Permeation of calcium ions through non-NMDA glutamate channels in retinal bipolar cells

Developmental regulation of mRNAs encoding rat brain kainate/AMPA receptors: a northern analysis study

Functionally diverse kainate/alpha-amino-3-hydroxy-5-methyl-4- isoxazolepropionic acid (AMPA) receptors are generated by assembly of glutamate receptor (GluR)1, 2, and 3 subunits into homomeric and heteromeric channels. We examined GluR1, 2, and 3 gene expression in embryonic, neonatal, and adult rat brain by northern analysis under conditions of high stringency. In the adult, hybridization to a GluR1 riboprobe revealed the presence of an abundant RNA species, 5.2 kb in size, and minor bands of 3.2 and 3.9 kb. GluR2 hybridized to two species, 3.9 and 5.9 kb, of comparable abundance, presumably attributable to alternate splice products. Hybridization to the GluR3 riboprobe showed a major species of 5.2 kb. This pattern of RNA species was invariant over all the brain regions examined. Examination of GluR expression in development revealed that in the postnatal period, GluR1, 2, and 3 mRNAs are regulated as a function of age. In adult rat brain, GluR1 and 2 mRNA expression was highest in hippocampus; GluR3 was expressed at highest density in hippocampus and frontal cortex. The three transcripts were first detected at embryonic day 16 and then exhibited changes in expression levels in a region-specific manner. In hippocampus, all three transcripts exhibited elevated expression in the late neonatal period; in frontal cortex, elevated expression was observed for GluR2 and 3 only. In striatum, all three transcripts were expressed at relatively low levels throughout development, with a modest peak at postnatal day 14. In cerebellum, the GluR1 mRNA level was high from postnatal day 28 to adult.(ABSTRACT TRUNCATED AT 250 WORDS)

Different glutamate receptor channels mediate fast excitatory synaptic currents in inhibitory and excitatory cortical neurons

Spontaneous excitatory postsynaptic currents (sEPSCs) and responses to rapid application of glutamate were recorded in excitatory spiny, pyramidal neurons and compared with those recorded in inhibitory aspiny interneurons. The sEPSC decay time constant was faster in aspiny interneurons (2.5 ms) compared with pyramidal neurons (4.6 ms). The decay time constant in response to a brief application (1 ms) of glutamate (10 mM) in patches excised from pyramidal and aspiny interneurons were similar (1.9 and 2.7 ms, respectively). However, the rate of desensitization was faster in patches from interneurons compared with pyramidal neurons (3.4 and 12.0 ms, respectively). In addition, single-channel conductance was larger in aspiny interneurons (27 pS) compared with pyramidal neurons (9 pS). These results indicate that pyramidal neurons and aspiny interneurons express different non-N-methyl-D-aspartate receptors and that selective desensitization of interneuron receptors may contribute to depression of inhibition.

Induction of cerebellar long-term depression in culture requires postsynaptic action of sodium ions

Cerebellar long-term depression (LTD) is a persistent attenuation of the parallel fiber-Purkinje neuron (PF-PN) synapse induced by conjunctive stimulation of PF and climbing fiber (CF) inputs. A similar phenomenon is seen in the voltage-clamped PN in tissue culture when iontophoretic quisqualate application and PN depolarization are substituted for PF and CF stimulation, respectively. In this model, LTD induction requires activation of both AMPA and metabotropic receptors, together with PN depolarization. We have sought to determine the role of the AMPA receptor in LTD induction. The AMPA receptor does not appear to exert its effect by directly gating Ca2+ influx. Replacement of external Na+ during quisqualate/depolarization conjunction with permeant ions caused a blockade of LTD induction, suggesting that Na+ influx through the AMPA-associated channel is necessary for this process. Similarly, pairing quisqualate pulses with depolarizing steps near ENa also failed to induce LTD. The present results indicate that postsynaptic Na+ influx is necessary for LTD induction. While a portion of the relevant Na+ influx is provided by voltage-gated channels, the AMPA-associated ion channel is most important in this regard.

Treatment with an AMPA antagonist 12 hours following severe normothermic forebrain ischemia prevents CA1 neuronal injury

The neuroprotective effects of 2,3-dihydroxy-6-nitro-7- sulfamoylbenzo(f)quinoxaline (NBQX), GYKI 52466, and MK-801 were tested following severe forebrain ischemia. Wistar rats were subjected to 10 min of normothermic ischemia and reperfused for 7 days. Necrotic hippocampal CA1 neurons were counted and expressed as a percentage (mean +/- SD). In Experiment 1, saline-treated rats sustained 81 +/- 20% damage to dorsal CA1. Rats given NBQX 30 mg/kg i.p. x3 lost 21 +/- 27% (p < 0.01). Neither MK-801 1 mg i.p. x3 alone, nor in combination with the cytoprotective dose of NBQX protected CA1, with 83 +/- 18 and 54 +/- 34% damage, respectively (NS). Giving NBQX 90 mg/kg i.v. did not protect cells (94 +/- 5%) and resulted in nephrotoxicity. In Experiment 2, rats were given saline or three doses of NBQX 30 mg/kg i.p. immediately at reperfusion (RP) or after a 6-, 12-, or 24-h delay. Saline-treated rats suffered 79 +/- 16% injury. NBQX given immediately resulted in 17 +/- 17% injury, and even if treatment was delayed by either 6 or 12 h, there was marked protection with only 27 +/- 32 and 25 +/- 17% injury, respectively (all p < 0.01). Delaying the initiation of treatment to 24 h was not successful, resulting in 50 +/- 28% injury (NS). In Experiment 3, saline-treated rats lost 81 +/- 19% of CA1 cells, while those given GYKI 52466 10 mg/kg i.p. x5 starting immediately following RP lost 80 +/- 14%.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronological atrophy after transient middle cerebral artery occlusion in rats

