Steady-State Free Ca2+ in Astrocytes Is Decreased by Experience and Impacts Arteriole Tone

Astrocytic NMDA Receptors

Astrocytic NMDA receptors (NMDARs) are heterotetramers, whose expression and properties are largely determined by their subunit composition. Astrocytic NMDARs are characterized by a low sensitivity to magnesium ions and low calcium conductivity. Their activation plays an important role in the regulation of various intracellular processes, such as gene expression and mitochondrial function. Astrocytic NMDARs are involved in calcium signaling in astrocytes and can act through the ionotropic and metabotropic pathways. Astrocytic NMDARs participate in the interactions of the neuroglia, thus affecting synaptic plasticity. They are also engaged in the astrocyte-vascular interactions and contribute to the regulation of vascular tone. Astrocytic NMDARs are involved in various pathologies, such as ischemia and hyperammonemia, and their blockade prevents negative changes in astrocytes during these diseases.

Astrocyte-induced mGluR1 activates human lung cancer brain metastasis via glutamate-dependent stabilization of EGFR

There are limited methods to stably analyze the interactions between cancer cells and glial cells in vitro, which hinders our molecular understanding. Here, we develop a simple and stable culture method of mouse glial cells, termed mixed-glial culture on/in soft substrate (MGS), which serves well as a platform to study cancer-glia interactions. Using this method, we find that human lung cancer cells become overly dependent on metabotropic glutamate receptor 1 (mGluR1) signaling in the brain microenvironment. Mechanistically, interactions with astrocytes induce mGluR1 in cancer cells through the Wnt-5a/prickle planar cell polarity protein 1 (PRICKLE1)/RE1 silencing transcription factor (REST) axis. Induced mGluR1 directly interacts with and stabilizes the epidermal growth factor receptor (EGFR) in a glutamate-dependent manner, and these cells then become responsive to mGluR1 inhibition. Our results highlight increased dependence on mGluR1 signaling as an adaptive strategy and vulnerability of human lung cancer brain metastasis.

Modeling the relationship between neuronal activity and the BOLD signal: contributions from astrocyte calcium dynamics

Functional magnetic resonance imaging relies on the coupling between neuronal and vascular activity, but the mechanisms behind this coupling are still under discussion. Recent experimental evidence suggests that calcium signaling may play a significant role in neurovascular coupling. However, it is still controversial where this calcium signal is located (in neurons or elsewhere), how it operates and how relevant is its role. In this paper we introduce a biologically plausible model of the neurovascular coupling and we show that calcium signaling in astrocytes can explain main aspects of the dynamics of the coupling. We find that calcium signaling can explain so-far unrelated features such as the linear and non-linear regimes, the negative vascular response (undershoot) and the emergence of a (calcium-driven) Hemodynamic Response Function. These features are reproduced here for the first time by a single model of the detailed neuronal-astrocyte-vascular pathway. Furthermore, we analyze how information is coded and transmitted from the neuronal to the vascular system and we predict that frequency modulation of astrocytic calcium dynamics plays a key role in this process. Finally, our work provides a framework to link neuronal activity to the BOLD signal, and vice-versa, where neuronal activity can be inferred from the BOLD signal. This opens new ways to link known alterations of astrocytic calcium signaling in neurodegenerative diseases (e.g. Alzheimer's and Parkinson's diseases) with detectable changes in the neurovascular coupling.

Astrocytes amplify neurovascular coupling to sustained activation of neocortex in awake mice

Functional hyperemia occurs when enhanced neuronal activity signals to increase local cerebral blood flow (CBF) to satisfy regional energy demand. Ca²⁺ elevation in astrocytes can drive arteriole dilation to increase CBF, yet affirmative evidence for the necessity of astrocytes in functional hyperemia in vivo is lacking. In awake mice, we discovered that functional hyperemia is bimodal with a distinct early and late component whereby arteriole dilation progresses as sensory stimulation is sustained. Clamping astrocyte Ca²⁺ signaling in vivo by expressing a plasma membrane Ca²⁺ ATPase (CalEx) reduces sustained but not brief sensory-evoked arteriole dilation. Elevating astrocyte free Ca²⁺ using chemogenetics selectively augments sustained hyperemia. Antagonizing NMDA-receptors or epoxyeicosatrienoic acid production reduces only the late component of functional hyperemia, leaving brief increases in CBF to sensory stimulation intact. We propose that a fundamental role of astrocyte Ca²⁺ is to amplify functional hyperemia when neuronal activation is prolonged.

