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ABSTRACT: In the developing nervous system, spontaneous neuronal activity arises independently of experience or any environmental input. This activity may play a major role in axonal pathfinding, refinement of topographic maps, dendritic morphogenesis, and the segregation of axonal terminal arbors. In the auditory system, endogenously released ATP in the cochlea activates inner hair cells to trigger bursts of action potentials (APs), which are transferred to the central auditory system. Here we show the modulatory role of purinergic signaling beyond the cochlea, i.e., the developmentally regulated and cell-type-specific depolarizing effects on auditory brainstem neurons of Mongolian gerbil. We assessed the effects of P2X receptors (P2XRs) on neuronal excitability from prehearing to early stages of auditory signal processing. Our results demonstrate that in neurons expressing P2XRs, extracellular ATP can evoke APs in sync with Ca(2+) signals. In cochlear nucleus (CN) bushy cells, ATP increases spontaneous and also acoustically evoked activity in vivo, but these effects diminish with maturity. Moreover, ATP not only augmented glutamate-driven firing, but it also evoked APs in the absence of glutamatergic transmission. In vivo recordings also revealed that endogenously released ATP in the CN contributes to neuronal firing activity by facilitating AP generation and prolonging AP duration. Given the enhancing effect of ATP on AP firing and confinement of P2XRs to certain auditory brainstem nuclei, and to distinct neurons within these nuclei, it is conceivable that purinergic signaling plays a specific role in the development of neuronal brainstem circuits.
Journal of Neuroscience 08/2012; 32(31):10699-712. · 7.11 Impact Factor
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ABSTRACT: Extracellular signals from the endbulb of Held-spherical bushy cell (SBC) synapse exhibit up to three component waves ('P', 'A' and 'B'). Signals lacking the third component (B) are frequently observed but as the origin of each of the components is uncertain, interpretation of this lack of B has been controversial: is it a failure to release transmitter or a failure to generate or propagate an action potential? Our aim was to determine the origin of each component. We combined single- and multiunit in vitro methods in Mongolian gerbils and Wistar rats and used pharmacological tools to modulate glutamate receptors or voltage-gated sodium channels. Simultaneous extra- and intracellular recordings from single SBCs demonstrated a presynaptic origin of the P-component, consistent with data obtained with multielectrode array recordings of local field potentials. The later components (A and B) correspond to the excitatory postsynaptic potential (EPSP) and action potential of the SBC, respectively. These results allow a clear interpretation of in vivo extracellular signals. We conclude that action potential failures occurring at the endbulb-SBC synaptic junction largely reflect failures of the EPSP to trigger an action potential and not failures of synaptic transmission. The data provide the basis for future investigation of convergence of excitatory and inhibitory inputs in modulating transmission at a fully functional neuronal system using physiological stimulation.
European Journal of Neuroscience 05/2010; 31(9):1574-81. · 3.63 Impact Factor
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ABSTRACT: Purinoreceptors of the P2 family contribute strongly to signaling in the cochlea, but little is known about the effects of purinergic neurotransmission in the central auditory system. Here we examine P2 receptor-mediated signaling in the large spherical bushy cells (SBCs) of Mongolian gerbils around the onset of acoustically evoked signal processing (P9-P14). Brief adenosine 5'-O-(3-thiotriphosphate) (ATPgammaS) application evoked inward current, membrane depolarization, and somatic Ca2+ signals. Moreover, ATPgammaS changed the SBCs firing pattern from phasic to tonic, when the application was synchronized with depolarizing current injection. This bursting discharge activity was dependent on [Ca2+]i and Ca2+-dependent protein kinase (PKC) activity and is presumably caused by modulation of low-threshold K+ conductance. Activation of P2Y1 receptors could not evoke these changes per se, thus it was concluded that the involvement of P2X receptors seems to be necessary. Ca2+ imaging data showed that both P2X and P2Y1 receptors mediate Ca2+ signals in SBCs where P2Y1 receptors most likely activate the PLC-IP3 (inositol trisphosphate) pathway and release Ca2+ from internal stores. Immunohistochemical staining confirmed the expression of P2X2 and P2Y1 receptor proteins in SBCs, providing additional evidence for the involvement of both receptors in signal transduction in these neurons. Purinergic signaling might modulate excitability of SBCs and thereby contribute to regulation of synaptic strength. Functionally, the increase in firing rate mediated by P2 receptors could reduce temporal precision of the postsynaptic firing, e.g., phase locking, which has an immediate effect on signal processing related to sound localization. This might provide a mechanism for adaptation to the ambient acoustic environment.
