Evidence for a role of presynaptic AMPA receptors in the control of neuronal glutamate release in the rat forebrain

Presynaptic glutamate receptors in nociception

Chronic pain is a frequent, distressing and poorly understood health problem. Plasticity of synaptic transmission in the nociceptive pathways after inflammation or injury is assumed to be an important cellular basis for chronic, pathological pain. Glutamate serves as the main excitatory neurotransmitter at key synapses in the somatosensory nociceptive pathways, in which it acts on both ionotropic and metabotropic glutamate receptors. Although conventionally postsynaptic, compelling anatomical and physiological evidence demonstrates the presence of presynaptic glutamate receptors in the nociceptive pathways. Presynaptic glutamate receptors play crucial roles in nociceptive synaptic transmission and plasticity. They modulate presynaptic neurotransmitter release and synaptic plasticity, which in turn regulates pain sensitization. In this review, we summarize the latest understanding of the expression of presynaptic glutamate receptors in the nociceptive pathways, and how they contribute to nociceptive information processing and pain hypersensitivity associated with inflammation / injury. We uncover the cellular and molecular mechanisms of presynaptic glutamate receptors in shaping synaptic transmission and plasticity to mediate pain chronicity, which may provide therapeutic approaches for treatment of chronic pain.

Presynaptic AMPA Receptors in Health and Disease

AMPA receptors (AMPARs) are ionotropic glutamate receptors that play a major role in excitatory neurotransmission. AMPARs are located at both presynaptic and postsynaptic plasma membranes. A huge number of studies investigated the role of postsynaptic AMPARs in the normal and abnormal functioning of the mammalian central nervous system (CNS). These studies highlighted that changes in the functional properties or abundance of postsynaptic AMPARs are major mechanisms underlying synaptic plasticity phenomena, providing molecular explanations for the processes of learning and memory. Conversely, the role of AMPARs at presynaptic terminals is as yet poorly clarified. Accruing evidence demonstrates that presynaptic AMPARs can modulate the release of various neurotransmitters. Recent studies also suggest that presynaptic AMPARs may possess double ionotropic-metabotropic features and that they are involved in the local regulation of actin dynamics in both dendritic and axonal compartments. In addition, evidence suggests a key role of presynaptic AMPARs in axonal pathology, in regulation of pain transmission and in the physiology of the auditory system. Thus, it appears that presynaptic AMPARs play an important modulatory role in nerve terminal activity, making them attractive as novel pharmacological targets for a variety of pathological conditions.

Traumatic Brain Injury-Induced Sex-Dependent Changes in Late-Onset Sensory Hypersensitivity and Glutamate Neurotransmission

Women approximate one-third of the annual 2.8 million people in the United States who sustain traumatic brain injury (TBI). Several clinical reports support or refute that menstrual cycle-dependent fluctuations in sex hormones are associated with severity of persisting post-TBI symptoms. Previously, we reported late-onset sensory hypersensitivity to whisker stimulation that corresponded with changes in glutamate neurotransmission at 1-month following diffuse TBI in male rats. Here, we incorporated intact age-matched naturally cycling females into the experimental design while monitoring daily estrous cycle. We hypothesized that sex would not influence late-onset sensory hypersensitivity and associated in vivo amperometric extracellular recordings of glutamate neurotransmission within the behaviorally relevant thalamocortical circuit. At 28 days following midline fluid percussion injury (FPI) or sham surgery, young adult Sprague-Dawley rats were tested for hypersensitivity to whisker stimulation using the whisker nuisance task (WNT). As predicted, both male and female rats showed significantly increased sensory hypersensitivity to whisker stimulation after FPI, with females having an overall decrease in whisker nuisance scores (sex effect), but no injury and sex interaction. In males, FPI increased potassium chloride (KCl)-evoked glutamate overflow in primary somatosensory barrel cortex (S1BF) and ventral posteromedial nucleus of the thalamus (VPM), while in females the FPI effect was discernible only within the VPM. Similar to our previous report, we found the glutamate clearance parameters were not influenced by FPI, while a sex-specific effect was evident with female rats showing a lower uptake rate constant both in S1BF and VPM and longer clearance time (in S1BF) in comparison to male rats. Fluctuations in estrous cycle were evident among brain-injured females with longer diestrus (low circulating hormone) phase of the cycle over 28 days post-TBI. Together, these findings add to growing evidence indicating both similarities and differences between sexes in a chronic response to TBI. A better understanding of the influence of gonadal hormones on behavior, neurotransmission, secondary injury and repair processes after TBI is needed both clinically and translationally, with potential impact on acute treatment, rehabilitation, and symptom management.

