Astrocyte-mediated distributed plasticity at hypothalamic glutamate synapses

Electroencephalographic field influence on calcium momentum waves

Macroscopic electroencephalographic (EEG) fields can be an explicit top-down neocortical mechanism that directly drives bottom-up processes that describe memory, attention, and other neuronal processes. The top-down mechanism considered is macrocolumnar EEG firings in neocortex, as described by a statistical mechanics of neocortical interactions (SMNI), developed as a magnetic vector potential A. The bottom-up process considered is Ca(2+) waves prominent in synaptic and extracellular processes that are considered to greatly influence neuronal firings. Here, the complimentary effects are considered, i.e., the influence of A on Ca(2+) momentum, p. The canonical momentum of a charged particle in an electromagnetic field, Π=p+qA (SI units), is calculated, where the charge of Ca(2+) is q=-2e, e is the magnitude of the charge of an electron. Calculations demonstrate that macroscopic EEG A can be quite influential on the momentum p of Ca(2+) ions, in both classical and quantum mechanics. Molecular scales of Ca(2+) wave dynamics are coupled with A fields developed at macroscopic regional scales measured by coherent neuronal firing activity measured by scalp EEG. The project has three main aspects: fitting A models to EEG data as reported here, building tripartite models to develop A models, and studying long coherence times of Ca(2+) waves in the presence of A due to coherent neuronal firings measured by scalp EEG. The SMNI model supports a mechanism wherein the p+qA interaction at tripartite synapses, via a dynamic centering mechanism (DCM) to control background synaptic activity, acts to maintain short-term memory (STM) during states of selective attention.

Activation of α7-containing nicotinic receptors on astrocytes triggers AMPA receptor recruitment to glutamatergic synapses

Astrocytes, an abundant form of glia, are known to promote and modulate synaptic signaling between neurons. They also express α7-containing nicotinic acetylcholine receptors (α7-nAChRs), but the functional relevance of these receptors is unknown. We show here that stimulation of α7-nAChRs on astrocytes releases components that induce hippocampal neurons to acquire more α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors post-synaptically at glutamatergic synapses. The increase is specific in that no change is seen in synaptic NMDA receptor clusters or other markers for glutamatergic synapses, or in markers for GABAergic synapses. Moreover, the increases in AMPA receptors on the neuron surface are accompanied by increases in the frequency of spontaneous miniature synaptic currents mediated by the receptors and increases in the ratio of evoked synaptic currents mediated by AMPA versus NMDA receptors. This suggests that stimulating α7-nAChRs on astrocytes can convert 'silent' glutamatergic synapses to functional status. Astrocyte-derived thrombospondin is necessary but not sufficient for the effect, while tumor necrosis factor-α is sufficient but not necessary. The results identify astrocyte α7-nAChRs as a novel pathway through which nicotinic cholinergic signaling can promote the development of glutamatergic networks, recruiting AMPA receptors to post-synaptic sites and rendering the synapses more functional. We find that activation of nicotinic receptors on astrocytes releases a component that specifically recruits AMPA receptors to glutamatergic synapses. The recruitment appears to occur preferentially at what may be 'silent synapses', that is, synapses that have all the components required for glutamatergic transmission (including NMDA receptors) but lack sufficient AMPA receptors to generate a response. The results are unexpected and open up new possibilities for mechanisms underlying network formation and synaptic plasticity.

Glial cells modulate the synaptic transmission of NTS neurons sending projections to ventral medulla of Wistar rats

There is evidence that sympathoexcitatory and respiratory responses to chemoreflex activation involve ventrolateral medulla-projecting nucleus tractus solitarius (NTS) neurons (NTS-VLM neurons) and also that ATP modulates this neurotransmission. Here, we evaluated whether or not astrocytes is the source of endogenous ATP modulating the synaptic transmission in NTS-VLM neurons. Synaptic activities of putative astrocytes or NTS-VLM neurons were recorded using whole cell patch clamp. Tractus solitarius (TS) stimulation induced TS-evoked excitatory postsynaptic currents (TS-eEPSCs) in NTS-VLM neurons as well in NTS putative astrocytes, which were also identified by previous labeling. Fluoracetate (FAC), an inhibitor of glial metabolism, reduced TS-eEPSCs amplitude (-85.6 ± 16 vs. -39 ± 7.1 pA, n = 12) and sEPSCs frequency (2.8 ± 0.5 vs. 1.8 ± 0.46 Hz, n = 10) in recorded NTS-VLM neurons, indicating a gliomodulation of glutamatergic currents. To verify the involvement of endogenous ATP a purinergic antagonist was used, which reduced the TS-eEPSCs amplitude (-207 ± 50 vs. -149 ± 50 pA, n = 6), the sEPSCs frequency (1.19 ± 0.2 vs. 0.62 ± 0.11 Hz, n = 6), and increased the paired-pulse ratio (PPR) values (∼20%) in NTS-VLM neurons. Simultaneous perfusion of Pyridoxalphosphate-6-azophenyl-2',5'-disulfonic acid (iso-PPADS) and FAC produced reduction in TS-eEPSCs similar to that observed with iso-PPADS or FAC alone, indicating that glial cells are the source of ATP released after TS stimulation. Extracellular ATP measurement showed that FAC reduced evoked and spontaneous ATP release. All together these data show that putative astrocytes are the source of endogenous ATP, which via activation of presynaptic P2X receptors, facilitates the evoked glutamate release and increases the synaptic transmission efficacy in the NTS-VLM neurons probably involved with the peripheral chemoreflex pathways.

Physiological regulation of magnocellular neurosecretory cell activity: integration of intrinsic, local and afferent mechanisms

The hypothalamic supraoptic and paraventricular nuclei contain magnocellular neurosecretory cells (MNCs) that project to the posterior pituitary gland where they secrete either oxytocin or vasopressin (the antidiuretic hormone) into the circulation. Oxytocin is important for delivery at birth and is essential for milk ejection during suckling. Vasopressin primarily promotes water reabsorption in the kidney to maintain body fluid balance, but also increases vasoconstriction. The profile of oxytocin and vasopressin secretion is principally determined by the pattern of action potentials initiated at the cell bodies. Although it has long been known that the activity of MNCs depends upon afferent inputs that relay information on reproductive, osmotic and cardiovascular status, it has recently become clear that activity depends critically on local regulation by glial cells, as well as intrinsic regulation by the MNCs themselves. Here, we provide an overview of recent advances in our understanding of how intrinsic and local extrinsic mechanisms integrate with afferent inputs to generate appropriate physiological regulation of oxytocin and vasopressin MNC activity.

