Blockade of desensitization augments quisqualate excitotoxicity in hippocampal neurons

Altered Synaptic and Extrasynaptic NMDA Receptor Properties in Substantia Nigra Dopaminergic Neurons From Mice Lacking the GluN2D Subunit

N-methyl-D-aspartate receptors (NMDARs) are ubiquitously expressed in the mammalian brain and are essential for neuronal development, survival and plasticity. GluN2 subunit composition has a profound effect on the properties of NMDARs. In substantia nigra dopaminergic (SNc-DA) neurons, pharmacological experiments suggest that the relatively rare GluN2D subunits form functional synaptic and extrasynaptic NMDARs. Given the importance of establishing this point, mice lacking the GluN2D subunit (Grin2D-null) were used in this study to further explore the contribution of the GluN2D subunit to NMDAR responses. Significantly less DQP-1105-sensitive NMDAR-EPSC and significantly more ifenprodil-sensitive NMDAR-EPSC was observed in SNc-DA neurons from Grin2D-null mice, indicating that in these animals a small population of synaptic GluN2D subunits is replaced with GluN2B. Significantly larger currents were seen in response to higher concentrations (1–10 mM) of NMDA in SNc-DA neurons from Grin2D-null mice, as well as significantly more desensitization: these data are consistent with the presence of GluN2D-containing whole-cell NMDARs in SNc-DA neurons, with low conductance and little desensitization. Brief applications of NMDA evoked responses that were significantly less sensitive to DQP-1105 in slices from Grin2D-null mice. Tonic NMDAR activity in response to ambient extracellular glutamate, determined by the sensitivity of tonic current to D-AP5 (50 μM), was significantly less in SNc-DA neurons from Grin2D-null mice. In the presence of the glutamate transporter blocker TBOA (30 μM), the D-AP5-sensitive current was also significantly less in Grin2D-null mice. Taken together, these data support the evidence for GluN2D subunit expression in functional NMDARs at both synaptic and extrasynaptic locations in SNc-DA neurons.

N-methyl-D-aspartate receptor subunit- and neuronal-type dependence of excitotoxic signaling through post-synaptic density 95

NMDA receptors (NMDARs) mediate excitatory synaptic transmission during repetitive or prolonged glutamate release, playing a critical role in synaptic plasticity or cell death, respectively. Evidence indicates that a major pathway of NMDAR signaling to cell death in cortical and hippocampal neurons requires the scaffolding protein post-synaptic density 95 (PSD-95) and activation of neuronal nitric oxide synthase. However, it is not known if this PSD-95-dependent pathway contributes to excitotoxicity in other brain regions. It is also unclear whether the neuroprotective effects of Tat-NR2B9c, a membrane-permeant peptide that disrupts PSD-95/NMDAR binding, correlate with uncoupling NR2B- and/or NR2A-type NMDARs from PSD-95. In this study, we used cultured hippocampal and striatal neurons to test the potency of Tat-NR2B9c on uncoupling NR2 subunits from PSD-95 and protecting against NMDA-induced excitotoxicity. We found that the concentration of Tat-NR2B9c required to dissociate 50% of PSD-95 was fourfold lower for NR2B than NR2A in cultured hippocampal and striatal neurons, and that this concentration correlated tightly with protection against NMDA-induced toxicity in hippocampal neurons without altering NMDAR current. In contrast, NMDAR signaling to cell death in cultured striatal neurons occurred independently of the NR2B/PSD-95 interaction or neuronal nitric oxide synthase activation. These results will facilitate development of neuronal type-specific protective therapies.

Magnesium induces neuronal apoptosis by suppressing excitability

In clinical obstetrics, magnesium sulfate (MgSO(4)) use is widespread, but effects on brain development are unknown. Many agents that depress neuronal excitability increase developmental neuroapoptosis. In this study, we used dissociated cultures of rodent hippocampus to examine the effects of Mg(++) on excitability and survival. Mg(++)-induced caspase-3-associated cell loss at clinically relevant concentrations. Whole-cell patch-clamp techniques measured Mg(++) effects on action potential threshold, action potential peak amplitude, spike number and changes in resting membrane potential. Mg(++) depolarized action potential threshold, presumably from surface charge screening effects on voltage-gated sodium channels. Mg(++) also decreased the number of action potentials in response to fixed current injection without affecting action potential peak amplitude. Surprisingly, Mg(++) also depolarized neuronal resting potential in a concentration-dependent manner with a +5.2 mV shift at 10 mM. Voltage ramps suggested that Mg(++) blocked a potassium conductance contributing to the resting potential. In spite of this depolarizing effect of Mg(++), the net inhibitory effect of Mg(++) nearly completely silenced neuronal network activity measured with multielectrode array recordings. We conclude that although Mg(++) has complex effects on cellular excitability, the overall inhibitory influence of Mg(++) decreases neuronal survival. Taken together with recent in vivo evidence, our results suggest that caution may be warranted in the use of Mg(++) in clinical obstetrics and neonatology.

NMDA receptor desensitization regulated by direct binding to PDZ1-2 domains of PSD-95

Regulation of N-methyl-d-aspartate receptor (NMDAR) activity by desensitization is important in physiological and pathological states; NMDAR desensitization contributes in shaping synaptic responses and may be protective by limiting calcium influx during sustained glutamate insults. We previously reported that glycine-independent desensitization decreases during hippocampal neuronal development, correlating with NMDAR synaptic localization and association with postsynaptic density 95 (PSD-95). PSD-95/Discs large/zona occludens (PDZ)-1,2 domains of PSD-95 bind to the C-terminus of NMDAR NR2 subunits. The role of PSD-95 in anchoring signaling proteins near NMDARs is well documented. To determine if PSD-95-induced changes in NMDAR desensitization occur because of direct binding to NR2 or due to recruitment of regulatory proteins, we tested the effects of various PSD-95 constructs on NMDAR currents in human embryonic kidney 293 (HEK293) cells and neurons. In HEK cells, wild-type PSD-95 significantly reduced wild-type NMDAR desensitization without altering currents of NMDARs containing NR2A-S1462A, a mutation that abolishes PSD-95 binding. The PSD-95 N-terminus truncated after the PDZ1-2 domains was sufficient for this effect in neurons with low endogenous PSD-95 levels; in NMDAR-expressing HEK cells, the effect persisted when PSD-95 multimerization was eliminated. Moreover other PSD-95 family members with highly homologous PDZ1-2 domains significantly reduced NMDAR desensitization. In mature neurons, disruption of PSD-95/NMDAR interaction through protein kinase C (PKC) activation increased desensitization to levels found in immature neurons, and this effect was not due to PKC direct regulation of NMDAR activity. We conclude that direct binding of PSD-95 increases stability of NMDAR responses to agonist exposure in neuronal and nonneuronal cells.

Conformational restriction blocks glutamate receptor desensitization

Desensitization is a universal feature of ligand-gated ion channels. Using the crystal structure of the GluR2 L483Y mutant channel as a guide, we attempted to build non-desensitizing kainate-subtype glutamate receptors. Success was achieved for GluR5, GluR6 and GluR7 with intermolecular disulfide cross-links but not by engineering the dimer interface. Crystallographic analysis of the GluR6 Y490C L752C dimer revealed relaxation from the active conformation, which functional studies reveal is not sufficient to trigger desensitization. The equivalent non-desensitizing cross-linked GluR2 mutant retained weak sensitivity to a positive allosteric modulator, which had no effect on GluR2 L483Y. These results establish that the active conformation of AMPA and kainate receptors is conserved and further show that their desensitization requires dimer rearrangements, that subtle structural differences account for their diverse functional properties and that the ligand-binding core dimer is a powerful regulator of ion-channel activity.

