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.     

Localization and Quantitative Autoradiography of Glutamatergic Ligand Binding Sites in Chick Brain

The anatomical localization of glutamate receptor subtype-selective ligand binding sites was investigated in 1-day-old chick brain using quantitative autoradiography. Under the conditions used, the regional distributions of [3H]glutamate, [3H]AMPA (a selective quisqualate receptor ligand) and [3H]kainate binding sites are manifestly different. [3H]l-glutamate binding is densely localized in the telencephalon, particularly in the neostriatum (2.8 pmol/mg protein). In addition, [3H]l-glutamate labels the thalamus, the nucleus mesencephalicus lateralis pars dorsalis, the superficial layers of the optic tectum and the molecular layer of the cerebellum. [3H]AMPA binding sites are most densely localized in the hippocampus (0.90 pmol/mg protein), with an otherwise relatively uniform distribution of binding within the telencephalon. [3H]AMPA also labels the striatum griseum et fibrosum superficiale of the optic tectum and the molecular layer of the cerebellum. [3H]Kainate binding sites are extremely densely packed in the molecular layer of the cerebellum (10 pmol/mg protein). Other regions of [3H]kainate binding include the hyperstriatum and the thalamus. The binding of the NMDA receptor channel blocker [3H]MK-801 is increased in the presence of 1 mM l-glutamate. [3H]MK-801 binding is generally widespread in the telencephalon but is notably absent from the ectostriatum. No evidence of [3H]MK-801 binding sites was detected in the cerebellum, even in the presence of 1 mM l-glutamate. The relatively high densities and the well-defined localizations of the glutamate receptor subtype binding sites suggest that chick brain provides a useful system for the further study of excitatory amino acid receptors.     

Ionotropic glutamate receptor modulation preferentially affects NMDA receptor expression in rat hippocampus

Electrophysiological data suggest that alterations in the function of one glutamate receptor subtype may affect the function of other subtypes. Further, previous studies have demonstrated that NMDA receptor antagonists affect NMDA and kainate receptor expression in rat hippocampus. In order to address the mutual regulation of NMDA, AMPA, and kainate receptor expression in rat hippocampus, we conducted two experiments examining the effects of NMDA and non-NMDA glutamate receptor modulators on NMDA, AMPA, and kainate receptor expression using in situ hybridization and receptor autoradiography. NMDA receptor expression was preferentially affected by systemic treatments, as all drugs significantly altered [(3)H]MK-801 binding, and several drugs increased [(3)H]ifenprodil binding. GYKI52466 and aniracetam treatments resulted in changes in both [(3)H]ifenprodil binding and NR2B mRNA levels, consistent with the association of this subunit and binding site in vitro. There were more modest effects on AMPA and kainate receptor expression, even by direct antagonists. Together, these data suggest that ionotropic glutamate receptors interact at the level of expression. These data also suggest that drug regimens targeting one ionotropic glutamate receptor subtype may indirectly affect other subtypes, potentially producing unwanted side effects.     

Analysis of NMDA Receptors in the Human Spinal Cord

NMDA receptors in postmortem human spinal cord were analyzed using [³H]MK-801 ligand binding and immunoblotting with NMDA receptor subunit-specific antibodies. The averageK_Dfor [³H]MK-801 binding was 1.77 nM with aB_maxof 0.103 pmol/mg. The EC₅₀for stimulation of [³H]MK-801 binding withl-glutamate was 0.34 μM. None of these parameters were affected by postmortem intervals up to 72 h. Immunoblotting of native NMDA receptors showed that NR1, NR2A, NR2C, and NR2D subunits could all be found in the human spinal cord of which NR1 was preferentially located to the dorsal half. Immunoprecipitation of solubilized receptors revealed that NR1, NR2C, and NR2D subunits coprecipitated with the NR2A subunit, indicating that native human spinal cord NMDA receptors are heteroligimeric receptors assembled by at least three different receptor subunits. These results provide a basis for the development of drugs selectively aimed at spinal cord NMDA receptors for the future treatment of spinal cord disorders.     

Alterations in spinal cord excitatory amino acid receptors in amyotrophic lateral sclerosis patients.