Two-photon excitation microscopy for the study of living cells and tissues

Two-photon excitation microscopy is an alternative to confocal microscopy that provides advantages for three-dimensional and deep tissue imaging. This unit will describe the basic physical principles behind two-photon excitation and discuss the advantages and limitations of its use in laser-scanning microscopy. The principal advantages of two-photon microscopy are reduced phototoxicity, increased imaging depth, and the ability to initiate highly localized photochemistry in thick samples. Practical considerations for the application of two-photon microscopy will then be discussed, including recent technological advances. This unit will conclude with some recent applications of two-photon microscopy that highlight the key advantages over confocal microscopy and the types of experiments which would benefit most from its application.

Calcium-induced outgrowth of astrocytic peripheral processes requires actin binding by Profilin-1

Peripheral astrocytic processes (PAPs) are highly motile structures that are strategically positioned in close proximity to synapses. Long-lasting PAP retraction in hypothalamus is known to alter synaptic transmission. The PAP motility is likely to be actin-based because they are known to contain actin-related proteins such as Ezrin. However, the link between dynamic activity-dependent changes in astrocytic morphology and the synaptic function has not been established experimentally, presumably due to lack of appropriate tools. To selectively suppress activity-dependent morphological plasticity of astrocytes, we developed a bicistronic construct that allows simultaneous tracing and manipulating the morphology of PAPs. The construct is designed for co-expression of (i) the mutant actin binding protein Profilin-1 (abdProf-1) with a single amino acid substitution (H119E) that prevents its binding to actin monomers with (ii) the membrane-targeted morphological tracer LckGFP. Cultured cortical astrocytes transfected with this construct showed abdProf-1 overexpression at a 5-fold level compared to the endogenous Profilin-1. The cells also expressed LckGFP at a level sufficient for precise morphological tracing. We found that photolysis of caged Ca²⁺ induced a pronounced outgrowth of PAPs, which was suppressed by abdProf-1 overexpression in terms of PAP number, growth rate and maximal length. In contrast, the morphological complexity of astrocytes, basal motility of their PAPs and major cytoskeletal structures were not affected by abdProf-1 overexpression. In summary, we identified the actin binding by Profilin-1 as a pivotal mechanism in activity-dependent morphological plasticity of PAPs in cultured astrocytes.

Bidirectional neuro-glial signaling modalities in the hypothalamus: role in neurohumoral regulation

Maintenance of bodily homeostasis requires concerted interactions between the neuroendocrine and the autonomic nervous systems, which generate adaptive neurohumoral outflows in response to a variety of sensory inputs. Moreover, an exacerbated neurohumoral activation is recognized to be a critical component in numerous disease conditions, including hypertension, heart failure, stress, and the metabolic syndrome. Thus, the study of neurohumoral regulation in the brain is of critical physiological and pathological relevance. Most of the work in the field over the last decades has been centered on elucidating neuronal mechanisms and pathways involved in neurohumoral control. More recently however, it has become increasingly clear that non-neuronal cell types, particularly astrocytes and microglial cells, actively participate in information processing in areas of the brain involved in neuroendocrine and autonomic control. Thus, in this work, we review recent advances in our understanding of neuro-glial interactions within the hypothalamic supraoptic and paraventricular nuclei, and their impact on neurohumoral integration in these nuclei. Major topics reviewed include anatomical and functional properties of the neuro-glial microenvironment, neuron-to-astrocyte signaling, gliotransmitters, and astrocyte regulation of signaling molecules in the extracellular space. We aimed in this review to highlight the importance of neuro-glial bidirectional interactions in information processing within major hypothalamic networks involved in neurohumoral integration.

Antagonism of purinergic signalling improves recovery from traumatic brain injury

The recent public awareness of the incidence and possible long-term consequences of traumatic brain injury only heightens the need to develop effective approaches for treating this neurological disease. In this report, we identify a new therapeutic target for traumatic brain injury by studying the role of astrocytes, rather than neurons, after neurotrauma. We use in vivo multiphoton imaging and show that mechanical forces during trauma trigger intercellular calcium waves throughout the astrocytes, and these waves are mediated by purinergic signalling. Subsequent in vitro screening shows that astrocyte signalling through the 'mechanical penumbra' affects the activity of neural circuits distant from the injury epicentre, and a reduction in the intercellular calcium waves within astrocytes restores neural activity after injury. In turn, the targeting of different purinergic receptor populations leads to a reduction in hippocampal cell death in mechanically injured organotypic slice cultures. Finally, the most promising therapeutic candidate from our in vitro screen (MRS 2179, a P2Y1 receptor antagonist) also improves histological and cognitive outcomes in a preclinical model of traumatic brain injury. This work shows the potential of studying astrocyte signalling after trauma to yield new and effective therapeutic targets for treating traumatic brain injury.

Computational quest for understanding the role of astrocyte signaling in synaptic transmission and plasticity

The complexity of the signaling network that underlies astrocyte-synapse interactions may seem discouraging when tackled from a theoretical perspective. Computational modeling is challenged by the fact that many details remain hitherto unknown and conventional approaches to describe synaptic function are unsuitable to explain experimental observations when astrocytic signaling is taken into account. Supported by experimental evidence is the possibility that astrocytes perform genuine information processing by means of their calcium signaling and are players in the physiological setting of the basal tone of synaptic transmission. Here we consider the plausibility of this scenario from a theoretical perspective, focusing on the modulation of synaptic release probability by the astrocyte and its implications on synaptic plasticity. The analysis of the signaling pathways underlying such modulation refines our notion of tripartite synapse and has profound implications on our understanding of brain function.

Genetically Encoded Calcium Indicators and Astrocyte Calcium Microdomains

The discovery of intracellular Ca2+ signals within astrocytes has changed our view of how these ubiquitous cells contribute to brain function. Classically thought merely to serve supportive functions, astrocytes are increasingly thought to respond to, and regulate, neurons. The use of organic Ca2+ indicator dyes such as Fluo-4 and Fura-2 has proved instrumental in the study of astrocyte physiology. However, progress has recently been accelerated by the use of cytosolic and membrane targeted genetically encoded calcium indicators (GECIs). Herein, we review these recent findings, discuss why studying astrocyte Ca2+ signals is important to understand brain function, and summarize work that led to the discovery of TRPA1 channel-mediated near-membrane Ca2+ signals in astrocytes and their indirect neuromodulatory roles at inhibitory synapses in the CA1 stratum radiatum region of the hippocampus. We suggest that the use of membrane-targeted and cytosolic GECIs holds great promise to explore the diversity of Ca2+ signals within single astrocytes and also to study diversity of function for astrocytes in different parts of the brain.

