Propagation of intercellular calcium waves in retinal astrocytes and Müller cells

Visualization of Ins(1,4,5)P3 dynamics in living cells: two distinct pathways for Ins(1,4,5)P3 generation following mechanical stimulation of HSY-EA1 cells

In the present study, the contribution of inositol (1,4,5)-trisphosphate [Ins(1,4,5)P(3)] generation on the mechanical-stimulation-induced Ca(2+) response was investigated in HSY-EA1 cells. Mechanical stimulation induced a local increase in the cytosolic concentration of Ins(1,4,5)P(3) ([IP(3)](i)), as indicated by the Ins(1,4,5)P(3) biosensor LIBRAvIII. The area of this increase expanded like an intracellular Ins(1,4,5)P(3) wave as [IP(3)](i) increased in the stimulated region. A small transient [IP(3)](i) increase was subsequently seen in neighboring cells. The phospholipase C inhibitor U-73122 abolished these Ins(1,4,5)P(3) responses and resultant Ca(2+) releases. The purinergic receptor blocker suramin completely blocked increases in [IP(3)](1) and the Ca(2+) release in neighboring cells, but failed to attenuate the responses in mechanically stimulated cells. These results indicate that generation of Ins(1,4,5)P(3) in response to mechanical stimulation is primarily independent of extracellular ATP. The speed of the mechanical-stimulation-induced [IP(3)](i) increase was much more rapid than that induced by a supramaximal concentration of ATP (1 mM). The contribution of the Ins(1,4,5)P(3)-induced Ca(2+) release was larger than that of Ca(2+) entry in the Ca(2+) response to mechanical stimulation in HSY-EA1 cells.

Quantitative analysis of cellular metabolic dissipative, self-organized structures

One of the most important goals of the postgenomic era is understanding the metabolic dynamic processes and the functional structures generated by them. Extensive studies during the last three decades have shown that the dissipative self-organization of the functional enzymatic associations, the catalytic reactions produced during the metabolite channeling, the microcompartmentalization of these metabolic processes and the emergence of dissipative networks are the fundamental elements of the dynamical organization of cell metabolism. Here we present an overview of how mathematical models can be used to address the properties of dissipative metabolic structures at different organizational levels, both for individual enzymatic associations and for enzymatic networks. Recent analyses performed with dissipative metabolic networks have shown that unicellular organisms display a singular global enzymatic structure common to all living cellular organisms, which seems to be an intrinsic property of the functional metabolism as a whole. Mathematical models firmly based on experiments and their corresponding computational approaches are needed to fully grasp the molecular mechanisms of metabolic dynamical processes. They are necessary to enable the quantitative and qualitative analysis of the cellular catalytic reactions and also to help comprehend the conditions under which the structural dynamical phenomena and biological rhythms arise. Understanding the molecular mechanisms responsible for the metabolic dissipative structures is crucial for unraveling the dynamics of cellular life.

A novel animal model of graded neuropathic pain: utility to investigate mechanisms of population heterogeneity

The mechanisms underlying neuropathic pain are not well understood, resulting in unsatisfactory treatment outcomes for many patients. Animal models underpin much of the current understanding of pain mechanisms due to their perceived ability to mimic pain hypersensitivities; however, are limited by their binomial approach (pain vs. control), which does not reflect the clinical heterogeneity in nociceptive hypersensitivity. We modified the chronic constriction injury model by varying the number of sciatic nerve chromic gut sutures. Each Sprague Dawley rat received 4 pieces of chromic gut to control for the inflammatory challenge posed by the gut. Treatment groups were neuronal sutures (N), subcutaneous sutures (S) N0S0, N0S4, N1S3, N2S2 and N4S0. At postoperative (PO) day 29, there was a 'dose-response' relationship between the number of perineural sutures and von Frey threshold (N0S4<N1S3<N2S2<N4S0, P<0.05). This graded model was applied to investigate lumbar dorsal spinal cord glial activation marker expression. Microglial CD11b expression was positively correlated with graded allodynia in the ipsilateral dorsal horn (P<0.05, r(2)>0.9) and associated in the dorsolateral funiculus (DLF; P=0.10, r(2)>0.8) at PO day 14. Astrocyte GFAP expression was positively associated with graded allodynia in the ipsilateral dorsal horn (P=0.18, r(2)>0.6) and ipsilateral DLF (P<0.05, r(2)>0.9). DLF glial activation may represent a contributor to contralateral pain. Our novel graded model has a dynamic range, allowing sensitive detection of interactions and subtle influences on neuropathic pain processing.

Stimulation of the P2X7 receptor kills rat retinal ganglion cells in vivo

The P2X(7) receptor is associated with the death of many cell types, and growing evidence supports its presence on neurons. Activation of the P2X(7) receptor on isolated retinal ganglion cells increases intracellular calcium levels and can kill the cells. Within the intact eye, however, glia and other cell types surrounding the ganglion cells may provide protection and attenuate the effects of receptor stimulation. This investigation thus asks whether stimulation of the P2X(7) receptor can actually kill retinal ganglion cells in vivo. Drugs were injected intravitreally into the superior/nasal region of Long Evans rats. Cell survival was determined by counting the number of remaining ganglion cells labeled with aminostilbamidine. The P2X(7) receptor agonist BzATP reduced ganglion cell survival as compared to eyes injected with saline solution. Ganglion cell death was inhibited by co-injection of the P2X(7) antagonists Brilliant Blue G and MRS 2540. The loss of ganglion cells following activation of the P2X(7) receptor was also prevented by the adenosine A(3) adenosine receptor agonist MRS 3558. In conclusion, stimulation of the P2X(7) receptor can kill retinal ganglion cells in vivo. The neuroprotective effects of A(3) receptor activation identified in isolated ganglion cells are also apparent in vivo. This implies that the balance between extracellular ATP and its protective metabolite adenosine can influence ganglion cell survival in the living eye.

Damage-induced cell-cell communication in different cochlear cell types via two distinct ATP-dependent Ca waves

Intercellular Ca(2+) waves can coordinate the action of large numbers of cells over significant distances. Recent work in many different systems has indicated that the release of ATP is fundamental for the propagation of most Ca(2+) waves. In the organ of hearing, the cochlea, ATP release is involved in critical signalling events during tissue maturation. ATP-dependent signalling is also implicated in the normal hearing process and in sensing cochlear damage. Here, we show that two distinct Ca(2+) waves are triggered during damage to cochlear explants. Both Ca(2+) waves are elicited by extracellular ATP acting on P2 receptors, but they differ in their source of Ca(2+), their velocity, their extent of spread and the cell type through which they propagate. A slower Ca(2+) wave (14 mum/s) communicates between Deiters' cells and is mediated by P2Y receptors and Ca(2+) release from IP(3)-sensitive stores. In contrast, a faster Ca(2+) wave (41 mum/s) propagates through sensory hair cells and is mediated by Ca(2+) influx from the external environment. Using inhibitors and selective agonists of P2 receptors, we suggest that the faster Ca(2+) wave is mediated by P2X(4) receptors. Thus, in complex tissues, the expression of different receptors determines the propagation of distinct intercellular communication signals.

