Postischemic enhancements of N-methyl-D-aspartic acid (NMDA) and non-NMDA receptor-mediated responses in hippocampal CA1 pyramidal neurons

Glutamate receptor-mediated responses were investigated by using a whole-cell recording and an intracellular calcium ion ([Ca2+]i) imaging in gerbil postischemic hippocampal slices prepared at 1, 3, 6, 9, 12, and 24 hours after 5-minute ischemia. Bath application of N-methyl-D-aspartic acid (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), and kainate showed that NMDA-, AMPA- and kainate-induced currents were enhanced in postischemic CA1 pyramidal neurons at 1 to 12 hours after 5-minute ischemia. NMDA and non-NMDA receptor-mediated excitatory postsynaptic currents (EPSC) were examined in postischemic CA1 pyramidal neurons at 3 hours after 5-minute ischemia to confirm whether synaptic responses are enhanced in the postischemic CA1 pyramidal neurons. The amplitudes of NMDA- and non-NMDA-receptor-mediated EPSC were enhanced in the postischemic CA1 pyramidal neurons. NMDA-, AMPA-, and kainate-induced [Ca2+]i elevations were also examined to determine whether the enhancement of currents is accompanied by the enhancement of [Ca2+]i elevation. The enhancements of NMDA-, AMPA-, and kainate-induced [Ca2+]i elevations were shown in the postischemic CA1. These results indicate that NMDA and non-NMDA receptor-mediated responses are persistently enhanced in the CA1 pyramidal neurons 1 to 12 hours after transient ischemia, and suggest that the enhancement of glutamate receptor-mediated responses may act as one of crucial factors in the pathologic mechanism responsible for leading postischemic CA1 pyramidal neurons to irreversible neuronal injury.     

Rapid Ca2+ entry through Ca2+-permeable AMPA/Kainate channels triggers marked intracellular Ca2+ rises and consequent oxygen radical production

The widespread neuronal injury that results after brief activation of highly Ca2+-permeable NMDA channels may, in large part, reflect mitochondrial Ca2+ overload and the consequent production of injurious oxygen radicals. In contrast, AMPA/kainate receptor activation generally causes slower toxicity, and most studies have not found evidence of comparable oxygen radical production. Subsets of central neurons, composed mainly of GABAergic inhibitory interneurons, express AMPA/kainate channels that are directly permeable to Ca2+ ions. Microfluorometric techniques were performed by using the oxidation-sensitive dye hydroethidine (HEt) to determine whether the relatively rapid Ca2+ flux through AMPA/kainate channels expressed on GABAergic neurons results in oxygen radical production comparable to that triggered by NMDA. Consistent with previous studies, NMDA exposures triggered increases in fluorescence in most cultured cortical neurons, whereas high K+ (50 mM) exposures (causing depolarization-induced Ca2+ influx through voltage-sensitive Ca2+ channels) caused little fluorescence change. In contrast, kainate exposure caused fluorescence increases in a distinct subpopulation of neurons; immunostaining for glutamate decarboxylase revealed the responding neurons to constitute mainly the GABAergic population. The effect of NMDA, kainate, and high K+ exposures on oxygen radical production paralleled the effect of these exposures on intracellular Ca2+ levels when they were monitored with the low-affinity Ca2+-sensitive dye fura-2FF, but not with the high-affinity dye fura-2. Inhibition of mitochondrial electron transport with CN- or rotenone almost completely blocked kainate-triggered oxygen radical production. Furthermore, antioxidants attenuated neuronal injury resulting from brief exposures of NMDA or kainate. Thus, as with NMDA receptor activation, rapid Ca2+ influx through Ca2+-permeable AMPA/kainate channels also may result in mitochondrial Ca2+ overload and consequent injurious oxygen radical production.     