We studied the development of brain atrophy after transient focal ischemia in rats. The animals were subjected to cerebral ischemia induced by embolization of the right middle cerebral artery (MCA) for 60 min. The brains were studied morphologically 7 days, 1 month, 3 months and 9 months after recirculation. In addition, the effects of a new calcium antagonist, KB-2796, and a glutamate receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX); were evaluated in this model 1 month after ischemia. The hemispheric volume of the ipsilateral ischemic side, expressed as a percentage of that on the contralateral non-ischemic side, was 99% in the sham operation group, 94% at 7 days, 87% at 1 month, 68% at 3 months and 65% at 9 months. Atrophy of the striatum and cortex, but not the hippocampus, was observed 1 month after ischemia. Atrophy of the ipsilateral substantia nigra and the thalamus, which are remote from the ischemic region, was observed 7 days and 1 month, respectively, after ischemia. Correlations between the extent of the atrophy in the striatum and that in the substantia nigra and between the extent of the atrophy in the cortex and in the thalamus were statistically significant. Treatment with KB-2796 or DNQX administered intraperitoneally at a dose of 10 mg/kg twice 30 min before ischemia and immediately after ischemia was effective in reducing the extent of atrophy in both the ipsilateral ischemic and non-ischemic regions. These results suggest that brain atrophy on the ipsilateral ischemic side develops time-sequentially after transient focal ischemia and that ischemia affects not only the primary ischemic focus but also remote regions through transsynaptic connections, and that KB-2796 and DNQX have beneficial effects on atrophy in the chronic phase after ischemia.

Glutamate and GABA in retinal circuitry

Glutamate and GABA have been identified as the major neurotransmitters in the radial and lateral synaptic pathways, respectively, of the vertebrate retina. Over the past year or so new information has appeared that has significantly increased the knowledge of how these compounds can elicit a range of responses. Key features of this new information are the identification and localization of many receptor subtypes within the retina, the recognition that glutamate can modulate membrane potential through cGMP-gated ion channels, and the finding that GABA can be released through non-vesicular pathways.

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.

Ionotropic non-N-methyl-D-aspartate agonists induce retraction of dendritic spinules from retinal horizontal cells

Horizontal cells invaginate the photoreceptors in the retina and form reciprocal synaptic connections in the cone pedicles. In fish retina the pattern of synaptic connections is plastic and modulated by the ambient light conditions. Numerous dendritic spinules protrude from the terminal horizontal-cell dendrites into the cone pedicle when the retina is light-adapted and are retracted during dark adaptation. The retraction of spinules can be induced during maintained illumination by an injection of the putative cone transmitter L-glutamate or its analogue kainic acid into the vitreous humor. The formation and the retraction of spinules have a time course of minutes. Activation of protein kinase C through phorbol esters initiates the formation of spinules, but the retraction has not yet been linked to a specific second messenger. Herein we report that physiological concentrations of the glutamate analogs quisqualic acid and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid induce retraction of spinules during maintained illumination. (+/-)-trans-1-Amino-1,3-cyclopentanedicarboxylic acid, an agonist for the metabotropic quisqualic acid receptor, was without effect on spinule retraction. N-Methyl-D-aspartate and L-2-amino-4-phosphonobutyric acid, agonists at other types of glutamate receptors, were also without any effect. The effects of the active agonists persisted when synaptic transmission was blocked. In the presence of the ionotropic quisqualate receptor antagonist 6-cyclo-7-nitro-quinoxaline-2,3-dione the effects of all active agonists were blocked. These results demonstrate that activation of ionotropic quisqualate receptors on the horizontal-cell membrane can induce dendritic spinule retraction, a process associated with dark adaptation.

Kainate elicits elevated nuclear calcium signals in retinal neurons via calcium-induced calcium release

Intracellular Ca2+ was imaged in cultured neonatal rat retinal neurons using the Ca(2+)-sensitive dye fluo-3 and confocal scanning laser microscopy. Depolarization via elevation of bath K+ concentration resulted in large cytoplasmic and nuclear Ca2+ signals; responses in the nucleus exceeded those of the cytoplasm. Glutamate or kainate application elicited the same intracellular pattern of elevated Ca2+ signals. Kainate stimulation was blocked by the non-NMDA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and greatly reduced by removing Ca2+ from the bath and adding ethylene glycol-bis (beta-amino-ethyl ether) N,N,N',N'-tetraacetic acid (EGTA). Kainate was equally effective in eliciting Ca2+ signals when bath Na+ was replaced with equimolar concentrations of choline, or in the presence of the NMDA receptor antagonist, 2-amino-5-phosphonovaleric acid (APV). Caffeine treatment significantly reduced the kainate-induced intracellular Ca2+ response. These results suggest that Ca2+ can enter through the kainate receptor of retinal neurons and amplify the Ca2+ signals in the cytoplasm and nucleus by releasing Ca2+ from intracellular stores.

Fractional contribution of calcium to the cation current through glutamate receptor channels

The Ca2+ fraction of the ion current flowing through glutamatergic NMDA and AMPA/kainate receptor channels was determined in forebrain neurons of the medial septum. The neurons were overloaded with the Ca2+ indicator dye fura-2 (1 mM) via the recording patch pipettes. This approach allowed the direct determination of the Ca2+ influx from changes in the Ca(2+)-sensitive fura-2 fluorescence. We found that, at negative membrane potentials and at an extracellular free Ca2+ concentration of 1.6 mM, the Ca2+ fraction of the current through the NMDA receptor channels is only 6.8%, about 2-fold lower than previously estimated from reversal potential measurements. Interestingly, a quite high fractional Ca2+ current of 1.4% was determined for the linearly conducting AMPA/kainate receptor channels found in these neurons.