Excitatory amino acids (EAA) have been implicated in the pathogenesis of amyotrophic lateral sclerosis (ALS). We have analyzed the distribution of the N-methyl-D-aspartate (NMDA) 1-(1-(2-thienyl)-cyclohexyl) piperidine (TCP), kainate and alpha-amino-3-hydroxy-5-methyl-4 isoxazole propionic acid (AMPA) quisqualate subtypes of EAA receptors using quantitative receptor autoradiography in the cervical and thoracic spinal cords of patients who have died with ALS, and of controls. We observed that in control spinal cords [3H]TCP/NMDA binding sites were located both in the ventral and dorsal horns with the highest densities being situated in lamina II. [3H]AMPA and [3H]kainate binding sites were present almost exclusively in the substantia gelatinosa of the dorsal horn. In ALS, the distribution of these 3 types of receptors was unchanged, but [3H]TCP/NMDA binding was decreased both in the dorsal and ventral horns. [3H]kainate binding was possibly decreased in substantia gelatinosa, of ALS cords. However, the limited sample size available for [3H]kainate binding did not permit statistical analysis. [3H]AMPA binding sites were unaltered in ALS. These results indicate that there is a preferential reduction in NMDA receptors in ALS. We suggest that should an excitotoxic mechanism be involved in the pathogenesis of ALS, then NMDA receptors may be the target of this effect.     

Crucial role of kainate receptors in mediating striatal kainate injection-induced decrease in acetylcholine M(1) receptor binding in rat forebrain

We investigated the roles of kainate-, alpha-amino-3-hydroxy-5-methylisoxazol-4-propionate (AMPA)- and N-methyl-D-aspartate (NMDA)-receptors in mediating striatal kainate injection-induced decrease in the binding of acetylcholine M(1) receptors in rat forebrain. After unilateral intrastriatal injection of kainate (4 nmol), the bindings of [3H]kainate (10 nM), [3H]MK-801 (4 nM) and [3H]pirenzepine (4 nM) to the rat ipsilateral forebrain membranes declined, reaching the lowest on day 2 to 4 and recovering on day 8. Saturation binding studies, performed on day 2 post-injection, showed that kainate (1, 2, 4 nmol) dose-dependently decreased B(max) and K(d) of the three ligands. (+)-5-Methyl-10, 11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801), a selective NMDA receptor channel blocker, antagonised (from a dose of 4 nmol) kainate-induced decreases in the bindings of [3H]kainate (up to approximately 20%), [3H]MK-801 (up to approximately 90%) and [3H]pirenzepine (up to approximately 70%). In contrast, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a selective non-NMDA receptor antagonist, almost completely abolished (from a dose of 12 nmol) kainate-induced decreases in the bindings of all the three ligands (up to approximately 95-98%). Cyclothiazide, a selective potentiator that enhances AMPA receptor-mediated responses, significantly enhanced (from a dose of 4 nmol) kainate-induced decrease in the binding of [3H]kainate but not that of [3H]pirenzepine or [3H]MK-801. In summary, these results indicate that striatal kainate injection-induced decrease in the binding of acetylcholine M(1) receptors in rat forebrain is dependent on activation of kainate receptors and, to a certain extent, a consequent involvement of NMDA receptors. These and previous studies provide some evidence showing that kainate receptors might play a crucial role in regulating excitatory amino acids (EAA)-modulated cholinergic neurotransmission in the central nervous system (CNS).     

Autoradiographical analysis of excitatory amino acid binding sites in rat hippocampus during the development of hippocampal kindling

Binding sites for excitatory amino acids have been determined by autoradiographical procedures in the rat hippocampus and striatum during hippocampal kindling. The binding sites measured were the N-methyl-D-aspartate (NMDA)-sensitive sites for L-[3H]glutamate and [3H]MK-801 sites (transmitter recognition site and ion channel of the NMDA receptor, respectively), [3H]alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) sites (quisqualate receptor), [3H]kainate sites (kainate receptor) and NMDA-insensitive sites for L-[3H]glutamate. In general, little change was apparent in the hippocampus or striatum for any of these binding sites when assessed 48 h after attaining stages 1/2, 3 or 5 of kindling. These results suggest that hippocampal kindling does not bring about a change in the excitatory amino acid receptor binding sites examined, and that the appearance of an NMDA receptor-mediated component to synaptic responses in the hippocampus produced by kindling, cannot be explained on this basis.     

Olfactory bulbectomy alters NMDA receptor levels in the rat prefrontal cortex

Olfactory bulbectomized (OBX) rats show a variety of behavioral and biochemical deficits that parallel human depression. We investigated the expression of glutamate receptor subtypes in cortical and subcortical brain regions following bilateral olfactory bulbectomy in adult rats. Quantitative receptor autoradiography using [(125)I]MK-801 (NMDA receptor), [(3)H]AMPA (AMPA receptor), and [(3)H]kainate (kainate receptor) was performed on brain sections at 1-5 weeks following olfactory bulbectomy. Our results show an elevation of NMDA receptors in the medial prefrontal cortex within 1 week following bulbectomy, which persisted up to at least 5 weeks post-bulbectomy. Neither kainate nor AMPA receptors were altered in any brain region examined. The potential significance of these results is discussed in light of experimental findings supporting a role for NMDA receptors in the mechanism of action of antidepressant drugs and the pathophysiology of major depression.     