Cobalt accumulation in neurons expressing ionotropic excitatory amino acid receptors in young rat spinal cord: morphology and distribution

Excitatory amino acids (EAA) acting on N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and kainate receptors play an important role in synaptic transmission in the spinal cord. Quantitative autoradiography and physiological experiments suggest that NMDA receptors are localized mainly in lamina II while kainate and AMPA receptors are found on both dorsal and ventral horn neurons. However the cell types expressing EAA receptors and their laminar distribution is not known. We have used a cobalt uptake method to study the morphology and distribution of spinal cord neurons expressing AMPA, kainate, or NMDA excitatory amino acid receptors in the lumbar enlargement of the rat spinal cord. The technique involved superfusion of hemisected spinal cords of 14 day-old rat pups in vitro with excitatory amino acid receptor ligands in the presence of CoCl2. Cobalt has been shown to enter cells through ligand-gated ion channels in place of Ca2+. Cells which accumulated cobalt ions following activation by ionotropic excitatory amino acid receptors were visualized histochemically. The cobalt uptake generated receptor-specific labeling of cells, as the NMDA receptor antagonist D-(-)-2-amino-(5)-phosphonovaleric acid (D-AP-5) (20 microM) blocked the NMDA, but not kainate-induced cobalt uptake. The kainate-induced cobalt labeling was reduced by the non-selective excitatory amino acid receptor antagonist kynurenic acid (4 mM). Passive opening of the voltage-gated Ca(2+)-channels by KCl (50 mM) did not result in cobalt uptake, indicating that cobalt enters the cells through ligand-gated Ca(2+)-channels. AMPA (500 microM), kainate (500 microM), or NMDA (500 microM) each induced cobalt uptake with characteristic patterns and distributions of neuronal staining. Overall, kainate induced cobalt uptake in the greatest number of neuronal staining. Overall, kainate induced cobalt uptake in the greatest number of neuronal perikarya while NMDA-induced uptake was the lowest. AMPA and kainate, but not NMDA superfusion, resulted in cobalt labeling of glial cells. Our results show that the cobalt uptake technique is a useful way to study the morphology and distribution of cells expressing receptors with ligand-gated Ca2+ channels.     

Regulation of intracellular Ca2+ in response to muscarinic and glutamate receptor agonists during the differentiation of NTERA2 human embryonal carcinoma cells into neurons

Single cell microfluorimetry was used to study intracellular calcium ion signals ([Ca(2+)](i)) evoked by acetylcholine (ACh), glutamate receptor agonists and by KCI-induced membrane depolarization, during neuronal differentiation of the human embryonal carcinoma (EC) cell line, NTERA2. In undifferentiated NTERA2 EC cells, [Ca(2+)](i) was elevated in response to ACh, but not to the glutamate receptor agonists NMDA, kainate or AMPA. The ACh-induced rise in [Ca(2+)](i) was dependent upon both Ca(2+) influx and Ca(2+) mobilization from cytoplasmic calcium stores. Three other human EC cell lines responded similarly to ACh but not to glutamate or KCI-induced depolarization. In neurons derived from NTERA2 cells by retinoic acid induction, [Ca(2+)](i) signals were evoked by ACh, NMDA, kainate and by an elevation of the extracellular KCI concentration. As in undifferentiated EC cells, the ACh-mediated increases in [Ca(2+)](i) were governed by both Ca(2+) influx and Ca(2+) mobilization. In contrast, the effects of NMDA, kainate and KCI did not involve intracellular Ca(2+) mobilization. The appearance of glutamate and KCI responsiveness was not detected in non-neuronal differentiated derivatives of NTERA2 cells. Using a number of pharmacologically defined muscarinic receptor antagonists we found that NTERA2 EC cells express M(1), M(3), M(4) and possibly M(5) receptor subtypes linked to changes in [Ca(2+)](i), whilst only M(3) and M(5) are present in NTERA2-derived neurons. The results were supported by PCR analysis of the muscarinic mRNA species expressed in the cells. The data demonstrate that differentiation of NTERA2 EC cells into neurons involves the induction of functional glutamate receptors coupled to rises in [Ca(2+)](i), and changes in the expression of muscarinic ACh receptor subtypes.     

Global ischemia induces downregulation of Glur2 mRNA and increases AMPA receptor-mediated Ca2+ influx in hippocampal CA1 neurons of gerbil

