Propagation of intercellular calcium waves in retinal astrocytes and Müller cells

Changes in P2Y and P2X purinoceptors in reactive glia following axonal degeneration in the rat optic nerve

Purinoceptors have been shown to be important in mediating Ca(2+) signalling in glial cells and it has been proposed that they may have a role in their response to injury. To investigate this, the glial response to adenosine 5'triphosphate (ATP) was measured in situ, in optic nerves from juvenile rats that were enucleated at postnatal day (P) 1; age-matched normal nerves were used as controls. The optic nerve is a typical central nervous system (CNS) white matter tract containing axons and glial cells, but not neurones or synapses. Following neonatal enucleation, axons degenerate and oligodendrocytes do not develop, so that the optic nerve is populated predominantly by reactive astrocytes, with a minor population of activated microglia. Application of 1 mM ATP evoked a large and rapid increase in glial [Ca(2+)](i) in fura-2 ratiometric whole nerve recordings from normal and gliotic axon-free nerves. Significantly, the response to ATP had a prolonged duration in gliotic axon-free nerves and there was as shift in the agonist rank order of potency from ATP = ADP > UTP >> alpha,beta-metATP to ATP > ADP = UTP = alpha,beta-metATP. The results indicate an in situ role for ATP signalling in reactive astrocytes, via metabotropic P2Y(1) and P2Y(2/4) purinoceptors and ionotropic P2X purinoreceptors. The change in the purinoceptor profile following axon degeneration suggests a special role for P2X purinoceptors in mediating the glial reaction to CNS injury.

Glutamate on demand: astrocytes as a ready source

The past decade of studies has changed our view of the integrative capacities and roles of glia. A picture is emerging in which neurons and astrocytes, a subtype of glial cell, are in a continuous regulatory dialogue. Initial studies demonstrated that chemical transmitters, which are released from neurons, induce elevations of astrocytic calcium. Furthermore, stimulation of neuronal afferents at modest frequencies induces a calcium response in astrocytes that is graded with stimulation frequency. The consequence of this astrocytic calcium response is now beginning to be appreciated in that changes in calcium level can induce the release of the chemical transmitter glutamate from this nonneuronal cell. During the past few years, it has been shown that by releasing glutamate, astrocytes can regulate synaptic transmission and contribute to certain forms of synaptic plasticity. The roles played in information processing by this glial feedback loop remain to be determined. However, it is likely that the results of these recent studies will signal a new way of thinking about the nervous system, in which the glial cell comes to the forefront of our attention.

P2X and P2Y purinoreceptors mediate ATP-evoked calcium signalling in optic nerve glia in situ

It is known that ATP acts as an extracellular messenger mediating Ca2+ signalling in glial cells. Here, the mechanisms involved in the ATP-evoked increase in glial [Ca2+]i were studied in situ, in the acutely isolated rat optic nerve. ATP and agonists for P2X (a,b-metATP) and P2Y (2MeSATP) purinoreceptors triggered raised glial [Ca2+]i, and there was no significant difference between cells identified morphologically as astrocytes and oligodendrocytes. Dose-response curves indicated that P2Y receptors were activated at nanomolar concentrations, whereas P2X purinoreceptors were only activated above 10 microM. The rank order of potency for several agonists indicated optic nerve glia expressed heterogeneous purinoreceptors, with P2Y1< or = P2Y2/4< or = P2X. The ATP evoked increase in [Ca2+]i was reversibly blocked by the P2X/Y purinoreceptor antagonist suramin (100 microM) and markedly reduced by thapsigargin (10 microM), which blocks IP3-dependent release of Ca2+ from intracellular stores. Removal of extracellular Ca2+ reduced the ATP evoked increase in [Ca2+]i and completely blocked its recovery, indicating that refilling of intracellular stores was ultimately dependent on Ca2+ influx from the extracellular milieu. The results implicate ATP as an important signal in CNS white matter astrocytes and oligodendrocytes in situ, and indicate that metabotropic P2Y purinoreceptors mobilize intracellular Ca2+ at physiological concentrations of ATP, whereas ionotropic P2X purinoreceptors induce Ca2+ influx across the plasmalemma only at high concentrations of ATP, such as occur following CNS injury.

