Properties of quisqualate-sensitive L-[3H]glutamate binding sites in rat brain as determined by quantitative autoradiography

Amantadine: reappraisal of the timeless diamond-target updates and novel therapeutic potentials

The aim of the current review was to provide a new, in-depth insight into possible pharmacological targets of amantadine to pave the way to extending its therapeutic use to further indications beyond Parkinson's disease symptoms and viral infections. Considering amantadine's affinities in vitro and the expected concentration at targets at therapeutic doses in humans, the following primary targets seem to be most plausible: aromatic amino acids decarboxylase, glial-cell derived neurotrophic factor, sigma-1 receptors, phosphodiesterases, and nicotinic receptors. Further three targets could play a role to a lesser extent: NMDA receptors, 5-HT3 receptors, and potassium channels. Based on published clinical studies, traumatic brain injury, fatigue [e.g., in multiple sclerosis (MS)], and chorea in Huntington's disease should be regarded potential, encouraging indications. Preclinical investigations suggest amantadine's therapeutic potential in several further indications such as: depression, recovery after spinal cord injury, neuroprotection in MS, and cutaneous pain. Query in the database http://www.clinicaltrials.gov reveals research interest in several further indications: cancer, autism, cocaine abuse, MS, diabetes, attention deficit-hyperactivity disorder, obesity, and schizophrenia.

Immunohistochemical detection by immersion fixation with Carnoy solution of particular non-N-methyl-D-aspartate receptor subunits in murine hippocampus

Immunoblotting analysis revealed heterologous distribution profiles of the non-N-methyl-D-aspartate (NMDA) receptor subunits, GluR1, GluR2 and GluR6, in membrane fractions prepared from murine discrete brain structures including hippocampus. In coronal sections fixed with paraformaldehyde (PA) solution after dissection from mice perfused with 4% PA, however, no marked immunoreactivity was detected to GluR6 subunit in any hippocampal subregions, with high immunoreactivities to both GluR1 and GluR2 subunits in the strata oriens, radiatum and lacunosum-moleculare of the CA1 and CA3 subfields and the stratum moleculare of the dentate gyrus in hippocampus. In coronal, sagittal and horizontal sections fixed with Carnoy solution after dissection from animals decapitated, by contrast, high immunoreactivity was additionally detected to GluR6 subunit in the stratum lucidum of hippocampus. The systemic administration of kainate not only resulted in marked neuronal losses along the CA1-CA4 pyramidal layers 1 week later, but also led to significant decreases in immunoreactivities to GluR1, GluR2 and GluR6 subunits in the CA1 and CA3 subfields on brain coronal sections prepared by immersion fixation with Carnoy solution. These results suggest that immersion fixation with Carnoy solution may be suitable and appropriate for reproducible and quantitative immunohistochemical detection of particular non-NMDA receptor subunits in murine hippocampus.

Autoradiography of glutamate receptor binding in adult Lurcher mutant mice

The mutation Lurcher, resulting from a gain of malfunction of the delta2 glutamate receptor expressed specifically by cerebellar Purkinje cells, causes a primary total loss of these neurons of the cerebellar cortex, as well as the secondary degeneration of cerebellar granule and inferior olive neurons. The distributions of glutamate receptors sensitive to amino-methylisoxazole-propionic acid (AMPA), to kainic acid (KA), and to N-methyl-D-aspartic acid (NMDA) as well as metabotropic sites (MET1 and MET2) were examined in wild type and Lurcher mice by quantitative autoradiography. This study was undertaken to determine the gene effect on the distribution of the various glutamate receptor subtypes, as well as how the cerebellar lesion affects the glutamatergic system in other brain regions. In cerebellum, there were postsynaptic AMPA and metabotropic receptors on Purkinje cells, postsynaptic NMDA receptors on granule cells, as well as KA receptors on granule cells or on parallel fibers. Taking into account surface areas, binding to all receptor subtypes was lower in the cerebellar cortex of Lurcher mutants than in wild type mice, while in the deep cerebellar nuclei only KA receptors were diminished. In other brain regions, the alterations followed always the same pattern characterized by a decrease of NMDA and KA receptors but with an increase of AMPA sites; these reciprocal changes were seen in thalamus. neostriatum, limbic regions, and motor cerebral cortical regions. Comparisons of glutamate receptor distribution in Lurcher mutants and in human autosomal cerebellar ataxia may permit further understanding of the role of glutamate-induced toxicity on neuronal death in these heredo-degenerative diseases.

Selective metabotropic receptor agonists distinguish non-ionotropic glutamate binding sites

Metabotropic glutamate receptors (mGluRs) are thought to mediate diverse processes in brain including synaptic plasticity and excitotoxicity. These receptors are often divided into three groups by their pharmacological profiles. [3H]Glutamate binding in the presence of compounds selective for ionotropic glutamate receptors can be used as a general assay for these receptors; subtypes of this non-ionotropic [3H]glutamate binding differ in both pharmacology and anatomical distribution, and are differentially sensitive to quisqualate. The characteristics of these binding sites are consistent with those of group 1 (high-affinity quisqualate) and group 2 (low-affinity quisqualate) mGluRs. Under our assay conditions, no [3H]glutamate binding to group 3-like (L-AP4 sensitive) sites could be demonstrated. We have attempted to characterize particular agents which may selectively measure [3H]glutamate binding to mGluR subtypes. We used two isomers of 2-(carboxycyclopropyl)glycine, L-CCG-I and L-CCG-II, and the (2S,1'R,2'R,3'R) isomer of 2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV) as competitors of non-ionotropic [3H]glutamate binding sites. DCG-IV clearly distinguishes two binding sites. Quantitative levels of DCG-IV binding by anatomic region correlate with quisqualate-defined binding subtypes: high-affinity DCG-IV binding correlates with low-affinity quisqualate binding, whereas low-affinity DCG-IV binding correlates with high-affinity quisqualate binding. L-CCG-II displaces only one type of non-ionotropic [3H]glutamate binding, corresponding to high-affinity quisqualate binding. Therefore DCG-IV and L-CCG-II at appropriate concentrations appear to distinguish binding to putative group 2 vs. group 1 mGluRs. L-CCG-I displaces both high- and low-affinity quisqualate binding sites, but unlike the other two compounds, does not clearly distinguish between them.