ELECTRONIC SUPPLEMENTARY MATERIAL

The online version of this article (doi:10.1007/s11302-010-9193-8) contains supplementary material, which is available to authorized users.

Involvement of the PI3K/AKT pathway in ATP-induced proliferation of developing retinal cells in culture

ATP induces the proliferation of chick retinal cells in culture through the activation of P2Y1 receptors, PKC and MAP kinases. Together with MAP kinases, the PI3K/AKT pathway has also been implicated as an important mediator in proliferative events during development. Here we investigated the participation of the PI3K/AKT signal pathway on ATP-induced proliferation of chick embryo retinal cells in culture. When retinal cultures obtained from 7-day-old embryos were cultivated for 1 day and treated with ATP, a transient and dose-dependent phosphorylation of both ERK and AKT was observed, an effect that could be mimicked by 500 microM ADP and blocked by 100 microM PPADS, a P2 receptor antagonist. Maximal stimulation of both enzymes was obtained with 100 microM ATP in 5 min, decreasing thereafter. Activation of these pathways by ATP seemed to be independent, since LY294002 and U0126, inhibitors of PI3K and MEK, did not block the activation of ERK and AKT, respectively, although each compound blocked its respective target. Moreover, when the cultures were incubated with ATP in the presence of LY294002, a decreased incorporation of [(3)H]-thymidine was observed, as compared to cultures treated only with ATP, a decline that was also obtained by incubating the cells with ATP plus 0.5 microM API-59CJ-Ome, an inhibitor of AKT. No decrease in cell viability was observed with this concentration of API-59CJ-Ome. An increase in cyclin D1 expression, that could be inhibited by 10 microM LY 294002 or 20 microM U0126, was observed when cells were incubated with 500 microM ADP. No effect of PI3K and MEK inhibitors was observed in the expression of p27kip1 in the cultures. These results suggest that, besides the involvement of the MAP kinases pathway, ATP-induced cell cycling of late developing retinal progenitors in culture also involves the activation of the PI3K/AKT pathway.

Role of nitric oxide on purinergic signalling in the cochlea

In the inner ear, there is considerable evidence that extracellular adenosine 5'-triphosphate (ATP) plays an important role in auditory neurotransmission as a neurotransmitter or a neuromodulator, although the potential role of adenosine signalling in the modulation of auditory neurotransmission has also been reported. The activation of ligand-gated ionotropic P2X receptors and G protein-coupled metabotropic P2Y receptors has been reported to induce an increase of intracellular Ca(2+) concentration ([Ca(2+)](i)) in inner hair cells (IHCs), outer hair cells (OHCs), spiral ganglion neurons (SGNs), and supporting cells in the cochlea. ATP may participate in auditory neurotransmission by modulating [Ca(2+)](i) in the cochlear cells. Recent studies showed that extracellular ATP induced nitric oxide (NO) production in IHCs, OHCs, and SGNs, which affects the ATP-induced Ca(2+) response via the NO-cGMP-PKG pathway in those cells by a feedback mechanism. A cross-talk between NO and ATP may therefore exist in the auditory signal transduction. In the present article, I review the role of NO on the ATP-induced Ca(2+) signalling in IHCs and OHCs. I also consider the possible role of NO in the ATP-induced Ca(2+) signalling in SGNs and supporting cells.

The renin-angiotensin system in retinal health and disease: Its influence on neurons, glia and the vasculature

Renin-Angiotensin System is classically recognized for its role in the control of systemic blood pressure. However, the retina is recognized to have all the components necessary for angiotensin II formation, suggestive of a role for Angiotensin II in the retina that is independent of the systemic circulation. The most well described effects of Angiotensin II are on the retinal vasculature, with roles in vasoconstriction and angiogenesis. However, it is now emerging that Angiotensin II has roles in modulation of retinal function, possibly in regulating GABAergic amacrine cells. In addition, Angiotensin II is likely to have effects on glia. Angiotensin II has also been implicated in retinal vascular diseases such as Retinopathy of Prematurity and diabetic retinopathty, and more recently actions in choroidal neovascularizaiton and glaucoma have also emerged. The mechanisms by which Angiotensin II promotes angiogensis in retinal vascular diseases is indicative of the complexity of the RAS and the variety of cell types that it effects. Indeed, these diseases are not purely characterized by direct effects of Angiotensin II on the vasculature. In retinopathy of prematurity, for example, blockade of AT1 receptors prevents pathological angiogenesis, but also promotes revascularization of avascular regions of the retina. The primary site of action of Angiotensin II in this disease may be on retinal glia, rather than the vasculature. Indeed, blockade of AT1 receptors prevents glial loss and promotes the re-establishment of normal vessel growth. Blockade of RAS as a treatment for preventing the incidence and progression of diabetic retinopathy has also emerged based on a series of studies in animal models showing that blockade of the RAS prevents the development of a variety of vascular and neuronal deficits in this disease. Importantly these effects may be independent of actions on systemic blood pressure. This has culminated recently with the completion of several large multi-centre clinical trials that showed that blockade of the RAS may be of benefit in some at risk patients with diabetes. With the emergence of novel compounds targeting different aspects of the RAS even more effective ways of blocking the RAS may be possible in the future.

Do astrocytes really exocytose neurotransmitters?

In the past 20 years, an extra layer of information processing, in addition to that provided by neurons, has been proposed for the CNS. Neuronally evoked increases of the intracellular calcium concentration in astrocytes have been suggested to trigger exocytotic release of the 'gliotransmitters' glutamate, ATP and D-serine. These are proposed to modulate neuronal excitability and transmitter release, and to have a role in diseases as diverse as stroke, epilepsy, schizophrenia, Alzheimer's disease and HIV infection. However, there is intense controversy about whether astrocytes can exocytose transmitters in vivo. Resolving this issue would considerably advance our understanding of brain function.

Effects of protons on macroscopic and single-channel currents mediated by the human P2X7 receptor

Human P2X7 receptors (hP2X7Rs) belong to the P2X family, which opens an intrinsic cation channel when challenged by extracellular ATP. hP2X7Rs are expressed in cells of the inflammatory and immune system. During inflammation, ATP and protons are secreted into the interstitial fluid. Therefore, we investigated the effect of protons on the activation of hP2X7Rs. hP2X7Rs were expressed in Xenopus laevis oocytes and activated by the agonists ATP or benzoyl-benzoyl-ATP (BzATP) at different pH values. The protons reduced the hP2X7R-dependent cation current amplitude and slowed the current deactivation depending on the type and concentration of the agonist used. These effects can be explained by (i) the protonation of ATP, which reduces the effective concentration of the agonist ATP(4-) at the high- and low-affinity ATP activation site of the hP2XR, and (ii) direct allosteric inhibition of the hP2X7R channel opening that follows ATP(4-) binding to the low-affinity activation site. Due to the hampered activation via the low-affinity activation site, a low pH (as observed in inflamed tissues) leads to a relative increase in the contribution of the high-affinity activation site for hP2X7R channel opening.

