Performance of three cognitive screening tools in a sample of older New Zealanders.

BACKGROUND: With the ubiquitous Mini-Mental State Exam now under copyright, attention is turning to alternative cognitive screening tests. The aim of the present study was to investigate three common cognitive screening tools: the Montreal Cognitive Assessment (MoCA), the Rowland Universal Dementia Assessment Scale (RUDAS), and the recently revised Addenbrooke's Cognitive Assessment Version III (ACE-III). METHODS: The ACE-III, MoCA and RUDAS were administered in random order to a sample of 37 participants with diagnosed mild dementia and 47 comparison participants without dementia. The diagnostic accuracy of the three tests was assessed. RESULTS: All the tests showed good overall accuracy as assessed by area under the ROC Curve, 0.89 (95% CI = 0.80-0.95) for the ACE-III, 0.84 (0.75-0.91) for the MoCA, and 0.86 (0.77-0.93) for RUDAS. The three tests were strongly correlated: r(84) = 0.85 (0.78-0.90) between the ACE-III and MoCA, 0.70 (0.57-0.80) between the ACE-III and RUDAS; and 0.65 (0.50-0.76) between the MoCA and RUDAS. The data derived optimal cut-off points for were lower than the published recommendations for the ACE-III (optimal cut-point ≤76, sensitivity = 81.1%, specificity = 85.1%) and the MoCA (≤20, sensitivity = 78.4%, specificity = 83.0%), but similar for the RUDAS (≤22, sensitivity = 78.4%, specificity = 85.1%). CONCLUSIONS: All three tools discriminated well overall between cases of mild dementia and controls. To inform interpretation of these tests in clinical settings, it would be useful for future research to address more inclusive and potentially age-stratified local norms.

Anticonvulsant action of excitatory amino acid antagonists.

Compounds that antagonize neuronal excitation induced by dicarboxylic amino acids were tested in two animal models of epilepsy, namely sound-induced seizures in DBA/2 mice and threshold pentylenetetrazol seizures in Swiss mice. Sound-induced seizures could be prevented by intracerebroventricular injection of compounds that block excitation due to N-methyl-D-aspartic acid. The most potent such compound, 2-amino-7-phosphonoheptanoic acid, was anticonvulsant in both test systems when given either intraperitoneally or intracerebroventricularly. Specific antagonists of excitation that is caused by amino acids provide a new class of anticonvulsant agents.

Glutamate release inhibiting properties of the novel mGlu(5) receptor antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP): complementary in vitro and in vivo evidence.

We have previously demonstrated that neuronal release of the excitatory amino acid glutamate is facilitated by the selective activation of presynaptic Group I metabotropic autoreceptors. Here we report the release inhibiting actions of the novel mGlu(5) receptor-selective antagonist, 2-methyl-6-(phenylethynyl)-pyridine (MPEP), both in vitro and in vivo. These data provide compelling evidence for the presence of functional positive modulatory mGlu(5) subtype autoreceptors in the mammalian central nervous system.

Anticonvulsant actions of DS 103-282. Pharmacological studies in rodents and the baboon, Papio papio.

The anticonvulsant actions of DS 103-282 [5-chloro-4-(2-imidazolin-2-yl-amino)-2,1,3- benzothiadazole , tizanidine], have been evaluated after intraperitoneal administration in DBA/2 mice (seizures induced by sound), in Swiss S mice (seizures induced by N-methyl-D,L-aspartate; NMDLA ) and following intravenous or oral administration in Papio papio (seizure responses to intermittent photic stimulation). Protection against sound-induced seizures occurred after intraperitoneal administration of DS 103-282 (0.66-3.33 mg/kg). The ED50 doses for suppression of the tonic, clonic and wild-running phases of sound-induced seizures were 0.53, 0.79 and 1.3 mg/kg respectively. This protective effect of DS 103-282 (1.5 mg/kg, i.p.) was maximal after 30 min and was maintained for 60-120 min. Seizures induced by NMDLA were not suppressed by DS 103-282 (3.3-10 mg/kg, i.p.). In the baboons, a transient protection against photomyoclonic responses was observed 1 hr after intravenous administration of DS 103-282 (2-4 mg/kg). A similar profile of action was seen after oral administration of larger doses of DS 103-282 (16-32 mg/kg). Unwanted effects of DS 103-282 included transient piloerection, slight disturbance of gait and a fall in rectal temperature in mice, and muscular hypotonia and signs of sedation in baboons. These studies demonstrate an anticonvulsant action of DS 103-282, in both rodent and primate models of epilepsy, but do not support a postsynaptic blockade in excitatory neurotransmission as the mechanism of this action.