Journal of Neurophysiology 08/2009; 102(3):1821-33. · 3.32 Impact Factor
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ABSTRACT: We assessed the potential of using multielectrode arrays (MEAs) to investigate several physiological properties of the calyx of Held synapse in the medial nucleus of the trapezoid body of gerbil. Due to the large size of the synapse, it became widely employed in studies on synaptic mechanisms. Electrical stimulation at the midline evoked a characteristic compound signal consisting of a presynaptic volley (C(1)) and a postsynaptic response (C(2)). The C(1) was blocked by tetrodotoxin, whilst the C(2) was blocked by perfusion of low Ca(2+) external solution, or the AMPA-R antagonists CNQX, and GYKI52466. NMDA-R blocker D-AP5, partially inhibited the postsynaptic response at P12, but showed no effect in P30 animals. The inhibitory effects of GABA or glycine on postsynaptic responses were reciprocal with regard to animal's maturity: GABA caused a pronounced reduction of C(2) amplitude in P20-22 animals, while glycine showed a stronger inhibition in P27-28 animals. Low-frequency super-threshold stimulation of the afferents induced facilitation of the postsynaptic C(2) amplitudes and only minor changes in temporal characteristics of the signals. At stimulation frequencies >200 Hz, however, significant depression occurs accompanied by increases in transmission delay and in the width of the postsynaptic response. This study suggests MEAs as a useful tool to study calyx of Held synapse by simultaneous recordings of pre- and postsynaptic elements of synaptically interconnected neurons in the auditory brainstem. Moreover, MEAs enable convenient analysis of activity-dependent depression and modulation of neuronal activity by glycine and GABA at later developmental stages not accessible to patch recordings.
Journal of Neuroscience Methods 08/2008; 174(2):227-36. · 1.98 Impact Factor
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ABSTRACT: Activation of the receptor for alpha2-macroglobulin (alpha2 M), the low-density lipoprotein-related protein (LRP1; CD91), has been suggested to represent a possible strategy for the inhibition of uncontrolled retinal cell proliferation via stimulation of the clearance of alpha2 M-bound growth factors and proteinases from the extracellular space. In order to prove this assumption, we investigated the effect of alpha2 M on the proliferation of Müller glial cells in vitro.
Proliferation assays using bromodeoxyuridine were carried out on cultured Müller glial cells of the guinea pig in the absence and presence of alpha2 M.
Activated alpha2 M evoked a slight increase of the cell proliferation at control conditions. Addition of alpha2 M to the culture medium inhibited the proliferation evoked by agonists of G-protein-coupled receptors [adenosine 5'-triphosphate (ATP), neuropeptide Y]. However, alpha2 M did not diminish the proliferation evoked by agonists of receptor tyrosine kinases (epidermal and platelet-derived growth factors) and by serum, respectively. Inhibition of LRP1 by a neutralizing antibody did not alter the ATP-evoked proliferation while it increased the proliferation in the presence of alpha2 M.
It is concluded that alpha2 M inhibits the proliferation evoked by agonists of G-protein-coupled receptors, possibly via enhanced growth factor clearance by LRP.