Acute inhibition of glial cells in the NTS does not affect respiratory and sympathetic activities in rats exposed to chronic intermittent hypoxia

Recent studies suggest that neuron-glia interactions are involved in multiple aspects of neuronal activity regulation. In the nucleus tractus solitarius (NTS) neuron-glia interactions are thought to participate in the integration of autonomic responses to physiological challenges. However, it remains to be shown whether NTS glial cells might influence breathing and cardiovascular control, and also if they could be integral to the autonomic and respiratory responses to hypoxic challenges. Here, we investigated whether NTS glia play a tonic role in the modulation of central respiratory and sympathetic activities as well as in the changes in respiratory-sympathetic coupling induced by exposure to chronic intermittent hypoxia (CIH), a model of central autonomic and respiratory plasticity. We show that bilateral microinjections of fluorocitrate (FCt), a glial cell inhibitor, into the caudal and intermediate subnuclei of the NTS did not alter baseline respiratory and sympathetic parameters in in situ preparations of juvenile rats. Similar results were observed in rats previously exposed to CIH. Likewise, CIH-induced changes in respiratory-sympathetic coupling were unaffected by FCt-mediated inhibition. However, microinjection of FCt into the ventral medulla produced changes in respiratory frequency. Our results show that acute glial inhibition in the NTS does not affect baseline respiratory and sympathetic control. Additionally, we conclude that NTS glial cells may not be necessary for the continuous manifestation of sympathetic and respiratory adaptations to CIH. Our work provides evidence that neuron-glia interactions in the NTS do not participate in baseline respiratory and sympathetic control.

The computational power of astrocyte mediated synaptic plasticity

Research in the last two decades has made clear that astrocytes play a crucial role in the brain beyond their functions in energy metabolism and homeostasis. Many studies have shown that astrocytes can dynamically modulate neuronal excitability and synaptic plasticity, and might participate in higher brain functions like learning and memory. With the plethora of astrocyte mediated signaling processes described in the literature today, the current challenge is to identify, which of these processes happen under what physiological condition, and how this shapes information processing and, ultimately, behavior. To answer these questions will require a combination of advanced physiological, genetical, and behavioral experiments. Additionally, mathematical modeling will prove crucial for testing predictions on the possible functions of astrocytes in neuronal networks, and to generate novel ideas as to how astrocytes can contribute to the complexity of the brain. Here, we aim to provide an outline of how astrocytes can interact with neurons. We do this by reviewing recent experimental literature on astrocyte-neuron interactions, discussing the dynamic effects of astrocytes on neuronal excitability and short- and long-term synaptic plasticity. Finally, we will outline the potential computational functions that astrocyte-neuron interactions can serve in the brain. We will discuss how astrocytes could govern metaplasticity in the brain, how they might organize the clustering of synaptic inputs, and how they could function as memory elements for neuronal activity. We conclude that astrocytes can enhance the computational power of neuronal networks in previously unexpected ways.

Neuromodulation by extracellular ATP and P2X receptors in the CNS

Extracellular adenosine 5' triphosphate (ATP) is a widespread cell-to-cell signaling molecule in the brain, where it activates cell surface P2X and P2Y receptors. P2X receptors define a protein family unlike other neurotransmitter-gated ion channels in terms of sequence, subunit topology, assembly, and architecture. Within milliseconds of binding ATP, they catalyze the opening of a cation-selective pore. However, recent data show that P2X receptors often underlie neuromodulatory responses on slower time scales of seconds or longer. Herein, we review these findings at molecular, cellular and systems levels. We propose that, while P2X receptors are fast ligand-gated cation channels, they are most adept at mediating slow neuromodulatory functions that are more widespread and more physiologically utilized than fast ATP synaptic transmission in the CNS.

Oxygen consumption and blood flow coupling in human motor cortex during intense finger tapping: implication for a role of lactate

Rates of cerebral blood flow (CBF) and glucose consumption (CMR(glc)) rise in cerebral cortex during continuous stimulation, while the oxygen-glucose index (OGI) declines as an index of mismatched coupling of oxygen consumption (cerebral metabolic rate of oxygen-CMRO(2)) to CBF and CMR(glc). To test whether the mismatch reflects a specific role of aerobic glycolysis during functional brain activation, we determined CBF and CMRO(2) with positron emission tomography (PET) when 12 healthy volunteers executed finger-to-thumb apposition of the right hand. Movements began 1, 10, or 20 minutes before administration of the radiotracers. In primary and supplementary motor cortices and cerebellum, CBF had increased at 1 minute of exercise and remained elevated for the duration of the 20-minute session. In contrast, the CMRO(2) numerically had increased insignificantly in left M1 and supplementary motor area at 1 minute, but had declined significantly at 10 minutes, returning to baseline at 20 minutes. As measures of CMR(glc) are impossible during short-term activations, we used measurements of CBF as indices of CMR(glc). The decline of CMRO(2) at 10 minutes paralleled a calculated decrease of OGI at this time. The implied generation of lactate in the tissue suggested an important hypothetical role of the metabolite as regulator of CBF during activation.

Astrocyte regulation of sleep circuits: experimental and modeling perspectives

Integrated within neural circuits, astrocytes have recently been shown to modulate brain rhythms thought to mediate sleep function. Experimental evidence suggests that local impact of astrocytes on single synapses translates into global modulation of neuronal networks and behavior. We discuss these findings in the context of current conceptual models of sleep generation and function, each of which have historically focused on neural mechanisms. We highlight the implications and the challenges introduced by these results from a conceptual and computational perspective. We further provide modeling directions on how these data might extend our knowledge of astrocytic properties and sleep function. Given our evolving understanding of how local cellular activities during sleep lead to functional outcomes for the brain, further mechanistic and theoretical understanding of astrocytic contribution to these dynamics will undoubtedly be of great basic and translational benefit.

Light-controlled tools

Spatial and temporal control over chemical and biological processes plays a key role in life, where the whole is often much more than the sum of its parts. Quite trivially, the molecules of a cell do not form a living system if they are only arranged in a random fashion. If we want to understand these relationships and especially the problems arising from malfunction, tools are necessary that allow us to design sophisticated experiments that address these questions. Highly valuable in this respect are external triggers that enable us to precisely determine where, when, and to what extent a process is started or stopped. Light is an ideal external trigger: It is highly selective and if applied correctly also harmless. It can be generated and manipulated with well-established techniques, and many ways exist to apply light to living systems--from cells to higher organisms. This Review will focus on developments over the last six years and includes discussions on the underlying technologies as well as their applications.