P2X7 receptors exert a permissive effect on the activation of presynaptic AMPA receptors in rat trigeminal caudal nucleus glutamatergic nerve terminals

Background

Purine receptors play roles in peripheral and central sensitization and are associated with migraine headache. We investigated the possibility that ATP plays a permissive role in the activation of AMPA receptors thus inducing Glu release from nerve terminals isolated from the rat trigeminal caudal nucleus (TCN).

Methods

Nerve endings isolated from the rat TCN were loaded with [³H]D-aspartic acid ([³H]D-ASP), layered into thermostated superfusion chambers, and perfused continuously with physiological medium, alone or with various test drugs. Radioactivity was measured to assess [³H]D-ASP release under different experimental conditions.

Results

Synaptosomal [³H]D-ASP spontaneous release was stimulated by ATP and to an even greater extent by the ATP analogue benzoylbenzoylATP (BzATP). The stimulation of [³H]D-ASP basal release by the purinergic agonists was prevented by the selective P2X7 receptor antagonist A438079. AMPA had no effect on basal [³H]D-ASP release, but the release observed when synaptosomes were exposed to AMPA plus a purinoceptor agonist exceeded that observed with ATP or BzATP alone. The selective AMPA receptor antagonist NBQX blocked this “excess” release. Co-exposure to AMPA and BzATP, each at a concentration with no release-stimulating effects, evoked a significant increase in [³H]D-ASP basal release, which was prevented by exposure to a selective AMPA antagonist.

Conclusions

P2X7 receptors expressed on glutamatergic nerve terminals in the rat TCN can mediate Glu release directly and indirectly by facilitating the activation of presynaptic AMPA receptors. The high level of glial ATP that occurs during chronic pain states can promote widespread release of Glu as well as can increase the function of AMPA receptors. In this manner, ATP contributes to the AMPA receptor activation involved in the onset and maintenance of the central sensitization associated with chronic pain.

Presynaptic PICK1 facilitates trafficking of AMPA-receptors between active zone and synaptic vesicle pool

Previous studies have indicated that presynaptic α-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors (AMPARs) contribute to the regulation of neurotransmitter release. In hippocampal synapses, the presynaptic surface expression of several AMPAR subunits, including GluA2, is regulated in a ligand-dependent manner. However, the molecular mechanisms underlying the presynaptic trafficking of AMPARs are still unknown. Here, using bright-field immunocytochemistry, western blots, and quantitative immunogold electron microscopy of the hippocampal CA1 area from intact adult rat brain, we demonstrate the association of AMPA receptors with the presynaptic active zone and with small presynaptic vesicles, in Schaffer collateral synapses in CA1 of the hippocampus. Furthermore, we show that GluA2 and protein interacting with C kinase 1 (PICK1) are colocalized at presynaptic vesicles. Similar to postsynaptic mechanisms, overexpression of either PICK1 or pep2m, which inhibit the N-ethylmaleimide sensitive fusion protein (NSF)-GluA2 interaction, decreases the concentration of GluA2 in the presynaptic active zone membrane. These data suggest that the interacting proteins PICK1 and NSF act as regulators of presynaptic GluA2-containing AMPAR trafficking between the active zone and a vesicle pool that may provide the basis of presynaptic components of synaptic plasticity.

