N-Methyl-D-Aspartate receptors in acquisition of the short-term memory in the honeybeeApis mellifera

Role of NMDA receptors in the process of associative learning has been studied in the honeybeeApis mellifera L. in behavior experiments, using method of conditional reflexes. To determine pharmacological profile of NMDA receptors, effects of Mg²⁺ ions, NMDA, glycine (Gly), antagonist of the glycine site 5,7-dichlorokynurenic acid (DCK), competitive antagonists of NMDA receptors: D,L-2-aminophosphovalerate (APV), L-2-aminophosphobutyrate (APB), and D-glutamyl-aminomethylphosphonic acid (GAMP) as well of antagonists of NMDA receptor ion channels MK-801 and ketamine (Ket) were studied on acquisition of alimentary conditional reflex and its retention in memory. NMDA increased capacity for learning by stimulating shortterm memory. The NMDA receptor co-agonist Gly activated this NMDA effect. DCK eliminated the NMDA and Gly stimulatoty effects. All tested antagonists at millimolar concentrations inhibited associative function. The data obtained confirm our hypothesis about participation of NMDA receptors in processes of formation of short-term memory in the honeybeeApis mellifera and suggest that functional characteristics of the NMDA receptors involved in the process of associative learning in the honeybee resemble those in mammals.     

Chronic Dosing with 1-Aminocyclopropanecarboxylic Acid,a Glycine Partial Agonist,Modulates NMDA Inhibition of Muscarinic-Coupled PI Hydrolysis in Rat Cortical Slices

Chronic dosing with the glycine partial NMDA agonist, 1-aminocyclopropanecarboxylic acid (ACPC) elicited an altered allosteric regulation of cortical NMDA receptor binding. The present study hypothesized that these allosteric receptor binding changes would be manifest as pharmacologically functional reductions in NMDA receptor activity following chronic ACPC dosing. NMDA inhibition of carbachol—induced phosphoinositide (PI) hydrolysis was used as a functional assay to assess NMDA receptor function in rat cerebral cortex. NMDA inhibition of stimulated PI turnover was similar in naive (46% ± 4.5%; mean ± SE; n = 34) and ACPC dosed rats (39% ± 2.3%; n = 34). While ACPC reversed NMDA's inhibitory effects in naive rats (80% ± 13%; n = 9), it was ineffective (P < 0.05) in ACPC pretreated rats (48% ± 9.8%; n = 9). In contrast, the NMDA antagonists, MK-801 (ion channel), 7-chlorokynurenic acid (glycine site) and AP-7 (glutamate site), effectively reversed NMDA's inhibitory effects in naive and ACPC treated rats. The potency of these antagonists were unaltered by prior ACPC dosing. Thus, chronic ACPC therapy does not alter the functioning of the NMDA ion channel or glutamate receptor sites, but elicits functional tolerance to ACPC's actions in the cortical NMDA complex.     

Kinetic analysis of antagonist action at N-methyl-D-aspartic acid receptors. Two binding sites each for glutamate and glycine.

Antagonism of glutamate-receptor responses activated by N-methyl-D-aspartic acid (NMDA) was studied using whole cell voltage clamp recording from mouse dissociated hippocampal neurons cultured for 10-15 d. The kinetics of onset of and recovery from NMDA receptor block during continuous application of NMDA together with either glycine, or L-alanine, were recorded in response to concentration jump application of NMDA- and glycine-binding site directed competitive antagonists, applied with a multibarrel flow pipe under conditions which allowed rapid solution changes around the cell less than 10 ms. Mathematical solutions for both one- and two-equivalent site models for competitive antagonism were determined according to the differential equations outlined by Colquhoun and Hawkes (1977. Proc. R. Soc. Lond. B. 199:231-262). The kinetics of action of D-CPP and D-AP5, NMDA binding site antagonists, and 7Cl-kynurenic acid, a glycine binding site antagonist, were examined for each model. For all these antagonists, the kinetics for the onset of and recovery from antagonism were better fit by the two-equivalent site model, which yielded antagonist microscopic kBoff/kBon values which closely approximated Ki values determined from analysis of equilibrium dose response curves. These results suggest that two molecules of NMDA and two molecules of glycine must bind to the NMDA receptor for activation of ion channel gating.     