Hypothalamic 2-arachidonoylglycerol regulates multistage process of high-fat diet preferences

Background

In this study, we examined alterations in the hypothalamic reward system related to high-fat diet (HFD) preferences. We previously reported that hypothalamic 2-arachidonoylglycerol (2-AG) and glial fibrillary acid protein (GFAP) were increased after conditioning to the rewarding properties of a HFD. Here, we hypothesized that increased 2-AG influences the hypothalamic reward system.

Methods

The conditioned place preference test (CPP test) was used to evaluate HFD preferences. Hypothalamic 2-AG was quantified by gas chromatography-mass spectrometry. The expression of GFAP was examined by immunostaining and western blotting.

Results

Consumption of a HFD over either 3 or 7 days increased HFD preferences and transiently increased hypothalamic 2-AG levels. HFD consumption over 14 days similarly increased HFD preferences but elicited a long-lasting increase in hypothalamic 2-AG and GFAP levels. The cannabinoid 1 receptor antagonist O-2050 reduced preferences for HFDs after 3, 7, or 14 days of HFD consumption and reduced expression of GFAP after 14 days of HFD consumption. The astrocyte metabolic inhibitor Fluorocitrate blocked HFD preferences after 14 days of HFD consumption.

Conclusions

High levels of 2-AG appear to induce HFD preferences, and activate hypothalamic astrocytes via the cannabinoid system. We propose that there may be two distinct stages in the development of HFD preferences. The induction stage involves a transient increase in 2-AG, whereas the maintenance stage involves a long lasting increase in 2-AG levels and activation of astrocytes. Accordingly, hypothalamic 2-AG may influence the development of HFD preferences.

Astrocyte-neuron communication: functional consequences

Astrocyte-neuron communication has recently been proposed as a potential mechanism participating to synaptic transmission. With the development of this concept and accumulating evidences in favor of a modulation of synaptic transmission by astrocytes, has emerged the term gliotransmission. It refers to the capacity of astrocytes to release various transmitters, such as ATP, glutamate, D-serine, and GABA in the vicinity of synapses. While the cellular mechanisms involved in gliotransmission still need to be better described and, for some, identified, the aim of more and more studies is to determine the role of astrocytes from a functional point of view. This review will summarize the principal studies that have investigated a potential role of astrocytes in the various functions regulated by the brain (sleep, breathing, perception, learning and memory…). This will allow us to highlight the similarities and discrepancies in the signaling pathways involved in the different areas of the brain related to these functions.

Receptor-selective diffusion barrier enhances sensitivity of astrocytic processes to metabotropic glutamate receptor stimulation

Metabotropic glutamate receptor (mGluR)-dependent calcium ion (Ca²+) signaling in astrocytic processes regulates synaptic transmission and local blood flow essential for brain function. However, because of difficulties in imaging astrocytic processes, the subcellular spatial organization of mGluR-dependent Ca²+ signaling is not well characterized and its regulatory mechanism remains unclear. Using genetically encoded Ca²+ indicators, we showed that despite global stimulation by an mGluR agonist, astrocyte processes intrinsically exhibited a marked enrichment of Ca²+ responses. Immunocytochemistry indicated that these polarized Ca²+ responses could be attributed to increased density of surface mGluR5 on processes relative to the soma. Single-particle tracking of surface mGluR5 dynamics revealed a membrane barrier that blocked the movement of mGluR5 between the processes and the soma. Overexpression of mGluR or expression of its carboxyl terminus enabled diffusion of mGluR5 between the soma and the processes, disrupting the polarization of mGluR5 and of mGluR-dependent Ca²+ signaling. Together, our results demonstrate an mGluR5-selective diffusion barrier between processes and soma that compartmentalized mGluR Ca²+ signaling in astrocytes and may allow control of synaptic and vascular activity in specific subcellular domains.

Adenosine trisphosphate appears to act via different receptors in terminals versus somata of the hypothalamic neurohypophysial system

ATP-induced ionic currents were investigated in isolated terminals and somata of the hypothalamic neurohypophysial system (HNS). Both terminals and somata showed inward rectification of the ATP-induced currents and reversal near 0 mV. In terminals, ATP dose-dependently evoked an inactivating, inward current. However, in hypothalamic somata, ATP evoked a very slowly inactivating, inward current with a higher density, and different dose dependence (EC(50) of 50 μm in somata versus 9.6 μm in terminals). The ATP-induced currents, in both the HNS terminals and somata, were highly and reversibly inhibited by suramin, suggesting the involvement of a purinergic receptor (P2XR). However, the suramin inhibition was significantly different in the two HNS compartments (IC(50) of 3.6 μm in somata versus 11.6 μm in terminals). Also, both HNS compartments show significantly different responses to the purinergic receptor agonists: ATP-γ-S and benzoyl-benzoyl-ATP. Finally, there was an initial desensitisation to ATP upon successive stimulations in the terminals, which was not observed in the somata. These differences in EC(50) , inactivation, desensitisation and agonist sensitivity in terminals versus somata indicate that different P2X receptors mediate the responses in these two compartments of HNS neurones. Previous work has revealed mRNA transcripts for multiple purinergic receptors in micropunches of the hypothalamus. In the HNS terminals, the P2X purinergic receptor types P2X2, 3, 4 and 7 (but not 6) have been shown to exist in AVP terminals. Immonohistochemistry now indicates that P2X4R is only present in AVP terminals and that the P2X7R is found in both AVP and oxytocin terminals and somata. We speculate that these differences in receptor types reflects the specific function of endogenous ATP in the terminals versus somata of these central nervous system neurones.

Glial regulation of neuronal function: from synapse to systems physiology

Classically, glia have been regarded as non-excitable cells that provide nourishment and physical scaffolding for neurones. However, it is now generally accepted that glia are active participants in brain function that can modulate neuronal communication via several mechanisms. Investigations of anatomical plasticity in the magnocellular neuroendocrine system of the hypothalamic paraventricular and supraoptic nuclei led the way in the development of much of our understanding of glial regulation of neuronal activity. In this review, we provide an overview of glial regulation of magnocellular neurone activity from a historical perspective of the development of our knowledge of the morphological changes that are evident in the paraventricular and supraoptic nuclei. We also focus on recent data from the authors' laboratories presented at the 9th World Congress on Neurohypophysial Hormones that have contributed to our understanding of the multiple mechanisms by which glia modulate the activity of neurones, including: gliotransmitter modulation of synaptic transmission; trans-synaptic modulation by glial neurotransmitter transporter regulation of neurotransmitter spillover; and glial neurotransmitter transporter modulation of excitability by regulation of ambient neurotransmitter levels and their action on extrasynaptic receptors. The magnocellular neuroendocrine system secretes oxytocin and vasopressin from the posterior pituitary gland to control birth, lactation and body fluid balance, and we finally speculate as to whether glial regulation of individual magnocellular neurones might co-ordinate population activity to respond appropriately to altered physiological circumstances.

