Calcium signals in the nucleus accumbens: activation of astrocytes by ATP and succinate

Astrocyte-neuron signaling in the mesolimbic dopamine system: the hidden stars of dopamine signaling

Astrocytes are fundamental components of brain information processing and possess the ability to respond to synaptic signaling with increases in cytoplasmic calcium and modulate neuronal activity with the subsequent release of neuroactive transmitters. Dopamine signaling is essential for brain physiology and pathology, participating in learning and memory, motor control, neurological diseases, and psychiatric diseases, and astrocytes are emerging as a key cellular target of dopamine signaling. The present review will examine evidence revealing that astrocytes respond to dopamine and modulate information processing in the primary brain regions implicated in the mesolimbic dopamine system. Astrocytes exhibit circuit-specific modulation of neuronal networks and have the potential to serve as a therapeutic target for interventions designed for dopamine pathologies.

Massive efflux of adenosine triphosphate into the extracellular space immediately after experimental traumatic brain injury

The aim of the current study was to determine effects of mild traumatic brain injury (TBI), with or without blockade of purinergic ATP Y1 (P2Y1) receptors or store-operated calcium channels, on extracellular levels of ATP, glutamate, glucose and lactate. Concentrations of ATP, glutamate, glucose and lactate were measured in cerebral microdialysis samples obtained from the ipsilateral cortex and underlying hippocampus of rats with mild unilateral controlled cortical impact (CCI) or sham injury. Immediately after CCI, a large release of ATP was observed in the cortex (3.53-fold increase of pre-injury value) and hippocampus (2.97-fold increase of pre-injury value), with ATP returning to the baseline levels within 20 min post-injury and remaining stable for during the 3-h sampling period. In agreement with the results of previous studies, there was a significant increase in glutamate 20 min after CCI, which was concomitant with a decrease in extracellular glucose (20 min) and an increase in lactate (40-60 min) in both brain regions after CCI. Addition of a selective P2Y1 receptor blocker (MRS2179 ammonium salt hydrate) to the microdialysis perfusate significantly lowered pre-injury ATP and glutamate levels, and eliminated the post-CCI peaks. Addition of a blocker of store-operated calcium channels [2-aminoethoxy diphenylborinate (2-APB)] to the microdialysis perfusate significantly lowered pre-injury ATP in the hippocampus, and attenuated the post-CCI peak in both the cortex and hippocampus. 2-APB treatment significantly increased baseline glutamate levels, but the values post-injury did not differ from those in the sham group. Pre-injury glucose levels, but not lactate levels, were increased by MRS2179 and decreased by 2-APB. However, none of these treatments substantially altered the CCI-induced reduction in glucose and increase in lactate in the cortex. In conclusion, the results of the present study demonstrated that a short although extensive release of ATP immediately after experimental TBI can be significantly attenuated by blockade of P2Y1 receptors or store-operated calcium channels.

G-Protein-Coupled Receptors in Astrocyte-Neuron Communication

Astrocytes, a major type of glial cell, are known to play key supportive roles in brain function, contributing to ion and neurotransmitter homeostasis, maintaining the blood-brain barrier and providing trophic and metabolic support for neurons. Besides these support functions, astrocytes are emerging as important elements in brain physiology through signaling exchange with neurons at tripartite synapses. Astrocytes express a wide variety of neurotransmitter transporters and receptors that allow them to sense and respond to synaptic activity. Principal among them are the G-protein-coupled receptors (GPCRs) in astrocytes because their activation by synaptically released neurotransmitters leads to mobilization of intracellular calcium. In turn, activated astrocytes release neuroactive substances called gliotransmitters, such as glutamate, GABA, and ATP/adenosine that lead to synaptic regulation through activation of neuronal GPCRs. In this review we will present and discuss recent evidence demonstrating the critical roles played by GPCRs in the bidirectional astrocyte-neuron signaling, and their crucial involvement in the astrocyte-mediated regulation of synaptic transmission and plasticity.

Multifaceted Actions of Succinate as a Signaling Transmitter Vary with Its Cellular Locations

Since the identification of succinate's receptor in 2004, studies supporting the involvement of succinate signaling through its receptor in various diseases have accumulated and most of these investigations have highlighted succinate's pro-inflammatory role. Taken with the fact that succinate is an intermediate metabolite in the center of mitochondrial activity, and considering its potential regulation of protein succinylation through succinyl-coenzyme A, a review on the overall multifaceted actions of succinate to discuss whether and how these actions relate to the cellular locations of succinate is much warranted. Mechanistically, it is important to consider the sources of succinate, which include somatic cellular released succinate and those produced by the microbiome, especially the gut microbiota, which is an equivalent, if not greater contributor of succinate levels in the body. Continue learning the critical roles of succinate signaling, known and unknown, in many pathophysiological conditions is important. Furthermore, studies to delineate the regulation of succinate levels and to determine how succinate elicits various types of signaling in a temporal and spatial manner are also required.

