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

Abstract

Calcium-permeable non- N-methyl- d-aspartate receptor channels are now characterized in much detail, but still little is known about the consequences of Ca 2+ influx through these channels in specific neuron types. We are interested in the role of Ca 2+-permeable non- N-methyl- d-aspartate receptor channels during differentiation of retinal ganglion cells. However, in view of the conflicting data on the relative Ca 2+ permeability of non- N-methyl- d-aspartate receptor channels in these neurons, a more systematic evaluation of permeation properties of different Na + substitutes was necessary before proceeding with the main goal of the present study—evaluating the effects of non- N-methyl- d-aspartate receptor 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 immunostaining and repetitive firing behaviour and submitted to patch-clamp recording in the whole-cell configuration. Non- N-methyl- d-aspartate receptor channels were activated by application of amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid or kainate. It was found that they were 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 TMA/P Cs=0.23). When using N-methyl- d-glucamine as a substitute for Na + to obtain bi-ionic conditions P Ca/P Cs varied between 0.08 to 1.40. Linear current-voltage relation or little outward rectification corresponded to a low Ca 2+ permeability (P Ca/P Cs=0.14). In about one third of the cells kainate-induced currents showed inward rectification and non- N-methyl- d-aspartate receptor agonists induced a substantially higher Ca 2+ influx (P Ca/P Cs=0.64). Activation of non- N-methyl- d-aspartate receptors by kainate profoundly altered the repetitive discharge of retinal ganglion cells. In contrast to the continuously firing controls, cells generated only a few spikes at the beginning of a steady depolarization after kainate exposure. Among the candidates regulating the firing behaviour of retinal ganglion cells voltage-activated Ca 2+ and K + conductances were tested for their sensitivity to kainate application. It was found that even short conditioning pulses of kainate decreased the peak amplitudes of both voltage-activated K + and voltage-activated Ca 2+ currents. Only the latter effect required extracellular Ca 2+ and was antagonized by increasing the intracellular Ca 2+ buffering strength. Thus, suppression of calcium currents was induced by a non- N-methyl- d-aspartate receptor-mediated rise of the intracellular calcium concentration. The reduction of K + currents did not depend on extracellular calcium and was insensitive to experimental manipulation of intracellular Ca 2+ buffer strength. The interaction between Ca 2+-permeable non- N-methyl- d-aspartate receptor channels and voltage-activated Ca 2+ and K + currents may represent an important regulatory mechanism to control the repetitive firing of developing retinal ganglion cells.