Enhanced expression of heme oxygenase-1 and carbon monoxide excitatory effects in oxytocin and vasopressin neurones during water deprivation

A growing body of evidence indiates that carbon monoxide (CO) acts as a gas neurotransmitter within the central nervous system. Although CO has been shown to affect neurohypophyseal hormone release in response to osmotic stimuli, the precise sources, targets and mechanisms underlying the actions of CO within the magnocellular neurosecretory system remain largely unknown. In the present study, we combined immunohistochemistry and patch-clamp electrophysiology to study the cellular distribution of the CO-synthase enzyme heme oxygenase type 1 (HO-1), as well as the actions of CO on oxytocin (OT) and vasopressin (VP) magnocellular neurosecretory cells (MNCs), in euhydrated (EU) and 48-h water-deprived rats (48WD). Our results show the expression of HO-1 immunoreactivity both in OT and VP neurones, as well as in a small proportion of astrocytes, both in supraoptic (SON) and paraventricular (PVN) nuclei. HO-1 expression, and its colocalisation with OT and VP neurones within the SON and PVN, was significantly enhanced in 48WD rats. Inhibition of HO activity with chromium mesoporphyrin IX chloride (CrMP; 20 μm) resulted in a slight membrane hyperpolarisation in SON neurones from EU rats, without significantly affecting their firing activity. In 48WD rats, on the other hand, CrMP resulted in a more robust membrane hyperpolarisation, significantly decreasing neuronal firing discharge. Taken together, our results indicate that magnocellular SON and PVN neurones express HO-1, and that CO acts as an excitatory gas neurotransmitter in this system. Moreover, we found that the expression and actions of CO were enhanced in water-deprived rats, suggesting that the state-dependent up-regulation of the HO-1/CO signalling pathway contributes to enhance MNCs firing activity during an osmotic challenge.

The age of enlightenment: evolving opportunities in brain research through optical manipulation of neuronal activity

Optical manipulation of neuronal activity has rapidly developed into the most powerful and widely used approach to study mechanisms related to neuronal connectivity over a range of scales. Since the early use of single site uncaging to map network connectivity, rapid technological development of light modulation techniques has added important new options, such as fast scanning photostimulation, massively parallel control of light stimuli, holographic uncaging, and two-photon stimulation techniques. Exciting new developments in optogenetics complement neurotransmitter uncaging techniques by providing cell-type specificity and in vivo usability, providing optical access to the neural substrates of behavior. Here we review the rapid evolution of methods for the optical manipulation of neuronal activity, emphasizing crucial recent developments.

Optogenetic activation of LiGluR-expressing astrocytes evokes anion channel-mediated glutamate release

Increases in astrocyte Ca(2+) have been suggested to evoke gliotransmitter release, however, the mechanism of release, the identity of such transmitter(s), and even whether and when such release occurs, are controversial, largely due to the lack of a method for selective and reproducible stimulation of electrically silent astrocytes. Here we show that photoactivation of the light-gated Ca(2+)-permeable ionotropic GluR6 glutamate receptor (LiGluR), and to a lesser extent the new Ca(2+)-translocating channelrhodopsin CatCh, evokes more reliable Ca(2+) elevation than the mutant channelrhodopsin 2, ChR2(H134R) in cultured cortical astrocytes. We used evanescent-field excitation for near-membrane Ca(2+) imaging, and epifluorescence to activate and inactivate LiGluR. By alternating activation and inactivation light pulses, the LiGluR-evoked Ca(2+) rises could be graded in amplitude and duration. The optical stimulation of LiGluR-expressing astrocytes evoked probabilistic glutamate-mediated signalling to adjacent LiGluR-non-expressing astrocytes. This astrocyte-to-astrocyte signalling was insensitive to the inactivation of vesicular release, hemichannels and glutamate-transporters, and sensitive to anion channel blockers. Our results show that LiGluR is a powerful tool to selectively and reproducibly activate astrocytes.

Bidirectional coupling between astrocytes and neurons mediates learning and dynamic coordination in the brain: a multiple modeling approach

In recent years research suggests that astrocyte networks, in addition to nutrient and waste processing functions, regulate both structural and synaptic plasticity. To understand the biological mechanisms that underpin such plasticity requires the development of cell level models that capture the mutual interaction between astrocytes and neurons. This paper presents a detailed model of bidirectional signaling between astrocytes and neurons (the astrocyte-neuron model or AN model) which yields new insights into the computational role of astrocyte-neuronal coupling. From a set of modeling studies we demonstrate two significant findings. Firstly, that spatial signaling via astrocytes can relay a "learning signal" to remote synaptic sites. Results show that slow inward currents cause synchronized postsynaptic activity in remote neurons and subsequently allow Spike-Timing-Dependent Plasticity based learning to occur at the associated synapses. Secondly, that bidirectional communication between neurons and astrocytes underpins dynamic coordination between neuron clusters. Although our composite AN model is presently applied to simplified neural structures and limited to coordination between localized neurons, the principle (which embodies structural, functional and dynamic complexity), and the modeling strategy may be extended to coordination among remote neuron clusters.

Activity-induced remodeling of olfactory bulb microcircuits revealed by monosynaptic tracing

The continued addition of new neurons to mature olfactory circuits represents a remarkable mode of cellular and structural brain plasticity. However, the anatomical configuration of newly established circuits, the types and numbers of neurons that form new synaptic connections, and the effect of sensory experience on synaptic connectivity in the olfactory bulb remain poorly understood. Using in vivo electroporation and monosynaptic tracing, we show that postnatal-born granule cells form synaptic connections with centrifugal inputs and mitral/tufted cells in the mouse olfactory bulb. In addition, newly born granule cells receive extensive input from local inhibitory short axon cells, a poorly understood cell population. The connectivity of short axon cells shows clustered organization, and their synaptic input onto newborn granule cells dramatically and selectively expands with odor stimulation. Our findings suggest that sensory experience promotes the synaptic integration of new neurons into cell type-specific olfactory circuits.