Transient, severe forebrain or global ischemia leads to delayed cell death of pyramidal neurons in the hippocampal CA1. The precise molecular mechanisms underlying neuronal cell death after global ischemia are as yet unknown. Glutamate receptor-mediated Ca2+ influx is thought to play a critical role in this cell death. In situ hybridization revealed that the expression of mRNA encoding GluR2 (the subunit that limits Ca2+ permeability of AMPA-type glutamate receptors) was markedly and specifically reduced in gerbil CA1 pyramidal neurons after global ischemia but before the onset of neurodegeneration. To determine whether the change in GluR2 expression is functionally significant, we examined the AMPA receptor-mediated rise in cytoplasmic free Ca2+ level ([Ca2+]i) in individual CA1 pyramidal neurons by optical imaging with the Ca2+ indicator dye fura-2 and by intracellular recording. Seventy-two hours after ischemia, CA1 neurons that retained the ability to fire action potentials exhibited a greatly enhanced AMPA-elicited rise in [Ca2+]i. Basal [Ca2+]i in these neurons was unchanged. These findings provide evidence for Ca2+ entry directly through AMPA receptors in pyramidal neurons destined to die. Downregulation of GluR2 gene expression and an increase in Ca2+ influx through AMPA receptors in response to endogenous glutamate are likely to contribute to the delayed neuronal death after global ischemia.     

Synaptic strengthening through activation of Ca2+-permeable AMPA receptors

Postsynaptic Ca2+ elevation during synaptic transmission is an important trigger for short- and long-term changes in synaptic strength in the vertebrate central nervous system. The AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate) receptors, a subfamily of glutamate receptors, mediate much of the excitatory synaptic transmission in the brain and spinal cord. It has been shown that a subtype of the AMPA receptor is Ca2+-permeable and is present in the subpopulations of neurons. When synaptically localized, these receptors should mediate postsynaptic Ca2+ influx, providing a trigger for changes in synaptic strength. Here we show that Ca2+-permeable AMPA receptors are synaptically localized on a subpopulation of dorsal horn neurons, and that they provide a synaptically gated route of Ca2+ entry, and that activation of these receptors strengthens synaptic transmission mediated by AMPA receptors. This pathway for postsynaptic Ca2+ influx may provide a new form of activity-dependent modulation of synaptic strength.     

NMDA and non-NMDA receptor antagonists protect against excitotoxic injury in the rat spinal cord

The neuroprotective properties of the N-methyl-D-aspartate (NMDA) antagonist dizocilpine (MK-801) and the non-NMDA antagonists 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo[f]quinoxaline (NBQX) and alpha-methyl-4-carboxyphenylglycine (MCPG) were evaluated against neuronal injury produced by the intraspinal injection of NMDA and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA). Forty-nine animals were divided into eight groups in order to evaluate the effects of different drug combinations: (a) NMDA; (b) NMDA + MCPG; (c) NMDA + NBQX; (d) NMDA + MK-801; (e) AMPA; (f) AMPA + MCPG; (g) AMPA + MK-801; and (h) AMPA + NBQX. Drugs were microinjected into spinal segments T12-L3 through a micropipette attached to a Hamilton microliter syringe. Spinal cords were evaluated after a survival period of 48 h at which time NMDA and AMPA were found to produce morphological changes over the concentration ranges of 125-500 mM and 75-500 microM, respectively. Neuronal loss following injections of NMDA + MK-801 or AMPA + NBQX was significantly less than that following injections of NMDA or AMPA alone. By contrast, neuronal loss following co-injections of NMDA or AMPA with inappropriate antagonists, i.e., NMDA + NBQX/MCPG or AMPA + MCPG/MK-801, was not significantly different from that produced by NMDA or AMPA. The results suggest that elevations in spinal levels of glutamate followed by prolonged activation of NMDA and AMPA receptor subtypes initiate an excitotoxic cascade resulting in neuronal injury. Blockade of NMDA and AMPA effects by MK-801 and NBQX respectively confirms the well documented neuroprotective effects of these drugs and lends support to the potential importance of NMDA and especially AMPA receptor antagonists as therapeutic agents in the treatment of acute spinal cord injury.     

Antagonists for group I mGluRs attenuate excitotoxic neuronal death in cortical cultures

Activation of ion channel-linked glutamate receptors, especially N-methyl-D-aspartate (NMDA) receptors, mediates the excitotoxic effects of glutamate upon central neurons. We examined the hypothesis that activation of group I metabotropic glutamate receptors (mGluRs) would increase NMDA receptor-mediated cortical neuronal death. Addition of the selective group I mGluR agonists, dihydroxyphenylglycine (DHPG) or trans-azetidine-2,4-dicarboxylic acid (t-ADA) potentiated NMDA-induced neuronal death, and application of the group I mGluR-selective antagonist, aminoindan-1,5-dicarboxylic acid (AIDA), as well as the non-selective antagonists methyl-4-carboxyphenylglycine (MCPG) or 4-carboxyphenylglycine (4CPG) reduced NMDA- and kainate-induced neuronal death in murine cortical cultures. The pro-excitotoxic effect of group I mGluR activation may be mediated largely by enhancement of glutamate release, as DHPG potentiated high potassium-stimulated glutamate release, and the protective effects of both AIDA and MCPG were abolished when NMDA and alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid (AMPA) receptors were blocked immediately after toxic NMDA receptor overstimulation. The present data support the possibility that antagonizing group I mGluRs may be a useful strategy for attenuating excitotoxic neuronal death in certain disease states.     