Postnatal ontogeny of glutamate receptors in the rat nucleus tractus solitarii and ventrolateral medulla

The nucleus tractus solitarii and the ventrolateral medulla are two brainstem regions involved in regulation of autonomic functions. Glutamate (Glu) receptors localized within these two regions play a key role in neural control of swallowing and breathing and in blood pressure regulation. In the present study, postnatal changes in global [3H]Glu binding and in [3H]Glu binding to N-methyl-D-aspartate (NMDA) receptors were analyzed in the nucleus tractus solitarii and the ventrolateral medulla using in vitro receptor autoradiography. Similar results were obtained in both regions. When expressed as density values (fmol/mg tissue), both global and NMDA-sensitive Glu binding increased by approximately 50-70% between birth and postnatal day 9 (P9) and then decreased until P30. When expressed as binding per nucleus (i.e., after correction for tissue growth), global Glu binding still increased between birth and P9 and decreased between P9 and P30 whereas NMDA-sensitive binding increased until P9 and remained stable thereafter. Saturation studies showed a postnatal increase in Glu receptor number per nucleus, which occurred mainly between birth and P9, and a decrease in Glu receptor affinity between P9 and adulthood. These results indicate that dramatic changes in glutamatergic neurotransmission occur in the nucleus tractus solitarii and the ventrolateral medulla during the first month of postnatal life. They suggest that both neonates and young animals may not be fully mature as regard to central regulation of autonomic functions.

The glutamate uptake inhibitor L-trans-2,4-pyrrolidine dicarboxylate is neurotoxic in neonatal rat brain

High-affinity glutamate uptake (HAGU) transporters rapidly remove released glutamate from the synaptic cleft. If HAGU is suppressed, neurotoxic concentrations of excitatory amino acids may accumulate. To seek further evidence in support of the neurotoxicity of endogenous glutamate in the developing brain, we assessed the neurotoxicity of the selective HAGU inhibitor L-trans-2,4-pyrrolidine dicarboxylate (L-PDC) in postnatal day 7 (PND 7) rats. The hippocampus of PND 7 rats is susceptible to EAA agonist-mediated injury; features of injury include atrophy and neuronal loss. Since HAGU is energy-dependent, we hypothesized that moderate hypoxia would increase L-PDC-mediated injury by further suppressing HAGU. L-PDC was stereotaxically injected into dorsolateral hippocampus of PND 7 rats (568 nmol, n = 20). Prior to return to the dam, rats were divided into two groups, one of which was subjected to moderate hypoxia (3 h, FiO2 = 0.08) (n = 11; 2 died acutely). On PND 12, hippocampal neuropathology was assessed by a blinded observer using a five-point scale and also by measuring hippocampal cross-sectional areas with computerized image analysis. Three brains were excluded from analysis, since markedly asymmetric tissue sectioning precluded valid side-to-side comparison of hippocampal areas. Injection of L-PDC alone elicited focal pyramidal cell loss (6/7); in the (L-PDC + hypoxia) group, injury was significantly increased (median scores: L-PDC = 2; [L-PDC + hypoxia] = 3.5; p < 0.005). Hippocampal atrophy was noted only after L-PDC + hypoxia (4/8) (percent right-left difference in mean hippocampal area [+/- SE]: L-PDC = 2.5% [+/- 2.6]; [L-PDC + hypoxia] = 8.9% [+/- 3.2]; p < 0.02). In tissue from PND 7 rats, L-PDC (10 microM) inhibited hippocampal synaptosomal HAGU by > 85%; at the same concentration, L-PDC did not displace [3H]glutamate from NMDA- or AMPA-sensitive hippocampal binding sites. These results support the hypothesis that increased synaptic accumulation of endogenous excitatory amino acid neurotransmitters may produce hippocampal injury in perinatal rodents.

Characteristics and localization of high-affinity kainate sites in slide-mounted sections of rat cerebellum

The characteristics and localization of high-affinity, kainic acid (KA)-sensitive glutamate sites have been investigated using a radioreceptor procedure to provide insights into specific high-affinity KA receptors identified in molecular biological studies. Binding sites identified by employing [3H]KA. in the presence of the AMPA-selective antagonist NBQX (1 microM), and slide-mounted, coronal sections of rat cerebellum were of high-affinity (Kd 6 nM) and possessed an unique pharmacological profile. Specific binding was to a single population of sites and fully inhibited by kainoids and glutamate, but essentially insensitive to AMPA and willardiines. Autoradiography revealed that the high-affinity KA sites were localized to the granule cell layer of cerebellum. The KA site resembled both the KA receptor found on spinal motoneurones and the KA-2 type of receptor.

GABA, muscarinic cholinergic, excitatory amino acid, neurotensin and opiate binding sites in the octavolateralis column and cerebellum of the skate Raja nasuta (Pisces: Rajidae)

As part of a study of signal processing in the electro- and mechanosensory systems we have screened the octavolateralis column of the skate for GABAA, muscarinic cholinergic, excitatory amino acid, neurotensin and opiate binding sites using autoradiography following in vitro labelling of cryostat sections with tritiated ligands. The presence and distribution of these binding sites is compared between the octavolateralis column and the corpus cerebellum. GABAA binding sites were located in high concentrations in the granule cell regions of the cerebellum and octaval columns, with much lower concentrations in the Purkinje cell layer of the corpus cerebellum. Little or no labelling was evident in all molecular layer areas. Displacement studies using the discriminating ligand CL218,872 indicated that the GABAA binding sites were predominantly of the GABAA/benzodiazepine Type II variety. M1 muscarinic cholinergic binding sites were found in high concentrations in all granule cell areas and in lower concentrations in the molecular layer of the octavolateralis column, with an absence of labelling in the molecular layer of the corpus cerebellum. Kainic acid and AMPA binding sites were present in very high concentrations in all molecular layer areas. Glutamate binding was present in the molecular layer of the octavolateralis column and in some restricted regions of the dorsal granular ridge, whereas phencyclidine binding sites were sparse or absent. Neurotensin binding sites were strongly present in all granule cell areas and evident in the molecular layer of the octavolateralis column. There was evidence for opiate binding sites in the molecular layer of both the dorsal and medial octavolateralis nucleus.(ABSTRACT TRUNCATED AT 250 WORDS)

Glutamate receptor binding in juvenile and adult Rana pipiens CNS

Autoradiographic methods were used to map NMDA- and quisqualate-sensitive glutamate binding sites in the brain of mature and juvenile Rana pipiens frogs. NMDA- and quisqualate-sensitive sites were consistently co-localized in the CNS. The highest glutamate binding occurred in the telencephalon, hypothalamus, and cerebellum. Glutamate binding sites were also specifically localized in visual pathways, including the superficial neuropil of the optic tectum, consistent with glutamate being the retinal ganglion cell neurotransmitter. The distribution of glutamate binding sites in the brain of juvenile postmetamorphic frogs was similar to that in adults. In general, Quis binding increased about twofold in adults compared to juveniles, whereas NMDA binding did not show a comparable developmental increase. To test whether glutamate binding sites are located on retinal axon terminals or on tectal cell dendrites in the optic tectum, juvenile postmetamorphic frogs were enucleated unilaterally, and receptor binding was performed following 1, 3, 7, and 14 days survival. The denervated tectal neuropil showed a delayed decrease in NMDA- and quisqualate-sensitive binding, consistent with the receptors being located on postsynaptic tectal cell dendrites.