Roles of the NMDA Receptor and EAAC1 Transporter in the Modulation of Extracellular Glutamate by Low and High Affinity AMPA Receptors in the Cerebellum in Vivo: Differential Alteration in Chronic Hyperammonemia

The roles of high- and low-affinity AMPA receptors in modulating extracellular glutamate in the cerebellum remain unclear. Altered glutamatergic neurotransmission is involved in neurological alterations in hyperammonemia, which differently affects high- and low-affinity AMPA receptors. The aims were to assess by in vivo microdialysis (a) the effects of high- and low-affinity AMPA receptor activation on extracellular glutamate in the cerebellum; (b) whether chronic hyperammonemia alters extracellular glutamate modulation by high- and/or low-affinity AMPA receptors; and (c) the contribution of NMDA receptors and EAAC1 transporter to AMPA-induced changes in extracellular glutamate. In control rats, high affinity receptor activation does not affect extracellular glutamate but increases glutamate if NMDA receptors are blocked. Low affinity AMPA receptor activation increases transiently extracellular glutamate followed by reduction below basal levels and return to basal values. The reduction is associated with transient increased membrane expression of EAAC1 and is prevented by blocking NMDA receptors. Blocking NMDA receptors with MK-801 induces a transient increase in extracellular glutamate which is associated with reduced membrane expression of EAAC1 followed by increased membrane expression of the glutamate transporter GLT-1. Chronic hyperammonemia does not affect responses to activation of low affinity AMPA receptors. Activation of high affinity AMPA receptors increases extracellular glutamate in hyperammonemic rats by an NMDA receptor-dependent mechanism. In conclusion, these results show that there is a tightly controlled interplay between AMPA and NMDA receptors and an EAAC1 transporter in controlling extracellular glutamate. Hyperammonemia alters high- but not low-affinity AMPA receptors.

Glutamate (mGluR-5) gene expression in brain regions of streptozotocin induced diabetic rats as a function of age: role in regulation of calcium release from the pancreatic islets in vitro

Metabotrophic glutamate receptors (mGluRs) modulate cellular activities involved in the processes of differentiation and degeneration. In this study, we have analysed the expression pattern of group-I metabotropic glutamate receptor (mGlu-5) in cerebral cortex, corpus striatum, brainstem and hippocampus of streptozotocin induced and insulin treated diabetic rats (D+I) as a function of age. Also, the functional role of glutamate receptors in intra cellular calcium release from the pancreatic islets was studied in vitro. The gene expression studies showed that mGlu-5 mRNA in the cerebral cortex increased siginficantly in 7 weeks old diabetic rats whereas decreased expression was observed in brainstem, corpus striatum and hippocampus when compared to control. 90 weeks old diabetic rats showed decreased expression in cerebral cortex, corpus striatum and hippocampus whereas in brainstem the expression increased significantly compared to their respective controls. In 7 weeks old D+I group, mGlu-5 mRNA expression was significantly decreased in cerebral cortex and corpus striatum whereas the expression increased significantly in brainstem and hippocampus. 90 weeks old D+I group showed an increased expression in cerebral cortex, while it was decreased significantly in corpus striatum, brainstem and hippocampus compared to their respective controls. In vitro studies showed that glutamate at lower concentration (10(-7) M) stimulated calcium release from the pancreatic islets. Our results suggest that mGlu-5 receptors have differential expression in brain regions of diabetes and D+I groups as a function of age. This will have clinical significance in management of degeneration in brain function and memory enhancement through glutamate receptors. Also, the regulatory role of glutamate receptors in calcium release has immense therapeutic application in insulin secretion and function.

Mobility of NMDA autoreceptors but not postsynaptic receptors at glutamate synapses in the rat entorhinal cortex

NMDA receptors (NMDAr) are known to undergo recycling and lateral diffusion in postsynaptic spines and dendrites. However, NMDAr are also present as autoreceptors on glutamate terminals, where they act to facilitate glutamate release, but it is not known whether these receptors are also mobile. We have used functional pharmacological approaches to examine whether NMDA receptors at excitatory synapses in the rat entorhinal cortex are mobile at either postsynaptic sites or in presynaptic terminals. When NMDAr-mediated evoked EPSCs (eEPSCs) were blocked by MK-801, they showed no evidence of recovery when the irreversible blocker was removed, suggesting that postsynaptic NMDAr were relatively stably anchored at these synapses. However, using frequency-dependent facilitation of AMPA receptor (AMPAr)-mediated eEPSCs as a reporter of presynaptic NMDAr activity, we found that when facilitation was blocked with MK-801 there was a rapid (approximately 30-40 min) anomalous recovery upon removal of the antagonist. This was not observed when global NMDAr blockade was induced by combined perfusion with MK-801 and NMDA. Anomalous recovery was accompanied by an increase in frequency of spontaneous EPSCs, and a variable increase in frequency-facilitation. Following recovery from blockade of presynaptic NMDAr with a competitive antagonist, frequency-dependent facilitation of AMPAr-mediated eEPSCs was also transiently enhanced. Finally, an increase in frequency of miniature EPSCs induced by NMDA was succeeded by a persistent decrease. Our data provide the first evidence for mobility of NMDAr in the presynaptic terminals, and may point to a role of this process in activity-dependent control of glutamate release.

