoglobulin cell adhesion molecules. We have previously found that OBCAM is localized preferentially to dendrites compared with somata and terminals of hypothalamic vasopressin-secreting magnocellular neurons. This localization indicates that OBCAM is one of the dendrite-associated cell adhesion molecules. In the present study, we further characterized the localization and the sorting mechanism, and activity-dependent changes of this molecule in vasopressin-secreting magnocellular dendrites. Confocal microscopic observation revealed the preferential localization of OBCAM at the neurosecretory granules in the vasopressin-positive dendrites. Electron microscopic observation using chromogen-intensified and gold-conjugated methods also demonstrated the OBCAM labeling at most of the neurosecretory granules within the dendrites, while the labeling within the somata was observed at only a few neurosecretory granules. I.c.v. colchicine administration resulted in the disappearance of OBCAM immunoreactivity from the dendrites and in its concomitant accumulation at the somata, suggesting that OBCAM is synthesized at the somata and transported to the dendrites by dendrite-associated neurosecretory granules. During the postnatal development, OBCAM immunoreactivity targeted to vasopressin-positive dendrites became clear from at least 3 weeks after birth, although it appeared at only a few somata 2 weeks after birth. Phosphatidylinositol specific phospholipase C treatment of the membrane fraction of the supraoptic homogenate solubilized OBCAM. Kilon, another IgLON member, was also shown to localize at the neurosecretory granules of vasopressin-positive dendrites via the glycosylphosphatidylinositol anchor. High K(+)-stimulation appeared to cause the diffusion of OBCAM-labeled gold particles from neurosecretory granules together with the exocytosis. These findings indicate that OBCAM is synthesized within the somata, attached to vasopressin neurosecretory granules via the glycosylphosphatidylinositol anchor, and transported to the dendrites. Moreover, the subcellular localization of OBCAM is changed in an activity-dependent manner.

Increased tonic activation of presynaptic metabotropic glutamate receptors in the rat supraoptic nucleus following chronic dehydration

Chronic dehydration induces structural changes in the hypothalamic supraoptic nucleus (SON), including increased glutamate synapses and retraction of astroglial processes. We performed whole-cell recordings in acute hypothalamic slices to determine whether these changes increase tonic activation of presynaptic metabotropic glutamate receptors (mGluRs) by increasing ambient glutamate in the SON. Activation of presynaptic group III mGluRs caused a decrease in the frequency of miniature excitatory postsynaptic currents (mEPSCs) in SON neurones that was significantly attenuated in slices from dehydrated rats (-27.8 %) compared with untreated rats (-41.7 %), suggesting a higher basal occupancy of mGluRs by ambient glutamate during dehydration. Blocking group III mGluRs caused an increase in the frequency of mEPSCs that was significantly higher in slices from dehydrated rats (+42.8 %) than untreated rats (+31.4 %), suggesting greater tonic activation of presynaptic mGluRs by ambient glutamate during dehydration. Increasing ambient glutamate levels by inhibiting astrocyte glutamate uptake resulted in a decrease in mEPSC frequency due to increased activation of presynaptic mGluRs. This was attenuated in slices from dehydrated rats (-35.4 %) compared with slices from untreated rats (-48.8 %), suggesting diminished astrocytic glutamate uptake during dehydration. Immunochemical analyses revealed a robust expression of the GLT-1 transporter protein in the SON, which was diminished in SON punches from dehydrated rats compared with untreated controls. Thus, dehydration leads to increased tonic activation of presynaptic mGluRs on glutamate terminals, consistent with a decrease in glutamate buffering capacity. The resulting reduction in glutamate release probability may compensate for the increase in glutamate release sites that occurs during dehydration.

The magnocellular oxytocin system, the fount of maternity: adaptations in pregnancy

Oxytocin secretion from the posterior pituitary gland is increased during parturition, stimulated by the uterine contractions that forcefully expel the fetuses. Since oxytocin stimulates further contractions of the uterus, which is exquisitely sensitive to oxytocin at the end of pregnancy, a positive feedback loop is activated. The neural pathway that drives oxytocin neurons via a brainstem relay has been partially characterised, and involves A2 noradrenergic cells in the brainstem. Until close to term the responsiveness of oxytocin neurons is restrained by neuroactive steroid metabolites of progesterone that potentiate GABA inhibitory mechanisms. As parturition approaches, and this inhibition fades as progesterone secretion collapses, a central opioid inhibitory mechanism is activated that restrains the excitation of oxytocin cells by brainstem inputs. This opioid restraint is the predominant damper of oxytocin cells before and during parturition, limiting stimulation by extraneous stimuli, and perhaps facilitating optimal spacing of births and economical use of the store of oxytocin accumulated during pregnancy. During parturition, oxytocin cells increase their basal activity, and hence oxytocin secretion increases. In addition, the oxytocin cells discharge a burst of action potentials as each fetus passes through the birth canal. Each burst causes the secretion of a pulse of oxytocin, which sharply increases uterine tone; these bursts depend upon auto-stimulation by oxytocin released from the dendrites of the magnocellular neurons in the supraoptic and paraventricular nuclei. With the exception of the opioid mechanism that emerges to restrain oxytocin cell responsiveness, the behavior of oxytocin cells and their inputs in pregnancy and parturition is explicable from the effects of hormones of pregnancy (relaxin, estrogen, progesterone) on pre-existing mechanisms, leading through relative quiescence at term inter alia to net increase in oxytocin storage, and reduced auto-inhibition by nitric oxide generation. Cyto-architectonic changes in parturition, involving evident retraction of glial processes between oxytocin cells so they get closer together, are probably a response to oxytocin neuron activation rather than being essential for their patterns of firing in parturition.