Excitotoxicity and ALS: what is unique about the AMPA receptors expressed on spinal motor neurons?

It has been repeatedly reported that spinal motor neurons are selectively vulnerable to AMPA receptor-mediated excitotoxicity. Therefore, identifying the uniqueness of AMPA receptors that are expressed on motor neurons, especially in individuals affected with sporadic amyotrophic lateral sclerosis (ALS) is essential for elucidating the etiology of this disorder. The mechanism that initiates motor neuronal death appears to be an exaggerated influx of Ca(2+) through AMPA receptors. The determinants that affect this Ca(2+) influx are Ca(2+) permeability, which is regulated by the presence of the GluR2 subunit and by RNA editing at the Q/R site of GluR2; channel desensitization, which is regulated by alternative splicing at the flip/flop site and by RNA editing at the R/G site of GluR subunits; and receptor density on the cell surface, which is controlled by many factors including regulatory proteins, direct phosphorylation and RNA editing at the Q/R site. This review focuses on recent progress on the molecular dynamics of AMPA receptors and discusses the pathophysiology of selective motor neuron death mediated by AMPA receptors in individuals affected with sporadic ALS.

Underlying mechanism for NMDA receptor antagonism by the anti-inflammatory drug, sulfasalazine, in mouse cortical neurons

Sulfasalazine (SULFA), of anti-inflammatory drugs, shows a protective action against NMDA-induced neuronal toxicity. Here, we used an electrophysiological study of the pharmacological effects of SULFA on NMDA receptors to examine the molecular mechanisms underlying the neuroprotective role of SULFA. The drug acted as a typical noncompetitive inhibitor with neither agonist- nor use-dependency, and antagonized NMDA-evoked responses in a voltage-independent manner, suggesting that SULFA is not an open channel blocker. Noise and single channel analyses showed that SULFA-blocked NMDA responses by reducing the number of NMDA channels available for activation, and also reduced the channel open probability without changing single channel conductance. Moreover, SULFA accelerated NMDA desensitization without affecting the affinity of the receptor for NMDA or glutamate. Taken together, these data indicate that SULFA blocks the NMDA response by reducing the number of NMDA channels available for activation. This appears to occur via a SULFA-induced decrease in the channel open probability, and a concomitant acceleration of the desensitization response, which is likely associated with a reduced affinity for glycine. SULFA indeed decreased the glycine-potentiated NMDA response without binding directly to the glycine site. Our results suggest that SULFA acts as a noncompetitive NMDA receptor antagonist with an allosteric glycine modulation.

Developmental decrease in NMDA receptor desensitization associated with shift to synapse and interaction with postsynaptic density-95

NMDA receptors (NMDARs) play a crucial role in neuronal development, synaptic plasticity, and excitotoxicity; therefore, regulation of NMDAR function is important in both physiological and pathological conditions. Previous studies indicate that the NMDAR-mediated synaptic current decay rate increases during development because of a switch in receptor subunit composition, contributing to developmental changes in plasticity. To test whether NMDAR desensitization also changes during development, we recorded whole-cell NMDA-evoked currents in cultured rat hippocampal neurons. We found that glycine-independent desensitization of NMDARs decreases during development. This decrease was not dependent on a switch in subunit composition or differential receptor sensitivity to agonist-, Ca2+-, or Zn2+-induced increase in desensitization. Instead, several lines of evidence indicated that the developmental decrease in desensitization was tightly correlated with synaptic localization of the receptor, suggesting that association of NMDARs with proteins selectively expressed at synapses in mature neurons might account for developmental alterations in desensitization. Accordingly, we tested the role of interactions between PSD-95 (postsynaptic density-95) and NMDARs in regulating receptor desensitization. Overexpression of PSD-95 reduced NMDAR desensitization in immature neurons, whereas agents that interfere with synaptic targeting of PSD-95, or induce movement of NMDARs away from synapses and uncouple the receptor from PSD-95, increased NMDAR desensitization in mature neurons. We conclude that synaptic localization and association with PSD-95 increases stability of hippocampal neuronal NMDAR responses to sustained agonist exposure. Our results elucidate an additional mechanism for differentially regulating NMDAR function in neurons of different developmental stages or the response of subpopulations of NMDARs in a single neuron.

Chloride influx aggravates Ca2+-dependent AMPA receptor-mediated motoneuron death

AMPA receptor-mediated excitotoxicity has been implicated in the pathogenesis of stroke, neurotrauma, epilepsy, and many neurodegenerative diseases such as motoneuron disease. We studied the role of Cl- in AMPA receptor-mediated Ca2+-dependent excitotoxicity in cultured rat spinal motoneurons. Using the gramicidin perforated patch-clamp technique, the intracellular Cl- concentration could be calculated from the reversal potential of the GABA-induced current. The membrane depolarization caused by AMPA receptor stimulation resulted in Cl- influx through 5-nitro-2(3-phenylpropyl-amino) benzoic acid- and niflumic acid-sensitive Cl- channels. Cl- influx during AMPA receptor stimulation aggravated excitotoxic motoneuron death by two mechanisms: an increase of AMPA receptor conductance and an elevation of the Ca2+ driving force through a partial repolarization. The Cl- influx during AMPA receptor stimulation was enhanced by coadministration of GABA. This resulted in an increased Ca2+ influx and an enhanced cell death, suggesting that concomitant GABAergic stimulation may aggravate excitotoxic motoneuron death.

Flip and flop splice variants of AMPA receptor subunits in the spinal cord of amyotrophic lateral sclerosis

Excitotoxicity mediated by AMPA receptors has been suggested to be implicated in the pathogenesis of amyotrophic lateral sclerosis (ALS). To investigate the relevance of AMPA receptors to motor neuron degeneration in ALS, we evaluated the expression of mRNAs coding for flip and flop splice variants of AMPA receptor subunits (GluR-A to GluR-D) in the cervical segment of the spinal cord from control individuals and patients with ALS using in situ hybridization histochemistry. Transcript mRNAs coding for flop variants were significantly decreased in the ventral horn of the spinal cord from patients with ALS, whereas the mRNAs for flip variants were preserved. These findings suggest that the relative abundance of flip variants vs. flop variants is increased in spinal motor neurons of ALS patients when compared to that of control individuals. Flip variants promote assemblies of slowly desensitizing AMPA receptors. These results imply that spinal motor neurons of ALS patients possess more slowly desensitizing AMPA receptors than those of control individuals. This expression change of AMPA receptors in ALS may account for vulnerability of motor neurons in this disease.

