Activity-dependent upregulation of presynaptic kainate receptors at immature CA3-CA1 synapses.

Presynaptic kainate-type glutamate receptors (KARs) regulate glutamate release probability and short-term plasticity in various areas of the brain. Here we show that long-term depression (LTD) in the area CA1 of neonatal rodent hippocampus is associated with an upregulation of tonic inhibitory KAR activity, which contributes to synaptic depression and causes a pronounced increase in short-term facilitation of transmission. This increased KAR function was mediated by high-affinity receptors and required activation of NMDA receptors, nitric oxide (NO) synthetase, and postsynaptic calcium signaling. In contrast, KAR activity was irreversibly downregulated in response to induction of long-term potentiation in a manner that depended on activation of the TrkB-receptor of BDNF. Both tonic KAR activity and its plasticity were restricted to early stages of synapse development and were lost in parallel with maturation of the network due to ongoing BDNF-TrkB signaling. These data show that presynaptic KARs are targets for activity-dependent modulation via diffusible messengers NO and BDNF, which enhance and depress tonic KAR activity at immature synapses, respectively. The plasticity of presynaptic KARs in the developing network allows nascent synapses to shape their response to incoming activity. In particular, upregulation of KAR function after LTD allows the synapse to preferentially pass high-frequency afferent activity. This can provide a potential rescue from synapse elimination by uncorrelated activity and also increase the computational dynamics of the developing CA3-CA1 circuitry.

Small GTPase Rab17 regulates the surface expression of kainate receptors but not α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in hippocampal neurons via dendritic trafficking of Syntaxin-4 protein.

Glutamate receptors are fundamental for control synaptic transmission, synaptic plasticity, and neuronal excitability. However, many of the molecular mechanisms underlying their trafficking remain elusive. We previously demonstrated that the small GTPase Rab17 regulates dendritic trafficking in hippocampal neurons. Here, we investigated the role(s) of Rab17 in AMPA receptor (AMPAR) and kainate receptor (KAR) trafficking. Although Rab17 knockdown did not affect surface expression of the AMPAR subunit GluA1 under basal or chemically induced long term potentiation conditions, it significantly reduced surface expression of the KAR subunit GluK2. Rab17 co-localizes with Syntaxin-4 in the soma, dendritic shaft, the tips of developing hippocampal neurons, and in spines. Rab17 knockdown caused Syntaxin-4 redistribution away from dendrites and into axons in developing hippocampal neurons. Syntaxin-4 knockdown reduced GluK2 but had no effect on GluA1 surface expression. Moreover, overexpression of constitutively active Rab17 promoted dendritic surface expression of GluK2 by enhancing Syntaxin-4 translocation to dendrites. These data suggest that Rab17 mediates the dendritic trafficking of Syntaxin-4 to selectively regulate dendritic surface insertion of GluK2-containing KARs in rat hippocampal neurons.

c-fos regulates neuronal excitability and survival.

Excitotoxicity is a process in which glutamate or other excitatory amino acids induce neuronal cell death. Accumulating evidence suggests that excitotoxicity may contribute to human neuronal cell loss caused by acute insults and chronic degeneration in the central nervous system. The immediate early gene (IEG) c-fos encodes a transcription factor. The c-Fos proteins form heterodimers with Jun family proteins, and the resulting AP-1 complexes regulate transcription by binding to the AP-1 sequence found in many cellular genes. Emerging evidence suggests that c-fos is essential in regulating neuronal cell survival versus death. Although c-fos is induced by neuronal activity, including kainic acid-induced seizures, whether and how c-fos is involved in excitotoxicity is still unknown. To address this issue, we generated a mouse in which c-fos expression is largely eliminated in the hippocampus. We found that these mutant mice have more severe kainic acid-induced seizures, increased neuronal excitability and neuronal cell death, compared with control mice. Moreover, c-Fos regulates the expression of the kainic acid receptor GluR6 and brain-derived neurotrophic factor (BDNF), both in vivo and in vitro. Our results suggest that c-fos is a genetic regulator for cellular mechanisms mediating neuronal excitability and survival.