Albrecht von Graæes Archiv für Ophthalmologie 09/2005; 243(8):811-6. · 2.17 Impact Factor
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ABSTRACT: Interleukin 8 (IL-8, CXCL8) is a pro-inflammatory chemokine which attracts neutrophils to sites of inflammation via an activation of the G-protein-coupled receptors, CXCR1 and CXCR2. However, both IL-8 and IL-8 receptors are widely expressed in various tissues and cell types, and have been suggested to be involved in other functions such as angiogenesis, tumor growth, or brain pathology. We examined the expression of IL-8 and IL-8 receptors in highly enriched primary cultures of guinea pig Muller glial cells. Immunoreactivity for CXCL8, CXCR1 and CXCR2 was observed in all cultured Muller cells. The expression of CXCL8 was confirmed by PCR, and the secretion of the CXCL8 protein from Muller cells was revealed by ELISA. Western blots showed prominent bands at approximately 40 kDa by using antibodies specific for human CXCR1 and CXCR2, and the expression of a putative CXCR2 receptor in Muller cells was confirmed by PCR. Furthermore, cultured Muller cells responded to application of recombinant human IL-8 with an increase of the cytosolic Ca(2+) concentration. If supernatants of cultured human retinal pigment epithelium (RPE) cells were applied to the Muller cell cultures, no obvious changes were observed in the CXCL8, CXCR1 and CXCR2 expression but (i) Muller cell proliferation was stimulated, and (ii) there was an increased number of CXCL8-responsive Muller cells and the amplitudes of the evoked calcium responses were enhanced. It is concluded that Muller glial cells may participate in the inflammatory response(s) of the retina during ocular diseases, and that this contribution may be modified by interactions with RPE cells.
Journal of Neuroimmunology 05/2005; 161(1-2):49-60. · 2.96 Impact Factor
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ABSTRACT: Glial cells in human retinas and in fibrocellular membranes from patients with proliferative vitreoretinopathy (PVR) have been described to upregulate their expression of Y1 receptors for neuropeptide Y (NPY) (Soler et al.: Glia 39:320, 2002). However, it is unknown whether Y1 receptor activation causes proliferation of retinal glial cells. We investigated whether NPY exerts a proliferation-stimulating effect on retinal glial cells, and compared the NPY-evoked signaling with the signaling of purinergic P2Y receptors.
Proliferation assays using bromodeoxyuridine were carried out on primarily cultured Muller glial cells of the guinea pig, in the absence and presence of blockers of Y1 receptors, of receptor tyrosine kinases (RTKs), of mitogen-activated protein kinases (MAPKs) and of phosphatidylinositol-3 kinase (PI3K).
NPY exerted a biphasic effect on Muller cell proliferation. At low concentrations (0.1 ng/ml and 1 ng/ml) it decreased the proliferation rate of the cells, while at higher concentration (100 ng/ml) it increased Muller cell proliferation. The NPY-evoked proliferation was mediated by Y1 receptor stimulation and by activation of the p44/p42 MAPKs and partially of the p38 MAPK. Moreover, Y1 receptor-induced activation of PI3K as well as transactivations of the platelet-derived and the epidermal growth factor RTKs were necessary for full mitogenic effect of NPY. Y1 and P2Y receptors share partially common signal transduction pathways in Muller cells.
It is suggested that NPY may be involved in stimulation of retinal glial cell proliferation during PVR when it is released at higher amounts into the injured retina.
Albrecht von Graæes Archiv für Ophthalmologie 12/2004; 242(11):944-50. · 2.17 Impact Factor
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ABSTRACT: To determine whether P2Y receptor-evoked proliferation of Müller glial cells depends on transactivation of receptor tyrosine kinases.
Primary cultures of Müller cells of the guinea pig were treated with test substances for 16 hours. The DNA synthesis rate was assessed by a bromodeoxyuridine (BrdU) immunoassay, and the phosphorylation states of the extracellular signal-regulated kinase (ERK1/2) and the p38 mitogen-activated protein kinase (p38 MAPK) were determined by Western blot analysis.
