Fractional calcium currents through recombinant GluR channels of the NMDA, AMPA and kainate receptor subtypes.

Abstract

1. Simultaneous fluorescence and whole-cell current measurements using the calcium indicator dye fura-2 were made in HEK 293 cells expressing recombinant glutamate receptor (GluR) channels, and fractional Ca2+ currents (the proportion of whole-cell current carried by CP2+ 4. In cells expressing a-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor (AMPAR) subunits which were unedited at the Q/R site of the putative transmembrane segment TM2 (Q-form), or in cells coexpressing unedited and edited subunits (R-form), the glutamateevoked Ca2+ inflow increased from 20 to -80 mV in an almost linear way. 5. Fractional Ca2+ currents through AMPAR channels depended on subunit composition.Pf values of Q-form homomeric channels at -60 mV and 1X8 mm [Ca2+]o were between 3-2 and 3 9%. They were slightly voltage dependent and increased with [Ca2+]o in the range 1 8-10 mm. Pf values in cells co-expressing Q-and R-form subunits were almost one order of magnitude smaller (0 54 %). 6. Relative concentrations of Q-form and R-form GluR-B subunit-specific cDNAs used for cell transfection determined the expression of functionally different heteromeric AMPARS.Pf decreased with increasing relative concentration of R-form encoding cDNAs from 3-4 to 1-4 %, demonstrating that editing of the Q/R site of GluR-B subunits decreases Ca2P inflow through heteromeric AMPARs. 7. Cells expressing the GluR-6 subunit of the kainate receptor (KAR) family were characterized by Pf values which depended on the editing in the TM1 and TM2 segments.Pf values were largest for the Q-form (1 -55-2 0 %) and lowest for R-form channels (< 0 2 %), suggesting that Q/R site editing also decreases Ca2P inflow through KAR channels. Cells coexpressing both subunit forms showed an intermediate value (0 58 %).

Calcium release via activation of presynaptic IP3 receptors contributes to kainate-induced IPSC facilitation in rat neocortex

“…Homomeric, unedited GluR5 receptors display a significantly higher Ca 2+ permeability than the edited forms (Egebjerg and Heinemann, 1993;Burnashev et al, 1995). It is possible that the extracellular Ca 2+ needed for KA facilitation was entering through Ca 2+ permeable KARs.…”

NMDA Receptor Heterogeneity During Postnatal Development of the Rat Brain: Differential Expression of the NR2A, NR2B, and NR2C Subunit Proteins

“…N-Methyl-D-aspartate (NMDA) receptors are glutamate-activated cation channels, characterized by a high Ca 2~INa~permeability ratio, a voltage-dependent Mg 2+ block, a requirement for glycine as coagonist, and slow activation and deactivation kinetics (Nowak et al, 1984;Ascher and Nowak, 1988;Johnson and Ascher, 1990;Stern et al, 1992;Schneggenburger et al, 1993;Monyer et al, 1994;Burnashev et al, 1995;Garaschuk et al, 1996). These properties underlie the neurophysiological functions of NMDA receptors, in particular in synaptic plasticity (Constantine-Paton et al, 1990;Komuro and Rakic, 1993;Malenka and NicoIl, 1993;Constantine-Paton, 1994;Kullmann and Siegelbaum, 1995; Rison and Stanton, 1995;Seeburg et al, 1995).…”

Human neuronal threonine-for-leucine-248 alpha 7 mutant nicotinic acetylcholine receptors are highly Ca2+ permeable

“…Activities, not concentrations, of ions were used for the calculations. The activity coefficients have been described (8).…”

“…The rise in cytosolic Ca 2ϩ concentration induced by nicotine was expressed as the ratio of time-resolved fluorescence variation over the basal fluorescence (⌬F͞F) and averaged from an ellipsoidal domain approximating the cell shape. For determining the fractional Ca 2ϩ current (P f ), the fluorescence emission elicited by an excitation wavelength of 380 nm (F 380 ) was used (8,10,12,31).…”

“…[1][2][3][4][5], and it is now well established that Ca 2ϩ may permeate the pore of various types of neurotransmitter-gated receptors in the brain. Nevertheless, direct determinations of the transmitter-activated Ca 2ϩ inflow lacked until the Ca 2ϩ entry into the cells was monitored by a combination of electrophysiological and optical techniques with Ca 2ϩ sensitive dyes (6)(7)(8)(9)(10)(11)(12). While trying to measure Ca 2ϩ influx through nicotinic receptors, we and others found that the adult muscle nAcChoR channel is much more permeable than the embryonic muscle nAcChoR (11,13) but less permeable than some neuronal nAcChoR channels (7,11,14).…”