Anticonvulsant and proconvulsant properties of a series of structural isomers of piperidine dicarboxylic acid.

Anticonvulsant and convulsant effects of various piperidine dicarboxylic acids have been evaluated following their intracerebroventricular (i.c.v.) or intraperitoneal (i.p.) injection in DBA/2 mice, a strain of mice genetically susceptible to sound-induced seizures. Protection against sound-induced seizures occurred after intraventricular administration of (+/-)cis-2,3-piperidine dicarboxylic acid (0.017-0.045 mumol), (+/-)trans-2,3-piperidine dicarboxylic acid (0.018-0.33 mumol) and (+/-)cis-2,4-piperidine dicarboxylic acid (0.57-1.68 mumol). Protection against sound-induced seizures occurred after intraperitoneal injection of (+/-)cis-2,3-piperidine dicarboxylic acid (0.52-1.8 mmol/kg). Myoclonus or convulsions occurred at various times after the intraventricular injection of cis-2,3-piperidine dicarboxylic acid, trans-2,3-, cis-2,4-, cis-2,5- and cis-2,6-, piperidine dicarboxylic acids, and after the intraperitoneal injection of trans-2,3-piperidine dicarboxylic acid. The latter effect was blocked by pretreatment with 2-amino-7-phosphonoheptanoic acid (0.33 mmol/kg, i.p.) a potent and specific antagonist of excitation induced by N-methyl-D-aspartate. The anticonvulsant action of cis-2,3-piperidine dicarboxylic acid and the convulsant action of trans-2,3-piperidine dicarboxylic acid were associated with predominant antagonist and agonist actions respectively, at receptors preferring N-methyl-D-aspartate.

Metabotropic glutamate autoreceptors of the mGlu(5) subtype positively modulate neuronal glutamate release in the rat forebrain in vitro.

In the present study we have examined the role of presynaptic group I metabotropic glutamate (mGlu) receptors in the control of neuronal glutamate release using rat forebrain slices pre-loaded with [(3)H]D-aspartate. We have also addressed the question of which group I mGlu receptor subtype, mGlu(1) or mGlu(5), mediates the facilitatory response observed by the use of a range of established and some more novel agonists and antagonists showing selectivity for these receptors. The electrically-stimulated release of pre-loaded [(3)H]D-aspartate from rat forebrain slices was markedly potentiated by the potent group I mGlu receptor agonist, L-quisqualic acid (L-QUIS), in a concentration-dependent manner (EC(50) 17.31 microM). This response was inhibited by the mGlu receptor antagonists (S)-MCPG (100 microM) and (RS)-MTPG (100 microM) but not by the AMPA-type ionotropic glutamate receptor antagonist, NBQX (100 microM). The selective group I mGlu receptor agonist (S)-3, 5-dihydroxyphenylglycine ((S)-DHPG) also enhanced electrically-stimulated efflux of label, although responses diminished with high (10-100 microM) concentrations of the agonist. Maximum responses were fully restored when (S)-DHPG (10 microM) was applied in the presence of the proposed mGlu(5) receptor desensitization inhibitor, cyclothiazide (10 microM). The positive modulatory response to (S)-DHPG (1 microM) was powerfully inhibited by (S)-MCPG (IC(50) 0.08 microM) but was resistant to the mGlu(1) receptor antagonists, (RS)-AIDA (1-500 microM), CPCCOEt (0.1-100 microM) and (+)-2-methyl-4-carboxyphenylglycine (LY367385) (0.1-10 microM). The recently developed, selective mGlu(5) receptor agonist (RS)-2-chloro-5-hydroxyphenylglycine ((RS)-CHPG) enhanced electrically-stimulated [(3)H]D-aspartate efflux from rat forebrain slices with a similar concentration-response profile to that of (S)-DHPG. Responses to this receptor subtype-selective agonist were also blocked by (S)-MCPG (IC(50) 1.13 microM) but were unaffected by (RS)-AIDA (500 microM), CPCCOEt (100 microM) or LY367385 (10 microM). These results indicate that the positive modulation of neuronal glutamate release seen in the rat forebrain in the presence of group I mGlu receptor agonists is mediated by presynaptically located mGlu(5) glutamate autoreceptors. The pharmacological profile of these receptors appears to be distinct from that of postsynaptic mGlu receptors. Novel antagonists acting at these presynaptic receptors may provide new drugs for the experimental therapy of a range of acute or chronic neurodegenerative disorders.