Mammalian ionotropic glutamate receptors

Exciting new milestones in glutamate receptor (GluR) channel research include the following: the cloning of N-methyl-D-aspartate (NMDA) receptors; delineation of molecular determinants for ion flow through glutamate-gated channels; the discovery that Ca2+ permeability of non-NMDA receptor channels is determined by RNA editing; the construction of antibodies and their use in immunocytochemical localizations of alpha-amino-3-hydroxy-5-methyl isoxazole-4-propionic acid (AMPA) receptor subunits in the rat brain; and the return to prominence of the high-affinity kainate site with the publication of cDNA sequences for subunits (GluR-5, -6, -7; KA-1, -2) constituting subtypes of this site. Major unresolved issues comprise the transmembrane topology and subunit stoichiometries of native receptor channels.

Rat retinal ganglion cells express Ca(2+)-permeable non-NMDA glutamate receptors during the period of histogenetic cell death

Local application of glutamate agonists to retinal ganglion cells (RGNs) was performed in retinae isolated from pigmented rats aged between 3 and 8 days postnatally. A vast majority of RGNs displayed current responses to glutamate (Glu), N-methyl-D-aspartate (NMDA), quisqualate (QA), alpha-amino-2,3-dihydro-5-methyl-3-oxo-4-isoazolepropanoic acid (AMPA), kainate (KA) and domoate (DA). In Na(+)-free extracellular solution with elevated Ca2+, non-NMDA agonists elicited large (up to 200 pA) inward currents that were completely blocked by 6,7-dinitroquinoxaline-2,3-dione (DNQX) and Cd2+. MK-801 also induced a partial block of cationic currents in Na(+)-free saline. In standard salt solutions, current-voltage relationships of Glu-R-mediated currents were often inwardly rectifying in the presence of D-aminophosphonovalerat (D-APV), as is typical of Ca(2+)-permeable non-NMDA receptors. The presence of inward rectification in the current voltage relationship was always associated with a high value of the cationic permeability ratio PCa2+/PCs+ (> 0.8). However, in about half of the investigated RGNs no inward rectification was observed under standard recording conditions. Our results lead to the suggestion that expression of Ca(2+)-permeable Glu receptor subunits may contribute to regulation of cell numbers in the postnatal retina.

Calcium permeability of non-N-methyl-D-aspartate receptor channels in immature cerebellar Purkinje cells: studies using fura-2 microfluorometry

Using fura-2 microfluorometry, I investigated the mechanism by which non-N-methyl-D-aspartate (NMDA) receptor agonists increase the cytosolic free calcium concentration ([Ca]in) in single cerebellar Purkinje cells isolated from 3-10-day-old rats. Kainate and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate dose-dependently increased the cytosolic free Na+ concentration, which was measured using sodium-binding benzofuran isophthalate microfluorometry, confirming the Na+ influx through ion channels linked to non-NMDA receptors. The [Ca2+] increases induced by relatively lower concentrations of agonists were entirely dependent on external Ca2+ and were reduced by removal of external Na+ or by addition of a Ca2+ channel blocker, D600. The results indicate that the non-NMDA agonist-induced [Ca]in increase was due mainly to Ca2+ influx through voltage-dependent Ca2+ channels, which were activated by a massive Na+ influx. On the other hand, higher concentrations of agonists dose-dependently increased [Ca]in under conditions in which activation of voltage-dependent Ca2+ channels were blocked by a combination of Na+ removal with D600. These [Ca]in increases were Ca2+ dependent and little affected by adding a competitive NMDA antagonist. Non-NMDA agonists also stimulated influxes of Mn2+ and Co2+, both of which can be monitored by quenching fura-2 fluorescence under the same conditions. These results suggest that ion channels linked to non-NMDA receptors on immature Purkinje cells are permeable to Ca2+, Mn2+, and Co2+.

Region-specific expression of subunits of ionotropic glutamate receptors (AMPA-type, KA-type and NMDA receptors) in the rat spinal cord with special reference to nociception

The present study attempted to explore the gene expression of the subunits (GluR1-4) of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type receptor, subunit (GluR5) of kainic acid (KA)-type receptor, NR1 [a subunit of N-methyl-D-aspartate (NMDA) receptors] and the possible glutamate-binding subunit of an NMDA receptor complex in the dorsal horn of the rat spinal cords using in situ hybridization histochemistry. These results were compared with those of the spinal motor neurons. Expression of the subunits of the AMPA-type receptor was also examined at the protein level using immunocytochemistry, with reference to the motor neurons. Although all the four subunits of the AMPA-type receptor were expressed throughout the dorsal horn, the pattern of expression was different according to the dorsal horn region and to the subunits. GluR2 showed the strongest expression in the dorsal horn. Huge numbers of strongly labelled cells formed a dense collection in lamina II and superficial parts of lamina III. Many neurons in lamina II and superficial parts of lamina III expressed GluR1 moderately. Scattered neurons moderately expressing GluR3 were also seen in these regions, while the expression of GluR4 was very low. Labelling of the dorsal horn neurons by the GluR5 probe was low, and NR1 probe and a glutamate-binding subunit of an NMDA receptor complex probe labelled them diffusely with low to moderate intensity. These findings show a close relationship between the glutamergic nociceptive primary afferent system and AMPA-type receptors in which GluR2 is especially highly expressed. The present study further showed that the expression pattern of the glutamate receptors in the spinal sensory neurons differs considerably from that of spinal motor neurons. Motor neurons very strongly express GluR3 and 4, while the expression of GluR2 and GluR1 is moderate and low, respectively. Expression of GluR5 is also low in the motor neurons. However, expression of NR1 and the glutamate-binding subunit of an NMDA receptor complex is very strong. These findings indicate that the subunit composition of the AMPA-type receptors regulating motor neurons is different from that of the AMPA-type receptors in the spinal sensory neurons, and that there are at least two kinds of glutamergic systems which regulate motor neurons: via AMPA-type receptors and via NMDA receptors.

