[Current topics on glutamate receptors].
Glutamate (Glu) receptors are classified into two major categories in the mammalian central nervous system: inotropic receptors linked to ion channels and metabotropic receptors linked to phosphatidylinositol (PI) metabolism. Classification of the inotropic Glu receptors is based on the differential sensitivity to excitement by N-methyl-D-aspartic acid (NMDA), DL-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and kainic acid (KA). The NMDA-sensitive subclass is supposed to be a receptor ionophore complex consisting of at least four different subcomponents, including an NMDA recognition site, a glycine (Gly) recognition site, a polyamine recognition site and a cation channel. The NMDA site is radiolabeled by both Glu and competitive antagonists, such as (+-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) and DL-(E)-2-amino-4-propyl-5-phosphono-3-pentenoic acid (CGP 39653). The Gly domain, which is labeled by both [3H]Gly and [3H]5,7-dichlorokynurenic acid, is sensitive to D-serine but insensitive to strychnine, and this domain seems to be absolutely required for an activation of the NMDA channel by agonists. The ionophore domain is identified by radiolabeled non-competitive NMDA antagonists that gain access to the binding sites within the channel only when it is gated by agonists. The opening of an NMDA channel is allosterically potentiated by Gly and several polyamines. In contrast, an activation of the NMDA channel is blocked by both H+ and divalent cations such as Mg2+ and Zn2+. [3H]AMPA binding displays pharmacological profiles of the AMPA-sensitive subclass with a rank order of agonistic potencies of quisqualic acid (QA) greater than or equal to AMPA greater than Glu greater than KA, which is apparently different from that found for the KA-sensitive subclass (domoic acid greater than or equal to KA greater than QA greater than Glu). In contrast, several quinoxaline derivatives competitively antagonize neuronal responses mediated not only by the AMPA receptor but also by the KA receptor. The metabotropic Glu receptors, which stimulate PI metabolism through an activation of the guanosine triphosphate-binding proteins, are activated by Glu, QA and trans-1-amino-cyclopentyl-1,3-di-carboxylic acid (ACPD). Responses mediated by the metabotropic receptors are competitively blocked by 2-amino-3-phosphonopropionic acid. Three or four cloned complementary deoxyribonucleic acids (cDNAs) encoding inotropic Glu receptors are isolated from a rat brain cDNA library. Pharmacological and electrophysiological properties of receptor-ion channels encoded by a transfection of these cDNAs are similar to those observed with the AMPA receptor as well as the KA receptor, but not with the NMDA receptor.
Yoneda
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Y
Department of Pharmacology, Faculty of Pharmaceutical Sciences, Setsunan University, Osaka, Japan.
Ogita
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K
D000818:Animals; D001665:Binding Sites; D002490:Central Nervous System / Q000378:metabolism; D005971:Glutamates / Q000378:metabolism / Q000502:physiology; D007473:Ion Channels / Q000378:metabolism; D007476:Ionophores / Q000378:metabolism; D018377:Neurotransmitter Agents; D010716:Phosphatidylinositols / Q000378:metabolism; D017470:Receptors, Glutamate; D017981:Receptors, Neurotransmitter / Q000145:classification / Q000378:metabolism / Q000502:physiology*
D005971:Glutamates; D007473:Ion Channels; D007476:Ionophores; D018377:Neurotransmitter Agents; D010716:Phosphatidylinositols; D017470:Receptors, Glutamate; D017981:Receptors, Neurotransmitter