Axons and astrocytes release ATP and glutamate to evoke calcium signals in NG2-glia

NG2-glia are an abundant population of cells in the adult CNS that make up a novel glial cell type. Here, we have examined calcium signals in NG2-glia identified by expression of the fluorescent protein DsRed under the control of the NG2 promoter in the white matter of the mouse optic nerve. We focused on mice aged postnatal day (P)12-16, after the main period of oligodendrocyte generation. Using fluo-4 and fura-2 calcium imaging in isolated intact nerves, we show that glutamate and ATP evoke Ca(2+) signals in NG2-glia in situ, acting on AMPA-type glutamate receptors and P2Y(1) and P2X(7) purine receptors; NMDA evoked a weak Ca(2+) signal in a small proportion of NG2-glia. We show that axonal action potentials and mechanical stimulation of astrocytes effect the release of glutamate and ATP to act on NG2-glia; ATP alone evokes robust Ca(2+) signals, whereas glutamate did not unless AMPA receptor desensitization was blocked with cyclothiazide. We identify the precise contacts that NG2-glia form with axons at nodes of Ranvier, and the intricate bipartite sheaths formed between the processes of NG2-glia and astrocytes. In addition, we provide evidence that NG2-glia express synaptophysin, indicating they have mechanisms for transmitting as well as receiving signals. This study places NG2-glia within a neuron-glial network, and identifies roles for glutamate and ATP in communication with astrocytes as well as axons.

A model for Ca2+ waves in networks of glial cells incorporating both intercellular and extracellular communication pathways

Networks of glial cells, and in particular astrocytes, are capable of sustaining calcium (Ca(2+)) waves both in vivo and in vitro. Experimentally, it has been shown that there are two separate modes of communication: the first by the passage of an agent (inositol 1,4,5-triphosphate, IP(3)) through gap junctions (GJs) joining cells; the second by the diffusion of an extracellular agent (adenosine triphosphate, ATP) that binds to receptors on the cells. In both cases, the outcome is the release of Ca(2+) from internal stores in the glial cells. These two modes of communication are not mutually exclusive, but probably work in conjunction in many cases. We present a model of a two-dimensional network of glial cells that incorporates regenerative intercellular (GJ) and extracellular (ATP) pathways. In the extreme cases of only one type of pathway, the results are in agreement with previous models. Adding an extracellular pathway to the GJ model increased the extent and duration of the Ca(2+) wave, but did not significantly change the speed of propagation. Conversely, adding GJs to the extracellular model did increase the wave speed. The model was modified to apply to the retina by extending it to include both astrocytes and Müller cells, with GJs the dominant coupling between astrocytes and ATP responsible for most of the remaining communication. It was found that both pathways are necessary to account for experimental results.

Endogenous purinergic signaling is required for osmotic volume regulation of retinal glial cells

Intense neuronal activity in the sensory retina is associated with a volume increase of neuronal cells (Uckermann et al., J. Neurosci. 2004, 24:10149) and a decrease in the osmolarity of the extracellular space fluid (Dmitriev et al., Vis. Neurosci. 1999, 16:1157). Here, we show the existence of an endogenous purinergic mechanism that prevents hypoosmotic swelling of retinal glial (Müller) cells in mice. In contrast to the cells from wild-type mice, hypoosmotic stress induced rapid swelling of glial cell somata in retinal slices from mice deficient in P2Y(1), adenosine A(1) receptors, or ecto-5'-nucleotidase (CD73). Consistently, glial cell bodies in retinal slices from wild-type mice displayed osmotic swelling when P2Y(1) or A(1) receptors, or CD73, were pharmacologically blocked. Exogenous ATP, UTP, and UDP inhibited glial swelling in retinal slices, while the swelling of isolated glial cells was prevented by ATP but not by UTP or UDP, suggesting that uracil nucleotides indirectly regulate the glial cell volume via activation of neuronal P2Y(4/6) and neuron-to-glia signaling. It is suggested that autocrine/paracrine activation of purinergic receptors and enzymes is crucially involved in the regulation of the glial cell volume.

Astrocyte-mediated distributed plasticity at hypothalamic glutamate synapses

Afferent activity can induce fast, feed-forward changes in synaptic efficacy that are synapse specific. Using combined electrophysiology, caged molecule photolysis, and Ca(2+) imaging, we describe a plasticity in which the recruitment of astrocytes in response to afferent activity causes a fast and feed-forward, yet distributed increase in the amplitude of quantal synaptic currents at multiple glutamate synapses on magnocellular neurosecretory cells in the hypothalamic paraventricular nucleus. The plasticity is largely multiplicative, consistent with a proportional increase or "scaling" in the strength of all synapses on the neuron. This effect requires a metabotropic glutamate receptor-mediated rise in Ca(2+) in the astrocyte processes surrounding the neuron and the release of the gliotransmitter ATP, which acts on postsynaptic purinergic receptors. These data provide evidence for a form of distributed synaptic plasticity that is feed-forward, expressed quickly, and mediated by the synaptic activation of neighboring astrocytes.

Calcium oscillations in the olfactory nonsensory cells of the goldfish, Carassius auratus

BACKGROUND

The olfactory nonsensory cells contribute to the maintenance of normal functions of the olfactory epithelium (OE). Specifically, the ciliated nonsensory cells of teleosts play important roles in the odorant detection by OE in aqueous environment. Their cilia show strong beating activities and cause water flow at the OE surface, making the detection of odorants by OE more efficient. Because intracellular Ca2+ level has been reported to play an important role in ciliary beating, the ciliary beating activity may be regulated by intracellular Ca2+ dynamics of these ciliated nonsensory cells.

METHODS

We performed Ca2+ imaging experiments to analyze the Ca2+ dynamics in acutely dissociated OE cells of the goldfish. Furthermore, we examined the contribution of the Ca2+ dynamics to the ciliary beating frequency (CBF) at the surface of the intact OE.

RESULTS

Olfactory nonsensory cells showed both spontaneous intracellular Ca2+ oscillations and propagating intercellular Ca2+ waves. Application of 2-aminoethoxydiphenylborate (2-APB), which antagonizes IP3-induced Ca2+ release from intracellular stores suppressed these Ca2+ oscillations. Furthermore, 2-APB application to the intact OE lamellae resulted in the decrease of CBF at the surface of the OE.

CONCLUSIONS

These results indicate that spontaneous intracellular calcium oscillations persistently up-regulate the ciliary beating at the surface of the OE in teleosts.

GENERAL SIGNIFICANCE

Ciliary beating activity at the surface of OE can be regulated by the Ca2+ dynamics of olfactory nonsensory cells. Because this ciliary movement causes inflow of external fluid into the nostril, this regulation is suggested to influence the efficiency of odorant detection by OE.