Interplay between expressed non-NMDA receptors and endogenous calcium-activated chloride currents in Xenopus laevis oocytes

Current-voltage (I-V) relationships of glutamate receptors activated by the non-NMDA receptor agonist, kainate (KA), were determined in Xenopus laevis oocytes injected with either calf or chick brain mRNA. In most injected oocytes (n = 44; 84%), I-V plots to 80 microM KA exhibited inward rectification; however, in some oocytes (n = 7; 16%), the KA-evoked membrane current showed only slight outward rectification. A comparison between the resting membrane properties of injected oocytes with the KA-evoked currents revealed that weak outward rectification was often observed in oocytes possessing predominant voltage-activated calcium-dependent chloride currents (ICl(Ca)). In these oocytes, replacement of extracellular calcium (2 mM Ca2+) with magnesium ions (Mg2+), or the addition of the chloride channel blocker, anthracene-9-carboxylic acid (500 microM to 1 mM A9C), reduced ICl(Ca) and also altered the nature of the KA I-V plot revealing inward rectification. It is proposed that the responses mediated by expressed non-NMDA receptors may be influenced by the activation of endogenous calcium-dependent membrane currents in Xenopus laevis oocytes.

Long-term potentiation in the avian hippocampus does not require activation of the N-methyl-D-aspartate (NMDA) receptor

Evoked potentials recorded from slices of pigeon hippocampus were reversibly attenuated by 2 mM kynurenic acid. High frequency stimulation (3 x 200 Hz for 1 sec, with 1 sec intervals) evoked a persistent increase in the evoked potential, lasting in a nondecremental form for at least 2 hr. This increase in the magnitude of the potential was not blocked by antagonists of the NMDA receptor (APV and MK-801). These data suggest that the synaptic facilitation observed in the pigeon hippocampus, which appears to be a form of long-term potentiation, does not depend on the activation of NMDA receptors.

The differential expression patterns of messenger RNAs encoding non-N-methyl-D-aspartate glutamate receptor subunits (GluR1-4) in the rat brain

The messenger RNA expression of non-N-methyl-D-aspartate glutamate receptor subunits (GluR1-4), considered alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid type, was investigated in rat brain by in situ hybridization histochemistry using oligonucleotide probes specific to each subunit sequence. GluR1-4 subunit messenger RNAs were expressed widely and abundantly throughout the CNS. However, the combination of expression pattern varied notably according to location. GluR2 messenger RNA was expressed most strongly and widely, with most areas except the Bergmann glia containing this messenger RNA. GluR4 messenger RNA was also present widely, although the expression level was low. However, we observed many areas which lacked or expressed very little GluR1 messenger RNA, such as some nuclei in the general motor system and auditory system. In addition, some nuclei in the hypothalamus and general somatosensory system lacked or expressed very little GluR3 messenger RNA. These results suggest that in the rat CNS non-N-methyl-D-aspartate receptors varied their composition according to the area where they were expressed, and that the combination pattern might be related to the functional role of neurons.

Synaptically activated increases in Ca2+ concentration in hippocampal CA1 pyramidal cells are primarily due to voltage-gated Ca2+ channels

Changes in intracellular Ca2+ concentration ([Ca2+]i) in the soma and dendrites of hippocampal CA1 pyramidal neurons were measured using intracellularly injected fura-2. A large component of the [Ca2+]i elevation caused by high frequency stimulation of the Schaffer collaterals was correlated with the Na+ spikes triggered by the excitatory postsynaptic potentials (EPSPs). These spikes were generated in the soma and proximal dendrites and stimulated Ca2+ entry through voltage-gated Ca2+ channels. Suppressing spikes by hyperpolarizing the soma or by injecting QX-314 revealed a smaller nonspike component of Ca2+ entry. A substantial fraction of this component was mediated by the action of the EPSPs on voltage-gated Ca2+ channels, because it persisted in 2-amino-5-phosphonovaleric acid and because it was usually reduced when Ca2+ channel activity was suppressed by hyperpolarization. Ca2+ entry through the N-methyl-D-aspartate receptor channel could not be detected with certainty, perhaps because it was highly localized.

Ca2+ permeability of unedited and edited versions of the kainate selective glutamate receptor GluR6

The Ca2+ permeability of the kainate selective glutamate receptor GluR6 depends on the editing of the RNA (or DNA). The unedited version of GluR6, GluR6Q, encodes a glutamine at position 621 (Q/R site) and exhibits a Ca2+/monovalent ion permeability ratio of 1.2, while the edited version of GluR6, GluR6R, encodes an arginine at position 621 and exhibits a permeability ratio of 0.47. Kainate activation of the GluR6 receptor results in currents that are modulated by extracellular calcium ions. Permeability ratios of other divalent ions indicate that the Q/R site is not the only determinant for divalent ion permeability. The level of editing of the receptor will determine the Ca2+ influx through the GluR6 receptor channels and, consequently, may modulate the synaptic activity.

