L-Glutamate has higher affinity than other amino acids for [3H]-D-AP5 binding sites in rat brain membranes

Abstract

Electrophysiological studies indicate the existence of several types of receptors for excitatory amino acids. Thus, responses induced by N-methyl-D-aspartate (NMDA) are potently and selectively blocked by D(-)-2-amino-5-phosphonopentanoic acid (D-AP5), while responses induced by such agonists as kainate and quisqualate are relatively resistant to this antagonist. Evidence is mounting that excitatory amino acid receptors are involved in synaptic excitation in many regions of the central nervous system (see refs 1 and 4 for reviews). Although the identity of the transmitter(s) acting at these receptors remains uncertain, L-aspartate has been considered the most likely transmitter at NMDA receptors and L-glutamate at kainate/quisqualate receptors. Other endogenous acidic amino acids proposed as possible transmitters include a range of sulphur-containing amino acids and the tryptophan metabolite, quinolinic acid. Ligand-binding studies offer a means not only of assessing receptor densities in different brain regions but also of comparing affinities of transmitter candidates for these receptors. However, to avoid difficulties of interpretation arising from the use of ligands which bind to more than one type of receptor, such as [3H]-L-glutamate and [3H]-L-aspartate (for example, refs 8-12), ligands with high receptor selectivity are required. Here, we report that [3H]-D-AP5 binds specifically to rat brain membranes, that the hippocampus and cerebral cortex are enriched in these sites relative to other brain areas and that L-glutamate has higher affinity for these receptors than have all other transmitter candidates tested.

Pharmacology of glutamate neurotoxicity in cortical cell culture: attenuation by NMDA antagonists

“…Although the binding data of Olverman et al (1984) suggest that the affinity of glutamate for D-APV binding sites approaches that of D-APV itself, we think it likely that these high concentrations of APV or D-APV effectively blocked 500 PM glutamate from activating most of the physiologically relevant NMDA receptors. O'Brien and Fischbath (1986) found 1 mM APV to be sufficient to eliminate the voltage-dependent component of the response to 1 mM glutamate on chick spinal motoneurons.…”

“…NMDA-activated membrane channels are thought to exhibit a number of special properties, including susceptibility to blockade by magnesium at hyperpolarized membrane potentials Nowak et al, 1984), rapid desensitization (Zorumski and Fischbach, 1985), and high calcium permeability Mayer et al, 1986). Although glutamate has a high affinity for the NMDA receptor (Olverman et al, 1984) magnesium blockade (at least near resting membrane potentials) and desensitization probably act to limit the contribution of NMDA channels, relative to that of QUIS and RAIN channels, to glutamate neuroexcitation. Glutamate neuroexcitation is thus completely blocked by high concentrations of broad spectrum antagonists such as kynurenate (Ganong et al, 1983;Jahr and Jessel, 1985;Huettner and Baughman, 1986) but is only partially attenuated by selective NMDA antagonists (Watkins and Evans, 1981;O'Brien and Fischbach, 1986).…”

Rapid desensitization of glutamate receptors in vertebrate central neurons.

“…If G1 receptors have a relatively high affinity for glutamate (19), a response that slowly rises and decays implies that there might be a 'significant population of extrasynaptic receptors.…”

Possible roles ofl-phosphoserine in the pathogenesis of Alzheimer’s disease

“…D-PSer had a K1 of 303 pM in the same assay. Olverman et al (1984Olverman et al ( , 1988 showed L-PSer to be a weak inhibiter of D- [3H]2-amino-5-phosphonopentanoiC acid (AP5) with an Ki of approx 1 mM. Likewise, Murphy et -al.…”