MEMBRANE CURRENTS EVOKED BY EXCITATORY AMINO-ACID AGONISTS IN ON BIPOLAR CELLS OF THE MUDPUPPY RETINA

1. Whole-cell patch-clamp recordings were obtained from ON bipolar cel ls in a retinal slice preparation of the mudpuppy, Necturus maculosus. The effects of excitatory amino acid (EAA) agonists applied in the pr esence of cobalt (2-5 mM) were examined. 2. At the holding potential o f -50 MV, L-2-amino-4-phosphonobutanoic acid (L-AP4, 5-10 muM) evoked an outward current accompanied by a conductance decrease. The zero cur rent potential of the L-AP4-evoked current was near 0 mV independent o f 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 cholin e 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 calci um. These results indicate that L-AP4 closes nonspecific cation channe ls that are blocked by extracellular calcium. 3. In 2 mM cobalt, lpha- amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA, 50-100 muM) evoked membrane currents that were accompanied by a conductance increa se. AMPA-evoked currents exhibited a significant chloride dependence a nd were suppressed by gamma-aminobutyric acid-A (GABA(A)) antagonists bicuculline and picrotoxin; a GABA uptake blocker, nipecotic acid; and a glycine antagonist, strychnine. AMPA-induced currents were virtuall y absent in the presence of 5 mM cobalt and nominally 0 mM extracellul ar calcium. These results indicate that the conductance increase induc ed by AMPA in the presence of 2 mM cobalt is largely the result of cal cium-dependent synaptic inputs onto GABA(A) and glycine receptors of O N bipolar cells. 4. N-methyl-D-aspartic acid (250 muM) was ineffective when applied in the presence of 100 muM cadmium or 2 mM cobalt. 5. 1S ,3R/1R,3S-1-aminocyclopentane-1,3-dicarboxylic acid 100-200 muM) evoke d 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 cel ls is the L-AP4 receptor and that L-AP4 receptor activation results in the closing of nonspecific cation channels that are blocked by extrac ellular calcium.