Effects of glutamate antagonists on the activity of aromatic L-amino acid decarboxylase

Summary This study examines the hypothesis that glutamate tonically suppresses the activity of the enzyme aromatic L-amino acid decarboxylase (AADC), and hence the biosynthesis of dopamine, to explain how antagonists of glutamate receptors might potentiale the motor actions of L-DOPA in animal models of Parkinson's disease. A variety of glutamate antagonists were therefore administered acutely to normal rats, which were sacrificed 30–60 min later and AADC activity assayed in the substantia nigra pars reticulate (SNr) and corpus striatum (CS). The NMDA receptor-ion channel antagonists MK 801, budipine, amantadine, memantine and dextromethorphan all caused a pronounced in creased in AADC activity, more especially in the SNr than CS. The NMDA glycine site antagonist (R)-HA 966 produced a modest increase in AADC activity in the CS but not SNr, whilst the NMDA polyamine site antagonist eliprodil, the NMDA competitive antagonist CGP 40116 and the AMPA antagonist NBQX were without effect. The results suggest that an increase in dopamine synthesis might contribute to the L-DOPA-facilitating actions of some glutamate antagonists.     

Role of Glycine in the N-Methyl-d-Aspartate-Mediated Neuronal Cytotoxicity

Current evidence indicates that glutamate acting via the N-methyl-D-aspartate (NMDA) receptor/ion channel complex plays a major role in the neuronal degeneration associated with a variety of neurological disorders. In this report the role of glycine in NMDA neurotoxicity was examined. We demonstrate that NMDA-mediated neurotoxicity is markedly potentiated by glycine and other amino acids, e.g., D-serine. Putative glycine antagonists HA-966 and 7-chlorokynurenic acid were highly effective in preventing NMDA neurotoxicity, even in the absence of added glycine. The neuroprotective action of HA-966 and 7-chlorokynurenic acid, but not that of NMDA antagonists 3-(2-carboxypiperazine-4-yl)propylphosphonate and MK-801, could be reversed by glycine. These results indicate that glycine, operating through a strychinine-insensitive glycine site, plays a central permissive role in NMDA-mediated neurotoxicity.     

Different capacities of various NMDA receptor antagonists to prevent ischemia-induced neurodegeneration in human cultured NT2 neurons

In the present study, human NT2 neurons obtained from embryonic teratocarcinoma (NT2) cells were established as human in-vitro model to investigate the mechanisms associated with hypoxia/ischemia-induced neuronal injury. NT2 neurons express functional NMDA receptors that are of particular significance for hypoxia/ischemia-related neuronal damage. In patch-clamp recordings under normoxic conditions, NMDA (plus 10 microM glycine)-induced inward currents (EC(50)=43.7 microM) were distinctly antagonized by memantine, a blocker of the receptor channel, but only slightly by 5,7-dichlorokynurenic acid (DCKA), a glycine(B) binding site antagonist. Immunohistochemistry demonstrated that the NT2 neurons are mostly GABAergic; they predominantly express the NMDA receptor subunits NR2B and NR2C, and lower levels of NR1 and, particularly, of NR2A. Upon glucose and oxygen deprivation for 3h the loss of cell viability measured directly after 3h was higher than after application of either hypoxia or aglycemia as assessed by propidium iodide flow cytometry. Ischemic conditions significantly reduced the NMDA responses associated with a decrease in EC(50) and decreased mitochondrial membrane potential as detected by JC-1 flow cytometry. Memantine (50 microM) and CGS19755 (a competitive NMDA receptor antagonist; 10 microM) reduced ischemia-induced cell death, in contrast to DCKA (10 microM). In conclusion, in the present human in-vitro model for studying the molecular mechanisms associated with ischemic injury, neuroprotection could be achieved with NMDA receptor antagonists but not with a glycine(B) binding site antagonist. Accordingly, glycine antagonists might not represent an optimal therapeutic strategy for preventing ischemic neuronal damage in contrast to NMDA receptor antagonists like memantine.     