ATP controls cell cycle and induces proliferation in the mouse developing retina

Previous data suggest that nucleotides are important mitogens in the developing chick retina. Here, we extended the study on the mitogenic effect of ATP to newborn mouse retinal explants. Our results showed that P2Y(1) receptors were widely distributed in C57bl/6 mice retina and that the majority of PCNA positive cells co-localized with P2Y(1) receptor. To evaluate proliferation, retinal explants obtained from newborn mice were incubated with 0.5 microCi [(3)H]-thymidine or 3 microM BrDU 1h before the end of culture. Our data showed that ATP induced a dose-dependent increase in [(3)H]-thymidine incorporation, an effect that was mimicked by ADP but not by UTP and was blocked by the P2 antagonist PPADS in a dose-dependent manner. The increase in [(3)H]-thymidine incorporation induced by ATP was only observed in explants cultured for 3 days or less and was mimicked by the ectoapyrase inhibitor ARL 67156. It corresponded to an increase in the number of BrdU(+) cells in the neuroblastic layer (NL) of the tissue, suggesting that ATP, through activation of P2Y(1) receptors, induced proliferation of late developing progenitors in retinal explants of newborn mice. The increase in the number of BrdU(+) cells was observed across the whole NL when explants were incubated with ATP for 24h and no increase in the number of p-histone H3 labeled cells could be noticed at this time point. In longer incubations of 48h with ATP or 24h with ATP followed by a period of 24h in fresh medium, an increase in the number of BrdU(+) cells promoted by ATP was observed only in the middle and outer, but not in the inner NL. In these conditions, an increase in the number of p-histone H3 labeled cells was detected in the outer NL, suggesting that ATP induced cells to enter S and progress to G2 phase of the cell cycle in the first 24h period of incubation. ATP also induced an increase and a decrease in the expression of cyclin D1 and p27(kip1), respectively, in retinal progenitors of the NL. While the increase in the expression of cyclin D1 was observed when retinal explants were incubated for 3h or longer periods of time, the decrease in the expression of p27(kip1) was noticed only after 6h incubation with ATP. Both effects were blocked by the P2 receptor antagonist PPADS. These data suggest that ATP induces cell proliferation in retinal explants by inducing late developing progenitors to progress from G1 to S phase of cell cycle.

Spatiotemporal characteristics of calcium dynamics in astrocytes

Although Ca(i)(2+) waves in networks of astrocytes in vivo are well documented, propagation in vivo is much more complex than in culture, and there is no consensus concerning the dominant roles of intercellular and extracellular messengers [inositol 1,4,5-trisphosphate (IP(3)) and adenosine-5'-triphosphate (ATP)] that mediate Ca(i)(2+) waves. Moreover, to date only simplified models that take very little account of the geometrical struture of the networks have been studied. Our aim in this paper is to develop a mathematical model based on realistic cellular morphology and network connectivity, and a computational framework for simulating the model, in order to address these issues. In the model, Ca(i) (2+) wave propagation through a network of astrocytes is driven by IP(3) diffusion between cells and ATP transport in the extracellular space. Numerical simulations of the model show that different kinetic and geometric assumptions give rise to differences in Ca(i)(2+) wave propagation patterns, as characterized by the velocity, propagation distance, time delay in propagation from one cell to another, and the evolution of Ca(2+) response patterns. The temporal Ca(i)(2+) response patterns in cells are different from one cell to another, and the Ca(i)(2+) response patterns evolve from one type to another as a Ca(i)(2+) wave propagates. In addition, the spatial patterns of Ca(i)(2+) wave propagation depend on whether IP(3), ATP, or both are mediating messengers. Finally, two different geometries that reflect the in vivo and in vitro configuration of astrocytic networks also yield distinct intracellular and extracellular kinetic patterns. The simulation results as well as the linear stability analysis of the model lead to the conclusion that Ca(i)(2+) waves in astrocyte networks are probably mediated by both intercellular IP(3) transport and nonregenerative (only the glutamate-stimulated cell releases ATP) or partially regenerative extracellular ATP signaling.

The A3 adenosine receptor attenuates the calcium rise triggered by NMDA receptors in retinal ganglion cells

The A(3) adenosine receptor is emerging as an important regulator of neuronal signaling, and in some situations receptor stimulation can limit excitability. As the NMDA receptor frequently contributes to neuronal excitability, this study examined whether A(3) receptor activation could alter the calcium rise accompanying NMDA receptor stimulation. Calcium levels were determined from fura-2 imaging of isolated rat retinal ganglion cells as these neurons possess both receptor types. Brief application of glutamate or NMDA led to repeatable and reversible elevations of intracellular calcium. The A(3) agonist Cl-IB-MECA reduced the response to both glutamate and NMDA. While adenosine mimicked the effect of Cl-IB-MECA, the A(3) receptor antagonist MRS 1191 impeded the block by adenosine, implicating a role for the A(3) receptor in response to the natural agonist. The A(1) receptor antagonist DPCPX provided additional inhibition, implying a contribution from both A(1) and A(3) adenosine receptors. The novel A(3) agonist MRS 3558 (1'S,2'R,3'S,4'R,5'S)-4-(2-chloro-6-(3-chlorobenzylamino)-9H-purin-9-yl)-2,3-dihydroxy-N-methylbicyclo [3.1.0] hexane-1-carboxamide and mixed A(1)/A(3) agonist MRS 3630 (1'S,2'R,3'S,4'R,5'S)-4-(2-chloro-6-(cyclopentylamino)-9H-purin-9-yl)-2,3-dihydroxy-N-methylbicyclo [3.1.0] hexane-1-carboxamide also inhibited the calcium rise induced by NMDA. Low levels of MRS 3558 were particularly effective, with an IC(50) of 400 pM. In all cases, A(3) receptor stimulation inhibited only 30-50% of the calcium rise. In summary, stimulation of the A(3) adenosine receptor by either endogenous or synthesized agonists can limit the calcium rise accompanying NMDA receptor activation. It remains to be determined if partial block of the calcium rise by A(3) agonists can modify downstream responses to NMDA receptor stimulation.

Involvement of Müller glial cells in epiretinal membrane formation

BACKGROUND

Proliferative retinopathies are considered to represent maladapted retinal wound repair processes driven by growth factor- and cytokine-induced overstimulation of proliferation, migration, extracellular matrix production and contraction of retinal cells. The formation of neovascular membranes represents an attempt to reoxygenize non-perfused retinal areas. Müller glial cells play a crucial role in the pathogenesis of proliferative retinopathies. This review summarizes the present knowledge regarding the role of Müller cells in periretinal membrane formation, especially in the early steps of epiretinal membrane formation, which involve an interaction of inflammatory and glial cells, and gives a survey of the factors which are suggested to be implicated in the induction of Müller cell gliosis and proliferation.

CONCLUSIONS

Alterations in the membrane conductance of Müller cells suggest that Müller cells may alter their phenotype into progenitor-like cells in the course of proliferative retinopathies; transdifferentiated Müller cells may have great impact for the development of new cell-based therapies.

