title: Membrane currents evoked by excitatory amino acid agonists in ON bipolar cells of the mudpuppy retina.
abstract: 1. Whole-cell patch-clamp recordings were obtained from ON bipolar cells in a retinal slice preparation of the mudpuppy, Necturus maculosus. The effects of excitatory amino acid (EAA) agonists applied in the presence of cobalt (2-5 mM) were examined. 2. At the holding potential of -50 mV, L-2-amino-4-phosphonobutanoic acid (L-AP4, 5-10 microM) evoked an outward current accompanied by a conductance decrease. The zero current potential of the L-AP4-evoked current was near 0 mV independent of whether the intracellular Ringer solution contained CsCl or CsCH3SO4. The currents evoked by light were also accompanied by a conductance decrease and reversed near 0 mV. Replacing external sodium with choline or N-methyl-D-glucamine generated an outward current and suppressed the response to L-AP4. The response to L-AP4 was enhanced by removing extracellular calcium and suppressed by increasing extracellular calcium. These results indicate that L-AP4 closes nonspecific cation channels that are blocked by extracellular calcium. 3. In 2 mM cobalt, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA, 50-100 microM) evoked membrane currents that were accompanied by a conductance increase. AMPA-evoked currents exhibited a significant chloride dependence and were suppressed by gamma-aminobutyric acid-A (GABAA) antagonists bicuculline and picrotoxin; a GABA uptake blocker, nipecotic acid; and a glycine antagonist, strychnine. AMPA-induced currents were virtually absent in the presence of 5 mM cobalt and nominally 0 mM extracellular calcium. These results indicate that the conductance increase induced by AMPA in the presence of 2 mM cobalt is largely the result of calcium-dependent synaptic inputs onto GABAA and glycine receptors of ON bipolar cells. 4. N-methyl-D-aspartic acid (250 microM) was ineffective when applied in the presence of 100 microM cadmium or 2 mM cobalt. 5. 1S,3R/1R,3S-1-aminocyclopentane-1,3-dicarboxylic acid (100-200 microM) evoked an outward current accompanied by a conductance decrease and appears to be an agonist at the L-AP4 receptor. 6. The findings of this study suggest that the only type of EAA receptor in mudpuppy ON bipolar cells is the L-AP4 receptor and that L-AP4 receptor activation results in the closing of nonspecific cation channels that are blocked by extracellular calcium.
authors.affiliation: Department of Physiology, University of Minnesota, Minneapolis 55455.
authors.forename: W B
authors.initials: WB
authors.lastname: Thoreson
authors.forename: R F
authors.initials: RF
authors.lastname: Miller
mesh_terms: D000596:Amino Acids / Q000502:physiology*; D000613:Aminobutyrates / Q000494:pharmacology; D000818:Animals; D002104:Cadmium / Q000494:pharmacology; D002462:Cell Membrane / Q000378:metabolism; D003035:Cobalt / Q000494:pharmacology; D003515:Cycloleucine / Q000031:analogs & derivatives / Q000494:pharmacology; D004594:Electrophysiology; D005971:Glutamates / Q000494:pharmacology; D018698:Glutamic Acid; D007473:Ion Channels / Q000378:metabolism*; D007608:Kainic Acid / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects; D016202:N-Methylaspartate / Q000494:pharmacology; D009338:Necturus maculosus; D018377:Neurotransmitter Agents / Q000494:pharmacology; D017459:Receptors, Amino Acid / Q000187:drug effects / Q000378:metabolism; D012160:Retina / Q000166:cytology / Q000378:metabolism*; D018350:alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid / Q000494:pharmacology
chemical_list: D000596:Amino Acids; D000613:Aminobutyrates; D005971:Glutamates; D007473:Ion Channels; D018377:Neurotransmitter Agents; D017459:Receptors, Amino Acid; D002104:Cadmium; D003515:Cycloleucine; C036361:1-amino-1,3-dicarboxycyclopentane; D003035:Cobalt; D018698:Glutamic Acid; D016202:N-Methylaspartate; D018350:alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid; C012729:2-amino-4-phosphonobutyric acid; D007608:Kainic Acid