Attenuation of febrile seizures in epileptic chicks by N-methyl-D-aspartate receptor antagonists

Experimental febrile seizures can be evoked in epileptic chicks by elevation of their body temperature. Both competitive N-methyl-D-aspartate (NMDA) receptor antagonists [(3-(+/- )2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP), DL-2-amino-7-phosphosphonoheptanoic acid (APH), DL-2-amino-5-phosphonovaleric acid (APV), D-alpha-aminoadipic acid (AAA), and DL-alpha, epsilon-diaminopimelic acid (DAP)] and the noncompetitive NMDA antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5, 10-imine maleate (MK-801) produced dose-dependent increases in latency to the onset of seizures. Of the drugs tested, MK-801 had the highest potency followed in order by CPP = APH greater than APV much greater than AAA greater than DAP. There was a high correlation (r = 0.995) between the dose capable of doubling seizure latency and the affinity of the competitive NMDA antagonists for the NMDA receptor as determined by in vitro binding assays. These data suggest that NMDA receptor mediated mechanisms may be involved in the production of seizures in response to hyperthermia.     

The Non-NMDA Glutamate Receptor Antagonists 6-Cyano-7-Nitroquinoxaline-2,3-dione and 2,3-Dihydroxy-6-Nitro-7-Sulfamoylbenzo(f)quinoxaline, but Not NMDA Antagonists, Block the Intrastriatal Neurotoxic Effect of MPP+

Altered glutamatergic neurotransmission appears to be central to the pathophysiology of Parkinson's disease; consequently, considerable effort has been made to elucidate neuroprotective mechanisms against such toxicity. In the present study, the possible neuroprotective effect of glutamate receptor antagonists against MPP+ neurotoxicity on dopaminergic terminals of rat striatum was investigated. Different doses of glutamate receptor antagonists were coinfused with 1.5 microg of MPP+ into the striatum; kynurenic acid, a nonselective antagonist of glutamate receptors (30 and 60 nmol), partially protected dopaminergic terminal degeneration in terms of rescue of dopamine levels and tyrosine hydroxylase immunohistochemistry. Dizocilpine, a channel blocker of the NMDA receptor (1, 4, and 8 nmol), and 7-chlorokynurenic acid, a selective antagonist at the glycine site of the NMDA receptor (1 and 10 nmol), failed to protect dopaminergic terminals from MPP+ toxicity. However, 6-cyano-7-nitroquinoxaline-2,3-dione (0.5 and 1 nmol) and 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(f)quinoxaline (1 nmol), two AMPA-kainate receptor antagonists, protected against MPP toxicity. Our findings suggest that the toxic effects of MPP+ on dopaminergic terminals are not mediated through a direct interaction with the NMDA subtype of glutamate receptor, but with the AMPA-kainate subtype.     

Ethanol Differentially Inhibits Homoquinolinic Acid- and NMDA-Induced Neurotoxicity in Primary Cultures of Cerebellar Granule Cells

