N-methyl-D-aspartate Binding Sites Research Articles

Pentylenetetrazole-induced seizures affect binding site densities for GABA, glutamate and adenosine receptors in the rat brain

Pentylenetetrazole (PTZ) is a convulsant used to model epileptic seizures in rats. In the PTZ-model, altered heat shock protein 27 (HSP-27) expression highlights seizure-affected astrocytes, which play an important role in glutamate and GABA metabolism. This raises the question whether impaired neurotransmitter metabolism leads to an imbalance in neurotransmitter receptor expression. Consequently, we investigated the effects of seizures on the densities of seven different neurotransmitter receptors in rats which were repeatedly treated with PTZ (40 mg/kg) over a period of 14 days. Quantitative in vitro receptor autoradiography was used to measure the regional binding site densities of the glutamate α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), kainate and N-methyl-d-aspartate (NMDA) receptors, the adenosine receptor type 1 (A1), which is part of the system controlling glutamate release, and the γ-aminobutyric acid (GABA) receptors GABAA and GABAB as well as the GABAA-associated benzodiazepine (BZ) binding sites in each rat. Our results demonstrate altered receptor densities in brain regions of PTZ-treated animals, including the HSP-27 expressing foci (i.e. amygdala, piriform and entorhinal cortex, dentate gyrus). A general decrease of kainate receptor densities was observed together with an increase of NMDA binding sites in the hippocampus, the somatosensory, piriform and the entorhinal cortices. Furthermore, A1 binding sites were decreased in the amygdala and hippocampal CA1 region (CA1), while BZ binding sites were increased in the dentate gyrus and CA1. Our data demonstrate the impact of PTZ induced seizures on the densities of kainate, NMDA, A1 and BZ binding sites in epileptic brain. These changes are not restricted to regions showing glial impairment. Thus, an altered balance between different excitatory (NMDA) and modulatory receptors (A1, BZ binding sites, kainate) shows a much wider regional distribution than that of glial HSP-27 expression, indicating that receptor changes are not following the glial stress responses, but may precede the HSP-27 expression.

Non-steroidal anti-inflammatory agents, tolmetin and sulindac, attenuate oxidative stress in rat brain homogenate and reduce quinolinic acid-induced neurodegeneration in rat hippocampal neurons

Alzheimer's disease (AD) is the most common form of neurodegenerative disease in the elderly. Anti-inflammatory agents have been shown to be beneficial in preventing neurodegenerative disorders such as AD. In this study we investigated the possible antioxidant and neuroprotective properties of two non-steroidal anti-inflammatory drugs (NSAIDS), tolmetin and sulindac, using quinolinic acid (QA)-induced neurotoxicity as a model. We used the thiobarbituric acid assay to measure the extent of lipid peroxidation and the nitroblue tetrazolium assay to measure the superoxide anion generated in rat brain homogenate. QA (1 mM) induced lipid peroxidation in rat brain homogenate was significantly curtailed by co-treatment of the homogenate with tolmetin and/or sulindac. Tolmetin and sulindac both reduced the generation of superoxide anions by the known neurotoxin, potassium cyanide (KCN). Intrahippocampal injections of QA induced neurotoxicity in rat hippocampus. N-Methyl-D-Aspartate (NMDA) receptor counts were conducted do give an indication of the amount protection offered by the NSAIDS. QA drastically reduced the number of NMDA binding sites by approximately 37%. This sharp decrease was considerably attenuated by the pre-treatment of the rats with tolmetin and sulindac (5 mg/kg/bd for five days). This study shows the antioxidant and neuroprotective properties of tolmetin and sulindac and hereby postulates that these drugs have important implications in the prevention or treatment of neurodegenerative diseases such as AD.

Effect of antioxidants and NMDA antagonists on the density of NMDA binding sites in rat hippocampal slices exposed to hypoxia/reoxygenation.