Extracellular ATP induces retinal photoreceptor apoptosis through activation of purinoceptors in rodents

We have previously demonstrated that photoreceptors express P2X(7) purinoceptors. These excitatory receptors are activated by extracellular adenosine 5'-triphosphate (ATP) and have been implicated in neurodegeneration in other parts of the central nervous system (CNS). In this study we examined whether extracellular ATP could contribute to photoreceptor degeneration in rodents through excessive activation of P2 purinoceptors. Intravitreal injection of high concentrations of extracellular ATP into normal rat eyes induced extensive and selective apoptosis of photoreceptors within 18 hours of injection. Five days after injection the outer nuclear layer was severely degenerated and electroretinographic responses were impaired. Preinjection of the purinergic antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) protected against ATP-mediated apoptosis. The initial phase of ATP-induced photoreceptor death did not temporally coincide with retinal pigment epithelium degeneration or microglial activation, suggesting that cell death was due to direct activation of purinergic receptors on photoreceptors. Finally, we demonstrate that intravitreal injection of PPADS results in a 30% increase in photoreceptor survival in the rd1 mouse, a model of human recessive retinitis pigmentosa (RP). These findings highlight the importance of extracellular ATP in retinal neurodegeneration and provide a potential new avenue for therapeutic intervention in RP.

Calcium responses mediated by type 2 IP3-receptors are required for osmotic volume regulation of retinal glial cells in mice

Prevention of osmotic swelling of retinal glial (Müller) cells is required to avoid detrimental decreases in the extracellular space volume during intense neuronal activity. Here, we show that glial cells in slices of the wildtype mouse retina maintain the volume of their somata constant up to approximately 4 min of perfusion with a hypoosmolar solution. However, calcium chelation with BAPTA/AM induced a rapid swelling of glial cell bodies. In glial cells of retinas from inositol-1,4,5-trisphosphate-receptor type 2-deficient (IP(3)R2(-/-)) mice, hypotonic conditions caused swelling of the cell bodies without delay. Exogenous ATP (acting at P2Y(1) receptors) prevented the swelling of glial cells in retinal slices from wildtype but not from IP(3)R2(-/-) mice. Müller cells from IP(3)R2(-/-) mice displayed a strongly reduced amplitude of the ATP-evoked calcium responses as compared to cells from wildtype mice. It is concluded that endogenous calcium signaling mediated by IP(3)R2 is required for the osmotic volume regulation of retinal glial cells.

ATP release from non-excitable cells

All cells release nucleotides and are in one way or another involved in local autocrine and paracrine regulation of organ function via stimulation of purinergic receptors. Significant technical advances have been made in recent years to quantify more precisely resting and stimulated adenosine triphosphate (ATP) concentrations in close proximity to the plasma membrane. These technical advances are reviewed here. However, the mechanisms by which cells release ATP continue to be enigmatic. The current state of knowledge on different suggested mechanisms is also reviewed. Current evidence suggests that two separate regulated modes of ATP release co-exist in non-excitable cells: (1) a conductive pore which in several systems has been found to be the channel pannexin 1 and (2) vesicular release. Modes of stimulation of ATP release are reviewed and indicate that both subtle mechanical stimulation and agonist-triggered release play pivotal roles. The mechano-sensor for ATP release is not yet defined.

The P2X(7) receptor in retinal ganglion cells: A neuronal model of pressure-induced damage and protection by a shifting purinergic balance

Retinal ganglion cells process the visual signal and transmit it along their axons in the optic nerve to the brain. Molecular, immunohistochemical, and functional analyses indicate that the majority of retinal ganglion cells express the ionotropic P2X(7) receptor. Stimulation of the receptor can lead to a rise in intracellular calcium and cell death, although death does not involve the opening of a large diameter pore. Adenosine acting at A(3) receptors can attenuate the rise in calcium and death accompanying P2X(7) receptor activation, suggesting that dephosphorylation of ATP into adenosine is neuroprotective and that the balance of extracellular purines can influence neuronal survival. Increased intraocular pressure can lead to release of excessive extracellular ATP in the retina and damage ganglion cells by acting on P2X(7) receptors, implicating a role for the receptor in the loss of ganglion cell activity in glaucoma. In summary, the activation of P2X(7) receptors has both physiologic and pathophysiologic implications for ganglion cell function. These characteristics may also provide an insight into the contributions the P2X(7) receptor makes to neurons elsewhere.

Glial-neuronal interactions--implications for plasticity and drug addiction

Among neuroscientists, astrocytes have for long played Cinderella to their neuron stepsisters. While the importance of glia in regulating brain activity was predicted by Ramon y Cajal more than a century ago (Garcia-Marin et al., Trends. Neurosci. 30:479-787, 2007), these cells, until recently, have been thought to play mainly a passive part in synaptic signaling. Results obtained over the last decade have begun to suggest otherwise. Experiments carried out in a number of labs have shown that glial cells, especially astrocytes, directly participate in synaptic signaling and potentially regulate synaptic plasticity and network excitability. The presence of signaling pathways on astrocytes that are analogous to those at presynaptic terminals suggests a role for these cells in network plasticity. Findings that the same signaling pathways can be activated by receptors for drugs of abuse present on astrocytes suggest a role for these cells in the addictive process. In this review, we summarize current understanding of astrocytic role in synaptic signaling and suggest that a complete understanding of the process of addiction requires a better understanding of the functional role of these cells.

Subsets of retinal neurons and glia express P2Y1 receptors

Recent evidence suggests that extracellular ATP modulates retinal processing and could play a role in modulating glial cells during retinal diseases. Here, we evaluated the localization of P2Y(1) receptors in the rat retina using indirect immunofluorescence immunocytochemistry. We observed labeling within defined populations of inner retinal neurons and Müller cell processes and end feet. Double labeling of P2Y(1) receptor with choline acetyltransferase revealed extensive colocalization indicating the expression of this receptor by cholinergic amacrine cells. Ganglion cell labeling for P2Y(1) receptors was also observed. Having established the normal pattern of immunolabeling within the retina, we next examined whether immunolabeling was altered by retinal disease. P2Y(1) receptor immunolabeling of Müller cells was of greater intensity following light-induced retinal degeneration, suggesting that Müller cell gliosis is accompanied by changes in P2Y(1) receptor expression. Overall, these data provide further evidence for a role of extracellular ATP in retinal signaling within subsets of retinal neurons as well as glia.

Radially expanding transglial calcium waves in the intact cerebellum

Multicellular glial calcium waves may locally regulate neural activity or brain energetics. Here, we report a diffusion-driven astrocytic signal in the normal, intact brain that spans many astrocytic processes in a confined volume without fully encompassing any one cell. By using 2-photon microscopy in rodent cerebellar cortex labeled with fluorescent indicator dyes or the calcium-sensor protein G-CaMP2, we discovered spontaneous calcium waves that filled approximately ellipsoidal domains of Bergmann glia processes. Waves spread in 3 dimensions at a speed of 4-11 microm/s to a diameter of approximately 50 microm, slowed during expansion, and were reversibly blocked by P2 receptor antagonists. Consistent with the hypothesis that ATP acts as a diffusible trigger of calcium release waves, local ejection of ATP triggered P2 receptor-mediated waves that were refractory to repeated activation. Transglial waves represent a means for purinergic signals to act with local specificity to modulate activity or energetics in local neural circuits.

