title: Microglial morphology and dynamic behavior is regulated by ionotropic glutamatergic and GABAergic neurotransmission.
abstract: OBJECTIVE
  Microglia represent the primary resident immune cells in the CNS, and have been implicated in the pathology of neurodegenerative diseases. Under basal or "resting" conditions, microglia possess ramified morphologies and exhibit dynamic surveying movements in their processes. Despite the prominence of this phenomenon, the function and regulation of microglial morphology and dynamic behavior are incompletely understood. We investigate here whether and how neurotransmission regulates "resting" microglial morphology and behavior.
  
  
  METHODS
  We employed an ex vivo mouse retinal explant system in which endogenous neurotransmission and dynamic microglial behavior are present. We utilized live-cell time-lapse confocal imaging to study the morphology and behavior of GFP-labeled retinal microglia in response to neurotransmitter agonists and antagonists. Patch clamp electrophysiology and immunohistochemical localization of glutamate receptors were also used to investigate direct-versus-indirect effects of neurotransmission by microglia.
  
  
  RESULTS
  Retinal microglial morphology and dynamic behavior were not cell-autonomously regulated but are instead modulated by endogenous neurotransmission. Morphological parameters and process motility were differentially regulated by different modes of neurotransmission and were increased by ionotropic glutamatergic neurotransmission and decreased by ionotropic GABAergic neurotransmission. These neurotransmitter influences on retinal microglia were however unlikely to be directly mediated; local applications of neurotransmitters were unable to elicit electrical responses on microglia patch-clamp recordings and ionotropic glutamatergic receptors were not located on microglial cell bodies or processes by immunofluorescent labeling. Instead, these influences were mediated indirectly via extracellular ATP, released in response to glutamatergic neurotransmission through probenecid-sensitive pannexin hemichannels.
  
  
  CONCLUSIONS
  Our results demonstrate that neurotransmission plays an endogenous role in regulating the morphology and behavior of "resting" microglia in the retina. These findings illustrate a mode of constitutive signaling between the neural and immune compartments of the CNS through which immune cells may be regulated in concert with levels of neural activity.
authors.affiliation: Unit on Neuron-Glia Interactions in Retinal Disease, National Eye Institute, National Institutes of Health, Bethesda, Maryland, United States of America.
authors.forename: Aurora M
authors.initials: AM
authors.lastname: Fontainhas
authors.forename: Minhua
authors.initials: M
authors.lastname: Wang
authors.forename: Katharine J
authors.initials: KJ
authors.lastname: Liang
authors.forename: Shan
authors.initials: S
authors.lastname: Chen
authors.forename: Pradeep
authors.initials: P
authors.lastname: Mettu
authors.forename: Mausam
authors.initials: M
authors.lastname: Damani
authors.forename: Robert N
authors.initials: RN
authors.lastname: Fariss
authors.forename: Wei
authors.initials: W
authors.lastname: Li
authors.forename: Wai T
authors.initials: WT
authors.lastname: Wong
mesh_terms: D000818:Animals; D048430:Cell Shape; D049452:Green Fluorescent Proteins; D007107:Immune System / Q000166:cytology; D066298:In Vitro Techniques; D051379:Mice; D017628:Microglia / Q000166:cytology*; D018715:Microscopy, Video; D018079:Receptors, GABA; D058468:Receptors, Ionotropic Glutamate; D012160:Retina / Q000166:cytology*; D009435:Synaptic Transmission / Q000502:physiology*
chemical_list: D018079:Receptors, GABA; D058468:Receptors, Ionotropic Glutamate; D049452:Green Fluorescent Proteins