Determination of degradation rates of transfer and ribosomal ribonucleic acids in cultured rat hepatocytes by measuring N6-threoninocarbonyladenosine, dihydrouridine, and pseudouridine in medium using high-performance liquid chromatography

The cause of the selective degeneration of motor neurons in amyotrophic lateral sclerosis (ALS) remains unexplained. One potential pathogenetic mechanism is chronic toxicity due to disturbances of the glutamatergic neurotransmitter system, mediated via alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-sensitive glutamate receptors. Functional AMPA receptors consist of various combinations of four subunits (designated GluR1-4). The GluR2 subunit is functionally dominant and renders AMPA receptors impermeable to calcium. Most native AMPA receptors in the mammalian central nervous system (CNS) contain the GluR2 subunit and are calcium impermeable. We have investigated the composition of AMPA receptors expressed on normal human spinal motor neurons by in situ hybridization to determine their likely subunit stoichiometry. Highly significant levels of mRNA were detected for the GluR1, GluR3, and GluR4 subunits. However, GluR2 subunit mRNA was not detectable in this cell group. The absence of detectable GluR2 mRNA in normal human spinal motor neurons predicts that they express calcium-permeable AMPA receptors unlike most neuronal groups in the human CNS. Expression of atypical calcium-permeable AMPA receptors by human motor neurons provides a possible mechanism whereby disturbances of glutamate neurotransmission in ALS may selectively injure this cell group.     

Enhancement of the N-methyl-D-aspartate response in spinal dorsal horn neurons by cAMP-dependent protein kinase

Glutamate-gated ion channels mediate excitatory synaptic transmission in the central nervous system and are involved in synaptic plasticity, neuronal development and excitotoxicity (5,24). These ionotropic glutamate receptors were classified according to their preferred agonists as AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid), KA (kainate), and NMDA (N-methyl-D-aspartate) receptors [Trends Pharmacol. Sci., 11 (1990) 25-33]. The present study of NMDA receptor channels expressed in acutely isolated spinal dorsal horn (DH) neurons of young rat reveals that they are subject to modulation through the adenylate cyclase cascade. Whole-cell voltage-clamp recording mode was used to examine the effect of adenosine 3',5'-monophosphate (cAMP)-dependent protein kinase (PKA) on the responses of DH neurons to NMDA. Whole-cell current response to NMDA was enhanced by 8 Br-cAMP, a membrane permeant analog of cAMP or by intracellular application of cAMP or catalytic subunit of PKA.     

Role of Ca2+ in alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-mediated polyphosphoinositide turnover in primary neuronal cultures

Excitatory amino acid (EAA) receptors and EAA-mediated stimulation of polyphosphoinositide (poly-PI) turnover were studied in cultured neurons at different days in vitro (DIV). Six main observations have emerged from these studies: (a) Neurons increased their sensitivity to EAAs as a function of time in culture, indicated by increasing EAA-mediated poly-PI turnover. (b) Extracellular Ca2+ concentration played an important role in alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-stimulated poly-PI turnover in cells at 4 DIV, whereas poly-PI turnover mediated by L-glutamate and trans-1-amino-cyclopentane-1,3-dicarboxylic acid was not Ca(2+)-dependent. (c) A marked stimulation of poly-PI turnover by AMPA was seen in the cultured neurons at 4 DIV, but not at 17 DIV, suggesting that a distinct EAA receptor sensitive to AMPA is transiently expressed. (d) The Ca2+ ionophore A23187 increased poly-PI turnover in cultured neurons, suggesting that Ca2+ entry is involved in stimulating poly-PI turnover. (e) Stimulation of poly-PI turnover by carbachol was greater in neurons at 17 DIV as compared with 4 DIV, and appeared to be Ca(2+)-dependent across DIV. (f) 6-Cyano-7-nitroquinoxaline-2,3-dione, an antagonist for non-N-methyl-D-aspartate ionotropic EAA receptors, inhibited 100% and 35% of AMPA- and quisqualate-induced poly-PI turnover, respectively, suggesting an involvement of ionotropic AMPA/quisqualate receptors in stimulating poly-PI turnover.     