AMPA Neurotoxicity in Rat Cerebellar and Hippocampal Slices: Histological Evidence for Three Mechanisms

Excitatory amino acid-induced death of central neurons may be mediated by at least two receptor types, the so-called NMDA (N-methyl-d-aspartate) and AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate) receptors. We have studied the neurodegenerative mechanisms set in motion by AMPA receptor activation using incubated slices of 8-day-old rat cerebellum and hippocampus. In both preparations, AMPA induced a pattern of degeneration that differed markedly from the one previously shown to be elicited by NMDA. In cerebellar slices, AMPA induced the degeneration of most Purkinje cells together with a population of Golgi cells; in hippocampal slices the neurons were affected in the order CA3 > CA1 > dentate granule cells. Three mechanisms could be discerned: an acute one in which neurons (e.g. cerebellar Golgi cells) underwent a rapid degeneration; a delayed one in which the neurons (Purkinje cells and hippocampal neurons) appeared to be only mildly affected immediately after a 30 min exposure but then underwent a protracted degeneration during the postincubation period (1.5 - 3 h); and finally a slow toxicity, which took place during long (2 h) exposures to AMPA (3 - 30 microM). Although Purkinje cells were vulnerable in both cases, the efficacy of AMPA was higher for the delayed mechanism than for the slow one. The pathology displayed by the acutely destroyed Golgi neurons was a classical oedematous necrosis, whereas most neurons vulnerable to the delayed and slow mechanisms displayed a 'dark cell degeneration', whose cytological features bore a close resemblance to those of neurons irreversibly damaged by ischaemia, hypoglycaemia or status epilepticus in vivo.

Characterization of l-2-Amino-4-Phosphonobutanoate Action Following Sensitization by Quisqualate in Rat Hippocampal Slice Cultures

An excitatory action of l-2-amino-4-phosphonobutanoate (l-AP4), a glutamate analogue, is observed following pre-exposure of tissue to quisqualate. We have studied the mechanism of sensitization of l-AP4 responses by quisqualate in voltage-clamped CA3 pyramidal cells in rat hippocampal slice cultures in the presence of tetrodotoxin. Prior to quisqualate addition, CA3 cells did not respond to l-AP4 (50 - 1000 microM). Following brief application of quisqualate (500 nM for 30 s), l-AP4 (50 - 200 microM) induced a complex excitatory response which could be obtained for >1 h. l-AP4 caused an ionotropic inward current associated with a conductance increase. This response was in part sensitive to 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX) and in part sensitive to d-2-amino-5-phosphonovalerate (d-AP5) and Mg2+ ions. At depolarizing potentials, in the presence of CNQX and d-AP5, l-AP4 caused excitation by depressing K+ currents, mimicking the metabotropic action of glutamate. This indicates that the action of l-AP4 is mediated by three different receptor types: N-methyl-d-aspartate (NMDA) receptors, alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA) receptors, and glutamatergic metabotropic receptors. The l-AP4 response persisted in solutions containing low Ca2+ and high Mg2+ concentrations or 100 - 200 microM Cd2+, suggesting that it is independent of extracellular Ca2+. We were unable to identify any substance other than quisqualate capable of sensitizing the l-AP4 action. This effect also occurred when quisqualate was applied in Ca2+-free solution or in solutions containing low concentrations of Na+ or Cl-. Sensitization of l-AP4 responses by quisqualate was not observed in acutely dissociated pyramidal cells recorded by means of the whole-cell recording mode, although ionotropic quisqualate responses were present. Sensitization was readily reversed by short applications of the endogenous excitatory amino acids glutamate, aspartate and homocysteate at concentrations of 10 - 100 microM. Our data are consistent with the hypothesis that the excitatory action of l-AP4 results from a Ca2+-independent release of endogenous excitatory amino acids from some presynaptic neuronal or glial site.

Low doses of AMPA exert anticonvulsant effects on pentylenetetrazol-kindled seizures

Excitatory amino acids (EAAs) are critically involved in the initiation and propagation of seizures. N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors appear to be of special interest in this regard. Besides receptor binding by antagonists, the function of glutamatergic synapses can be altered via autoreceptor-mediated mechanisms or by receptor desensitisation. Therefore, the effect of AMPA (1, 10 or 100 pmol per animal, intracerebroventricular injection) was tested on acutely induced pentylenetetrazol (PTZ) seizures. The lowest dose exerted clear anticonvulsant effects. Furthermore, 1 and 10 pmol AMPA were tested for their efficacy to suppress PTZ kindling. The lower dose reduced seizure severity significantly but 10 pmol AMPA was ineffective. In reaction to a test dose of PTZ, the kindled groups pretreated with AMPA reached seizure scores similar to saline-pretreated kindled rats, suggesting that the kindled state was reached. In a further experiment, we tested the effect of cyclothiazide (CYC, which blocks AMPA receptor desensitisation) on the 1 pmol AMPA-mediated anticonvulsant effect. The AMPA response was not altered. These results suggest that autoreceptor-mediated mechanisms rather than desensitisation might contribute to the anticonvulsant effect found.

AMPA receptor current density, not desensitization, predicts selective motoneuron vulnerability

Spinal motoneurons are more susceptible to AMPA receptor-mediated injury than are other spinal neurons, a property that has been implicated in their selective degeneration in amyotrophic lateral sclerosis (ALS). The aim of this study was to determine whether this difference in vulnerability between motoneurons and other spinal neurons can be attributed to a difference in AMPA receptor desensitization and/or to a difference in density of functional AMPA receptors. Spinal motoneurons and dorsal horn neurons were isolated from embryonic rats and cultured on spinal astrocytes. Single-cell RT-PCR quantification of the relative abundance of the flip and flop isoforms of the AMPA receptor subunits, which are known to affect receptor desensitization, did not reveal any difference between the two cell populations. Examination of AMPA receptor desensitization by patch-clamp electrophysiological measurements on nucleated and outside-out patches and in the whole-cell mode also yielded similar results for the two cell groups. However, AMPA receptor current density was two- to threefold higher in motoneurons than in dorsal horn neurons, suggesting a higher density of functional AMPA receptors in motoneuron membranes. Pharmacological reduction of AMPA receptor current density in motoneurons to the level found in dorsal horn neurons eliminated selective motoneuron vulnerability to AMPA receptor activation. These results suggest that the greater AMPA receptor current density of spinal motoneurons may be sufficient to account for their selective vulnerability to AMPA receptor agonists in vitro.

Dorsomedial medulla is more susceptible than rostral ventrolateral medulla to hypoxic insult in cats

We investigated the responses of systemic arterial pressure and vertebral sympathetic nerve activity to glutamate microinjections (0. 1 M, 70 nl) in the dorsomedial (DM) and the rostral ventrolateral medulla (RVLM) before hypoxia and after reoxygenation (posthypoxia) after various degrees of hypoxia in anesthetized cats. Hypoxia was produced by ventilating 5% O(2) and 95% N(2) for different durations (hypoxia I-III). In intact cats, the glutamate-induced systemic arterial pressure and vertebral nerve activity responses of the DM were depressed after all degrees of hypoxia. Posthypoxic depression in the RVLM, however, was not observed until hypoxia II and III. Precollicular decerebration prevented depression in the RVLM, but, for the DM, it was effective only for hypoxia I. Baro- and chemoreceptor denervation abolished all posthypoxic depression in both the DM and the RVLM. Pressor responses to tyramine (100-400 microg/kg iv) remained unchanged after all degrees of hypoxia. These results suggest that the DM is more susceptible to hypoxia than the RVLM. The peripheral baro- and chemoreceptors and the suprapontine structures apparently play an important role in posthypoxic depression. Moreover, the depression is not due to the postganglionic norepinephrine depletion.

