The Possible Roll of a Neuron-Astrocyte Signaling Pathway in Regulating Vascular Tone

1. The Possible Roll of a Neuron-Astrocyte Signaling Pathway in Regulating Vascular Tone Presented by Chris Sherman

2. Presentation Structure 1) Critical look at Neuron to Astrocyte Signaling Pathways -Research conducted by Cornell-Bell et al. -Calcium Wave Propagation 2) Critical look at Astrocyte to Neuron Signaling Pathways -Astrocytic Glutamate release -Research conducted by Parpura et al. 3) Neuron-Astrocyte-Endothelial Cell Signaling Pathways -Research conducted by Zonta et al -Regulation of Gap Junctional Expression -Neural activity-dependent vasodilation 4) Implications of a ‘Three-Part Synapse’ 5) Future Research

3. Neuron to Astrocyte Signaling As activity increases at the synapse, more astroccytic mGluR are activated. This ultimately leads to an increased level of vesicular-bound Ca++ release into the cytoplasm of the astrocyte. When Cornell-Bell et al published this finding over a decade ago, it forced science to take a more critical look at the role astrocytes play in the brain. IV. Intracellular levels of Ca++ rise. Cytoplasmic InsP activates the first of the Ca++ vesicles, but once levels of the later rise, free Ca++ releases other pools of vesicular-bound Ca++. II. Metabotropic receptors for Glutamate (mGluR) located on astrocyte bind synaptic Glutamate. Subsequent intracellular Phospholipase C release leads to Inositol Triphosphate (InsP3) production. III. Vesicular ionotropic InsP3-mediated receptors bind InsP3. Ion channels open, allowing vesicular-bound pools of Ca++ into the intracellular enviornment. I. Glutamate release from pre-synaptic neuron

4. Glutamate Induces Calcium Waves in Cultured Astrocytes: A Form of Long-Range Glial Signalling Cornell-Bell et al Major Findings Low and High Neural Activity encoded by Astrocytes in the Frequency of Ca++ Oscillations Neural Activity Period between Calcium Peaks Existance of two GluR subtypes on Astrocytic membrane: Kainate and Quisqualate Probability of Neurotransmitter release in synapse* Astrocytic Intracellular [Ca++] Synaptic cleft [Ca++] Kainate Receptors: (ionotropic) Quisqualate Receptors: (metabotropic) Vesicular release of Ca++ into Cytoplasm of Astrocyte Inositol Triphosphate in Cytoplasm 2nd messenger Phospholipase C * (Katz and Miledi et al.)

5. Glutamate Induces Calcium Waves in Cultured Astrocytes: A Form of Long-Range Glial Signalling Cornell-Bell et al It is unlikely that sufficient neurotransmitter at one astrocytically-enveloped synapse could provoke a calcium wave. An individual Astrocyte, however, can envelop many synapses in vivo. Exposure to neurotransmitters at multiple synapses is most likely the necessary factor in calcium wave production.

6. Astrocyte to Neuron Signaling Pathways Inhibitory Action on Pre-synaptic Neuron: Kainate GluR’s have been shown to modulate inhibitory transmissions, possibly through the release of GABA (Qing-song Liu et al., 2003). Excitatory Action on Pre-Synaptic Neuron: NMDA Ionotropic Receptors bind Glutamate; ion channel opens and influx of Ca++ follows (Parpura et al. 1994). Excitatory action stems from subsequent exocytosis of neurotransmitter-containing vesicles; this occurs due to the increase in intracellular [Ca++]. Specific action of Ca++ on vesicles is not known. I. Ca++ wave from synctyium reaches distal Astrocyte; ions cause glutamate-containing vesicles to release their payload into the synaptic cleft via exocytosis. In addition to signaling to these vesicles, the Ca++ ions also move into the synaptic cleft themselves (Nedergaard et al., 1994). 30 years ago demonstrated that free Ca++ presence in the synaptic cleft increases the probability of pre-synaptic neurotransmitter release(Katz and Miledi, 1967). Excitatory Action on Post-Synaptic Neuron: NMDA Ionotropic Receptors bind Glutamate; ion channel opens and influx of Ca++ follows. This causes a membrane-wide depolarization.

