Sodium signals in cerebellar Purkinje neurons and Bergmann glial cells evoked by glutamatergic synaptic transmission

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.

Homeostatic function of astrocytes: Ca(2+) and Na(+) signalling

The name astroglia unifies many non-excitable neural cells that act as primary homeostatic cells in the nervous system. Neuronal activity triggers multiple homeostatic responses of astroglia that include increase in metabolic activity and synthesis of neuronal preferred energy substrate lactate, clearance of neurotransmitters and buffering of extracellular K(+) ions to name but a few. Many (if not all) of astroglial homeostatic responses are controlled by dynamic changes in the cytoplasmic concentration of two cations, Ca(2+) and Na(+). Intracellular concentration of these ions is tightly controlled by several transporters and can be rapidly affected by the activation of respective fluxes through ionic channels or ion exchangers. Here, we provide a comprehensive review of astroglial Ca(2+) and Na(+) signalling.

Cellular and metabolic origins of flavoprotein autofluorescence in the cerebellar cortex in vivo

Flavoprotein autofluorescence imaging, an intrinsic mitochondrial signal, has proven useful for monitoring neuronal activity. In the cerebellar cortex, parallel fiber stimulation evokes a beam-like response consisting of an initial, short-duration increase in fluorescence (on-beam light phase) followed by a longer duration decrease (on-beam dark phase). Also evoked are parasagittal bands of decreased fluorescence due to molecular layer inhibition. Previous work suggests that the on-beam light phase is due to oxidative metabolism in neurons. The present study further investigated the metabolic and cellular origins of the flavoprotein signal in vivo, testing the hypotheses that the dark phase is mediated by glia activation and the inhibitory bands reflect decreased flavoprotein oxidation and increased glycolysis in neurons. Blocking postsynaptic ionotropic and metabotropic glutamate receptors abolished the on-beam light phase and the parasagittal bands without altering the on-beam dark phase. Adding glutamate transporter blockers reduced the dark phase. Replacing glucose with lactate (or pyruvate) or adding lactate to the bathing media abolished the on-beam dark phase and reduced the inhibitory bands without affecting the light phase. Blocking monocarboxylate transporters eliminated the on-beam dark phase and increased the light phase. These results confirm that the on-beam light phase is due primarily to increased oxidative metabolism in neurons. They also show that the on-beam dark phase involves activation of glycolysis in glia resulting in the generation of lactate that is transferred to neurons. Oxidative savings in neurons contributes to the decrease in fluorescence characterizing the inhibitory bands. These findings provide strong in vivo support for the astrocyte-neuron lactate shuttle hypothesis.

Ionotropic receptors in neuronal-astroglial signalling: what is the role of "excitable" molecules in non-excitable cells

Astroglial cells were long considered to serve merely as the structural and metabolic supporting cast and scenery against which the shining neurones perform their illustrious duties. Relatively recent evidence, however, indicates that astrocytes are intimately involved in many of the brain's functions. Astrocytes possess a diverse assortment of ionotropic transmitter receptors, which enable these glial cells to respond to many of the same signals that act on neurones. Ionotropic receptors mediate neurone-driven signals to astroglial cells in various brain areas including neocortex, hippocampus and cerebellum. Activation of ionotropic receptors trigger rapid signalling events in astroglia; these events, represented by local Ca(2+) or Na(+) signals provide the mechanism for fast neuronal-glial signalling at the synaptic level. Since astrocytes can detect chemical transmitters that are released from neurones and can release their own extracellular signals, gliotransmitters, they are intricately involved in homocellular and heterocellular signalling mechanisms in the nervous system. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Synaptically induced sodium signals in hippocampal astrocytes in situ

Astrocytes are in close contact to excitatory synapses and express transporters which mediate the sodium-dependent uptake of glutamate. In cultured astrocytes, selective activation of glutamate transport results in sodium elevations which stimulate Na(+)/K(+)-ATPase and glucose uptake, indicating that synaptic release of glutamate might couple excitatory neuronal activity to glial sodium homeostasis and metabolism. Here, we analysed intracellular sodium transients evoked by synaptic stimulation in acute mouse hippocampal slices using quantitative sodium imaging with the sodium-sensitive fluorescent indicator dye SBFI (sodium-binding benzofuran isophthalate). We found that short bursts of Schaffer collateral stimulation evoke sodium transients in the millimolar range in both CA1 pyramidal neurons and in SR101-positive astrocytes of the stratum radiatum. At low stimulation intensities, glial sodium transients were confined to one to two primary branches and adjacent fine processes and only weakly invaded the soma. Increasing the number of activated afferent fibres by increasing the stimulation intensity elicited global sodium transients detectable in the processes as well as the somata of astrocytes. Pharmacological analysis revealed that neuronal sodium signals were mainly attributable to sodium influx through ionotropic glutamate receptors. Activation of ionotropic receptors also contributed to glial sodium transients, while TBOA-sensitive glutamate transport was the major pathway responsible for sodium influx into astrocytes. Our results thus establish that glutamatergic synaptic transmission in the hippocampus results in sodium transients in astrocytes that are mainly mediated by activation of glutamate transport. They support the proposed link between excitatory synaptic activity, glutamate uptake and sodium signals in astrocytes of the hippocampus.