Crucial role of kainate receptors in mediating striatal kainate injection-induced decrease in acetylcholine M1 receptor binding in rat forebrain

We investigated the roles of kainate-, α-amino-3-hydroxy-5-methylisoxazol-4-propionate (AMPA)- and N-methyl- -aspartate (NMDA)-receptors in mediating striatal kainate injection-induced decrease in the binding of acetylcholine M₁ receptors in rat forebrain. After unilateral intrastriatal injection of kainate (4 nmol), the bindings of [³H]kainate (10 nM), [³H]MK-801 (4 nM) and [³H]pirenzepine (4 nM) to the rat ipsilateral forebrain membranes declined, reaching the lowest on day 2 to 4 and recovering on day 8. Saturation binding studies, performed on day 2 post-injection, showed that kainate (1, 2, 4 nmol) dose-dependently decreased B_max and K_d of the three ligands. (+)-5-Methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801), a selective NMDA receptor channel blocker, antagonised (from a dose of 4 nmol) kainate-induced decreases in the bindings of [³H]kainate (up to 20%), [³H]MK-801 (up to 90%) and [³H]pirenzepine (up to 70%). In contrast, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a selective non-NMDA receptor antagonist, almost completely abolished (from a dose of 12 nmol) kainate-induced decreases in the bindings of all the three ligands (up to 95–98%). Cyclothiazide, a selective potentiator that enhances AMPA receptor-mediated responses, significantly enhanced (from a dose of 4 nmol) kainate-induced decrease in the binding of [³H]kainate but not that of [³H]pirenzepine or [³H]MK-801. In summary, these results indicate that striatal kainate injection-induced decrease in the binding of acetylcholine M₁ receptors in rat forebrain is dependent on activation of kainate receptors and, to a certain extent, a consequent involvement of NMDA receptors. These and previous studies provide some evidence showing that kainate receptors might play a crucial role in regulating excitatory amino acids (EAA)-modulated cholinergic neurotransmission in the central nervous system (CNS).     

Quantitative autoradiographic distribution of glutamate receptors in the cervical segment of the spinal cord of the wobbler mouse

The reduction of glutamate content has been observed in the spinal cord of the wobbler mouse, a purported model of amyotrophic lateral sclerosis (ALS). To elucidate glutamate receptors in the wobbler spinal cord, we measured densities ofN-methyl-d-aspartate (NMDA), kainate, -amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and metabotropic glutamate (mGlu) binding sites using in vitro autoradiography. In wobbler mice, NMDA, kainate, and AMPA binding sites were increased in the dorsal horn and kainate binding sites were also increased in the intermediate zone. However, mGlu binding was unchanged. These results disagree with those observed in ALS spinal cords, in which NMDA and kinate binding sites are decreased. The wobbler mouse may have the glutamate dysfunction, but in a different way from ALS.     

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.     

Inhibition of N-methyl-D-aspartate evoked sodium flux by MK-801

The inhibition of N-methyl-D-aspartate (NMDA) stimulated 22Na+ efflux from rat hippocampal slices was studied using competitive and non-competitive receptor antagonists. There was a good correlation between the abilities of the competitive antagonists to block NMDA evoked 22Na+ efflux and their potencies as inhibitors of L-[3H]glutamate binding. The recently reported novel NMDA receptor antagonist, (+)-5-methyl-16,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801) was shown to non-competitively inhibit NMDA stimulated 22Na+ efflux with an IC50 value of 0.4 microM. Relatively high (10 microM) concentrations of MK-801 had no effects on quisqualic acid, alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA), or kainic acid stimulated efflux. However, MK-801 was able to block 22Na+ efflux induced by ibotenic acid and L-homocysteic acid, amino acids that act as NMDA receptor agonists. MK-801, (-)-MK-801, and non-competitive NMDA receptor antagonists of the arylcyclohexylamine and dioxolane classes inhibited NMDA stimulated 22Na+ efflux with potencies that reflected their abilities to compete for [3H]MK-801 binding sites in rat cortical membranes. These results indicate the utility of the 22Na+ efflux assay in studying the properties of NMDA receptors and confirm the nature and selectivity of the inhibition of NMDA receptor linked ion channel activation by MK-801.     

