Kainate-binding proteins: phylogeny, structures and possible functions

Association study of GRIK1 gene polymorphisms in schizophrenia: case-control and family-based studies

Glutamatergic function is one of the major hypotheses for schizophrenia. Within the glutamate system, the glutamate receptor ionotropic kainate-1 (GRIK1) gene is thought to be particularly involved in schizophrenia because of the reported reduction of GRIK1 in the dorsolateral prefrontal cortex of patients.

OBJECTIVE

We examined single-nucleotide polymorphisms (SNPs) in the GRIK1 gene for possible association with schizophrenia.

METHODS

We analyzed eight SNPs across the GRIK1 gene in 202 case-control pairs and 108 small nuclear families.

RESULTS

For the case-control study, we found nominal significant associations in the analysis of rs469472 (p = 0.028) and its haplotypes. In the family-based study, nominal significant association was also observed for rs469472 (p = 0.046), as well as rs455892 (p = 0.024). The marker rs469472 was associated with schizophrenia when we combined the case-control and family samples (p = 0.027). The association findings did not survive correction for multiple testing.

CONCLUSIONS

Because we observed similar association findings with marker rs469472 in two independent samples, further analyses in larger samples are warranted.

AMPA receptor assembly: atomic determinants and built-in modulators

Glutamate-gated ion channels (iGluRs) predominantly operate as heterotetramers to mediate excitatory neurotransmission at glutamatergic synapses. The subunit composition of the receptors determines their targeting to synaptic sites and signalling properties and is therefore a fundamental parameter for neuronal computations. iGluRs assemble as obligatory or preferential heteromers; the mechanisms underlying this selective assembly are only starting to emerge. Here we review recent work in the field and provide an in-depth update on atomic determinants in the assembly domains, which have been facilitated by recent advances in iGluR structural biology. We also discuss the role of alternative RNA processing in the ligand-binding domain, which modulates a central subunit interface and has the capacity to modulate receptor formation in response to external cues. Finally, we review the emerging physiological significance of signalling via distinct iGluR heterotetramers and provide examples of how recruitment of functionally diverse receptors modulates excitatory neurotransmission under physiological and pathological conditions.

Decreased numerical density of kainate receptor-positive neurons in the orbitofrontal cortex of chronic schizophrenics

We utilised postmortem brain tissue to quantify sections of left and right orbitofrontal cortex (area 11) from nine schizophrenic and eight control patients from the Charing Cross Prospective Schizophrenia Study immunostained for the presence of the kainate receptor (GluR5/6/7). The numerical density of neurons immunopositive for kainate receptor was measured. Other sections from the same blocks were stained with cresyl violet to determine the total neuronal numerical density. All measurements were made blind: diagnoses were only revealed by a third party after measurements were completed. There was a significant reduction (21%) in numerical density of kainate receptor-positive neurons in both cortices in the schizophrenic group (488 cells/mm2) compared to that in the control group (618 cells/mm2) (P=0.033). Nissl-stained tissue showed no significant difference in total neuronal numerical density between control and schizophrenic groups. These observations suggest that there are actually fewer kainate receptor-positive neurons in schizophrenic orbitofrontal cortex. There was no correlation of reduced kainate receptor-positive cell number with age at death, postmortem interval, or other possibly confounding neuropathology. Our results support the concept of there being reduced glutamatergic activity in frontal cortex in schizophrenia.

Revisiting the postulated "unitary glutamate receptor": electrophysiological and pharmacological analysis in two heterologous expression systems fails to detect evidence for its existence

Several years ago evidence for a so-called "unitary glutamate receptor" was published. This unique type of glutamate receptor was reported to be activated by the traditional agonists of all three major glutamate receptor subfamilies [i.e., alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA), kainate, and N-methyl-d-aspartate (NMDA)] in a glycine-dependent as well as magnesium-blockable manner and was reported to consist of an NR1 subunit coexpressed with the kainate binding protein (KBP) from Xenopus laevis, XenU1. To re-examine the existence of such a receptor, we cloned two splice variants of the X. laevis NMDA receptor subunit NR1, XenNR1-4a and XenNR1-4b, and expressed them in X. laevis oocytes as well as in human embryonic kidney (HEK) 293 cells, either alone or with the X. laevis KBP subunit XenU1. In addition, we coexpressed XenU1 separately with all eight splice variants of the rat NR1 subunit. In no case did we see evidence of a unitary glutamate receptor pharmacology. In HEK293 cells, we did not get receptor response unless an NR2 subunit was coexpressed. In X. laevis oocytes, we did observe responses to glutamate/glycine as well as small responses to glycine alone, but these were independent of coexpressed XenU1. Neither AMPA nor kainate ever elicited significant responses. Because we verified that XenU1 is expressed and inserted into the plasma membrane of HEK293 cells, we conclude that XenU1 and NR1 do not form the postulated unitary glutamate receptor. Furthermore, successful amplification of a fragment of a X. laevis NR2 subunit indicates that X. laevis uses NR2 subunits and not XenU1 to form heteromeric complexes with NR1.

