Behavioural activity of (S)-3,5-DHPG, a selective agonist of group I metabotropic glutamate receptors.

The influence of intracerebroventricular (i.c.v.) injections of (S)-3,5-dihydroxyphenyl-glycine (S)-3,5-DHPG, a selective agonist of group I metabotropic glutamate receptors (mGluRs), on the activity of the central nervous system was examined in male rats. (S)-3,5-DHPG at doses of 25, 50 and 100 nmol significantly attenuated crossings of squares and rearings, but not bar approaches, in an 'open field' test and failed to change apomorphine-induced stereotypy. (S)-3,5-DHPG at the above doses, given immediately after the learning trial, significantly facilitated the consolidation process in a passive avoidance situation, but given before the learning trial and before the retention testing did not have any influence on acquisition and retrieval processes, respectively. Moreover, (S)-3,5-DHPG did not influence recognition memory evaluated in an object recognition test. These results may suggest that activation of group I mGluRs takes part in the consolidation process in affectively-motivated memory, but is probably not necessary for processing of recognition memory, and that (S)-3,5-DHPG memory facilitation seems to be independent of glutamatergic and dopaminergic interaction.     

Influence of AIDA on certain behaviors in rats subjected to experimental hypoxia

The influence of the blockade of group I metabotropic glutamate receptors (mGluRs) by AIDA [(RS)-1-aminoindan-1,5-dicarboxylic acid] on some behavioral effects was tested in control groups of rats and in rats that underwent short-term hypoxia. We used the following methods: the open field test, the passive avoidance test and the elevated "plus" maze test. In rats without hypoxia, AIDA (100 nmol icv) decreased the number of crossings in the open field test, impaired acquisition, improved consolidation and did not influence retrieval in the passive avoidance situation and was ineffective in the elevated "plus" maze. Short-term hypoxia (2% O2, 98% N2), as a model of experimentally induced amnesia, significantly inhibited locomotor and exploratory activity and profoundly impaired acquisition, consolidation and retrieval processes and did not exhibit proanxiogenic or anxiolytic effect in elevated "plus" maze. AIDA (100 nmol icv) used before hypoxia significantly improved consolidation and retrieval processes, but had no effect on acquisition and did not significantly influence all parameters of the elevated "plus" maze test. The obtained results suggest that AIDA, the selective antagonist of group I mGluRs, had beneficial effects on consolidation and retrieval of passive avoidance in rats undergoing hypoxia.     

2R,4R-APDC influence on hypoxia-induced impairment of learning and memory processes in passive avoidance test

We investigated the effects of 2R,4R-APDC, a selective group II metabotropic glutamate receptor (II mGluR) agonist, on certain behaviors in rats subjected and non-subjected to hypoxia. Short-term hypoxia was used as a model of experimentally induced amnesia. 2R,4R-APDC given intracerebroventricularly (icv) at doses of 1 mumol and 100 nmol decreased the number of crossings and rearings in the open field, impaired acquisition and consolidation but improved retrieval in the passive avoidance tests. It also shortened the time spent in open arms and prolonged the time spent in closed arms, reduced the number of open and closed arms entries in an elevated "plus" maze, which is a measure of anxiety. Four-minute hypoxia (2% O(2), 98% N(2)) retrieval of conditioned responses, and exhibited an anxiogenic effect in the elevated "plus" maze in rats, i.e. it reduced the time spent in open arms and the number of entries to closed and open arms. 2R,4R-APDC effect on locomotor and exploratory activity was not changed after hypoxia, i.e. we observed inhibition of motility. This agonist of II mGluRs used at both doses before hypoxia significantly improved acquisition and retrieval, and had dual effect on consolidation, viz. at a dose of 1 mumol, it impaired this process and at a dose of 100 nmol it improved it. In the elevated "plus" maze, rats pretreated with 2R,4R-APDC and then subjected to hypoxia shortened the time spent in open arms and prolonged the time spent in closed arms, reduced the time spent in open arms, i.e. the drug exhibited anxiogenic effect. We conclude, therefore, that 2R,4R-APDC itself impaired acquisition and consolidation, enhanced retrieval but in rats undergoing hypoxia, it improved acquisition, retrieval and when used at the dose of 100 nmol enhanced consolidation. 2R,4R-APDC had beneficial effect in hypoxia-induced memory impairment in passive avoidance test.     