TRPA1 channels regulate astrocyte resting calcium and inhibitory synapse efficacy through GAT-3

Astrocytes contribute to the formation and function of synapses and are found throughout the brain, where they show intracellular store-mediated Ca(2+) signals. Here, using a membrane-tethered, genetically encoded calcium indicator (Lck-GCaMP3), we report the serendipitous discovery of a new type of Ca(2+) signal in rat hippocampal astrocyte-neuron cocultures. We found that Ca(2+) fluxes mediated by transient receptor potential A1 (TRPA1) channels gave rise to frequent and highly localized 'spotty' Ca(2+) microdomains near the membrane that contributed appreciably to resting Ca(2+) in astrocytes. Mechanistic evaluations in brain slices showed that decreases in astrocyte resting Ca(2+) concentrations mediated by TRPA1 channels decreased interneuron inhibitory synapse efficacy by reducing GABA transport by GAT-3, thus elevating extracellular GABA. Our data show how a transmembrane Ca(2+) source (TRPA1) targets a transporter (GAT-3) in astrocytes to regulate inhibitory synapses.

A tale of two stories: astrocyte regulation of synaptic depression and facilitation

Short-term presynaptic plasticity designates variations of the amplitude of synaptic information transfer whereby the amount of neurotransmitter released upon presynaptic stimulation changes over seconds as a function of the neuronal firing activity. While a consensus has emerged that the resulting decrease (depression) and/or increase (facilitation) of the synapse strength are crucial to neuronal computations, their modes of expression in vivo remain unclear. Recent experimental studies have reported that glial cells, particularly astrocytes in the hippocampus, are able to modulate short-term plasticity but the mechanism of such a modulation is poorly understood. Here, we investigate the characteristics of short-term plasticity modulation by astrocytes using a biophysically realistic computational model. Mean-field analysis of the model, supported by intensive numerical simulations, unravels that astrocytes may mediate counterintuitive effects. Depending on the expressed presynaptic signaling pathways, astrocytes may globally inhibit or potentiate the synapse: the amount of released neurotransmitter in the presence of the astrocyte is transiently smaller or larger than in its absence. But this global effect usually coexists with the opposite local effect on paired pulses: with release-decreasing astrocytes most paired pulses become facilitated, namely the amount of neurotransmitter released upon spike i+1 is larger than that at spike i, while paired-pulse depression becomes prominent under release-increasing astrocytes. Moreover, we show that the frequency of astrocytic intracellular Ca(2+) oscillations controls the effects of the astrocyte on short-term synaptic plasticity. Our model explains several experimental observations yet unsolved, and uncovers astrocytic gliotransmission as a possible transient switch between short-term paired-pulse depression and facilitation. This possibility has deep implications on the processing of neuronal spikes and resulting information transfer at synapses.

Astrocytes are endogenous regulators of basal transmission at central synapses

Basal synaptic transmission involves the release of neurotransmitters at individual synapses in response to a single action potential. Recent discoveries show that astrocytes modulate the activity of neuronal networks upon sustained and intense synaptic activity. However, their ability to regulate basal synaptic transmission remains ill defined and controversial. Here, we show that astrocytes in the hippocampal CA1 region detect synaptic activity induced by single-synaptic stimulation. Astrocyte activation occurs at functional compartments found along astrocytic processes and involves metabotropic glutamate subtype 5 receptors. In response, astrocytes increase basal synaptic transmission, as revealed by the blockade of their activity with a Ca(2+) chelator. Astrocytic modulation of basal synaptic transmission is mediated by the release of purines and the activation of presynaptic A(2A) receptors by adenosine. Our work uncovers an essential role for astrocytes in the regulation of elementary synaptic communication and provides insight into fundamental aspects of brain function.

Prostaglandin E2 release from astrocytes triggers gonadotropin-releasing hormone (GnRH) neuron firing via EP2 receptor activation

Astrocytes in the hypothalamus release prostaglandin E(2) (PGE(2)) in response to cell-cell signaling initiated by neurons and glial cells. Upon release, PGE(2) stimulates the secretion of gonadotropin-releasing hormone (GnRH), the neuropeptide that controls reproduction, from hypothalamic neuroendocrine neurons. Whether this effect on GnRH secretion is accompanied by changes in the firing behavior of these neurons is unknown. Using patch-clamp recording we demonstrate that PGE(2) exerts a dose-dependent postsynaptic excitatory effect on GnRH neurons. These effects are mimicked by an EP2 receptor agonist and attenuated by protein kinase A (PKA) inhibitors. The acute blockade of prostaglandin synthesis by indomethacin (INDO) or the selective inhibition of astrocyte metabolism by fluoroacetate (FA) suppresses the spontaneous firing activity of GnRH neurons in brain slices. Similarly, GnRH neuronal activity is reduced in mice with impaired astrocytic PGE(2) release due to defective erbB signaling in astrocytes. These results indicate that astrocyte-to-neuron communication in the hypothalamus is essential for the activity of GnRH neurons and suggest that PGE(2) acts as a gliotransmitter within the GnRH neurosecretory system.

Bulk loading of calcium indicator dyes to study astrocyte physiology: key limitations and improvements using morphological maps

Calcium signaling has been studied in astrocyte cell bodies using bulk loading of calcium indicator dyes, and astrocytes are known to display intracellular calcium transients. An assumption in recent data on the neuronal impact of somatic astrocyte calcium transients has been that bulk loading reflects signaling in relevant astrocyte compartments such as processes. We assessed bulk loading using Sholl analysis (Sholl, 1953) of astrocytes loaded with common calcium indicator dyes and compared these data with Sholl analysis of astrocyte morphology. In the CA1 region of the hippocampus from rats, we found that bulk loading of calcium indicator dyes only reports on calcium signals within the soma and in the most proximal processes, leaving ∼90% of the area of an astrocyte and its extensive processes unsampled. By using morphological reconstructions as "maps" after the imaging session, we present simple procedures that remedy these shortfalls and permit reliable detection of calcium transients in distal astrocyte processes. The data thus reveal limitations in the interpretation of astrocyte calcium imaging data gathered with bulk loading and provide refinements to minimize these shortcomings.

Astrocytes display complex and localized calcium responses to single-neuron stimulation in the hippocampus

Astrocytes show a complex structural and physiological interplay with neurons and respond to neuronal activation in vitro and in vivo with intracellular calcium elevations. These calcium changes enable astrocytes to modulate synaptic transmission and plasticity through various mechanisms. However, the response pattern of astrocytes to single neuronal depolarization events still remains unresolved. This information is critical for fully understanding the coordinated network of neuron-glial signaling in the brain. To address this, we developed a system to map astrocyte calcium responses along apical dendrites of CA1 pyramidal neurons in hippocampal slices using single-neuron stimulation with channelrhodopsin-2. This technique allowed selective neuronal depolarization without invasive manipulations known to alter calcium levels in astrocytes. Light-evoked neuronal depolarization was elicited and calcium events in surrounding astrocytes were monitored using the calcium-sensitive dye Calcium Orange. Stimulation of single neurons caused calcium responses in populations of astrocytes along the apical axis of CA1 cell dendrites. Calcium responses included single events that were synchronized with neuronal stimulation and poststimulus changes in calcium event frequency, both of which were modulated by glutamatergic and purinergic signaling. Individual astrocytes near CA1 cells showed low ability to respond to repeated neuronal depolarization events. However, the response of the surrounding astrocyte population was remarkably accurate. Interestingly, the reliability of responses was graded with respect to astrocyte location along the CA1 cell dendrite, with astrocytes residing in the primary dendrite subregion being most responsive. This study provides a new perspective on the dynamic response property of astrocyte ensembles to neuronal activity.