In Müller cells, the mitogenic effect of P2Y receptor activation by extracellular adenosine triphosphate (ATP) depended on transactivation of both the platelet-derived growth factor (PDGF) and the epidermal growth factor (EGF) receptor tyrosine kinases, as suggested by the blocking effects of the tyrphostins AG1296 and AG1478 on the ATP-induced proliferation and phosphorylation of ERK1/2. Moreover, the PDGF-induced proliferation may depend on transactivation of the EGF receptor kinase. Antibodies against heparin-binding EGF (HB-EGF) or PDGF, as well as inhibition of matrix metalloproteinases (MMPs) blocked ATP-evoked proliferation. At least one metalloproteinase (MMP-9), was implicated in the signal transfer from P2Y to EGF receptors. In contrast, the mitogenic effect of fetal calf serum was independent of growth factor receptor activity. P2Y receptor activation stimulated Müller cell proliferation by activating the ERK1/2 and the phosphatidylinositol 3 (PI3) kinase signaling pathways, whereas the p38 MAPK pathway was not involved in mitogenic signaling.
The present data suggest that P2Y-receptor-induced mitogenic signaling in Müller cells is mediated by transactivation of the PDGF and EGF receptor tyrosine kinases. The transactivation may be mediated by release of PDGF and MMP-dependent shedding of HB-EGF from the Müller cell matrix, respectively. The transactivation of the receptor tyrosine kinases may result in activation of ERK1/2 and PI3 kinase and an increase in the proliferation rate.
Investigative Ophthalmology & Visual Science 04/2003; 44(3):1211-20. · 3.60 Impact Factor
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ABSTRACT: To understand the role of different K(+) channel subtypes in glial cell-mediated spatial buffering of extracellular K(+), immunohistochemical localization of inwardly rectifying K(+) channel subunits (Kir2.1, Kir2.2, Kir2.3, Kir4.1, and Kir5.1) was performed in the retina of the mouse. Stainings were found for the weakly inward-rectifying K(+) channel subunit Kir4.1 and for the strongly inward-rectifying K(+) channel subunit Kir2.1. The most prominent labeling of the Kir4.1 protein was found in the endfoot membranes of Müller glial cells facing the vitreous body and surrounding retinal blood vessels. Discrete punctate label was observed throughout all retinal layers and at the outer limiting membrane. By contrast, Kir2.1 immunoreactivity was located predominantly in the membrane domains of Müller cells that contact retinal neurons, i.e., along the two stem processes, over the soma, and in the side branches extending into the synaptic layers. The results suggest a model in which the glial cell-mediated transport of extracellular K(+) away from excited neurons is mediated by the cooperation of different Kir channel subtypes. Weakly rectifying Kir channels (Kir4.1) are expressed predominantly in membrane domains where K(+) currents leave the glial cells and enter extracellular "sinks," whereas K(+) influxes from neuronal "sources" into glial cells are mediated mainly by strongly rectifying Kir channels (Kir 2.1). The expression of strongly rectifying Kir channels along the "cables" for spatial buffering currents may prevent an unwarranted outward leak of K(+), and, thus, avoid disturbances of neuronal information processing.
Glia 10/2002; 39(3):292-303. · 4.82 Impact Factor
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ABSTRACT: One of the main functions of Müller glial cells is the performance of retinal K+ homeostasis which is thought to be primarily mediated by K+ fluxes through inwardly rectifying K+ (Kir) channels expressed in Müller cell membranes. Until now, there is limited knowledge about the types of Kir channel subunits expressed by Müller cells. Using RT-PCR, we investigated the expression of mRNA encoding different Kir channel subunits in the retina of the guinea pig. In order to verify expression by Müller cells, primary cultures of guinea pig Müller cells were also investigated. Both retinae and cultured Müller cells express mRNA for a diversity of Kir channel subtypes which include members of at least four channel subfamilies: Kir2.1, Kir2.2, Kir2.4, Kir3.1, Kir 3.2, Kir4.1, Kir6.1, and Kir6.2. mRNAs for the following Kir channel subtypes were not detected in Müller cells: Kir1.1, Kir2.3, Kir3.3, Kir3.4, Kir4.2, and Kir5.1. It is concluded that the spatial buffering of extracellular K+ by Müller cells may be mediated by cooperation of different subtypes of Kir channels, and that the distinct Kir channel types involved in this function may change depending on the physiological or metabolic state of the retina.