The potency of ethanol to inhibit N-methyl-D-aspartate (NMDA) receptor functions may depend on the subunit composition of the NMDA receptors. We used a NR2A-B subunit-selective NMDA receptor agonist, homoquinolinic acid (HQ), and a subunit-unselective agonist, NMDA, to induce neurotoxicity in cerebellar granule cells and examined the neuroprotective actions of ethanol, as well as NR2A- and NR2B-subunit selective antagonists, respectively. HQ was a more potent neurotoxic agent than NMDA, as measured by the MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) assay. NR2A- and NR2B-selective NMDA receptor antagonists displayed quite similar neuroprotective potencies against the NMDA- and HQ-produced cell death, indicating that the higher potency of HQ to induce neurotoxicity cannot be simply explained by NR2A- or NR2B-subunit selectivity. As expected, ethanol (25 and 50 mM) attenuated the NMDA-induced neurotoxicity in a non-competitive manner by significantly reducing the maximum neurotoxicity produced by NMDA. By contrast, ethanol inhibited the HQ-induced neurotoxicity in a manner resembling a competitive-like interaction significantly increasing the EC₅₀ value for HQ, without reducing the maximum neurotoxicity produced by HQ. These results suggest that HQ reveals either a novel site or a not previously observed mechanism of interaction between ethanol and NMDA receptors in rat cerebellar granule cell cultures.     

N-Methyl-D-Aspartate Recognition Site Ligands Modulate Activity at the Coupled Glycine Recognition Site

In synaptic plasma membranes from rat forebrain, the potencies of glycine recognition site agonists and antagonists for modulating [3H]1-[1-(2-thienyl)cyclohexyl]piperidine ([3H]TCP) binding and for displacing strychnine-insensitive [3H]glycine binding are altered in the presence of N-methyl-D-aspartate (NMDA) recognition site ligands. The NMDA competitive antagonist, cis-4-phosphonomethyl-2-piperidine carboxylate (CGS 19755), reduces [3H]glycine binding, and the reduction can be fully reversed by the NMDA recognition site agonist, L-glutamate. Scatchard analysis of [3H]glycine binding shows that in the presence of CGS 19755 there is no change in Bmax (8.81 vs. 8.79 pmol/mg of protein), but rather a decrease in the affinity of glycine (KD of 0.202 microM vs. 0.129 microM). Similar decreases in affinity are observed for the glycine site agonists, D-serine and 1-aminocyclopropane-1-carboxylate, in the presence of CGS 19755. In contrast, the affinity of glycine antagonists, 1-hydroxy-3-amino-2-pyrrolidone and 1-aminocyclobutane-1-carboxylate, at this [3H]glycine recognition site increases in the presence of CGS 19755. The functional consequence of this change in affinity was addressed using the modulation of [3H]TCP binding. In the presence of L-glutamate, the potency of glycine agonists for the stimulation of [3H]TCP binding increases, whereas the potency of glycine antagonists decreases. These data are consistent with NMDA recognition site ligands, through their interactions at the NMDA recognition site, modulating activity at the associated glycine recognition site.     

Pentobarbital-like effects of N-methyl-D-aspartate antagonists in mice

The effects of the competitive N-methyl-D-aspartate (NMDA) antagonist, 3-[(+/-)-2-carboxypiperazin-4-yl]propyl-1-phosphonic acid (CPP), and of the noncompetitive NMDA antagonist, dizocilpine (MK-801), were determined in mice trained to discriminate pentobarbital (20 mg/kg i.p.) from saline under a standard two-lever fixed-ratio 20 schedule of sweetened milk reinforcement. CPP substituted for pentobarbital; however, pentobarbital-lever responding was usually associated with decreases in response rates. Dizocilpine produced a maximum average of only 62% pentobarbital-lever responding, accompanied by a 50% decrease in response rates. These results suggest that pentobarbital-like discriminative stimulus effects are more likely to be produced by competitive than by noncompetitive NMDA antagonists. This extends previous observation in rats and provides further evidence for differences in the behavioral effects of competitive and noncompetitive NMDA antagonists and for an overlap in the behavioral pharmacology of NMDA antagonists and classical CNS depressants.     

