Sergei Kirischuk

Sodium dynamics: another key to astroglial excitability?

Astroglial excitability is largely mediated by fluctuations in intracellular ion concentrations. In addition to generally acknowledged Ca²⁺ excitability of astroglia, recent studies have demonstrated that neuronal activity triggers transient increases in the cytosolic Na⁺ concentration ([Na⁺](i)) in perisynaptic astrocytes. These [Na⁺](i) transients are controlled by multiple Na⁺-permeable channels and Na⁺-dependent transporters; spatiotemporally organized [Na⁺](i) dynamics in turn regulate diverse astroglial homeostatic responses such as metabolic/signaling utilization of lactate and glutamate, transmembrane transport of neurotransmitters and K⁺ buffering. In particular, near-membrane [Na⁺](i) transients determine the rate and the direction of the transmembrane transport of GABA and Ca²⁺. We discuss here the role of Na⁺ in the regulation of various systems that mediate fast bidirectional communication between neurones and glia at the single synapse level.

Electrical activity patterns and the functional maturation of the neocortex.

At the earliest developmental stages, sensory neocortical areas in various species reveal distinct patterns of spontaneous neuronal network activity. These activity patterns either propagate over large neocortical areas or synchronize local neuronal ensembles. In vitro and in vivo experiments indicate that these spontaneous activity patterns are generated from neuronal networks in the cerebral cortex, in subcortical structures or in the sensory periphery (retina, cochlea, whiskers). At early stages spontaneous periphery‐driven and also sensory evoked activity is relayed to the developing cerebral cortex via the thalamus and the neocortical subplate, which amplifies the afferent sensory input. These early local and large‐scale neuronal activity patterns influence a variety of developmental processes during corticogenesis, such as neurogenesis, apoptosis, neuronal migration, differentiation and network formation. The experimental data also indicate that disturbances in early neuronal patterns may have an impact on the development of cortical layers, columns and networks. In this article we review our current knowledge on the origin of early electrical activity patterns in neocortical sensory areas and their functional implications on shaping developing cortical networks.

Spontaneous Neuronal Activity in Developing Neocortical Networks: From Single Cells to Large-Scale Interactions

Neuronal activity has been shown to be essential for the proper formation of neuronal circuits, affecting developmental processes like neurogenesis, migration, programmed cell death, cellular differentiation, formation of local and long-range axonal connections, synaptic plasticity or myelination. Accordingly, neocortical areas reveal distinct spontaneous and sensory-driven neuronal activity patterns already at early phases of development. At embryonic stages, when immature neurons start to develop voltage-dependent channels, spontaneous activity is highly synchronized within small neuronal networks and governed by electrical synaptic transmission. Subsequently, spontaneous activity patterns become more complex, involve larger networks and propagate over several neocortical areas. The developmental shift from local to large-scale network activity is accompanied by a gradual shift from electrical to chemical synaptic transmission with an initial excitatory action of chloride-gated channels activated by GABA, glycine and taurine. Transient neuronal populations in the subplate (SP) support temporary circuits that play an important role in tuning early neocortical activity and the formation of mature neuronal networks. Thus, early spontaneous activity patterns control the formation of developing networks in sensory cortices, and disturbances of these activity patterns may lead to long-lasting neuronal deficits.

Role of tonic GABAergic currents during pre- and early postnatal rodent development.

In the last three decades it became evident that the GABAergic system plays an essential role for the development of the central nervous system, by influencing the proliferation of neuronal precursors, neuronal migration and differentiation, as well as by controlling early activity patterns and thus formation of neuronal networks. GABA controls neuronal development via depolarizing membrane responses upon activation of ionotropic GABA receptors. However, many of these effects occur before the onset of synaptic GABAergic activity and thus require the presence of extrasynaptic tonic currents in neuronal precursors and immature neurons. This review summarizes our current knowledge about the role of tonic GABAergic currents during early brain development. In this review we compare the temporal sequence of the expression and functional relevance of different GABA receptor subunits, GABA synthesizing enzymes and GABA transporters. We also refer to other possible endogenous agonists of GABAA receptors. In addition, we describe functional consequences mediated by the GABAergic system during early developmental periods and discuss current models about the origin of extrasynaptic GABA and/or other endogenous GABAergic agonists during early developmental states. Finally, we present evidence that tonic GABAergic activity is also critically involved in the generation of physiological as well as pathophysiological activity patterns before and after the establishment of functional GABAergic synaptic connections.