Characterization of a domoic acid binding site from Pacific razor clam

The Pacific razor clam, Siliqua patula, is known to retain domoic acid, a water-soluble glutamate receptor agonist produced by diatoms of the genus Pseudo-nitzschia. The mechanism by which razor clams tolerate high levels of the toxin, domoic acid, in their tissues while still retaining normal nerve function is unknown. In our study, a domoic acid binding site was solubilized from razor clam siphon using a combination of Triton X-100 and digitonin. In a Scatchard analysis using [3H]kainic acid, the partially-purified membrane showed two distinct receptor sites, a high affinity, low capacity site with a KD (mean +/- S.E.) of 28 +/- 9.4 nM and a maximal binding capacity of 12 +/- 3.8 pmol/mg protein and a low affinity, high capacity site with a mM affinity for radiolabeled kainic acid, the latter site which was lost upon solubilization. Competition experiments showed that the rank order potency for competitive ligands in displacing [3H]kainate binding from the membrane-bound receptors was quisqualate > ibotenate > iodowillardiine = AMPA = fluorowillardiine > domoate > kainate > L-glutamate. At high micromolar concentrations, NBQX, NMDA and ATPA showed little or no ability to displace [3H]kainate. In contrast, Scatchard analysis using [3H]glutamate showed linearity, indicating the presence of a single binding site with a KD and Bmax of 500 +/- 50 nM and 14 +/- 0.8 pmol/mg protein, respectively. These results suggest that razor clam siphon contains both a high and low affinity receptor site for kainic acid and may contain more than one subtype of glutamate receptor, thereby allowing the clam to function normally in a marine environment that often contains high concentrations of domoic acid.

Glutamate regulates Oct-2 DNA-binding activity through α-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors in cultured chick Bergmann glia cells

Ionotropic glutamate receptors in cerebellar Bergmann glial cells are linked to transcriptional regulation and, by these means, are thought to play an important role in plasticity, learning and memory and in several neuropathologies. Within the CNS, the transcription factors of the POU family bind their target DNA sequences after a growth factor-dependent phosphorylation-dephosphorylation cascade. Exposure of cultured Bergmann glial cells to glutamate leads to a time- and dose-dependent increase in Oct-2 DNA-binding activity. The use of specific pharmacological tools established the involvement of Ca2+-permeable alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors. Furthermore, the signaling cascade includes phosphatidyl inositol 3-kinase as well as protein kinase C activation. Interestingly, transcriptional as well as translational inhibitors abolish the glutamate effect, suggesting a transcriptional up-regulation of the oct-2 gene. These data demonstrate that Oct-2 expression is not restricted to neurons and further strengthen the notion that the glial glutamate receptors participate in the modulation of glutamatergic cerebellar neurotransmission.

Comparative anticonvulsant activity of some 2,3-benzodiazepine derivatives in rodents

The anticonvulsant activities of some 2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA)/kainate receptor antagonists, noncompetitive (2,3-benzodiazepines) and a competitive 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)-quinoxaline (NBQX), were compared in different experimental seizure models. In particular, compounds were evaluated against audiogenic seizure in DBA/2 mice, maximal electroshock seizure (MES) test and various chemoconvulsant models; both groups showed a protective action against audiogenic seizure, MES- and pentylenetetrazole (PTZ)-induced seizures. All 2,3-benzodiazepines were also protective against clonic and tonic seizures and lethality induced by 4-aminopyridine, kainate, AMPA and 3-mercaptopropionic acid but were ineffective against NMDA-induced seizures. NBQX was unable to affect 4-aminopyridine-, mercaptopropionic acid- and NMDA-induced seizures. The duration of anticonvulsant action of 33 micromol/kg of some 2,3-benzodiazepine in DBA/2 mice, genetically susceptible to audiogenic seizures, was also investigated. The derivatives possessing a thiocarbonyl group at the C-4 position of heptatomic ring showed higher anticonvulsant activities and longer lasting protective effects. We conclude that all 2,3-benzodiazepines studied are effective against various models of experimental epilepsy and the presence of thiocarbonyl groups at the C-4 position of heptatomic ring is able to increase the anticonvulsant effect of these compounds.

