Permeation of calcium through excitatory amino acid receptor channels in cultured rat hippocampal neurones.

Abstract

1. N-methyl-D-aspartate (NMDA)-, quisqualate- and kainate-induced currents were recorded in cultured rat hippocampal neurones using the whole-cell voltage-clamp technique. To isolate the inward currents carried by Ca2+ and other divalent cations (Sr2+, Ba2+, Mn2+ and Mg2+), both Na+ and K+ in the control external solution were replaced with the impermeant cation N-methylglucamine (NMG). 2. Replacement of Na+, K+ and Ca2+ with NMG abolished NMDA-, quisqualate- and kinate-induced inward currents. In Na+-, K+-free (abbreviated simply as Na+-free) solution containing 10 mM-Ca2+ NMDA caused prominent inward currents at -60 mV. In this solution with the internal solution containing 165 mM-Cs+, the reversal potential of the NMDA-induced current was -5.0 .+-. 0.7 mV (n = 36), indicating a value of PCa/PCs = 6.2 for the ratio of the permeability coefficients of Ca2+ and Cs+ according to the constant-field equation. 3. NMDA elicited inward current responses at -60 mV in Na+-, Ca2+-free solution containing 10 mM-Sr2+, Ba2+, or Mn2+, but not in a Na+-free, 10 mM-Mg2+ solution. On the basis of reversal potential measurements, the permeability sequence of NMDA receptor channels among the divalent cations was determined to be Ba2+ (1.2) > Ca2+ (1.0) > Sr2+ (0.8) > Mn2+ (0.3) .mchgt. Mg2+ (< 0.02). 4. The reversal potential of the quisqualate-induced current was more negative than -80 mV in the Na+-free, 10 mM-Ca2+ solution, indicating a value of PCa/PCs < 0.18. 5. Kainate-induced current responses were classified into two types. In the type I response the reversal potential of the kainate-induced current was more negative than -80 mV in Na+-free, 10 mM-Ca2+ solution, indicating that the Ca2+ permeability of this type of kainate channel is as low as that of the quisqualate channel. In the neurones which showed a type I response, there was a tendency of outward rectification in the current-voltage plots of the kainate response in control solution. 6. In the type II response kainate caused prominent inward currents at -60 mV in Na+-free, 10 mM-Ca2+ solution. The reversal potential was -23.3 .+-. 5.6 mV (n = 17), indicating a permeability ratio PCa/PCs = 2.3. In the neurones which showed a type II response, a remarkable inward rectification was observed in the current-voltage plots of the kainate response in control solution. 7. Type II kainate channels showed relatively poor selectivity among divalent cations. The permeability sequence was Ba2+ (1.3) > Ca2+ (1.0) > Sr2+ (0.9) > Mg2+ (0.8) > Mn2+ (0.7).