GABA uptake-dependent Ca(2+) signaling in developing olfactory bulb astrocytes

We studied GABAergic signaling in astrocytes of olfactory bulb slices using confocal Ca(2+) imaging and two-photon Na(+) imaging. GABA evoked Ca(2+) transients in astrocytes that persisted in the presence of GABA(A) and GABA(B) receptor antagonists, but were suppressed by inhibition of GABA uptake by SNAP 5114. Withdrawal of external Ca(2+) blocked GABA-induced Ca(2+) transients, and depletion of Ca(2+) stores with cyclopiazonic acid reduced Ca(2+) transients by approximately 90%. This indicates that the Ca(2+) transients depend on external Ca(2+), but are mainly mediated by intracellular Ca(2+) release, conforming with Ca(2+)-induced Ca(2+) release. Inhibition of ryanodine receptors did not affect GABA-induced Ca(2+) transients, whereas the InsP(3) receptor blocker 2-APB inhibited the Ca(2+) transients. GABA also induced Na(+) increases in astrocytes, potentially reducing Na(+)/Ca(2+) exchange. To test whether reduction of Na(+)/Ca(2+) exchange induces Ca(2+) signaling, we inhibited Na(+)/Ca(2+) exchange with KB-R7943, which mimicked GABA-induced Ca(2+) transients. Endogenous GABA release from neurons, activated by stimulation of afferent axons or NMDA application, also triggered Ca(2+) transients in astrocytes. The significance of GABAergic Ca(2+) signaling in astrocytes for control of blood flow is demonstrated by SNAP 5114-sensitive constriction of blood vessels accompanying GABA uptake. The results suggest that GABAergic signaling is composed of GABA uptake-mediated Na(+) rises that reduce Na(+)/Ca(2+) exchange, thereby leading to a Ca(2+) increase sufficient to trigger Ca(2+)-induced Ca(2+) release via InsP(3) receptors. Hence, GABA transporters not only remove GABA from the extracellular space, but may also contribute to intracellular signaling and astrocyte function, such as control of blood flow.

Ammonium-evoked alterations in intracellular sodium and pH reduce glial glutamate transport activity

The clearance of extracellular glutamate is mainly mediated by pH- and sodium-dependent transport into astrocytes. During hepatic encephalopathy (HE), however, elevated extracellular glutamate concentrations are observed. The primary candidate responsible for the toxic effects observed during HE is ammonium (NH(4) (+)/NH(3)). Here, we examined the effects of NH(4) (+)/NH(3) on steady-state intracellular pH (pH(i)) and sodium concentration ([Na(+)](i)) in cultured astrocytes in two different age groups. Moreover, we assessed the influence of NH(4) (+)/NH(3) on glutamate transporter activity by measuring D-aspartate-induced pH(i) and [Na(+)](i) transients. In 20-34 days in vitro (DIV) astrocytes, NH(4) (+)/NH(3) decreased steady-state pH(i) by 0.19 pH units and increased [Na(+)](i) by 21 mM. D-Aspartate-induced pH(i) and [Na(+)](i) transients were reduced by 80-90% in the presence of NH(4) (+)/NH(3), indicating a dramatic reduction of glutamate uptake activity. In 9-16 DIV astrocytes, in contrast, pH(i) and [Na(+)](i) were minimally affected by NH(4) (+)/NH(3), and D-aspartate-induced pH(i) and [Na(+)](i) transients were reduced by only 30-40%. Next we determined the contribution of Na(+), K(+), Cl(-)-cotransport (NKCC). Immunocytochemical stainings indicated an increased expression of NKCC1 in 20-34 DIV astrocytes. Moreover, inhibition of NKCC with bumetanide prevented NH(4) (+)/NH(3)-evoked changes in steady-state pH(i) and [Na(+)](i) and attenuated the reduction of D-aspartate-induced pH(i) and [Na(+)](i) transients by NH(4) (+)/NH(3) to 30% in 20-34 DIV astrocytes. Our results suggest that NH(4) (+)/NH(3) decreases steady-state pH(i) and increases steady-state [Na(+)](i) in astrocytes by an age-dependent activation of NKCC. These NH(4) (+)/NH(3)-evoked changes in the transmembrane pH and sodium gradients directly reduce glutamate transport activity, and may, thus, contribute to elevated extracellular glutamate levels observed during HE.