Kainate-binding proteins are rendered functional ion channels upon transplantation of two short pore-flanking domains from a kainate receptor

Kainate-binding proteins belong to an elusive class of putative ionotropic glutamate receptors that to date have not been shown to form functional ion channels in heterologous expression systems, despite binding glutamatergic agonists with high affinity. To test the hypothesis that inefficient or interrupted signal transduction from the ligand-binding site via linker domains to the ion pore (gating) might be responsible for this apparent lack of function, we transplanted the short homologous linker sequences from the fully functional rat kainate receptor GluR6 into frog kainate-binding protein. We were able to generate chimeric receptors that are functional in the Xenopus oocyte expression system and in human embryonic kidney 293 cells. The linker domains A and B in particular appear to be crucial for gating, because a functional kainate-binding protein was observed when at least parts of both linkers were derived from GluR6. We speculate that to enable signal transduction from the ligand-binding site to the ion pore of the frog kainate-binding protein, the linker structure of the protein has to undergo an essential conformational alteration, possibly mediated by an as yet unknown subunit or modulatory protein.

Synergistic effects of corticosterone and kainic acid on neurite outgrowth in axotomized dorsal root ganglion

Corticosterone is the main adrenal glucocorticoids induced by stress in rats. Therapeutic use of high concentration of synthetic glucocorticoids in clinical treatment of spinal cord injury suggests that pharmacological action of glucocorticoids might be beneficial for nerve repair. In this article we cultured axotomized rat dorsal root ganglion neurons to investigate the effects of corticosterone and a glutamate receptor agonist kainic acid on neurite outgrowth. Our results revealed a synergistic effect of corticosterone and kainic acid in promoting neurite outgrowth when applied as early as one and two days in vitro, but not effective at three and four days in vitro. In addition, applied corticosterone and kainic acid were neurotoxic at three and four days in vitro but not at one and two days in vitro. The minimal concentrations of corticosterone and kainic acid to be effective were 10 microM and 1 mM, respectively. The neurotrophic effect of corticosterone and kainic acid was attenuated by the receptor tyrosine kinase A (TrkA) inhibitor AG-879. Western blot analysis and immunocytochemical studies revealed an increase of expressions of both TrkA and growth-associated protein GAP-43 in dorsal root ganglion neurons with combined treatment of corticosterone and kainic acid. Immunocytochemistry showed that corticosterone+kainic acid increase nerve growth factor immunoreactivity in dorsal root ganglion neurites and enhance GAP-43 immunointensity in dorsal root ganglion neurons. These results suggest that the neurotrophic effect of glucocorticoids on axonal regeneration might require facilitation of excitatory stimulation at an early stage of nerve injury, and nerve growth factor may mediate a growth signaling to accomplish the effect.

Glutamate-dependent transcriptional regulation of the chkbp gene: signaling mechanisms

Glutamate, the major excitatory neurotransmitter, induces a signal from the membrane to the nucleus that regulates gene expression. The gene encoding the chick kainate binding protein undergoes a glutamate-dependent transcriptional regulation via an activator protein-1 site within its promoter region. To characterize this event, cultured chick Bergmann glia cells were exposed to glutamate, and a dose-dependent increase in promoter activity was established. The glutamate effect is mediated through Ca(2+)-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate receptors. The signaling cascade includes phosphatidyl inositol 3-kinase, Ca(2+)/calmodulin-dependent protein kinase II, mitogen-activated protein kinase, and p90 ribosomal S6 kinase activation. The cAMP response-element binding protein becomes phosphorylated and activates fos transcription. Finally, the activator protein-1 complex binds to the glutamate response element in the chick kainate binding protein promoter region inducing its activity. We propose that the mitogen-activated protein kinase/p90 ribosomal S6 kinase pathway plays a critical role in glutamate-induced gene transcription.

Interspecific differences in the expression of the AMPA-type glutamate receptors and parvalbumin in the nucleus of Edinger-Westphal of chicks and pigeons

The distribution of AMPA-type glutamate receptor (GluR) subunits was studied in the Edinger-Westphal nucleus (EW) of chicks and pigeons. GluR1, GluR2, GluR3 and GluR4 subunits appeared to be present in EW neurons of both species, but interspecific differences were observed in the abundance of the different types of subunits found in EW neurons. Of particular note, GluR2 immunoreactivity was present in the vast majority (ca. 80%) of neurons of pigeon EW but was found in only a small fraction (ca. 15%) of chick EW neurons. Scarcity of the GluR2 subunit in chick EW was confirmed by in situ hybridization. Because of the tendency for parvalbumin to be localized to neurons that are selectively deficient in GluR2, we also studied the localization of parvalbumin, as well as other calcium-binding proteins, in EW of chick and pigeon. Parvalbumin was found in more than 50% of chick EW neurons but was not detected in pigeon EW neurons. Our results suggest that there are major glutamatergic inputs to EW neurons in both pigeons and chicks. Furthermore, there are likely to be more AMPA-type calcium-permeable glutamate receptors in EW neurons of chick than in pigeon, since it is known that the subtype containing the edited GluR2 subunit is not calcium permeable.