Receptor autoradiographic mapping of the mesial motor and premotor cortex of the macaque monkey

This study analyzes regional and laminar distribution patterns of neurotransmitter binding sites in the motor areas of the macaque mesial frontal cortex. Differences in distribution patterns are compared with the cytoarchitectonic parcellation. Binding sites were analyzed with quantitative in vitro receptor autoradiography in unfixed brains of five macaque monkeys. Alpha-amino-3-hydroxy-5-methyl-4-isoxalone propionic acid (AMPA), kainate, and N-methyl-D-aspartate (NMDA) binding sites were labeled with [³H]AMPA, [³H]kainate, and [³H]MK-801, respectively, muscarinic binding sites with [³H]pirenzepine or [³H]oxotremorine-M, noradrenergic binding sites with [³H]prazosin or [³H]UK-14304, γ-aminobutyric acid (GABA)_A binding sites with [³H]muscimol, and serotoninergic binding sites with [³H]ketanserine. Adjacent sections were stained with a modified Nissl method for cytoarchitectonic analysis. In the motor areas F1, F3, and F6, [³H]AMPA, [³H]pirenzepine, and [³H]oxotremorine-M binding was maximal in layers II, III, and V, and [³H]kainate binding was maximal in layers V and VI. Clear-cut changes in laminar distribution patterns of [³H]AMPA, [³H]kainate, and [³H]oxotremorine-M binding sites very closely matched corresponding cytoarchitectonic borders. Mean areal binding densities of all ligands to F1, F3, and F6 were plotted as polar plots for each area. A polygon was obtained for each area (“neurochemical fingerprint”) when all the density values belonging to one area were connected with each other. The “neurochemical fingerprints” of F1, F3, and F6 were virtually identical in shape but increased in size from F1 to F6. This result reflects the functional similarity of these motor-related areas and possibly correlates with their differential involvement in motor control. Areas F1, F3, and F6 can thus be grouped into one “neurochemical family” of areas. J. Comp. Neurol. 397:231–250, 1998. © 1998 Wiley-Liss, Inc.     

Excitatory amino acid receptor regulation after subthalamic nucleus lesions in the rat

The subthalamic nucleus plays a pivotal role in the regulation of basal ganglia output. Recent electrophysiologic, lesion and immunocytochemical studies suggest that the subthalamic nucleus uses an excitatory amino acid as a neurotransmitter. After complete ablation of the subthalamic nucleus, we have examined the NMDA, AMPA, kainate and metabotropic subtypes of excitatory amino acid receptors in two major subthalamic projection areas (globus pallidus and substantia nigra pars reticulata) with quantitative autoradiography. Two weeks after ablation, binding sites for [³H]AMPA and [³H]kainate increased in substantia nigra pars reticulata ipsilateral to the lesion. In globus pallidus on the lesioned side, [³H]glutamate binding to the NMDA recognition site decreased. The results suggest that glutamate receptors regulate after interruption of subthalamic nucleus output.     

Characterization of neuronal migration disorders in neocortical structures: quantitative receptor autoradiography of ionotropic glutamate, GABAA and GABAB receptors

Epileptiform activity was previously described [ Luhmann et al. (1998 ) Eur.J. Neurosci., 10, 3085–3094] in the neocortex of the adult rat following freeze lesioning of the newborn neocortex. After a survival time of 3 months, a small area of dysplastic cortex surrounded by histologically normal (exofocal) neocortex was observed. The dysplastic cortex is characterized by the formation of a small sulcus and a three- to four-layered architecture. Two questions are addressed here: (i) is the hyperexcitability associated with changes in binding to major excitatory and inhibitory transmitter receptors in the dysplastic cortex?; and (ii) do such changes also occur in the exofocal cortex? Alterations in binding to glutamatergic N-methyl-d -aspartate (NMDA), (±)-α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), kainate and GABA_A and GABA_B (γ-aminobutyric acid) receptors are demonstrated with quantitative in vitro receptor autoradiography by using the ligands [³H]MK-801, [³H]AMPA, [³H]kainate, [³H]muscimol and [³H]baclofen, respectively. In the dysplastic cortex, the binding to NMDA, AMPA and kainate receptors is significantly increased, whereas the binding to GABA_A and GABA_B receptors is reduced. Exofocal areas of the lesioned hemisphere show an imbalance between excitatory and inhibitory receptor binding with an up-regulation of the binding to AMPA and kainate, and a down-regulation to GABA_A receptors. The binding to GABA_B and NMDA receptors is not significantly changed in the exofocal areas. The imbalance between excitatory and inhibitory receptors may cause the hyperexcitability, as previously found in the identical experimental model, and may also induce epileptiform activity in the human cortex with migration disorders.     