Emerging role of astrocytes in oxytocin-mediated control of neural circuits and brain functions

The neuropeptide oxytocin has been in the focus of scientists for decades due to its profound and pleiotropic effects on physiology, activity of neuronal circuits and behaviors, among which sociality. Until recently, it was believed that oxytocinergic action exclusively occurs through direct activation of neuronal oxytocin receptors. However, several studies demonstrated the existence and functional relevance of astroglial oxytocin receptors in various brain regions in the mouse and rat brain. Astrocytic signaling and activity is critical for many important physiological processes including metabolism, neurotransmitter clearance from the synaptic cleft and integrated brain functions. While it can be speculated that oxytocinergic action on astrocytes predominantly facilitates neuromodulation via the release of specific gliotransmitters, the precise role of astrocytic oxytocin receptors remains elusive. In this review, we discuss the latest studies on the interaction between the oxytocinergic system and astrocytes, including detailed information about intracellular cascades, and speculate about future research directions on astrocytic oxytocin signaling.

Extracellular Calcium Influx Pathways in Astrocyte Calcium Microdomain Physiology

Astrocytes are complex glial cells that play many essential roles in the brain, including the fine-tuning of synaptic activity and blood flow. These roles are linked to fluctuations in intracellular Ca2+ within astrocytes. Recent advances in imaging techniques have identified localized Ca2+ transients within the fine processes of the astrocytic structure, which we term microdomain Ca2+ events. These Ca2+ transients are very diverse and occur under different conditions, including in the presence or absence of surrounding circuit activity. This complexity suggests that different signalling mechanisms mediate microdomain events which may then encode specific astrocyte functions from the modulation of synapses up to brain circuits and behaviour. Several recent studies have shown that a subset of astrocyte microdomain Ca2+ events occur rapidly following local neuronal circuit activity. In this review, we consider the physiological relevance of microdomain astrocyte Ca2+ signalling within brain circuits and outline possible pathways of extracellular Ca2+ influx through ionotropic receptors and other Ca2+ ion channels, which may contribute to astrocyte microdomain events with potentially fast dynamics.

Decreased parenchymal arteriolar tone uncouples vessel-to-neuronal communication in a mouse model of vascular cognitive impairment

Chronic hypoperfusion is a key contributor to cognitive decline and neurodegenerative conditions, but the cellular mechanisms remain ill-defined. Using a multidisciplinary approach, we sought to elucidate chronic hypoperfusion-evoked functional changes at the neurovascular unit. We used bilateral common carotid artery stenosis (BCAS), a well-established model of vascular cognitive impairment, combined with an ex vivo preparation that allows pressurization of parenchymal arterioles in a brain slice. Our results demonstrate that mild (~ 30%), chronic hypoperfusion significantly altered the functional integrity of the cortical neurovascular unit. Although pial cerebral perfusion recovered over time, parenchymal arterioles progressively lost tone, exhibiting significant reductions by day 28 post-surgery. We provide supportive evidence for reduced adenosine 1 receptor-mediated vasoconstriction as a potential mechanism in the adaptive response underlying the reduced baseline tone in parenchymal arterioles. In addition, we show that in response to the neuromodulator adenosine, the action potential frequency of cortical pyramidal neurons was significantly reduced in all groups. However, a significant decrease in adenosine-induced hyperpolarization was observed in BCAS 14 days. At the microvascular level, constriction-induced inhibition of pyramidal neurons was significantly compromised in BCAS mice. Collectively, these results suggest that BCAS uncouples vessel-to-neuron communication-vasculo-neuronal coupling-a potential early event in cognitive decline.