Distinct functional roles of subunits within the heteromeric kainate receptor.

Kainate receptors (KARs) have been implicated in a number of neurological disorders, including epilepsy. KARs are tetrameric, composed of a combination of GluK1-GluK5 subunits. We examined the contribution of GluK2 and GluK5 subunits to activation and desensitization of the heteromeric receptor. Heteromeric GluK2/K5 receptors expressed in HEK-293T cells showed markedly higher glutamate sensitivity than GluK2 homomers and did not desensitize at low glutamate concentrations. Mutation of residue E738 in GluK2 substantially lowered its glutamate sensitivity. However, heteromeric KARs containing this mutant GluK2 [GluK2(E738D)] assembled with wild-type GluK5 showed no change in glutamate EC(50) compared with wild-type heteromeric KARs. Instead, higher concentrations of glutamate were required to produce desensitization. This suggested that, within the heteromeric receptor, glutamate binding to the high-affinity GluK5 subunit alone was sufficient for channel activation but not desensitization, whereas agonist binding to the low-affinity GluK2 subunit was not necessary to open the channel but instead caused the channel to enter a closed, desensitized state. To test this hypothesis in wild-type receptors, we used the competitive antagonist kynurenate, which has higher affinity for the GluK2 than the GluK5 subunit. Coapplication of kynurenate with glutamate to heteromeric receptors reduced the onset of desensitization without affecting the peak current response, consistent with our hypothesis. Our results suggest that GluK2 and GluK5 subunits can be individually activated within the heteromeric receptor and that these subunits serve dramatically different functional roles.

Mechanisms underlying lateral GABAergic feedback onto rod bipolar cells in rat retina.

GABAergic feedback inhibition from amacrine cells shapes visual signaling in the inner retina. Rod bipolar cells (RBCs), ON-sensitive cells that depolarize in response to light increments, receive reciprocal GABAergic feedback from A17 amacrine cells and additional GABAergic inputs from other amacrine cells located laterally in the inner plexiform layer. The circuitry and synaptic mechanisms underlying lateral GABAergic inhibition of RBCs are poorly understood. A-type and rho-subunit-containing (C-type) GABA receptors (GABA(A)Rs and GABA(C)Rs) mediate both forms of inhibition, but their relative activation during synaptic transmission is unclear, and potential interactions between adjacent reciprocal and lateral synapses have not been explored. Here, we recorded from RBCs in acute slices of rat retina and isolated lateral GABAergic inhibition by pharmacologically ablating A17 amacrine cells. We found that amacrine cells providing lateral GABAergic inhibition to RBCs receive excitatory synaptic input mostly from ON bipolar cells via activation of both Ca(2+)-impermeable and Ca(2+)-permeable AMPA receptors (CP-AMPARs) but not NMDA receptors (NMDARs). Voltage-gated Ca(2+) (Ca(v)) channels mediate the majority of Ca(2+) influx that triggers GABA release, although CP-AMPARs contribute a small component. The intracellular Ca(2+) signal contributing to transmitter release is amplified by Ca(2+)-induced Ca(2+) release from intracellular stores via activation of ryanodine receptors. Furthermore, lateral nonreciprocal feedback is mediated primarily by GABA(C)Rs that are activated independently from receptors mediating reciprocal feedback inhibition. These results illustrate numerous physiological differences that distinguish GABA release at reciprocal and lateral synapses, indicating complex, pathway-specific modulation of RBC signaling.

Fluoxetine affects GluK2 editing, glutamate-evoked Ca(2+) influx and extracellular signal-regulated kinase phosphorylation in mouse astrocytes.