The selective AMPA receptor antagonist GYKI 53784 blocks action potential generation and excitotoxicity in the guinea pig cochlea

The role of AMPA receptors in cochlear synaptic transmission and excitotoxicity was investigated by comparing the actions of a selective AMPA antagonist GYKI 53784 (LY303070) with additional AMPA/kainate antagonists, GYKI 52466 and DNQX, and the NMDA antagonist, D-AP5, in several electrophysiological, neurotoxicological and histochemical tests. GYKI 53784 had the same potency as DNQX and was 10 times more potent than GYKI 52466 in reducing auditory nerve activity. The NMDA antagonist D-AP5 had no effect on auditory nerve activity. When single-fiber activity was blocked with GYKI 53784, the effects of AMPA or kainate were also antagonized. GYKI 53784 completely blocked excitotoxicity (i.e. destruction of the afferent nerve endings) induced by AMPA and kainate. The histochemical detection of Co(2+) uptake was used to study Ca(2+) influx within the primary auditory nerve cells. Application of AMPA induced no significant Co(2+) uptake into the cells, suggesting that these receptors normally have a very low permeability to Ca(2+). Application of kainate induced significant Co(2+) uptake that was blocked by the AMPA receptor antagonist GYKI 53784 suggesting that kainate stimulated Ca(2+) entry through AMPA receptor channels. Results suggest that AMPA-preferring receptors are functionally located at the sensory cell-afferent synapse whereas NMDA and kainate receptors are not.

FGF-2 potentiates Ca(2+)-dependent inactivation of NMDA receptor currents in hippocampal neurons

Peptide growth factors such as the neurotrophins and fibroblast growth factors have potent effects on synaptic transmission, development, and cell survival. We report that chronic (hours) treatment with basic fibroblast growth factor (FGF-2) potentiates Ca(2+)-dependent N-methyl-D-aspartate (NMDA) receptor inactivation in cultured hippocampal neurons. This effect is specific for the NMDA-subtype of ionotropic glutamate receptor and FGF-2. The potentiated inactivation requires ongoing protein synthesis during growth factor treatment and the activity of protein phosphatase 2B (PP2B or calcineurin) during agonist application. These results suggest a mechanism by which FGF-2 receptor signaling may regulate neuronal survival and synaptic plasticity.

Enhancement of AMPA-mediated current after traumatic injury in cortical neurons

Overactivation of ionotropic glutamate receptors has been implicated in the pathophysiology of traumatic brain injury. Using an in vitro cell injury model, we examined the effects of stretch-induced traumatic injury on the AMPA subtype of ionotropic glutamate receptors in cultured neonatal cortical neurons. Recordings made using the whole-cell patch-clamp technique revealed that a subpopulation of injured neurons exhibited an increased current in response to AMPA. The current-voltage relationship of these injured neurons showed an increased slope conductance but no change in reversal potential compared with uninjured neurons. Additionally, the EC(50) values of uninjured and injured neurons were nearly identical. Thus, current potentiation was not caused by changes in the voltage-dependence, ion selectivity, or apparent agonist affinity of the AMPA channel. AMPA-elicited current could also be fully inhibited by the application of selective AMPA receptor antagonists, thereby excluding the possibility that current potentiation in injured neurons was caused by the activation of other, nondesensitizing receptors. The difference in current densities between control and injured neurons was abolished when AMPA receptor desensitization was inhibited by the coapplication of AMPA and cyclothiazide or by the use of kainate as an agonist, suggesting that mechanical injury alters AMPA receptor desensitization. Reduction of AMPA receptor desensitization after brain injury would be expected to further exacerbate the effects of increased postinjury extracellular glutamate and contribute to trauma-related cell loss and dysfunctional synaptic information processing.

Pyruvate and lactate protect striatal neurons against N-methyl-D-aspartate-induced neurotoxicity

A sustained release of glutamate contributes to neuronal loss during cerebral ischaemia. Using cultured mouse striatal neurons, we observed that glucose deprivation, which occurs in this pathological process, enhanced the N-Methyl-D-aspartate (NMDA)- or alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)-induced neurotoxicity. The end products of glycolysis, lactate and pyruvate, strongly protected neurons from these neurotoxic effects. The neuroprotective effect of pyruvate (which is more prominent in the absence of glucose) was not related to its ability to react with H2O2 by a decarboxylation process. Pyruvate and L-lactate strongly counteracted the deep decrease in the neuronal ATP content induced by NMDA, indicating that they might protect striatal neurons by rescuing cellular energy charge. Addition of MK-801 after the NMDA withdrawal completely protected neurons, suggesting that NMDA neurotoxicity resulted from a delayed NMDA receptor activation probably linked to a delayed release of an endogenous agonist in the extracellular medium. The strong accumulation of extracellular glutamate which was found in both sham and NMDA-treated cultures was markedly decreased by pyruvate. Thus, pyruvate might also exert its protecting activity by decreasing the delayed accumulation of glutamate which seemed to be neurotoxic only after a preexposure of neurons to NMDA.

Glycine-independent NMDA receptor desensitization: localization of structural determinants

In studying chimeras of NR2A and NR2C subunits of the NMDA receptor, we have found that glycine-independent desensitization depends on two regions of the extracellular N-terminal domain. One corresponds to a stretch of approximately 190 amino acids preceding the glutamate-binding domain S1. The other localizes at the interface between the N-terminal segment and the first transmembrane domain of NR2A subunits and involves A555 and S556. Both regions support desensitization in the absence of the other with different time courses. Desensitization did not develop with time in receptors containing the entire N-terminal region of NR2C. The introduction of A555 into the corresponding position of NR2C subunits enabled the receptors to manifest time-dependent increase in desensitization. Thus, this determinant behaves as an allosteric effector for glycine-independent desensitization.

Characterization of YM90K, a selective and potent antagonist of AMPA receptors, in rat cortical mRNA-injected Xenopus oocytes

The inhibitory potencies of 6-(1H-imidazol-1-yl)-7-nitro-2,3(1H,4H)-quinoxalinedione hydrochloride (YM90K), 2-3,dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline (NBQX) and 1-(4-amino-phenyl)-4-methyl-7,8-methyl-endioxyl-5H-2,3-benzodiazep ine (GYKI 52466) at excitatory amino acid receptors were examined in rat cortical mRNA-injected Xenopus oocytes using a two-electrode voltage clamp. Schild analysis of YM90K and NBQX inhibition of kainate currents yielded pA2 values of 6.83 +/- 0.01 and 7.24 +/- 0.01, respectively. GYKI 52466 reduced the maximum kainate response and increased the kainate EC50 in a dose-dependent manner, suggesting that the antagonism of AMPA receptors by GYKI 52466 is mixed competitive and non-competitive for kainate. Schild analysis of YM90K and NBQX inhibition of kainate currents in the presence of 30 microM cyclothiazide yielded pA2 values of 6.62 +/- 0.03 (slope: 1.02 +/- 0.01) and 7.10 +/- 0.02 (slope: 1.00 +/- 0.02), respectively, consistent with competitive antagonism. Cyclothiazide potentiated the AMPA response as well as the kainate response and increased the apparent Hill coefficients in a concentration-dependent manner. The potency of YM90K to inhibit AMPA-induced current could be reduced by increasing the concentration of cyclothiazide. We showed that YM90K is a potent and competitive antagonist for AMPA receptors and the apparent affinity of competitive antagonists was reduced by cyclothiazide. Cyclothiazide can affect the interaction between receptors and both agonists and antagonists, suggesting that it might allosterically alter the affinity of agonists and competitive antagonists for their binding site on the AMPA receptor complex.

