DIFFERENTIAL ROLE OF 2 CA2-PERMEABLE NON-NMDA GLUTAMATE CHANNELS IN RAT RETINAL GANGLION-CELLS - KAINATE-INDUCED CYTOPLASMIC AND NUCLEAR CA2+ SIGNALS()

1. The permeability of non-N-methyl-D-aspartate (non-NMDA) glutamate c hannels to divalent cations and specifically the entry of Ca2+ and sub sequent elevations in cytoplasmic and nuclear Ca2+ signals were invest igated in cultured neonatal rat retinal ganglion cells using the whole cell patch-clamp technique and Ca2+ imaging with confocal microscopy. In addition, divalent-permeable non-NMDA receptor channels were studi ed in retinal slices using a Co2+ staining technique. 2. Using Ca2+ (2 .5 mM) as the only permeable cation in the external solution, stimulat ion with 100 mu M kainate produced nondesensitizing, nonselective cati on currents with either low or high Ca2+ permeability. Both currents w ere reversibly blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Neurons with the low divalent-permeable currents (type 1) had reversa l potentials of -41.5 +/- 4.4 mV (mean +/- SD), and neurons with the h igh divalent-permeable currents (type 2) had reversal potentials of -2 2.6 +/- 5.5 mV. The permeability ratio P-Ca/P-Cs, was 3.3 for the type 1 currents and 8.5 for the type 2 currents, indicating a 2.5-fold gre ater permeability to Ca2+ for the type 2 non-NMDA glutamate channels. 3. Both types of non-NMDA glutamate channels showed relatively little selectivity between Ca2+ and Co2+. The type 1 neurons had a slightly h igher permeability to Co2+ than to Ca2+, whereas the type 2 neurons we re equally permeable to both divalent cations. The type 2 neurons had a much higher permeability for both divalent cations compared with the type 1 neurons. 4. Staining for Co2+ uptake through kainate-stimulate d non-NMDA glutamate channels in retinal slices provided additional ev idence for the presence of the two ganglion cell populations. Activati on of the neurons by kainate in conditions isolating the non-NMDA glut amate channel caused differential uptake of Co2+. In contrast, depolar ization in the presence of the non-NMDA antagonist CNQX failed to caus e Co2+ influx. 5. Imaging experiments using confocal microscopy showed that kainate stimulation induced an increase in intracellular Ca2+ in both types of retinal ganglion cells, but only the type 2 neurons sho wed a substantial increase in cytoplasmic and nuclear Ca2+ signals. Ka inate-induced Ca2+ signals in the type 2 neurons were almost nine time s greater than those of the type 1 neurons. 6. When intracellular Ca2 stores were depleted by brief treatment with thapsigargin, kainate-in duced Ca2+ signals in the type 1 neurons were unchanged. However, in t he type 2 neurons kainate no longer induced large Ca2+ signals in the cytoplasm and nucleus, despite normal influx of Ca2+. After thapsigarg in treatment, kainate-induced Ca2+ signals in the type 2 neurons were reduced to the levels of the treated or untreated type 1 neurons. Calc ium influx via the type 2 receptor channels therefore appears capable of triggering the release of intracellular Ca2+ stores. 7. The two typ es of non-NMDA glutamate channels described here are likely to contrib ute to a variety of Ca2+-dependent intracellular processes. Glutamate may differentially affect intracellular Ca2+ in retinal ganglion cells . In particular, it appears that Ca2+ entry via the high divalent-perm eable type 2 non-NMDA glutamate channel can trigger Ca2+-induced Ca2release and thereby amplify intracellular Ca2+ signals.