Chondroitin sulfate proteoglycan phosphacan/RPTPbeta in the hypothalamic magnocellular nuclei

The hypothalamo-neurohypophysial system synthesizes and releases arginine vasopressin (AVP) and oxytocin (OXT) with physiological stimulation. In the present study, we investigated localization of a chondroitin sulfate proteoglycan (CSPG), phosphacan/RPTPbeta, in the supraoptic nucleus (SON) and paraventricular nucleus (PVN) of adult rats at both the light and electron microscopic levels. Immunohistochemical analyses demonstrated stronger phosphacan/RPTPbeta immunoreactivity within the SON and PVN compared with adjacent hypothalamic areas. Double labeling experiments showed phosphacan/RPTPbeta immunoreactivity constituting punctate networks to surround the somata and dendrites of AVP- and OXT-secreting magnocellular neurons. Electron microscopic examination further revealed strong phosphacan/RPTPbeta immunoreactivity at extracellular membrane surface of some axons, somata, and dendrites of the SON, but not of synaptic junctions. Interestingly, phosphacan/RPTPbeta immunoreactivity was also observed at extracellular surface membrane between astrocytic processes and neurons rather than between magnocellular neurons. The present results indicate the high expression of the CSPG, phosphacan/RPTPbeta at the extracellular space in the hypothalamic AVP- and OXT-secreting magnocellular neurons.

Neuronal-glial remodeling: a structural basis for neuronal-glial interactions in the adult hypothalamus

Increasing evidence is establishing that adult neurons and their associated glia can undergo state-dependent changes in their morphology and in consequence, in their relationships and functional interactions. A neuronal system that illustrates this kind of neuronal-glial plasticity in an exemplary fashion is that responsible for the secretion of the neurohormone oxytocin (OT). As shown by comparative ultrastructural analysis, during physiological conditions like lactation and dehydration, which result in enhanced peripheral and central release of the peptide, astrocytic coverage of OT neurons is markedly reduced and their surfaces are left directly juxtaposed. Such reduced glial coverage is of consequence to neuronal activity since it modifies extracellular ionic homeostasis and glutamate neurotransmission. In addition, it is probably prerequisite to the synaptic remodeling that occurs concurrently, and results in an enhanced number of inhibitory (GABAergic) and excitatory (glutamatergic, noradrenergic) synapses, thus further affecting neuronal function. The neuronal-glial and synaptic changes occur rapidly, within a matter of hours, and are reversible with termination of stimulation. The adult OT system retains many juvenile molecular features that may allow such plasticity, including expression of cell adhesion molecules implicated in neuronal-glial interactions during development, like polysialylated NCAM, F3/contactin and its ligand, the matrix glycoprotein, tenascin-C. On the other hand, OT itself can induce the changes since in vivo (ventricular microinfusion) or in vitro (on acute hypothalamic slices) application leads to glial and neuronal transformations similar to those induced by physiological stimuli.

Activity-related, dynamic neuron-glial interactions in the hypothalamo-neurohypophysial system

Magnocellular neurons located in the supraoptic nucleus send their principal axons to terminate in the neurohypophysis, where they release vasopressin and oxytocin into the blood circulation. This magnocellular hypothalamo-neurohypophysial system is known to undergo dramatic activity-dependent structural plasticity during chronic physiological stimulation, such as dehydration and lactation. This structural plasticity is accompanied not only by synaptic remodeling, increased direct neuronal membrane apposition, and dendritic bundling in the supraoptic nucleus, but also organization of neurovascular contacts in the neurohypophysis. The adjacent glial cells actively participate in these plastic changes in addition to magnocellular neurons themselves. Many molecules that are possibly concerned with dynamic structural remodeling are highly expressed in the hypothalamo-neurohypophysial system, although they are generally at low expression levels in other regions of adult brains. Interestingly, some of them are highly expressed only in embryonic brains. On the basis of function, these molecules are classified mainly into two categories. Cytoskeletal proteins, such as tubulin, microtubule-associated proteins, and intermediate filament proteins, are responsible for changing both glial and neuronal morphology and location. Cell adhesion molecules, belonging to immunoglobulin superfamily proteins and extracellular matrix glycoproteins, also participate in neuronal-glial, neuronal-neuronal, and glial-glial recognition and guidance. Thus, the hypothalamo-neurohypophysial system is an interesting model for elucidating physiological significance and molecular mechanisms of activity-dependent structural plasticity in adult brains.