Apoptosis induced by beta-N-oxalylamino-L-alanine on a motoneuron hybrid cell line

It has been suggested that beta-N-oxalylamino-L-alanine, a non-protein amino acid present in the Lathyrus Sativus seeds, may play a role in the etiopathogenesis of neurolathyrism, a toxic form of motor neuron disease clinically characterized by a severe spastic paraparesis. In order to investigate the mechanisms of beta-N-oxalylamino-L-alanine-mediated cell death, we studied the effect of this neurotoxin as well as other excitatory amino acids agonists on the growth and survival of motoneuron hybrid ventral spinal cord 4.1 cells. beta-N-oxalylamino-L-alanine was toxic to ventral spinal cord 4.1 cells in a concentration-dependent fashion (0.5-10 mM). Among the excitatory amino acids tested, only glutamate (1-10 mM), quisqualate (1 mM) and, with less extent, beta-N-methylamino-L-alanine (10 mM) induced a significant reduction of cell survival. The effect of Lathyrus Sativus neurotoxin was a slow process, becoming apparent only after 24-48 h of incubation. Interestingly, a mathematical analysis applied to the time course and dose curve of beta-N-oxalylamino-L-alanine toxicity suggested that even for very low concentrations of the amino acid it is theoretically possible to predict a time-dependent effect. The cell death was not blocked by antagonists of N-methyl-D-aspartate or non-N-methyl-D-aspartate receptors; aurintricarboxylic acid and alpha-tocopherol gave a partial protection; cysteine (1 mM) prevented the toxic effect of both Lathyrus Sativus neurotoxin and glutamate as well as quisqualate. Morphologically, in the presence of either beta-N-oxalylamino-L-alanine, glutamate or quisqualate, ventral spinal cord 4.1 cells showed apoptotic features also confirmed by ISEL technique and agarose gel electrophoresis of genomic DNA. Thus, our results suggest that in ventral spinal cord 4.1 motoneuron hybrid cells, in the absence of functional synaptic excitatory amino acid receptors, beta-N-oxalylamino-L-alanine induces cell degeneration through an apoptotic mechanism, possibly mediated by a block of cystine/glutamate Xc antiporter.

New developments in the molecular pharmacology of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate and kainate receptors

Separation of non-N-methyl-D-aspartate subtypes of glutamate receptors, known as alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and kainate receptors, is traced through conventional pharmacology to molecular biology. The physiology and pharmacology of recombinant receptor subtypes (GluR1-7 and KA1-2) are described. Competitive antagonists, e.g., the quinoxalinedione, 2,3-dihyroxy-6-nitro-7-sulphamoyl-benz(F)quinoxaline, and the decahydroisoquinoline, 3S,4aR,6R, 8aR-6-[2-(1(2)H-tetrazol-5-yl)ethyl]-decahydroisoquinolin e-3-carboxylate, have a broad antagonist spectrum, except that the latter is inactive on GluR6. The 2,3-benzodiazepines noncompetitively antagonise the AMPA receptor GluR1-4. Desensitisation of AMPA (GluR1-4) and kainate (GluR5-7 and KA1-2) receptors is blocked by cyclothiazide and concanavalin A, respectively. Polyamine toxins block AMPA receptors not containing GluR2 and unedited kainate receptors (GluR5-6). These data correlate well with results on native neurons characterised by techniques such as in situ hybridisation.

AMPA receptors in cerebellar granule cells during development in culture

The survival and maturation of differentiating cerebellar granule cells in culture are known to be promoted by excitatory amino acids (EAAs) which, however, compromise the survival of mature cells. In contrast to the trophic effect, the toxic effect of alpha-amino-3-hydroxy-5-methyl-4-isoxasolepropiate (AMPA) could only be elicited when the desensitisation of AMPA receptors was blocked, cyclothiazide being used in this study. Nevertheless, even under these conditions, toxicity induced by AMPA in contrast to kainate was, at 9 DIV, only half of the maximal toxicity attained by 13-16 DIV. Since cellular responses to AMPA depend so dramatically on the maturational stage of granule cells, we examined here whether this characteristic is related to developmental changes in AMPA receptor properties, which may result from changes in the subunit composition of the receptor. In contrast to toxicity, AMPA-induced 45Ca2+ influx (determined in the presence of cyclothiazide and the NMDA receptor blocker MK-801) reached a maximum already at 9 DIV. This also applied to a fraction of the 45Ca2+ uptake which persisted either after Cd2+ application or under Na(+)-free conditions and therefore presumably was mediated directly through AMPA receptor channels. Quantitative analysis of Western blots showed that the amounts of GluR4 and to a lesser extent GluR2/3/4c are substantial already at 2 DIV, remaining fairly constant until 9 DIV, followed by an increase by 16 DIV. However GluR1, which is hardly detectable in granule cells in vivo and is also low early in vitro, increased almost linearly with cultivation time.(ABSTRACT TRUNCATED AT 250 WORDS)

The opioid peptide dynorphin modulates AMPA and kainate responses in acutely isolated neurons from the dorsal horn

In freshly isolated spinal dorsal horn (DH) neurons (laminae I-IV) of the young rat, the effects of dynorphin A1-17, U-50,488H and U-69,593 on inward currents induced by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate (KA) were studied under whole-cell voltage-clamp conditions. When the cells were clamped to a holding potential of -60 mV, co-application of dynorphin A1-17 (10(-6) M) and AMPA (2 x 10(-5) M) reversibly decreased the peak amplitude of the initial transient component of the AMPA-induced current in 72% of the examined cells. In addition, dynorphin (10 microM) in perforated patch-recordings consistently produced a decrease in the steady-state component of the AMPA response. The depressant effect was concentration-dependent (IC50 = 86 nM) and reversible. The dynorphin A1-17-induced depression of the AMPA response was associated with slowing of the response kinetics, including both a 10-90% rise-time and time constant of decay. The AMPA-induced currents were modulated by dynorphin not only during the co-administration but also after the removal of the peptide. Dynorphin increased the initial peak AMPA current in 42% of the examined cells. Similar as with dynorphin A1-17, the peak amplitude of the AMPA-induced current was reversibly suppressed in the presence of 1 microM U-50,488H and U-69,593 in 75% and 86% of the examined cells, respectively. Naloxone and the kappa 1-selective antagonist norbinaltorphimine (nor-BNI) blocked the initial depressant but not late excitatory effects of dynorphin A1-17 and U-50,488H. This antagonistic effect of naloxone and norbinaltorphimine suggests that the depressant effect of dynorphin A1-17 on the AMPA-activated conductance is a true opioid, probably kappa 1-opioid receptor-mediated event. In contrast, the dynorphin-induced late potentiation of AMPA/KA responses appears to be a non-opioid effect since it was not inhibited by nor-BNI, CTAP and naltrindole, the selective kappa-, mu- and delta-opioid receptor blocking agents, respectively. Pretreatment of DH neurons with pertussis toxin blocked the depressant action of dynorphin A1-17, indicating that a Gi- or Go-type G protein was required for this effect on AMPA-activated currents. Intracellular dialysis with a highly specific peptide inhibitor (peptide 6-22) of the cAMP-activated protein kinase (PKA), and with Rp-cAMPS, prevented the depressant effect of dynorphin A1-17. In addition, staurosporine, a nonselective kinase inhibitor, blocked the dynorphin depression of the AMPA response.(ABSTRACT TRUNCATED AT 400 WORDS)