Photostimulation of astrocytes with femtosecond laser pulses

The involvement of astrocytes in brain functions rather than support has been identified and widely concerned. However the lack of an effective stimulation of astrocytes hampers our understanding of their essential roles. Here, we employed 800-nm near infrared (NIR) femtosecond laser to induce Ca2+ wave in astrocytes. It was demonstrated that photostimulation of astrocytes with femtosecond laser pulses is efficient with the advantages of non-contact, non-disruptiveness, reproducibility, and high spatiotemporal precision. Photostimulation of astrocytes would facilitate investigations on information processing in neuronal circuits by providing effective way to excite astrocytes.

Limited intravascular coupling in the rodent brainstem and retina supports a role for glia in regional blood flow

Regional synaptic activity induces local increases in perfusion that are coupled to upstream vasodilation and improved blood flow. In the cerebral circulation, it has been proposed that astrocytes mediate the link between the initiating stimulus and local vasodilation through propagated intracellular calcium waves. In the systemic circulation the mechanism by which local vasodilation triggers upstream alterations in blood flow involves electrotonic propagation of hyperpolarization via endothelial gap junctions, although less is known concerning the cerebral circulation. The present study aimed to investigate the extent of coupling in microvessels of the rodent brainstem and retina and the subtypes of intracellular calcium stores that might mediate astrocytic signaling. Within the brainstem, connexins (Cxs) 37 and 40 were restricted to the endothelium of pial vessels and larger penetrating arterioles, whereas astrocytic Cxs30 and 43 were found closely associated with pre- and postsynaptic neurons and nearby microvessels. Within the rat retina, Cxs37 and 40 were expressed in large radiating arterioles, but were not found in smaller vessels on the retinal surface or in the deeper retinal layers. These Cxs were absent from all retinal vessels in mice. Astrocytes, expressing Cxs30 and 43 in the rat, but only Cx43 in the mouse, were found closely associated with superficial, but not deeper blood vessels. Inositol-trisphosphate receptors (IP(3)R) 1 and 2 were expressed within brainstem astrocytes, whereas IP(3)R1 and 3 were expressed within retinal astrocytes. Limited intravascular coupling and the proximity of astrocytic networks to blood vessels supports a role for glia in activity-dependent alterations in central blood flow.

Characterization of Calcium-Mediated Intracellular and Intercellular Signaling in the rMC-1 Glial Cell Line

Retinal Müller glial cells, in addition to providing homeostatic support to retinal neurons, have been shown to engage in modulation of neuronal activity and regulate vasomotor responses in the retina, among other functions. Calcium-mediated signaling in Müller cells has been implicated to play a significant role in the intracellular and intercellular interactions necessary to carry out these functions. Although the basic molecular mechanisms of calcium signaling in Müller cells have been described, the dynamics of calcium responses in Müller cells have not been fully explored. Here, we provide a quantitative characterization of calcium signaling in an in vitro model of Müller cell signaling using the rMC-1 cell line, a well-established line developed from rat Müller cells. rMC-1 cells displayed robust intracellular calcium transients and the capacity to support calcium transient-mediated intercellular calcium waves with signaling dynamics similar to that reported for Müller cells in in situ retinal preparations. Furthermore, pharmacological perturbations of intracellular calcium transients with thapsigargin and intercellular calcium waves with purinergic receptor antagonists and gap junction blockers (PPADS and FFA, respectively) suggest that the molecular mechanisms that underlie calcium signaling in rMC-1 cells has been conserved with those of Müller cells. This model provides a robust in vitro system for investigating specific mechanistic hypotheses of intra- and intercellular calcium signaling in Müller cells.

The P2X(7) receptor in retinal ganglion cells: A neuronal model of pressure-induced damage and protection by a shifting purinergic balance

Retinal ganglion cells process the visual signal and transmit it along their axons in the optic nerve to the brain. Molecular, immunohistochemical, and functional analyses indicate that the majority of retinal ganglion cells express the ionotropic P2X(7) receptor. Stimulation of the receptor can lead to a rise in intracellular calcium and cell death, although death does not involve the opening of a large diameter pore. Adenosine acting at A(3) receptors can attenuate the rise in calcium and death accompanying P2X(7) receptor activation, suggesting that dephosphorylation of ATP into adenosine is neuroprotective and that the balance of extracellular purines can influence neuronal survival. Increased intraocular pressure can lead to release of excessive extracellular ATP in the retina and damage ganglion cells by acting on P2X(7) receptors, implicating a role for the receptor in the loss of ganglion cell activity in glaucoma. In summary, the activation of P2X(7) receptors has both physiologic and pathophysiologic implications for ganglion cell function. These characteristics may also provide an insight into the contributions the P2X(7) receptor makes to neurons elsewhere.

Elevated pressure triggers a physiological release of ATP from the retina: Possible role for pannexin hemichannels

Increased hydrostatic pressure can damage neurons, although the mechanisms linking pressure to neurochemical imbalance or cell injury are not fully established. Throughout the body, mechanical perturbations such as shear stress, cell stretching, or changes in pressure can lead to excessive release of ATP. It is thus possible that increased pressure across neural tissues triggers an elevated release of ATP into extracellular space. As stimulation of the P2X(7) receptor for ATP on retinal ganglion cells leads to elevation of intracellular calcium and excitotoxic death, we asked whether increased levels of extracellular ATP accompanied an elevation in pressure across the retina. The hydrostatic pressure surrounding bovine retinal eyecups was increased and the ATP content of the vitreal compartment adjacent to the retina was determined. A step increase of only 20 mm Hg induced a threefold increase in the vitreal ATP concentration. The ATP levels correlated closely with the degree of pressure increase over 20-100 mm Hg. The increase was transient at lower pressures but sustained at higher pressures. The rise in vitreal ATP was the same regardless of whether nitrogen or air was used to increase pressure, implying changes in oxygen partial pressure did not contribute. Lactate dehydrogenase activity was not affected by pressure, ruling out a substantial contribution from cell lysis. The ATP increase was largely inhibited by either 30 muM 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB) or 10 muM carbenoxolone (CBX). While this pharmacological profile is consistent with physiological release of ATP through pannexins hemichannels, a contribution from anion channels, vesicular release or other mechanisms cannot be ruled out. In conclusion, a step elevation in pressure leads to a physiologic increase in the levels of extracellular ATP bathing retinal neurons. This excess extracellular ATP may link increased pressure to the death of ganglion cells in acute glaucoma, and suggests a possible role for ATP in the neuronal damage accompanying increased intracranial pressure.