Oxytocin-secreting neurons: A physiological model of morphological neuronal and glial plasticity in the adult hypothalamus

Oxytocin-secreting neurons of the hypothalamoneurohypophysial system undergo reversible morphological changes whenever they are strongly stimulated. In the hypothalamus, such structural plasticity is represented by modifications in the size and shape of their somata and dendrites, in the extent to which their surfaces are covered by glia, and in the density of their synapses. In the neurohypophysis, there is a parallel reduction in glial (pituicyte) coverage of their axons together, with retraction of pituicyte processes from the perivascular basal lamina and an increase in the number and size of their terminals. These changes occur rapidly, within a few hours. On the other hand, the system returns to its prestimulated condition on arrest of stimulation at a rate that depends on the length of time it has remained activated. Such neuronal-glial changes have several functional consequences. In the hypothalamic nuclei, reduction in astrocytic coverage of oxytocinergic neurons and their synapses modifies extracellular ionic homeostasis and glutamate clearance and, therefore, their overall excitability. Since it results in extensive dendritic bundling, it may also lead to ephaptic interactions and may facilitate dendritic electrotonic coupling. A most important indirect effect may be to permit synaptic remodeling that occurs concomitantly and that results in significant increases in the number of excitatory and inhibitory synapses driving their activity. In the stimulated neurohypophysis, glial retraction results in increased levels of extracellular K+ which can enhance neurohormone release while an enlarged neurovascular contact zone may facilitate diffusion of neurohormone into the circulation. Ongoing work aims to unravel the cell mechanisms and factors underlying such plasticity and has revealed that neurons and glia of the hypothalamoneurohypophysial system continue to express juvenile molecular features associated with similar neuronglial interactions and synaptic events during development and regeneration. They include strong expression of cell surface adhesion molecules like F3/contactin and polysialylated neural cell adhesion molecule, extracellular matrix glycoproteins like tenascin C, and cytoskeletal proteins like vimentin and microtubule-associated protein 1D. Some of these molecules reach the cell surface constitutively while others follow the activity-dependent regulated pathway. We consider many of these molecular features permissive, allowing oxytocin neurons and their glia to undergo morphological remodeling throughout life, provided the proper stimulus intervenes. In the hypothalamic nuclei, one such stimulus is centrally released oxytocin; in the neurohypophysis, an adrenergic, cAMP-mediated mechanism appears responsible.

Plasticity of neurohypophysial terminals with increased hormonal release during dehydration: ultrastructural and biochemical analyses

Arginine vasopressin- (AVP) and oxytocin- (OXT) secreting magnocellular neurons undergo gross structural changes with chronic physiological stimulation. Here, we investigated subcellular aspects of plasticity in rat neurohypophysial terminals during dehydration. Ultrastructural analyses demonstrated that chronic dehydration by 2% NaCl drinking for 7 days significantly decreased the numbers of neurosecretory granules and microvesicles but not the numbers of mitochondria. Moreover, in dehydrated rats, terminals making neurovascular contacts enlarged, whereas terminals in apposition to astrocytes, i.e., neuroglial contacts, became smaller. Western blot analyses demonstrated significant decreases in the levels of F3 and Thy-1 together with those of AVP- and OXT-neurophysin, but the levels of synaptophysin, SNAP-25, and GAP-43 were unchanged. Both F3 and Thy-1 were recovered in the buffer-insoluble pellet, and phosphatidyl inositol-specific phospholipase C treatment released both molecules from the crude membrane fraction, indicating that they are attached to terminal membranes by glycosylphosphatidyl inositol anchors. Confocal microscopic observations demonstrated that F3 colocalized with Thy-1 in the same terminals of magnocellular neurons. In contrast, the level of calretinin, a Ca(2+) binding protein was significantly increased with chronic dehydration. Thus, the present results suggest that enhancement of neurovascular contacts results from rearrangement of terminal-astrocyte and terminal-vessel contacts rather than enlargement or sprouting of magnocellular terminals themselves. The down-regulation of F3 and Thy-1 may contribute to enhancement of neurovascular contacts that accompany increased peptide release during dehydration.

Expression of the IgLON cell adhesion molecules Kilon and OBCAM in hypothalamic magnocellular neurons

The vasopressin (AVP) and oxytocin (OXT) magnocellular neurons in the hypothalamic supraoptic (SON) and paraventricular nuclei (PVN) display reversible structural plasticity of neurons and glial cells under different conditions of neuropeptide secretion. In the present study, we investigated the expression of two immunoglobulin superfamily (IgSF) proteins, Kilon and OBCAM, in the magnocellular neurons by using monoclonal antibodies. Anti-Kilon antibody reacted specifically with the bacterially expressed recombinant Kilon but not with the recombinant OBCAM, and similarly anti-OBCAM antibody specifically recognized the recombinant OBCAM. Western blotting analysis revealed the specific expression of Kilon and OBCAM in the SON homogenates. Although Kilon and OBCAM of the SON homogenates were present as the insoluble form, most Kilon was present in the Triton-insoluble fraction, and OBCAM was localized mainly in the Triton-soluble fraction. Immunocytochemistry revealed Kilon and OBCAM immunoreactivity in the magnocellular neurons of the SON and PVN of the rat hypothalamus compared with outside of the SON and PVN in the hypothalamus. The double-labeling study with confocal microscopy further demonstrated that Kilon immunoreactivity was observed mainly in the dendrites of AVP-secreting neurons and also occasionally OXT-secreting neurons. However, OBCAM immunoreactivity was exclusively seen in the dendrites of AVP-secreting magnocellular neurons. Chronic physiological stimulation by 2% NaCl had no effect on the expression levels of either IgLON protein in the SON. Our study thus demonstrated specific expression of Kilon and OBCAM in the hypothalamic magnocellular neurons, particularly in dendrites, suggesting that they confer on magnocellular neurons the ability to rearrange dendritic connectivity.