2-Amino-6-trifluoromethoxy benzothiazole, a possible antagonist of excitatory amino acid neurotransmission--I. Anticonvulsant properties.

2-Amino-6-trifluoromethoxy benzothiazole (PK 26124) prevented convulsions induced in rodents by maximal electroshock, inhibitors of the synthesis of gamma-aminobutyric acid (GABA) and ouabain, but was inactive against seizures provoked by GABA antagonists, unlike diazepam, chlordiazepoxide, phenobarbital and valproic acid. 2-Amino-6-trifluoromethoxy benzothiazole prevented seizures induced by sound stimuli in DBA/2 mice (ED50 = 0.66; 2.1 and 4.1 mg/kg, i.p. according to the seizure component), postural seizures in El mice (ED50 = 7.5 mg, i.p.) and seizures induced by photic stimulation in the baboon, Papio papio, at 4 and 8 mg/kg (i.v.). This spectrum of anticonvulsant activity closely resembles that reported previously for dicarboxylic amino acid antagonists. Indeed, PK 26124 prevented seizures induced by L-glutamate (ED50 = 8.5 mg/kg, i.p.) or by kainate (ED50 = 9.25 mg/kg, i.p.) and tremors induced by harmaline (ED50 = 2.5 mg/kg, i.p.) In these tests diazepam was inactive (L-glutamate) or as potent as PK 26124 (kainate, harmaline), whereas it was 10-20 times more potent than PK 26124 against seizures induced by inhibitors of the synthesis of GABA. Together, these data suggest that PK 26124 possesses antagonistic properties of excitatory dicarboxylic amino acids, which may contribute to its anticonvulsant action.

Actions of Group I and Group II metabotropic glutamate receptor ligands on 5-hydroxytryptamine release in the rat cerebral cortex in vivo: differential roles in the regulation of central serotonergic neurotransmission.