Competitive AMPA receptor antagonists

Glutamic acid (Glu) is the major excitatory neurotransmitter in the mammalian central nervous system (CNS) where it is involved in the physiological regulation of different processes. It has been well established that excessive endogenous Glu is associated with many acute and chronic neurodegenerative disorders such as cerebral ischaemia, epilepsy, amiotrophic lateral sclerosis, Parkinson's, and Alzheimer's disease. These data have consequently added great impetus to the research in this field. In fact, many Glu receptor antagonists acting at the N-methyl-D-aspartic acid (NMDA), 2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA), and/or kainic acid (KA) receptors have been developed as research tools and potential therapeutic agents. Ligands showing competitive antagonistic action at the AMPA type of Glu receptors were first reported in 1988, and the systemically active 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo[f]quinoxaline (NBQX) was first shown to have useful therapeutic effects in animal models of neurological disease in 1990. Since then, the quinoxaline template has represented the backbone of various competitive AMPA receptor antagonists belonging to different classes which had been developed in order to increase potency, selectivity and water solubility, but also to prolong the "in vivo" action. Compounds that present better pharmacokinetic properties and less serious adverse effects with respect to the others previously developed are undergoing clinical evaluation. In the near future, the most important clinical application for the AMPA receptor antagonists will probably be as neuroprotectant in neurodegenerative diseases, such as epilepsy, for the treatment of patients not responding to current therapies. The present review reports the history of competitive AMPA receptor antagonists from 1988 up to today, providing a systematic coverage of both the open and patent literature.

Ultrastructural localisation and differential agonist-induced regulation of AMPA and kainate receptors present at the presynaptic active zone and postsynaptic density

Activity-dependent changes in ionotropic glutamate receptors at the postsynaptic membrane are well established and this regulation plays a central role in the expression of synaptic plasticity. However, very little is known about the distributions and regulation of ionotropic receptors at presynaptic sites. To determine if presynaptic receptors are subject to similar regulatory processes we investigated the localisation and modulation of AMPA (GluR1, GluR2, GluR3) and kainate (GluR6/7, KA2) receptor subunits by ultrasynaptic separation and immunoblot analysis of rat brain synaptosomes. All of the subunits were enriched in the postsynaptic fraction but were also present in the presynaptic and non-synaptic synaptosome fractions. AMPA stimulation resulted in a marked decrease in postsynaptic GluR2 and GluR3 subunits, but an increase in GluR6/7. Conversely, GluR2 and GluR3 increased in the presynaptic fraction whereas GluR6/7 decreased. There were no significant changes in any of the compartments for GluR1. NMDA treatment decreased postsynaptic GluR1, GluR2 and GluR6/7 but increased presynaptic levels of these subunits. NMDA treatment did not evoke changes in GluR3 localisation. Our results demonstrate that presynaptic and postsynaptic subunits are regulated in opposite directions by AMPA and NMDA stimulation.

Facilitation of glutamatergic synaptic transmission in hippocampal CA1 area of rats with traumatic brain injury

We investigated the effects of traumatic brain injury (TBI) on the glutamatergic synaptic transmission in the hippocampal CA1 area. A moderate impact (3.8-4.8atm) was applied onto the left parietal cerebral cortex by a fluid percussion injury (FPI) device. Conventional intracellular recordings were made from hippocampal CA1 pyramidal neurons in vitro. Electrophysiological properties of these neurons were compared between three groups (control, FPI-ipsilateral, and FPI-contralateral). The excitability of postsynaptic membrane of CA1 pyramidal neurons was not altered by the moderate FPI; however, the evoked glutamatergic excitatory synaptic transmission in the pyramidal neurons of post-FPI-CA1 was enhanced. Paired-pulse facilitation (PPF) was significantly suppressed in both the FPI-ipsilateral and FPI-contralateral groups and the frequencies of mEPSPs in neurons from the bilateral FPI groups were greater than the frequency in the control group. These results suggest that the glutamatergic synaptic transmission in the hipppocampal CA1 area is facilitated through presynaptic mechanisms after TBI.

