Laminar distribution of MK-801, kainate,AMPA, and muscimol binding sites in cat visual cortex: A developmental study

We used quantitative autoradiography to determine whether the development of glutamate receptors correlates with the sensitive period for monocular deprivation in the visual cortex. To study glutamate receptors, we incubated sections of cat visual cortex with tritiated (+)-5-methyl-10, 11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10imine-maleate (MK-801), tritiated kainate, and tritiated amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA). [³H]MK-801 is a noncompetitive ligand for the N-methyl-D-aspartate (NMDA) receptor. [³H]kainate and [³H]AMPA are competitive ligands for non-NMDA receptors. We used [³H]muscimol, which binds to GABA_A receptors, so that we would have one control ligand that binds to a nonglutamate receptor. When all layers were combined, the results confirmed our previous studies with homogenate binding. [³H]MK-801 and [³H]kainate binding were significantly greater at 42 days than at earlier or later times. [³H]AMPA and [³H]muscimol binding did not show such a peak. This suggests that MK-801 and kainate binding sites are more likely to be involved in plasticity than are AMPA and muscimol binding sites. In layers 2/3, MK-801 had the greatest age-dependent changes; in layers 5 and 6, kainate binding changed most with age. This suggests that the mechanisms of plasticity may vary with cortical layer. © 1996 Wiley-Liss, Inc.     

Development of MK-801, kainate,AMPA, and muscimol binding sites and the effect of dark rearing in rat visual cortex

We used quantitative autoradiography to determine whether the development of glutamate receptors correlates with the plastic period for monocular deprivation in rat visual cortex. To study glutamate receptors, we incubated sections of rat visual cortex with tritiated (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10imine maleate (MK-801), tritiated kainate, and tritiated amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA). [³H]MK-801 is a noncompetitive ligand for the N-methyl-D-aspartate (NMDA) receptor. [³H]kainate and [³H]AMPA are competitive ligands for non-NMDA receptors. To compare glutamate binding sites with a nonglutamate binding site, we studied [³H]muscimol, which binds to γ-aminobutyric acid (GABA)_A receptors. [³H]MK-801 binding was maximal at postnatal day 26 (P26) and decreased in adulthood. [³H]AMPA binding was maximal at P18. [³H]kainate binding and [³H]muscimol binding were not age dependent. Dark rearing partially prevented the age-dependent decrease in [³H]MK-801 binding but had no effect on [³H]kainate or [³H]AMPA binding. Dark rearing decreased muscimol binding in adult animals. These results suggest that NMDA receptors, but not other glutamate receptors or GABA_A receptors, are likely to be critical for developmental plasticity in rat visual cortex. J. Comp. Neurol. 383:73–81, 1997. © 1997 Wiley-Liss, Inc.     

Impaired maze performance in aged rats is accompanied by increased density of NMDA, 5-HT1A, and alpha-adrenoceptor binding in hippocampus

Using quantitative receptor autoradiography, we assessed binding site densities and distribution patterns of glutamate, GABA(A), acetylcholine (ACh), and monoamine receptors in the hippocampus of 32-month-old Fischer 344/Brown Norway rats. Prior to autoradiography, the rats were divided into two groups according to their retention performance in a water maze reference memory task, which was assessed 1 week after 8 days of daily maze training. The animals of the inferior group showed less long-term retention of the hidden-platform task but did not differ from superior rats in their navigation performance during place training and cued trials. The decreased retention performance in the group of inferior learners was primarily accompanied by increased alpha(1)-adrenoceptors in all hippocampal subregions under inspection (CA1-CA4 and dentate gyrus), while elevated alpha(2)-adrenoceptor binding was observed in the CA1 region and DG. Furthermore, inferior learners had higher NMDA binding in the CA2 and CA4 and increased 5-HT(1A) binding sites in the CA2, CA3, and CA4 region. No significant differences between inferior and superior learners were evident with regard to AMPA, kainate, GABA(A), muscarinergic M(1), dopamine D(1), and 5-HT(2) binding densities in any hippocampal region analyzed. These results show that increased NMDA, 5-HT(1A), and alpha-adrenoceptor binding in the hippocampus is associated with a decline in spatial memory. The increased receptor binding observed in the group of old rats with inferior maze performance might be the result of neural adaptation triggered by age-related changes in synaptic connectivity and/or synaptic activity.     