Calcium signaling in neuroglia

Glial cells exploit calcium (Ca²⁺) signals to perceive the information about the activity of the nervous tissue and the tissue environment to translate this information into an array of homeostatic, signaling and defensive reactions. Astrocytes, the best studied glial cells, use several Ca²⁺ signaling generation pathways that include Ca²⁺ entry through plasma membrane, release from endoplasmic reticulum (ER) and from mitochondria. Activation of metabotropic receptors on the plasma membrane of glial cells is coupled to an enzymatic cascade in which a second messenger, InsP₃ is generated thus activating intracellular Ca²⁺ release channels in the ER endomembrane. Astrocytes also possess store-operated Ca²⁺ entry and express several ligand-gated Ca²⁺ channels. In vivo astrocytes generate heterogeneous Ca²⁺ signals, which are short and frequent in distal processes, but large and relatively rare in soma. In response to neuronal activity intracellular and inter-cellular astrocytic Ca²⁺ waves can be produced. Astrocytic Ca²⁺ signals are involved in secretion, they regulate ion transport across cell membranes, and are contributing to cell morphological plasticity. Therefore, astrocytic Ca²⁺ signals are linked to fundamental functions of the central nervous system ranging from synaptic transmission to behavior. In oligodendrocytes, Ca²⁺ signals are generated by plasmalemmal Ca²⁺ influx, or by release from intracellular stores, or by combination of both. Microglial cells exploit Ca²⁺ permeable ionotropic purinergic receptors and transient receptor potential channels as well as ER Ca²⁺ release. In this contribution, basic morphology of glial cells, glial Ca²⁺ signaling toolkit, intracellular Ca²⁺ signals and Ca²⁺-regulated functions are discussed with focus on astrocytes.

Non-canonical glutamate signaling in a genetic model of migraine with aura

Migraine with aura is a common but poorly understood sensory circuit disorder. Monogenic models allow an opportunity to investigate its mechanisms, including spreading depolarization (SD), the phenomenon underlying migraine aura. Using fluorescent glutamate imaging, we show that awake mice carrying a familial hemiplegic migraine type 2 (FHM2) mutation have slower clearance during sensory processing, as well as previously undescribed spontaneous "plumes" of glutamate. Glutamatergic plumes overlapped anatomically with a reduced density of GLT-1a-positive astrocyte processes and were mimicked in wild-type animals by inhibiting glutamate clearance. Plume pharmacology and plume-like neural Ca2+ events were consistent with action-potential-independent spontaneous glutamate release, suggesting plumes are a consequence of inefficient clearance following synaptic release. Importantly, a rise in basal glutamate and plume frequency predicted the onset of SD in both FHM2 and wild-type mice, providing a novel mechanism in migraine with aura and, by extension, the other neurological disorders where SD occurs.

Whole brain irradiation in mice causes long-term impairment in astrocytic calcium signaling but preserves astrocyte-astrocyte coupling

Whole brain irradiation (WBI) therapy is an important treatment for brain metastases and potential microscopic malignancies. WBI promotes progressive cognitive dysfunction in over half of surviving patients, yet, the underlying mechanisms remain obscure. Astrocytes play critical roles in the regulation of neuronal activity, brain metabolism, and cerebral blood flow, and while neurons are considered radioresistant, astrocytes are sensitive to γ-irradiation. Hallmarks of astrocyte function are the ability to generate stimulus-induced intercellular Ca2+ signals and to move metabolic substrates through the connected astrocyte network. We tested the hypothesis that WBI-induced cognitive impairment associates with persistent impairment of astrocytic Ca2+ signaling and/or gap junctional coupling. Mice were subjected to a clinically relevant protocol of fractionated WBI, and 12 to 15 months after irradiation, we confirmed persistent cognitive impairment compared to controls. To test the integrity of astrocyte-to-astrocyte gap junctional coupling postWBI, astrocytes were loaded with Alexa-488-hydrazide by patch-based dye infusion, and the increase of fluorescence signal in neighboring astrocyte cell bodies was assessed with 2-photon microscopy in acute slices of the sensory-motor cortex. We found that WBI did not affect astrocyte-to-astrocyte gap junctional coupling. Astrocytic Ca2+ responses induced by bath administration of phenylephrine (detected with Rhod-2/AM) were also unaltered by WBI. However, an electrical stimulation protocol used in long-term potentiation (theta burst), revealed attenuated astrocyte Ca2+ responses in the astrocyte arbor and soma in WBI. Our data show that WBI causes a long-lasting decrement in synaptic-evoked astrocyte Ca2+ signals 12-15 months postirradiation, which may be an important contributor to cognitive decline seen after WBI.