Dizocilpine, ketamine and ethanol reverse NMDA-induced EEG changes and convulsions in rats and mice.

Electroencephalographic (EEG) activity in neocortex of rats following intracerebroventricular (icv) administration of NMDA (0.25-2 nmol/10 microliters) and its modification by noncompetitive NMDA-receptor antagonists, dizocilpine (MK-801) (0.025-0.1 mg/kg, ip) and ketamine (10-50 mg/kg, ip) was recorded at 0, 0.5, 4, 8 and 24 hr with chronically implanted electrodes. NMDA (0.25 and 1 nmol) showed longer lasting decrease in frequency in cortical neurons while 2 nmol produced convulsions and death. Administration of MK 801 (0.05 mg/kg) and ketamine (50 mg/kg) prior to NMDA offered protection in 40% of animals against NMDA-induced convulsions and blocked NMDA-induced long term influence. However, ketamine and MK 801 showed an increase in percent amplitude and also had long lasting effects per se. In conscious mice, NMDA (0.5-10 nmol/microliters icv) induced dose dependent convulsions. Both MK 801 and ketamine showed potent anticonvulsant effect. Ethanol (0.5-2 g/kg, ip) also offered significant protection against NMDA-induced convulsions. MK 801 (0.1 mg/kg) when administered concurrently with ethanol (0.5 g/kg) exhibited synergistic anticonvulsant effect. The EEG study in rats and effect of NMDA in conscious mice provide a direct evidence for the role of NMDA-receptor system in convulsions and in anticonvulsant action of ethanol.     

Effect of (S)-3,5-DHPG on learning,exploratory activity and anxiety in rats with experimental hypoxia

We investigated the effects of (S)-3,5-DHPG, a selective agonist of group I metabotropic glutamate receptors (I mGluRs), on certain behaviors in rats after short-term hypoxia as a model of experimentally induced amnesia. The effect of (S)-3,5-DHPG administered intracerebroventriculary (icv) at doses of 0.01, 0.1 and 1.0 nmol was assessed using behavioral tests: the open field test, the passive avoidance response and the elevated "plus" maze test. (S)-3,5-DHPG did not change the number of crossed fields and rearings and only at a dose of 0.01 nmol it increased the number of bar approaches in the open field test. (S)-3,5-DHPG used at all doses improved consolidation, and at doses of 0.01 and 1.0 nmol it improved retrieval in the passive avoidance test. (S)-3,5-DHPG did not produce any significant effects in control rats in the elevated "plus" maze test. Hypoxia inhibited locomotor and exploratory activity of rats, significantly impaired consolidation and retrieval processes. We observed tendency to shortening the time spent in open arms and to decrease in the number of entries into open arms in the elevated "plus" maze in rats with underwent hypoxia. In hypoxia-treated groups of rats, (S)-3,5-DHPG inhibited locomotor and exploratory activity in comparison with the control groups administered (S)-3,5-DHPG. Hypoxia significantly inhibited beneficial effects of (S)-3,5-DHPG on consolidation and retrieval in passive avoidance. (S)-3,5-DHPG only at the dose of 1.0 nmol used before hypoxia improved consolidation and at the dose of 0.01 nmol enhanced retrieval in comparison with saline-treated group subjected to hypoxia. (S)-3,5-DHPG only at the dose of 1.0 nmol in hypoxia-treated group shortened the time spent in closed arms and increased the number of entries into closed and open arms in the elevated "plus" maze vs saline-treated group subjected to hypoxia. In rats subjected to hypoxia, (S)-3,5-DHPG, the agonist of I mGluRs, improved consolidation and retrieval and exhibited anxiolytic activity in dose-dependent manner.     