P2 purinoceptor blocker suramin antagonises NMDA receptors and protects against excitatory behaviour caused by NMDA receptor agonist (RS)-(tetrazol-5-yl)-glycine in rats

It has been reported that suramin, an anthelminthic, trypanocidal agent and an inhibitor of P₂ receptors, may antagonise N-methyl-D-aspartate (NMDA) subtype of the excitatory amino acid receptors. Both NMDA receptors and P_2X subclass of P₂ receptors are ligand-gated Ca²⁺-selective channels and, since the increased influx of Ca²⁺ into neurons has been linked to neurotoxicity, simultaneous inhibition of P_2X and NMDA receptors in vivo by suramin could represent an effective neuroprotective treatment. We have found that suramin inhibited the binding of [³H]CGP 39653 to NMDA receptor binding sites in vitro and reduced the frequency of NMDA channel openings in patch-clamp studies. Suramin (1 mM) had no effect on [³H]kainate binding in vitro. In vivo, intracerebroventricular (ICV) injections of suramin (70 nmol/brain) antagonised convulsive effects of the NMDA agonist (RS)-(tetrazol-5-yl)-glycine (TZG, LY 285265). Suramin, however, did not prevent neurotoxic lesions in the hippocampus caused by ICV administration of TZG. Increasing the dose of suramin resulted in death from severe respiratory depression. J. Neurosci. Res. 49:627–638, 1997. © 1997 Wiley-Liss, Inc.     

Localization of ionotropic glutamate receptors in caudate-putamen and nucleus accumbens septi of rat brain: Comparison of NMDA,AMPA, and kainate receptors

Changes in binding of selective radioligands at NMDA ([³H]MK-801), AMPA ([³H]CNQX), and kainate ([³H]kainic acid) glutamate (GLU) ionotropic receptors in rat caudate-putamen (CPu) and nucleus accumbens (NAc) were examined by quantitative autoradiography following: 1) unilateral surgical ablation of frontal cerebral cortex to remove descending corticostriatal GLU projections, 2) unilateral injection of kainic acid (KA) into CPu or NAc to degenerate local intrinsic neurons, or 3) unilateral injections of 6-hydroxydopamine (6-OH-DA) into substantia nigra to degenerate ascending nigrostriatal dopamine (DA) projections. Cortical ablation significantly decreased NMDA receptor binding in ipsilateral medial CPu (20%), and NAc (16%), similar to previously reported losses of DA D₄ receptors. KA lesions produced large losses of NMDA receptor labeling in CPu and NAc (both by 52%), AMPA (41% and 45%, respectively), and kainate receptors (40% and 45%, respectively) that were similar to the loss of D₂ receptors in CPu and NAc after KA injections. Nigral 6-OH-DA lesions yielded smaller but significant losses in NMDA (17%), AMPA (12%), and kainate (11%) receptor binding in CPu. The results indicate that most NMDA, AMPA, and kainate receptors in rat CPu and NAc occur on intrinsic postsynaptic neurons. Also, some NMDA, but not AMPA or kainate, receptors are also found on corticostriatal projections in association with D₄ receptors; these may, respectively, represent excitatory presynaptic NMDA autoreceptors and inhibitory D₄ heteroceptors that regulate GLU release from corticostriatal axons in medial CPu and NAc. Conversely, the loss of all three GLU receptor subtypes after lesioning DA neurons supports their role as excitatory heteroceptors promoting DA release from nigrostriatal neurons. Synapse 30:227–235, 1998. © 1998 Wiley-Liss, Inc.     