Glutamate: its role in learning, memory, and the aging brain

L-Glutamate is the most abundant of a group of endogenous amino acids in the mammalian central nervous system which presumably function as excitatory neurotransmitters and under abnormal conditions may behave as neurotoxins. As neurotransmitters, these compounds are thought to play an important role in functions of learning and memory. As neurotoxins, they are believed to be involved in the pathogenesis of a variety of neurodegenerative disorders in which cognition is impaired. Moreover, brain structures which are considered anatomical substrata for learning and memory may be particularly vulnerable to the neurotoxic actions of these excitatory amino acids, especially in the elderly who are also the segment of the population most susceptible to impairments of mnemonic function. This paper is a review of data concerning the role of excitatory amino acids in the processes of learning and memory and in the pathogenesis and treatment of disorders thereof.

Blockade of NMDA receptor-mediated mobilization of intracellular Ca2+ prevents neurotoxicity

NMDA receptor activation leads to elevated Ca2+ in cultured rat cortical and retinal ganglion cell neurons. If excessive, this Ca2+ response is associated with delayed neurotoxicity. We used dantrolene and ionomycin to test if the Ca2+ response to NMDA was due to mobilization of intracellular Ca2+ stores rather than merely to Ca2+ influx. In the presence of EGTA, ionomycin resulted in release and subsequent depletion of intracellular Ca2+ stores. Henceforth, despite normal extracellular Ca2+, NMDA elicited only about half of its former Ca2+ response. Similarly, when dantrolene was used to block Ca2+ release from intracellular stores, we observed > 50% smaller NMDA-evoked Ca2+ responses. These results quite surprisingly indicate that at least half the Ca2+ response to NMDA is due to release of intracellular Ca2+, a process triggered by influx of extracellular Ca2+. Dantrolene also protected neurons from NMDA receptor-mediated neurotoxicity. Release of intracellular Ca2+ may therefore be a necessary step in the cascade leading to neuronal damage induced by excessive NMDA receptor stimulation and may be amenable to pharmacological intervention.

Differential roles of NMDA and non-NMDA receptor activation in induction and maintenance of thermal hyperalgesia in rats with painful peripheral mononeuropathy

Central activation of excitatory amino acid receptors has been implicated in neuropathic pain following nerve injury. In a rat model of painful peripheral mononeuropathy, we compared the effects of non-competitive NMDA receptor antagonists (MK 801 and HA966) and a non-NMDA receptor antagonist (CNQX) on induction and maintenance of thermal hyperalgesia induced by chronic constrictive injury (CCI) of the rat common sciatic nerve. Thermal hyperalgesia to radiant heat was assessed by using a foot-withdrawal test and NMDA/non-NMDA receptor antagonists were administered intrathecally onto the lumbar spinal cord before and after nerve injury. Four daily single treatments with 20 nmol HA966 or CNQX beginning 15 min prior to nerve ligation (pre-injury treatment), reliably reduced thermal hyperalgesia in CCI rats on days 3, 5, 7 and 10 after nerve ligation. Thermal hyperalgesia was also reduced in CCI rats receiving a single post-injury treatment with HA966 (20 or 80 nmol) or MK 801 (5 or 20 nmol) on day 3 after nerve ligation when thermal hyperalgesia was well developed. In contrast, a single post-injury CNQX (20 or 80 nmol) treatment failed to reduce thermal hyperalgesia or to potentiate effects of HA966 or MK 801 (5 or 20 nmol) on thermal hyperalgesia in CCI rats. Moreover, multiple post-injury CNQX treatments utilizing the same dose regime as employed for the pre-injury treatment attenuated thermal hyperalgesia but only when the treatment began 1 or 24 h (but not 72 h) after nerve ligation.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular diversity of glutamate receptors and implications for brain function

The glutamate receptors mediate excitatory neurotransmission in the brain and are important in memory acquisition, learning, and some neurodegenerative disorders. This receptor family is classified in three groups: the N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-kainate, and metabotropic receptors. Recent molecular studies have shown that many receptor subtypes exist in all three groups of the receptors and exhibit heterogeneity in function and expression patterns. This article reviews the molecular and functional diversity of the glutamate receptors and discusses their implications for integrative brain function.

Effects of L- and N-type Ca2+ channel antagonists on excitatory amino acid-evoked dopamine release

In the present study we tested the effect of dihydropyridine (DHP) Ca2+ channel antagonists and of omega-conotoxin GVIA on [3H]dopamine (DA) release evoked by the activation of excitatory amino acid (EAA) receptors in cultures of fetal rat ventral mesencephalon, in order to investigate the role of voltage-sensitive L- and N-type Ca2+ channels in these EAA-mediated processes. Micromolar concentrations (10-30 microM) of DHP L-type Ca2+ channel antagonists inhibited [3H]DA release evoked by N-methyl-D-aspartate (NMDA), kainate, quisqualate or veratridine. [3H]DA release evoked by the L-type Ca2+ channel agonist, Bay K 8644, was inhibited by lower concentrations (0.1-1 microM) of the DHP antagonist, nitrendipine, than was the release evoked by EAAs. The DHP antagonist, (+)-PN 200-110, was more potent than (-)-PN 200-110 in inhibiting [3H]DA release evoked by Bay K 8644, but the two stereoisomers were equipotent in inhibiting NMDA-evoked release. These results indicate that activation of L-type Ca2+ channels is able to evoke [3H]DA release. However activation of L-type channels is not involved in EAA-induced [3H]DA release and therefore inhibition of EAA-induced [3H]DA release by micromolar concentrations of DHPs must be mediated by actions other than inhibition of L-type Ca2+ channels. omega-Conotoxin GVIA (3 microM) had no effect on [3H]DA release evoked by Bay K 8644, indicating that the toxin may selectively inhibit N-type channels in this preparation.(ABSTRACT TRUNCATED AT 250 WORDS)

Are Ca(2+)-permeable kainate/AMPA receptors more abundant in immature brain?

