AMPA distinguishes multiple types of quisqualate binding sites in rat brain.

Abstract

Glutamate is the major neurotransmitter mediating excitatory signalling within the mammalian central nervous system. Glutamate exerts its effects via its interactions with different types of excitatory amino acid receptors, commonly classified as N-methyl- sc D-aspartate (NMDA), kainate, and quisqualate receptors. Quantitative autoradiographic assays for quisqualate-sensitive ($\sp3$H) glutamate binding and ($\sp3$H) (RS)-$\alpha$-amino-3-hydroxy-5-methylisoxazole-4-propionic acid ( ($\sp3$H) AMPA) were developed to study the properties of quisqualate receptors. Quisqualate-sensitive ($\sp3$H) glutamate binding was nonuniformly distributed in the rat brain, and the pharmacological properties of this binding distinguished it from ($\sp3$H) glutamate binding to astrocytic membrane sites, enzymes, or chloride-dependent ($\sp3$H) glutamate uptake sites. Quisqualate-sensitive ($\sp3$H) glutamate binding was only partially displaced by AMPA and had a different regional distribution than ($\sp3$H) AMPA binding. Regional comparisons and competition studies indicated that AMPA interacted with a subset of ($\sp3$H) glutamate binding sites recognized by quisqualate. Potassium thiocyanate (KSCN) increased both overall ($\sp3$H) AMPA binding and the ability of unlabelled AMPA to compete for ($\sp3$H) glutamate binding. ($\sp3$H) AMPA saturation curves performed in different concentrations of KSCN revealed two binding sites for ($\sp3$H) AMPA existed, and that KSCN increased both the density of high affinity sites and the affinity of low affinity sites. Comparison of ($\sp3$H) AMPA binding in the absence and presence of KSCN suggest that high and low affinity sites are interconvertible to some extent. The portion of ($\sp3$H) glutamate binding which was displaced by quisqualate but not AMPA (AMPA-insensitive, quisqualate-sensitive ($\sp3$H) glutamate binding) had high affinity for quisqualate. Glutamate and ibotenate also competed for AMPA-insensitive, quisqualate-sensitive ($\sp3$H) glutamate binding. The pharmacology of this binding site corresponds to a novel type of quisqualate receptor linked to phosphoinositide metabolism. In summary, two populations of ($\sp3$H) glutamate binding sites recognized by quisqualate exist: AMPA-sensitive and AMPA-insensitive. The AMPA-sensitive population corresponds to the ionotropic quisqualate/AMPA receptor while the AMPA-insensitive population corresponds to the metabotropic quisqualate receptor. Additionally, AMPA distinguishes two affinity states of the ionotropic receptor. These states are interconvertible to some extent, and their interconversion is dependent on KSCN. The term quisqualate receptors therefore denotes multiple entities: a metabotropic receptor insensitive to AMPA, and an AMPA-sensitive ionotropic receptor which itself has multiple states.