Excitotoxic death of a subset of embryonic rat motor neurons in vitro

We have used cultures of purified embryonic rat spinal cord motor neurons to study the neurotoxic effects of prolonged ionotropic glutamate receptor activation. NMDA and non-NMDA glutamate receptor agonists kill a maximum of 40% of the motor neurons in a concentration- and time-dependent manner, which can be blocked by receptor subtype-specific antagonists. Subunit-specific antibodies stain all of the motor neurons with approximately the same intensity and for the same repertoire of subunits, suggesting that the survival of the nonvulnerable population is unlikely to be due to the lack of glutamate receptor expression. Extracellular Ca2+ is required for excitotoxicity, and the route of entry initiated by activation of non-NMDA, but not NMDA, receptors is L-type Ca2+ channels. Ca2+ imaging of motor neurons after application of specific glutamate receptor agonists reveals a sustained rise in intracellular Ca2+ that is present to a similar degree in most motor neurons, and can be blocked by appropriate receptor/channel antagonists. Although the lethal effects of glutamate receptor agonists are seen in only a subset of cultured motor neurons, the basis of this selectivity is unlikely to be simply the glutamate receptor phenotype or the level/pattern of rise in agonist-evoked intracellular Ca2+.     

Autophosphorylation-dependent targeting of calcium/ calmodulin-dependent protein kinase II by the NR2B subunit of the N-methyl- D-aspartate receptor

Activation and Thr286 autophosphorylation of calcium/calmodulindependent kinase II (CaMKII) following Ca2+ influx via N-methyl-D-aspartate (NMDA)-type glutamate receptors is essential for hippocampal long term potentiation (LTP), a widely investigated cellular model of learning and memory. Here, we show that NR2B, but not NR2A or NR1, subunits of NMDA receptors are responsible for autophosphorylation-dependent targeting of CaMKII. CaMKII and NMDA receptors colocalize in neuronal dendritic spines, and a CaMKII.NMDA receptor complex can be isolated from brain extracts. Autophosphorylation induces direct high-affinity binding of CaMKII to a 50 amino acid domain in the NR2B cytoplasmic tail; little or no binding is observed to NR2A and NR1 cytoplasmic tails. Specific colocalization of CaMKII with NR2B-containing NMDA receptors in transfected cells depends on receptor activation, Ca2+ influx, and Thr286 autophosphorylation. Translocation of CaMKII because of interaction with the NMDA receptor Ca2+ channel may potentiate kinase activity and provide exquisite spatial and temporal control of postsynaptic substrate phosphorylation.     

Subcellular and subsynaptic distribution of the NR1 subunit of the NMDA receptor in the neostriatum and globus pallidus of the rat: co-localization at synapses with the GluR2/3 subunit of the AMPA receptor

Glutamatergic neurotransmission in the neostriatum and the globus pallidus is mediated through NMDA-type as well as other glutamate receptors and is critical in the expression of basal ganglia function. In order to characterize the cellular, subcellular and subsynaptic localization of NMDA receptors in the neostriatum and globus pallidus, multiple immunocytochemical techniques were applied using antibodies that recognize the NR1 subunit of the NMDA receptor. In order to determine the spatial relationship between NMDA receptors and AMPA receptors, double labelling was performed with the NR1 antibodies and an antibody that recognizes the GluR2 and 3 subunits of the AMPA receptor. In the neostriatum all neurons with characteristics of spiny projection neurons, some interneurons and many dendrites and spines were immunoreactive for NR1. In the globus pallidus most perikarya and many dendritic processes were immunopositive. Immunogold methods revealed that most NR1 labelling is associated with asymmetrical synapses and, like the labelling for GluR2/3, is evenly spread across the synapse. Double immunolabelling revealed that in neostriatum, over 80% of NR1-positive axospinous synapses are also positive for GluR2/3. In the globus pallidus most NR1-positive synapses are positive for GluR2/3. In both regions many synapses labelled only for GluR2/3 were also detected. These results, together with previous data, suggest that NMDA and AMPA receptor subunits are expressed by the same neurons in the neostriatum and globus pallidus and that NMDA and AMPA receptors are, at least in part, colocalized at individual asymmetrical synapses. The synaptic responses to glutamate in these regions are thus likely be mediated by both AMPA and NMDA receptors at the level of individual synapses.     