Neuronal-glial interactions and behaviour

Both neurons and glia interact dynamically to enable information processing and behaviour. They have had increasingly intimate, numerous and differentiated associations during brain evolution. Radial glia form a scaffold for neuronal developmental migration and astrocytes enable later synapse elimination. Functionally syncytial glial cells are depolarised by elevated potassium to generate slow potential shifts that are quantitatively related to arousal, levels of motivation and accompany learning. Potassium stimulates astrocytic glycogenolysis and neuronal oxidative metabolism, the former of which is necessary for passive avoidance learning in chicks. Neurons oxidatively metabolise lactate/pyruvate derived from astrocytic glycolysis as their major energy source, stimulated by elevated glutamate. In astrocytes, noradrenaline activates both glycogenolysis and oxidative metabolism. Neuronal glutamate depends crucially on the supply of astrocytically derived glutamine. Released glutamate depolarises astrocytes and their handling of potassium and induces waves of elevated intracellular calcium. Serotonin causes astrocytic hyperpolarisation. Astrocytes alter their physical relationships with neurons to regulate neuronal communication in the hypothalamus during lactation, parturition and dehydration and in response to steroid hormones. There is also structural plasticity of astrocytes during learning in cortex and cerebellum.

Expression of Cx36 in mammalian neurons

Cx36 is the first mammalian member of a novel subgroup of the connexin family, characterized by a long cytoplasmic loop, a peculiar gene structure and a preferential expression in cell types of neural origin. In the present review we summarize the evidence in favour of its predominant expression in neuronal cells in the mammalian central nervous system, such as results from experiments with specific neurotoxins and co-localization of Cx36 mRNA and a neuronal marker. We also report a detailed description of Cx36 mRNA distribution in the rat and human central nervous system by in situ hybridization and, for each brain region, we correlate the novel findings with previous morphological or functional demonstrations of neuronal gap junctions in the same area.

Effects of serotonin inhibition on neuronal and astrocyte plasticity in the phrenic nucleus 4 h following C2 spinal cord hemisection

C2 spinal cord hemisection results in synaptic and astroglial changes in the phrenic nucleus which have been associated with the recovery of the ipsilateral hemidiaphragm during expression of the crossed phrenic phenomenon. As part of our ongoing analysis of the neurotransmitters involved, the present study investigated the effects of systemic administration of para-chlorophenylalanine (p-CPA), a serotonin (5-HT) synthesis inhibitor, on plasticity in the rat phrenic nucleus 4 h following C2 hemisection. Hemisected control rats demonstrated typical morphological changes in the ipsilateral phrenic nucleus including: (1) an increased number and length of synaptic active zones and (2) an increased number and length of dendrodendritic membrane appositions. p-CPA treatment 3 days prior to hemisection reduced 5-HT levels and resulted in an attenuation of these changes in the ipsilateral phrenic nucleus 4 h following hemisection compared to hemisected controls. In addition, p-CPA treatment attenuated injury-induced alterations in immunohistochemical staining of glial fibrillary acidic protein (GFAP), although Western blot analysis demonstrated that overall levels of GFAP did not differ significantly between groups. The results suggest that inhibition of 5-HT synthesis by p-CPA attenuates hemisection-induced plasticity in the phrenic nucleus 4 h following an ipsilateral C2 hemisection.

Osmotic activation of the hypothalamo-neurohypophysial system reversibly downregulates the NMDA receptor subunit, NR2B, in the supraoptic nucleus of the hypothalamus

NMDA receptor activation produces a characteristic pattern of neuronal firing in magnocellular neuroendocrine cells (MNCs) of the supraoptic nucleus of the hypothalamus (SON) which has been associated with greater hormone release in vivo and in vitro. In addition, i.c.v. administered NMDA receptor blockers suppress the dehydration-induced rise in plasma vasopressin and drinking. To investigate the role of NMDA receptor subunits in the neuroendocrine functions of the magnocellular neuroendocrine cells of the hypothalamus, we examined the effects of osmotic stimulation on the protein expression of the NMDA receptor subunits, NR1 and NR2B, important in binding glycine and glutamate, respectively. Homogenates of SON, paraventricular nucleus of the hypothalamus (PVN), cortex and lateral hypothalamus from control rats and rats given 2% saline water to drink for 4-10 days were subjected to SDS-PAGE and Western blot analysis. This saline water drinking regimen produced a significant rise in plasma osmolality levels. NR1 and NR2B immunoreactivity was detected in SON, PVN, lateral hypothalamus and cortex but not in liver homogenates using subunit-specific polyclonal antibodies and quantified using computer-assisted densitometry. Mean NR2B immunoreactivity was significantly lower in SON (29%) and PVN homogenates (23%) from saline-treated rats than in those from control rats. In addition, the effect of dehydration on NR2B was regionally specific since no significant changes in NR2B expression were observed in homogenates of cortex and lateral hypothalamus. Rehydration allowed recovery of plasma osmolality as well as NR2B protein levels in the SON. These results suggest that changes in NMDA receptor subunit expression contribute to the plasticity manifested by in magnocellular neuroendocrine cells in response to osmotic activation of the hypothalamo-neurohypophysial system. In addition, our results indicate that NMDA receptors on SON and PVN MNCs may contribute to neuroendocrinological functions associated with body fluid homeostasis.