BACKGROUND: We sought to study the effects of chronic exposure to fluoxetine - a selective serotonin reuptake inhibitor (SSRI) and specific 5-HT(2B) receptor agonist in astrocytes - on the expression of kainate receptors (GluK1-5) in cultured astrocytes and in intact brains in mice and on GluK2 editing by adenosine deaminase acting on RNA (ADAR), as well as the ensuing effects of fluoxetine on glutamate-mediated Ca(2+) influx and extracellular signal-regulated kinase (ERK)(1/2) phosphorylation in astrocytes. METHODS: We performed reverse transcription-polymerase chain reaction (PCR) to assess mRNA expression. We analyzed RNA editing with amplification refractory mutation system PCR and complementary DNA sequencing. Protein expression and ERK phosphorylation were assessed using Western blots. We studied gene silencing with specific small interfering RNAs (siRNA), and we studied intracellular Ca(2+) using fluorometry. RESULTS: All GluK subunits were present in the brain in vivo, and GluK2-5 subunits were present in cultured astrocytes. Fluoxetine upregulated GluK2 and ADAR2. Enhanced GluK2 editing by fluoxetine abolished glutamate-mediated increases in intra cellular Ca(2+) and ERK(1/2) phosphorylation. Enhanced editing of GluK2 was prevented by siRNA against the 5-HT(2B) receptor or ADAR2. LIMITATIONS: Limitations of our study include the use of an in vitro system, but our cultured cells in many respects behave like in vivo astrocytes. CONCLUSION: Fluoxetine alters astrocytic glutamatergic function.

Convergent genomic studies identify association of GRIK2 and NPAS2 with chronic fatigue syndrome.

BACKGROUND: There is no consistent evidence of specific gene(s) or molecular pathways that contribute to the pathogenesis, therapeutic intervention or diagnosis of chronic fatigue syndrome (CFS). While multiple studies support a role for genetic variation in CFS, genome-wide efforts to identify associated loci remain unexplored. We employed a novel convergent functional genomics approach that incorporates the findings from single-nucleotide polymorphism (SNP) and mRNA expression studies to identify associations between CFS and novel candidate genes for further investigation. METHODS: We evaluated 116,204 SNPs in 40 CFS and 40 nonfatigued control subjects along with mRNA expression of 20,160 genes in a subset of these subjects (35 CFS subjects and 27 controls) derived from a population-based study. RESULTS: Sixty-five SNPs were nominally associated with CFS (p<0.001), and 165 genes were differentially expressed (≥4-fold; p≤0.05) in peripheral blood mononuclear cells of CFS subjects. Two genes, glutamate receptor, ionotropic, kinase 2 (GRIK2) and neuronal PAS domain protein 2 (NPAS2), were identified by both SNP and gene expression analyses. Subjects with the G allele of rs2247215 (GRIK2) were more likely to have CFS (p=0.0005), and CFS subjects showed decreased GRIK2 expression (10-fold; p=0.015). Subjects with the T allele of rs356653 (NPAS2) were more likely to have CFS (p=0.0007), and NPAS2 expression was increased (10-fold; p=0.027) in those with CFS. CONCLUSION: Using an integrated genomic strategy, this study suggests a possible role for genes involved in glutamatergic neurotransmission and circadian rhythm in CFS and supports further study of novel candidate genes in independent populations of CFS subjects.

Identification of amino acid residues that control functional behavior in GluR5 and GluR6 kainate receptors.

GluR5 and GluR6 kainate receptors differ in their responses to a variety of agonists, despite their relatively high primary sequence homology. We carried out a structure-function study to identify amino acids underlying these divergent responses. Patch clamp analysis of chimeric GluR5-GluR6 receptors indicated that several functionally dominant sites were localized to the C-terminal side of M1. All nonconserved amino acids in the region between M3 and M4 of GluR6 were then individually mutated to their GluR5 counterparts. We found that a single amino acid (N721 in GluR6) controls both AMPA sensitivity and domoate deactivation rates. Additionally, mutation of A689 in GluR6 slowed kainate desensitization. These functional effects were accompanied by alterations in binding affinities. These results support a critical role for these residues in receptor binding and gating activity.

Selective modulation of desensitization at AMPA versus kainate receptors by cyclothiazide and concanavalin A.