Major interest is currently focused on the development and evaluation of effective strategies for the pharmacological therapy of human stroke and cerebral ischemia, as well as some neurodegenerative disorders in which increased production of free oxygen radicals and the neurotoxic effect of excitatory amino acids may take place. Selected N-methyl-D-aspartate (NMDA) antagonists and antioxidants in the model of experimental oxidative stress induced by hypoxia and reoxygenation in rat hippocampal slices were tested. The putative antiglutamatergic effect of the antioxidant stobadine and its neuroprotective effect during oxidative stress was studied. NMDA antaonists 2-amino-5-phosphonovaleric acid (APV) and Mg2+, as well as the antioxidants stobadine and trolox, prevented the decrease of NMDA binding site number induced by hypoxia/reoxygenation in rat hippocampal slices. Moreover, stobadine, APV and Mg2+ prevented the decrease of NMDA binding site number due to glutamic acid incubation. Stobadine does not inhibit [3H]-glutamate binding and therefore does not seem to interact directly with glutamate binding sites. Thus, its neuroprotective effect in rat hippocampus exposed to hypoxia/reoxygenation does not seem to be based on a direct antiglutamatergic effect. The protective action of stobadine against the decrease of NMDA binding site number elicited by hypoxia/reoxygenation in rat hippocampus could rather be due to its antioxidant and antiradical effect.

Behavioral and neurochemical alterations following repeated phencyclidine administration in rats with neonatal ventral hippocampal lesions

Exposure to chronic phencyclidine (PCP) has been reported to mimic certain aspects of schizophrenia in normal subjects as well as to exacerbate symptoms in schizophrenic patients. Analogous to schizophrenics, adult rats with neonatal ventral hippocampal (VH) lesions have been shown to display enhanced sensitivity to both stress and psychostimulants. In order to examine whether repeated PCP treatment can modulate behavior when administered to neonatal VH-lesioned animals, we examined locomotor activity and immobility time in the forced swimming test (FST) in neonatal VH-lesioned rats following repeated PCP treatment. Receptor autoradiography studies were also performed for dopamine (DA) and N-methyl-D-aspartate (NMDA) receptors to identify neurochemical correlates of the altered behavior in these animals. Though repeated PCP administration resulted in increased levels of locomotor activity and rearing in both VH-lesioned as well as sham rats, the effects were much more enhanced in the lesioned rats compared to sham. However, repeated PCP treatment induced hypolocomotion during the habituation period in both sham and lesioned rats. In the FST paradigm, lesioned rats displayed an altered retention of acquired immobility. Repeated PCP administration increased DA D1-like receptors in the caudate-putamen in lesioned rats and decreased striatal D2-like receptors in both sham and lesioned rats. Moreover, repeated PCP administration in lesioned rats decreased NMDA binding sites in the prefrontal cortex while increasing labelling in the subcortical regions. These results suggest that repeated administration of PCP can qualitatively and quantitatively affect behaviors in neonatal VH-lesioned rats related to abnormal neurodevelopmental processes presumably via prefrontal glutamatergic and subcortical dopaminergic dysfunctions.

Blockade of NMDA receptors in the nucleus accumbens elicits spontaneous tail-flicks in rats