Astroglial modulation of neurotransmitter/peptide release from the neurohypophysis: present status

Reviewed in this article are those studies that have contributed heavily to our current conceptualizations of glial participation in the functioning of the magnocellular hypothalamo-neurohypophysial system. This system undergoes remarkable morphological and functional reorganization induced by increased demand for peptide synthesis and release, and this reorganization involves the astrocytic elements in primary roles. Under basal conditions, these glia appear to be vested with the responsibility of controlling the neuronal microenvironment in ways that reduce neuronal excitability, restrict access to neuronal membranes by neuroactive substances and deter neuron neuron interactions within the system. With physiological activation, the glial elements, via receptor-mediated mechanisms, take up new positions. This permissively facilitates neuron neuron interactions such as the exposure of neuronal membranes to released peptides and the formation of gap junctions and new synapses, enhances and prolongs the actions of those excitatory neurotransmitters for which there are glial uptake mechanisms, and facilitates the entry of peptides into the blood. In addition, subpopulations of these glia either newly synthesize or increase synthesis of neuroactive peptides for which their neuronal neighbors have receptors. Release of these peptides by the glia or their functional roles in the system have not yet been demonstrated.

Factors governing activity-dependent structural plasticity of the hypothalamoneurohypophysial system

1. The adult hypothalamoneurohypophysial system (HNS) undergoes reversible morphological changes in response to physiological stimulation. 2. In the hypothalamus, stimulation of neurohormone secretion results in reduced astrocytic coverage of oxytocinergic somata and dendrites so that their surfaces become directly juxtaposed. Concurrently, there is a significant increase in the number of GABAergic, glutamatergic. and noradrenergic synapses impinging on the neurons. 3. In the neurohypophysis, stimulation induces retraction of pituicyte processes from the perivascular area and enlargement and multiplication of neurosecretory terminals. 4. These neuronal-glial and synaptic changes are reversible with cessation of stimulation, thus rendering the HNS an excellent model to study physiologically linked structural neuronal plasticity in the adult CNS. 5. We still do not know the cellular mechanisms and factors underlying such plasticity. Recent studies indicate, however, that the adult HNS expresses molecular characteristics normally associated with histogenesis and/or tissue reorganization in developing or regenerating neural systems. They include expression of cell adhesion molecules such as the highly sialylated isoform of the neural cell adhesion molecule, PSA-NCAM, and the glycoproteins, F3 and tenascin-C. 6. The expression of PSA-NCAM and tenascin-C does not show striking differences in terms of age, sex or physiological condition but that of F3 varies considerably with neurohypophysial stimulation. 7. We postulate that such molecular features allow magnocellular neurons and their glia to undergo neuronal-glial and synaptic plasticity throughout life, provided the proper stimulus intervenes. 8. Thus, in the hypothalamic nuclei, centrally released oxytocin acting in synergy with steroids can induce such plasticity, while adrenaline, acting through beta-adrenergic mechanisms, does so in the neurohypophysis.

Dehydration and rehydration selectively and reversibly alter glial fibrillary acidic protein immunoreactivity in the rat supraoptic nucleus and subjacent glial limitans

Ultrastructural studies of the supraoptic nucleus (SON) of the hypothalamus suggest that an active retraction and extension of astrocytic processes (structural plasticity) from between magnocellular neuroendocrine neurons plays a role in the release of oxytocin, vasopressin, or both peptides that accompanies parturition, lactation, and dehydration. In support of this, Salm et al. (1985) previously demonstrated a lactation-associated reduction in immunoreactive glial fibrillary acidic protein (GFAP), an astrocyte-specific cytoskeletal constituent. To determine if similar changes occur in response to dehydration, and if they are reversible, the present study examined GFAP-immunoreactivity (IR) in the SON under various hydration states. Rats were dehydrated for 7 days by substitution of drinking water with 2% saline (n = 3), or dehydrated for 7 days followed by 7 days of rehydration (n = 3). A control group (n = 3) with free access to tap water was used for comparisons. The optical density of GFAP-IR was obtained from the SON, globus pallidus, and lateral hypothalamic regions. The areas of the ventral glial limitans subjacent to the SON (SON-VGL) and of linearly equivalent segments of glial limitans more distant from the SON were also determined. Dehydration resulted in a significant reduction in GFAP-IR in the SON compared to control and rehydrated levels. We also found that the area of the SON-VGL was significantly larger than that of linearly equivalent segments of glial limitans elsewhere and that it was significantly reduced in dehydrated rats, returning to control levels with rehydration. GFAP-IR and glial limitans thickness in regions unrelated to body fluid homeostasis lateral to the SON, overlying to dorsal cortex, and subjacent to the optic chiasm were not significantly changed by hydration state. These results are similar to the changes of GFAP-IR reported for lactating rats and provide further evidence for a role of structural plasticity of astrocytes in events surrounding the selective functional activation of local neurons.