Electrophysiological alterations and upregulation of ATP receptors in retinal glial Müller cells from rats infected with the Borna disease virus

Infection with the neurotropic Borna disease virus (BDV) causes an immune-mediated neurological disease in a broad range of species. In addition to encephalitis, BDV-infected Lewis rats develop a retinitis histologically characterized by the loss of most retinal neurons. By contrast, the dominating retinal macroglia, the Müller cells, do not degenerate. It is known from several models of neurodegeneration that glial cells may survive but undergo significant alterations of their physiological parameters. This prompted us to study the electrophysiology and ATP-induced changes of intracellular Ca(2+)-concentration ([Ca(2+)](i)) in Müller cells from BDV-infected rat retinae. Freshly isolated cells were used for whole-cell patch-clamp recordings. Whereas neither zero current potentials nor membrane resistances showed significant alterations, the membrane capacitance increased in cells from BDV-infected rats during survival times of up to 8 months. This process was accompanied by a decrease in K(+) current densities. Müller cells from BDV-infected rats were characterized by expression of a prominent fast-inactivating A-type K(+) current which was rarely found in control cells. Moreover, the number of cells displaying Na(+) currents was slightly increased after BDV-infection. ATP evoked increases in [Ca(2+)](i) in Müller cells within retinal wholemounts of both control and BDV-infected animals. However, the number of ATP-responding isolated cells increased from 24% (age-matched controls) to 78% (cells from animals > or =18 weeks after infection). We conclude that in BDV-induced retinopathy, reactive rat Müller cells change their physiological parameters but these changes are different from those in Müller cells during proliferative vitreoretinopathy in man and rabbit.

Ca2+ signaling regulated by an ATP-dependent autocrine mechanism in astrocytes

Although the mechanisms of Ca2+ wave propagation in astrocytes induced by mechanical stimulation have been well studied, it is still not known how the [Ca2+]i increases in the stimulated cells. Here, we have analyzed the mechanisms of [Ca2+]i increase in single, isolated astrocytes. Our results showed that there was an autocrine mechanism of Ca2+ regulation mediated by ATP in mechanically stimulated astrocytes. This autocrine mechanism induced the activation of phospholipase C via a G-protein, resulting in Ca2+ release from intracellular Ca2+ stores. A second pathway mediating a [Ca2+]i increase was via a Ca2+ influx from the extracellular space, which, interestingly, suppressed an intracellular Ca2+ oscillation. These two different Ca2+ cascades are involved in signal transduction and may function separately during intercellular communication.

Folliculostellate cell network: a route for long-distance communication in the anterior pituitary

All higher life forms critically depend on hormones being rhythmically released by the anterior pituitary. The proper functioning of this master gland is dynamically controlled by a complex set of regulatory mechanisms that ultimately determine the fine tuning of the excitable endocrine cells, all of them heterogeneously distributed throughout the gland. Here, we provide evidence for an intrapituitary communication system by which information is transferred via the network of nonendocrine folliculostellate (FS) cells. Local electrical stimulation of FS cells in acute pituitary slices triggered cytosolic calcium waves, which propagated to other FS cells by signaling through gap junctions. Calcium wave initiation was because of the membrane excitability of FS cells, hitherto classified as silent cells. FS cell coupling could relay information between opposite regions of the gland. Because FS cells respond to central and peripheral stimuli and dialogue with endocrine cells, the form of large-scale intrapituitary communication described here may provide an efficient mechanism that orchestrates anterior pituitary functioning in response to physiological needs.