Modulation of respiratory responses to chemoreflex activation by L-glutamate and ATP in the rostral ventrolateral medulla of awake rats

Presympathetic neurons in the different anteroposterior aspects of rostral ventrolateral medulla (RVLM) are colocalized with expiratory [Bötzinger complex (BötC)] and inspiratory [pre-Bötzinger complex (pre-BötC)] neurons of ventral respiratory column (VRC), suggesting that this region integrates the cardiovascular and respiratory chemoreflex responses. In the present study, we evaluated in different anteroposterior aspects of RVLM of awake rats the role of ionotropic glutamate and purinergic receptors on cardiorespiratory responses to chemoreflex activation. The bilateral ionotropic glutamate receptors antagonism with kynurenic acid (KYN) (8 nmol/50 nl) in the rostral aspect of RVLM (RVLM/BötC) enhanced the tachypneic (120 ± 9 vs. 180 ± 9 cpm; P < 0.01) and attenuated the pressor response (55 ± 2 vs. 15 ± 1 mmHg; P < 0.001) to chemoreflex activation (n = 7). On the other hand, bilateral microinjection of KYN into the caudal aspect of RVLM (RVLM/pre-BötC) caused a respiratory arrest in four awake rats used in the present study. Bilateral P2X receptors antagonism with PPADS (0.25 nmol/50 nl) in the RVLM/BötC reduced chemoreflex tachypneic response (127 ± 6 vs. 70 ± 5 cpm; P < 0.001; n = 6), but did not change the chemoreflex pressor response. In addition, PPADS into the RVLM/BötC attenuated the enhancement of the tachypneic response to chemoreflex activation elicited by previous microinjections of KYN into the same subregion (188 ± 2 vs. 157 ± 3 cpm; P < 0.05; n = 5). Our findings indicate that: 1) L-glutamate, but not ATP, in the RVLM/BötC is required for pressor response to peripheral chemoreflex and 2) both transmitters in the RVLM/BötC are required for the processing of the ventilatory response to peripheral chemoreflex activation in awake rats.

ATP: a ubiquitous gliotransmitter integrating neuron-glial networks

Astrocytes are ideally situated to integrate glial and neuronal functions and neurovascular coupling by way of their multiple contacts with neurons, glia and blood vessels. There is a high degree of specialisation of astroglial membranes at the different sites of contact, including the expression of neurotransmitter receptors, ion channels, transporters and gap junctional proteins. An apparently universal property of astrocytes throughout the CNS is their responsiveness to ATP acting via metabotropic P2Y receptors, with a prominent role for the P2Y1 receptor subtype. Activation of astroglial P2Y receptors triggers a rise in intracellular calcium, which is the substrate for astroglial excitability and intercellular communication. In addition, astrocytes have a number of mechanisms for the release of ATP, which can be considered a 'gliotransmitter'. Astrocytes may be the most widespread source of ATP release in the CNS, and astroglial ATP and its metabolite adenosine activate purine receptors on neurons, microglia, oligodendrocytes and blood vessels. There is compelling evidence that astroglial ATP and adenosine regulate neuronal synaptic strength, although the physiological significance of this astrocyte-to-neuron signalling is questioned. A less appreciated aspect of astrocyte signalling is that they also release neurotransmitters onto other glia. Notably, both ATP and adenosine control microglial behaviour and regulate oligodendrocyte differentiation and myelination. P2 receptors also mediate injury responses in all glial cell types, with a prominent role for the P2X7 receptor subtype. In addition, ATP is a potent vasoconstrictor and astrocytes provide a route for coupling blood flow to neuronal activity by way of their synaptic and perivascular connections. Thus, astrocytes are the fulcrum of neuron-glial-vascular networks and purinergic signalling is the primary mechanism by which astrocytes can integrate the functions of these diverse elements.

Learning an operant conditioning task differentially induces gliogenesis in the medial prefrontal cortex and neurogenesis in the hippocampus

Circuit modification associated with learning and memory involves multiple events, including the addition and remotion of newborn cells trough adulthood. Adult neurogenesis and gliogenesis were mainly described in models of voluntary exercise, enriched environments, spatial learning and memory task; nevertheless, it is unknown whether it is a common mechanism among different learning paradigms, like reward dependent tasks. Therefore, we evaluated cell proliferation, neurogenesis, astrogliogenesis, survival and neuronal maturation in the medial prefrontal cortex (mPFC) and the hippocampus (HIPP) during learning an operant conditioning task. This was performed by using endogenous markers of cell proliferation, and a bromodeoxiuridine (BrdU) injection schedule in two different phases of learning. Learning an operant conditioning is divided in two phases: a first phase when animals were considered incompletely trained (IT, animals that were learning the task) when they performed between 50% and 65% of the responses, and a second phase when animals were considered trained (Tr, animals that completely learned the task) when they reached 100% of the responses with a latency time lower than 5 seconds. We found that learning an operant conditioning task promoted cell proliferation in both phases of learning in the mPFC and HIPP. Additionally, the results presented showed that astrogliogenesis was induced in the medial prefrontal cortex (mPFC) in both phases, however, the first phase promoted survival of these new born astrocytes. On the other hand, an increased number of new born immature neurons was observed in the HIPP only in the first phase of learning, whereas, decreased values were observed in the second phase. Finally, we found that neuronal maturation was induced only during the first phase. This study shows for the first time that learning a reward-dependent task, like the operant conditioning, promotes neurogenesis, astrogliogenesis, survival and neuronal maturation depending on the learning phase in the mPFC-HIPP circuit.