Vesicular osmometers, vasopression secretion and aquaporin-4: a new mechanism for osmoreception?

This review cites new evidence suggesting a link between the recently discovered membrane bound water-selective channel, aquaporin-4 (AQP4), and the mechanism of central osmoreception. AQP4 is found in a number of brain regions associated with the osmoregulation of vasopressin secretion and thirst, including the supraoptic nucleus (SON) and subfornical organ (SFO). AQP4 expression is restricted to ependymal cell membranes in the SFO and astrocyte membranes in the SON, especially perivascular end foot processes, suggesting that glial cells may correspond to Verney's hypothalamic 'vesicular osmometers'. Information on osmotic status may thus be conveyed to the neuronal elements of the 'osmoreceptor complex' by a neurone-glial interaction.

Function-related plasticity in hypothalamus

Physiological activation of the magnocellular hypothalamo-neurohypophysial system induces a coordinated astrocytic withdrawal from between the magnocellular somata and the parallel-projecting dendrites of the supraoptic nucleus. Neural lobe astrocytes release engulfed axons and retract from their usual positions along the basal lamina. Occurring on a minutes-to-hours time scale, these changes are accompanied by increased direct apposition of both somatic and dendritic membrane, the formation of dendritic bundles, the appearance of novel multiple synapses in both the somatic and dendritic zones, and increased neural occupation of the perivascular basal lamina. Reversal, albeit with varying time courses, is achieved by removing the activating stimuli. Additionally, activation results in interneuronal coupling increases that are capable of being modulated synaptically via second messenger-dependent mechanisms. These changes appear to play important roles in control and coordination of oxytocin and vasopressin release during such conditions as lactation and dehydration.

Differential expression of tenascin by astrocytes associated with the supraoptic nucleus (SON) of hydrated and dehydrated adult rats

The present study evaluated the expression of tenascin by astrocytes in the supraoptic nucleus and associated ventral glial limitans (SON-VGL) under conditions that induce reversible changes in neuronal organization (dehydration and rehydration). Immunostaining of astroglia cultured from rat neonatal SON-VGL confirmed that these cells are capable of both expressing and secreting tenascin. Observations of immunostained tissue sections from adult rats revealed tenascin immunoreactivity primarily in the VGL and dendritic zone, subjacent to SON neuronal somata. Comparison of immunostained tissues from hydrated and dehydrated animals showed an apparent decrease in the intensity of immunostaining with dehydration. Subsequent Western blots of similar tissues confirmed the presence of the 210-220-kDa tenascin protein in the SON-VGL. SON-VGL tissues from control, dehydrated, and rehydrated rats were then studied by using SDS-PAGE and quantitative gel densitometry. A consistent decrease in tenascin concentration was observed by 6 days of dehydration that, with rehydration, reversed back toward or beyond control levels. Together, these observations indicate that SON-VGL astrocytes variably express tenascin and that this protein may play a role in adult SON plasticity.

Oestradiol acutely stimulates exocytosis of oxytocin and vasopressin from dendrites and somata of hypothalamic magnocellular neurons

Oestrogen has many direct or indirect actions upon the magnocellular system of the hypothalamus. We have examined the possibility of acute actions of oestrogen upon the magnocellular system by stimulating slices of supraoptic nucleus in vitro with various concentrations of oestrogen, for varying lengths of time, and assessing the intrahypothalamic release of oxytocin and vasopressin under these conditions. Slices were stimulated in the presence of tannic acid, which precipitates extracellular protein and thereby stabilizes exocytosed neurosecretory granule cores. Stimulation for 5 or 20 min of slices of hypothalamus containing the supraoptic nuclei with 2.66 nM-26.6 microM 17 beta-oestradiol benzoate caused the exocytosis of granules from both dendrites and cell bodies of the magnocellular neurons; exocytosis from the dendrites predominated. Granules of both oxytocin- and vasopressin-producing cells were exocytosed to a similar extent. The incidence of exocytosis of both hormones after stimulation by oestrogen was significantly higher than after exposure either to physiological saline or to 17 alpha-oestradiol, but significantly lower than after stimulation by 56 mM potassium. The various doses of steroid and durations of stimulation all resulted in similar amounts of captured exocytosis. Furthermore, the oestradiol-induced release was not inhibited by removal of extracellular calcium, whereas the potassium-stimulated release was abolished. Exposure for 20 min to either testosterone or progesterone did not induce intranuclear release of significant numbers of neurosecretory granules from the magnocellular neurons. In contrast to its effect on the hypothalamus, 26.6 microM oestradiol for either 20 min or 5 min did not induce exocytosis of neurosecretory granules from the posterior pituitary. We conclude that oestrogen can exert acute non-genomic actions on the magnocellular neurons to promote intrahypothalamic release of oxytocin and vasopressin. This effect is probably direct on the magnocellular neurons as it is not dependent on external calcium. Such actions may be important in the development of the functional and morphological plasticity of the magnocellular system that occurs in parturition and lactation.