Pharmacological characterization of glutamate binding sites in cultured cerebellar granule cells and cortical astrocytes

Membranes prepared from cerebellar granule cells and cortical astrocytes exhibited specific, saturable binding of L-[3H]glutamate. The apparent binding constant KD was 135 nM and 393 nM and the maximal binding capacity Bmax 42 and 34 mumol/kg in granule cells and astrocytes, respectively. In granule cells the binding was strongly inhibited by the glutamate receptor agonists kainate, quisqualate, N-methyl-D-aspartate (NMDA), L-homocysteate and ibotenate, and the antagonist DL-5-aminophosphonovalerate. In astrocytes, only quisqualate among these was effective. L-Aspartate, L-cysteate, L-cysteinesulphinate and gamma-D-glutamylglycine were inhibitors in both cell types. The binding was totally displaced in both cell types by L-cysteinesulphinate with IC50 in the micromolar range. In astrocytes the binding was also totally displaced by quisqualate, but in granule cells only partially by NMDA, kainate and quisqualate in turn. It is concluded from the relative potencies of agonists and antagonists in [3H]glutamate binding that cerebellar granule cells express the NMDA, kainate and quisqualate types of the glutamate receptor, while only the quisqualate-sensitive binding seems to be present in cortical astrocytes.

Excitatory amino acid AMPA receptor mRNA localization in several regions of normal and neurological disease affected human brain. An in situ hybridization histochemistry study

In situ hybridization histochemistry was used to localize the mRNAs coding for four alpha-aminoisoxazole propionic acid-sensitive glutamate receptor subunits in human brain (age range 51-95 years, postmortem delay 4.5-10 h). High levels of the B receptor subunit mRNA were present in all the studied regions, followed by the A-subunit and the C-subunit. Only very low levels of the D-subunit mRNA were detected. In hippocampus, the mRNA coding for the B-subunits of the glutamate receptor was observed in granule cells of dentate gyrus and in the pyramidal cells of Ammon's horn. In cortex, the highest levels of glutamate receptor subunit mRNAs were found in layer I and layers III-IV of entorhinal and temporal cortex, although significant levels were also observed in the other cell layers. A differential distribution was seen in cerebellum where the A-subunit mRNA is expressed mainly by Purkinje cells, while the B-subunit mRNA is present in the internal granule cell layer. These results correlate well with previous data from autoradiographic studies on the localization of excitatory amino acid binding sites in human brain and pinpoint the cells where these receptors are synthesized. In situ hybridization in the hippocampus of patients affected by Alzheimer's disease (age range 77-82 years, postmortem delay 19-25.5 h) revealed a decrease on the content of the mRNAs coding for these excitatory amino acid receptors, while an increase was detected in surgically dissected epileptic human hippocampi. These results corroborate and extend the previous data from in vitro autoradiography and suggest alteration of the excitatory amino acid disfunction during these neurodegenerative processes.

The chemical nature of the main central excitatory transmitter: a critical appraisal based upon release studies and synaptic vesicle localization

The chemical nature of the central transmitter responsible for fast excitatory events and other related phenomena is analysed against the historical background that has progressively clarified the structure and function of central synapses. One of the problems posed by research in this field has been whether one or more of the numerous excitatory substances endogenous to the brain is responsible for fast excitatory synaptic transmission, or if such a substance is, or was, a previously unknown one. The second question is related to the presence in the CNS of three main receptor types related to fast excitatory transmission, the so-called alpha-amino-3-hydroxy-5-methylisoxazole propionic acid, kainate and N-methyl-D-aspartate receptors. This implies the possibility that each receptor type might have its own endogenous agonist, as has sometimes been suggested. To answer such questions, an analysis was done of how different endogenous substances, including L-glutamate, L-aspartate, L-cysteate, L-homocysteate, L-cysteine sulfinate, L-homocysteine sulfinate, N-acetyl-L-aspartyl glutamate, quinolinate, L-sulfoserine, S-sulfo-L-cysteine, as well as possible unknown compounds, were able to fulfil the more important criteria for transmitter identification, namely identity of action, induced release, and presence in synaptic vesicles. The conclusion of this analysis is that glutamate is clearly the main central excitatory transmitter, because it acts on all three of the excitatory receptors, it is released by exocytosis and, above all, it is present in synaptic vesicles in a very high concentration, comparable to the estimated number of acetylcholine molecules in a quantum, i.e. 6000 molecules. Regarding a possible transmitter role for aspartate, for which a large body of evidence has been presented, it seems, when this evidence is carefully scrutinized, that it is either inconclusive, or else negative. This suggests that aspartate is not a classical central excitatory transmitter. From this analysis, it is suggested that the terms alpha-amino-3-hydroxy-5-methylisoxazole propionic acid, kainate and N-methyl-D-aspartate receptors, should be changed to that of glutamate receptors, and, more specifically, to GLUA, GLUK and GLUN receptors, respectively. When subtypes are described, a Roman numeral may be added, as in GLUNI, GLUNII, and so on.

Age-related changes in excitatory amino acid receptors in two mouse strains

The present study examined the binding of [3H]-L-glutamate to NMDA receptors, [3H]-kainate to kainate receptors, and [3H]-AMPA to AMPA/quisqualate receptors in the brains of C57Bl and BALB/c mice as a function of increasing age. Significant decreases in binding to NMDA receptors occurred with increasing age (3 to 30 months) in a majority of cortical and hippocampal brain regions from the C57Bl and BALB/c strains of mice. Significant decreases in binding to kainate and AMPA receptors were found in the inner frontal and parietal cortices and stratum lacunosum/moleculare of CA1 in both strains. These regions also exhibited the greatest percent decline in NMDA binding sites with aging. The loss of NMDA receptors in the stratum lacunosum/moleculare of CA1 was greater in the BALB/c mice than the C57Bl strain. These results demonstrate that a few brain regions have age-associated reductions in all three ionotropic EAA receptors; however, the NMDA receptor appears to be selectively vulnerable to the aging process throughout much of the cerebral cortex and hippocampus.