We have previously shown that the release of central neurotransmitters can be modulated by the activation of Group I and Group II subtypes of G-protein-linked metabotropic glutamate (mGlu) receptors. To date, however, very little is known about the regulation of serotonergic neurotransmission by these receptor subtypes. In the present study, we have utilized in vivo intracerebral microdialysis to elucidate the roles of Group I and Group II mGlu receptors in the regulation of neuronal 5-hydroxytryptamine (5-HT) release in the frontal cortex of conscious, freely moving rats. Dialysate 5-HT was of neuronal origin with basal release showing strong calcium dependency and tetrodotoxin sensitivity and marked elevation following K(+)-induced depolarization. The broad-spectrum mGlu receptor agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD; 1-3 mM] did not significantly modify basal cerebrocortical 5-HT release. Similarly, the Group I mGlu receptor-specific agonist (RS)-3,5-dihydroxyphenylglycine [(RS)-3,5-DHPG; 1-3 mM] showed no marked effect on cortical dialysate 5-HT levels. To eliminate the possibility that these findings were the result of receptor desensitization, the effects of lower concentrations of (RS)-DHPG (100-300 microM) and shorter ligand exposure time (15 min) were also evaluated. Dialysate 5-HT levels remained unmodified by these manipulations. In comparison, the Group II mGlu receptor agonist, (2S,1'S,2'S)-2-(carboxycyclopropyl)glycine (L-CCG-1; 500 microM), evoked a marked facilitation of release (approximately 150% of basal) which was fully reversed by the Group I/II antagonist, (S)-alpha-methyl-4-carboxyphenylglycine [(S)-MCPG; 3 mM]. The modulatory action of L-CCG-1 showed a bell-shaped concentration-response relationship. (S)-MCPG (3 mM) and the potent and selective mGlu(5) receptor antagonist, 2-methyl-6-(phenylethynyl)pyridine (MPEP; 100 microM), when given alone, did not significantly modify 5-HT levels.The current data provide strong evidence to suggest that while the release of neuronal 5-HT in the rat frontal cortex is not subject to regulation by facilitatory Group I mGlu receptors, it may be positively modulated by activation of Group II mGlu receptors. Taken together with data from other studies, the present investigation lends emphasis to the notion that neuromodulation by mGlu receptors is a region-specific phenomenon and also proposes that the heterogeneous distribution of these receptors is neurone-specific in its complexity. The failure of (S)-MCPG alone to modify cortical 5-HT release suggests that Group II mGlu receptors do not tonically modulate serotonergic neurotransmission in the cerebral cortex but this does not preclude an important functional role for these receptors during pathological conditions when endogenous neurotransmitter levels become excessively elevated. The strategic development of new subtype-specific mGlu receptor ligands may provide novel therapeutic agents for the treatment of a range of neurological and psychiatric disorders.

Presynaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor-mediated stimulation of glutamate and GABA release in the rat striatum in vivo: a dual-label microdialysis study.

The existence of presynaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)-type glutamate autoreceptors on glutamate nerve terminals in vitro has recently been demonstrated using synaptosomal and brain slice preparations. In the present study we have used a modification of a rapid dual-label intracerebral microdialysis method, previously developed by Young and co-workers(80,81) for the study of presynaptic mechanisms of neurotransmitter release, to investigate whether presynaptic AMPA receptors also play a role in the control of striatal glutamate release in vivo. For comparative purposes, the action of locally applied AMPA on striatal GABA release in vivo was also monitored. Local application of AMPA (0.01-100 microM), by reverse dialysis, into the striatum resulted in concentration-dependent increases in the Ca(2+)-dependent efflux of both [3H]L-glutamate and [14C]GABA. Maximum responses reached 142.0+/-6.5% and 166.8+/-7.7% of basal efflux for [3H]L-glutamate and [14C]GABA, respectively. No marked behavioural changes were observed at any dose of the agonist. Unexpectedly, the AMPA-evoked responses were not potentiated by the AMPA receptor desensitization inhibitors cyclothiazide (10-100microM) or aniracetam (1mM). Consistent with this finding, AMPA-stimulated [3H]L-glutamate and [14C]GABA efflux were significantly attenuated by co-perfusion with the selective, competitive AMPA receptor antagonist 6-nitro-7-sulphamoylbenzo(F)quinoxaline-2,3-dione (100microM) but not 1-(aminophenyl)-4-methyl-7,8-methylendioxy-5H-2,3-benzodiazepine (100microM), a non-competitive AMPA receptor antagonist known to interact with the cyclothiazide site to control AMPA receptor function. The broad spectrum ionotropic glutamate receptor antagonist, kynurenic acid (100-1000microM) also markedly inhibited the AMPA-evoked responses in the striatum in vivo. None of the antagonists, when given alone, influenced basal efflux of [3H]L-glutamate suggesting a lack of tonic regulatory control of glutamate release via presynaptic AMPA-type autoreceptors in the rat striatum. These results demonstrate the presence of presynaptic AMPA receptors, of a novel cyclothiazide- and aniracetam-insensitive subtype, on presynaptic nerve terminals in the rat striatum in vivo, acting to enhance glutamate and GABA release. Our data support the concept of AMPA receptor heterogeneity in vivo, a finding which may facilitate the development of novel, more selective drugs for the treatment of a range of neurological disorders associated with abnormal cerebral glutamate release. The pharmacological profile of these novel presynaptic receptors is currently under investigation.