Molecular physiology of kainate receptors

A decade ago, our understanding of the molecular properties of kainate receptors and their involvement in synaptic physiology was essentially null. A plethora of recent studies has altered this situation profoundly such that kainate receptors are now regarded as key players in the modulation of transmitter release, as important mediators of the postsynaptic actions of glutamate, and as possible targets for the development of antiepileptic and analgesic drugs. In this review, we summarize our current knowledge of the properties of kainate receptors focusing on four key issues: 1) their structural and biophysical features, 2) the important progress in their pharmacological characterization, 3) their pre- and postsynaptic mechanisms of action, and 4) their involvement in a series of physiological and pathological processes. Finally, although significant progress has been made toward the elucidation of their importance for brain function, kainate receptors remain largely an enigma and, therefore, we propose some new roads that should be explored to obtain a deeper understanding of this young, but intriguing, class of proteins.

Water maze and radial maze learning and the density of binding sites of glutamate, GABA, and serotonin receptors in the hippocampus of inbred mouse strains

Correlations between the densities of ionotropic glutamate, GABA(A), and serotonin binding sites in the hippocampus of seven inbred mouse strains and strain-specific learning capacities in two types of maze were studied. Binding site densities were measured with quantitative receptor autoradiography. Learning capacities were determined in a water maze task as well as in spatial and nonspatial versions of an eight-arm radial maze. The densities of most binding sites differed significantly between the strains in the subfields of Ammon's horn (CA1 and CA3) and the dentate gyrus, except for serotonin binding sites in CA1. By comparing the different strains, significant receptor-behavioral correlations between the densities of the GABA(A) receptors and the activity-dependent behavior in the water maze as well as the spatial learning in the radial maze were found. The densities of D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxalone propionate (AMPA) and kainate receptors correlated positively with learning capacity in the spatial eight-arm radial maze. We conclude that hereditary variations mainly in AMPA, kainate, and GABA(A) receptor densities are involved in behavioral variations in spatial and nonspatial learning tasks.

Glutamate regulates kainate-binding protein expression in cultured chick Bergmann glia through an activator protein-1 binding site

The expression of the chick kainate-binding protein, a member of the ionotropic glutamate receptor family, is restricted to the cerebellum, specifically to Bergmann glia. Glutamate induces a membrane to nuclei signaling involved in gene expression regulation. Exposure of cultured chick Bergmann glia cells to glutamate leads to an increase in kainate binding protein and mRNA levels, suggesting a transcriptional level of regulation. The 5' proximal region of the chick kainate binding gene was cloned and transfected 4into Bergmann glia cells. Three main regulatory regions could be defined, a minimal promoter region, a negative regulatory region, and interestingly, a glutamate-responsive element. Deletion of this element abolishes the agonist effect. Moreover, electrophoretic mobility shift assays, cotransfection experiments, and site-directed mutagenesis clearly suggest that the glutamate effect is mediated through an AP-1 site by a Fos/Jun heterodimer. The present results favor the notion of a functional role of kainate-binding protein in glutamatergic cerebellar neurotransmission.

The activation of glutamate receptors by kainic acid and domoic acid

The neurotoxins kainic acid and domoic acid are potent agonists at the kainate and alphaamino-5-methyl-3-hydroxyisoxazolone-4-propionate (AMPA) subclasses of ionotropic glutamate receptors. Although it is well established that AMPA receptors mediate fast excitatory synaptic transmission at most excitatory synapses in the central nervous system, the role of the high affinity kainate receptors in synaptic transmission and neurotoxicity is not entirely clear. Kainate and domoate differ from the natural transmitter, L-glutamate, in their mode of activation of glutamate receptors; glutamate elicits rapidly desensitizing responses while the two neurotoxins elicit non-desensitizing or slowly desensitizing responses at AMPA receptors and some kainate receptors. The inability to produce desensitizing currents and the high affinity for AMPA and kainate receptors are undoubtedly important factors in kainate and domoate-mediated neurotoxicity. Mutagenesis studies on cloned glutamate receptors have provided insight into the molecular mechanisms responsible for these unique properties of kainate and domoate.