Localization and possible function of P2Y(4) receptors in the rodent retina

Extracellular ATP acts as a neurotransmitter in the retina, via the activation of ionotropic P2X receptors and metabotropic P2Y receptors. The expression of various P2X and P2Y receptor subtypes has been demonstrated in the retina, but the localization of P2Y receptors and their role in retinal signaling remains ill defined. In this study, we were interested in determining the localization of the P2Y(4) receptor subtype in the rat retina, and using the electroretinogram (ERG) to assess whether activation of these receptors modulated visual transmission. Using light and electron microscopy, we demonstrated that P2Y(4) receptors were expressed pre-synaptically in rod bipolar cells and in processes postsynaptic to cone bipolar cells. Furthermore, we show that the expression of P2Y(4) receptors on rod bipolar cell axon terminals is reduced following dark adaptation, suggesting receptor expression may be dependent on retinal activity. Finally, using the electroretinogram, we show that intravitreal injection of uridine triphosphate, a P2Y receptor agonist, decreases the amplitude of the rod PII, supporting a role for P2Y receptors in altering inner retinal function. Taken together, these results suggest a role for P2Y(4) receptors in the modulation of inner retinal signaling.

Nucleotides in ocular secretions: their role in ocular physiology

The eye is the sense organ that permits the detection of light owing to the existence of a sophisticated neuronal array, called the retina, which is responsive to photons. The correct functioning of this complex system requires the coordination of several intraocular structures that ultimately permit the perfect focusing of images on the neural retina. Light has to pass through different media: the tear, the cornea, aqueous humour, lens, and vitreous humour before it reaches the retina. Moreover, the composition and structure of some of these media can change due to several physiological mechanisms. Nucleotides are active components of the humours bathing relevant ocular structures. The tear contains nucleotides and dinucleotides that control the process of tearing, wound healing and protects of superficial infections. In the inner eye, the aqueous humour also presents a collection of mono and dinucleotides that affect pupil contraction, aqueous humour production and accommodation. Behind the lens and between this structure and the retina the vitreous humour can modify the physiology of the retinal cells, mostly the ganglion cells. By investigating the actions of nucleotides and dinucleotide present in the ocular humours we will be able not only to understand the functioning of the ocular structures but also to develop new pharmacological therapies for pathologies such as dry eye, glaucoma or retinal detachment.

Mechanisms of ATP- and glutamate-mediated calcium signaling in white matter astrocytes

Neurotransmitters released at synapses mediate Ca2+ signaling in astrocytes in CNS grey matter. Here, we show that ATP and glutamate evoke these Ca2+ signals in white matter astrocytes of the mouse optic nerve, a tract that contains neither neuronal cell bodies nor synapses. We further demonstrate that action potentials along white matter axons trigger the release of ATP and the intercellular propagation of astroglial Ca2+ signals. These mechanisms were studied in astrocytes in intact optic nerves isolated from transgenic mice expressing enhanced green fluorescent protein (EGFP) under control of the human glial fibrillary acidic protein promoter (GFAP) by Fura-2 ratiometric Ca2+ imaging. ATP evoked astroglial Ca2+ signals predominantly via metabotropic P2Y1 and ionotropic P2X7 purinoceptors. Glutamate acted on both AMPA- and NMDA-type receptors, as well as on group I mGlu receptors to induce an increase in astroglial [Ca2+]i. The direct Ca2+ signal evoked by glutamate was small, and the main action of glutamate was to trigger the release of the "gliotransmitter" ATP by a mechanism involving P2X7 receptors; propagation of the glutamate-mediated Ca2+ signal was significantly reduced in P2X7 knock-out mice. Furthermore, axonal action potentials and mechanical stimulation of astrocytes both induced the release of ATP, to propagate Ca2+ signals in astrocytes and neighboring EGFP-negative glia. Our data provide a model of multiphase axon-glial signaling in the optic nerve as follows: action potentials trigger axonal release of ATP, which evokes further release of ATP from astrocytes, and this acts by amplifying the initiating signal and by transmitting an intercellular Ca2+ wave to neighboring glia.

Voltage-activated ion channels and Ca(2+)-induced Ca (2+) release shape Ca (2+) signaling in Merkel cells

Ca(2+) signaling and neurotransmission modulate touch-evoked responses in Merkel cell-neurite complexes. To identify mechanisms governing these processes, we analyzed voltage-activated ion channels and Ca(2+) signaling in purified Merkel cells. Merkel cells in the intact skin were specifically labeled by antibodies against voltage-activated Ca(2+) channels (Ca(V)2.1) and voltage- and Ca(2+)-activated K(+) (BK(Ca)) channels. Voltage-clamp recordings revealed small Ca(2+) currents, which produced Ca(2+) transients that were amplified sevenfold by Ca(2+)-induced Ca(2+) release. Merkel cells' voltage-activated K(+) currents were carried predominantly by BK(Ca) channels with inactivating and non-inactivating components. Thus, Merkel cells, like hair cells, have functionally diverse BK(Ca) channels. Finally, blocking K(+) channels increased response magnitude and dramatically shortened Ca(2+) transients evoked by mechanical stimulation. Together, these results demonstrate that Ca(2+) signaling in Merkel cells is governed by the interplay of plasma membrane Ca(2+) channels, store release and K(+) channels, and they identify specific signaling mechanisms that may control touch sensitivity.

A review of the role of glial cells in understanding retinal disease

Retinal vascular diseases such as diabetic retinopathy and retinopathy of prematurity are major causes of visual loss. Although the focus of a great deal of research has been on the aetiology of vascular growth, it is now emerging that anomalies in other retinal cell types, especially glial cells, occur very early in the course of the disease. Glial cells have major roles in every stage of disease, from the earliest subtle variations in neural function, to the development of epi-retinal membranes and tractional detachment. Therefore, having a firm understanding of the function of retinal glia is important in our understanding of retinal disease and is crucial for the development of new treatment strategies.

Müller glia as an active compartment modulating nervous activity in the vertebrate retina: neurotransmitters and trophic factors

Müller cells represent the main type of glia present in the retina interacting with most, if not all neurons in this tissue. Müller cells have been claimed to function as optic fibers in the retina delivering light to photoreceptors with minimal distortion and low loss [Franze et al (2007) Proc Natl Acad Sci 104:8287-8292]. Most of the mediators found in the brain are also detected in the retinal tissue, and glia cells are active players in the synthesis, release, signaling and uptake of major mediators of synaptic function. Müller glia trophic factors may regulate many different aspects of neuronal circuitry during synaptogenesis, differentiation, neuroprotection and survival of photoreceptors, Retinal Ganglion Cells (RGCs) and other targets in the retina. Here we review the role of several transmitters and trophic factors that participate in the neuron-glia loop in the retina.

Innexins form two types of channels

Injury to the central nervous system triggers glial calcium waves in both vertebrates and invertebrates. In vertebrates the pannexin1 ATP-release channel appears to provide for calcium wave initiation and propagation. The innexins, which form invertebrate gap junctions and have sequence similarity with the pannexins, are candidates to form non-junctional membrane channels. Two leech innexins previously demonstrated in glia were expressed in frog oocytes. In addition to making gap junctions, innexins also formed non-junctional membrane channels with properties similar to those of pannexons. In addition, carbenoxolone reversibly blocked the loss of carboxyfluorescein dye into the bath from the giant glial cells in the connectives of the leech nerve cord, which are known to express the innexins we assayed.