Prefrontal cortex lesions cause only minor effects on the hyperlocomotion induced by MK-801 and its reversal by clozapine

The non-competitive NMDA receptor antagonist MK-801 elicits a behavioural syndrome in rodents characterized by hyperlocomotion and stereotypies, which is antagonized by antipsychotic drugs. NMDA receptor antagonists increase prefrontal cortex (PFC) activity in rodents, as assessed by electrophysiological and neurochemical measures. The increase in glutamate outflow induced by systemic MK-801 administration in the medial PFC (mPFC) is prevented by the local administration of clozapine (Clz). In the present study, we examine whether a PFC lesion alters the behavioural syndrome induced by MK-801 in rats and the Clz-induced antagonism of MK-801 actions. We evaluated the hyperlocomotion, stereotypies and other behavioural changes induced by MK-801 in the open field and the effect of electrolytic lesions of the mPFC, and of cortical transection on the behavioural syndrome induced by MK-801 and its reversal by Clz. MK-801 (0.1-0.2 mg/kg i.p.) reduced rearings but only the higher dose induced hyperlocomotion. At this dose, MK-801 also increased disorganized movements, head weavings, and induced ataxia signs. An electrolytic lesion of the mPFC markedly reduced the number of rearings pre-treatment but caused a very slight attenuation of MK-801-induced hyperlocomotion. Cortical transection did not significantly alter MK-801 effects. Clz administration (1 mg/kg s.c.) significantly attenuated hyperlocomotion, head weavings and ataxia signs induced by MK-801 but did not prevent the decrease in rearings. The effect of Clz was essentially unaffected by electrolytic lesions of the mPFC. These results show that MK-801-induced motor syndrome and its reversal by Clz are mostly independent on PFC integrity.     

Behavioral effects of D-AP7 in rats subjected to experimental hypoxia

We investigated the effects of D-AP7 [D-(-)-2-amino-7-phosphonoheptanoic acid], a specific, potent antagonist of NMDA receptor on certain forms of behavior in control groups of rats and in rats submitted to hypoxia. D-AP7 given intracerebroventrically (icv) at a dose of 5 nmol was tested in the open field test, passive avoidance test and in elevated plus maze test. D-AP7 did not significantly change the locomotor and exploratory activity, but it exhibited a tendency to enhance motility of rats in the open field test. It impaired the acquisition and did not influence the consolidation and retrieval in the passive avoidance situation. D-AP7 did not produce any significant effects in the elevated plus maze in rats which did not undergo hypoxia. Short-term hypoxia (about 3 min) decreased the crossings, rearings and bar approaches in the open field test and impaired acquisition, consolidation and retrieval processes. It did not evoke any changes in elevated plus maze. In hypoxia-treated groups of rats, D-AP7 enhanced locomotor and exploratory activity and it did not change the acquisition and retrieval processes. D-AP7 administrated before hypoxia impaired the consolidation in the passive avoidance test vs. D-AP7-treated group of rats. D-AP7 shortened the time spent in open arms and decreased the number of entries in open arms in hypoxia-treated groups of rats. In conclusion, in hypoxia-treated groups of rats, D-AP7 enhanced motility, exhibited anxiogenic-like effect and impaired consolidation in passive avoidance.     

ACPD-mediated slow-onset potentiation is associated with cell death in the rat CA1 region in vivo

Slow-onset potentiation of synaptic transmission in the hippocampus in vitro and in vivo is induced by application of the metabotropic glutamate receptor (mGluR) agonist, 1S,3R-amino cyclopentane 2,3-dicarboxylic acid (ACPD). This study investigated the cellular response in the CA1 region of freely moving rats to ACPD application. Drugs were applied via the lateral cerebral ventricle, and measurements were obtained from the CA1 region via permanently implanted electrodes. ACPD (20 nmol/5 microl) produced a dose-dependent slow-onset potentiation in the CA1 region which lasted over 4 h. Histological evaluation at either 4 h or 7 days following ACPD-injection indicated that slow-onset potentiation was associated with gradual but marked cell death in the CA1 region. Whereas 20 nmol ACPD produced significant CAI neurotoxicity, concentrations which did not induce potentiation had little or no neurotoxic effect. Both the general mGluR antagonist R,S-alpha-methyl-carboxyphenylglycine (1 micromol/5 microl), and the group 1 mGluR antagonist (S)-4-carboxyphenylglycine (4CPG, 100 nmol/5 microl) significantly inhibited ACPD-induced neuropathology. In addition, 4CPG inhibited the expression of ACPD-mediated slow-onset potentiation. These results confirm previous findings that in the CA1 region in vivo, slow-onset potentiation is mediated group 1 mGluRs, and indicate that slow-onset potentiation may involve pathological processes.     

NMDA receptor antagonists protect against seizures and wet-dog shakes induced by 4-aminopyridine.