Kainate receptors coupled to G(i)/G(o) proteins in the rat hippocampus

Kainate receptors are a subtype of ionotropic glutamate receptors, permeable to cations and thus expected to have an excitatory depolarizing action on neurons. However, kainate receptor activation inhibits gamma-aminobutyric acid release in the hippocampus through activation of protein kinase C in a pertussis toxin-dependent manner, suggesting a coupling of kainate receptors to G proteins. Thus, we directly investigated the G protein coupling of kainate receptors in the rat hippocampus by using a selective kainate receptor agonist, [(3)H](2S,4R)-4-methylglutamate ([(3)H]MGA). [(3)H]MGA bound to a single site to hippocampal membranes with a K(D) value of 32 nM and a B(max) value of 1024 fmol/mg protein. This binding likely represents kainate receptors because it was displaced by domoate (K(i) = 4 nM), kainate (K(i) = 11 nM), and 6-cyano-7-nitroquinoxaline-2,3-dione (K(i) = 1.4 microM), but not by alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (K(i) > 10 microM), (RS)-alpha-methyl-4-phosphonophenylglycine (K(i) > 10 microM), or (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (K(i) > 10 microM). Guanylylimidodiphosphate (30 microM), which uncouples all G protein-coupled receptors, shifted to the right the saturation curve of [(3)H]MGA (K(D) = 133 nM). This effect was mimicked by pretreatment of hippocampal membranes with modifiers of G(i)/G(o) proteins [30 microM N-ethylmaleimide (K(D) = 98 nM) or 25 microgram/ml pertussis toxin (K(D) = 95 nM)] but not by a modifier of G(s) proteins [50 microgram/ml cholera toxin (K(D) = 32 nM)]. Treatment of solubilized hippocampal membranes with pertussis toxin (25 microgram/ml) decreased [(3)H]MGA affinity (K(D) = 105-113 nM), which was recovered by reconstitution of these pretreated solubilized hippocampal membranes with G(i)/G(o) proteins (K(D) = 41-76 nM). These results indicate that hippocampal kainate receptors are coupled to G(i)/G(o) proteins.

A new pyrrolyl-quinoxalinedione series of non-NMDA glutamate receptor antagonists: pharmacological characterization and comparison with NBQX and valproate in the kindling model of epilepsy

Antagonists at the ionotropic non-NMDA [AMPA (amino-methyl proprionic acid)/kainate] type of glutamate receptors have been suggested to possess several advantages compared to NMDA (N-methyl-D-aspartate) receptor antagonists, particularly in terms of risk/benefit ratio, but the non-NMDA receptor antagonists available so far have not fulfilled this promise. From a large series of pyrrolyl-quinoxalinedione derivatives, we selected six new competitive non-NMDA receptor antagonists. The basis of selection was high potency and selectivity for AMPA and/or kainate receptors, high in vivo potency after systemic administration, and an acceptable ratio between neuroprotective or anticonvulsant effects and adverse effects. Pharmacological characteristics of these novel compounds are described in this study with special emphasis on their effects in the kindling model of temporal lobe epilepsy, the most common type of epilepsy in humans. In most experiments, NBQX and the major antiepileptic drug valproate were used for comparison with the novel compounds. The novel non-NMDA receptor antagonists markedly differed in their AMPA and kainate receptor affinities from NBQX. Thus, while NBQX essentially did not bind to kainate receptors at relevant concentrations, several of the novel compounds exhibited affinity to rat brain kainate receptors or recombinant kainate receptor subtypes in addition to AMPA receptors. One compound, LU 97175, bound to native high affinity kainate receptors and rat GluR5-GluR7 subunits, i.e. low affinity kainate binding sites, with much higher affinities than to AMPA receptors. All compounds potently blocked AMPA-induced cell death in vitro and, except LU 97175, AMPA-induced convulsions in vivo. In the kindling model, compounds with a high affinity for GluR7 (LU 97175) or compounds (LU 115455, LU 136541) which potently bind to AMPA receptors and low affinity kainate receptor subunits were potent anticonvulsants in the kindling model, whereas the AMPA receptor-selective LU 112313 was the least selective compound in this model, indicating that non-NMDA antagonists acting at both AMPA and kainate receptors are more effective in this model than AMPA receptor-selective drugs. Three of the novel compounds, i.e. LU 97175, LU 115455 and LU 136541, exerted potent anticonvulsant effects without inducing motor impairment in the rotarod test. This combination of actions is thought to be a prerequisite for selective anticonvulsant drug action.

The GluR5 subtype of kainate receptor regulates excitatory synaptic transmission in areas CA1 and CA3 of the rat hippocampus

Activation of kainate receptors depresses excitatory synaptic transmission in the hippocampus. In the present study, we have utilised a GluR5 selective agonist, ATPA [(RS)-2-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl)propanoic acid], and a GluR5 selective antagonist, LY294486 [(3SR,4aRS,6SR,8aRS)-6-([[(1H-tetrazol-5-y l)methyl]oxy]methyl)-1,2,3,4,4a,5,6,7,8,8a-decahydroisoquinoline-3 -carboxylic acid], to determine whether GluR5 subunits are involved in this effect. ATPA mimicked the presynaptic depressant effects of kainate in the CA1 region of the hippocampus. It depressed reversibly AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid) receptor-mediated field excitatory postsynaptic potentials (field EPSPs) with an IC50 value of approximately 0.60 microM. The dual-component excitatory postsynaptic current (EPSC) and the pharmacologically isolated NMDA (N-methyl-D-aspartate) receptor-mediated EPSC were depressed to a similar extent by 2 microM ATPA (61 +/- 7% and 58 +/- 6%, respectively). Depressions were associated with an increase in the paired-pulse facilitation ratio suggesting a presynaptic locus of action. LY294486 (20 microM) blocked the effects of 2 microM ATPA on NMDA receptor-mediated EPSCs in a reversible manner. In area CA3, 1 microM ATPA depressed reversibly mossy fibre-evoked synaptic transmission (by 82 +/- 10%). The effects of ATPA were not accompanied by any changes in the passive properties of CA1 or CA3 neurones. However, in experiments where K+, rather than Cs+, containing electrodes were used, a small outward current was observed. These results show that GluR5 subunits comprise or contribute to a kainate receptor that regulates excitatory synaptic transmission in both the CA1 and CA3 regions of the hippocampus.