Molecular Plasticity of the Nucleus Accumbens Revisited-Astrocytic Waves Shall Rise

Part of the ventral striatal division, the nucleus accumbens (NAc) drives the circuit activity of an entire macrosystem about reward like a "flagship," signaling and leading diverse conducts. Accordingly, NAc neurons feature complex inhibitory phenotypes that assemble to process circuit inputs and generate outputs by exploiting specific arrays of opposite and/or parallel neurotransmitters, neuromodulatory peptides. The resulting complex combinations enable versatile yet specific forms of accumbal circuit plasticity, including maladaptive behaviors. Although reward signaling and behavior are elaborately linked to neuronal circuit activities, it is plausible to propose whether these neuronal ensembles and synaptic islands can be directly controlled by astrocytes, a powerful modulator of neuronal activity. Pioneering studies showed that astrocytes in the NAc sense citrate cycle metabolites and/or ATP and may induce recurrent activation. We argue that the astrocytic calcium, GABA, and Glu signaling and altered sodium and chloride dynamics fundamentally shape metaplasticity by providing active regulatory roles in the synapse- and network-level flexibility of the NAc.

Predicting Reactive Astrogliosis Propagation by Bayesian Computational Modeling: the Repeater Stations Model

Reactive astrogliosis occurs upon focal brain injury and in neurodegenerative diseases. The mechanisms that propagate reactive astrogliosis to distal parts of the brain, in a rapid wave that activates astrocytes and other cell types along the way, are not completely understood. It is proposed that damage-associated molecular patterns (DAMP) released by necrotic cells from the injury core have a major role in the reactive astrogliosis initiation but whether they also participate in reactive astrogliosis propagation remains to be determined. We here developed a Bayesian computational model to define the most probable model for reactive astrogliosis propagation. Starting with experimental data from GFAP-immunostained reactive astrocytes, we defined five types of astrocytes based on morphometrical cues and registered the position of each reactive astrocyte cell type in the hemisphere ipsilateral to the injured site after 3 and 7 days post-ischemia. We developed equations for the changes in DAMP concentration (due to diffusion, binding to receptors or degradation), soluble mediators secretion, and for the evolution reactive astrogliosis. We tested four predefined models based on abovementioned previous hypothesis and modifications to it. Our results showed that DAMP diffusion alone has not justified the reactive astrogliosis propagation as previously assumed. Only two models succeeded in accurately reproducing the experimentally measured data and they highlighted the role of microglia and the glial secretion of soluble mediators to sustain the reactive signal and activating neighboring astrocytes. Thus, our in silico analysis proposes that glial cells behave as repeater stations of the injury signal in order to propagate reactive astrogliosis.

Connexin 43 Differentially Regulates Epileptiform Activity in Models of Convulsive and Non-convulsive Epilepsies

The influence of astrocytic cell networks on neuronal network activity is an emerging issue in epilepsy. Among the various mechanisms by which astrocytes modulate neuronal function, synchronization of astrocytes via gap junction channels is widely considered to be a crucial mechanism in epileptic conditions, contributing to the synchronization of the neuronal cell networks, possibly inducing recurrent epileptiform activity. Here, we explored whether modulation of astrocytic gap junctions could alter epileptic seizures in different types of epilepsy. Opening of gap junctions by trimethylamine intensifies seizure-like events (SLEs) in the low-[Mg2+] in vitro model of temporal lobe epilepsy, while alleviates seizures in the in vivo WAG/Rij rat model of absence epilepsy. In contrast, application of the gap junction blocker carbenoxolone prevents the appearance of SLEs in the low-[Mg2+] epilepsy model, but aggravates seizures in non-convulsive absence epilepsy, in vivo. Pharmacological dissection of neuronal vs. astrocytic connexins shows that astrocytic Cx43 contribute to seizure formation to a significantly higher extent than neuronal Cx36. We conclude that astrocytic gap junctions are key players in the formation of epileptiform activity and we provide a scheme for the different mode of action in the convulsive and non-convulsive epilepsy types.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Extensive astrocyte synchronization advances neuronal coupling in slow wave activity in vivo