Potentiation by cyclothiazide of recombinant glutamate receptor responses in Xenopus oocytes showed absolute selectivity for AMPA versus kainate receptors. In contrast, concanavalin A strongly potentiated responses at kainate but not AMPA receptors. Rapid desensitization in HEK 293 cells transfected with AMPA receptors was blocked by cyclothiazide, but only weakly attenuated by concanavalin A. Desensitization at kainate receptors was blocked by concanavalin A but unaffected by cyclothiazide. Selective effects of these modulators following coexpression of subunits from different families suggest independent assembly of functional AMPA and kainate receptors. Northern blot analysis of mRNA for dorsal root ganglia revealed a predominant expression of GluR5, indicating that modulation of desensitization by concanavalin A but not cyclothiazide in sensory neurons accurately predicts subunit expression for native glutamate receptors.

Critical roles for the M3-S2 transduction linker domain in kainate receptor assembly and postassembly trafficking.

Kainate receptors (KARs) are neuronal proteins that exhibit a highly polarized distribution in the mammalian CNS. Assembly, intracellular trafficking, and synaptic targeting of KARs and other ionotropic glutamate receptors are processes controlled, in part, by various determinants within the constituent subunit proteins themselves. Here, we demonstrate that the linker region between the M3 and S2 domains, which in current structural models is thought to transduce ligand-binding energy into channel opening, additionally has an essential role in receptor biogenesis. Our results show that this gating-associated domain is engaged at two distinct critical stages of KAR biogenesis: first, during the transition from dimeric to tetrameric assembly states and, second, at a postassembly trafficking checkpoint within the endoplasmic reticulum. Alteration of a basic residue, arginine 663, altered the desensitization properties of the GluR6 kainate receptor in response to glutamate application, and these changes were weakly correlated with intracellular retention of the mutant receptors. Elimination of the positive charge also significantly attenuated oligomerization and stability of the intracellular subunit protein. Furthermore, charge swapping with an adjacent residue, glutamate 662, normalized the receptor physiological behavior and reversed the deficits in assembly and degradation, but only partially restored plasma membrane expression of the receptors. These results reveal a new role for this linker domain in glutamate receptor biogenesis and contribute to understanding the cellular controls of receptor assembly and trafficking, which will be important for relating receptor stoichiometry to their neuronal targeting and function.

mGlu1 Receptors Monopolize the Synaptic Control of Cerebellar Purkinje Cells by Epigenetically Down-Regulating mGlu5 Receptors.

In cerebellar Purkinje cells (PCs) type-1 metabotropic glutamate (mGlu1) receptors play a key role in motor learning and drive the refinement of synaptic innervation during postnatal development. The cognate mGlu5 receptor is absent in mature PCs and shows low expression levels in the adult cerebellar cortex. Here we found that mGlu5 receptors were heavily expressed by PCs in the early postnatal life, when mGlu1α receptors were barely detectable. The developmental decline of mGlu5 receptors coincided with the appearance of mGlu1α receptors in PCs, and both processes were associated with specular changes in CpG methylation in the corresponding gene promoters. It was the mGlu1 receptor that drove the elimination of mGlu5 receptors from PCs, as shown by data obtained with conditional mGlu1α receptor knockout mice and with targeted pharmacological treatments during critical developmental time windows. The suppressing activity of mGlu1 receptors on mGlu5 receptor was maintained in mature PCs, suggesting that expression of mGlu1α and mGlu5 receptors is mutually exclusive in PCs. These findings add complexity to the the finely tuned mechanisms that regulate PC biology during development and in the adult life and lay the groundwork for an in-depth analysis of the role played by mGlu5 receptors in PC maturation.

Neurotransmitters. Elusive glutamate receptors.

Kainate-preferring glutamate receptors appear to be abundant in the central nervous system. We have recently begun to understand their properties, but their functions remain to be described.

Identification of C-terminal domain residues involved in protein kinase A-mediated potentiation of kainate receptor subtype 6.

