INTERACTION OF CALCIUM-PERMEABLE NON-N-METHYL-D-ASPARTATE RECEPTOR CHANNELS WITH VOLTAGE-ACTIVATED POTASSIUM AND CALCIUM CURRENTS IN RAT RETINAL GANGLION-CELLS IN-VITRO

Calcium-permeable non-N-methyl-D-aspartate receptor channels are now c haracterized in much detail, but still little is known about the conse quences of Ca2+ influx through these channels in specific neuron types . We are interested in the role of Ca2+-permeable non-N-methyl-D-aspar tate receptor channels during differentiation of retinal ganglion cell s. However, in view of the conflicting data on the relative Ca2+ perme ability of non-N-methyl-D-aspartate receptor channels in these neurons , a more systematic evaluation of permeation properties of different N a+ substitutes was necessary before proceeding with the main goal of t he present study-evaluating the effects of non-N-methyl-D-aspartate re ceptor activation on repetitive firing and voltage-activated K+ and Ca 2+ conductances. Retinal ganglion cells were dissociated from the rat retina on postnatal day 5. They were selected by vital anti-Thy-1 immu nostaining and repetitive firing behaviour and submitted to patch-clam p recording in the whole-cell configuration. Non-N-methyl-D-aspartate receptor channels were activated by application of amino-3-hydroxy-5-m ethyl-4-isoxazolepropionic acid or kainate. It was found that they wer e essentially impermeable to N-methyl-D-glucamine (P-NMDG/P-Cs<0.02), but not to choline (P-choline/P-Cs=0.24) and tetramethylammonium (P-TM A/P-Cs=0.23). When using N-methyl-D-glucamine as a substitute for Naf to obtain bi-ionic conditions P-Ca/P-Cs varied between 0.08 to 1.40. L inear current-voltage relation or little outward rectification corresp onded to a low Ca2+ permeability (P-Ca/P-Cs=0.14). In about one third of the cells kainate-induced currents showed inward rectification and non-lv-methyl-D-aspartate receptor agonists induced a substantially hi gher Ca2+ influx (P-Ca/P-Cs=0.64). Activation of non-N-methyl-D-aspart ate receptors by kainate profoundly altered the repetitive discharge o f retinal ganglion cells. In contrast to the continuously firing contr ols, cells generated only a few spikes at the beginning of a steady de polarization after kainate exposure. Among the candidates regulating t he firing behaviour of retinal ganglion cells voltage-activated Ca2+ a nd K+ conductances were tested for their sensitivity to kainate applic ation. It was found that even short conditioning pulses of kainate dec reased the peak amplitudes of both voltage-activated Kt and voltage-ac tivated Ca2+ currents. Only the latter effect required extracellular C a2+ and was antagonized by increasing the intracellular Ca2+ buffering strength. Thus, suppression of calcium currents was induced by a non- lv-methyl-D-aspartate receptor-mediated rise of the intracellular calc ium concentration. The reduction of K+ currents did not depend on extr acellular calcium and was insensitive to experimental manipulation of intracellular Ca2+ buffer strength. The interaction between Ca2+-perme able non-N-methyl-D-aspartate receptor channels and voltage-activated Ca2+ and K+ currents may represent an important regulatory mechanism t o control the repetitive firing of developing retinal ganglion cells. (C) 1998 IBRO. Published by Elsevier Science Ltd.