Autoradiographic evidence that NMDA receptor-coupled channels are located postsynaptically and not presynaptically in the perforant path-dentate granule cell system of the rat hippocampal formation

In vitro quantitative autoradiography with [3H]MK-801 was used to determine Kd and Bmax values for the NMDA receptor-coupled channel in subregions of the rat hippocampal formation. A single form of the channel with an apparent Kd in the 15-20 nM range was found for [3H]MK-801 binding in the presence of both 1 microM glutamate and 1 microM glycine. Specific binding was highest in the molecular layer of the dentate gyrus, followed by CA1 stratum radiatum and CA1 stratum oriens. Fewer binding sites were observed in the hilus of the dentate gyrus, cerebral cortex, CA1 stratum pyramidale, CA3 subregion (stratum oriens, stratum pyramidale, stratum radiatum), and thalamus. Selective destruction of dentate granule cells by colchicine microinjections reduced the amount of specific [3H]MK-801 binding by half in the molecular layer of the dentate, compared to intact tissue. [3H]MK-801 binding did not change in other hippocampal subregions as a consequence of colchicine injection. Electrolytic entorhinal cortical lesions produced no changes in regional MK-801 binding site density in any of the regions under study. To address the tissue shrinkage following entorhinal cortex lesions, detailed analysis of the binding site density per fixed (16 microns) length of granule cell dendrite, and of the aggregate density across the entire molecular layer revealed no change in the number of MK-801 binding sites per unit length of dendrite in the molecular layer of the dentate gyrus. These findings indicate that NMDA receptor-coupled channels are confined to a postsynaptic location in the perforant path-dentate granule cell system of the adult rat.     

Glutamate receptors in striatum and substantia nigra: Effects of medial forebrain bundle lesions

We examined NMDA-sensitive [³H]glutamate, [³H]AMPA, [³H]kainate and metabotropic-sensitive [³H]glutamate binding sites in neostriatum and substantia nigra pars reticulata (SNr) in rats after unilateral 6-hydroxydopamine lesions of the medial forebrain bundle. One week after the lesion, NMDA, AMPA, kainate and metabotropic receptors were decreased in the ipsilateral neostriatum, whereas at three months NMDA receptors were increased while AMPA, kainate and metabotropic receptors were not changed. In the SNr at one week, only AMPA and metabotropic receptors were significantly decreased whereas three months after the lesion NMDA, AMPA and kainate binding sites were decreased. The early decrease of excitatory amino acid receptors in the striatum is likely to reflect degeneration of dopaminergic fibers, suggesting that specific subpopulations of excitatory amino acid binding sites are located on dopaminergic terminals.     

Alterations in regional brain metabolism in genetic and pharmacological models of reduced NMDA receptor function

A mouse line has been developed that expresses low levels of the NMDA R1 (NR1) subunit of the NMDA receptor [Cell 98 (1999) 427]. These NR1 hypomorphic mice represent an experimental model of reduced NMDA receptor function that may be relevant to the pathophysiology of schizophrenia. To further characterize the neurobiological phenotype resulting from developmental NMDA receptor hypofunction, regional brain metabolic activity was assessed by autoradiographic analysis of 14C-2-deoxyglucose (2-DG) uptake. In addition, ligand binding to NMDA, AMPA, and kainate receptors was measured by quantitative autoradiography. MK-801 binding to NMDA receptors was reduced markedly throughout the brain of the NR1 hypomorphic mice. However, no alteration in 3H-AMPA or 3H-kainate binding was apparent in any region examined. Neuroanatomically specific alterations in regional 2-DG uptake were observed in the NR1 hypomorphic animals. Reduced relative 2-DG uptake was observed in the medial prefrontal and anterior cingulate cortices. Altered patterns of 2-DG uptake were also found in neocortical regions, with selective reductions of uptake in layer 6 in frontal regions of somatosensory and motor cortices. These data indicate alterations in cortical circuitry in the NR1 hypomorphic animals and are consistent with functional imaging studies in chronic schizophrenia patients which typically show reduced frontal cortical metabolic activity. Reduced relative 2-DG uptake was also found in the caudate, accumbens, hippocampus, and select thalamic regions in the NR1-deficient mice. However, in many other brain regions no alteration in 2-DG uptake was observed. The alterations in 2-DG uptake in the NR1 hypomorphic mice were distinctly different compared to those observed after acute challenge with the selective NMDA antagonist MK-801 in wild-type mice. The altered patterns of brain 2-DG uptake in the NR1 hypomorphic mice found in the present work, together with the altered behavioral phenotypes previously described, suggest that the mice may provide a valuable model to study novel therapeutic strategies to counteract the neurobiological consequences of chronic developmental NMDA receptor hypofunction.