Slow wave activity (SWA) is a characteristic brain oscillation in sleep and quiet wakefulness. Although the cell types contributing to SWA genesis are not yet identified, the principal role of neurons in the emergence of this essential cognitive mechanism has not been questioned. To address the possibility of astrocytic involvement in SWA, we used a transgenic rat line expressing a calcium sensitive fluorescent protein in both astrocytes and interneurons and simultaneously imaged astrocytic and neuronal activity in vivo. Here we demonstrate, for the first time, that the astrocyte network display synchronized recurrent activity in vivo coupled to UP states measured by field recording and neuronal calcium imaging. Furthermore, we present evidence that extensive synchronization of the astrocytic network precedes the spatial build-up of neuronal synchronization. The earlier extensive recruitment of astrocytes in the synchronized activity is reinforced by the observation that neurons surrounded by active astrocytes are more likely to join SWA, suggesting causality. Further supporting this notion, we demonstrate that blockade of astrocytic gap junctional communication or inhibition of astrocytic Ca²⁺ transients reduces the ratio of both astrocytes and neurons involved in SWA. These in vivo findings conclusively suggest a causal role of the astrocytic syncytium in SWA generation.

The nature of early astroglial protection-Fast activation and signaling

Our present review is focusing on the uniqueness of balanced astroglial signaling. The balance of excitatory and inhibitory signaling within the CNS is mainly determined by sharp synaptic transients of excitatory glutamate (Glu) and inhibitory γ-aminobutyrate (GABA) acting on the sub-second timescale. Astroglia is involved in excitatory chemical transmission by taking up i) Glu through neurotransmitter-sodium transporters, ii) K⁺ released due to presynaptic action potential generation, and iii) water keeping osmotic pressure. Glu uptake-coupled Na⁺ influx may either ignite long-range astroglial Ca²⁺ transients or locally counteract over-excitation via astroglial GABA release and increased tonic inhibition. Imbalance of excitatory and inhibitory drives is associated with a number of disease conditions, including prevalent traumatic and ischaemic injuries or the emergence of epilepsy. Therefore, when addressing the potential of early therapeutic intervention, astroglial signaling functions combating progress of Glu excitotoxicity is of critical importance. We suggest, that excitotoxicity is linked primarily to over-excitation induced by the impairment of astroglial Glu uptake and/or GABA release. Within this framework, we discuss the acute alterations of Glu-cycling and metabolism and conjecture the therapeutic promise of regulation. We also confer the role played by key carrier proteins and enzymes as well as their interplay at the molecular, cellular, and organ levels. Moreover, based on our former studies, we offer potential prospect on the emerging theme of astroglial succinate sensing in course of Glu excitotoxicity.

Recurrent seizure-like events are associated with coupled astroglial synchronization

Increasing evidence suggest that astrocytes significantly modulate neuronal function at the level of the tripartite synapse both in physiological and pathophysiological conditions. The global control of the astrocytic syncytium over neuronal networks, however, is still less recognized. Here we examined astrocytic signaling during epileptiform activity which is generally attributed to large-scale neuronal synchronization. We show that seizure-like events in the low-[Mg(2+)] in vitro epilepsy model initiate massive, long-range astrocytic synchronization which is spatiotemporally coupled to the synchronized neuronal activity reaching its maximum at the electrographic tonic/clonic transition. Cross-correlation analysis of neuronal and astrocytic Ca(2+) signaling demonstrates that high degree of synchronization arises not only among astrocytes, but also between neuronal and astrocyte populations, manifesting in astrocytic seizure-like events. We further show that astrocytic gap junction proteins contribute to astrocytic synchronization since their inhibition by carbenoxolone (CBX) or Cx43 antibody increased the interictal interval and in 41% of slices completely prevented recurrent seizure-like activity. In addition, CBX also induced unsynchronized Ca(2+) transients associated with decreasing incidence of epileptiform discharges afterwards. We propose therefore that local, unsynchronized astrocytic Ca(2+) transients inhibit, while long-range, synchronized Ca(2+) signaling contributes to the propagation of recurrent seizure-like events.

The psychostimulant modafinil enhances gap junctional communication in cortical astrocytes

Sleep-wake cycle is characterized by changes in neuronal network activity. However, for the last decade there is increasing evidence that neuroglial interaction may play a role in the modulation of sleep homeostasis and that astrocytes have a critical impact in this process. Interestingly, astrocytes are organized into communicating networks based on their high expression of connexins, which are the molecular constituents of gap junction channels. Thus, neuroglial interactions should also be considered as the result of the interplay between neuronal and astroglial networks. Here, we investigate the effect of modafinil, a wakefulness-promoting agent, on astrocyte gap junctional communication. We report that in the cortex modafinil injection increases the expression of mRNA and protein of connexin 30 but not those of connexin 43, the other major astroglial connexin. These increases are correlated with an enhancement of intercellular dye coupling in cortical astrocytes, which is abolished when neuronal activity is silenced by tetrodotoxin. Moreover, gamma-hydroxybutyric acid, which at a millimolar concentration induces sleep, has an opposite effect on astroglial gap junctions in an activity-independent manner. These results support the proposition that astroglia may play an important role in complex physiological brain functions, such as sleep regulation, and that neuroglial networking interaction is modified during sleep-wake cycle. This article is part of the Special Issue Section entitled 'Current Pharmacology of Gap Junction Channels and Hemichannels'.

