title: Carbenoxolone blocks the light-evoked rise in intracellular calcium in isolated melanopsin ganglion cell photoreceptors.
abstract: BACKGROUND
  Retinal ganglion cells expressing the photopigment melanopsin are intrinsically photosensitive (ipRGCs). These ganglion cell photoreceptors send axons to several central targets involved in a variety of functions. Within the retina ipRGCs provide excitatory drive to dopaminergic amacrine cells via glutamatergic signals and ipRGCs are coupled to wide-field GABAergic amacrine cells via gap junctions. However, the extent to which ipRGCs are coupled to other retinal neurons in the ganglion cell layer via gap junctions is unclear. Carbenoxolone, a widely employed gap junction inhibitor, greatly reduces the number of retinal neurons exhibiting non-rod, non-cone mediated light-evoked Ca(2+) signals suggesting extensive intercellular coupling between ipRGCs and non-ipRGCs in the ganglion cell layer. However, carbenoxolone may directly inhibit light-evoked Ca(2+) signals in ipRGCs independent of gap junction blockade.
  
  
  RESULTS
  To test the possibility that carbenoxolone directly inhibits light-evoked Ca(2+) responses in ipRGCs, the light-evoked rise in intracellular Ca(2+) ([Ca(2+)](i)) was examined using fura-2 imaging in isolated rat ipRGCs maintained in short-term culture in the absence and presence of carbenoxolone. Carbenoxolone at 50 and 100 µM concentrations completely abolished the light-evoked rise in [Ca(2+)](i) in isolated ipRGCs. Recovery from carbenoxolone inhibition was variable.
  
  
  CONCLUSIONS
  We demonstrate that the light-evoked rise in [Ca(2+)](i) in isolated mammalian ganglion cell photoreceptors is inhibited by carbenoxolone. Since the light-evoked increase in [Ca(2+)](i) in isolated ipRGCs is almost entirely due to Ca(2+) entry via L-type voltage-gated calcium channels and carbenoxolone does not inhibit light-evoked action potential firing in ipRGCs in situ, carbenoxolone may block the light-evoked increase in [Ca(2+)](i) in ipRGCs by blocking L-type voltage-gated Ca(2+) channels. The ability of carbenoxolone to block evoked Ca(2+) responses must be taken into account when interpreting the effects of this pharmacological agent on retinal or other neuronal circuits, particularly if a change in [Ca(2+)](i) is the output being measured.
authors.affiliation: School of Veterinary Medicine and Biomedical Sciences, University of Nebraska, Lincoln, Nebraska, United States of America.
authors.forename: Jayne R
authors.initials: JR
authors.lastname: Bramley
authors.forename: Erin M
authors.initials: EM
authors.lastname: Wiles
authors.forename: Patricia J
authors.initials: PJ
authors.lastname: Sollars
authors.forename: Gary E
authors.initials: GE
authors.lastname: Pickard
mesh_terms: D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D000897:Anti-Ulcer Agents / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002229:Carbenoxolone / Q000494:pharmacology*; D002478:Cells, Cultured; D016257:Fura-2; D007074:Immunoglobulin G / Q000276:immunology; D008027:Light; D010446:Peptide Fragments / Q000276:immunology; D010775:Photic Stimulation; D010786:Photoreceptor Cells / Q000187:drug effects / Q000378:metabolism*; D011817:Rabbits; D051381:Rats; D020318:Rats, Long-Evans; D012160:Retina / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D012165:Retinal Ganglion Cells / Q000187:drug effects* / Q000378:metabolism* / Q000528:radiation effects; D017299:Rod Opsins / Q000276:immunology / Q000378:metabolism*; D000099210:Melanopsin
chemical_list: D000897:Anti-Ulcer Agents; D002118:Calcium; D002229:Carbenoxolone; D016257:Fura-2; D007074:Immunoglobulin G; D010446:Peptide Fragments; D017299:Rod Opsins; D000099210:Melanopsin