The role of group I metabotropic glutamate receptors in schizophrenia

Summary. It has been proposed that glutamatergic transmission, in particular NMDA receptor function, might be altered in schizophrenia. This hypothesis is mainly based on the observation that uncompetitive NMDA receptor antagonists, e.g. phencyclidine, evoke psychotic symptoms in healthy subjects, whereas agonists interacting at the glycine site of the NMDA receptor complex, e.g. glycine or D-serine, administered jointly with typical neuroleptics, can alleviate schizophrenic symptoms. The function of NMDA receptors may be modulated by group I mGluRs (mGluR1 and mGluR5), which have also been shown to be altered in schizophrenia. In rodents, mGluR5 antagonists, but not mGluR1 ones, potentiate the locomotor activity and the deficit of prepulse inhibition (PPI) induced by uncompetitive NMDA receptor antagonists. These antagonists (of either type) administered alone are not active in the above tests. Hence, antagonists of mGluR1 and mGluR5 may evoke different effects on the NMDA receptor antagonists-induced behavior and, possibly, on schizophrenic symptoms.     

N-methyl-d-aspartate andl-aspartate activate distinct receptors in piriform cortex

The effects of ionophoretically applied N-methyl-DL-aspartate (NMDA) and aspartate on identified pyramidal neurons in rat piriform cortex were examined in isolated, submerged, and perfused brain slices. NMDA was more potent than aspartate in eliciting neuronal discharge. Perfusion of the acidic amino acid antagonists, DL-2-amino-5-phosphonovalerate (APV), 10(-6) or 10(-5) M, DL-2-amino-7-phosphonoheptanoate (APH), 10(-5) M, and gamma-D-glutamylglycine (gamma DGG), 10(-5) M, selectively blocked the response to NMDA without effect on the response to aspartate. At higher concentrations which blocked responses to both NMDA and aspartate, gamma DGG blocked kainate responses and depressed glutamate and quisqualate responses. These results suggest that in piriform neurons NMDA and aspartate act at distinct receptor sites, not a common receptor site, and that both of these sites are distinct from those that mediate responses to glutamate, quisqualate, and kainate.     

Post-ischemic recovery of acetylcholine release in vitro: influence of different excitatory amino acid receptor subtype antagonists

The release of endogenous acetylcholine was measured in electrically (5–20 Hz) stimulated guinea pig cerebral cortex and caudate nucleus slices under ischemic (hypoxic and glucose-free) conditions. Ischemia reduced acetylcholine release by 40–90%; the inhibition depended on the duration of ischemia (10–30 min) while the extent of post-ischemic recovery was inversely related to it. Caudate nucleus slices displayed a higher sensitivity to ischemia than did cortical slices. To test the effects of excitatory amino acid receptor antagonists on the ischemia-induced reduction of acetylcholine release and on its post-ischemic recovery, the following drugs were used: 5-methyl-10,11-dihydro-5-H-dibenzo-[a,b]-cyclohepten-5,10-imine (MK-801, a blocker of the N-methyl-D-aspartate [NMDA] receptor-linked channel), 7-chloro-kynurenic acid (7-Cl-KYN) and (E)-3-[2(phenylcarbamoyl)ethenyl]-4,6-dichloroindole-2-carboxylic acid sodium salt (GV150526A, blockers of the glycine site of the NMDA receptor), eliprodil, (an antagonist at the polyamine site of the NMDA receptor), and 6-cyano- 7-nitro-quinoxalin-2,3-dione (CNQX, a D,L--amino-3-hydroxy-5-methyl-4-isoxalone propionic acid [AMPA] receptor antagonist). These did not modify the time-course and the extent of ischemia-induced inhibition but improved post-ischemic recovery in a concentration dependent manner. GV 150526A and CNQX appeared to be more effective in the cerebral cortex. Only eliprodil was devoid of any effect in both areas.

The evaluation of acetylcholine release from brain slices represents a suitable in vitro model to quantify the effectiveness of drugs in favouring recovery from the cholinergic presynaptic failure induced by ischemic conditions. The different effects of the excitatory amino acid receptor antagonists cited above, depending on the brain areas considered and the receptor subtypes involved, may be of interest in view of their therapeutic potential.     