More than just summed neuronal activity: how multiple cell types shape the BOLD response

Functional neuroimaging techniques are widely applied to investigations of human cognition and disease. The most commonly used among these is blood oxygen level-dependent (BOLD) functional magnetic resonance imaging. The BOLD signal occurs because neural activity induces an increase in local blood supply to support the increased metabolism that occurs during activity. This supply usually outmatches demand, resulting in an increase in oxygenated blood in an active brain region, and a corresponding decrease in deoxygenated blood, which generates the BOLD signal. Hence, the BOLD response is shaped by an integration of local oxygen use, through metabolism, and supply, in the blood. To understand what information is carried in BOLD signals, we must understand how several cell types in the brain-local excitatory neurons, inhibitory neurons, astrocytes and vascular cells (pericytes, vascular smooth muscle and endothelial cells), and their modulation by ascending projection neurons-contribute to both metabolism and haemodynamic changes. Here, we review the contributions of each cell type to the regulation of cerebral blood flow and metabolism, and discuss situations where a simplified interpretation of the BOLD response as reporting local excitatory activity may misrepresent important biological phenomena, for example with regards to arousal states, ageing and neurological disease. This article is part of the theme issue 'Key relationships between non-invasive functional neuroimaging and the underlying neuronal activity'.

Seizures elevate gliovascular unit Ca2+ and cause sustained vasoconstriction

Seizures can result in a severe hypoperfusion/hypoxic attack that causes postictal memory and behavioral impairments. However, neither postictal changes to microvasculature nor Ca²⁺ changes in key cell types controlling blood perfusion have been visualized in vivo, leaving essential components of the underlying cellular mechanisms unclear. Here, we use 2-photon microvascular and Ca²⁺ imaging in awake mice to show that seizures result in a robust vasoconstriction of cortical penetrating arterioles, which temporally mirrors the prolonged postictal hypoxia. The vascular effect was dependent on cyclooxygenase 2, as pretreatment with ibuprofen prevented postictal vasoconstriction. Moreover, seizures caused a rapid elevation in astrocyte endfoot Ca²⁺ that was confined to the seizure period, and vascular smooth muscle cells displayed a significant increase in Ca²⁺ both during and following seizures, lasting up to 75 minutes. Our data show enduring postictal vasoconstriction and temporal activities of 2 cell types within the neurovascular unit that are associated with seizure-induced hypoperfusion/hypoxia. These findings support prevention of this event may be a novel and tractable treatment strategy in patients with epilepsy who experience extended postseizure impairments.

Seizures cause enduring microvascular constriction via long lasting calcium elevations in vascular smooth muscle cells.