Structural reorganization in the supraoptic nucleus of withdrawn rats following long-term alcohol consumption

We have recently shown in the supraoptic nucleus (SON) of the rat that prolonged ethanol consumption induces cell degeneration and enlargement of the surviving neurons and of their subcellular organelles. We analyzed the SON of withdrawn rats to evaluate whether it displays any evidence of morphological reorganization following abstinence from ethanol, inasmuch as in this condition the ethanol-induced changes in the plasma levels of neurohormones and plasma osmolality are no longer detectable. A group of 18-month-old withdrawn rats was compared with age-matched, pair-fed control and ethanol-treated rats. To differentiate between the effects of withdrawal and the effects of rehydration, a group of 18-month-old rehydrated rats was also included in this study and compared with age-matched, pair-fed control and dehydrated rats. We estimated the volume of SON, and the total number and mean volume of its neurons. The cross-sectional areas of the vasopressinergic and oxytocinergic populations were also evaluated. At the ultrastructural level, we determined the volumes and surface areas of the rough endoplasmic reticulum and Golgi apparatus, and the volumes of neurosecretory granules and nucleoli. In withdrawn animals, the total number of SON neurons was smaller than in controls, although the neuronal volume was greater. The number of SON neurons did not differ between withdrawn and ethanol-treated rats, despite the reduced volume of SON in the former animals. The decrease of SON volume correlated with and was caused by a reduction in the volume of SON neurons and in the size of the organelles involved in neuro-hormone synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional role of intrahypothalamic release of oxytocin and vasopressin: consequences and controversies

This brief review of vasopressin (VP) and oxytocin (OT) release into the extracellular space of the supraoptic (SON) and paraventricular (PVN) nuclei focuses on recent data illustrating the significance of their intranuclear release and the potential functional consequences. With the use of in vitro techniques, it has been demonstrated that administration of exogenous OT causes local peptide release and that, in vivo, this facilitates the milk ejection reflex. These findings lead to the idea that endogenous peptides are released into the hypothalamic nuclei. Microperfusion techniques have been used to monitor the dynamics of intranuclear OT and VP release in response to distinct stimuli. It is clear that intranuclear release of OT plays a role during reproductive states (parturition and lactation) and that intranuclear release of VP and OT is involved in osmoregulation. This review discusses 1) the origin of the intranuclearly released peptides, 2) stimuli which cause release into the hypothalamic nuclei, and 3) the function of intranuclear VP and OT, e.g., regulation of local morphology, feedback mechanisms and synchronization, and the possible role in regulating autonomic function and behavior.

Phorbol ester-stimulated stellation in primary cultures of astrocytes from different brain regions

Stellation is the process by which astrocytes change from epithelial-like to process-bearing cells. Stellation occurs following activation of either cyclic AMP-dependent protein kinase or protein kinase C. This process occurs through tubulin-dependent rearrangement of the cytoskeleton. We have evaluated the ability of phorbol, 12-myristate, 13-acetate (PMA) to induce astrocyte stellation. Astrocytes from five brain regions (cerebellum, cerebral cortex, hippocampus, diencephalon, and brain-stem) were examined to determine if all astrocytes would exhibit similar responses to this activator of protein kinase C. Stellation was evaluated following cell fixation by either phase optics using conventional light microscopy, or scanning laser confocal light microscopy of cultures prepared using immunocytochemistry for tubulin and glial fibrillary acidic protein. Both the number of cells responding to PMA and the sensitivity to PMA varied for astrocytes from each brain region. PMA-induced stellation was most robust in cerebellar and brainstem astrocytes, with greater than 70% responding. Less than 40% of hippocampal and diencephalic astrocytes responded to PMA at the maximum dose (10(-5) M). PMA also induced different numbers of processes or branching patterns of processes on astrocytes from different brain regions. The protein kinase C induced stellation response in astrocytes supports the hypothesis that astrocytes contribute to neural plasticity.

The influence of age on supraoptic nucleus glial cells of the mouse

In 2-year-old mice, astrocytes of the supraoptic nucleus (SON) of the hypothalamus, placed in close contact with the meningeal covering contained concentric arrangements of cisterns in their cytoplasm called glial concentric bodies (GCBs). They are formed by a variable number of flattened cisterns without pores, and no anastomosis was observed between adjacent cisterns. Rough endoplasmic reticulum (RER) cisterns do not show continuity with fully developed GCBs, although a close continuity between RER cisterns and GCBs was present in images suggesting GCB formation. Concomitantly with the appearance of GCBs, ventral neurosecretory neurons of the SON showed a cytoplasmic membrane reinforced by the apposition of glial prolongations. The possible origin and function of these GCBs and lamellae are discussed here.

Ultrastructural demonstration of NADPH-diaphorase histochemical activity in the supraoptic nucleus of normal and dehydrated rats

The ultrastructural location of the NADPH-diaphorase, likely to represent the nitric oxide synthase, was studied in the supraoptic nucleus of control and dehydrated rats. The NADPH-diaphorase activity was found in nuclear envelope, inner and outer membranes of mitochondria, Golgi apparatus, and endoplasmic reticulum in both neurons and astrocytes. We observed an increase in the number of stained mitochondria and an increased staining of the endoplasmic membranes in prolonged dehydration stress. Based on available data, the supraoptic NADPH-diaphorase/nitric oxide synthase seems to be involved in adaptative function of the hypothalamic neurosecretory system.

