title: Glutamate modulation of GABA transport in retinal horizontal cells of the skate.
abstract: Transport of the amino acid GABA into neurons and glia plays a key role in regulating the effects of GABA in the vertebrate retina. We have examined the modulation of GABA-elicited transport currents of retinal horizontal cells by glutamate, the likely neurotransmitter of vertebrate photoreceptors. Enzymatically isolated external horizontal cells of skate were examined using whole-cell voltage-clamp techniques. GABA (1 mM ) elicited an inward current that was completely suppressed by the GABA transport inhibitors tiagabine (10 microM) and SKF89976-A (100 microM), but was unaffected by 100 microM picrotoxin. Prior application of 100 microM glutamate significantly reduced the GABA-elicited current. Glutamate depressed the GABA dose-response curve without shifting the curve laterally or altering the voltage dependence of the current. The ionotropic glutamate receptor agonists kainate and AMPA also reduced the GABA-elicited current, and the effects of glutamate and kainate were abolished by the ionotropic glutamate receptor antagonist 6-cyano-7-nitroquinoxaline. NMDA neither elicited a current nor modified the GABA-induced current, and metabotropic glutamate analogues were also without effect. Inhibition of the GABA-elicited current by glutamate and kainate was reduced when extracellular calcium was removed and when recording pipettes contained high concentrations of the calcium chelator BAPTA. Caffeine (5 mM) and thapsigargin (2 nM), agents known to alter intracellular calcium levels, also reduced the GABA-elicited current, but increases in calcium induced by depolarization alone did not. Our data suggest that glutamate regulates GABA transport in retinal horizontal cells through a calcium-dependent process, and imply a close physical relationship between calcium-permeable glutamate receptors and GABA transporters in these cells.
authors.affiliation: Department of Biological Sciences, University of Illinois at Chicago, Chicago, IL 60607, USA.
authors.forename: Matthew A
authors.initials: MA
authors.lastname: Kreitzer
authors.forename: Kristen A
authors.initials: KA
authors.lastname: Andersen
authors.forename: Robert Paul
authors.initials: RP
authors.lastname: Malchow
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D002352:Carrier Proteins / Q000378:metabolism* / Q000502:physiology; D004305:Dose-Response Relationship, Drug; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D018756:GABA Antagonists / Q000494:pharmacology; D050485:GABA Plasma Membrane Transport Proteins; D018698:Glutamic Acid / Q000008:administration & dosage / Q000494:pharmacology / Q000502:physiology*; D007425:Intracellular Membranes / Q000378:metabolism; D007608:Kainic Acid / Q000494:pharmacology; D008565:Membrane Proteins / Q000378:metabolism* / Q000502:physiology; D026901:Membrane Transport Proteins; D027361:Organic Anion Transporters; D018408:Patch-Clamp Techniques; D012160:Retina / Q000166:cytology / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology; D012860:Skates, Fish / Q000502:physiology*; D018350:alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid / Q000494:pharmacology
chemical_list: D002352:Carrier Proteins; D018691:Excitatory Amino Acid Antagonists; D018756:GABA Antagonists; D050485:GABA Plasma Membrane Transport Proteins; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; D027361:Organic Anion Transporters; D018698:Glutamic Acid; D018350:alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid; D007608:Kainic Acid; D002118:Calcium