Pharmacological characterization, localization, and regulation of ionotropic glutamate receptors in skate horizontal cells.
Glutamate is believed to be the primary excitatory neurotransmitter in the vertebrate retina, and its fast postsynaptic effects are elicited by activating NMDA-, kainate-, or AMPA-type glutamate receptors. We have characterized the ionotropic glutamate receptors present on retinal horizontal cells of the skate, which possess a unique all-rod retina simplifying synaptic circuitry within the outer plexiform layer (OPL). Isolated external horizontal cells were examined using whole-cell voltage-clamp techniques. Glutamate and its analogues kainate and AMPA, but not NMDA, elicited dose-dependent currents. The AMPA receptor antagonist GYKI 52466 at 100 microm abolished glutamate-elicited currents. Desensitization of glutamate currents was removed upon coapplication of cyclothiazide, known to potentiate AMPA receptor responses, but not by concanavalin A, which potentiates kainate receptor responses. The dose-response curve to glutamate was significantly broader in the presence of the desensitization inhibitor cyclothiazide. Polyclonal antibodies directed against AMPA receptor subunits revealed prominent labeling of isolated external horizontal cells with the GluR2/3 and GluR4 antibodies. 1-Naphthylacetyl spermine, known to block calcium-permeable AMPA receptors, significantly reduced glutamate-gated currents of horizontal cells. Downregulation of glutamate responses was induced by increasing extracellular ion concentrations of Zn2+ and H+. The present study suggests that Ca2+-permeable AMPA receptors likely play an important role in shaping the synaptic responses of skate horizontal cells and that alterations in extracellular concentrations of calcium, zinc, and hydrogen ions have the potential to regulate the strength of postsynaptic signals mediated by AMPA receptors within the OPL.
Kreitzer
Matthew A
MA
Department of Biological Sciences, University of Illinois at Chicago, Chicago, Illinois, USA. matthew.kreitzer@indwes.edu
Birnbaum
Andrea D
AD
Qian
Haohua
H
Malchow
Robert Paul
RP
D000818:Animals; D001569:Benzodiazepines / Q000494:pharmacology; D001581:Benzothiadiazines / Q000494:pharmacology; D001703:Biophysics; D002118:Calcium / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D004558:Electric Stimulation / Q000379:methods; D018683:Excitatory Amino Acid Agents / Q000494:pharmacology*; D018690:Excitatory Amino Acid Agonists / Q000494:pharmacology; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D018698:Glutamic Acid / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D015640:Ion Channel Gating / Q000187:drug effects; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D018408:Patch-Clamp Techniques / Q000379:methods; D018091:Receptors, AMPA / Q000378:metabolism*; D012160:Retina / Q000166:cytology; D051248:Retinal Horizontal Cells / Q000187:drug effects* / Q000378:metabolism*; D012860:Skates, Fish / Q000033:anatomy & histology* / Q000378:metabolism*; D015032:Zinc / Q000494:pharmacology; D018350:alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid / Q000494:pharmacology
D001581:Benzothiadiazines; D018683:Excitatory Amino Acid Agents; D018690:Excitatory Amino Acid Agonists; D018691:Excitatory Amino Acid Antagonists; D018091:Receptors, AMPA; C061950:GYKI 52466; D001569:Benzodiazepines; D018698:Glutamic Acid; D018350:alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid; D015032:Zinc; C004639:cyclothiazide; D002118:Calcium