Pharmacological characterization, anatomical distribution and sex differences of the non-NMDA excitatory amino acid receptors in the quail brain as identified by CNQX binding

The distribution of non-N-methyl-D-aspartate binding sites was studied in coronal and sagittal sections through the brain of adult Japanese quail by quantitative autoradiography, using tritiated 6-cyano-7-nitroquinoxaline-2,3-dione as a radioligand. Saturation binding experiments were, in addition, carried out in areas showing high levels of binding (cerebellar molecular layer, nucleus anterior medialis and nucleus infundibularis) and demonstrated that the binding of tritiated ligand was specific and saturable. Competition studies with alpha-amino-3-hydroxy-methyl-4-isoxazole propionic acid and kainic acid indicated that kainic acid strongly inhibited ligand binding in all brain areas. alpha-Amino-3-hydroxy-methyl-4-isoxazole propionic acid was only a weak inhibitor in the hypothalamic nuclei whereas in the cerebellar molecular layer both high and low affinity inhibitions were detected. The highest binding levels of tritiated ligand were observed in the molecular layer of the cerebellum. Very high levels of binding were detected in various preoptic/hypothalamic sites including the nucleus suprachiasmaticus pars medialis, nucleus anterior medialis hypothalami, nucleus infundibularis, nucleus mammillaris medialis, nucleus posteromediale hypothalami and nucleus hypothalami ventromedialis. High levels of binding were also detected in the bulbus olfactorius, bed nucleus commissuralis anterior, bed nucleus commissuralis pallii, nucleus accumbens, bed nucleus striae terminalis and nucleus interpeduncularis. In the preoptic area/hypothalamus, high levels of binding were clearly present in all areas that contain gonadotropin releasing hormone cells or fibers. In the pons and mesencephalon, moderate levels of binding were associated with catecholaminergic areas such as the area ventralis tegmentalis (area ventralis of Tsai) and the locus coeruleus. Saturation analysis demonstrated the presence of a higher number of binding sites in females than in males in the cerebellar molecular layer, nucleus infundibularis and nucleus anterior medialis. This latter difference was confirmed in the one point assays that also identified higher levels of specific binding in the nucleus suprachiasmaticus pars medialis of males as compared with females. These anatomical data suggest a possible implication of non-N-methyl-D-aspartate receptors in the synthesis and/or release of both gonadotropin releasing hormone and catecholaminergic neurotransmitters that should now be tested by pharmacological experiments.

The regulation of AMPA receptor-binding sites

A wide variety of mechanisms have been identified that can regulate the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)-receptor complex. Modulation has been shown to occur at the nucleic acid level via RNA editing and alternative splicing. At the posttranslational level, processes such as phosphorylation, glycosylation, chemical modification of reactive groups on the receptor proteins, interaction with a putative receptor-associated modulatory protein, and changes in the lipid environment have been reported to regulate receptor binding and function. In this review, we discuss general aspects of the cell biology, pharmacology, and function of AMPA receptors. In particular, we focus on some factors shown to modulate agonist binding and discuss possible molecular mechanisms underlying the regulation observed.

Kainate binding proteins possess functional ion channel domains

Kainate binding proteins (KBPs) are highly homologous to ionotropic glutamate receptors; however, no ion channel function has been demonstrated for these proteins. To investigate possible reasons for the apparent lack of ion channel function we transplanted the ion channel domains of five KBPs into glutamate receptors GluR 6 and GluR1. In each case we obtained functional chimeric receptors in which glutamatergic agonists were able to open the KBP-derived ion channel with EC50 values identical to those of the subunit contributing the ligand binding domain. Maximal current amplitudes were significantly smaller than those of the parent clones, however. We also show that the KBP ion channels are highly permeable for calcium and have certain pharmacological properties that are distinct from all other glutamate receptor (GluR) subunits. Thus, all five known KBPs, in addition to their well characterized functional ligand binding sites, have functional ion permeation pathways. Our data suggest that the lack of ion channel function in wild-type KBPs results from a failure to translate ligand binding into channel opening. We interpret our findings to indicate the requirement for a modulatory protein or an additional subunit serving to alter the structure of the KBP subunit complex such that signal transduction is enabled from the ligand binding site to the intrinsically functional ion pore.