The open channel blocker at N-methyl- d-aspartate (NMDA) receptors, dizocilpine, stereospecifically elicited spontaneous tail-flicks in rats — a reaction similar to those elicited by other drugs (tenocyclidine, phencyclidine and ketamine) acting as open channel blockers. Their relative potencies were strongly correlated with affinities at NMDA binding sites and labeled by [ 3H]dizocilpine in the frontal cortex ( r=0.94) and, as determined previously [Millan, M.J., Seguin, L., 1994. Chemically-diverse ligands at the glycine B site coupled to N-methyl- d-aspartate (NMDA) receptors selectively block the late phase of formalin-induced pain in mice, Neurosci. Lett., 178 (1994) 139–143], potency for eliciting antinociception (0.93). The competitive antagonists at the NMDA receptor recognition site, (±)3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP), 4-phosphonomethyl-2-piperidine carboxylic acid (CGS19755), d, l-( E)-2-amino-4-methylphosphono-3-pentanoic acid (CGP37849) and (3 E)-1-ethyl ester-2-amino-4-methyl-5-phosphono-3-pentenoic acid (CGP39551), likewise dose-dependently evoked spontaneous tail-flick. In contrast, antagonists/weak partial agonists at the coupled, glycine B site, 7-chloro-4-hydroxy-3-(3-phenoxy) phenyl-2( H)-quinolinone (L701,324), (+)-1-hydroxy-3-aminopyrrolidine-2-one ((+)-HA966), (3 R,4 R)-3-amino-1-hydroxy-4-methyl-2-pyrrolidinone (L687,414), 6,7-dichloro-1, 4-dihydro-5-nitro, 2,3 quinoxalinedione (ACEA1021) and 2-carboxy-4,6-dichloro (1 H)-indole-3-propanoic acid (MDL29,951), were inactive. NMDA abolished induction of spontaneous tail-flick by CPP and CGS19755, but not by dizocilpine. Upon bilateral injection into the nucleus accumbens, dizocilpine immediately and dose-dependently elicited spontaneous tail-flick, but it was ineffective in the ventrotegmental area and striatum. Similarly, injection of CPP into the nucleus accumbens elicited spontaneous tail-flick. Neither dizocilpine nor CPP elicited spontaneous tail-flick upon administration onto lumbar spinal cord. In conclusion, a pharmacologically specific spontaneous tail-flick-response is elicited by both open channel blockers and recognition site antagonists, but not glycine B site antagonists, at NMDA receptors. Their actions, mediated in the nucleus accumbens, may be differentiated by their respective resistance and sensitivity to NMDA.

Reduction in the MK-801 binding sites of the NMDA sub-type of glutamate receptor in a mouse model of congenital hyperammonemia: prevention by acetyl- l-carnitine

Our earlier studies on the pharmacotherapeutic effects of acetyl- l-carnitine (ALCAR), in sparse-fur (spf) mutant mice with X linked ornithine transcarbamylase deficiency, have shown a restoration of cerebral ATP, depleted by congenital hyperammonemia and hyperglutaminemia. The reduced cortical glutamate and increased quinolinate may cause a down-regulation of the N-methyl- d-aspartate (NMDA) receptors, observed by us in adult spf mice. We have now studied the kinetics of [ 3H]-MK-801 binding to NMDA receptors in spf mice of different ages to see the effect of chronic hyperammonemia on the glutamate neurotransmission. We have also studied the Ca 2+-dependent and independent (4-aminopyridine (AP) and veratridine-mediated) release of glutamate and the uptake of [ 3H]-glutamate in synaptosomes isolated from mutant spf mice and normal CD-1 controls. All these studies were done with and without ALCAR treatment (4 mmol/kg wt i.p. daily for 2 weeks), to see if its effect on ATP repletion could correct the glutamate neurotransmitter abnormalities. Our results indicate a normal MK-801 binding in 12-day-old spf mice but a significant reduction immediately after weaning (21 day), continuing into the adult stage. The Ca 2+-independent release of endogenous glutamate from synaptosomes was significantly elevated at 35 days, while the uptake of glutamate into synaptosomes was significantly reduced in spf mice. ALCAR treatment significantly enhanced the MK-801 binding, neutralized the increased glutamate release and restored the glutamate uptake into synaptosomes of spf mice. These studies point out that: (a) the developmental abnormalities of the NMDA sub-type of glutamate receptor in spf mice could be due to the effect of sustained hyperammonemia, causing a persistent release of excess glutamate and inhibition of the ATP-dependent glutamate transport, (b) the modulatory effects of ALCAR on the NMDA binding sites could be through a repletion of ATP, required by the transporters to efficiently remove extracellular glutamate.

Autoradiography of [ 3H]glutamate binding during pentobarbital tolerance and withdrawal in the rat

The influence of centrally administered pentobarbital on [ 3H]glutamate receptor binding in the rat brain was examined. Animals were rendered tolerant by intracerebroventricular (i.c.v.) infusion through osmotic minipumps with pentobarbital (300 μg/10 μl/h, for 6 days), and dependent, by 24 h after withdrawal from pentobarbital. Pentobarbital tolerant rats have significant increases in [ 3H]glutamate binding in the cortex and hippocampus area. Pentobarbital withdrawal produced increases in glutamate binding in many regions, e.g., the cortex, hippocampus area, thalamus, and cerebellum. These results show that chronic i.c.v. infusion with pentobarbital increases N-methyl- d-aspartate (NMDA) displaceable [ 3H]glutamate binding, suggesting that an increase in NMDA binding sites may play an important role in the development of tolerance to and withdrawal from pentobarbital.