Decreased hippocampal NMDA,but not kainate or AMPA receptors in bipolar disorder

Objectives: The purpose of this study was to determine whether there are changes in the density of ionotropic glutamate receptors in the hippocampus of subjects with bipolar disorder. Methods: Using in situ radioligand binding with semiquantitative autoradiography, we measured the density of [³H]MK‐801, [³H]CGP39653, [³H]AMPA and [³H]kainate binding in hippocampi, obtained postmortem, from eight subjects with type 1 bipolar disorder and 8 age‐ and sex‐matched controls. Results: In subjects with bipolar disorder there were significant decreases in the density of [³H]MK‐801 binding in the Cornu Ammonis (CA) 3 (mean ± SEM; 108.8 ± 12.2 versus 166.2 ± 18.0 fmol/mg ETE, p < 0.005) as well as the pyramidal (102.8 ± 9.2 versus 136.6 ± 11.2 fmol/mg ETE, p < 0.05) and polymorphic (21.73 ± 6.5 versus 53.26 ± 11.6 fmol/mg ETE, p < 0.05) layers of the subiculum. In addition, two‐way analysis of variance (ANOVA) revealed a decrease in the density of [³H]CGP39653 binding across the hippocampal formation in bipolar subjects, which did not reach significance in any subregion. There were no changes in the densities of [³H]AMPA or [³H]kainate binding in these subjects. Conclusions: [³H]CGP39653 and [³H]MK‐801 bind to the glutamate binding site and open ion channel of the n ‐methyl‐d ‐aspartate (NMDA) receptor, respectively. Therefore, these data suggest that there is a decrease in the number of open ion channels associated with no significant change in the apparent density of NMDA receptors in regions of the hippocampus from subjects with bipolar disorder.     

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 α-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. J. Comp. Neurol. 384:200-210, 1997. © 1997 Wiley-Liss, Inc.     

Characterization of striatal activity in conscious rats: contribution of NMDA and AMPA/kainate receptors to both spontaneous and glutamate-driven firing

Single-unit activity in the striatum of unrestrained, conscious rats was characterized by extracellular recording in combination with iontophoresis. To avoid the confounding effect of motor-related changes in firing rate, measurements were restricted to periods when animals were at quiet rest. Recording electrodes were lowered stepwise through 4.0 mm of anterior striatum in 36 equal ventral movements of 111 microm to assess the ratio of spontaneously active vs. silent neurons. Spontaneous activity was assessed at each step followed by iontophoretic glutamate (GLU) application to expose silent neurons. Eleven such experimental sessions resulted in a total of 100 spontaneously active and 264 silent neurons, indicating that without overt movement the large majority (72.7%) of striatal cells are silent. Spontaneously active neurons, moreover, discharged at low rates (4.85 +/- 0.85 spikes/s). In separate experiments, both the AMPA/kainate (CNQX: 6-cyano-2,3-dihydroxy-7-nitro-quinoxaline disodium salt) and NMDA (AP5: D-(-)-2-amino-5-phosphonovaleric acid) GLU-receptor antagonists blocked the activity of most spontaneously active (83% CNQX, 69% AP5), and GLU-stimulated silent (68% CNQX, 98% AP5) units. Collectively, our results are consistent with an overall low level of striatal activity in the absence of strong excitatory input. When neuronal activity is initiated, however, it appears that both NMDA and AMPA/kainate receptors are critical for maintaining continuous impulse activity.     

Quantitative autoradiography of binding sites for [H]AMPA, a structural analogue of glutamic acid

Binding sites for the potent glutamate agonist [³H]α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) were localized in rat brain frozen sections by quantitative autoradiography. Highest levels of binding were seen in stratum radiatum and stratum oriens of the CA1 hippocampal subfield and in the dorsal subiculum. Substantially less but still high amounts of [³H]AMPA binding occurred in other hippocampal subfields and in rostral forebrain structures. The heterogeneous nature of [³H]AMPA binding is discussed in relation to [³H]glutamate binding visualized by similar methods. From these data it is suggested that [³H]AMPA may label a particular subclass of the glutamate receptor population which exhibits a high affinity for quisqualic acid.     

Ultrastructural analysis of NMDA,AMPA, and kainate receptors on unmyelinated and myelinated axons in the periphery