Neuroreport 07/2002; 13(8):1037-40. · 1.66 Impact Factor
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ABSTRACT: To determine whether activation of P2Y receptors may increase the DNA synthesis rate of cultured Müller cells and to investigate whether adenosine 5'-triphosphate (ATP)-induced Müller cell proliferation is mediated by an intracellular calcium increase.
Primary cultures of Müller cells of the guinea pig were treated with test substances for 16 hours. The DNA synthesis rate was assessed by a bromodeoxyuridine immunoassay, and ATP-induced elevations of the intracellular calcium concentration were recorded by fura-2 imaging.
ATP or uridine triphosphate (UTP) increased the DNA synthesis rate whereas alpha,beta-methylene-ATP, 2-methyl-thio-ATP, and adenosine were ineffective, indicating that the action of ATP was through P2Y receptors. The effect of ATP was dose dependent, with an EC(50) of 5.9 microM. The mitogenic effect of ATP required an elevation of the intracellular calcium and a calcium influx into Müller cells. Blockers of calcium-permeable channels (nickel ions) or of calcium-dependent potassium (BK) channels (iberiotoxin, charybdotoxin) inhibited the ATP-stimulated DNA synthesis. In calcium-imaging experiments, ATP-evoked intracellular calcium transients were significantly shortened in the presence of extracellular nickel ions or of iberiotoxin. A correlation was found between the duration of the ATP-evoked calcium transients and the basal proliferation rate of the cultures.
The results indicate that the ATP-induced elevation of Müller glial DNA synthesis is dependent on an influx of calcium ions from the extracellular space and that the inhibiting effect of BK channel blockers on ATP-evoked DNA synthesis is caused by an inhibition of this influx. The amount of the calcium influx seems to be directly correlated to the strength of the ATP-evoked proliferation.
Investigative Ophthalmology & Visual Science 04/2002; 43(3):766-73. · 3.60 Impact Factor
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Mike Francke,
Michael Weick,
Thomas Pannicke,
Ortrud Uckermann,
Jens Grosche,
Iwona Goczalik, Ivan Milenkovic,
Susanne Uhlmann,
Frank Faude,
Peter Wiedemann,
Andreas Reichenbach,
Andreas Bringmann
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ABSTRACT: To test whether in an animal model of proliferative vitreoretinopathy (PVR) the Müller glial cells displayed an upregulation of purinergic P2 receptor-mediated responses.
PVR was induced by intravitreal injection of the proteolytic enzyme, dispase, in the eyes of adult rabbits. The developing PVR was examined ophthalmoscopically. After 3 weeks, small retinal pieces were wholemounted and used for calcium imaging, freshly dissociated Müller cells were subjected to calcium imaging, and patch-clamp recordings were made. The presence of P2 receptor-mediated Ca(2+) responses was determined both directly--that is, fluorometrically--and indirectly, by electrophysiological recording of Ca(2+)-activated K(+) currents.
According to earlier observations in another model of retinal detachment and PVR, the reactive Müller cells displayed hypertrophy, downregulation of inwardly rectifying K(+) currents, and depolarization of the resting membrane potential, all dependent on the severity of the PVR. Further, significant PVR-induced increase was observed in the number of Müller cells responding to adenosine 5'-triphosphate (ATP), with a transient elevation of their [Ca(2+)](i). If isolated Müller cells were exposed to ATP, 13% of the control cells, but 29% (moderate PVR) or 53% (massive PVR) of the reactive cells, showed fluorometric Ca(2+) increases. An increase of Ca(2+)-activated K(+) currents was measured in 11% of the control cells, but in 83% (moderate PVR) and 90% (massive PVR) of the reactive cells. Confocal images of retinal wholemounts revealed similar results. Because similar responses were elicited by uridine triphosphate (UTP), the dominant involvement of metabotropic (P2Y type) purinergic receptors is suggested.
An upregulation of purinergic receptors is part of the reactive changes of Müller cells during PVR. It is suggested that ATP-evoked Ca(2+) responses may support the proliferation of Müller cells during PVR.
Investigative Ophthalmology & Visual Science 04/2002; 43(3):870-81. · 3.60 Impact Factor