NMDA receptors sustain but do not initiate neuronal depolarization in spreading depolarization

Spreading depolarization (SD) represents a neurological process characterized by a massive, self-sustaining wave of brain cell depolarization. Understanding its mechanism is important for treating ischemic or hemorrhagic stroke and migraine with aura. Many believed that ion fluxes through NMDA receptors (NMDARs) are responsible for neuronal transmembrane currents of SD. However, the explicit role of NMDARs remains ambiguous. This is in part due to the limitation of traditional pharmacological approaches in resolving the contribution of NMDARs in different intercellular and intracellular processes of SD. Here, we applied single-cell blockade and genetic deletion methods to remove functional NMDARs from individual hippocampal CA1 neurons in order to examine the role of NMDARs in the depolarization mechanism without affecting the propagation of SD. We analyzed neuronal membrane potential changes to demonstrate that NMDARs are not required for initiating the depolarization. Consistently, neuronal input resistance (R_N) revealed a sharp decline at the start of SD, which was unaffected by blocking NMDARs. Instead, the recovery of both membrane potential and R_N during the late phase of SD was facilitated by inhibition of NMDARs, indicating that NMDARs are responsible for sustaining the depolarization. Our results strongly indicate that NMDAR activation is not a determinant of the initiation of depolarization but is important for sustaining transmembrane ion fluxes during SD.

The Emerging Nature of Astrocyte Diversity

Astrocytes are morphologically complex, ubiquitous cells that are viewed as a homogeneous population tiling the entire central nervous system (CNS). However, this view has been challenged in the last few years with the availability of RNA sequencing, immunohistochemistry, electron microscopy, morphological reconstruction, and imaging data. These studies suggest that astrocytes represent a diverse population of cells and that they display brain area- and disease-specific properties and functions. In this review, we summarize these observations, emphasize areas where clear conclusions can be made, and discuss potential unifying themes. We also identify knowledge gaps that need to be addressed in order to exploit astrocyte diversity as a biological phenomenon of physiological relevance in the CNS. We thus provide a summary and a perspective on astrocyte diversity in the vertebrate CNS.

Pharmacological or genetic depletion of senescent astrocytes prevents whole brain irradiation-induced impairment of neurovascular coupling responses protecting cognitive function in mice

Whole brain irradiation (WBI, also known as whole brain radiation therapy or WBRT) is a mainstream therapy for patients with identifiable brain metastases and as a prophylaxis for microscopic malignancies. WBI accelerates brain aging, causing progressive cognitive dysfunction in ~ 50% of surviving patients, thus compromising quality of life. The mechanisms responsible for this WBI side effect remain obscure, and there are no effective treatments or prevention strategies. Here, we test the hypothesis that WBI induces astrocyte senescence, which contributes to impaired astrocytic neurovascular coupling (NVC) responses and the genesis of cognitive decline. To achieve this goal, we used transgenic p16-3MR mice, which allows the detection and selective elimination of senescent cells. We subjected these mice to a clinically relevant protocol of fractionated WBI (5 Gy twice weekly for 4 weeks). WBI-treated and control mice were tested for spatial memory performance (radial arm water maze), astrocyte-dependent NVC responses (whisker-stimulation-induced increases in cerebral blood flow, assessed by laser speckle contrast imaging), NVC-related gene expression, astrocytic release of eicosanoid gliotransmitters and the presence of senescent astrocytes (by flow cytometry, immunohistochemistry and gene expression profiling) at 6 months post-irradiation. WBI induced senescence in astrocytes, which associated with NVC dysfunction and impaired performance on cognitive tasks. To establish a causal relationship between WBI-induced senescence and NVC dysfunction, senescent cells were depleted from WBI-treated animals (at 3 months post-WBI) by genetic (ganciclovir treatment) or pharmacological (treatment with the BCL-2/BCL-xL inhibitor ABT263/Navitoclax, a known senolytic drug) means. In WBI-treated mice, both treatments effectively eliminated senescent astrocytes, rescued NVC responses, and improved cognitive performance. Our findings suggest that the use of senolytic drugs can be a promising strategy for preventing the cognitive impairment associated with WBI.