The present study determines the proportions of unmyelinated cutaneous axons at the dermal–epidermal junction in glabrous skin and of myelinated and unmyelinated axons in the sural and medial plantar nerves that immunostain for subunits of the ionotropic glutamate receptors. Approximately 20% of the unmyelinated cutaneous axon profiles at the dermal–epidermal junction immunostain for either N-methyl-D-aspartate (NMDA), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), or kainate receptor subunits. These findings are consistent with previous observations that NMDA and non-NMDA antagonists ameliorate nociceptive behaviors that result from noxious peripheral stimulation. In the sural nerve, where the large majority of myelinated fibers are sensory, approximately half of the myelinated axon profiles immunostain for the NMDA receptor 1 (R1) subunit, 28% immunostain for the glutamate receptor 1 (GluR1) AMPA subunit, and 11% for the GluR5,6,7 kainate subunits. Even higher proportions immunostain for these receptors in the medial plantar nerve, a mixed sensory and motor nerve. In the sural nerve, 20% of the unmyelinated axon profiles immunostain for NMDAR1 and only 7% label for GluR1 or GluR5,6,7. Because the sural nerve innervates hairy skin, these data suggest that glutamate will activate a higher proportion of unmyelinated axons in glabrous skin than in hairy skin. Measurements of fiber diameters indicate that all sizes of myelinated axon profiles, including Aδ and Aβ, are positively labeled for the ionotropic receptors. The presence of glutamate receptors on large-diameter myelinated axons suggests that these mechanosensitive receptors, presumably transducing touch and pressure, may also respond to local glutamate and thus be chemosensitive. J. Comp. Neurol. 391: 78–86, 1998. © 1998 Wiley-Liss, Inc.     

New boundaries and dissociation of the mouse hippocampus along the dorsal‐ventral axis based on glutamatergic,GABAergic and catecholaminergic receptor densities

In rodents, gene‐expression, neuronal tuning, connectivity and neurogenesis studies have postulated that the dorsal, the intermediate and the ventral hippocampal formation (HF) are distinct entities. These findings are underpinned by behavioral studies showing a dissociable role of dorsal and ventral HF in learning, memory, stress and emotional processing. However, up to now, the molecular basis of such differences in relation to discrete boundaries is largely unknown. Therefore, we analyzed binding site densities for glutamatergic AMPA, NMDA, kainate and mGluR_2/3, GABAergic GABA_A (including benzodiazepine binding sites), GABA_B, dopaminergic D_1/5 and noradrenergic α₁ and α₂ receptors as key modulators for signal transmission in hippocampal functions, using quantitative in vitro receptor autoradiography along the dorsal‐ventral axis of the mouse HF. Beside general different receptor profiles of the dentate gyrus (DG) and Cornu Ammonis fields (CA1, CA2, CA3, CA4/hilus), we detected substantial differences between dorsal, intermediate and ventral subdivisions and individual layers for all investigated receptor types, except GABA_B. For example, striking higher densities of α₂ receptors were detected in the ventral DG, while the dorsal DG possesses higher numbers of kainate, NMDA, GABA_A and D_1/5 receptors. CA1 dorsal and intermediate subdivisions showed higher AMPA, NMDA, mGluR_2/3, GABA_A, D_1/5 receptors, while kainate receptors are higher expressed in ventral CA1, and noradrenergic α₁ and α₂ receptors in the intermediate region of CA1. CA2 dorsal was distinguished by higher kainate, α₁ and α₂ receptors in the intermediate region, while CA3 showed a more complex dissociation. Our findings resulted not only in a clear segmentation of the mouse hippocampus along the dorsal‐ventral axis, but also provides insights into the neurochemical basis and likely associated physiological processes in hippocampal functions. Therein, the presented data has a high impact for future studies modeling and investigating dorsal, intermediate and ventral hippocampal dysfunction in relation to neurodegenerative diseases or psychiatric disorders.     

Differential expression of NMDA and AMPA receptor subunits in DARPP-32-containing neurons of the cerebral cortex, hippocampus and neostriatum of rats