Neurotransmitter regulation of dopamine neurons in the ventral tegmental area

Over the last 10 years there has been important progress towards understanding how neurotransmitters regulate dopaminergic output. Reasonable estimates can be made of the synaptic arrangement of afferents to dopamine and non-dopamine cells in the ventral tegmental area (VTA). These models are derived from correlative findings using a variety of techniques. In addition to improved lesioning and pathway-tracing techniques, the capacity to measure mRNA in situ allows the localization of transmitters and receptors to neurons and/or axon terminals in the VTA. The application of intracellular electrophysiology to VTA tissue slices has permitted great strides towards understanding the influence of transmitters on dopamine cell function, as well as towards elucidating relative synaptic organization. Finally, the advent of in vivo dialysis has verified the effects of transmitters on dopamine and gamma-aminobutyric acid transmission in the VTA. Although reasonable estimates can be made of a single transmitter's actions under largely pharmacological conditions, our knowledge of how transmitters work in concert in the VTA to regulate the functional state of dopamine cells is only just emerging. The fact that individual transmitters can have seemingly opposite effects on dopaminergic function demonstrates that the actions of neurotransmitters in the VTA are, to some extent, state-dependent. Thus, different transmitters perform similar functions or the same transmitter may perform opposing functions when environmental circumstances are altered. Understanding the dynamic range of a transmitter's action and how this couples in concert with other transmitters to modulate dopamine neurons in the VTA is essential to defining the role of dopamine cells in the etiology and maintenance of neuropsychiatric disorders. Further, it will permit a more rational exploration of drugs possessing utility in treating disorders involving dopamine transmission.

Biochemical, autoradiographic and functional studies on a unique glutamate binding site in adrenal gland

L-Glutamate is known to function as a major excitatory neurotransmitter in the mammalian central nervous system, and recent reports suggest the existence of receptors for glutamate in several peripheral tissues. In the present study, the characteristics of the binding of [3H]L-glutamate to sections of bovine adrenal gland were studied, and the localisation of this binding was investigated in adrenal glands from cow, dog, rat and guinea pig. In addition, the effects of glutamate on catecholamine release from the perfused isolated bovine adrenal gland were investigated. Binding of [3H]L-glutamate to slide-mounted sections of bovine adrenal gland was of high affinity (Kd 0.4 microM), rapid, saturable, reversible, stereospecific and to a single population of sites. The pharmacological profile of this binding site appeared to be unique, and did not correspond to any of the central receptor subtypes for glutamate so far identified. In the adrenal gland of the cow, rat and guinea pig, the binding density of [3H]L-glutamate was higher in cortex than medulla, while this pattern was reversed in the canine adrenal gland. Glutamate had no effect on the basal secretion of noradrenaline or adrenaline from the perfused isolated bovine adrenal gland, and neither glutamate nor the glutamate receptor antagonist kynurenate altered the nicotine-stimulated release of these catecholamines. These results suggest the existence of a novel peripheral binding site for glutamate in the adrenal gland. The differential autoradiographic localisation of this binding site in the adrenal glands of the various species studied may reflect different functional properties of glutamate in these species, and suggests possible roles for glutamate in the modulation of adrenal function.

Classical conditioning selectively increases AMPA receptor binding in rabbit hippocampus

The NMDA and AMPA receptors have been shown to play critical roles in various forms of synaptic plasticity (learning and memory, long-term potentiation). The present study investigated the involvement of these two receptors in a well-characterized classical conditioning paradigm. Following classical conditioning of the rabbit nictitating membrane the binding properties of these two subclasses of excitatory amino acid transmitter receptors were analyzed in dorsal hippocampi by quantitative autoradiography. [3H] TCP and [3H] AMPA were used to identify the NMDA and AMPA receptors, respectively. The binding of [3H]TCP to the NMDA receptor remained unchanged in all the experimental groups tested. Paired presentations of the conditioned and unconditioned stimuli resulted in increased [3H] AMPA binding to the AMPA receptor in several subfields of the hippocampus, while unpaired presentations had no significant effects. The increase in binding was due to an increased affinity of the low-affinity component of the AMPA receptor. The results support the hypothesis that changes in glutamate receptors participate in the synaptic plasticity involved in certain forms of learning.

Effect of age on alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) binding sites in the rat brain studied by in vitro autoradiography

Receptors for excitatory amino acid, L-glutamate, have been classified into three subtypes named as N-methyl-D-aspartate (NMDA), quisqualate (QA) and kainate receptors. In the present study, an effect of age on binding sites of [3H]alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (3H-AMPA), a QA agonist, was studied in the rat brain through quantitative in vitro autoradiography. 3H-AMPA binding sites were most concentrated in the hippocampus and cerebral cortex where glutamate receptors have been demonstrated to play a role in synaptic transmission. In aged rats, 3H-AMPA binding sites in the hippocampus and cerebral cortex were not significantly changed. In our previous studies, it was noticed that strychnine-insensitive glycine receptors, which functionally coupled with NMDA receptors, showed marked age-dependent decreases in telencephalic regions. It has been shown that the glutamatergic neuronal system is involved in learning and memory. Nevertheless, it is considered that AMPA binding sites are not involved in the decline of neuronal functions, especially impairment of learning and memory, accompanying with aging process.

Altered excitatory and inhibitory amino acid receptor binding in hippocampus of patients with temporal lobe epilepsy

We examined binding to excitatory amino acid and inhibitory amino acid receptors in frozen hippocampal sections prepared from surgical specimens resected from 8 individuals with medically refractory temporal lobe epilepsy. The excitatory receptors studied included N-methyl-D-aspartate (NMDA), strychnine-insensitive glycine, phencyclidine, and quisqualate. The inhibitory receptors studied were gamma-aminobutyric acid type A (GABAA) and benzodiazepine. Excitatory and inhibitory amino acid receptor binding were differentially altered in the patients with temporal lobe epilepsy in comparison to 8 age-comparable autopsy control subjects, and changes in receptor binding were regionally selective in four areas. Binding to phencyclidine receptors associated with the NMDA channel was reduced by 35 to 70% in all regions in the hippocampi of the patients. In contrast, binding to the NMDA recognition site and its associated glycine modulatory site was elevated by 20 to 110% in the cornu ammonis (CA) 1 area and dentate gyrus of the hippocampus of the patients. Binding to these sites was unaffected in area CA4. Binding to the quisqualate-type excitatory amino acid receptor was unchanged in all regions except the stratum lacunosum moleculare CA1, where it was increased by 63%. GABAA and benzodiazepine receptor binding was reduced by 20 to 60% in CA1 and CA4, but unchanged in dentate gyrus. The data indicate that excitatory and inhibitory amino acid receptors are altered in the hippocampus of patients with temporal lobe epilepsy.