Mechanisms of L-Cysteine Neurotoxicity

We review here the possible mechanisms of neuronal degeneration caused by L-cysteine, an odd excitotoxin. L-Cysteine lacks the omega carboxyl group required for excitotoxic actions via excitatory amino acid receptors, yet it evokes N-methyl-D-aspartate (NMDA) -like excitotoxic neuronal death and potentiates the Ca2+ influx evoked by NMDA. Both actions are prevented by NMDA antagonists. One target for cysteine effects is thus the NMDA receptor. The following mechanisms are discussed now: (1) possible increase in extracellular glutamate via release or inhibition of uptake/degradation, (2) generation of cysteine alpha-carbamate, a toxic analog of NMDA, (3) generation of toxic oxidized cysteine derivatives, (4) chelation of Zn2+ which blocks the NMDA receptor-ionophore, (5) direct interaction with the NMDA receptor redox site(s), (6) generation of free radicals, and (7) formation of S-nitrosocysteine. In addition to these, we describe another new alternative for cytotoxicity: (8) generation of the neurotoxic catecholamine derivative, 5-S-cysteinyl-3,4-dihydroxyphenylacetate (cysdopac).     

The Glycine Site of the N-Methyl-D-Aspartate Receptor Channel: Differences Between the Binding of HA-966 and of 7-Chlorokynurenic Acid

The mechanisms of action of three different glycine-site antagonists of the N-methyl-D-aspartate (NMDA)-receptor channel were analyzed employing [3H]glycine direct binding assays, as well as functional glycine- and glutamate-induced uncompetitive blocker binding assays. The latter assays measure apparent channel opening. All three antagonists tested, viz., 7-chlorokynurenic acid (7-Cl-KYNA), kynurenic acid (KYNA), and 1-hydroxy-3-aminopyrrolidone-2 (HA-966), inhibited the binding of [3H]glycine to the NMDA receptor in a dose-dependent manner. These antagonists also inhibited the glycine-induced increase in accessibility of the uncompetitive blocker [3H]N-[1-(2-thienyl)cyclohexyl]-piperidine ([3H]TCP) to the channel. 7-Cl-KYNA and KYNA, but not HA-966, completely blocked the glutamate-induced binding of [3H]TCP, in a manner similar to the non-competitive manner in which the selective NMDA antagonist D-(-)-2-amino-5-phosphonovaleric acid (AP-5) inhibited glycine-induced [3H]TCP binding. The inhibitory effects of HA-966 and of AP-5 on glutamate-induced [3H]TCP binding were overcome when glutamate concentrations were increased. Of the three antagonists, 7-Cl-KYNA appears to be the most potent (Ki = 0.4-1.0 microM) and the most selective glycine antagonist. KYNA was found to act at both the glycine (Ki = 40-50 microM) and the glutamate sites. In contrast, HA-966 (Ki = 6-17 microM) appears to act either on a domain distinct from the glutamate and the glycine sites, but tightly associated with the latter, or at the glycine site, but according to a mechanism distinct from that of 7-Cl-KYNA.(ABSTRACT TRUNCATED AT 250 WORDS)     

Characterization of the non-competitive antagonist binding site of the NMDA receptor in dark Agouti rats

The ability of non-competitive NMDA antagonists and other selected compounds to inhibit [3H]MK-801 binding to the NMDA receptor in brain membranes was evaluated in female, dark Agouti rats. In homologous competition binding studies the average apparent affinity (KD) of [3H]MK-801 for its binding site was 5.5 nM and the binding site density (Bmax) was 1.83 pmol/mg protein. Inhibition of [3H]MK-801 binding by non-competitive NMDA antagonists was best described with a one-site competition model and the average Hill coefficients were -1. A series of eight non-competitive NMDA antagonists inhibited [3H]MK-801 binding with the following rank order of affinity (K(i), nM): MK-801 (5.5) > dexoxadrol (21.5) > or = TCP (24.2) > phencyclidine (100.8) > (+)-SKF 10,047 (357.7) > dextrorphan (405.2) > ketamine (922.2) > dextromethorphan (2913). These inhibition binding constants determined in dark Agouti rat brain membranes were significantly correlated (P = 0.0002; r2 = 0.95) with previously reported values determined in Sprague-Dawley rats [Wong et al., 1988, J. Neurochem. 50, 274-281]. Despite significant differences in metabolic capability between these strains, the central nervous system NMDA receptor ion channel shares similar characteristics.     