In vivo hypoxia-induced neuronal damage in dentate gyrus of rat hippocampus: changes in NMDA receptors and the effect of MK-801

Hypoxia is a major cause of ischaemia-induced neuronal damage. In the present study, we examined the effects of in vivo hypoxia on N-methyl-D-aspartate receptors (NMDAR) in the rat hippocampus. This model of in vivo hypoxia involved placing rats in a hypoxic chamber containing 5% O2 and 95% N2 for 30 min. In the hippocampus, neuronal cells in the CA3, the hilus of the dentate gyrus and the dentate gyrus (DG) were damaged. In the CA1, which is known to be vulnerable to ischaemic damage, neuronal cells did not show hypoxia-induced damage. In vivo hypoxia-induced damage caused morphological changes in neuronal cells, such as shrunken, spindle or triangular shapes accompanied by pyknotic nuclei, but did not induce the loss of neuronal cells. On the other hand, the number of binding sites for [3H]-1-[1-(2-thienyl)cyclohexyl]-3,4-piperidine hydrochloride (TCP) gradually decreased on and after 7 days, and then maximally decreased by 25% at 21 days after hypoxia. The number of NMDAR1-immunopositive cells was decreased by 22% in the DG, but was unchanged in the CA3. Furthermore, we examined the effect of a non-competitive NMDA antagonist, (+)-5-methyl-10, 11-dihydro-5H-dibenzo[a,b] cyclohepten-5,10-imine hydrogen maleate (MK-801), on against in vivo hypoxia. The administration of MK-801 (3 mg/kg, i.p.), 30 min before hypoxia treatment, partly protected against neuronal damage in the DG, but not in the CA3. These results suggest that hypoxia-induced neuronal damage in the DG involves, in part, the activation of NMDAR.

Distribution of kainate receptor subunit mRNAs in human hippocampus, neocortex and cerebellum, and bilateral reduction of hippocampal GluR6 and KA2 transcripts in schizophrenia

The mRNAs encoding kainic acid (KA) preferring glutamate receptor subunits (GluR5-7, KA1 and KA2) are differentially expressed in rat brain. We have used regional and cellular in situ hybridization histochemistry with subunit-specific 35S-labelled oligodeoxyribonucleotides to examine these mRNAs in adult human hippocampus, neocortex and cerebellum. GluR5 mRNA was detected only in Purkinje cells and a few scattered hippocampal neurons. GluR6 mRNA was relatively abundant in all areas, notably in dentate gyrus, pyramidal neurons of CA3, and cerebellar granule cells, as well as being present in superficial and deep laminae of the neocortex. Moderate signal for GluR7 mRNA was seen in deep laminae of the neocortex with a weak signal in the dentate gyrus; in dipped sections GluR7 mRNA was also apparent over some pyramidal and non-pyramidal cells in hippocampus and over putative cerebellar stellate/basket cells. KA1 mRNA was detected in the dentate gyrus but not reliably elsewhere. The expression profile and abundance of KA2 mRNA was similar to that of GluR6 mRNA. For all five transcripts, concurrent hybridization of rat brain sections produced the anticipated distribution of signal. The data indicate that the regional and cellular distribution of KA receptor subunit mRNAs in human hippocampus, neocortex and cerebellum largely parallels that in the corresponding areas of rat brain, albeit at lower levels, especially with regard to GluR5 and KA1 transcripts. In schizophrenia there is a partial loss of hippocampal non-NMDA receptors, but there are no data concerning KA receptor subunit expression. KA2 and GluR6 mRNAs were sufficiently abundant for a comparison in the left and right hippocampus between 11 schizophrenics and 13 controls. Using film autoradiography, both mRNAs were significantly reduced in the schizophrenics, having controlled for the effects of brain pH, post mortem interval and age. GluR6 mRNA was also quantitated in cerebellum, wherein no differences were found between cases and controls. In conjunction with earlier findings of reduced hippocampal GluR1 and GluR2 expression and a loss of [3H]KA binding sites, these data show that schizophrenia is associated with impaired expression of both AMPA- and KA-preferring ionotropic glutamate receptors. These deficits are likely to contribute to the glutamatergic component of the disease pathophysiology.

