Inwardly rectifying and Ca2+-permeable AMPA-type glutamate receptor channels in rat neocortical neurons. Shun-ichi Itazawa, Tadashi Isa and Seiji Ozawa. Department of Physiology, Gunma University School of Medicine, 3-39-22 Showa-machi, Maebashi, Gunma, Japan 371, and Department of Integrative Physiology, The National Institute for Physiological Sciences, Myodaiji, Okazaki, Japan 444.
APStracts 4:142N, 1997.
ABSTRACT
Current-voltage (I-V) relations and Ca2+ permeability of a-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 non-desensitizing agonist. A patch pipette was filled with solution containing 100 mM 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 (mean S.D.= 0.08 0.02, n = 8) for type I neurons and from 0.14 to 1.29 (0.60 0.37, n = 11) for the intermediate neurons, respectively. 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 non-pyramidal 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(CNQX)-sensitive fast and D-2-amino-5-phosphono- valerate(APV)-sensitive slow 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 more prominently suppressed (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. 

Received 28 April 1997; accepted in final form 1 July 1997.
APS Manuscript Number J332-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 27 August 1997