Calcium elevation in astrocytes causes an NMDA receptor-dependent increase in the frequency of miniature synaptic currents in cultured hippocampal neurons

Astrocytes exhibit a form of excitability and communication on the basis of intracellular Ca2+ variations (Cornell-Bell et al., 1990; Charles et al., 1991) that can be initiated by neuronal activity (Dani et al., 1992; Porter and McCarthy, 1996). A Ca2+ elevation in astrocytes induces the release of glutamate (Parpura et al., 1994; Pasti et al., 1997; Araque et al., 1998;Bezzi et al., 1998), which evokes a slow inward current in neurons and modulates action potential-evoked synaptic transmission between cultured hippocampal cells (Araque et al., 1998), suggesting that astrocytes and neurons may function as a network with bidirectional communication. Here we show that a Ca2+ elevation in astrocytes increases the frequency of excitatory as well as inhibitory miniature postsynaptic currents (mPSCs), without modifying their amplitudes. Thapsigargin incubation, microinjection of the Ca2+ chelator BAPTA, and photolysis of the Ca2+ cage NP-EGTA demonstrate that a Ca2+ elevation in astrocytes is both necessary and sufficient to modulate spontaneous transmitter release. This Ca2+-dependent release of glutamate from astrocytes enhances mPSC frequency by acting on NMDA glutamate receptors, because it is antagonized by D-2-amino-5-phosphonopentanoic acid (AP5) or extracellular Mg2+. These NMDA receptors are located extrasynaptically, because blockage specifically of synaptic NMDA receptors by synaptic activation in the presence of the open channel blocker MK-801 did not impair the AP5-sensitive astrocyte-induced increase of mPSC frequency. Therefore, astrocytes modulate spontaneous excitatory and inhibitory synaptic transmission by increasing the probability of transmitter release via the activation of NMDA receptors.     

The AMPA receptor interacts with and signals through the protein tyrosine kinase Lyn

Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system. The ionotropic glutamate receptors are classified into two groups, NMDA (N-methyl-D-aspartate) receptors and AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate) receptors. The AMPA receptor is a ligand-gated cation channel that mediates the fast component of excitatory postsynaptic currents in the central nervous system. Here we report that AMPA receptors function not only as ion channels but also as cell-surface signal transducers by means of their interaction with the Src-family non-receptor protein tyrosine kinase Lyn. In the cerebellum, Lyn is physically associated with the AMPA receptor and is rapidly activated following stimulation of the receptor. Activation of Lyn is independent of Ca2+ and Na+ influx through AMPA receptors. As a result of activation of Lyn, the mitogen-activated protein kinase (MAPK) signalling pathway is activated, and the expression of brain-derived neurotrophic factor (BDNF) messenger RNA is increased in a Lyn-kinase-dependent manner. Thus, AMPA receptors generate intracellular signals from the cell surface to the nucleus through the Lyn-MAPK pathway, which may contribute to synaptic plasticity by regulating the expression of BDNF.     

Ultrastructural localization of glutamate receptor subunits (NMDAR1, AMPA GluR1 and GluR2/3) and spinothalamic tract cells

The associations of glutamate receptor subunits (NMDAR1, AMPA GluR1 and GluR2/3) and spinothalamic tract neurons in the rat lumbar spinal cord dorsal horn were investigated. Staining for NMDAR1 and AMPA GluR1 and GluR2/3 receptor subunits was observed throughout the spinothalamic tract soma and dendrites, particularly in association with the rough endoplasmic reticulum and some postsynaptic membrane sites. Immunostaining for NMDAR1 and AMPA GluR2/3 was also noted in presynaptic membrane sites. Localization of both NMDA and AMPA glutamate receptor subunits in association with spinothalamic tract neurons provides anatomical evidence in support of the various interactions reported for glutamate receptors in nociception. Presynaptic localization of the AMPA GluR2/3 receptor subunit suggests that spinothalamic tract cells may also be affected presynaptically by AMPA glutamate receptor interactions.