The role of nucleotides in the neuron--glia communication responsible for the brain functions

Accumulating findings indicate that nucleotides play an important role in cell-to-cell communication through P2 purinoceptors, even though ATP is recognized primarily to be a source of free energy and nucleotides are key molecules in cells. P2 purinoceptors are divided into two families, ionotropic receptors (P2X) and metabotropic receptors (P2Y). P2X receptors (7 types; P2X(1)-P2X(7)) contain intrinsic pores that open by binding with ATP. P2Y (8 types; P2Y(1, 2, 4, 6, 11, 12, 13,) and (14)) are activated by nucleotides and couple to intracellular second-messenger systems through heteromeric G-proteins. Nucleotides are released or leaked from non-excitable cells as well as neurons in physiological and pathophysiological conditions. One of the most exciting cells in non-excitable cells is the glia cells, which are classified into astrocytes, oligodendrocytes, and microglia. Astrocytes express many types of P2 purinoceptors and release the 'gliotransmitter' ATP to communicate with neurons, microglia and the vascular walls of capillaries. Microglia also express many types of P2 purinoceptors and are known as resident macrophages in the CNS. ATP and other nucleotides work as 'warning molecules' especially through activating microglia in pathophysiological conditions. Microglia play a key role in neuropathic pain and show phagocytosis through nucleotide-evoked activation of P2X(4) and P2Y(6) receptors, respectively. Such strong molecular, cellular and system-level evidence for extracellular nucleotide signaling places nucleotides in the central stage of cell communications in glia/CNS.

Acute increase of intraocular pressure releases ATP into the anterior chamber

Throughout the body, mechanical perturbations are transduced into neurochemical signals by the release of ATP from non-neuronal cells. As an increased intraocular pressure (IOP) can initiate mechanical changes, we hypothesized that extracellular levels of ATP might be increased in the anterior chamber of human patients with primary acute angle closure glaucoma (PAACG). ATP levels in aqueous humor samples were determined using the luciferin-luciferase assay. Samples were obtained from 18 controls scheduled for cataract extraction and from 14 subjects with angle closure glaucoma during paracentesis. All subjects with angle closure glaucoma had remarkably elevated levels of ATP in their aqueous humor. The mean ATP concentration was ninefold higher for patients with angle closure glaucoma than for control. The concentration of ATP was correlated with IOP levels, further supporting a causal relationship. ATP levels were not influenced by the duration of the PAACG attack, suggesting the rate of ATP release was sustained. We conclude that increased IOP leads to increased levels of extracellular ATP in the anterior chamber. This elevated ATP may contribute to the extreme pain experienced by subjects with angle closure glaucoma, and may serve as a source of elevated adenosine in the anterior chamber.

Astrocyte signaling in the presence of spatial inhomogeneities

Astrocytes, a special type of glial cells, were considered to have just a supporting role in information processing in the brain. However, several recent studies have shown that they can be chemically stimulated by various neurotransmitters, such as ATP, and can generate Ca2+ and ATP waves, which can propagate over many cell lengths before being blocked. Although pathological conditions, such as spreading depression and epilepsy, have been linked to abnormal wave propagation in astrocytic cellular networks, a quantitative understanding of the underlying characteristics is still lacking. Astrocytic cellular networks are inhomogeneous, in the sense that the domain they occupy contains passive regions or gaps, which are unable to support wave propagation. Thus, this work focuses on understanding the complex interplay between single-cell signal transduction, domain inhomogeneity, and the characteristics of wave propagation and blocking in astrocytic cellular networks. The single-cell signal transduction model that was employed accounts for ATP-mediated IP3 production, the subsequent Ca2+ release from the ER, and ATP release into the extracellular space. The model is excitable and thus an infinite range of wave propagation is observed if the domain of propagation is homogeneous. This is not always the case for inhomogeneous domains. To model wave propagation in inhomogeneous astrocytic networks, a reaction-diffusion framework was developed and one-gap as well as multiple-gap cases were simulated using an efficient finite-element algorithm. The minimum gap length that blocks the wave was computed as a function of excitability levels and geometric characteristics of the inhomogeneous network, such as the length of the active regions (cells). Complex transient patterns, such as wave reflection, wave trapping, and generation of echo waves, were also predicted by the model, and their relationship to the geometric characteristics of the network was evaluated. Therefore, the proposed model can help in the formulation of testable hypotheses to explain the observed abnormal wave propagation in pathological situations.

Ectonucleotidases in Müller glial cells of the rodent retina: Involvement in inhibition of osmotic cell swelling

Extracellular nucleotides mediate glia-to-neuron signalling in the retina and are implicated in the volume regulation of retinal glial (Müller) cells under osmotic stress conditions. We investigated the expression and functional role of ectonucleotidases in Müller cells of the rodent retina by cell-swelling experiments, calcium imaging, and immuno- and enzyme histochemistry. The swelling of Müller cells under hypoosmotic stress was inhibited by activation of an autocrine purinergic signalling cascade. This cascade is initiated by exogenous glutamate and involves the consecutive activation of P2Y₁ and adenosine A1 receptors, the action of ectoadenosine 5′-triphosphate (ATP)ases, and a nucleoside-transporter-mediated release of adenosine. Inhibition of ectoapyrases increased the ATP-evoked calcium responses in Müller cell endfeet. Müller cells were immunoreactive for nucleoside triphosphate diphosphohydrolases (NTPDase)2 (but not NTPDase1), ecto-5′-nucleotidase, P2Y₁, and A1 receptors. Enzyme histochemistry revealed that ATP but not adenosine 5′-diphosphate (ADP) is extracellularly metabolised in retinal slices of NTPDase1 knockout mice. NTPDase1 activity and protein is restricted to blood vessels, whereas activity of alkaline phosphatase is essentially absent at physiological pH. The data suggest that NTPDase2 is the major ATP-degrading ectonucleotidase of the retinal parenchyma. NTPDase2 expressed by Müller cells can be implicated in the regulation of purinergic calcium responses and cellular volume.

ATP-induced relaxation of porcine retinal arterioles depends on the perivascular retinal tissue and acts via an adenosine receptor

PURPOSE

Purinergic compounds and cyclooxygenase inhibitors are involved in the tone regulation of isolated retinal arterioles in vitro, but it is unknown whether the perivascular retinal tissue influences these effects.

METHODS

Adenosine-and ATP-induced vasodilation of porcine retinal arterioles was studied in a wire myograph before and after removal of the perivascular tissue.

RESULTS

Both adenosine and ATP caused relaxation of the studied arterioles. This effect depended on the perivascular tissue and could be blocked by antagonists but was unaffected by ibuprofen.

CONCLUSIONS

The relaxation of porcine retinal arterioles induced by purinergic compounds is modulated by the perivascular retinal tissue.