Hydrodynamic and pharmacological characterization of putative alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate-sensitive L-glutamate receptors solubilized from pig brain

L-[3H]Glutamate binding sites with characteristics resembling that of membrane-bound alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate-subtype L-glutamate receptors have been solubilized from pig brain synaptic junctions by Triton X-114. Binding of [3H]AMPA to these soluble sites in the presence of KSCN results in a curvilinear Scatchard plot that can be resolved into a high-affinity component and a low-affinity component. These Triton-X-114-solubilized sites can be further separated into two species of binding sites by gel-filtration chromatography or sucrose-density-gradient centrifugation. The pharmacological profiles of these two species of binding site are almost identical, and the rank orders of potency for glutamatergic drugs in displacing L-[3H]glutamate binding to these sites are quisqualate > 6,7-dinitroquinoxaline-2,3-dione > 6-cyano-7-nitroquinoxaline-2,3-dione > AMPA > L-glutamate > kainate >> N-methyl-D-aspartate = L-2-amino-4-phosphonobutyrate. Both sites are found to bind [3H]AMPA, and in the presence of KSCN the binding activities are significantly enhanced. Analysis of the hydrodynamic behaviour of these binding sites by sucrose-density-gradient centrifugation in H2O- and 2H2O-based solvents and gel-filtration chromatography has revealed that one of these sites (Stokes radius 8.3 nm, sedimentation coefficient 18.5 S) consists of 562 kDa protein and 281 kDa detergent, and the other site (Stokes radius 9.6 nm, sedimentation coefficient 13.4 S) consists of 352 kDa protein and 569 kDa detergent. Frictional coefficients of these sites indicate that these receptor-detergent complexes are asymmetrical in structure, consistent with large transmembrane proteins.

Kainate-binding proteins: phylogeny, structures and possible functions

Recent advances have demonstrated that the family of [3H]kainate-binding proteins and kainate receptors comprise a number of related polypeptides. In all the cases so far investigated, the kainate-binding proteins from non-mammalian vertebrates have M(r) values in the range of 40-50 kDa whereas mammalian kainate receptors and kainate-binding proteins have M(r) values in the order of 100 kDa. There have not, as yet, been any reports of 40-50 kDa kainate-binding proteins in mammalian CNS and, despite the cloning of increasing numbers of cDNAs encoding new kainate-binding proteins, the relationships between these two general groups of polypeptides remain unclear. Nonetheless, there is now a wealth of phylogenetic, structural and molecular biological data available about these proteins. In this review, Jeremy Henley outlines the properties and structures of kainate-binding proteins and offers some possibilities as to the roles of these often hugely abundant proteins.

Excitatory amino acid receptors: a gallery of new targets for pharmacological intervention

The excitatory amino acids (EAAs) L-glutamate and L-aspartate are the most abundant amino acids in brain and play a number of roles in maintaining neuronal function. Among these are their use as protein constituents, as key intermediates in ammonia metabolism, and as precursors for other neurotransmitters. Given the widespread distribution of EAA-containing neurons, these transmitters are likely to be involved in virtually all central nervous system functions, with abnormalities in neurotransmission contributing to the symptoms of a host of neurological and psychiatric disorders. Because of the importance of EAAs in maintaining the functional integrity of the central nervous system, efforts are underway to design agents capable of regulating the activity of these transmitters for therapeutic gain. Inasmuch as potential side effects preclude a generalized modification of this system, strategies must be found to alter EAA neurotransmission in selected brain regions. In this regard, pharmacological data suggest several functionally distinct EAA receptors, a finding confirmed by cloning studies which hint at an even larger family of sites. Moreover, it appears that some excitatory amino acid receptor complexes are composed of interacting sites which orchestrate receptor function, and there is evidence that EAA receptors may influence the activity of one another. Thus, there appear to be numerous sites that can be targeted to selectively modify excitatory amino acid neurotransmission in brain. Besides the agonist recognition site for each receptor subtype, other targets include regulatory subunits, ion channels and components of receptor-coupled second messenger systems.

Excitotoxic neuronal damage and neuropsychiatric disorders

Excitatory amino acids (EAA) serve important physiological functions in the vertebrate CNS, including participation in fast excitatory synaptic transmission, modulation of synaptic plasticity and regulation of neuronal morphology during development. However, paradoxically they also harbor neurotoxic (excitotoxic) potential, which, if unleashed, can cause widespread degeneration of CNS neurons. Accumulating evidence suggests a role for excitotoxins in a variety of human neuropsychiatric disorders. This paper reviews the classes of EAA receptors in the CNS, the mechanisms underlying EAA-mediated neuronal damage and the role of EAA in specific human disorders.

Zinc potentiates agonist-induced currents at certain splice variants of the NMDA receptor

We have determined the gene structure for the NMDA receptor subunit gene NMDAR1. We found eight splice variants that arise from different combinations of a single 5' terminal exon insertion and three different 3' terminal exon deletions, relative to NMDAR1. We analyzed the modulation by Zn2+ of currents through homomeric receptors assembled from these splice variants and found that, in addition to its well-known inhibitory effect at high concentrations, Zn2+ potentiates agonist-induced currents at submicromolar concentrations (EC50 = 0.50 microM). This potentiation is observed only with a subset of NMDAR1 splice variants that show additional differences in pharmacological properties. Zn2+ potentiation is rapidly reversible, noncompetitive with either glutamate or glycine, and voltage independent. Zn2+ potentiation is mimicked by Cd2+, Cu2+, and Ni2+, but not by Mn2+, Co2+, Fe3+, Sn2+, or Hg2+. Our results suggest a possible role for Zn2+ as a positive modulator of NMDA receptors in certain regions of the brain.

GYKI 52466 protects against non-NMDA receptor-mediated excitotoxicity in primary rat hippocampal cultures

Glutamate excitotoxicity is mediated by both N-methyl-D-aspartate (NMDA)-receptor and non-NMDA receptor (alpha-amino-3-hydroxy-5-methyl-isoxazolepropionate (AMPA)/kainate (KA)) mechanisms but the lack of specific antagonists has limited the characterization of AMPA/KA receptor-mediated excitotoxicity. The 2,3-benzodiazepine GYKI 52466 is a newly described non-competitive AMPA/KA receptor antagonist. We have investigated the neuroprotective efficacy of GYKI 52466 in an embryonic rat hippocampal culture model of non-NMDA receptor-mediated excitotoxicity using KA as an agonist at the AMPA/KA receptor. Overnight treatment with 500 microM KA resulted in prominent neuronal excitotoxicity as assessed by lactate dehydrogenase efflux. GYKI 52466 attenuated KA excitotoxicity in a dose-dependent manner with an IC50 of 9 microM. Together with competitive antagonists (e.g., various quinoxalinediones), non-competitive antagonists like GYKI 52466 can now be used to dissect mechanisms of non-NMDA receptor mediated excitotoxicity.