Local Resting Ca2+ Controls the Scale of Astroglial Ca2+ Signals

Astroglia regulate neurovascular coupling while engaging in signal exchange with neurons. The underlying cellular machinery is thought to rely on astrocytic Ca2+ signals, but what controls their amplitude and waveform is poorly understood. Here, we employ time-resolved two-photon excitation fluorescence imaging in acute hippocampal slices and in cortex in vivo to find that resting [Ca2+] predicts the scale (amplitude) and the maximum (peak) of astroglial Ca2+ elevations. We bidirectionally manipulate resting [Ca2+] by uncaging intracellular Ca2+ or Ca2+ buffers and use ratiometric imaging of a genetically encoded Ca2+ indicator to establish that alterations in resting [Ca2+] change co-directionally the peak level and anti-directionally the amplitude of local Ca2+ transients. This relationship holds for spontaneous and for induced (for instance by locomotion) Ca2+ signals. Our findings uncover a basic generic rule of Ca2+ signal formation in astrocytes, thus also associating the resting Ca2+ level with the physiological "excitability" state of astroglia.

Neurovascular Coupling under Chronic Stress Is Modified by Altered GABAergic Interneuron Activity

Neurovascular coupling (NVC), the interaction between neural activity and vascular response, ensures normal brain function by maintaining brain homeostasis. We previously reported altered cerebrovascular responses during functional hyperemia in chronically stressed animals. However, the underlying neuronal-level changes associated with those hemodynamic changes remained unclear. Here, using in vivo and ex vivo experiments, we investigate the neuronal origins of altered NVC dynamics under chronic stress conditions in adult male mice. Stimulus-evoked hemodynamic and neural responses, especially beta and gamma-band local field potential activity, were significantly lower in chronically stressed animals, and the NVC relationship, itself, had changed. Further, using acute brain slices, we discovered that the underlying cause of this change was dysfunction of neuronal nitric oxide synthase (nNOS)-mediated vascular responses. Using FISH to check the mRNA expression of several GABAergic subtypes, we confirmed that only nNOS mRNA was significantly decreased in chronically stressed mice. Ultimately, chronic stress impairs NVC by diminishing nNOS-mediated vasodilation responses to local neural activity. Overall, these findings provide useful information in understanding NVC dynamics in the healthy brain. More importantly, this study reveals that impaired nNOS-mediated NVC function may be a contributory factor in the progression of stress-related diseases.

SIGNIFICANCE STATEMENT The correlation between neuronal activity and cerebral vascular dynamics is defined as neurovascular coupling (NVC), which plays an important role for meeting the metabolic demands of the brain. However, the impact of chronic stress, which is a contributory factor of many cerebrovascular diseases, on NVC is poorly understood. We therefore investigated the effects of chronic stress on impaired neurovascular response to sensory stimulation and their underlying mechanisms. Multimodal approaches, from in vivo hemodynamic imaging and electrophysiology to ex vivo vascular imaging with pharmacological treatment, patch-clamp recording, FISH, and immunohistochemistry revealed that chronic stress-induced dysfunction of nNOS-expressing interneurons contributes to NVC impairment. These findings will provide useful information to understand the role of nNOS interneurons in NVC in normal and pathological conditions.

Cellular Signaling in Müller Glia: Progenitor Cells for Regenerative and Neuroprotective Responses in Pharmacological Models of Retinal Degeneration

Retinal degenerative diseases are a leading cause of visual impairment or blindness. There are many therapies for delaying the progression of vision loss but no curative strategies currently. Stimulating intrinsic neuronal regeneration is a potential approach to therapy in retinal degenerative diseases. In contrast to stem cells, as embryonic/pluripotent stem cell-derived retinal progenitor cell or mesenchymal stem cells, Müller glia provided an endogenous cellular source for regenerative therapy in the retina. Müller glia are a major component of the retina and considerable evidence suggested these cells can be induced to produce the lost neurons in several species. Understanding the specific characteristic of Müller glia to generate lost neurons will inspire an attractive and alternative therapeutic strategy for treating visual impairment with regenerative research. This review briefly provides the different signal transduction mechanisms which are underlying Müller cell-mediated neuroprotection and neuron regeneration and discusses recent advances about regeneration from Müller glia-derived progenitors.