Dopamine and cyclic adenosine 3',5'-monophosphate-regulated phosphoprotein, 32 kDa (DARPP-32) is a key element of dopamine/D1/DARPP-32/protein phosphatase-1 (PP-1) signaling cascades of mammalian brain. We are interested in the expression patterns of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors in DARPP-32-containing neurons, which may constitute morphological basis for interaction between dopamine and ionotropic glutamate receptors in dopaminoceptive cells. Double immunofluorescence was performed to visualize neurons showing coexpression of DARPP-32 with NMDA or AMPA receptor subunits (i.e., NR1, NR2a/b, glutamate receptor subunit 1 [GluR1], GluR2/3, and GluR4) in the forebrains of rats. Distribution of DARPP-32-positive neurons completely or partially overlapped with that of NMDA receptor- or AMPA receptor-immunoreactive ones in the frontal and parietal cortex, hippocampus and neostriatum, and neurons double-labeled with DARPP-32/NR1, DARPP-32/NR2a/b, DARPP-32/GluR1, DARPP-32/GluR2/3, or DARPP-32/GluR4 immunoreactivity were numerously observed. Semiquantification analysis indicated that most of DARPP-32-containing neurons (86-98%) expressed NR1, NR2a/b and GluR2/3, while less of them (14-90%) expressed GluR1 and GluR4. Although high rates (90-98%) of DARPP-32-positive cells expressed NMDA receptors in all regions above, variant percentages of them expressing AMPA receptor subunits were observed among the cortex (54-90%), hippocampus (59-97%) and neostriatum (14-97%). The study presents differential expression patterns of NMDA and AMPA receptors in DARPP-32-postive neurons in these forebrain regions. Taken together with previous reports, the present data suggest that interaction between dopamine and glutamate receptors may occur in the dopaminoceptive neurons with distinct receptor compositions and may be involved in modulating neuronal properties and excitotoxicity in mammalian forebrain.     

Effect of Temperature and Calcium on the Binding Properties of the AMPA Receptor in Frozen Rat Brain Sections

The elucidation of the mechanisms regulating the properties of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) subtype of glutamate receptors is important for understanding glutamatergic transmission. Here we report that qualitative as well as quantitative analysis of tritiated ligand binding to the AMPA receptor on thin frozen rat brain tissue sections reveals the existence of several mechanisms regulating the binding properties of AMPA receptors. Preincubation of tissue sections at 35 degrees C results in a decreased amount of [3H]AMPA binding as compared to that measured following preincubation at 0 degrees C. The decrease in binding appears to be mainly localized to cell bodies as evaluated by autoradiography, and could be due to proteolysis. Preincubation with calcium at 35 degrees C produces increased levels of [3H]AMPA binding. The effect of calcium is mimicked by manganese and to a lesser extent by magnesium; it is concentration-dependent with a 50% effective concentration for calcium of approximately 150 microM, time-dependent and temperature-dependent. The calcium-induced increase in [3H]AMPA binding is different among various brain structures, being larger in area CA1 of the hippocampus and in the superficial layers of the cerebral cortex. The effect of calcium is partly reduced by preincubation with the calpain inhibitor leupeptin and potentiated by preincubation with purified calpain II. The calcium-induced increase in [3H]AMPA binding is associated with a decrease in the binding of an antagonist of AMPA receptors, [3H]6-nitro-7-cyanoquinoxaline-2,3-dione. The results indicate that the binding properties of the AMPA receptor are rapidly regulated by calcium-dependent processes, and possibly by calcium-dependent proteases. They suggest that modulation of the binding properties involves changes in the configuration of the receptor, producing opposite changes in the affinities of the receptor for agonists and antagonists. Finally, these results strengthen the hypothesis that changes in the properties of AMPA receptors might underlie various forms of synaptic plasticity.     

Bidirectional redistribution of AMPA but not NMDA receptors after perforant path simulation in the adult rat hippocampus in vivo

Long-term potentiation (LTP) in vitro reveals dynamic regulation of synaptic glutamate receptors. AMPA receptors may be inserted into synapses to increase neurotransmission, whereas NMDA receptors may redistribute within the synapse to alter the probability of subsequent plasticity. To date, the only evidence for these receptor dynamics in the hippocampus is from the studies of dissociated neurons and hippocampal slices taken from young animals. Although synaptic plasticity is induced easily, the extent of AMPA and NMDA receptor mobility after LTP is unknown in the adult, intact hippocampus. To test whether AMPA or NMDAR subunits undergo activity-dependent modifications in adult hippocampal synapses, we induced LTP at perforant path-dentate gyrus (DG) synapses in anesthetized adult rats, using high frequency stimulation (HFS), verified layer-specific Arc induction, and analyzed the distribution of postsynaptic AMPA and NMDAR subunits, using immunogold electron microscopy. The number of synapses with AMPA receptor labeling increased with LTP-inducing HFS in the stimulated region of the dendrite relative to the nonstimulated regions. The opposite trend was noted with low frequency stimulation (LFS). Moreover, HFS increased and LFS decreased the ratio of synaptic to extrasynaptic AMPA receptor labeling in the postsynaptic membrane. In contrast, HFS did not significantly alter NMDAR labeling. Thus, LTP in the adult hippocampus in vivo selectively enhanced AMPA but not NMDAR labeling specifically in synapses undergoing activity-dependent plasticity relative to the remainder of the dendritic tree. The results suggest a mechanism by which rapid adjustments in synaptic strength can occur through localized AMPA receptor mobility and that this process may be competitive across the dendritic tree.     