The effect of N-methyl-D-aspartate (NMDA) and non-NMDA receptor antagonists on the generalized tonic-clonic convulsions and wet-dog shakes induced by the intraperitoneal (i.p.) or the intrahippocampal (i.h., stereotaxic microinjection into the CA1 region) administration of 4-aminopyridine (4-AP) was studied in rats. Pretreatment with NMDA competitive and non-competitive antagonists resulted in potent protection against the motor effects of both the i.p. and the i.h. administration of 4-AP. MK-801 (0.25 mg/kg i.p.) and 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP, 0.8 nmol intracerebroventricular, i.c.v.) showed the most powerful anticonvulsive effect, since they prevented the occurrence of generalized tonic convulsions and the death of the animals in convulsions after i.p. 4-AP. The i.c.v. injection (10 nmol) of the NMDA competitive antagonists 2-amino-5-phosphonopentanoate (AP-5) and 2-amino-5-phosphonoheptanoate (AP-7) also showed a clear though less potent protective effect. Similarly, the frequency of wet-dog shakes induced by i.h. 4-AP was markedly decreased by pretreating the animals with i.p. MK-801 or with i.c.v. CPP or AP-7. However, the co-injection of CPP with 4-AP failed to protect against the occurrence of wet-dog shakes. The i.c.v. pretreatment with the unselective antagonist, kynurenate (up to 68 nmol) or with the non-NMDA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (0.5 nmol), did not significantly modify the effects of 4-AP when administered either i.p. or i.h. We conclude that NMDA receptors are involved in the mechanism of the convulsive activity induced by 4-AP, probably because this drug induces the release of glutamate.     

AIDA influences behavior in rats pretreated with baclofen.

The influence of the blockade of group I metabotropic glutamate receptors (I mGluRs) by AIDA on some behavioral effects of rats pretreated with baclofen, an agonist of GABA-B receptor, was investigated using behavioral tests: the open field, the passive avoidance response and the elevated "plus" maze. Baclofen, applied intraperitoneally (ip) at a dose of 0.25 mg/kg, increased the number of crossed fields and bar approaches in rats in the open field test, and prolonged the time spent in the closed arms, shortened the time spent in the open arms and decreased the number of entries to the open arms in the elevated "plus" maze, but did not affect retrieval in the passive avoidance response. AIDA administered intracerebroventricularly (icv) alone at a dose of 100 nmol reduced crossings and rearings in the open field test, however, it had no effect on retrieval in the passive avoidance situation, nor did it show any influence in the elevated "plus" maze. AIDA given 15 min after baclofen significantly decreased mobility of rats (in the case of crossings to the level observed when AIDA was given alone), i.e. AIDA changed the effects of baclofen in the open field test. We also noted significant impairment of retrieval in rats pretreated with baclofen, which later received AIDA. AIDA significantly reduced the effect of baclofen on this memory process. In the elevated "plus" maze test, AIDA did not influence the behavior of rats pretreated with baclofen in comparison with the group treated with baclofen alone.     

Pharmacology of postsynaptic metabotropic glutamate receptors in rat hippocampal CA1 pyramidal neurones.