Glutamate receptors are the major excitatory receptors in the vertebrate CNS and have been implicated in a number of physiological and pathological processes. Previous work has shown that glutamate receptor function may be modulated by protein kinase A (PKA)-mediated phosphorylation, although the molecular mechanism of this potentiation has remained unclear. We have investigated the phosphorylation of specific amino acid residues in the C-terminal cytoplasmic domain of the rat kainate receptor subtype 6 (GluR6) as a possible mechanism for regulation of receptor function. The C-terminal tail of rat GluR6 can be phosphorylated by PKA on serine residues as demonstrated using [gamma-32P]ATP kinase assays. Whole cell recordings of transiently transfected human embryonic kidney (HEK) 293 cells showed that phosphorylation by PKA potentiates whole cell currents in wildtype GluR6 and that removal of the cytoplasmic C-terminal domain abolishes this potentiation. This suggested that the C-terminal domain may contain residue(s) involved in the PKA-mediated potentiation. Single mutations of each serine residue in the C-terminal domain (S815A, S825A, S828A, and S837A) and a truncation after position 855, which removes all threonines (T856, T864, and T875) from the domain, do not abolish PKA potentiation. However, the S825A/S837A mutation, but no other double mutation, abolishes potentiation. These results demonstrate that phosphorylation of the C-terminal tail of GluR6 by PKA leads to potentiation of whole cell response, and the combination of S825 and S837 in the C-terminal domain is a vital component of the mechanism of GluR6 potentiation by PKA.

Lateral components in the cone terminals of the rabbit retina: horizontal cell origin and glutamate receptor expression.

We examined the identities of horizontal cell (HC) lateral components in cone terminals and the expression of glutamate receptors on the tips of HC dendrites. We injected A-type horizontal cells (AHCs) with neurobiotin and demonstrated that neurobiotin labeled completely all AHCs within a patch of retina. We converted neurobiotin by using diaminobenzidine and considered labeled processes to be from AHCs and unlabeled processes to be from B-type horizontal cells (BHCs). Three possible combinations of HC dendrites could exist in cone pedicles: both lateral components originating from AHCs, both from BHCs, or one from an AHC and the other from a BHC. EM observations revealed that a majority of cone terminals contained about equal numbers of lateral components originating from each of the two types of HCs and that each of the three possible combinations was present in equal numbers. Localization of different types of glutamate receptors on HC dendritic tips showed that 55% of AHC dendritic tips expressed AMPA receptors and 30% expressed kainate receptors, whereas, in the case of BHCs, 22% of dendritic tips expressed AMPA receptors and 33% expressed kainate receptors. This study suggests that cone photoreceptors feed the light signal equally into networks of AHCs and BHCs and that differential expression of AMPA/kainate receptors by different HCs could account for different functions.

Gene expression regulation following behavioral sensitization to cocaine in transgenic mice lacking the glucocorticoid receptor in the brain.

Several findings suggest that glucocorticoid hormones influence the propensity of an individual to develop cocaine abuse. These hormones activate two related transcription factors, the glucocorticoid receptor and the mineralocorticoid receptor. We have shown previously that mice carrying a mutation of the glucocorticoid receptor gene specifically in neural cells, glucocorticoid receptor knock-out in the brain, show a dramatic decrease in cocaine-induced self-administration and no behavioral sensitization to this drug, two experimental procedures considered relevant models of addiction. Here, we investigated in glucocorticoid receptor knock-out in the brain mice the consequences of this mutation at the level of the expression of neuropeptide, dopamine receptor and glutamate receptor subunit mRNAs. We quantified mRNA levels in the cortex, striatum and accumbens under basal conditions and following acute or repeated cocaine treatments. Our results show that, under basal conditions, neuropeptide (substance P, dynorphin) and dopamine receptor (D1, D2) mRNAs were decreased in glucocorticoid receptor knock-out in the brain mice in the dorsal striatum but not in the accumbens. However, cocaine-induced changes in the levels of these mRNAs were not modified in glucocorticoid receptor knock-out in the brain mice. In contrast, mutant mice showed altered response in mRNA levels of N-methyl-D-aspartate, GLUR5 and GLUR6 glutamate receptor subunits as well as of enkephalin following cocaine administration. These modifications may be associated to decrease of behavioral effects of cocaine observed in glucocorticoid receptor knock-out in the brain mice.

