title: Tight Coupling of Astrocyte pH Dynamics to Epileptiform Activity Revealed by Genetically Encoded pH Sensors.
abstract: Astrocytes can both sense and shape the evolution of neuronal network activity and are known to possess unique ion regulatory mechanisms. Here we explore the relationship between astrocytic intracellular pH dynamics and the synchronous network activity that occurs during seizure-like activity. By combining confocal and two-photon imaging of genetically encoded pH reporters with simultaneous electrophysiological recordings, we perform pH measurements in defined cell populations and relate these to ongoing network activity. This approach reveals marked differences in the intracellular pH dynamics between hippocampal astrocytes and neighboring pyramidal neurons in rodent in vitro models of epilepsy. With three different genetically encoded pH reporters, astrocytes are observed to alkalinize during epileptiform activity, whereas neurons are observed to acidify. In addition to the direction of pH change, the kinetics of epileptiform-associated intracellular pH transients are found to differ between the two cell types, with astrocytes displaying significantly more rapid changes in pH. The astrocytic alkalinization is shown to be highly correlated with astrocytic membrane potential changes during seizure-like events and mediated by an electrogenic Na(+)/HCO3 (-) cotransporter. Finally, comparisons across different cell-pair combinations reveal that astrocytic pH dynamics are more closely related to network activity than are neuronal pH dynamics. This work demonstrates that astrocytes exhibit distinct pH dynamics during periods of epileptiform activity, which has relevance to multiple processes including neurometabolic coupling and the control of network excitability.
  
  
  
  Dynamic changes in intracellular ion concentrations are central to the initiation and progression of epileptic seizures. However, it is not known how changes in intracellular H(+) concentration (ie, pH) differ between different cell types during seizures. Using recently developed pH-sensitive proteins, we demonstrate that astrocytes undergo rapid alkalinization during periods of seizure-like activity, which is in stark contrast to the acidification that occurs in neighboring neurons. Rapid astrocytic pH changes are highly temporally correlated with seizure activity, are mediated by an electrogenic Na(+)/HCO3- cotransporter, and are more tightly coupled to network activity than are neuronal pH changes. As pH has profound effects on signaling in the nervous system, this work has implications for our understanding of seizure dynamics.
authors.affiliation: Department of Pharmacology, University of Oxford, Oxford OX1 3QT, United Kingdom, Division of Physiology, Department of Human Biology, Faculty of Health Sciences, University of Cape Town, Cape Town 7925, South Africa, UCT/MRC Receptor Biology Unit, Department of Integrative Biomedical Sciences and Institute of Infectious Disease and Molecular Medicine, Faculty of Health Sciences, University of Cape Town, Cape Town 7925, South Africa.
authors.forename: Joseph V
authors.identifier: 0000-0002-8266-3128
authors.initials: JV
authors.lastname: Raimondo
authors.affiliation: Division of Physiology, Department of Human Biology, Faculty of Health Sciences, University of Cape Town, Cape Town 7925, South Africa.
authors.forename: Hayley
authors.identifier: 0000-0002-4495-539X
authors.initials: H
authors.lastname: Tomes
authors.affiliation: Department of Pharmacology, University of Oxford, Oxford OX1 3QT, United Kingdom.
authors.forename: Agnese
authors.identifier: 0000-0003-1624-0386
authors.initials: A
authors.lastname: Irkle
authors.affiliation: Department of Pharmacology, University of Oxford, Oxford OX1 3QT, United Kingdom.
authors.forename: Louise
authors.initials: L
authors.lastname: Kay
authors.affiliation: Division of Physiology, Department of Human Biology, Faculty of Health Sciences, University of Cape Town, Cape Town 7925, South Africa.
authors.forename: Lauriston
authors.initials: L
authors.lastname: Kellaway
authors.affiliation: Brain Mind Institute, École Polytechnique Fédérale de Lausanne, Lausanne CH-1015, Switzerland, and.
authors.forename: Henry
authors.initials: H
authors.lastname: Markram
authors.affiliation: UCT/MRC Receptor Biology Unit, Department of Integrative Biomedical Sciences and Institute of Infectious Disease and Molecular Medicine, Faculty of Health Sciences, University of Cape Town, Cape Town 7925, South Africa, Centre for Neuroendocrinology and Mammal Research Institute, University of Pretoria, Pretoria 0001, South Africa.
authors.forename: Robert P
authors.initials: RP
authors.lastname: Millar
authors.affiliation: Department of Pharmacology, University of Oxford, Oxford OX1 3QT, United Kingdom, colin.akerman@pharm.ox.ac.uk.
authors.forename: Colin J
authors.identifier: 0000-0001-6844-4984
authors.initials: CJ
authors.lastname: Akerman
mesh_terms: D000818:Animals; D000831:Animals, Newborn; D001253:Astrocytes / Q000378:metabolism*; D004195:Disease Models, Animal; D004827:Epilepsy / Q000209:etiology / Q000473:pathology* / Q000503:physiopathology; D049452:Green Fluorescent Proteins / Q000235:genetics / Q000378:metabolism; D006624:Hippocampus / Q000166:cytology*; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007425:Intracellular Membranes / Q000378:metabolism*; D018613:Microscopy, Confocal; D009474:Neurons / Q000378:metabolism; D009924:Organ Culture Techniques; D051381:Rats; D017208:Rats, Wistar; D027982:Sodium-Bicarbonate Symporters / Q000235:genetics*; D019108:Tight Junctions / Q000502:physiology*; D014161:Transduction, Genetic
chemical_list: D027982:Sodium-Bicarbonate Symporters; C485184:enhanced green fluorescent protein; D049452:Green Fluorescent Proteins
keywords: astrocytes; epileptiform activity; intracellular pH; seizures