1. Activation of metabotropic glutamate receptors (mGluRs) in hippocampal CA1 pyramidal neurones leads to a depolarization, an increase in input resistance and a reduction in spike frequency adaptation (or accommodation). At least eight subtypes of mGluR have been identified which have been divided into three groups based on their biochemical, structural and pharmacological properties. It is unclear to which group the mGluRs which mediate these excitatory effects in hippocampal CA1 pyramidal neurones belong. We have attempted to address this question by using intracellular recording to test the effects of a range of mGluR agonists and antagonists, that exhibit different profiles of subtype specificity, on the excitability of CA1 pyramidal neurones in rat hippocampal slices. 2. (2S, 1'S,2'S)-2-(2'-carboxycyclopropyl)glycine (L-CCG1) caused a reduction in spike frequency adaptation and a depolarization (1-10 mV) associated with an increase in input resistance (10-30%) at concentrations (> or = 50 microM) that have been shown to activate mGluRs in groups I, II and III. Similar effects were observed with concentrations (50-100 microM) of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid ((1S,3R)-ACPD) and (1S,3S)-ACPD that exhibit little or no activity at group III mGluRs but which activate groups I and II mGluRs. 3. Inhibition of the release of endogenous neurotransmitters through activation of GABAB receptors, by use of 200 microM (+/-)-baclofen, did not alter the effects of (1S,3R)-ACPD (50-100 microM), (1S,3S)-ACPD (100 microM) or L-CCG1 (100 microM). This suggests that mGluR agonists directly activate CA1 pyramidal neurones. 4. Like these broad spectrum mGluR agonists, the racemic mixture ((SR)-) or resolved (S)-isomer of the selective group I mGluR agonist 3,5-dihydroxyphenylglycine ((SR)-DHPG (50-100 microM) or (S)-DHPG (20-50 microM)) caused a reduction in spike frequency adaptation concomitant with postsynaptic depolarization and an increase in input resistance. In contrast, 2S,1'R,2'R,3'R-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV; 100 microM) and (S)-2-amino-4-phosphonobutanoic acid (L-AP4; 100-500 microM), which selectively activate group II mGluRs and group III mGluRs, respectively, had no effect on the passive membrane properties or spike frequency adaptation of CA1 pyramidal neurones. 5. The mGluR antagonists (+)-alpha-methyl-4-carboxyphenylglycine ((+)-MCPG; 1000 microM) and (S)-4-carboxyphenylglycine ((S)-4CPG; 1000 microM), which block groups I and II mGluRs and group I mGluRs, respectively, had no effect on membrane potential, input resistance or spike frequency adaptation per se.(ABSTRACT TRUNCATED AT 400 WORDS)     

Interaction between metabotropic and NMDA glutamate receptors in the periaqueductal grey pain modulatory system

The formalin test was used to investigate the interactive role of periaqueductal grey (PAG) N-methyl-D-aspartate (NMDA) and metabotropic glutamate (mGlu) receptors in the modulation of persistent noxious stimulation in mice. Intra-PAG microinjections of 1 or 3 nmol NMDA, a selective agonist at NMDA-subtype receptors, decreased the nociceptive response (-94+/-5% with 3 nmol) during the latter phase of the test. This effect was antagonized by MK-801, a selective antagonist at NMDA receptors. No change in the early nociceptive phase was observed after NMDA injection. Pretreatment either with 2-methyl-6-phenylethynylpyridine (MPEP, 25 nmol/mouse), a selective antagonist at mGlu5 receptors, or with (2S)-alpha-ethylglutamic acid [(2S)-alpha-EGlu, 30 nmol/mouse], a selective antagonist at group-II mGluRs, prevented the NMDA-induced antinociceptive effect during the late hyperalgesic phase. Pretreatment with (R,S)-alpha-methylserine-O-phosphate [(R,S)-alpha-MSOP, 70 nmol/mouse], a selective antagonist at group-III mGlu receptors, had no effect on the NMDA-induced antinociception. None of the antagonists changed the formalin-induced nociceptive behaviour per se with the dosages used in combination with NMDA. MPEP at 50 nmol/mouse, however, potentiated the early nociceptive phase whilst 100 nmol/mouse attenuated the late phase. Similarly, at the higher dose of 140 nmol/mouse, (R,S)-alpha-MSOP decreased the late hyperalgesic phase. These results provide additional evidence that NMDA and mGlu receptors participate in modulating the hyperalgesia induced by peripheral noxious stimulation. In particular, mGlu receptors may modulate the NMDA receptors in the PAG since their physiological stimulation seems to be required for the NMDA-induced effect. This suggests that, together with ionotropic glutamate receptors, mGlu receptors also play a role in modulating a type of spinal cord neuroplasticity (i.e. wind-up) that has been proposed to mediate hyperalgesia.     