Autoradiographic visualisation of [3H]DTG binding to sigma receptors, [3H]TCP binding sites, and L-[3H]glutamate binding to NMDA receptors in human cerebellum

The autoradiographic distributions of [3H]1,3-di-ortho-tolyguanidine ([3H]DTG), [3H]1-[1-(2-thienyl) cyclohexyl] piperidine ([3H]TCP) and L-[3H]glutamate were studied in the human cerebellum. [3H]DTG is a selective label for the sigma receptor, while L-[3H]glutamate binding was carried out under conditions selective for the N-methyl-D-aspartate (NMDA) receptor. [3H]TCP binding sites and sigma receptors showed marked enrichment in the Purkinje cell layer, while L-[3H]glutamate-labelled NMDA receptors showed virtually no binding in the Purkinje cell layer. The results confirm the existence of [3H]TCP binding sites which are not linked to NMDA receptors in the human cerebellum, having a distribution which is more similar to that of the haloperidol-sensitive sigma receptor.

Quantitative autoradiographic study of L-glutamate binding sites in normal and atrophic human cerebellum

In the present work the distribution of L-glutamate binding sites in the different layers of human cerebellum of normal individuals and of seven patients who died with olivopontocerebellar atrophy (OPCA) was examined with the technique of quantitative autoradiography. Specific L-[3H]glutamate binding was higher in the molecular than in the granule cell layer of normal cerebellar tissue. A significant decrease of L-[3H]glutamate specific binding was observed in the molecular layer of all OPCA tissues. In the granule cell layer L-[3H]glutamate binding was decreased only in two patients who suffered from late-onset sporadic OPCA and in one patient who suffered from a form of OPCA inherited in a dominant manner. Quisqualate-sensitive binding sites were the most abundant binding sites in the molecular layer of normal cerebella, whereas N-methyl-D-aspartic acid (NMDA)-sensitive binding sites were the most abundant type in the granule cell layer. A significant decrease of quisqualate-sensitive and an increase in NMDA-sensitive binding sites were observed in the molecular layer of OPCA cerebellar tissues. No significant changes were observed in the granule cell layer of these tissues.

Autoradiographic characterization of [3H]L-glutamate binding sites in developing mouse cerebellar cortex

Postnatal changes of [3H]L-glutamate binding sites in mouse cerebellum were studied by in vitro autoradiography. These sites were already present at birth, their density globally increased until postnatal day 25, and at all ages it was higher when Cl- and Ca2+ were present in the incubation buffer. At birth, these binding sites were diffused through the whole cerebellar mass, but became distinctly concentrated in the molecular and the internal granular layers by postnatal day 10. From this age on, binding site sensitivity to ions and glutamate analogues takes a different course in each layer. The external granular layer and the white matter never displayed significant amounts of binding. In the molecular layer the Cl-/Ca2+ effect increased during ontogeny until, in adults, the ion-dependent binding was threefold higher than the ion-independent binding. Quisqualate-sensitive sites accounted for 80% of the total binding sites already at postnatal day 15, while displacement by alpha-amino-3-hydroxy-methyl-4-isoxazolepropionic and ibotenic acids attained the maximum (68%) at postnatal day 60. N-Methyl-D-aspartate displaced glutamate binding (50%) only in the presence of Cl- and Ca2+. Starting from postnatal day 15, binding site density in the molecular layer of lobules VIb and VII of the vermis was lower than in other lobules. In the internal granular layer, the Cl-/Ca2+ effect observed in young animals decreased during development. These transient binding sites were sensitive to quisqualic and ibotenic acid. In adults, the majority of glutamate binding sites were ion-independent and mainly sensitive to D,L-amino-5-phospho-valeric acid and N-methyl-D-aspartate. Throughout development and in both layers, sites displaced by kainate were present at low density and sites displaced by D,L-2-amino-4-phosphonobutyric acid were not detected. The localized postnatal changes of the [3H]L-glutamate binding sites were correlated with the events occurring during growth and maturation of cerebellar structures. The increase of the Cl-/Ca(2+)-dependent binding in the molecular layer is simultaneous with the growth of Purkinje cell dendrites and of parallel fibres and with the formation of the synapses between them. This suggests that these binding sites are localized in these synapses. The changing pattern of sensitivity to different agonists during development might correspond to the maturation of these synapses. The low density of [3H]L-glutamate binding in the molecular layer of lobules VIb and VII probably indicates the presence of specific nerve projections to these areas.(ABSTRACT TRUNCATED AT 400 WORDS)

Cerebellar excitatory amino acid binding sites in normal, granuloprival, and Purkinje cell-deficient mice

Using quantitative autoradiography, the cellular localization and characterization of cerebellar excitatory amino acid binding sites in normal, Purkinje cell-deficient and granuloprival (granule cell-deficient) mouse cerebella were investigated. In the molecular layer of normal mouse cerebellum, the quisqualate subtype of excitatory amino acid receptor (assayed by [3H](RS)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate, quisqualate-sensitive L-[3H]glutamate, and [3H]6-cyano-7-nitroquinoxaline-2,3-dione binding) predominated. In the granule cell layer of the cerebellum, N-methyl-D-aspartate-sensitive L-[3H]glutamate and [3H]glycine binding sites were predominant. In the molecular layer of Purkinje cell-deficient mutant mice, [3H](RS)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate binding sites and [3H]6-cyano-7-nitro-quinoxaline-2,3-dione binding were reduced to 24% (P less than 0.01) and 36% (P less than 0.001) of control, respectively, while quisqualate-sensitive [3H]glutamate binding sites were reduced to 54% of control (P less than 0.01). N-Methyl-D-aspartate-sensitive [3H]glutamate and [3H]glycine binding were unchanged. In the granule cell layer of these mouse cerebella, there was no change in excitatory amino acid receptor binding. In the molecular layer of granuloprival mouse cerebella, [3H](RS)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate binding was increased to 205% of control (P less than 0.01), [3H]6-cyano-7-nitro-quinoxaline-2,3-dione binding was increased to 136% of control (P less than 0.02), and quisqualate-sensitive [3H]glutamate binding was increased to 152% of control (P less than 0.01). N-Methyl-D-aspartate-sensitive [3H]glutamate and [3H]glycine binding were unchanged. In areas of granule cell depletion N-methyl-D-aspartate-sensitive [3H]glutamate and [3H]glycine binding were reduced to 68% (P less than 0.01) and 59% (P less than 0.01) of control, respectively. In the granule cell layer, binding to quisqualate receptors was not significantly different from binding in controls with any of the ligands tested. These results suggest that three different receptor assays: [3H](RS)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate, quisqualate-sensitive L-[3H]glutamate, and [3H]6-cyano-7-nitro-quinoxaline-2,3-dione binding can be used to demonstrate that quisqualate receptor specific binding sites are located on Purkinje cell dendrites in the molecular layer of cerebellum, and that these binding sites apparently up-regulate in response to granule cell ablation and Purkinje cell deafferentation.