7. Glutamate-dependent Astrocyte Modulation of Synaptic Transmission Between Cultured Hippocampal Neurons Parpura et al., 1994 To confirm that the increase in neuronal Ca++ was due to an astrocytic-dependant pathway (in contrast to synaptic), Parpura introduced a mGluR antagonist, d-glutamylglycine, into the cell co-culture. As expected, neuronal [Ca++] remained constant. Parpura measured neuronal [Ca++] after Bradykinin injection, and found that Ca++ waves in astrocytes induced a neural Ca++ rise. This leads to a greater potential for synaptic activity. When [Ca++] rises in astrocytes adjacent to the co-cultured neurons, glutamate is released (through exocytosis) and binds to ionotropic glutamate receptors on the neural membrane. This opens Ca++ ion channels to, and extrasynaptic Ca++ flows into the neuron. Parpura et al. introduced Bradykinin, an exogenous neuro-ligand, into the neuron-astrocyte co-culture. This glutamate receptor agonist bound to metabotropic glutamate receptor sites on a distal Astrocyte. Intracellular [Ca++] rises, eventually propagating into a global wave.

8. Cerebral Circulation As Neural activity there is an Energy requirement To solve this… Astrocytic uptake of Glutamate leads to> ADP leads to> Glycolysis within Astrocytic endfeetwhich finally leads to>Lactate delivered to neuron =Energy demand met! But what about OXYGEN? Waste? Other nutrients? With increased neural activity, there MUST be an increase in LOCAL CIRCULATION OF BLOODFLOW

9. Neuron-to-astrocyte signaling is central to the dynamic control of brain microcirculation Zonta et al., 2003 Neural Activity Ca++ propagation throughout astrocytic syncytium [Ca++] at endfeet attached to endothelial cells Vesicular release of prostanoids Relaxation of capillary walls; decrease in vascular tone Bloodflow

10. Synchronous Firing Groups- Astrocytic Regulation of Neural Networks

11. Gap Junctions Neuronal or Astrocytic control? Capillary-Tropic Neuronal Upregulation

12. Future Research -fMRI studies -Pathology -Specific action of Ca++ on Astrocytic vesicles (this is not specific to Astrocytes; research on the general mechanisms behind neuronal vesicular release of neurotransmitters is lacking as well) -Inhibitory action of Kainate receptors (both neuronal and astrocytic) -In vivo research -Cognitive functioning -Pharmacology -Overall models of how neurons integrate information

13. References 1) Katz B, Miledi R. The timing of calcium action during neuromuscular transmission. J Physiol (Lond) 1967;189: 535-544. 2) Cornell-Bell AH, Finkbeiner SM, Cooper MS, Smith SJ. Glutamate induces calcium waves in cultured astrocytes: long-range glial signaling. Science 1990;247:470-473. 3) Katz B, Miledi R. The timing of calcium action during neuromuscular transmission. J Physiol (Lond) 1967;189: 535-544. 4) Parpura V, Basarsky TA, Liu F, Jeftinija K, Jeftinija S, Haydon PG. Glutamate-mediated astrocyte-neuron signaling. Nature 1994;369:744-747. 5) Qing-song Liu, Qiwu Xu, Gregory Arcuino, Jian Kang, and Maiken Nedergaard. Astrocyte-mediated activation of neuronal kainate receptors. PNAS. 2004; 101: 3172-3177 6) Nedergaard M. Direct signaling from astrocytes to neurons in cultures of mammalian brain cells. Science 1994; 263:1768-1771. 7) Micaela Zonta, María Cecilia Angulo, Sara Gobbo, Bernhard Rosengarten, A. Hossmann, Tullio Pozzan and Giorgio Carmignoto. Neuron-to-astrocyte signaling is central to the dynamic control of brain microcirculation. Nat. Neurosci. 6, 43–50 (2003).