The permeability properties of kainate/AMPA receptors are determined by subunit composition. The GluR1 and GluR3 subunits form Ca(2+)-permeable channels and exhibit inward rectification; heteromeric receptors containing the GluR2 subunit are Ca(2+)-impermeable and electrically linear. These observations raise the possibility of a developmental 'switch' in which turning on or off of GluR2 expression regulates the level of Ca2+ permeable kainate/AMPA receptors. We examined the ratio of GluR1 and GluR3 to GluR2 gene expression in developing and adult rat brain by in situ hybridization. A larger value of this ratio is likely to be associated with greater Ca2+ permeability. Our data suggest that in neocortex, striatum and cerebellum the number of Ca(2+)-permeable kainate/AMPA receptors is high at P4 and declines monotonically with age. In hippocampus, the number increases from P7 to P21, after which it declines. These findings provide evidence for a developmental 'switch' in which Ca2+ permeable glutamate receptors are turned off following early developmental events.

Regulation of neurotransmitter enzyme by quisqualate subtype glutamate receptors in cultured cerebellar and hippocampal neurons

The possible involvement of ionotropic and metabotropic quisqualate (QA) receptors in neuronal plasticity was studied in cultured glutamatergic cerebellar or hippocampal cells in terms of the specific activity of phosphate-activated glutaminase, an enzyme important in the synthesis of the putative neurotransmitter pool of glutamate. When cerebellar or hippocampal neurons were treated with QA, it elevated the specific activity of glutaminase in a dose-dependent manner. The half-maximal effect was obtained at about 0.1 microM, the maximum increase was at about 1 microM, but levels higher than 10 microM QA produced progressive reduction in glutaminase activity. In contrast, QA had little effects on the activities of lactate dehydrogenase and aspartate aminotransferase and the amount of protein, indicating that the increase in glutaminase was relatively specific. The QA-mediated increase in glutaminase was mimicked by the ionotropic QA receptor agonist alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA; EC50, about 0.5 microM), but not by the metabotropic QA receptor agonist trans-(+-)-1-amino-cyclopentyl-1,3,dicarboxylate (t-ACPD; up to 0.5 mM). The specific ionotropic QA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) inhibited QA- and AMPA-mediated increases in glutaminase activity in a dose-dependent manner, whereas other glutamate receptor antagonists, D,L-2-amino-5-phosphonovalerate, gamma-D-glutamyl aminomethyl sulphonic acid and gamma-D-glutamyl diethyl ester were ineffective. The elevation of neurotransmitter enzyme was Ca(2+)-dependent.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

Magnesium-dependent inhibition of agonist-stimulated phosphoinositide breakdown in rat cortical slices by excitatory amino acids

The excitatory amino acid agonists kainate, N-methyl-D-aspartate (NMDA), and quisqualate inhibited ligand-stimulated phosphoinositide hydrolysis in rat cortical slices. The NMDA channel blocker MK-801 antagonized the inhibition by NMDA but had no effect on the inhibition due to kainate or quisqualate. The antagonist 6-cyano-7-nitroquinoxaline-2,3-dione blocked the effects of quisqualate and kainate but not the effect of NMDA. These data indicate that activation of the NMDA, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid, and kainate types of ionotropic receptors has the same effect. In membranes prepared from cortical slices, there was no inhibition of carbachol-stimulated phosphoinositidase C activity by excitatory amino acids, suggesting that excitatory amino acids indirectly affect carbachol-stimulated phosphoinositide hydrolysis. The inhibition by excitatory amino acids of carbachol-stimulated phosphoinositide breakdown was dependent on extracellular Mg2+ and was abolished by procedures that increase intracellular Ca2+. Veratridine inhibition of carbachol-stimulated phosphoinositide hydrolysis was reversed by ouabain but not by other procedures that increase intracellular Ca2+. In contrast to excitatory amino acids, veratridine potentiated carbachol-stimulated phosphoinositide breakdown in the presence of 10 mM extracellular Mg2+. These data suggest that excitatory amino acids inhibit carbachol-stimulated phosphoinositide breakdown in rat cortex by lowering intracellular Ca2+ through a mechanism dependent on extracellular Mg2+.

Determinants of the calcium permeation of ligand-gated cation channels

Fast synaptic transmission in the vertebrate brain is mediated by ligand-gated channel receptors. As some of these receptors have been implicated in learning and memory, it is important to understand their mechanism of action at a molecular level. Excitatory receptors are members of large gene families of related channels that are gated by acetylcholine, serotonin, and the most abundant neurotransmitter, glutamate. Within the last year, a number of important studies have focused on the ability of these channels to flux calcium ions. Calcium entry into neurons through some of these channels triggers biochemical cascades, which can lead to changes in synaptic efficacy, presumed to be a requisite for memory formation, or if it occurs in excess, to cell death. Recent studies that attempt to determine the channel structures responsible for this calcium conductance will be discussed.

Developmental changes in the effects of drugs acting at NMDA or non-NMDA receptors on synaptic transmission in the chick cochlear nucleus (nuc. magnocellularis)

The developmental pharmacology of excitatory amino acid (EAA) receptors in the chick cochlear nucleus (nucleus magnocellularis, NM) was studied by means of bath application of drugs and recording of synaptically-evoked field potentials in brain slices taken from chicks aged embryonic day (E) 14 through hatching (E21). The abilities of various EAA agonists (N-methyl-D-aspartate [NMDA], kainic acid, and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid [AMPA]) to suppress postsynaptic responses by depolarization block and of EAA antagonists ((3-[RS]-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid [CCP], dizocilpine [MK-801], 6-nitro-7-sulfamoyl-benzo(F)quinoxaline-2,3 dione [NBQX], 6-cyano-7-nitroquinoxaline-2,3-dione [CNQX] and 6,7-dinitroquinoxaline-2,3-dione [DNQX]) to suppress these responses directly were assessed quantitatively. The results support the existence of NMDA receptors in NM and suggest that the ability of these receptors to influence synaptically-evoked responses declines dramatically during the last week of embryonic life. The results similarly suggest that the non-NMDA receptors in NM undergo changes in density and/or function during a period of development when the cochlear nucleus is undergoing a variety of morphological and functional transformations.