Polyamidoamine dendrimer impairs mitochondrial oxidation in brain tissue

Background

The potential nanocarrier polyamidoamine (PAMAM) generation 5 (G5-NH₂) dendrimer has been shown to evoke lasting neuronal depolarization and cell death in a concentration-dependent manner. In this study we explored the early progression of G5-NH₂ action in brain tissue on neuronal and astroglial cells.

Results

In order to describe early mechanisms of G5-NH₂ dendrimer action in brain tissue we assessed G5-NH₂ trafficking, free intracellular Ca²⁺ and mitochondrial membrane potential (Ψ_MITO) changes in the rat hippocampal slice by microfluorimetry. With the help of fluorescent dye conjugated G5-NH₂, we observed predominant appearance of the dendrimer in the plasma membrane of pyramidal neurons and glial cells within 30 min. Under this condition, G5-NH₂ evoked robust intracellular Ca²⁺ enhancements and Ψ_MITO depolarization both in pyramidal neurons and astroglial cells. Intracellular Ca²⁺ enhancements clearly preceded Ψ_MITO depolarization in astroglial cells. Comparing activation dynamics, neurons and glia showed prevalence of lasting and transient Ψ_MITO depolarization, respectively. Transient as opposed to lasting Ψ_MITO changes to short-term G5-NH₂ application suggested better survival of astroglia, as observed in the CA3 stratum radiatum area. We also showed that direct effect of G5-NH₂ on astroglial Ψ_MITO was significantly enhanced by neuron-astroglia interaction, subsequent to G5-NH₂ evoked neuronal activation.

Conclusion

These findings indicate that the interaction of the PAMAM dendrimer with the plasma membrane leads to robust activation of neurons and astroglial cells, leading to mitochondrial depolarization. Distinguishable dynamics of mitochondrial depolarization in neurons and astroglia suggest that the enhanced mitochondrial depolarization followed by impaired oxidative metabolism of neurons may be the primary basis of neurotoxicity.

Activation of astroglial calcium signaling by endogenous metabolites succinate and gamma-hydroxybutyrate in the nucleus accumbens

Accumulating evidence suggests that different energy metabolites play a role not only in neuronal but also in glial signaling. Recently, astroglial Ca²⁺ transients evoked by the major citric acid cycle metabolite succinate (SUC) and gamma-hydroxybutyrate (GHB) that enters the citric acid cycle via SUC have been described in the brain reward area, the nucleus accumbens (NAc). Cells responding to SUC by Ca²⁺ transient constitute a subset of ATP-responsive astrocytes that are activated in a neuron-independent way. In this study we show that GHB-evoked Ca²⁺ transients were also found to constitute a subset of ATP-responsive astrocytes in the NAc. Repetitive Ca²⁺ dynamics evoked by GHB suggested that Ca²⁺ was released from internal stores. Similarly to SUC, the GHB response was also characterized by an effective concentration of 50 μM. We observed that the number of ATP-responsive cells decreased with increasing concentration of either SUC or GHB. Moreover, the concentration dependence of the number of ATP-responsive cells were highly identical as a function of both [SUC] and [GHB], suggesting a mutual receptor for SUC and GHB, therefore implying the existence of a distinct GHB-recognizing astroglial SUC receptor in the brain. The SUC-evoked Ca²⁺ signal remained in mice lacking GABA_B receptor type 1 subunit in the presence and absence of the N-Methyl-d-Aspartate (NMDA) receptor antagonist (2R)-amino-5-phosphonovaleric acid (APV), indicating action mechanisms independent of the GABA_B or NMDA receptor subtypes. By molecular docking calculations we found that residues R99, H103, R252, and R281 of the binding crevice of the kidney SUC-responsive membrane receptor SUCNR1 (GPCR91) also predict interaction with GHB, further implying similar GHB and SUC action mechanisms. We conclude that the astroglial action of SUC and GHB may represent a link between brain energy states and Ca²⁺ signaling in astrocytic networks.