A practical guide to the synthesis and use of membrane-permeant acetoxymethyl esters of caged inositol polyphosphates

This protocol describes a method for efficient chemical synthesis of an analog of inositol-1,4,5-trisphosphate (IP(3)) hexakis acetoxymethyl ester having an ortho-nitroveratryl photochemical caging group on the 6-hydroxyl position. The six esters render the probe membrane permeant, such that it can be loaded into intact living cells in vitro or in vivo. Inside cells, the caged IP(3) is inert until activated by two-photon excitation at 720 nm. The photoliberated signaling molecule can mobilize release of Ca(2+) from intracellular stores on the endoplasmic reticulum. When co-loaded with the fluorescent Ca(2+) indicator rhod-2, one laser can be used for stimulating and monitoring intracellular Ca(2+) signaling with single-cell resolution. This protocol has chemistry and biology sections; the former describes the organic synthesis of the caged IP(3), which requires 12 d, and the latter an application to a day-long study of astrocyte-regulated neuronal function in living brain slices acutely isolated from rats. As Ca(2+) is the single most important intracellular second messenger and the IP(3)-Ca(2+) signaling cascade is used by many cells to produce increases in Ca(2+) concentration, this method should be widely applicable for the study of a variety of physiological processes in intact biological systems.

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

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

Monitoring astrocyte calcium microdomains with improved membrane targeted GCaMP reporters

Astrocytes are involved in synaptic and cerebrovascular regulation in the brain. These functions are regulated by intracellular calcium signalling that is thought to reflect a form of astrocyte excitability. In a recent study, we reported modification of the genetically encoded calcium indicator (GECI) GCaMP2 with a membrane-tethering domain, Lck, to generate Lck-GCaMP2. This GECI allowed us to detect novel microdomain calcium signals. The microdomains were random and 'spotty' in nature. In order to detect such signals more reliably, in the present study we further modified Lck-GCaMP2 to carry three mutations in the GCaMP2 moiety (M153K, T203V within EGFP and N60D in the CaM domain) to generate Lck-GCaMP3. We directly compared Lck-GCaMP2 and Lck-GCaMP3 by assessing their ability to monitor several types of astrocyte calcium signals with a focus on spotty microdomains. Our data show that Lck-GCaMP3 is between two- and four-times better than Lck-GCaMP2 in terms of its basal fluorescence intensity, signal-to-noise and its ability to detect microdomains. The use of Lck-GCaMP3 thus represents a significantly improved way to monitor astrocyte calcium signals, including microdomains, and will facilitate detailed exploration of their molecular mechanisms and physiological roles.

Is astrocyte calcium signaling relevant for synaptic plasticity?

Astrocytes constitute a major group of glial cells which were long regarded as passive elements, fulfilling nutritive and structural functions for neurons. Calcium rise in astrocytes propagating to neurons was the first demonstration of direct interaction between the two cell types. Since then, calcium has been widely used, not only as an indicator of astrocytic activity but also as a stimulator switch to control astrocyte physiology. As a result, astrocytes have been elevated from auxiliaries to neurons, to cells involved in processing synaptic information. Curiously, while there is evidence that astrocytes play an important role in synaptic plasticity, the data relating to calcium's pivotal role are inconsistent. In this review, we will detail the various mechanisms of calcium flux in astrocytes, then briefly present the calcium-dependent mechanisms of gliotransmitter release. Finally, we will discuss the role of calcium in plasticity and present alternative explanations that could reconcile the conflicting results published recently.

Astrocytes in the retrotrapezoid nucleus sense H+ by inhibition of a Kir4.1-Kir5.1-like current and may contribute to chemoreception by a purinergic mechanism

Central chemoreception is the mechanism by which CO(2)/pH sensors regulate breathing in response to tissue pH changes. There is compelling evidence that pH-sensitive neurons in the retrotrapezoid nucleus (RTN) are important chemoreceptors. Evidence also indicates that CO(2)/H(+)-evoked adenosine 5'-triphosphate (ATP) release in the RTN, from pH-sensitive astrocytes, contributes to chemoreception. However, mechanism(s) by which RTN astrocytes sense pH is unknown and their contribution to chemoreception remains controversial. Here, we use the brain slice preparation and a combination of patch-clamp electrophysiology and immunohistochemistry to confirm that RTN astrocytes are pH sensitive and to determine mechanisms by which they sense pH. We show that pH-sensitive RTN glia are immunoreactive for aldehyde dehydrogenase 1L1, a marker of astrocytes. In HEPES buffer the pH-sensitive current expressed by RTN astrocytes reversed near E(K(+)) (the equilibrium potential for K(+)) and was inhibited by Ba(2+) and desipramine (blocker of Kir4.1-containing channels), characteristics most consistent with heteromeric Kir4.1-Kir5.1 channels. In bicarbonate buffer, the sodium/bicarbonate cotransporter also contributed to the CO(2)/H(+)-sensitive current in RTN astrocytes. To test the hypothesis that RTN astrocytes contribute to chemoreception by a purinergic mechanism, we used fluorocitrate to selectively depolarize astrocytes while measuring neuronal activity. We found that fluorocitrate increased baseline activity and pH sensitivity of RTN neurons by a P2-receptor-dependent mechanism, suggesting that astrocytes may release ATP to activate RTN chemoreceptors. We also found in bicarbonate but not HEPES buffer that P2-receptor antagonists decreased CO(2) sensitivity of RTN neurons. We conclude that RTN astrocytes sense CO(2)/H(+) in part by inhibition of a Kir4.1-Kir5.1-like current and may provide an excitatory purinergic drive to pH-sensitive neurons.

Perisynaptic glia discriminate patterns of motor nerve activity and influence plasticity at the neuromuscular junction

In the nervous system, the induction of plasticity is coded by patterns of synaptic activity. Glial cells are now recognized as dynamic partners in a wide variety of brain functions, including the induction and modulation of various forms of synaptic plasticity. However, it appears that glial cells are usually activated by stereotyped, sustained neuronal activity, and little attention has been given to more subtle changes in the patterns of synaptic activation. To this end, we used the mouse neuromuscular junction as a simple and useful model to study glial modulation of synaptic plasticity. We used two patterns of motor nerve stimulation that mimic endogenous motor-neuronal activity. A continuous stimulation induced a post-tetanic potentiation and a phasic Ca(2+) response in perisynaptic Schwann cells (PSCs), glial cells at this synapse. A bursting pattern of activity induced a post-tetanic depression and oscillatory Ca(2+) responses in PSCs. The different Ca(2+) responses in PSCs indicate that they decode the pattern of synaptic activity. Furthermore, the chelation of glial Ca(2+) impaired the production of the sustained plasticity events indicating that PSCs govern the outcome of synaptic plasticity. The mechanisms involved were studied using direct photo-activation of PSCs with caged Ca(2+) that mimicked endogenous plasticity. Using specific pharmacology and transgenic knock-out animals for adenosine receptors, we showed that the sustained depression was mediated by A1 receptors while the sustained potentiation is mediated by A(2A) receptors. These results demonstrate that glial cells decode the pattern of synaptic activity and subsequently provide bidirectional feedback to synapses.