Induction of cobalt accumulation by excitatory amino acids within neurons of the hippocampal slice

Computer-assisted image analysis was used to establish the dose response of excitatory amino acid (EAA) analogs on the induction of cobalt accumulation within pyramidal and granule cell neurons in 400 microns slices of gerbil hippocampus. Slices were incubated 20 min at 22 degrees C in a solution containing 5 mM CoCl2 and 0-1,000 microM EAA analog. The cobalt was visualized by development in (NH4)2S, and the slices were digitized for quantitative densitometry. Kainic acid (KA) had the largest effect and induced cobalt accumulation in the dentate gyrus and CA1, 180% and 150% above control, respectively, with an ED50 = 30 microM. alpha-Amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) induced accumulations of cobalt in CA1 and hilar neurons 130% above control with an ED50 = 30 microM, but had little effect on dentate granule cells. The accumulations induced by KA and AMPA were blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), but not by DL-2-amino-5-phosphonovaleric acid (AP5) or voltage-dependent calcium channel blockers. N-Methyl-D-aspartate (NMDA) induced accumulation in the dentate and CA1 150% above control in a pattern similar to KA, but with an ED50 of 100 microM. The accumulation was blocked by both AP5 and CNQX. These data indicate that cobalt-permeable, receptor-activated divalent cation channels are differentially distributed within the gerbil hippocampus, and have differential sensitivities to non-NMDA agonists. The localization of KA-activated, cobalt-permeable channels appears to be coincident with the flop form of the AMPA-selective calcium-permeable glutamate receptor-activated channel.(ABSTRACT TRUNCATED AT 250 WORDS)

Divalent ion permeability of AMPA receptor channels is dominated by the edited form of a single subunit

Functionally diverse GluR channels of the AMPA subtype are generated by the assembly of GluR-A, -B, -C, and -D subunits into homo- and heteromeric channels. The GluR-B subunit is dominant in determining functional properties of heteromeric AMPA receptors. This subunit exists in developmentally distinct edited and unedited forms, GluR-B(R) and GluR-B(Q), which differ in a single amino acid in transmembrane segment TM2 (Q/R site). Homomeric GluR-B(R) channels expressed in 293 cells display a low divalent permeability, whereas homomeric GluR-B(Q) and GluR-D channels exhibit a high divalent permeability. Mutational analysis revealed that both the positive charge and the size of the amino acid side chain located at the Q/R site control the divalent permeability of homomeric channels. Coexpression of Q/R site arginine- and glutamine-containing subunits generates cells with varying divalent permeabilities depending on the amounts of expression vectors used for cell transfection. Intermediate divalent permeabilities were traced to the presence of both divalent permeant homomeric and impermeant heteromeric channels. It is suggested that the positive charge contributed by the arginine of the edited GluR-B(R) subunit determines low divalent permeability in heteromeric GluR channels and that changes in GluR-B(R) expression regulate the AMPA receptor-dependent divalent permeability of a cell.

Differential expression of glutamate receptor genes (GluR1-5) in the rat retina

The recent isolation of at least five different cDNAs encoding functional subunits of glutamate receptors (GluR1 to GluR5) has revealed a diversity whose function is not understood. To learn more about how these different receptor subunits are used in the brain, we undertook an in situ hybridization study of the retina to define how the different glutamate receptor genes are expressed. We chose the retina because the glutamate sensitivities of its different cell types have been characterized, and these different neurons reside in different laminae. Hybridization of [35S]UTP-labeled cRNA probes with transverse sections and freshly dissociated cells reveals that all five receptor subunits are expressed in the retina. Hybridization signal is detected in different, but overlapping, sets of cells in the retina. GluR1, GluR2, and GluR5 are expressed by many somata, and GluR4 by a few, in the outer third of the inner nuclear layer, where the horizontal cells reside. Transcripts for GluR1, GluR2, and GluR5 are found in the somata within the middle third of the inner nuclear layer, which is where the bipolar cell somata are located, and GluR2 probes label freshly dissociated rod bipolar cells. All of the probes produce labeling over the cells at the inner edge of the inner nuclear layer, which are probably amacrine cells, as well as over the cell bodies in the ganglion cell layer.

Further evidence for calcium permeability of non-NMDA receptor channels in hippocampal neurons

We have previously suggested that kainic acid (KA) increases the fluorescence of a fluoroprobe for Ca2+ via activation of the non-NMDA (N-methyl-D-aspartic acid) receptor in cultured hippocampal cells. The present study provides further evidence to support this suggestion. KA and NMDA increased the fluorescence occasionally in different subcellular loci of a single cell for each agonist, excluding the possibility that KA activated the NMDA receptor. KA elevated fluorescence in the cell whose membrane was voltage-clamped, thus eliminating possible Ca2+ entry through voltage-dependent channels. A monophasic response to KA was found to occur in the cells identified immunochemically as neurons. Even when the cells were cultured at different embryonic stages, KA responsiveness reached a peak at a rather fixed timepoint through the total age of neurons, while NMDA responsiveness continued to increase. This suggests that the Ca(2+)-permeable non-NMDA receptor is expressed according to a genetic schedule, but not to an artifactual effect of cell culture.