NMDA Receptor Activation in the Aged Rat: Electrophysiological Investigations in the CA1 Area of the Hippocampal Slice Ex Vivo

The effects of aging on activation of N-methyl- d-aspartate (NMDA) receptors were studied in the CA1 field of hippocampal slices from young (2–4 months old) and aged (25–32 months old) Sprague–Dawley rats with the use of ex vivo extra- and intracellular electrophysiological recording techniques. No significant age-related changes of the unitary NMDA-receptor mediated excitatory postsynaptic potentials (EPSPs), recorded from the pyramidal cells after stimulation of the stratum radiatum in a magnesium-free medium and isolated in the presence of the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione, were found. Simultaneously, the magnitude of synaptic plasticity which involved NMDA receptor activation was not altered. No significant age-related modifications in the mechanisms controlling glutamate release and of postsynaptic NMDA receptor responsiveness were revealed. Considering the 30–40% decrease in NMDA binding sites in the aged hippocampus, our results suggest the occurrence of compensatory mechanisms which are discussed.

Substituted indole-2-carboxylates as in vivo potent antagonists acting as the strychnine-insensitive glycine binding site.

A series of indole-2-carboxylates bearing suitable chains at the C-3 position of the indole nucleus was synthesized and evaluated in terms of in vitro affinity using [3H]glycine binding assay and in vivo potency by inhibition of convulsions induced by N-methyl-D-aspartate (NMDA) in mice. 3-[2-[(Phenylamino)carbonyl]ethenyl]-4,6-dichloroindole-2-carboxyl ic acid (8) was an antagonist at the strychnine-insensitive glycine binding site (noncompetitive inhibition of the binding of [3H]TCP, pA2 = 8.1) displaying nanomolar affinity for the glycine binding site (pKi = 8.5), coupled with high glutamate receptor selectivity (> 1000-fold relative to the affinity at the NMDA, AMPA, and kainate binding sites). This indole derivative inhibited convulsions induced by NMDA in mice, when administered by both iv and po routes (ED50 = 0.06 and 6 mg/kg, respectively). The effect of the substituents on the terminal phenyl ring of the C-3 side chain was investigated. QSAR analysis suggested that the pKi value decreases with lipophilicity and steric bulk of substituents and increases with the electron donor resonance effect of the groups present in the para position of the terminal phenyl ring. According to these results the terminal phenyl ring of the C-3 side chain should lie in a nonhydrophobic pocket of limited size, refining the proposed pharmacophore model of the glycine binding site associated with the NMDA receptor.

Glycine enhances binding to the NMDA receptor complex in aged mice, but does not correct the aging change.

The purpose of this study was to determine whether glycine is effective in correcting the age-related change in the N-methyl-D-aspartate (NMDA) receptor complex. Autoradiographic analysis of [3H]MK801 and NMDA-displaceable [3H]glutamate binding was performed with 0, 10, 30 and 100 microM glycine on brain tissue from 3-, 10-, and 24-30-month-old C57B1 mice. Glycine, even at 10 microM, enhanced binding to both the NMDA and MK801 binding sites in all age groups. Glycine was not able to reduce the absolute difference in [3H]glutamate binding to NMDA sites between 3- and 24-month-olds. Low micromolar concentrations of glycine exacerbated the aging difference in [3H]MK801 binding between young and old mice, but 100 microM glycine produced smaller aging differences that were similar to those seen with no glycine. This study suggests that glycine may enhance NMDA receptor function in old animals, but it does not appear to correct the underlying problem with the NMDA receptor in aged animals.