Quisqualate- and NMDA-sensitive [3H]glutamate binding in primate brain

Excitatory amino acids (EAA) such as glutamate and aspartate are probably the neurotransmitters of a majority of mammalian neurons. Only a few previous studies have been concerned with the distribution of the subtypes of EAA receptor binding in the primate brain. We examined NMDA- and quisqualate-sensitive [3H]glutamate binding using quantitative autoradiography in monkey brain (Macaca fascicularis). The two types of binding were differentially distributed. NMDA-sensitive binding was most dense in dentate gyrus of hippocampus, stratum pyramidale of hippocampus, and outer layers of cerebral cortex. Quisqualate-sensitive binding was most dense in dentate gyrus of hippocampus, inner and outer layers of cerebral cortex, and molecular layer of cerebellum. In caudate nucleus and putamen, quisqualate- and NMDA-sensitive binding sites were nearly equal in density. However, in globus pallidus, substantia nigra, and subthalamic nucleus, quisqualate-sensitive binding was several-fold greater than NMDA-sensitive binding. In thalamus, [3H]glutamate binding was generally low for both subtypes of binding except for the anterior ventral, lateral dorsal, and pulvinar nuclei. In the brainstem, low levels of binding were found, and strikingly the red nucleus and pons, which are thought to receive glutamatergic projections, had approximately 1/20 the binding observed in cerebral cortex. These results demonstrate that NMDA- and quisqualate-sensitive [3H]glutamate binding are observed in all regions of primate brain, but that in some regions one subtype predominates over the other. In addition, certain areas thought to receive glutamatergic projections have low levels of both types of binding.

NMDA and kainic acid receptors have a complementary distribution to AMPA receptors in the human cerebellum

The distributions of N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-S-methyl-4-isoxazole propionic acid (AMPA) and kainic acid (KA) receptors were determined in the human cerebellum using autoradiography. In contrast to the cerebral cortex, where KA receptors have a complementary distribution to NMDA and AMPA receptors, AMPA receptors were concentrated in the cerebellar molecular layer while NMDA and KA receptors were concentrated in the granular layer.

L-[3H]glutamate labels the metabotropic excitatory amino acid receptor in rodent brain

A quantitative autoradiographic assay for a novel L-[3H]glutamate binding site in rodent brain has been developed. Binding to this site was distinguished by its high affinity for quisqualate (QA), ibotenate, glutamate and trans-1-amino-cyclopentyl-1,3-dicarboxylic acid (trans-ACPD), but low affinity for [RS]-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), kainate and N-methyl-D-aspartate (NMDA). 'AMPA-insensitive, QA-sensitive [3H]glutamate binding' (AiQsGB) had a heterogeneous distribution in rat brain with high levels observed in molecular layer of cerebellum, striatum, and lateral septum. AiQsGB was reduced in molecular layer of cerebellum in mice lacking Purkinje cells. AiQsGB appears to represent binding to the 'metabotropic' neuronal excitatory amino acid receptor linked to phosphoinositide metabolism.

Characterization of the quisqualate receptor linked to phosphoinositide hydrolysis in neurocortical cultures

Activation of phosphoinositide metabolism is an early event in signal transduction for a number of neurotransmitters and hormones. In primary cultures of rat neurocortical cells, various excitatory amino acids stimulate inositol phosphate production with a rank order of potency of quisqualate greater than ibotenate greater than glutamate greater than kainate, N-methyl-D-aspartate greater than alpha-amino-3-hydroxyl-5-methyl-4-isoxazole propionate. This response to excitatory amino acids was insensitive to a variety of excitatory amino acid antagonists including 6-cyano-7-nitroquinoxaline-2,3-dione, 3-3(2-carboxypiperazine-4-yl)propyl-1-phosphonate, and 2-amino-4-phosphonobutyrate. The individual responses of quisqualate-, ibotenate-, and kainate-stimulated inositol phosphate production were not additive. These results suggest that phosphoinositide metabolism activated by excitatory amino acids is mediated by a unique quisqualate-preferring receptor that is not antagonized by known N-methyl-D-aspartate and non-N-methyl-D-aspartate antagonists, and is relatively insensitive to alpha-amino-3-hydroxyl-5-methyl-4-isoxazole propionate.

Characterization of binding sites for spider toxin, [3H]NSTX-3, in the rat brain

A group of spider toxins (JSTX, NSTX, argiopin, argiotoxin etc.) share a basic common structure and have been reported to block strongly quisqualate- and kainate-sensitive glutamate responses in vertebrate and invertebrate nervous systems. They are presumed to be potent antagonists of both quisqualate and kainate receptors and may serve as useful tools for characterizing these receptors. We report here the synthesis of tritium-labeled NSTX-3 and the characterization of its binding sites in the rat brain. We found that high- and low-affinity binding sites exist in the cerebellum (Kd = 7.75 and 202 nM, Bmax = 0.37 and 5.54 pmol/mg protein, respectively). Synthetic NSTX analogs strongly inhibited [3H]NSTX-3 binding in the cerebellum (IC50 = 10(-7)-10(-6) M), whereas competitive agonists of glutamate receptors (AMPA, quisqualate, NMDA, kainate, glutamate and aspartate) exhibited weak or no inhibitory effects.

Excitatory amino acid receptors in the brain: membrane binding and receptor autoradiographic approaches

In last month's article in this series, Lodge and Johnson discussed the contribution of noncompetitive excitatory amino acid antagonists to understanding of these receptors. In this third article, Anne Young and Graham Fagg describe how radioligand binding experiments have helped to fuel the recent burst of progress in understanding excitatory amino acid receptors in the brain. New and selective radioligands have facilitated mapping the distributions of the major excitatory receptor subtypes in normal and diseased brain, examining allosteric interactions within the NMDA receptor, searching for novel therapeutic agents and determining drug mechanisms, and making first steps along the path to defining receptor structure at the molecular level.