Cyclothiazide binding to functionally active AMPA receptor reveals genuine allosteric interaction with agonist binding sites

The agonist, [3H](-)[S]-1-(2-amino-2-carboxyethyl)-5-fluoro-pyrimidine-2,4-dione ([3H](S)F-Willardiine) binding to functional alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors of resealed plasma membrane vesicles and nerve endings freshly isolated from the rat cerebral cortex displayed two binding sites (K(D1)=33+/-7 nM, B(MAX1)=1.6+/-0.3 pmol/mg protein, K(D2)=720+/-250 nM and B(MAX2)=7.8+/-4.0 pmol/mg protein). The drug which impairs AMPA receptor desensitisation, 6-chloro-3,4-dihydro-3-(2-norbornene-5-yl)-2H-1,2,4-benzothiadiazine-7-sulphonamide-1,1-dioxide (cyclothiazide, CTZ) fully displaced the [3H](S)F-Willardiine binding at a concentration of 500 microM. In the presence of 100 microM CTZ (K(I(CTZ))=60+/-6 microM), both the antagonist [3H]-1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo(F)quinoxaline-7-sulfonamide ([3H]NBQX: K(D)=24+/-4 nM, B(MAX)=12.0+/-0.1 pmol/mg protein) and the high-affinity agonist binding showed similar affinity reduction ([3H](S)F-Willardiine: K(D)=140+/-19 nM, B(MAX)=2.9+/-0.5 pmol/mg protein; [3H]NBQX: K(D)=111+/-34 nM, B(MAX)=12+/-3 pmol/mg protein). To disclose structural correlates underlying genuine allosteric binding interactions, molecular mechanics calculations of CTZ-induced structural changes were performed with the use of PDB data on extracellular GluR2 binding domain dimeric crystals available by now. Hydrogen-bonding and root mean square (rms) values of amino acid residues recognising receptor agonists showed minor alterations in the agonist binding sites itself. Moreover, CTZ binding did not affect dimeric subunit structures significantly. These findings indicated that the structural changes featuring the non-desensitised state could possibly occur to a further site of the extracellular GluR2 binding domain. The increase of agonist efficacy on allosteric CTZ binding may be interpreted in terms of a mechanism involving AMPA receptor desensitisation sequential to activation.

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.

Characterisation of an uridine-specific binding site in rat cerebrocortical homogenates

Parameters of [3H]uridine binding to synaptic membranes isolated from rat brain cortex (K(D)=71+/-4 nM, B(max)=1.37+/-0.13 pmol/mg protein) were obtained. Pyrimidine and purine analogues displayed different rank order of potency in displacement of specifically bound [3H]uridine (uridine>5-F-uridine>5-Br-uridine approximately adenosine>>5-ethyl-uridine approximately suramin>theophylline) and in the inhibition of [14C]uridine uptake (adenosine>uridine>5-Br-uridine approximately 5-F-uridine approximately 5-ethyl-uridine) into purified cerebrocortical synaptosomes. Furthermore, the effective ligand concentration for the inhibition of [14C]uridine uptake was about two order of magnitude higher than that for the displacement of specifically bound [3H]uridine. Adenosine evoked the transmembrane Na(+) ion influx, whereas uridine the transmembrane Ca(2+) ion influx much more effectively. Also, uridine was shown to increase free intracellular Ca(2+) ion levels in hippocampal slices by measuring Calcium-Green fluorescence. Uridine analogues were found to be ineffective in displacing radioligands that were bound to various glutamate and adenosine-recognition and modulatory-binding sites, however, increased [35S]GTPgammaS binding to membranes isolated from the rat cerebral cortex. These findings provide evidence for a rather specific, G-protein-coupled site of excitatory action for uridine in the brain.