Dizocilpine binding to cerebral cortical membranes from developing and ageing mice

The binding of [ 3H]dizocilpine (MK-801) to the N-methyl-D-aspartate (NMDA)-gated ion channel was characterized in cerebral cortical membranes during the major portion of the mouse life-span (from 7-day- to 22-month-olds). The binding was saturable, consisting of only one component at all ages studied. The maximal binding capacity B max was very substantial in 14-day-old mice when compared to adults (3-month-olds), decreasing there-after during ageing. The binding constant K D remained unchanged during development and increased only slightly in aged mice. Glutamate and glycine potentiated dizocilpine binding concentration-dependently. Their efficacy varied markedly with age. Both glutamate and glycine had considerably less effect on the immature cerebral cortex and in the oldest group of mice (22-month-old) than in young adults. The marked increase in dizocilpine binding sites at the age of 2 weeks coincides with the previously reported transient increase in NMDA binding sites in the cerebral cortex. The weak potentiation of dizocilpine binding by glutamate and glycine in the immature brain could be a factor which protects neurons during this period from excitotoxicity and increased susceptibility to seizures induced by acidic amino acids. The decrease in the number of dizocilpine binding sites during ageing could result partly from the loss of cortical neurons.

Glutamate and N-methyl- d-aspartate binding sites in the guinea pig hippocampus: Ontogeny and effect of acute in vitro ethanol exposure

The objectives of this study were to characterize the ontogeny of the l-glutamate (glutamate) and N-methyl- d-aspartate (NMDA) binding sites in the developing guinea pig hippocampus, and to determine the effect of acute in vitro ethanol exposure on these binding sites. Specific [ 3H]glutamate binding and NMDA-sensitive [ 3H]glutamate binding were determined using a guinea pig hippocampal synaptic membrane preparation (HSMP). To characterize the ontogeny of the density ( B max) and affinity ( K d) of the glutamate and NMDA binding sites, saturation analysis was conducted on HSMP of guinea pigs at gestational day (GD) 50 (immature fetus; term, GD 68), GD 62 (mature, near-term fetus), postnatal day (PD) 13 (neonate), and PD > 60 (adult). To examine the effect of ethanol on the glutamate and NMDA binding sites, HSMP of guinea pigs at GD 50, GD 62, PD 13, and PD > 60 was incubated with ethanol (0–100 mM), followed by determination of specific ['H]glutamate binding and NMDA-sensitive [ 3H]glutamate binding. To determine the effect of 50 mM ethanol on the B max, and K d of the glutamate and NMDA binding sites, HSMP of guinea pigs at GD 62 and PD > 60 was incubated with 0 or 50 mM ethanol followed by saturation analysis. The B max, values of the hippocampal glutamate and NMDA binding sites were greater at GD 62 and PD 13 compared with GD 50 and PD > 60, but there was no change in the K d of the binding sites throughout development. Ethanol did not alter hippocampal specific [ 3H]glutamate binding or NMDA-sensitive [ 3H]glutamate binding at any of the ages studied, and did not alter the hippocampal B max or K d of the glutamate or NMDA binding sites at GD 62 and PD > 60.

Effect of chronic maternal ethanol administration on glutamate and N-methyl- d-aspartate binding sites in the hippocampus of the near-term fetal guinea pig

The objective of this study was to determine the effect of chronic maternal administration of ethanol on hippocampal l-glutamate (glutamate) and N-methyl- d-aspartate (NMDA) binding sites in the near-term fetal guinea pig. Starting on gestational day (GD) 2, pregnant guinea pigs received one of the following oral treatments up to and including GD 62 (term, about GD 68): 4 g ethanol · kg maternal body weight −1 · day −1; isocaloric sucrose and pair-feeding; or water. Maternal blood ethanol concentration was determined at 1 h after the daily ethanol dose on GD 59. Fetuses were studied at GD 63 (mature, near-term fetus). Fetal body weight and brain weight were determined. The density ( B max) and affinity ( K d) of the glutamate and NMDA binding sites in the fetal hippocampus were measured using a radioligand membrane binding assay; saturation analysis was conducted on hippocampal synaptic membrane preparation (HSMP). Maternal blood ethanol concentration on GD 59 was 269 ± 111 (SD) mg/dl (59 ± 24 mM). There was no maternal or embryonic fetal lethality in any of the three treatment groups, and ethanol treatment did not affect maternal body weight gain compared with sucrose or water treatment. Fetal brain weight, but not body weight, was decreased in the ethanol treatment group compared with the sucrose and water treatment groups. The B max values of the glutamate and NMDA binding sites were decreased in the ethanol treatment group compared with the sucrose and water treatment groups; there was no difference in the K d values of the glutamate and NMDA binding sites among the three treatment groups. The data demonstrate that, in the guinea pig, chronic maternal administration of a binge-type ethanol regimen throughout gestation restricts brain growth and decreases the density of glutamate and NMDA binding sites in the hippocampus, as manifested in the mature fetus.