Autoradiographic characterization and localization of quisqualate binding sites in rat brain using the antagonist [3H]6-cyano-7-nitroquinoxaline-2,3-dione: comparison with (R,S)-[3H]alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid binding sites

Using quantitative autoradiography, we have investigated the binding sites for the potent competitive non-N-methyl-D-aspartate (non-NMDA) glutamate receptor antagonist [3H]6-cyano-7-nitro-quinoxaline-2,3-dione ([3H]-CNQX) in rat brain sections. [3H]CNQX binding was regionally distributed, with the highest levels of binding present in hippocampus in the stratum radiatum of CA1, stratum lucidum of CA3, and molecular layer of dentate gyrus. Scatchard analysis of [3H]CNQX binding in the cerebellar molecular layer revealed an apparent single binding site with a KD = 67 +/- 9.0 nM and Bmax = 3.56 +/- 0.34 pmol/mg protein. In displacement studies, quisqualate, L-glutamate, and kainate also appeared to bind to a single class of sites. However, (R,S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) displacement of [3H]CNQX binding revealed two binding sites in the cerebellar molecular layer. Binding of [3H]AMPA to quisqualate receptors in the presence of potassium thiocyanate produced curvilinear Scatchard plots. The curves could be resolved into two binding sites with KD1 = 9.0 +/- 3.5 nM, Bmax = 0.15 +/- 0.05 pmol/mg protein, KD2 = 278 +/- 50 nM, and Bmax = 1.54 +/- 0.20 pmol/mg protein. The heterogeneous anatomical distribution of [3H]CNQX binding sites correlated to the binding of L-[3H]glutamate to quisqualate receptors and to sites labeled with [3H]AMPA. These results suggest that the non-NMDA glutamate receptor antagonist [3H]CNQX binds with equal affinity to two states of quisqualate receptors which have different affinities for the agonist [3H]AMPA.

MK-801 pretreatment enhances N-methyl-D-aspartate-mediated brain injury and increases brain N-methyl-D-aspartate recognition site binding in rats

Direct intracerebral administration of N-methyl-D-aspartate typically produces focal brain injury. (+)-5-Methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-immi ne maleate (MK-801), a non-competitive N-methyl-D-aspartate antagonist, can protect against N-methyl-D-aspartate-mediated brain injury when administered shortly before or after an intracerebral injection of N-methyl-D-aspartate. However, in this study we report that in perinatal rats if MK-801 (1 mg/kg) is administered intraperitoneally 24 h prior to a unilateral intrastriatal N-methyl-D-aspartate injection, N-methyl-D-aspartate-mediated brain injury is paradoxically enhanced. The severity of resulting brain injury is 15-25% greater in groups that received MK-801 in comparison with saline-treated controls (P less than 0.001, linear regression analysis). In contrast, the severity of brain injury resulting from intrastriatal injection of the glutamate agonist quisqualate is not altered by a similar 24 h MK-801 pretreatment. Furthermore, the enhanced toxicity of N-methyl-D-aspartate produced by a 24 h pretreatment with MK-801 is completely blocked if a second dose of MK-801 is administered 15 min after the intrastriatal injection of N-methyl-D-aspartate. To determine if MK-801 produced alterations in glutamate receptor pharmacology co-incident with the enhanced toxicity of N-methyl-D-aspartate, in vitro quantitative autoradiography for excitatory amino acid receptor subtypes was performed with [3H]glutamate and [3H]N-1-(2-thienyl)cyclohexyl-3,4-piperidine in seven-day-old rats killed 2 or 24 h after MK-801 (1 mg/kg) administration. A 2 h MK-801 pretreatment produced a 30-50% increase in [3H]glutamate binding at N-methyl-D-aspartate preferring recognition sites in all four brain regions examined (areas CA1 and CA3 of the hippocampus, corpus striatum, cingulate cortex) in comparison with saline-treated controls (P less than 0.05, ANOVA). [3H]N-1-(2-Thienyl)cyclohexyl-3,4-piperidine binding to the phencyclidine site associated with the N-methyl-D-aspartate receptor was reduced by 60-80% in all brain regions examined (P less than 0.001). Quisqualate-sensitive [3H]glutamate binding was not altered by a 2 h MK-801 pretreatment. In animals that received a 24 h MK-801 pretreatment.(ABSTRACT TRUNCATED AT 400 WORDS)

Regional ontogeny of a unique glutamate recognition site in rat brain: an autoradiographic study

The developmental pattern of L-[3H]glutamate binding to rat brain in the presence of saturating concentrations of unlabeled N-methyl-D-aspartate (NMDA), kainate (KA) and quisqualate (QQA) was examined in an autoradiographic assay. The unique glutamate binding site defined by this assay displayed four distinct, regionally specific patterns of development. (1) In reticular nucleus of thalamus there was an initial very high level of binding at postnatal day 1 (PND1) followed by a progressive 80% decline in binding during maturation. (2) In entorhinal cortex, a progressive 500-1100% increase in binding was seen during development. (3) In ventral posterior medial nucleus of thalamus, there was an initial transient 200-300% increase in binding, peaking at PND10, followed by a steady decline in binding. (4) In frontal cortex, binding remained relatively stable throughout development. At all stages of development, the distribution of these recognition sites was different from NMDA, KA or QQA receptors. The function of this glutamate binding site remains to be determined, but the distinct regional and temporal patterns of binding suggest that it may be important in normal development of the central nervous system.

Two pharmacological classes of quisqualate-induced electrical responses in rat retinal ganglion cells in vitro

The pharmacological properties of steady state responses elicited by the excitatory amino acid agonists quisqualate and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) have been examined in isolated rat retinal ganglion cells with patch electrodes. Based upon testing retinal ganglion cells with both agonists at saturating concentrations (30 microM for quisqualate and 60 microM for AMPA), these neurons can be grouped into three different categories: (a) type I cells, in which AMPA steady state responses were larger than those induced by quisqualate; (b) type II cells, in which quisqualate steady state responses were larger than or equal to those produced by AMPA; and (c) type III cells, in which neither AMPA nor quisqualate elicited responses. Regardless of cell type (I or II), AMPA responses were substantially inhibited by the antagonists kynurenate (750 microM) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM). In contrast, the effects of these antagonists on quisqualate responses varied depending on the cell type, with both kynurenate and CNQX being more effective in type II than in type I cells. Based upon further pharmacological manipulations, it is proposed that quisqualate-induced steady state responses in retinal ganglion cells are divisible into two pharmacological classes that exist in varying proportions in these cells in vitro.

Excitatory amino acids and Alzheimer's disease

Excitatory amino acids (EAA) such as glutamate and aspartate are major transmitters of the cerebral cortex and hippocampus, and EAA mechanisms appear to play a role in learning and memory. Anatomical and biochemical evidence suggests that there is both pre- and postsynaptic disruption of EAA pathways in Alzheimer's disease. Dysfunction of EAA pathways could play a role in the clinical manifestations of Alzheimer's disease, such as memory loss and signs of cortical disconnection. Furthermore, EAA might be involved in the pathogenesis of Alzheimer's disease, by virtue of their neurotoxic (excitotoxic) properties. Circumstantial evidence raises the possibility that the EAA system may partially determine the distribution of pathology in Alzheimer's disease and may be important in producing the neurofibrillary tangles, RNA reductions and dendritic changes which characterize this devastating disorder. In this article, we will review the evidence suggesting a role for EAA in the clinical manifestations and pathogenesis of Alzheimer's disease.

