Constituents involved in glutamate receptor signaling

The 1980s: D-AP5, LTP and a Decade of NMDA Receptor Discoveries

In the 1960s and 70s, biochemical and pharmacological evidence was pointing toward glutamate as a synaptic transmitter at a number of distinct receptor classes, known as NMDA and non-NMDA receptors. The field, however, lacked a potent and highly selective antagonist to block these putative postsynaptic receptors. So, the discoveries in the early 1980s of d-AP5 as a selective NMDA receptor antagonist and of its ability to block synaptic events and plasticity were a major breakthrough leading to an explosion of knowledge about this receptor subtype. During the next 10 years, the role of NMDA receptors was established in synaptic transmission, long-term potentiation, learning and memory, epilepsy, pain, among others. Hints at pharmacological heterogeneity among NMDA receptors were followed by the cloning of separate subunits. The purpose of this review is to recognize the important contributions made in the 1980s by Graham L. Collingridge and other key scientists to the advances in our understanding of the functions of NMDA receptors throughout the central nervous system.

The history of the pharmacology and cloning of ionotropic glutamate receptors and the development of idiosyncratic nomenclature

In this article, the beginnings of glutamate pharmacology are traced from the early doubts about 'non-specific' excitatory effects, through glutamate- and aspartate-preferring receptors, to NMDA, quisqualate/AMPA and kainate subtypes, and finally to the cloning of genes for these receptor subunits. The development of selective antagonists, crucial to the subtype classification, allowed the fundamental importance of glutamate receptors to synaptic activity throughout the CNS to be realised. The ability to be able to express and manipulate cloned receptor subunits is leading to huge advances in our understanding of these receptors. Similarly the tortuous path of the nomenclature is followed from naming with reference to exogenous agonists, through abortive early attempts at generic schemes, and back to the NC-IUPHAR system based on the natural agonist, the defining exogenous agonist and the gene names.

Role of NMDA receptor functional domains in excitatory cell death

The mechanisms by which the NMDA receptor (NMDAR) induces excitotoxicity were investigated using a novel assay. We quantitated the capacity of wild type and mutant receptors for cell killing in CHO cells and cultured cortical neurons by measuring the activity of a co-transfected firefly luciferase expression plasmid. NR1 subunit pore mutations that block Ca(2+) influx, and deletion of the NR1 cytoplasmic C-terminal domain, which functions in Ca(2+) regulation of receptor currents, decreased NMDAR mediated cell killing. We also transfected the NR1 pore mutants and C-terminal truncations in the presence of co-expressed exogenous wild type subunits. The pore and C-terminal truncation mutants acted in a dominant negative fashion and increased the survival of NMDAR-expressing CHO cells. Although physiological studies of similar NMDA receptor mutants have been carried out in heterologous cell lines, their functions in neurons remain relatively unknown. We show that expression of pore mutants and specific C terminal truncation mutants in cultured cortical neurons also exerts dominant negative function and protects these primary cells from endogenous receptor induced excitotoxic death. These results implicate positive actions of the selectivity filter and of the NR1 C-terminal domain in a Ca(2+)-dependent mechanism for NMDAR excitotoxicity. They also indicate that the mutant receptors which show diminished excitotoxicity and dominant negative action in heterologous cells can co-assemble with endogenous subunits in primary neurons and block NMDAR-dependent excitotoxic death.

Distribution of glutamate receptors GluR 2/3 and NR1 in the developing rat cerebellum