Cyclothiazide treatment unmasks AMPA excitotoxicity in rat primary hippocampal cultures

Mechanisms of non-NMDA receptor-mediated excitotoxicity were studied in embryonic rat hippocampal cultures using kainic acid (KA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) as agonists. Under basal culture conditions, overnight treatment with AMPA resulted in negligible excitotoxicity as assessed by phase-contrast microscopy and measurement of lactate dehydrogenase (LDH) release. In contrast, similar treatment with KA resulted in marked excitotoxic morphologic changes and release of LDH. Cotreatment of cultures with AMPA but not NMDA effectively blocked KA toxicity, suggesting that AMPA-induced rapid desensitization of the AMPA/KA receptor could account for the lack of prominent direct toxicity as well as AMPA's ability to block KA toxicity. To test this hypothesis, cultures were briefly pretreated with 10 microM cyclothiazide, a drug reported to block desensitization of the AMPA/KA receptor, and then exposed overnight to cyclothiazide plus AMPA and/or KA. Cyclothiazide-treated cultures were now vulnerable to AMPA as well as KA; moreover, AMPA was unable to block KA toxicity completely, suggesting that cyclothiazide impaired AMPA/KA receptor desensitization. These and related studies suggest that a regulatory site may exist on the AMPA/KA receptor that modulates non-NMDA receptor-mediated excitotoxicity.

Blockade of ionotropic quisqualate receptor desensitization in rat hippocampal neurons by wheatgerm agglutinin and other lectins

Previous experiments with wheatgerm agglutinin, an inhibitor of ionotropic quisqualate receptor desensitization, suggest that desensitization regulates quisqualate receptor-mediated synaptic transmission and excitotoxicity. Using whole-cell recordings from cultured postnatal rat hippocampal neurons, we have examined the wheatgerm agglutinin effect in further detail and compared it to other lectins. Wheatgerm agglutinin and other lectins belonging to the glucose/mannose, N-acetylglucosamine, and sialic acid classes inhibited desensitization. However, wheatgerm agglutinin was the most effective and had the most rapid onset of action. The inhibition was dose-dependent, and it was reduced and reversed by N-acetylglucosamine and N-acetylneuraminic acid. Treating neurons with neuraminidase, which cleaves sialic acid residues from the surface of cells, also diminished the effect. These results suggest that wheatgerm agglutinin reversibly inhibits ionotropic quisqualate receptor desensitization by interacting with carbohydrate residues on or near the quisqualate receptor complex. Future studies using the lectins may help to clarify the functional role of carbohydrate chains on the ionotropic quisqualate receptor.

Diazoxide blocks glutamate desensitization and prolongs excitatory postsynaptic currents in rat hippocampal neurons

1. The effects of diazoxide (DZ) on synaptic transmission and upon responses to exogenously applied glutamate agonists were examined in cultured hippocampal neurons. 2. DZ reversibly increased the peak amplitude of evoked excitatory postsynaptic currents (EPSCs) to 150 +/- 100% of control and prolonged the EPSC decay time constant (tau) from 5.9 +/- 1.2 ms to 14 +/- 6.2 ms (240% of control). 3. Peak and steady-state glutamate (Glu) and quisqualate (QA) currents activated by exogenous application were dramatically increased by DZ at concentrations which did not influence N-methyl-D-aspartate (NMDA), kainate (KA), or GABA currents. These effects were rapidly and completely reversible. Active and passive membrane properties were unaffected by DZ. 4. Inhibitory postsynaptic currents (IPSCs) were unaffected by the same DZ concentrations. 5. These experiments indicate that desensitization plays an important role in terminating excitatory transmission between mammalian central neurons. DZ and perhaps related compounds will ultimately help us identify the regions of the AMPA/KA receptor responsible for desensitization.

Evans blue reduces macroscopic desensitization of non-NMDA receptor mediated currents and prolongs excitatory postsynaptic currents in cultured rat thalamic neurons

Fast application of L-glutamate, AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid) or kainate to cultured rat thalamic neurons revealed properties of non-NMDA (N-methyl-D-aspartate) receptors similar to those described in hippocampal neurons. The kinetics of non-NMDA receptor-mediated currents were altered by the addition of the dye Evans Blue (EB). Macroscopic desensitization was reduced and activation and deactivation kinetics were slowed. Delayed addition of EB, after desensitization of non-NMDA receptors, resulted in reactivation of desensitized receptors. Thus, both ion channel gating and entry into the desensitized state were affected. Evans blue also slowed the activation and the decay of glutamatergic miniature EPSCs (excitatory postsynaptic currents), demonstrating that receptor kinetics determine the time course of the synaptic response.

Properties of vertebrate glutamate receptors: calcium mobilization and desensitization

Glutamate is now recognized as a major excitatory neurotransmitter in the vertebrate CNS, participating in a number of physiological and pathological processes. The importance of glutamate in the mobilization of intracellular Ca2+ as well as the relationship between excitatory and toxic properties has made it important to understand factors that regulate the responsivity of glutamate receptors. In recent years considerable insight has been gained about regulatory sites on NMDA receptors, with the recognition that these receptors are modulated by multiple endogenous and exogenous agents. Less is known about the regulation of responses mediated by AMPA, kainate, ACPD or APB receptors. Desensitization represents a potentially powerful means by which glutamate responses may be regulated. Indeed, two agents closely linked to the physiology of NMDA receptors, glycine and Ca2+, appear to modulate different types of desensitization. In the case of glycine, alteration of a rapid form of desensitization may be important in the role of this amino acid as a necessary cofactor for NMDA receptor activation. Additionally, changes in the affinity of the receptor complex for glycine may underlie the use-dependent decline in NMDA responses under certain conditions. Likewise, Ca2+ is a crucial player in the synaptic and toxic effects mediated by NMDA receptors, and is involved in a slower form of desensitization, in effect helping to regulate its own influx into neurons. The site and mechanism of the Ca2+ regulatory effects remain uncertain with evidence supporting both intracellular and ion channel sites of action. A clear role for Ca(2+)-dependent desensitization in the function of NMDA receptors under physiological conditions has not yet been demonstrated. AMPA receptor desensitization has been an area of intense investigation in recent years. The rapidity and degree of this process, coupled with its apparent rapid recovery, has suggested that desensitization is a key mechanism for the short-term regulation of responses mediated by these receptors. Furthermore, rapid desensitization appears to be one factor determining the time course and efficacy of fast excitatory synaptic transmission mediated by AMPA receptors, highlighting the physiological relevance of the process. The molecular mechanisms underlying desensitization remain uncertain. Traditionally, desensitization, like inactivation of voltage-gated channels, has been thought to represent a conformational change in the ion channel complex (Ochoa et al., 1989). However, it is unknown to what extent desensitization, in particular rapid AMPA receptor desensitization, has mechanistic features in common with inactivation. In voltage-gated channels, conformational changes in the channel protein restrict ion flow through the channel (Stuhmer, 1991).(ABSTRACT TRUNCATED AT 400 WORDS)

Spermine regulates N-methyl-D-aspartate receptor desensitization

The action of the endogenous polyamine spermine on NMDA-induced responses (in the presence of glycine) was evaluated in cultured spinal cord neurons under voltage- and concentration-clamp conditions. Spermine potentiated NMDA-induced responses in a dose-dependent manner. It was more effective in potentiating steady-state currents (i.e., desensitized response) than the peak phase of the response, indicating that the degree of desensitization was reduced in the presence of the polyamine. Kinetic analysis revealed that the desensitization onset rate, but not recovery rate, was affected by spermine. The effect was voltage independent and was seen in thoroughly dialyzed cells, in which desensitization becomes independent of glycine. Spermine potentiation showed fast on-off kinetics, and intracellular spermine, loaded in the recording pipette, did not occlude potentiation by extracellularly applied spermine. These results are consistent with the existence of a modulatory site for polyamines in the extracellular domain of the NMDA receptor, the activation of which potentiates NMDA receptor function by regulating its desensitization kinetics.