Modulation of [3H]MK-801 binding to NMDA receptors in vivo and in vitro

[3H]MK-801 binding in vivo was used to determine the occupancy of NMDA receptor ligands shown to allosterically modulate binding in vitro. ED(50) values (mg/kg) were obtained for the channel blockers (+)-5-methyl-10,11-dihydro-5,4-dibenzo[a,d]cyclohepten-5,10-imine maleate ((+)-MK-801, 0.2), 1-(1-phenylcyclohexyl)piperidine (phencyclidine, PCP, 1.7) and ketamine (4.4). Antagonists at the glutamate (DL-(2-carboxypiperazine-4-yl)propyl-1-phosphonate (DL-CPP, 5.7)) and glycine site (7-Chloro-4-hydroxy-3-(3-phenoxy)-phenyl-2(H)quinolinone (L-701,324, 14.1), 3R(+)cis-4-methyl-pyrrollid-2-one (L-687,414, 15.1)) inhibited [3H]MK-801 binding in vivo to varying maximum levels (69%, 103% and 45%, respectively). NR2B subunit-selective compounds acting at the ifenprodil site inhibited [3H]MK-801 in vivo by a maximum of 52-72% and gave ED(50) values (mg/kg) of: (+/-)-(1S*, 2S*)-1-(4-hydroxyphenyl)-2-(4-hydroxy-4-phenylpiperidino)-1-propanol ((+/-)CP-101,606), 1.9; (+/-)-(3R, 4S)-3-[4-(4-fluorophenyl)-4-hydroxypiperidin-1-yl]chroman-4,7-diol ((+/-)CP-283,097), 1.8; (+/-)-(R*, S*)-alpha-(4-hydroxyphenyl)-beta-methyl-4-(phenylmethyl)-1-piperidine propanol ((+/-)Ro 25-6981), 1.0; ifenprodil, 6.0. The glycine site agonist D-serine stimulated binding to 151% of control with an ED(50) of 1.7 mg/kg. Results show that [3H]MK-801 binding in vivo may be used to measure receptor occupancy of ligands acting not only within the ion channel but also at modulatory sites on the NMDA receptor complex.     

Binding of [3H]MK-801 in subcellular fractions of Schistosoma mansoni: evidence for interaction with nicotinic receptors

Several studies have suggested that l-glutamate is a putative neurotransmitter in Schistosoma mansoni. Recently, we detected the presence of low-affinity binding sites for [(3)H]kainic acid in the heterogeneous (P(1)) subcellular fraction of S. mansoni. In an attempt to characterize N-methyl-d-aspartate (NMDA) receptors in this worm, we performed binding assays with [(3)H]MK-801, a NMDA non-competitive antagonist, in the P(1) fraction of adult S. mansoni. In competition experiments, MK-801 (IC(50) approximately 200 microM) and ketamine (IC(50) approximately 500 microM) exhibited a low affinity for the sites labeled with [(3)H]MK-801. Along with the lack of modulation of this binding by glutamatergic agonists and antagonists and the absence of stereoselectivity for MK-801 isomers, these results suggest that [(3)H]MK-801 could label a site different from the classical NMDA receptor in S. mansoni. Based on the evidences that MK-801 interacts with mammalian muscle and central nervous system nicotinic receptors as a low-affinity noncompetitive antagonist, we have investigated the effects of MK-801 on the nicotine-induced flaccid paralysis of the worm, in vivo. The motility of S. mansoni was quantified by image analysis through a measure of displacement of the worm's extremities. In the presence of (-)-nicotine (10-100 microM), we observed an immediate paralysis of the worms, that was inhibited by 1mM MK-801. Besides nicotine, choline (10-50mM) was also able to inhibit the worm's motility. As a conclusion, we suggest that [(3)H]MK-801 binds to nicotinic receptors, and not NMDA receptors, in subcellular fractions of S. mansoni.     

Non-competitive N-methyl-D-aspartate antagonists protect against sound-induced seizures in DBA/2 mice

Non-competitive antagonists of the N-methyl-D-aspartate (NMDA) receptor have been evaluated as anticonvulsants against sound-induced seizures in DBA/2 mice. The ED50 values for protection against sound-induced clonic seizures 15 min following the intracerebroventricular (i.c.v.) and intraperitoneal (i.p.) administration are: MK-801, ED50 = 0.5 nmol (i.c.v.); 0.14 mumol/kg (i.p.); phencyclidine, ED50 = 14 nmol (i.c.v.); 1.9 mumol/kg (i.p.); dextrorphan, ED50 = 35 nmol (i.c.v.); 18.5 mumol/kg (i.p.); tiletamine, ED50 = 40 nmol (i.c.v.); 5.6 mumol/kg (i.p.); SKF-10047, ED50 = 50 nmol (i.c.v.); 23.5 mumol/kg (i.p.); dextromethorphan, ED50 = 70 nmol (i.c.v.); 28.0 mumol/kg (i.p.); ketamine, ED50 = 110 nmol (i.c.v.); 15.5 mumol/kg (i.p.). The anticonvulsant effects of ketamine and tiletamine are of short duration (10-30 min), whereas the anticonvulsant effects of MK-801 and dextromethorphan last for 45 min or longer. The effects of phencyclidine, SKF-10047 and dextrorphan are of intermediate duration. Mild to moderate behavioural excitation is associated with the anticonvulsant activity of all the non-competitive NMDA antagonists. For MK-801, phencyclidine, dextrorphan, SKF-10047 and ketamine there is a close correlation between their relative anticonvulsant potencies and their potencies for displacing [3H]MK-801. The anticonvulsant effect is likely to be primarily mediated via NMDA antagonism at the PCP/MK-801 site.     