The distribution of glutamate receptors GluR2/3 and NR1 was analysed immunohistochemically during development of the rat cerebellum. GluR2/3 immunoreactivity appeared by postnatal day P0 in somata of Purkinje cells. Throughout P7, P15, P20 and adulthood, GluR2/3 immunoreactivity was found in the entire Purkinje cell dendritic arbor reaching to the external granular layer and, by P15, the surface of the cerebellum. By P7, the granular layer revealed scattered, mildly reactive, cells. NR-1 immunoreactivity first gained prominence about P7 in the region of the multi-layered Purkinje cell somata. By P15, NR1 was prominent in Purkinje cell somata and Golgi cells. The reaction product extended into the primary main dendrite of Purkinje cells. By P21, stellate and basket cells had intense reactivity throughout the molecular layer and reactive large-diameter dendrites of Golgi cells projected toward the molecular layer. Granule cells remained very weak among strongly reactive Golgi cell somata and dendrites. Ultrastructural immunohistochemistry revealed NR1 reaction product in Purkinje cell somata, in stellate cell somata and dendrites and on postsynaptic membranes of scattered spines throughout the molecular layer. The later appearance and restricted location of NR1 in somata and proximal dendrites of Purkinje cells contrasted markedly with GluR2/3 which appeared before birth and remained prominent throughout Purkinje cell dendritic arbors of adults. The time of NR1 expression correlated with the generation of granule cells, their synaptogenesis on Purkinje cells, the formation of stellate/baske cells and the shift of climbing fibre synapses from distal to proximal dendrites. The developmental appearance of stellate/basket cells and Golgi cells as well as their high reactivity remaining into adulthood suggest that these inhibitory molecular and granular layer interneurons are the principal targets of glutamate axons serving NR1 synaptic properties while Purkinje cells and brush type granule cells are targets for glutamate connections with GluR2/3 characteristics.

Relative concentrations and seizure-induced changes in mRNAs encoding three AMPA receptor subunits in hippocampus and cortex

In situ hybridization was used to determine 1) the relative concentrations of mRNAs encoding different subunits of the alpha-amino 3-hydroxy-5-methyl-4- isoxazolepropionate receptor family in select regions of rat forebrain and 2) whether limbic seizures alter the balances of the subunit mRNAs. GluR1 and GluR2 mRNA levels were about equal and were much greater than GluR3 mRNA levels in the principal neurons of each hippocampal subdivision. Probable interneurons in hippocampal molecular layers had much higher levels of GluR1 mRNA than of either GluR2 or GluR3 mRNA. Pyramidal cell layers in neo- and paleocortex had a balance of mRNAs that was significantly different from the balance in hippocampus: GluR1 mRNA and GluR3 mRNA levels were about equal and were substantially lower than those of GluR2 mRNA. Lesion-induced limbic seizures caused transient changes in mRNA levels that were differentiated with regard to subunit and brain region. All three mRNAs were decreased in the pyramidal layers of cortex, and changes in hippocampal pyramidal cells were smaller. Seizure-induced changes in granule cells of the dentate gyrus differed from all other regions examined: GluR1 mRNA was reduced to a greater degree than GluR2 mRNA, whereas GluR3 mRNA content was markedly increased. These data strongly suggest that the subunit composition of alpha-amino 3-hydroxy-5-methyl-4-isoxazolepropionate receptors differs significantly between areas of the cortical telencephalon. Furthermore, the data indicate that aberrant patterns of physiological activity differentially influence the expression of subunit mRNAs in a region-specific and/or cell-type-specific manner.

Hippocampal loss of N-methyl-D-aspartate receptor subunit 1 mRNA in chronic temporal lobe epilepsy

The hippocampal distribution of mRNA for the N-methyl-D-aspartate (NMDA) receptor subunit 1 (NR1) was examined by non-radioactive in situ hybridization in 21 archival formalin-fixed and paraffin-embedded surgical specimens from patients with pharmacoresistant chronic epilepsy and in normal control specimens obtained at autopsy. Using the digoxigenin-labeling procedure, ribonucleotide probes were found to be significantly more sensitive than synthetic oligonucleotide probes. In normal autopsy specimens and in surgical specimens without Ammon's horn sclerosis there was intense NR1 expression in a great majority of the dentate gyrus granular cells. Many neurons in the hippocampal pyramidal cell layer also revealed a strong signal intensity. The strata oriens and moleculare of Ammon's horn and the molecular layer of the dentate gyrus contained only few labeled neurons. In the subiculum and entorhinal cortex most neurons throughout various layers were positive. In hippocampal specimens of patients with chronic epilepsy there was a loss of NR1-positive cells that was closely related to the overall neuronal loss in the respective specimen and to Ammon's horn sclerosis. These data suggest that the loss of NR1 expression is a secondary phenomenon rather than an event that is relevant for the pathogenesis of epileptic seizures.