Excitatory amino acid receptors on isolated retinal ganglion cells from the goldfish

Abstract

1. Whole-cell currents activated by the excitatory amino acids L-glutamic acid (glutamate, Glu), L-aspartic acid (Asp), and their analogues N-methyl-D-aspartate (NMDA), kainic acid (KA), quisqualic acid (QA), and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) were recorded from ganglion cells enzymatically dissociated from goldfish retina and grown in culture. All agonists induced detectable whole-cell responses in the majority of cells cultured from 2 to 72 h. 2. Currents activated by each of the agonists were selective for cations (Na+) over anions (Cl-). The responses to Glu, NMDA, and Asp were each potentiated when 1 microM glycine was coapplied. Extracellular Mg2+ blocked completely the response to NMDA plus glycine in cells held at negative potentials, but the block was relieved when cells were more depolarized. 3. Dose-response measurements revealed a rank order of sensitivity to the Glu analogues in the presence of 1 microM glycine and zero extracellular Mg2+; QA greater than AMPA greater than NMDA greater than KA. Cells were not responsive to APB (L-2-aminophosphonobutyric acid). 4. Kynurenic acid (Ky) produced a noncompetitive block of the NMDA response in the presence of 1 microM glycine with a Ki of 40 microM. Responses to KA, QA, and AMPA were blocked competitively by Ky with Kis of 72, 148, and 656 microM, respectively. QA and AMPA competitively blocked the response to KA with Kis of 114 microM and 1 mM, respectively. NMDA single channels had a mainstate slope conductance of 27-30 pS and two subconductance levels of 5 and 24 pS at 13 degrees C in symmetrical Na+. 5. Whole-cell responses to QA and AMPA were highly correlated, suggesting that QA and AMPA activated the same receptor or class of receptors; whereas, responses to QA and KA were not well correlated, suggesting that these agonists at least in part activated separate receptor populations.