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 of N-methyl- d-aspartate (NMDA), kainate, α-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.

An increase in NMDA-sensitive [ 3H]glutamate and [ 3H]kainate binding in hippocampus 24 hours after PCP

Considerable research has identified a variety of acute PCP-induced biochemical changes in brain; but, little study has been devoted to characterizing delayed PCP-induced actions. These could potentially be associated with the prolonged psychotomimetic effects of the drug in humans. Here we studied delayed PCP-induced alterations in glutamate receptor subtype binding across a range of PCP doses, based on our previous findings of delayed regional cerebral metabolism changes with PCP. We report that 24 h after a single dose, PCP increases N- methyl- d - aspartate (NMDA)-sensitive [ 3H]glutamate binding in hippocampus (CA 1) in an apparent dose-sensitive manner; no other dose-sensitive regional changes in NMDA binding sites were apparent in a sampling of 19 brain regions. [ 3H]kainate binding sites were increased in CA 3 and dentate gyrus, but only at the high drug dose. Moreover, PCP appeared to have a general delayed effect in upregulating NMDA receptor binding in limbic-associated brain areas at its middle dose, and in upregulating [ 3H]kainate binding in neocortical and limbic area at its high dose. No PCP effects were noted on AMPA receptor binding. These delayed actions of PCP may be informative about the mechanism of PCP psychosis.

Kinetics of AP5 dissociation from NMDA receptors: evidence for two identical cooperative binding sites

1. N-methyl-D-aspartate (NMDA) channels have two agonist binding sites that have similar binding rates for glutamate. However, it is not known whether the dissociation rates at these two sites, and hence their affinities, are similar. The competitive antagonist, D-2-amino-5-phosphonopentanoic acid (AP5), was used to study dissociation kinetics from NMDA receptors in outside-out patches from cultured hippocampal neurons. 2. Rapid steps from AP5 into NMDA produced currents with a sigmoidal activation time course that was limited by AP5 dissociation. Ensemble average currents were well fitted using kinetic models with two identical, cooperative antagonist binding sites per channel. The results suggest that the two NMDA binding sites have similar affinities, but that occupation of one site reduces the affinity of the other. 3. The agonist and antagonist binding kinetics are consistent with an approximately homogeneous population of NMDA channels in cultured hippocampal neurons.

Polyamine potentiation and inhibition of NMDA-mediated increases of intracellular free Ca 2+ in cultured chick cortical neurons

Polyamine potentiation and inhibition of N- methyl- d-aspartate (NMDA) receptor-mediated CA 2+ changes was studied in cultured chick cortical neurons. Spermidine and spermine potentiated the effect of saturating concentrations of NMDA and glycine. No effect of spermidine or spermine was observed in the absence of NMDA or in the presence of either kainate or quisqualate. Similarly, antagonism of the NMDA receptor complex with dizocilpine (an open channel blocker), or with competitive antagonists to the NMDA or glycine binding sites greatly attenuated or completely abolished the combined effects of polyamines plus NMDA and glycine. N-Acetylspermine and N-acetylspermidine, in the presence or absence of NMDA and glycine, were without effect. These data strongly suggest that spermidine and spermine are potent and selective agonists at the polyamine binding site. Putrescine and diethylenetriamine were ineffective as antagonists of NMDA-mediated intracellular free Ca 2+ increases in the presence or absence of added spermine or spermidine. Arcaine and 1,10-diaminodecane, however, antagonized NMDA-mediated intracellular free Ca 2+ increases in the presence and absence of spermine and spermidine, and therefore appear to act either as inverse agonists at the polyamine binding site or as open channel blockers of the NMDA receptor.