Astrocytes Integrate Behavioral State and Vascular Signals during Functional Hyperemia

Dynamic changes in astrocyte free Ca²⁺ regulate synaptic signaling and local blood flow. Although astrocytes are poised to integrate signals from synapses and the vasculature to perform their functional roles, it remains unclear what dictates astrocyte responses during neurovascular coupling under realistic conditions. We examined peri-arteriole and peri-capillary astrocytes in the barrel cortex of active mice in response to sensory stimulation or volitional behaviors. We observed an AMPA and NMDA receptor-dependent elevation in astrocyte endfoot Ca²⁺ that followed functional hyperemia onset. This delayed astrocyte Ca²⁺ signal was dependent on the animal's action at the time of measurement as well as a neurovascular pathway that linked to endothelial-derived nitric oxide. A similar elevation in endfoot Ca²⁺ was evoked using vascular chemogenetics or optogenetics, and opto-stimulated dilation recruited the same nitric oxide pathway as functional hyperemia. These data show that behavioral state and microvasculature influence astrocyte Ca²⁺ in active mice. VIDEO ABSTRACT.

Cholecystokinin Switches the Plasticity of GABA Synapses in the Dorsomedial Hypothalamus via Astrocytic ATP Release

Whether synapses in appetite-regulatory brain regions undergo long-term changes in strength in response to satiety peptides is poorly understood. Here we show that following bursts of afferent activity, the neuromodulator and satiety peptide cholecystokinin (CCK) shifts the plasticity of GABA synapses in the dorsomedial nucleus of the hypothalamus of male Sprague Dawley rats from long-term depression to long-term potentiation (LTP). This LTP requires the activation of both type 2 CCK receptors and group 5 metabotropic glutamate receptors, resulting in a rise in astrocytic intracellular calcium and subsequent ATP release. ATP then acts on presynaptic P2X receptors to trigger a prolonged increase in GABA release. Our observations demonstrate a novel form of CCK-mediated plasticity that requires astrocytic ATP release, and could serve as a mechanism for appetite regulation.SIGNIFICANCE STATEMENT Satiety peptides, like cholecystokinin, play an important role in the central regulation of appetite, but their effect on synaptic plasticity is not well understood. The current data provide novel evidence that cholecystokinin shifts the plasticity from long-term depression to long-term potentiation at GABA synapses in the rat dorsomedial nucleus of the hypothalamus. We also demonstrate that this plasticity requires the concerted action of cholecystokinin and glutamate on astrocytes, triggering the release of the gliotransmitter ATP, which subsequently increases GABA release from neighboring inhibitory terminals. This research reveals a novel neuropeptide-induced switch in the direction of synaptic plasticity that requires astrocytes, and could represent a new mechanism by which cholecystokinin regulates appetite.

Heterogeneity and function of hippocampal macroglia

The contribution of glial cells to normal and impaired hippocampal function is increasingly being recognized, although important questions as to the mechanisms that these cells use for their crosstalk with neurons and capillaries are still unanswered or lead to controversy. Astrocytes in the hippocampus are morphologically variable and a single cell contacts with its processes more than 100,000 synapses. They predominantly express inward rectifier K⁺ channels and transporters serving homeostatic function but may also release gliotransmitters to modify neuronal signaling and brain circulation. Intracellular Ca²⁺ transients are key events in the interaction of astrocytes with neurons and the vasculature. Hippocampal NG2 glia represent a population of cells with proliferative capacity throughout adulthood. Intriguingly, they receive direct synaptic input from pyramidal neurons and interneurons and express a multitude of ion channels and receptors. Despite in-depth knowledge about the features of these transmembrane proteins, the physiological impact of NG2 glial cells and their synaptic input remain nebulous. Because of the low abundance of oligodendrocytes in the hippocampus, limited information is available about their specific properties. Given the multitude of signaling molecules expressed by the various types of hippocampal glial cells (and because of space constraints), we focus, in this review, on those properties that are considered key for the interaction of the respective cell type with its neighborhood.