title: Calcium imaging reveals a network of intrinsically light-sensitive inner-retinal neurons.
abstract: BACKGROUND
  Mice lacking rod and cone photoreceptors (rd/rd cl) are still able to regulate a range of responses to light, including circadian photoentrainment, the pupillary light reflex, and suppression of pineal melatonin by light. These data are consistent with the presence of a novel inner-retinal photoreceptor mediating non-image-forming irradiance detection.
  
  
  RESULTS
  We have examined the nature and extent of intrinsic light sensitivity in rd/rd cl retinae by monitoring the effect of light stimulation (470 nm) on intracellular Ca(2+) via FURA-2 imaging. Using this approach, which does not rely on pharmacological or surgical isolation of ganglion cells from the rod and cone photoreceptors, we identified a population of light-sensitive neurons in the ganglion cell layer (GCL). Retinal illumination induced an increase of intracellular Ca(2+) in approximately 2.7% of the neurons. The light-evoked Ca(2+) fluxes were dependent on the intensity and duration of the light stimulus. The light-responsive units formed an extensive network that could be uncoupled by application of the gap junction blocker carbenoxolone. Three types of light-evoked Ca(2+) influx were observed: sustained, transient, and repetitive, which are suggestive of distinct functional classes of GCL photoreceptors.
  
  
  CONCLUSIONS
  Collectively, our data reveal a heterogeneous syncytium of intrinsically photosensitive neurons in the GCL coupled to a secondary population of light-driven cells, in the absence of rod and cone inputs.
authors.affiliation: Department of Integrative and Molecular Neuroscience, Division of Neuroscience and Psychological Medicine, Imperial College London, Charing Cross Hospital, London, United Kingdom.
authors.forename: Sumathi
authors.initials: S
authors.lastname: Sekaran
authors.forename: Russell G
authors.initials: RG
authors.lastname: Foster
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Lucas
authors.forename: Mark W
authors.initials: MW
authors.lastname: Hankins
mesh_terms: D000818:Animals; D001693:Biological Transport, Active; D002118:Calcium / Q000378:metabolism*; D016257:Fura-2 / Q000378:metabolism; D007150:Immunohistochemistry; D017136:Ion Transport; D051379:Mice; D008856:Microscopy, Fluorescence; D010775:Photic Stimulation; D020419:Photoreceptor Cells, Vertebrate / Q000378:metabolism* / Q000502:physiology*; D012165:Retinal Ganglion Cells / Q000502:physiology*; D017299:Rod Opsins / Q000378:metabolism; D000099210:Melanopsin
chemical_list: D002118:Calcium; D016257:Fura-2; D017299:Rod Opsins; D000099210:Melanopsin