NMDA receptor blockade in the periaqueductal grey prevents stress-induced analgesia in attacked mice

Microinjections of N-methyl-D-aspartate (NMDA, 0.1 and 1.0 nmol) into the periaqueductal grey (PAG) of the mouse resulted in potential antinociception. In a social conflict situation, attacked mice exhibited a marked analgesia that was prevented by prior injection of the competitive NMDA antagonist, AP-7 (2.0 nmol) or naloxone (6.0 nmol) into the PAG and also by i.p. injection of the non-competitive NMDA antagonist, MK-801 (33 nmol). These results demonstrate that NMDA receptors are involved in endogenous analgesic mechanisms activated by stress.     

Spermidine enhancement of [3H]MK-801 binding to the NMDA receptor complex in human cortical membranes

The effects of spermidine on the binding of [3H]MK-801 to the N-methyl-D-aspartate (NMDA) receptor complex was studied in human cerebral cortical membranes. [3H]MK-801 binding was increased from 56 +/- 5 fmol/mg protein (mean +/- S.E.M., n = 7) to 319 +/- 71 fmol/mg protein in the presence of 200 microM spermidine. The ED50 for spermidine stimulation of [3H]MK-801 binding was 89 +/- 22 microM (mean +/- S.E.M., n = 6). In the presence of glutamate (1 microM) plus glycine (1 microM) the ED50 was reduced to 5.5 +/- 0.7 microM. The increase in binding in the presence of spermidine was characterised by an increase in the rate of association of [3H]MK-801. In the presence of spermidine. [3H]MK-801 was inhibited by AP5. 7-chlorokynurenic acid and ifenprodil with IC50 values of 0.5 +/- 0.3 24 +/- 19 and 91 +/- 28 microM, respectively. None of these antagonists was a competitive inhibitor of the spermidine stimulation of [3H]MK-801 binding. Thus spermidine modulates the NMDA receptor complex in human brain, providing further evidence that the complex is similar in rat and human cortex.     

Structure-activity relationships for a series of phenylglycine derivatives acting at metabotropic glutamate receptors (mGluRs).

1. The actions of a series of twelve phenylglycine derivatives at metabotropic glutamate receptors (mGluRs) linked to both phosphoinositide hydrolysis (PI) and cyclic AMP were investigated. 2. PI hydrolysis was determined by the accumulation of [3H]-inositol-monophosphate ([3H]-IP1) in neonatal ral cortical slices prelabelled with [3H]-myo-inositol. The non-selective mGluR agonist (1S,3R)-1-aminocyclopentane-1, 3-dicarboxylic acid ((1S,3R)-ACPD) produced a concentration-dependent increase in [3H]-IP1 (EC50 approximately 20 microM). This agonist was subsequently used to investigate potential antagonist activity of the phenylglycine derivatives. Of the compounds tested (+)-alpha-methyl-4-carboxyphenylglycine (M4CPG) and (RS)-alpha-ethyl-4-carboxyphenylglycine (E4CPG) were the most active with KP values of 0.184 +/- 0.04 mM and 0.367 +/- 0.2 mM respectively. 3. Activity at adenylyl cylase-coupled mGluRs was investigated by determining the accumulation of [3H]-cyclic AMP in adult rat cortical slices. [3H]-cyclic AMP accumulation, elicited by 30 microM forskolin, was inhibited by (2S,3S,4S)-alpha-(carboxycyclopropyl)glycine (L-CCG-1) and L-2-amino-4-phosphonobutanoate (L-AP4) with respective EC50 values of 0.3 microM and 10 microM. Neither agonist was able to inhibit completely forskolin stimulated cyclic AMP accumulation; this is evidence that not all adenylyl cyclase is susceptible to modulation by mGluRs. Phenylglycine derivatives were examined for their ability to antagonize the inhibition of [3H]-cyclic AMP accumulation by L-CCG-1 or L-AP4 at their EC50 concentrations. 4. A rank order of potency of the phenylglycine derivatives as antagonists of L-AP4 and L-CCG-1 was obtained. The most effective compound. (RS)-alpha-methyl-3-carboxymethylphenylglycine (M3CMPG) had IC50 values in the order of 1 microM against L-AP4 and 0.4 microM against L-CCG-1. 5. The results from this study indicate that phenylglycine-derived compounds can discriminate between groups of metabotropic glutamate receptors and may also display some selective activity between subtypes within groups. Future work based on these findings may lead to the development of more selective and potent compounds as important pharmacological tools.     