Quantitative autoradiographic localization of NMDA, quisqualate and PCP receptors in the frog tectum

An organizing role for the N-methyl-D-aspartate (NMDA) receptor/channel has been suggested in the development of the retinotectal projection in Rana pipiens. The regional distributions of NMDA, phencyclidine (PCP) and quisqualic acid (QA) receptors were quantified using in vitro autoradiography in the tectum of normal and surgically produced 3-eyed juvenile frogs. NMDA and QA receptor binding was highest in the pretectum. Of the tectal layers, the superficial retinotectal synaptic zone, layer 9, had the highest amount of NMDA and QA receptor binding. Moderate binding was observed in layer 5, with little binding in the cellular layer 6. No specific [3H]N-(1-[2-thienyl]cyclohexyl) piperidine ([3H]TCP) binding was observed in any of the tectal regions.

Contamination of commercially available quisqualic acid by glutamate-like and aspartate-like substances

Six different batches of the glutamic acid analogue quisqualic acid were analyzed with high-pressure liquid chromatography (HPLC). All batches examined showed contaminant peaks. Different batches had different contaminant peaks and differing amounts of each contaminant. Every batch of quisqualic acid tested demonstrated a contaminant peak which co-eluted with exogenously added glutamic acid. Certain batches possessed a contaminant which co-eluted with aspartic acid. The levels of glutamate-like contamination ranged from 0.08 to 0.60%, and the levels of aspartate-like contamination ranged from undetectable amounts to 0.80%. The amount of combined glutamate- and aspartate-like contamination of each batch of quisqualate correlated very highly with the ability of that batch to interact with non-quisqualate receptors in an autoradiographic binding assay. These non-quisqualate receptors are likely N-methyl-D-aspartate (NMDA) receptors. Thus, when high concentrations of quisqualate are used experimentally, contamination is likely to produce spurious effects at non-quisqualate glutamate receptors. Quisqualate itself may be a more specific agonist than assumed previously.

Excitatory amino acid receptors in the human cerebral cortex: a quantitative autoradiographic study comparing the distributions of [3H]TCP, [3H]glycine, L-[3H]glutamate, [3H]AMPA and [3H]kainic acid binding sites

The excitatory amino acids are probably the major neurotransmitters in the cerebral cortex, and they act through at least three receptors: the N-methyl-D-aspartate, the quisqualate and the kainic acid receptors. Under the appropriate conditions, [3H]1-(1-(2-thienyl)-cyclohexyl)piperidine [( 3H]TCP), [3H]glycine and L-[3H]glutamate label different sites on the N-methyl-D-aspartate receptor, [3H]-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid [( 3H]AMPA) labels the quisqualate receptor and [3H]kainic acid the kainic acid receptor. The anatomical localizations of these binding sites were studied in sections of blocks removed from the cerebral cortices of eight post-mortem human brains. The results showed that, in the human cerebral cortex, [3H]TCP, [3H]glycine and L-[3H]glutamate binding sites had congruent distributions, with [3H]AMPA binding sites showing a similar distribution. In the hippocampus, these four ligands had high binding site densities in the CA1 region and the dentate gyrus molecular layer. With the exception of the striate cortex, in the neocortex, a tri-laminar pattern was seen consisting of a high density across laminae I-III, a layer of low density corresponding to the region of lamina IV, and a band of moderate density across laminae V and VI, except for [3H]AMPA where the middle zone of low density was usually wider. [3H]Kainic acid showed a binding pattern which was generally complementary to that of the other four ligands. There were low levels of [3H]kainic acid binding sites in the CA1 region of the hippocampus with higher levels in the CA3 region, the hilus, and the inner third of the dentate gyrus molecular layer. In the neocortex there was a band of high density corresponding to laminae V and VI, with a thin band of moderate binding corresponding to lamina I and the outer region of lamina II. An exception was the motor cortex where the highest level of [3H]kainic acid binding was in laminae I and II. The high degree of congruence between the binding patterns of [3H]TCP, [3H]glycine and L-[3H]glutamate (using conditions appropriate for the N-methyl-D-aspartate receptor) supports data indicating that these ligands bind to different regions of the same receptor complex. The similar distribution of [3H]AMPA binding sites, with the exception of the striate cortex, supports observations made in rodents that N-methyl-D-aspartate receptors and quisqualate receptors have similar distributions and perform different but related functions in excitatory transmission.(ABSTRACT TRUNCATED AT 400 WORDS)

Thiocyanate stabilizes AMPA binding to the quisqualate receptor

Calcium and chloride ions stimulated [3H]glutamate binding to quisqualate-sensitive [3H]glutamate binding sites 4-fold, as measured by quantitative autoradiography, whereas 100 mM potassium thiocyanate had no additional effect. In contrast, calcium and chloride had little effect on the binding of [3H](RS)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid ([3H]AMPA), but 100 mM thiocyanate stimulated binding 4-fold. AMPA displaced little [3H]glutamate binding from quisqualate-sensitive binding sites in the molecular layer of the cerebellum in the absence of thiocyanate. However, in the presence of thiocyanate AMPA became a more effective displacer, but still displaced only 44% of the quisqualate-sensitive [3H]glutamate binding. The distribution of [3H]glutamate binding to quisqualate-sensitive sites was similar to but not identical with that of [3H]AMPA binding. However, the distribution of AMPA-displaceable [3H]glutamate binding correlated highly (r = 0.97, P less than 0.0005) with that of [3H]AMPA binding. The results suggest that AMPA binds to a subclass of quisqualate-sensitive [3H]glutamate binding sites that are highly influenced by ionic environment and that quisqualate-sensitive binding sites exist in several states.

NMDA receptor losses in putamen from patients with Huntington's disease

N-Methyl-D-aspartate (NMDA), phencyclidine (PCP), and quisqualate receptor binding were compared to benzodiazepine, gamma-aminobutyric acid (GABA), and muscarinic cholinergic receptor binding in the putamen and cerebral cortex of individuals with Huntington's disease (HD). NMDA receptor binding was reduced by 93 percent in putamen from HD brains compared to binding in normal brains. Quisqualate and PCP receptor binding were reduced by 67 percent, and the binding to other receptors was reduced by 55 percent or less. Binding to these receptors in the cerebral cortex was unchanged in HD brains. The results support the hypothesis that NMDA receptor-mediated neurotoxicity plays a role in the pathophysiology of Huntington's disease.