Electrical coupling between glial cells in the rat retina

The strength of electrical coupling between retinal glial cells was quantified with simultaneous whole-cell current-clamp recordings from astrocyte-astrocyte, astrocyte-Müller cell, and Müller cell-Müller cell pairs in the acutely isolated rat retina. Experimental results were fit and space constants determined using a resistive model of the glial cell network that assumed a homogeneous two-dimensional glial syncytium. The effective space constant (the distance from the point of stimulation to where the voltage falls to 1/e) equaled 12.9, 6.2, and 3.7 microm, respectively for astrocyte-astrocyte, astrocyte-Müller cell, and Müller cell-Müller cell coupling. The addition of 1 mM Ba(2+) had little effect on network space constants, while 0.5 mM octanol shortened the space constants to 4.7, 4.4, and 2.6 microm for the three types of coupling. For a given distance separating cell pairs, the strength of coupling showed considerable variability. This variability in coupling strength was reproduced accurately by a second resistive model of the glial cell network (incorporating discrete astrocytes spaced at varying distances from each other), demonstrating that the variability was an intrinsic property of the glial cell network. Coupling between glial cells in the retina may permit the intercellular spread of ions and small molecules, including messengers mediating Ca(2+) wave propagation, but it is too weak to carry significant K(+) spatial buffer currents.

Release of ATP by a human retinal pigment epithelial cell line: potential for autocrine stimulation through subretinal space

1. Stimulation of purinergic receptors on retinal pigment epithelial (RPE) cells can increase the rate of fluid transport or decrease phagocytosis. This study aims to: determine whether the purine ATP can be released from RPE cells, begin probing the mechanism of any release and test whether cells degrade ATP extracellularly. 2. ATP release was monitored from cultured human ARPE-19 cells with the luciferin-luciferase assay. Biphasic release of ATP was triggered by basic fibroblast growth factor (bFGF), by the pyrimidine uridine triphosphate (UTP) and by hypotonicity. 3. The Cl(-) channel blocker 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) inhibited release of ATP, suggesting that release was associated with Cl(-) channels. 4. Elevating intracellular Ca(2+) directly with ionomycin was insufficient to trigger ATP release. 5. UTP induced a biphasic elevation in intracellular Ca(2+). NPPB inhibited the second phase, suggesting autostimulation by released ATP. 6. Cells grown on permeable supports showed apical release of ATP, analogous to release into subretinal space in vivo. 7. The presence of ecto-ATPases on ARPE-19 cell membranes was suggested by the degradation of ATP added to intact cells. 8. Phagocytosis of fluorescent beads was inhibited by ATP, but the ecto-5'-nucleotidase inhibitor alpha, beta-methylene ADP prevented this, suggesting that inhibition was mediated by extracellular conversion of ATP to adenosine. 9. These results suggest that growth factors, pyrimidines and changes in tonicity could trigger ATP release into subretinal space. The levels of ATP released may be capable of autocrine stimulation of ATP receptors, while conversion to adenosine by ecto-enzymes could alter phagocytosis.

A neuron-glia signalling network in the active brain

Glial cells are active partners of neurons in processing information and synaptic integration. They receive coded signals from synapses and elaborate modulatory responses. The active properties of glia, including long-range signalling and regulated transmitter release, are beginning to be elucidated. Recent insights suggest that the active brain should no longer be regarded as a circuitry of neuronal contacts, but as an integrated network of interactive neurons and glia.

Ectopic pancreas cyst in the mesocolon

Neural activity increases local blood flow in the central nervous system (CNS), which is the basis of BOLD (blood oxygen level dependent) and PET (positron emission tomography) functional imaging techniques. Blood flow is assumed to be regulated by precapillary arterioles, because capillaries lack smooth muscle. However, most (65%) noradrenergic innervation of CNS blood vessels terminates near capillaries rather than arterioles, and in muscle and brain a dilatory signal propagates from vessels near metabolically active cells to precapillary arterioles, suggesting that blood flow control is initiated in capillaries. Pericytes, which are apposed to CNS capillaries and contain contractile proteins, could initiate such signalling. Here we show that pericytes can control capillary diameter in whole retina and cerebellar slices. Electrical stimulation of retinal pericytes evoked a localized capillary constriction, which propagated at approximately 2 microm s(-1) to constrict distant pericytes. Superfused ATP in retina or noradrenaline in cerebellum resulted in constriction of capillaries by pericytes, and glutamate reversed the constriction produced by noradrenaline. Electrical stimulation or puffing GABA (gamma-amino butyric acid) receptor blockers in the inner retina also evoked pericyte constriction. In simulated ischaemia, some pericytes constricted capillaries. Pericytes are probably modulators of blood flow in response to changes in neural activity, which may contribute to functional imaging signals and to CNS vascular disease.