Effects of chronic alcohol consumption and of dehydration on the supraoptic nucleus of adult male and female rats

Ethanol ingestion affects the hypothalamo-neurohypophysial system resulting in increased diuresis, dehydration and hyperosmolality. We studied the supraoptic nucleus, of the hypothalamus, in ethanol-treated rats, to determine if ethanol alone and/or the associated disturbances of water metabolism lead to structural alterations in a nucleus known to play a central role in fluid homeostasis. Groups of male and female rats were ethanol-treated until 12 and 18 months of age and compared with age-matched pair-fed controls. Twelve and 18-month-old control groups and 12-month-old water control groups (rats submitted to chronic dehydration) were also included in this study in an attempt to differentiate between the effects of undernutrition and dehydration/hyperosmolality, and the specific neurotoxic effects of ethanol. We estimated the volume of the supraoptic nucleus and the numerical density of its neurons and calculated the total number of supraoptic neurons. The volume of both supraoptic neurons and neuropil were also estimated. In immunostained material the ratio of vasopressin to oxytocin neurons and the cross-sectional areas of the two neuronal types were evaluated. There was marked neuronal loss in alcohol-treated rats, but the volume of the supraoptic nucleus was increased. The increase in the volume of the supraoptic nucleus correlated with and was due to increases in the volume was particularly marked for vasopressin neurons. No significant differences were found between controls and pair-fed controls in any of the parameters investigated. In water control rats, the volume of the supraoptic nucleus and of the supraoptic neurons and neuropil was also greater than in pair-fed controls. However, the variations found were not as marked as in ethanol-treated rats and there was no cell loss. These findings reveal, for the first time, that chronic ethanol consumption affects the morphology of supraoptic neurons and neuropil and, consequently, the structure of the entire supraoptic nucleus. Moreover, this study supports the view that ethanol has direct neurotoxic effects on supraoptic neurons because the alterations that occur are not mimicked in animals in which water metabolism alone is disturbed.

Astrocyte subtypes in the rat olfactory bulb: morphological heterogeneity and differential laminar distribution

Despite increased recognition of the importance and heterogeneity of astrocyte functions throughout the central nervous system (CNS) relatively little attention has been paid to morphological diversity among astrocytes. Recent studies have indicated that subsets of astrocytes are involved in glial-axonal interactions critical to both development and reinnervation of the rat olfactory bulb. Here, we have characterized the morphologies and distribution of astrocytes within anatomically and functionally distinct layers of the adult main olfactory bulb (MOB). Using a known immunohistochemical marker for astrocytes, glial fibrillary acidic protein (GFAP), and the classic gold sublimate method, we identified six astrocyte subtypes based on their morphology and distribution: (1) unipolar, (2) irregular, (3) wedge-shape, (4) circular, (5) semicircular, and (6) elongate. Unipolar, irregular and wedge-shape astrocytes have not been previously described in the CNS. The unipolar and irregular types are located exclusively in the olfactory nerve layer. Wedge-shape astrocytes are unique to, and are the major subtype in, the glomerular layer. These three morphologically unique astrocyte subtypes may correspond to olfactory nerve layer (ONL) and glomerular layer (GL) astrocytes, which express molecules that regulate axonal growth or synaptogenesis during development and/or regeneration of the olfactory nerve. In the glomerular layer, astrocytes are highly organized with respect to the glomeruli. Individual astrocytes are loyal to a single glomerulus. In the external plexiform layer, astrocytes are spaced relatively uniformly. In the granule cell layer, astrocytes appear to compartmentalize granule cell aggregates, recently shown to be coupled by tight junctions. The distribution and patterns of astrocyte processes and the density of GFAP immunoreactivity are distinctive for each of the layers of the olfactory bulb. The spacing of astrocytes and the organization of their processes may be important to compartmentalization of neuronal functions. High levels of GFAP immunoreactivity correlated with layers of high neuronal plasticity. The morphological diversity and differential distribution of astrocytes in the olfactory bulb reported here support growing evidence for functional diversity of astrocytes and important interactions among specific astrocyte and neuron subtypes. It is reasonable to hypothesize, therefore, that as for neurons, morphologically distinctive astrocyte subtypes may correspond to functionally specific classes.

Synthesis and release of neuroactive substances by glial cells

Glia contain, synthesize, or release more than 20 neuroactive compounds including neuropeptides, amino acid transmitters, eicosanoids, steroids, and growth factors. The stimuli that elicit release differ among compounds but include neuropeptides, neurotransmitters, receptor agonists, and elevated external [K+]. The mechanisms of release are poorly understood in most cases. Many of the neuroactive compounds are localized in discrete subpopulations of glia. Thus, glia are equipped to send as well as receive chemical messages and appear to be present as classes of cells with differing abilities to communicate chemically. It is possible that glia are as diverse as neurons in their functional characteristics.