Endogenous sulphur-containing amino acids: potent agonists at presynaptic metabotropic glutamate autoreceptors in the rat central nervous system.

We have recently demonstrated that presynaptically located metabotropic glutamate (mGlu) autoreceptors regulate synaptic glutamate release both in vitro and in vivo. We now report a positive modulatory action of the sulphur-containing amino acids (SCAAs), L-cysteic acid (CA) and L-cysteine sulphinic acid (CSA), at presynaptic group I mGlu receptors, specifically of the mGlu5 subtype, acting to enhance synaptic glutamate release from the rat forebrain in vitro. Neuronal glutamate release was monitored using electrically-evoked efflux of preloaded [(3)H]-D-aspartate from rat forebrain hemisections. Both CA (3 - 100 muM) and CSA (1 - 100 microM), in addition to the selective group I mGlu receptor agonist, (S)-3,5-dihydroxyphenylglycine ((S)-DHPG), concentration-dependently enhanced electrically-stimulated efflux of [(3)H]-D-aspartate from the rat forebrain slices. Basal efflux of label remained unchanged. The inhibitory activity of the broad spectrum mGlu receptor antagonist, (+/-)-alpha-methyl-4-carboxyphenylglycine ((+/-)-MCPG; 200 microM), coupled with the inactivity of the selective mGlu1 receptor antagonists, (R,S)-1-aminoindan-1,5-dicarboxylic acid ((R,S)-AIDA; 100 - 500 microM) and the more potent (+)-2-methyl-4-carboxyphenylglycine (LY367385; 10 microM) against these responses, indicates an action of the SCAAs at the mGlu5 receptor subtype. This proposal is supported by the potent inhibition of these responses by the selective, non-competitive mGlu5 receptor antagonist, 2-methyl-6-(phenylethynyl)pyridine (MPEP; 10 microM). The observed enhancement of the responses to high concentrations of CA by the selective mGlu5 receptor desensitization inhibitor, cyclothiazide (CYZ; 10 microM), is also consistent with this concept. Administration of the agonists in the presence of bovine serum albumin (BSA; 5 - 15 mg ml(-1)) markedly attenuated the positive modulatory responses observed, strongly supporting a role for arachidonic acid in the expression of these mGlu5 receptor-mediated responses. The regulatory actions of SCAAs on synaptic glutamate release demonstrated in the present study may provide a physiological function for these putative neurotransmitter amino acids in the mammalian brain. These central actions of the SCAAs may have wide-ranging implications for a range of neurological and neuropsychiatric disease states and their treatment.

Comparison of human recombinant adenosine A2B receptor function assessed by Fluo-3-AM fluorometry and microphysiometry.

1. The aim of this study was to establish the utility of a fluorometric imaging plate reader (FLIPR) assay to assess human adenosine A(2B) receptor function by characterizing its receptor pharmacology and comparing this profile to that obtained using a microphysiometer. 2. FLIPR was used, in conjunction with a Ca(2+)-sensitive dye (Fluo-3-AM), to measure rapid rises in intracellular calcium in a Chinese Hamster Ovary (CHO-K1) cell line stably transfected with both the human A(2B) receptor and a promiscuous G(alpha16) protein. Microphysiometry was used to measure rapid changes in the rate of extracellular acidification in a Human Embryonic Kidney (HEK-293) cell line also stably transfected with human A(2B) receptor. 3. Activation of A(2B) receptors by various ligands caused a concentration-dependent increase in both the intracellular calcium concentration and the extracellular acidification rate in the cells tested, with a similar rank order of potency for agonists: NECA > N(6)-Benzyl NECA > adenosine > or = R-PIA > CPA > S-PIA > CHA > CGS 21680. No comparable effects were observed in the non-transfected control cell lines. 4. The rank order of potency of the agonists examined was the same in all studies, whereas absolute potency and efficacy varied. Thus, all compounds exhibited greater potency in FLIPR than the microphysiometer and the efficacies obtained with CHO-K1 + G(alpha16) + A(2B) cell line and FLIPR were greater than those obtained with HEK-293 + A(2B) cell line in the microphysiometer. 5. ZM-241385 was the most potent of a range of adenosine antagonists tested with a pA(2) of 8.0 in both the FLIPR and microphysiometer assays. 6. In conclusion, the profile of the responses to both A(2B) receptor agonists and antagonists in FLIPR were similar to those obtained by the microphysiometer, although both potency and efficacy values were higher in the FLIPR assay. With this caveat in mind, this study shows that FLIPR coupled with a cell line transfected with both the human A(2B) receptor and a promiscuous G(alpha16) protein provides a useful, high throughput method for the assessment of A(2B) receptor function.