Effects of intracerebroventricular NMDA and non-NMDA receptor agonists or antagonists on general anesthesia of propofol in mice

The effects of intracerebroventricular (icv) agonists and antagonists of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptors on the general anesthesia of propofol were studied. A total of 144 Kunming mice, male and female with body mass of (22±3) g, were used. Part One of the Experiment: a total of 104 Kunming mice, male and female, were randomly divided into 13 groups. Intracerebroventricular artificial cerebral fluid (aCSF) or different doses of NMDA, AMPA, MK-801 or NBQX was injected immediately after intravenously administered propofol 25 mg/kg and the recovery time following the loss of righting reflex (LORR) was recorded. Part Two of the Experiment: a total of 40 Kunming female mice were divided randomly into 5 groups and injected with icv aCSF or NMDA, AMPA, MK-801 or NBQX after intraperitoneally administered propofol 50 mg/kg. The pain threshold of the mice was then investigated by hot-plate test (HPPT). NMDA (0.05 or 0.075μg, icv) or AMPA (0.05 μg, icv) exhibited no effects on the LORR, but NMDA (0.1 μg, icv) or AMPA (0.075 or 0.1 μg, icv) prolonged the LORR significantly compared with the aCSF group (P<0.05, P<0.01). The LORR of the 2 μg MK-801 group had no changes, while those of the 4 or 8 μg MK-801 groups were prolonged significantly. The LORR of the 0.5, 2 or 4 μg NBQX groups were all prolonged significantly. NMDA 0.05 μg or AMPA 0.05 μg decreased the pain threshold slightly but did not differ in effect compared with the aCSF group; 2 μg MK-801 or 0.5 μg NBQX both increased the pain threshold significantly. Our results indicate that propofol produces general anesthesia partly through an interaction with brain NMDA and AMPA receptors in mice.     

Intrathecal injection of glutamate receptor antagonists/agonist selectively attenuated rat pain-related behaviors induced by the venom of scorpion Buthus martensi Karsch

The present study investigated the involvement of spinal glutamate receptors in the induction and maintenance of the pain-related behaviors induced by the venom of scorpion Buthus martensi Karsch (BmK). (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5-10-imine hydrogen maleate (MK-801; 40 nmol; a non-competitive NMDA receptor antagonist), 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 40 nmol; a non-NMDA receptor antagonist), dl-amino-3-phosphonopropionic acid (dl-AP3; 100 nmol; a group I metabotropic glutamate receptor antagonist) and 4-aminopyrrolidine-2,4-dicarboxylate (APDC; 100 nmol; a group II metabotropic glutamate receptor agonist) were employed. On intrathecal injection of glutamate receptor antagonists/agonist before BmK venom administration by 10 min, BmK venom-induced spontaneous nociceptive responses could be suppressed by all tested agents. Primary thermal hyperalgesia could be inhibited by MK-801 and dl-AP3, while bilateral mechanical hyperalgesia could be inhibited by CNQX and dl-AP3 and contralateral mechanical hyperalgesia could be inhibited by APDC. On intrathecal injection of glutamate receptor antagonists/agonist after BmK venom injection by 4.5 h, primary thermal hyperalgesia could be partially reversed by all tested agents, while bilateral mechanical hyperalgesia could only be inhibited by APDC. The results suggest that the role of spinal glutamate receptors may be different on the various manifestations of BmK venom-induced pain-related behaviors.