Intracellular Na+ concentration influences short-term plasticity of glutamate transporter-mediated currents in neocortical astrocytes

Fast synaptic transmission requires a rapid clearance of the released neurotransmitter from the extracellular space. Glial glutamate transporters (excitatory amino acid transporters, EAATs) strongly contribute to glutamate removal. In this work, we investigated the paired‐pulse plasticity of synaptically activated, glutamate transporter‐mediated currents (STCs) in cortical layer 2/3 astrocytes. STCs were elicited by local electrical stimulation in layer 4 in the presence of ionotropic glutamate (AMPA and NMDA), GABAA, and GABAB receptor antagonists. In experiments with low [Na+]i (5 mM) intrapipette solution, STCs elicited by paired‐pulse stimulation demonstrated paired‐pulse facilitation (PPF) at short (<250 ms) interstimulus intervals (ISIs) and paired‐pulse depression at longer ISIs. In experiments with close to physiological, high [Na+]i (20 mM) intrapipette solution, PPF of STCs at short ISIs was significantly reduced. In addition, the STC kinetics was slowed in the presence of high [Na+]i. Exogenous GABA increased astrocytic [Na+]i, reduced the mean STC amplitude, decreased PPF at short ISIs, and slowed STC kinetics. All GABA‐induced changes were blocked by NO‐711 and SNAP‐5114, GABA transporter (GATs) antagonists. In experiments with the low intrapipette solution, GAT blockade under control conditions decreased PPF at short ISIs both at room and at near physiological temperatures. Dialysis of single astrocyte with low [Na+]i solution increased the amplitude and reduced PPR of evoked field potentials recorded in the vicinity of the astrocyte. We conclude that (1) endogenous GABA via GATs may influence EAAT functioning and (2) astrocytic [Na+]i modulates the short‐term plasticity of STCs and in turn the efficacy of glutamate removal.

Cajal–Retzius cells: Update on structural and functional properties of these mystic neurons that bridged the 20th century

Cajal-Retzius cells (CRc) represent a mostly transient neuronal cell type localized in the uppermost layer of the developing neocortex. The observation that CRc are a major source of the extracellular matrix protein reelin, which is essential for the laminar development of the cerebral cortex, attracted the interest in this unique cell type. In this review we will (i) describe the morphological and molecular properties of neocortical CRc, with a special emphasize on the question which markers can be used to identify CRc, (ii) summarize reports that identified the different developmental origins of CRc, (iii) discuss the fate of CRc, including recent evidence for apoptotic cell death and a possible persistence of some CRc, (iv) provide a detailed description of the electrical membrane properties and transmitter receptors of CRc, and (v) address the role of CRc in early neuronal circuits and cortical development. Finally, we speculate whether CRc may provide a link between early network activity and the structural maturation of neocortical circuits.

Early GABAergic circuitry in the cerebral cortex.

In the cerebral cortex GABAergic signaling plays an important role in regulating early developmental processes, for example, neurogenesis, migration and differentiation. Transient cell populations, namely Cajal-Retzius in the marginal zone and thalamic input receiving subplate neurons, are integrated as active elements in transitory GABAergic circuits. Although immature pyramidal neurons receive GABAergic synaptic inputs already at fetal stages, they are integrated into functional GABAergic circuits only several days later. In consequence, GABAergic synaptic transmission has only a minor influence on spontaneous network activity during early corticogenesis. Concurrent with the gradual developmental shift of GABA action from excitatory to inhibitory and the maturation of cortical synaptic connections, GABA becomes more important in synchronizing neuronal network activity.

Astrocyte sodium signaling and the regulation of neurotransmission.