Anticonvulsant activity of intracerebroventricularly administered valproate and valproate analogues. A dose-dependent correlation with changes in brain aspartate and GABA levels in DBA/2 mice.

Valproate and the branch-chain valproate analogues, 2-propyl-hexanoate and 2-ethyl-hexanoate, block sound-induced seizures in DBA/2 mice following their intracerebroventricular administration, the ED50 values for the suppression the clonic phase of the seizures being 6.0, 5.0, and 10.2 mumoles respectively. The straight-chain analogues, butyrate and pentanoate , have no anticonvulsant activity. Systemic administration of branched-chain valproate analogues has previously shown a progressive increase in the anticonvulsant activity of the analogues with an increase in the chain-length of the molecules. This relationship is abolished when the same analogues are injected directly into the ventricles. The icv administration of the 2 straight-chain, inactive analogues produces no effect on brain aspartate and GABA levels (with the exception of a butyrate-induced 34% rise in forebrain GABA level). There is a dose-dependent decrease in aspartate levels in cerebellum and forebrain (up to 45-55% reduction), and a dose-dependent increase in GABA levels in forebrain (up to 30-75% rise) 30 min after the icv administration of the 3 anticonvulsant branched-chain fatty acids. 2-Propyl-hexanoate does not affect the cerebellar GABA level significantly, while valproate and 2-ethyl-hexanoate administration produce 20-40% increases in cerebellar GABA levels.

The influence of strychnine-insensitive glycine receptor agonists and antagonists on generalized seizure thresholds.

We have examined the influence of strychnine-insensitive glycine (gly2) receptor agonists and antagonists on the expression of generalized seizure activity (generalized seizure threshold (GST), afterdischarge duration and motor seizure response) in fully amygdala kindled rats. Intra-amygdaloid administration of the selective gly2 receptor antagonist, 7-chlorokynurenic acid (7-C1KYN) (10-50 nmol) dose-dependently raised GSTs in a glycine-reversible manner. The same doses had no significant effect on other parameters of seizure expression. (+/-)-3-Amino-1-hydroxy-2-pyrrolidone (HA-966), a proposed low-efficacy partial agonist at the gly2 receptor17, showed similar but weaker anticonvulsant activity in this model. The active, R-(+)-enantiomer of HA-966 showed greater anticonvulsant efficacy than the racemic mixture, but was still clearly less active than 7-C1KYN. The gly2 receptor agonists glycine (10-50 nmol), D-serine (50 nmol) and D-alanine (50 nmol) failed to influence any of the parameters of the seizure response, suggesting that gly2 receptors in the basolateral amygdala are fully saturated in vivo. The behavioural impairment induced by high doses of 7-C1KYN did not appear to be associated with gly2 receptor blockade. These results support the concept that potent and selective gly2 receptor antagonists may provide a useful, novel class of anticonvulsant agents.

Kindling of full limbic seizures by repeated microinjections of excitatory amino acids into the rat amygdala.