The taste of monosodium glutamate: membrane receptors in taste buds

Receptor proteins for photoreception have been studied for several decades. More recently, putative receptors for olfaction have been isolated and characterized. In contrast, no receptors for taste have been identified yet by molecular cloning. This report describes experiments aimed at identifying a receptor responsible for the taste of monosodium glutamate (MSG). Using reverse transcriptase (RT)-PCR, we found that several ionotropic glutamate receptors are present in rat lingual tissues. However, these receptors also could be detected in lingual tissue devoid of taste buds. On the other hand, RT-PCR and RNase protection assays indicated that a G-protein-coupled metabotropic glutamate receptor, mGluR4, also is expressed in lingual tissues and is limited only to taste buds. In situ hybridization demonstrated that mGluR4 is detectable in 40-70% of vallate and foliate taste buds but not in surrounding nonsensory epithelium, confirming the localization of this metabotropic receptor to gustatory cells. Expression of mGluR4 in taste buds is higher in preweaning rats compared with adult rats. This may correspond to the known higher sensitivity to the taste of MSG in juvenile rodents. Finally, behavioral studies have indicated that MSG and L-2-amino-4-phosphonobutyrate (L-AP4), a ligand for mGluR4, elicit similar tastes in rats. We conclude that mGluR4 may be a chemosensory receptor responsible, in part, for the taste of MSG.

Presynaptic ionotropic receptors

Recent studies have provided new insights into the role of presynaptic ligand-gated ion channels in modifying synaptic transmission. Along with a growing list of different types of presynaptic ionotropic receptors and the cell types that express them, there have been advances in characterizing the molecular components of the receptors as well as the signaling processes that link receptor activation to changes in neurotransmitter release. Perhaps most striking is the recent convergence of data from biochemical, molecular and electrophysiological studies, implicating presynaptic ionotropic receptors in the effects of psychoactive and addictive drugs.

Corticosteroid actions on the expression of kainate receptor subunit mRNAs in rat hippocampus

Previous studies have shown that corticosteroid hormones affect kainate-induced excitotoxic processes in the rat hippocampus. In the present study we employed in situ hybridization to examine the effect of adrenalectomy, and subsequent treatment with a low or a high lose of corticosterone on the mRNA levels for kainate receptor subunits in the hippocampus. We observed that adrenalectomy by itself does not affect the expression pattern for the GluR6, GluR7, KAR1 and KAR2 subunits. However, treatment of the adrenalectomized animals with a low dose of corticosterone (3 micrograms/100 g bodyweight) resulted in an enhanced expression of the KAR1, KAR2 and GluR6 subunit mRNAs, when compared to the expression levels in the untreated rats or the sham operated controls. Treatment with a high dose of corticosterone (1 mg/100 g bodyweight) yielded expression levels which were significantly lower than those observed in animals treated with a low corticosterone dose, for the KAR1, KAR2 and GluR7 subunit mRNAs; the levels did not differ from those in untreated rats or in the sham group. We conclude that changes in corticosteroid receptor occupancy, which may occur daily due to circadian or stress-induced variations in the circulating corticosterone level, potentially regulate high affinity kainate receptor activation and the processes in which these receptors are involved.

Excitatory amino acid-induced AP-1 DNA binding activity in Müller glia

The effect of L-glutamate (L-Glu) and its structural analogs N-methyl-D-aspartate (NMDA), quisqualate (QA), and kainate (KA) on the DNA binding activity of the Activator Protein 1 (AP-1) and the Ca2+/cAMP Responsive Element Binding Protein (CREB) families of transcription factors was examined in cultured chick retinal Müller glia cells. L-Glu, NMDA, and KA evoked a dose and time dependent increase in AP-1 DNA binding activity and had no effect on CREB binding. The order of potency for stimulating AP-1 DNA binding was NMDA > or = Glu > KA >> QA. L-Glu responses were partially blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and by 3-[RS)-2-carboxypiperazin-4-yl)]-propyl-1-phosphonate (CPP) indicating that the increase in DNA binding is mediated both by an alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/low affinity KA and a NMDA subtypes of L-Glu receptors. Since Müller glia L-Glu receptors are probably mediators of the efficacy of the excitatory transmission in the retina, the present findings suggest that a stimulus-transcription coupling triggered by L-Glu in the glial cells might have a role in the long-term modulation of these synapses.

Interaction of guanine nucleotides with the kainate binding protein from chick cerebellum

The interaction of guanine nucleotides with the chick cerebellar kainate binding protein (chKBP) was studied using binding assays, immunoblotting, and in vitro phosphorylation experiments. Guanosine 5'-triphosphate (GTP) was found to reduce [3H]kainic acid (KA) binding in a concentration-dependent manner. Similarly, an inhibition of [3H]GTP binding by KA was observed. No G protein co-purified with chKBP. chKBP phosphorylation by the cAMP-dependent protein kinase (PKA) was prevented both by KA and by GTP. Neither KA nor GTP blocked each other's effect in chKBP phosphorylation. The present findings suggest that chKBP harbours two agonistic binding sites, one in the micromolar range, detected by binding techniques and one in the millimolar range detected by phosphorylation assays. Guanine nucleotides interact with both sites.