Unequal expression of allelic kainate receptor GluR7 mRNAs in human brains.

We describe here the first example of an exonic polymorphism that affects the primary structure of a human ionotropic glutamate receptor. The human kainate receptor GluR7 gene contains a thymine (T)/guanine (G) nucleotide variation that determines a serine or alanine at position 310 in the extracellular region of GluR7 receptor subunits. Our finding contrasts with a previous report that suggested that GluR7 transcripts were RNA-edited at this site. Whole-cell patch-clamp recordings did not detect differences in receptor activation and desensitization between the human GluR7 receptor isoforms expressed in HEK-293 cells. Analysis of 41 tissue samples obtained from 30 human brains revealed expression level differences between GluR7 alleles expressed in the same brain. The expression level of the allelic GluR7 mRNAs differed in 27 samples from 1.2- to 12.7-fold. Unequal expression level of allelic mRNAs is characteristic for genes that are affected by genomic imprinting or that contain mutations. Genomic imprinting in most cases is conserved between human and mice. However, we did not detect unequal expression of allelic GluR7 mRNAs in mice. Our results are important for future studies that explore a potential role or roles for GluR7 receptors in the brain and for neurological disorders.

Heterogeneity of Ca2+-permeable AMPA/kainate channel expression in hippocampal pyramidal neurons: fluorescence imaging and immunocytochemical assessment.

The presence of Ca2+-permeable AMPA/kainate (Ca-A/K) channels on hippocampal pyramidal neurons (HPNs) has been controversial, although they are present on many forebrain GABAergic neurons. We combined high-resolution fluorescence Ca2+ imaging with surface AMPA receptor (AMPAR) subunit immunocytochemistry to examine the expression of functional Ca-A/K channels in dissociated hippocampal neurons at the subcellular level. In GABAergic neurons [identified by glutamate decarboxylase (GAD) immunocytochemistry], focal application of AMPA induced large dendrosomatic intracellular [Ca2+] ([Ca2+]i) rises, consistent with their known strong Ca-A/K channel expression. Surface immunostaining for the AMPAR subunits GluR1 and GluR2 revealed abundant dendritic GluR1 puncta containing little or no GluR2, which, when present, was limited to diffuse staining in the soma and proximal dendrites. In contrast, the majority of HPNs (putatively identified by morphological criteria and lack of GAD labeling) showed little or no AMPA-induced [Ca2+]i rise. Correspondingly, most HPNs showed strong dendritic labeling for both GluR1 and GluR2 that colocalized extensively. A subpopulation of HPNs, however, displayed noticeable [Ca2+]i rises that began and often reached their highest levels in discrete dendritic regions. In these HPNs, levels of GluR1 relative to GluR2 were higher, and GluR1 was often present without overlying GluR2. The present studies, which are the first to directly examine the relationship between the local complement of cell surface AMPAR and the presence of dendritic Ca-A/K channels, clearly indicate that considerable cell surface GluR2 does not preclude the presence of Ca-A/K channels and further show that HPNs display considerable heterogeneity in terms of apparent Ca-A/K channel expression.

Subcellular and subsynaptic localization of presynaptic and postsynaptic kainate receptor subunits in the monkey striatum.