Fully developed limbic seizures were kindled by repeated (every second day) microinjections of an L-glutamate plus L-aspartate (Glu/Asp) mixture (1:3 ratio; 1.5 mumol total dose). Glu alone (1.5 mumol) or Asp alone (1.5 mumol), into the rat amygdala. This excitatory amino acid (EAA)-induced kindling was durable, persisting for at least several months, and showed strong positive transfer to electrical kindling. Fully EAA kindled seizures were inhibited by focally applied NMDA-receptor antagonists. EAA kindling and electrical kindling are shown to have many similar properties. This strongly suggests that they may also have neurochemical mechanisms in common. These results further highlight the important role of EAAs in basic mechanisms involved in the generation and expression of epilepsy.

NMDA receptor blockade inhibits glutamate-induced kindling of the rat amygdala.

Full limbic seizures were kindled in rats by repeated, bi-daily microinjections of glutamate (1.5 mumol) into the basolateral amygdala. Co-administration of the N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-7-phosphonoheptanoic acid with the glutamate, prevented the development of both electroencephalographic and motor signs of the kindling response. These results indicate an important functional role for NMDA receptors in the development of excitatory amino acid-induced kindling.

Anticonvulsant dicarboxyphenylglycines differentially modulate excitatory amino acid release in the rat cerebral cortex.

The 3,4-dicarboxyphenylglycines (3,4-DCPGs) have recently been shown to be effective new anticonvulsant agents in a rodent model of epilepsy, with the racemic mixture showing significantly greater potency than either isomer alone. The (R)-isomer has been identified as a competitive AMPA-type ionotropic glutamate receptor antagonist, whilst (S)-3,4-DCPG is a highly potent and selective metabotropic glutamate receptor 8 (mGlu8 receptor) agonist. We now report the inhibitory activity of (R)- and (RS)-3,4-DCPG, but not (S)-3,4-DCPG, against both 35 mM and 50 mM KCl-evoked glutamate release in the rat cerebral cortex in vitro. In contrast to the anticonvulsant actions of the 3,4-DCPGs, no evidence was obtained for a synergistic inhibitory interaction between the separate isomers. We conclude that whilst inhibition of cortical excitatory amino acid release may contribute to the anticonvulsant actions of (RS)-3,4-DCPG, it does not represent the sole mechanism of action. Synergistic interactions between ligands acting at different subtypes of ionotropic and metabotropic glutamate receptors remains a promising new strategy for the treatment of currently drug-refractory seizure states.

Specific group II metabotropic glutamate receptor activation inhibits the development of kindled epilepsy in rats.

The effects of intracerebral administration of the group II metabotropic glutamate receptor agonist, 2R,4R-APDC, were tested on both the development of amygdaloid kindling and on fully developed stage 5 amygdala kindled seizures. The development of amygdaloid kindling was significantly retarded in 2R,4R-APDC (10 nmol in 0.5 microl) treated animals compared to control animals over a period of 8 days. At a low dose, 2R,4R-APDC (0.1 nmol) caused a 42.5+/-26.6% increase of the generalised seizure threshold in fully kindled animals. As higher doses were administered, however, the changes in generalised seizure threshold were less marked, and even a small decrease in the threshold was seen (-19.6+/-5.36% at 10 nmol). The agonist 2R,4R-APDC inhibited depolarization-induced release of [3H]d-aspartate from cortical synaptosomes with an IC50 value of 0. 29 microM. This effect was maximal at 1 microM, and decreased with dose thereafter. These findings suggest that the selective activation of the group II metabotropic glutamate receptors by agonists such as 2R,4R-APDC may be of therapeutic potential in the treatment of seizure disorders.

Anticonvulsant and glutamate release-inhibiting properties of the highly potent metabotropic glutamate receptor agonist (2S,2'R, 3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV).