Calcium influx through nicotinic receptor in rat central neurons: its relevance to cellular regulation

The Ca2+ permeability of a nicotinic acetylcholine receptor (nAChR) in the rat CNS was determined using both current and fluorescence measurements on medial habenula neurons. The elementary slope conductance of the nAChR channel was 11 pS in pure external Ca2+ (100 mM) and 42 pS in standard solution. Ca2+ influx through nAChRs resulted in the rise of cytosolic Ca2+ concentration ([Ca2+]i) to the micromolar range. This increase was maximal under voltage conditions (below -50 mV) in which Ca2+ influx through voltage-activated channels was minimal. Ca2+ influx through nAChRs directly activated a Ca(2+)-dependent Cl- conductance. In addition, it caused a decrease in the GABAA response that outlasted the rise in [Ca2+]i. These results underscore the physiological significance of Ca2+ influx through nAChR channel in the CNS.

Blockade of the AMPA receptor prevents CA1 hippocampal injury following severe but transient forebrain ischemia in adult rats

The cytoprotective effect of NBQX, a selective AMPA receptor antagonist, was tested following 10 min of severe forebrain ischemia using the 4-vessel occlusion model. Immediately, and at 15 and 30 min following reperfusion, adult Wistar rats received intraperitoneal injections of either saline (n = 5), 1 mg lithium chloride (n = 17) or 30 mg/kg of the lithium salt of NBQX (n = 18). In saline-treated animals 82 +/- 12% of CA1 hippocampal neurons were lost. Of those treated with lithium 70 +/- 23% were injured, while those given NBQX sustained only 40 +/- 34% CA1 necrosis (P less than 0.01). Twelve of 18 NBQX-treated animals had less than 30% CA1 injury as compared with 1 of 17 lithium-treated animals. The AMPA receptor may play a more important role than the NMDA receptor in selective ischemic necrosis of hippocampal neurons.

Transferrin can alter physiological properties of retinal neurons

The role of transferrin as a possible neurotransmitter was examined in cultured chick retinal cells. Brief exposure to transferrin caused a dramatic and transient increase in intracellular calcium levels in approximately 20% of the total population of cultured retinal neurons. The increase in intracellular calcium was observed in cell bodies and neuronal processes. Electrophysiological analysis of a subset of the population, bipolar-like neurons, demonstrated that more than half of these cells responded to the application of transferrin with a transient membrane depolarization. Under voltage clamp conditions, the currents evoked by transferrin were similar to glutamate in that they both displayed non-linear voltage dependence. Furthermore, acute transferrin exposure resulted in a 200% increase in the amount of Na+ independent [3H]glutamate binding observed in these cultures. These results suggest that transferrin may function as a neurotransmitter or neuromodulator in the developing vertebrate nervous system.

Intradendritic release of calcium induced by glutamate in cerebellar Purkinje cells

The ability of excitatory amino acids to induce increases in the intracellular Ca2+ concentration ([Ca2+]i) of cerebellar Purkinje cells was examined by digital fluorescence ratio imaging of voltage-clamped Purkinje cells dialyzed with the Ca2+ indicator fura-2. Purkinje cells responded with large inward currents accompanied by increases in dendritic [Ca2+]i when challenged with the excitatory amino acid agonists glutamate and quisqualate. The rise in [Ca2+]i was transient and reached peak values of several hundred nanomolar. The response subsisted in the absence of extracellular Ca2+, a condition that eliminates Ca2+ entry through voltage-gated Ca2+ channels, indicating that Ca2+ arose in large part from an intracellular compartment. In support of this hypothesis, only the first agonist application elicited a [Ca2+]i increase in slices maintained in Ca(2+)-free medium, as expected if the intracellular stores become depleted. These results indicate that metabotropic glutamate receptors are functional in Purkinje cells and point to glutamate as a possible modulator of [Ca2+]i in these neurons.

Agonist-activated cobalt uptake identifies divalent cation-permeable kainate receptors on neurons and glial cells

Activation of kainate receptors causes Co2+ influx into neurons, type-2 astrocytes, and O-2A progenitor cells. Agonist-activated Co2+ uptake can be performed using cultured cells or fresh tissue slices. Based on the pattern of response to kainate, glutamate, and quisqualate, three functionally different kainate-activated ion channels (K1, K2, and K3) can be discriminated. Co2+ uptake through the K1 receptor was only activated by kainate. Both kainate and glutamate activated Co2+ uptake through the K2 receptor. Co2+ uptake through the K3 receptor was activated by all three ligands: kainate, glutamate, and quisqualate. Co2+ uptake occurred through a nonselective cation entry pathway permeable to Co2+, Ca2+, and Mn2+. The agonist-dependent activation of divalent cation influx through different kainate receptors could be correlated with expression of certain kainate receptor subunit combinations. These results are indicative of kainate receptors that may contribute to excitatory amino acid-mediated neurotoxicity.

Identification of a site in glutamate receptor subunits that controls calcium permeability

The neurotransmitter glutamate mediates excitatory synaptic transmission throughout the brain. A family of genes encoding subunits of the non-N-methyl-D-aspartate (non-NMDA) type of glutamate receptor has been cloned. Some combinations of these subunits assemble into receptors with a substantial permeability to calcium, whereas others do not. To investigate the structural features that control ion permeation through these ligand-gated channels, mutant receptor subunits with single-amino acid changes were constructed. Mutation of a certain amino acid that results in a net charge change (from glutamine to arginine or vice versa) alters both the current-voltage relation and the calcium permeability of non-NMDA receptors. A site has thus been identified that regulates the permeation properties of these glutamate receptors.