The localization and functions of kainate receptors (KARs) in the CNS are still poorly known. In the striatum, GluR6/7 and KA2 immunoreactivity is expressed presynaptically in a subpopulation of glutamatergic terminals and postsynaptically in dendrites and spines. The goal of this study was to further characterize the subcellular and subsynaptic localization of kainate receptor subunits in the monkey striatum. Immunoperoxidase data reveal that the relative abundance of GluR6/7- and KA2-immunoreactive terminals is homogeneous throughout the striatum irrespective of the differential degree of striatal degeneration in Huntington's disease. Pre-embedding and post-embedding immunogold data indicate that >70% of the presynaptic or postsynaptic GluR6/7 and KA2 labeling is expressed intracellularly. In material stained with the post-embedding immunogold method, approximately one-third of plasma membrane-bound gold particles labeling in axon terminals and spines is associated with asymmetric synapses, thereby representing synaptic kainate receptor subunits. On the other hand, >60% of the plasma-membrane bound labeling is extrasynaptic. Both GluR6/7 and KA2 labeling in glutamatergic terminals often occurs in clusters of gold particles along the membrane of large vesicular organelles located at various distances from the presynaptic grid. Anterograde labeling from the primary motor cortex or the centromedian thalamic nucleus indicate that both corticostriatal and thalamostriatal terminals express presynaptic GluR6/7 and KA2 immunoreactivity in the postcommissural putamen. In conclusion, these data demonstrate that kainate receptors in the striatum display a pattern of subcellular distribution different from other ionotropic glutamate receptor subtypes, but consistent with their metabotropic-like functions recently shown in the hippocampus.

Kainate receptors expressed by a subpopulation of developing nociceptors rapidly switch from high to low Ca2+ permeability.

Dorsal root ganglion (DRG) neurons first express kainate receptor subunits, predominantly GluR5, during embryonic development. In the DRG and throughout the nervous system, substantial editing of GluR5 mRNA occurs with developmental maturation (Bernard et al., 1999). The accompanying change in Ca(2+) permeability of functional kainate receptors that is the predicted outcome of this developmental regulation of mRNA editing has not been investigated. Here we report that kainate receptors on DRG neurons from late embryonic and newborn rats are predominantly Ca(2+) permeable but then become fully Ca(2+) impermeable later in the first postnatal week. Using multiple markers for nociceptor subpopulations, we show that this switch in Ca(2+) permeability is not caused by the appearance of a new subpopulation of nociceptors with different receptor properties. Instead, the change in Ca(2+) permeability matches the time course of post-transcriptional RNA editing of GluR5 at the Q/R site within the pore of the channel, indicating that the change is probably caused by developmentally regulated RNA editing. We also report that, on the basis of the strong correlation of receptor expression with expression of the surface markers LA4, isolectin B4, and LD2, kainate receptors are present on C-fiber-type neurons projecting to lamina II of spinal cord dorsal horn. These results raise the possibility that kainate receptors in their Ca(2+)-permeable form serve a developmental role in synapse formation between this population of C-fibers and their targets in the spinal cord dorsal horn. Thereafter, the receptors may serve a new function that does not require Ca(2+) permeability.

Synchronized overproduction of AMPA, kainate, and NMDA glutamate receptors during human spinal cord development.

Quantitative receptor autoradiography was used to map the distribution in the developing human spinal cord of the three types of ionotropic glutamate receptors. N-methyl-D-Aspartate (NMDA) receptors were labeled with [3H]glutamate, kainic acid (KA) receptors were labeled with [3H]KA, and alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionate (AMPA) receptors were labeled with [3H]AMPA. In the adult, labeling of all three receptor subtypes is largely restricted to the substantia gelatinosa (SG) in the dorsal horn, with very low level labeling elsewhere in the spinal gray matter. In marked distinction, in late fetal life, high level ligand binding is seen throughout the spinal gray matter. In early postnatal life, binding sites diminish in all regions, but least so in the SG, until the adult pattern emerges. Thus a coordinated transient high level of ionotropic glutamate receptor expression occurs within the developing spinal cord. Saturation analysis of ligand binding shows that the affinity of [3H]KA and [3H]AMPA binding is not developmentally regulated. In contrast, the affinity of [3H]glutamate binding to the NMDA receptor in the fetal ventral horn is three-fold greater than in the adult ventral horn. Thus, in addition to quantitative changes in glutamate receptor expression, qualitative changes occur in the expression of NMDA receptors during development. The distinct glutamate receptor phenotype of fetal and early postnatal spinal cord cells suggests that alterations in the excitable properties of these cells plays an important role in activity-dependent development and in susceptibility to excitotoxic injury.