The anticonvulsant effects of intracerebral administration of the highly potent group II metabotropic glutamate receptor agonist, DCG-IV, were tested in fully kindled rats following daily electrical stimulation of the basolateral amygdala. The agonist caused a dose-dependent increase in the generalized seizure threshold (GST) of these seizure susceptible animals within the dose range tested (0. 01-1.0 nmol). The estimated GST100 value (dose causing a 100% increase in GST) for this effect was 0.22 nmol. The anti-seizure activity of DCG-IV was fully inhibited in the presence of the group II metabotropic glutamate receptor antagonist (2S,1'S, 2'S)-2-methyl-2-(carboxycyclopropyl)glycine (MCCG; 40 nmol), while MCCG alone showed no significant inhibitory effect on seizure activity. DCG-IV also powerfully inhibited depolarization-induced release of [3H]D-aspartate from rat cerebrocortical synaptosomes, with an IC50 value of 0.39 microM. In this respect, DCG-IV was approximately 70-fold more potent than the clinically effective anticonvulsant drug lamotrigine (IC50=27.7 microM), a proposed neurotransmitter release inhibitor known to inhibit glutamate release, also tested in this assay. These findings demonstrate the high potency of DCG-IV as an anticonvulsant agent and confirm a key role for group II metabotropic glutamate receptors in the control of seizure activity via their modulatory action on neuronal glutamate release.

gamma-D-Glutamylaminomethylsulphonic acid (GAMS), a kainate and quisqualate antagonist, prevents sound-induced seizures in DBA/2 mice.

The effect of gamma-D-glutamylaminomethylsulphonic acid (GAMS) on sound-induced seizures has been assessed in DBA/2 mice. All phases of the seizure response are suppressed following intracerebroventricular injection (ED50 values: 0.077 mumol for tonic seizure, 0.132 mumol for wild running) or intraperitoneal injection (ED50: 1.21 mmol/kg for tonic seizure, 4.95 mmol/kg for wild running). The results are discussed in relation to the involvement of 'kainate' and 'quisqualate' receptors in the generation and propagation of seizures.

Anti-epileptogenic and anticonvulsant activity of L-2-amino-4-phosphonobutyrate, a presynaptic glutamate receptor agonist.

The protective effect of amygdaloid (focally administered) doses of the presynaptic metabotropic glutamate receptor agonist, L-2-amino-4-phosphonobutyrate (L-AP4) was tested on the development of electrical kindling and in fully kindled animals. L-AP4 inhibited epileptogenesis at 10 nmol in 0.5 microl buffer, by preventing the increase in both seizure score and afterdischarge duration. The effects were reversible after withdrawal of the drug, with all treated animals subsequently progressing to the fully kindled state at the same rate as control animals. The same concentration of the drug was also effective when injected into fully kindled animals. It significantly decreased the mean seizure score by 88% (P < 0.005) and increased the mean generalized seizure threshold (GST) by 85% (P < 0.005). The increase in GST was accompanied by a significant delay before the onset of generalized seizure and by a 37% reduction in generalized seizure duration. MPPG ((RS)-alpha-methyl-4-phosphonophenyl glycine) a selective antagonist of L-AP4 at glutamate pre-synaptic receptors inhibited the depressant effect of L-AP4 in a dose-dependent manner. MPPG (10 nmol) inhibited the antiseizure activity of L-AP4, whilst MPPG (40 nmol) reduced both the anti-epileptogenic and antiseizure activities of L-AP4. MPPG (40 nmol) by itself had no effect on generalized seizure activity, and it had no detectable influence on the normal rate of kindled epileptogenesis. During in vitro studies using a microsuperfusion method, L-AP4 inhibited depolarization-induced release of [3H]D-aspartate from rat cortical synaptosomes (IC50 125.1 microM) and decreased the depolarization-evoked uptake of 45Ca2+ in a dose-dependent manner. Both actions of L-AP4 were reduced by the selective antagonist MPPG. When applied alone MPPG (200 microM) had no detectable action on veratridine-evoked 45Ca2+ uptake by the synaptosomes. These results suggest the mechanisms by which presynaptically active glutamate receptor agonists block the development of the chronically epileptic state induced by electrical kindling, and indicate that their anticonvulsive activity is due to inhibition of presynaptic glutamate and/or aspartate release following blockade of presynaptic Ca2+ entry.