The transmembrane Na+ concentration gradient is an important source of energy required not only to enable the generation of action potentials in excitable cells, but also for various transmembrane transporters both in excitable and non‐excitable cells, like astrocytes. One of the vital functions of astrocytes in the central nervous system (CNS) is to regulate neurotransmitter concentrations in the extracellular space. Most neurotransmitters in the CNS are removed from the extracellular space by Na+‐dependent neurotransmitter transporters (NeuTs) expressed both in neurons and astrocytes. Neuronal NeuTs control mainly phasic synaptic transmission, i.e., synaptically induced transient postsynaptic potentials, while astrocytic NeuTs contribute to the termination of phasic neurotransmission and modulate the tonic tone, i.e., the long‐lasting activation of extrasynaptic receptors by neurotransmitter that has diffused out of the synaptic cleft. Consequently, local intracellular Na+ ([Na+]i) transients occurring in astrocytes, for example via the activation of ionotropic neurotransmitter receptors, can affect the driving force for neurotransmitter uptake, in turn modulating the spatio‐temporal profiles of neurotransmitter levels in the extracellular space. As some NeuTs are close to thermodynamic equilibrium under resting conditions, an increase in astrocytic [Na+]i can stimulate the direct release of neurotransmitter via NeuT reversal. In this review we discuss the role of astrocytic [Na+]i changes in the regulation of uptake/release of neurotransmitters. It is emphasized that an activation of one neurotransmitter system, including either its ionotropic receptor or Na+‐coupled co‐transporter, can strongly influence, or even reverse, other Na+‐dependent NeuTs, with potentially significant consequences for neuronal communication. GLIA 2016;64:1655–1666

Transporter-mediated replacement of extracellular glutamate for GABA in the developing murine neocortex

During early development, cortical neurons migrate from their places of origin to their final destinations where they differentiate and establish synaptic connections. During corticogenesis, radially migrating cells move from deeper zone to the marginal zone, but they do not invade the latter. This “stop” function of the marginal zone is mediated by a number of factors, including glutamate and γ‐aminobutyric acid (GABA), two main neurotransmitters in the central nervous system. In the marginal zone, GABA has been shown to be released via GABA transporters (GAT)‐2/3, whereas glutamate transporters (EAATs) operate in the uptake mode. In this study, GABAergic postsynaptic currents (GPSCs) were recorded from Cajal‐Retzius cells in the marginal zone of murine neonatal neocortex using a whole‐cell patch‐clamp technique. Minimal electrical stimulation was applied to elicit evoked GPSCs using a paired‐pulse protocol. EAAT blockade with dl‐threo‐b‐benzyloxyaspartic acid (dl‐TBOA), a specific non‐transportable EAAT antagonist, abolishes constitutive GAT‐2/3‐mediated GABA release. In contrast to dl‐TBOA, d‐aspartate, an EAAT substrate, fails to block GAT‐2/3‐mediated GABA release. SNAP‐5114, a specific GAT‐2/3 antagonist, induced an elevation of intracellular sodium concentration ([Na+]i) under resting conditions and in the presence of d‐aspartate, indicating that GAT‐2/3 operates in reverse mode. In the presence of dl‐TBOA, however, SNAP‐5114 elicited a [Na+]i decrease, demonstrating that GAT‐2/3 operates in uptake mode. We conclude that EAATs via intracellular Na+ signaling and/or cell depolarization can govern the strength/direction of GAT‐mediated GABA transport.

Sodium fluxes and astroglial function.

Astrocytes exhibit their excitability based on variations in cytosolic Ca(2+) levels, which leads to variety of signalling events. Only recently, however, intracellular fluctuations of more abundant cation Na(+) are brought in the limelight of glial signalling. Indeed, astrocytes possess several plasmalemmal molecular entities that allow rapid transport of Na(+) across the plasma membrane: (1) ionotropic receptors, (2) canonical transient receptor potential cation channels, (3) neurotransmitter transporters and (4) sodium-calcium exchanger. Concerted action of these molecules in controlling cytosolic Na(+) may complement Ca(2+) signalling to provide basis for complex bidirectional astrocyte-neurone communication at the tripartite synapse.