Involvement of γ-Aminobutyric Acid and N-Methyl-D-Aspartate Receptors in the Inhibitory Effect of Ethanol on Pentylenetetrazole-Induced c-fos Expression in Rat Brain

The expression of c-fos mRNA in rat brain was induced by intraperitoneal administration of pentylenetetrazole (PTZ) and picrotoxin, which act on the picrotoxin binding site of the gamma-aminobutyric acid-benzodiazepine (GABA-BZ) receptor complex, by N-methyl-D-aspartate (NMDA) and kainic acid, agonists of different classes of glutamate receptors and by caffeine, an antagonist of adenosine receptors. The actions of PTZ and picrotoxin but not that of NMDA were blocked by ethanol and the NMDA-receptor antagonist, MK-801. Ro 15-4513 partially reversed the inhibitory effect of ethanol on PTZ-induced c-fos mRNA synthesis. Acute ethanol administration blocked the actions of PTZ and NMDA without affecting the response to kainic acid or caffeine. Taken together, these data suggest that ethanol blocks c-fos gene activation by noncompetitive antagonists of the GABA-BZ receptor via actions on both the NMDA and GABA receptors.     

The role of striatal glutamate receptors in models of Parkinson's disease

Summary The aim of the study was to examine the effect of antagonists of the NMDA receptor on the parkinsonian-like muscle rigidity in rats. Reserpine and haloperidol increased the muscle resistance of the hind foot to passive movements, as well as the reflex electromyographic (EMG) activity in the gastrocnemius and tibialis anterior muscles. MK-801 (0.32-1.28 mg/kg sc), an uncompetitive antagonist of the NMDA receptor, and L-701,324 (5-40 mg/ kg ip), an antagonist of the glycine site, reduced the muscle tone and the reflex EMG activity enhanced by reserpine or haloperidol. AP-5 (2 and 5 ,g/ 0.5 pl), a competitive antagonist of the NMDA receptor, and 5,7-dichlorokynurenic acid (1.0-4.5g/0.5 pl), the glycine site antagonist injected bilaterally into the rostral striatum, inhibited the muscle rigidity induced by haloperidol. In contrast, AP-5, injected alone bilaterally into the intermediate-caudal striatum induced muscle rigidity. The present results suggest that: (1) the inhibitory effect of the NMDA receptor antagonists on the parkinsonian-like muscle rigidity depends, at least partly, on their action on the rostral striatum; (2) the blockade of NMDA receptors in the intermediate-caudal striatum may reduce the beneficial impact of these compounds.     

A Glycine Site Associated with N-Methyl-d-Aspartic Acid Receptors: Characterization and Identification of a New Class of Antagonists

Membranes from rat telencephalon contain a single class of strychnine-insensitive glycine sites. That these sites are associated with N-methyl-D-aspartic acid (NMDA) receptors is indicated by the observations that [3H]glycine binding is selectively modulated by NMDA receptor ligands and, conversely, that several amino acids interacting with the glycine sites increase [3H]N-[1-(2-thienyl)cyclohexyl]piperidine ([3H]TCP) binding to the phencyclidine site of the NMDA receptor. The endogenous compound kynurenate and several related quinoline and quinoxaline derivatives inhibit glycine binding with affinities that are much higher than their affinities for glutamate binding sites. In contrast to glycine, kynurenate-type compounds inhibit [3H]TCP binding and thus are suggested to form a novel class of antagonists of the NMDA receptor acting through the glycine site. These results suggest the existence of a dual and opposite modulation of NMDA receptors by endogenous ligands.