It is AMPA receptor, not kainate receptor, that contributes to the NBQX-induced antinociception in the spinal cord of rats

Studies demonstrated that intrathecal 1,2,3,4-tetrahydro-6-nitro-2, 3-dioxo[f]quinoxaline-7-sulfonamide disodium (NBQX), an antagonist of AMPA/kainate receptors, induced antinociception in the spinal cord of rats. The present study demonstrated that the NBQX-induced increases in hindpaw withdrawal latencies (HWLs) were dose-dependently attenuated by intrathecal pretreatment of the AMPA receptor desensitization inhibitor, diazoxide. The effect was unrelated to the opening of K+ channels by diazoxide. On the other hand, intrathecal pretreatment of concanavalin A, which selectively inhibits the desensitization of kainate receptor, produced no significant influence on the NBQX-induced antinociception. The results suggest that the NBQX-induced antinociception was mediated by AMPA receptors, not by kainate receptors, in the spinal cord of rats.     

Expression and regulation of kainate and AMPA receptors in the rat neural tube

We analyzed the expression and regulation of glutamate receptor subunits in the rat neural tube (10 day embryos) and in cell cultures derived from this tissue. In the cultures, all cells were stained with antibodies against the neural progenitor marker nestin. More than 50% of the cells were also stained by the monoclonal antibodies LB1 or A2B5, which bind to neuronal and glial progenitors. Approximately 6% of the cells were stained with antibodies for the low affinity NGF receptor, a neural crest cell marker. A small percentage of cells differentiated to neurons or astrocytes, as determined by staining with anti-neurofilament and anti-GFAP antibodies, respectively. RT-PCR analysis of neural tube tissue and culture mRNAs demonstrated that the AMPA receptor subunits GluR3 and 4 and the kainate receptor subunits GluR6, 7, KA1 and KA2 were detectable at E10. The kainate receptor subunits GluR6 and KA2 were upregulated by culture conditions which stimulated cell differentiation, as determined by concomitant downregulation of nestin mRNA. Both in neural tube tissue and in cultured cells, GluR6 was 100% unedited. Finally, both GluR6 and KA2 proteins could be detected in subpopulations of neural progenitors and differentiated neurons. Our data indicate that kainate receptor genes are expressed in undifferentiated progenitor cells of the neural tube at E10, and are upregulated during neural cell differentiation. J. Neurosci. Res. 52:356–368, 1998. Published 1998 Wiley-Liss, Inc.

1 This article is a US Government work and, as such, is in the public domain in the United States of America.

     

Metabotropic glutamate receptors are involved in calcium-induced LTP of AMPA and NMDA receptor-mediated responses in the rat hippocampus

Effects of metabotropic glutamate (mGlu) receptors on calcium-induced long-term potentiation (LTP) of α-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate (AMPA) and N-methyl-d-aspartate (NMDA) receptor-mediated components were investigated in rat hippocampal slices using whole-cell patch-clamp recordings of excitatory postsynaptic currents (EPSCs). Calcium-induced LTP comprises a parallel, long-lasting increase of AMPA and NMDA receptor-mediated components. The calcium-induced LTP of the AMPA receptor-mediated component can be significantly attenuated by the use of a selective NMDA antagonist. (R.S)-α-methyl-4-carboxyphenylglycine (MCPG), a selective antagonist of mGlu receptors, abolished the long-lasting increase of both AMPA and NMDA receptor-mediated components observed in calcium-induced LTP. In current clamp mode, the application of a high calcium alone or Schaffer fiber stimulation alone (20 Hz) only generated a short-term increase in the firing rate of evoked action potentials. Conversely, a long-term increase in the firing rate was observed if Schaffer fiber stimulation (20 Hz) accompanied the perfusion of high calcium. These results suggest that calcium-induced LTP involves a parallel, long-lasting enhancement in ionotropic AMPA and NMDA receptor-mediated components. More importantly, the mGlu receptor plays a critical role in the establishment of both AMPA and NMDA receptor-mediated components underlying calcium-induced LTP. In addition, the present study also described an experimental condition in which the coapplication of the high calcium pulse and Schaffer fiber stimulation (20 Hz) can synergistically elicit a long-term increase of neuronal excitability.     