Modulation of excitatory synaptic transmission by drugs that reduce desensitization at AMPA/kainate receptors

Desensitization at AMPA/kainate receptors has been proposed to contribute to the decay of excitatory synaptic currents. We examined the action of aniracetam, wheat germ agglutinin (WGA), and concanavalin A (Con A), drugs that act via separate mechanisms to reduce desensitization evoked by L-glutamate in rat hippocampal neurons. The decay of excitatory synaptic currents, and sucrose-evoked miniature excitatory postsynaptic currents (EPSCs) was slowed 2- to 3-fold by aniracetam. In contrast, WGA increased the EPSC decay time constant only 1.3-fold and Con A had no effect. Aniracetam increased the magnitude of stimulus-evoked EPSCs 1.9-fold; variance analysis suggests a postsynaptic mechanism of action. WGA and Con A reduced EPSC amplitude via a presynaptic mechanism. Aniracetam increased the burst length of L-glutamate-activated single-channel responses. Simulations suggest that aniracetam either slows entry into a desensitized state or decreases the closing rate constant for ion channel gating.

Solubilization, characterization, and partial purification of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-, quisqualate-, kainate-sensitive L-glutamate binding sites from porcine brain synaptic junctions

L-[3H]Glutamate binding sites were solubilized from porcine brain synaptic junctions by Triton X-114 in the presence of KCl. The solubilized binding sites bound L-[3H]glutamate reversibly with KD and Bmax values of 1.48 +/- 0.18 microM and 178.2 +/- 15.9 pmol/mg of protein, respectively. These binding sites appeared to be integral membrane glycoproteins, with sugar moieties recognized by wheat germ agglutinin. A 49.3-fold purification of these binding sites was achieved by Triton X-114 solubilization, anion-exchange chromatography, and affinity chromatography using wheat germ agglutinin-Sepharose. The apparent molecular mass of the partially purified binding sites was 620 +/- 50 kDa. L-[3H]Glutamate bound to the solubilized preparation could be effectively displaced by agonists of non-N-methyl-D-aspartate (NMDA) L-glutamate receptors but not by NMDA or alpha-amino-4-phosphonobutyrate. The rank order for the competitive ligands in displacing L-[3H]glutamate was: quisqualate greater than alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid greater than L-glutamate greater than kainate.

2,4,5-Trihydroxyphenylalanine (6-hydroxy-dopa) displaces [3H]AMPA binding in rat striatum

Excitatory amino acid (EAA) receptor-mediated events have recently been implicated in dopaminergic mechanisms of neurotoxicity. 2,4,5-Trihydroxyphenylalanine (6-hydroxy-DOPA, TOPA), the ortho-hydroxylated derivative of the dopamine precursor 2,4-dihydroxyphenylalanine (L-DOPA), has recently been reported to have neurotoxic properties which are blocked by CNQX, a specific antagonist of the AMPA class of non-N-methyl-D-aspartate (non-NMDA) EAA receptors. We report here that 6-hydroxy-DOPA is a selective displacer of [3H]AMPA binding in rodent brain. 6-Hydroxy-DOPA was as potent as kainate in displacing [3H]AMPA binding, with an IC50 value of 32 microM. Ineffective displacers of [3H]AMPA binding included dopamine, 6-hydroxydopamine, L-DOPA, D-DOPA, carbidopa, DOPAC, beta-methylamino-L-alanine, 2,4-dihydroxyphenylacetyl-L-asparagine, homogentisic acid, 2,4-dihydroxyphenylacetic acid, amantadine, and threo-DOPS. 6-Hydroxy-DOPA (100 microM) also displaced 20% of [3H]kainate binding, but did not displace binding to NMDA, phencyclidine (PCP), or dopaminergic (D1 and D2) receptors. These data raise the possibility that 6-hydroxy-DOPA or another abnormal metabolite of L-DOPA could act as an excitotoxic agent via action at AMPA receptors. Given that non-NMDA receptors are postulated to play a role in neurotoxic events, these data provide an additional mechanism via which EAA receptor-mediated events could produce neurodegeneration in areas of brain with dopaminergic innervation.

Both NMDA and non-NMDA subtypes of glutamate receptors are concentrated at synapses on cerebral cortical neurons in culture

N-methyl-D-aspartate (NMDA) and non-NMDA receptors play a key role in synaptic transmission and plasticity in the vertebrate central nervous system. Previous studies have suggested that although both receptor types are present at synapses, the NMDA receptors may be relatively uniformly distributed. We have combined iontophoretic mapping of NMDA and non-NMDA receptors with immunohistochemical localization of synaptic vesicles along dendrites of single neocortical neurons to determine the relationship between NMDA and non-NMDA receptor distribution and the location of synapses. We find that when corrections for glutamate diffusion are made, NMDA responses are concentrated at focal "hot spots" that coincide with non-NMDA hot spots and that there is an excellent correlation between these hot spots and synapses.

The effect of xanthine/xanthine oxidase generated reactive oxygen species on synaptic transmission

The effect of reactive oxygen species generated by the interaction of xanthine and xanthine oxidase on synaptic transmission was examined at the squid giant synapse and the lobster neuromuscular junction. Exposure of these synaptic regions to xanthine/xanthine oxidase produced a significant depression in evoked release, with no change in either resting membrane properties or in the action potential. Addition of catalase to the xanthine/xanthine oxidase-containing media partially blocked the synaptic depression, indicating that H2O2 contributes to the synaptic changes induced by exposure to xanthine/xanthine oxidase. H2O2 applied directly to the perfusing media also produced a decrease in synaptic efficacy. The results demonstrate that reactive oxygen species, in general, depress evoked synaptic transmission.

The non-NMDA receptors: types, protein structure and molecular biology

In this article in the TiPS series on the pharmacology of excitatory amino acids Eric Barnard and Jeremy Henley report on the very recent advances in protein purification and molecular biology of their receptors. Although the NMDA receptor, which is the best characterized of these pharmacologically, has not yet yielded to recombinant DNA technology, several different clones encoding subunits of non-NMDA excitatory amino acid receptors have been reported. From the properties of these and of a purified non-NMDA receptor, and the pharmacology of the native responses to AMPA and kainate, the authors conclude that multiple non-NMDA subtypes exist and that one of these is a unitary receptor that can respond to both kainate and AMPA.