Inwardly Rectifying and Ca²⁺-Permeable AMPA-Type Glutamate Receptor Channels in Rat Neocortical Neurons

Abstract

Inwardly rectifying and Ca2+-permeable AMPA-type glutamate receptor channels in rat neocortical neurons. J. Neurophysiol. 78: 2592-2605, 1997. Current-voltage (I-V) relations and Ca2+ permeability of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)type glutamate receptor channels were investigated in neurons of rat neocortex by using the whole cell patch-clamp technique in brain slices. To activate AMPA receptor channels, kainate was used as a nondesensitizing agonist. A patch pipette was filled with solution containing 100 mu M spermine to maintain the inward rectification of Ca2+-permeable AMPA receptor channels. Three types of responses to kainate were observed: type I response with outwardly rectifying I-V relation, type II response with I-V relation of marked inward rectification, and intermediate response with I-V relation of weaker inward rectification. Neurons with type I, type II and intermediate I-V relations were referred to as type I, type II, and intermediate neurons, respectively. Of a total of 223 recorded cells, 90 (40.4%) were type I, 129 (57.8%) intermediate, and 4 (1.8%) type II neurons. Properties of AMPA receptor channels were examined in the former two types of neurons. The value of PCa:PCs, the ratio of the permeability coefficients of Ca2+ and Cs+, was estimated from the reversal potentials of kainate responses in the outside-out patches bathed in Na+-free solution containing 100 mM Ca2+ according to the constant-field equation. They ranged from 0.05 to 0.10 (0.08 +/- 0. 02, mean +/- SD, n = 8) for type I neurons and from 0.14 to 1.29 (0. 60 +/- 0.37, n = 11) for the intermediate neurons. There was a close correlation between the inward rectification and the Ca2+ permeability in AMPA receptor channels in these neurons. Intermediate neurons stained with biocytin were nonpyramidal cells with ellipsoidal-shaped somata. Type I neurons had either triangular- or ellipsoidal-shaped somata. Excitatory postsynaptic currents (EPSCs) recorded in both type I and intermediate neurons had 6-cyano-7-nitroquinoxaline-2,3-dione-sensitive fast and -2-amino-5-phosphonovalerate-sensitiveslow components. The I-V relation of the fast component exhibited inward rectification in the intermediate neuron, whereas that in the type I neuron showed slight outward rectification. The fast component of EPSCs in the intermediate neuron was suppressed more prominently (to 56 +/- 15% of the control, n = 12) than that in the type I neuron (to 78 +/- 6% of the control, n = 6) by bath application of 1 mM spermine. These results indicate that inwardly rectifying and Ca2+-permeable AMPA receptor channels are expressed in a population of neurons of rat neocortex and are involved in excitatory synaptic transmission.

Evidence for Ca2+-permeable AMPA receptors in the olfactory bulb

“…This finding is similar to results demonstrated in other brain regions suggesting that CP-AMPARs are principally expressed by GABAergic interneurons (23,28,33,36,44,71). For example, AMPARs expressed by dentate gyrus basket cells in the hippocampus, which are thought to be GABAergic interneurons, are highly Ca 2ϩ permeable (36).…”

“…The resulting degree of current inhibition was expressed as a percentage of the control (KA alone) current and estimated using the following formula: [amplitude of KA ϩ NAS (or JSTX) current/ amplitude of KA-alone current] ϫ 100. Similarly to methods used in previous studies (28), KA rather than AMPA was used as an agonist to avoid the effects of receptor desensitization. Because KA receptors mediate relatively small currents and desensitize rapidly, measurement of the steady-state amplitudes of these nondesensitizing currents likely reflects currents mediated by AMPARs.…”

“…However, AMPARs that lack or contain unedited GluR2 generate inwardly rectifying currents and are highly Ca 2ϩ permeable (7,21,64). These Ca 2ϩ -permeable AMPARs (CP-AMPARs) have been found in a variety of regions (e.g., hippocampus, neocortex, spinal cord) (13,15,16,33,36,44) and have been implicated in physiological processes including synaptic transmission (16,24,25,28,44) and long-term depression (40). Increasing evidence suggests that CP-AMPARs also play a role in some forms of neuropathology (48,63,67).…”

Novel Application of Stem Cell-Derived Neurons to Evaluate the Time- and Dose-Dependent Progression of Excitotoxic Injury

“…For measurement of Mg 2+ -free ERB NMDAR currents, the perfusate was formulated as above, but without Mg 2+ . For AMPAR I-V curves, Kainate (100 uM; KA) was used as a non-desensitizing agonist of AMPAR channels to avoid neurotoxicity induced by simultaneous application of AMPA and cyclothioheximide [14], [15].…”

Synaptic Conductances and Spike Generation in Cortical Cells

“…Trying to predict when APs should occur is a difficult problem, even in this simplified case: the current shunted through GABA receptors during spikes is seen to be a sensitive function of the membrane potential, reversing in sign during the integration up to individual spikes, vanishing when g GABA drops, while the current through AMPA-Rs also becomes highly voltage dependent near to its reversal potential, ''choking'' off the inward current, indeed reversing transiently during each spike. Note, though, that in small, parvalbumin-expressing inhibitory neurons, AMPA-Rs lacking GluRB subunits give rise to complete inwardly rectification (and calcium permeability) of AMPA receptors (Itazawa, Isa et al 1997), due to block by intracellular spermine. Conductance injection shows that this rectification boosts the excitation (A. Harsch and H.P.C.…”