Neurotransmitter receptor sites in human hippocampus: a quantitative autoradiographic study

The distribution of serotonin₁, cholinergic-muscarinic, α-adrenergic and opiate-type receptors was studied in the human hippocampus by quantitative autoradiography. Each receptor type exhibited a unique distribution. Serotonin₁ receptors were found to predominate in the subiculum, whereas α₁-adrenergic receptor are absent in that area and present at high levels in the dentate and the CA fields. High densities of muscarinic receptors appear in the subiculum and dentate gyrus. Opiate receptors are restricted to a medial aspect of the subiculum, with lower levels in the dentate and CA fields. The high anatomical resolution and quantitative character of the data may make it useful in the investigation of hippocampal pathology in humans.     

Selective synaptic distribution of AMPA and kainate receptor subunits in the outer plexiform layer of the carp retina.

The subunit composition of ionotropic glutamate receptors (GluRs) is extremely diverse and responsible for the diversity of postsynaptic responses to the release of glutamate, which is the major excitatory neurotransmitter in the retina. To understand the functional consequences of this diversity, it is necessary to reveal the synaptic localization and subunit composition of GluRs. We have used immuno light and electron microscopy to localize AMPA and kainate (GluR1, GluR2/3, GluR4, GluR5-7) subunits in identified carp retinal neurons contributing to the outer plexiform layer. GluR1 could not be detected within the outer plexiform layer. Rod and cone horizontal cells all express only GluR2/3 at the tips of their invaginating dendrites. These receptors are also inserted into the membrane of spinules, light-dependent protrusions of the horizontal cell dendrites, flanking the synaptic ribbon of the cone synapse. Bipolar cells express GluR2/3, GluR4, and GluR5-7 at their terminal dendrites invaginating cone pedicles and rod spherules. Colocalization data suggest that each subunit is expressed by a distinct bipolar cell type. The majority of bipolar cells expressing these receptors seem to be of the functional OFF-type; however, in a few instances, GluR2/3 could also be detected on dendrites of bipolar cells that, based on their localization within the cone synaptic complex, appeared to be of the functional ON-type. The spatial arrangement of the different subunits within the cavity of the cone pedicle appeared not to be random: GluR2/3 was found predominantly at the apex of the cavity, GluR4 at its base and GluR5-7 dispersed between the two.     

Subunit selective decrease of AMPA and metabotropic glutamate receptor mRNA expression in rat brain by systemic administration of the NMDA receptor blocker MK-801

Glutamate mediates its effects in mammals through both ionotropic and metabotropic receptors. Antagonists of ionotropic N-methyl-d-aspartate (NMDA) glutamate receptors elicit neuroprotective and neurotropic effects that have been attributed to Ca²⁺ block through the membrane ion channel. Nonetheless, molecular and biochemical effects of NMDA receptor antagonism on other glutamate receptor subunits remain poorly understood. We investigated the effects of acute administration of the noncompetitive NMDA receptor antagonist MK-801 on the mRNA expression of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and metabotropic glutamate receptor (mGluR) subunits to determine the contribution of different glutamate receptors in response to blockade of NMDA receptor channels. In situ hybridization to rat brain sections revealed that AMPA receptor subunits GluR3 and GluR4, and mGluR3 were modestly but significantly decreased ∼10–20%, 8 h following 5 mg/kg MK-801 administration. A time course and dose response study revealed that the effect on mGluR3 was reversed by 24 h and occurred significantly at a dose range from 1 to 5 mg/kg. These results indicate that selected AMPA and mGluR subunit mRNAs respond at the RNA level to the blockade of NMDA receptors.