A cluster analysis study of acetyl-l-carnitine effect on NMDA receptors in aging

Aging is associated with a reduction in the maximum density of n-methyl-d-aspartate (NMDA)-sensitive glutamate binding sites in the hippocampus of Fischer 344 rats. This study was designed to investigate the effect of acetyl-l-carnitine (ALCAR) on NMDA receptors in the old rat (24 months) after chronic or single-dose treatments. The number of NMDA receptors was significantly decreased in the old rat hippocampus by 19.5% compared with the young rat. A six-month treatment with ALCAR in the old rat attenuated the loss of NMDA binding sites in the hippocampus. A single-dose treatment with ALCAR in the old rat increased the B max value by 35%, while no change was observed in the young group. We conclude that ALCAR can exert two actions: a trophic/neuro-preserving one when chronically administered during aging, and a stimulatory one when given at a single dose in the aged rat.

Glutamate Receptor Binding Sites in MPTP-Treated Mice

Changes in excitatory amino acid (EAA) neurotransmission are thought to play an important role int he development of parkinsonian symptoms. We examined EAA receptor binding sites in substantia nigra, striatum, globus pallidus, and cortex at 2 weeks and 2 months after MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) injection in C57b16 mice. At 2 weeks striatal dopamine content in MPTP-treated mice was reduced to 7% of control and N-methyl- d-aspartate (NMDA)-sensitive [ 3H]glutamate and [ 3H]α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) binding sites were decreased in substantia nigra to 57 and 76% of control, respectively. In globus pallidus only [ 3H]AMPA binding sites were decreased to 80% of control; no significant changes were found in striatum or cortex. [ 3H]Kainate binding sites remained unchanged. At 2 months striatal dopamine content was reduced to 31% and no changes in EAA binding sites could be detected in any of the structures examined. [ 3H]Mazindol binding to striatal monoamine-uptake sties was decreased to 17% of control at 2 weeks versus 37% at 2 months. Our data indicate that modulation of NMDA and AMPA binding sites in substantia nigra and globus pallidus, the major projection areas of the subthalamic nucleus, takes place only after severe impairment of the nigrostriatal system.

Steroid hormone effects on NMDA receptor binding and NMDA receptor mRNA levels in the hypothalamus and cerebral cortex of the adult rat.

Previous work has demonstrated that N-methyl-D-aspartate (NMDA) is capable of stimulating luteinizing hormone release in a variety of species. Interestingly, the ability of NMDA to stimulate luteinizing hormone release is significantly compromised in castrated male and female rats as compared to intact animals. The purpose of the present study was to determine if a difference exists in the number or affinity of NMDA receptors in the hypothalamus of intact or castrated adult male and female rats and whether steroid replacement has any effect on NMDA receptor binding. NMDA receptor mRNA levels were also determined in the respective models. The cerebral cortex was used as a control to check for specificity of any observed differences. The number of NMDA binding sites in the hypothalamus was found to be approximately 25% of that found in the cerebral cortex and the equilibrium association constant was similar in both tissues. In the female rat, neither ovariectomy nor ovariectomy with estrogen pellet replacement or estrogen and progesterone injections altered NMDA receptor binding or the equilibrium association constant in the hypothalamus or cerebral cortex as compared to intact controls. Similar to the case in the female, NMDA receptor binding in the hypothalamus and cerebral cortex of male rats did not change after castration or after treatment with testosterone propionate. Neither ovariectomy nor ovariectomy with estradiol replacement brought about any change in the NMDA receptor mRNA levels in the hypothalamus. However, in the cerebral cortex ovariectomy with estrogen replacement brought about a small but significant increase in NMDA receptor mRNA levels.(ABSTRACT TRUNCATED AT 250 WORDS)