Solubilization and immunological identification of presynaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors in the rat hippocampus

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors have been identified mostly as postsynaptic receptors mediating fast glutamatergic synaptic transmission. However, neurochemical studies based on the modulation of neurotransmitter release have suggested the existence of presynaptic AMPA receptors. We have used a recently described technique that allows a high-purity fractionation of the pre- and postsynaptic proteins of synaptic junctions to evaluate the distribution of the different AMPA receptor subunits in rat hippocampal synapses. Surprisingly, we found very high levels of GluR1- and GluR2/3-like immunoreactivity in the presynaptic fraction, but also in the postsynaptic and extrasynaptic fractions. GluR4-like immunoreactivity was much less abundant but was still detected, predominantly in the postsynaptic fraction. This methodology appears to be far more sensitive than the classical immunogold electron microscopy to determine the localization of synaptic receptors.

AMPA antagonist ZK200775 in patients with acute ischemic stroke: possible glial cell toxicity detected by monitoring of S-100B serum levels

BACKGROUND AND PURPOSE

S-100B and neuron-specific enolase (NSE) serum concentrations can be used as peripheral markers of glial cell and neuronal damage, respectively. We investigated these markers in a clinical trial with the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) antagonist ZK200775 in acute ischemic stroke patients.

METHODS

In a multicenter, double-blind, randomized, placebo-controlled phase 2 trial, 61 ischemic stroke patients were treated with either placebo or active drug in a dose-finding design. Twenty-five patients received placebo, 12 patients received a total dose of 262.5 mg in 48 hours (dose group 1), and 13 patients received a total dose of 525 mg in 48 hours (dose group 2). Eleven patients received a total dose of 105 mg over a period of 6 hours (dose group 3; reduction of total dose and infusion time because of adverse events in group 2). Serum concentrations of S-100B and NSE were analyzed with the use of a monoclonal sandwich immunoluminometric assay. Neurological outcome was assessed with the National Institutes of Health Stroke Scale (NIHSS).

RESULTS

In group 2 there was a significant transient worsening in the mean NIHSS score 48 hours after the start of treatment. The mean increase was 11 points. This was due to reduction of consciousness (stupor and coma) in 8 of 13 patients. Neurological deterioration in group 2 was associated with a higher increase of S-100B concentrations, but not of NSE concentrations, than in the placebo group. The trial was stopped prematurely for safety reasons.

CONCLUSIONS

The AMPA antagonist ZK200775 transiently worsened the neurological condition in patients with acute ischemic stroke. Our results suggest that in addition to neuronal dysfunction, glial cell toxicity may have occurred. It may be useful to introduce monitoring of serum markers of brain damage in phase 2 trials with glutamate receptor antagonists.

Low doses of AMPA exert anticonvulsant effects on pentylenetetrazol-kindled seizures

Excitatory amino acids (EAAs) are critically involved in the initiation and propagation of seizures. N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors appear to be of special interest in this regard. Besides receptor binding by antagonists, the function of glutamatergic synapses can be altered via autoreceptor-mediated mechanisms or by receptor desensitisation. Therefore, the effect of AMPA (1, 10 or 100 pmol per animal, intracerebroventricular injection) was tested on acutely induced pentylenetetrazol (PTZ) seizures. The lowest dose exerted clear anticonvulsant effects. Furthermore, 1 and 10 pmol AMPA were tested for their efficacy to suppress PTZ kindling. The lower dose reduced seizure severity significantly but 10 pmol AMPA was ineffective. In reaction to a test dose of PTZ, the kindled groups pretreated with AMPA reached seizure scores similar to saline-pretreated kindled rats, suggesting that the kindled state was reached. In a further experiment, we tested the effect of cyclothiazide (CYC, which blocks AMPA receptor desensitisation) on the 1 pmol AMPA-mediated anticonvulsant effect. The AMPA response was not altered. These results suggest that autoreceptor-mediated mechanisms rather than desensitisation might contribute to the anticonvulsant effect found.

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.

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.

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.

Synaptic and non-synaptic AMPA receptors permeable to calcium

For a long time, alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) receptors permeable to calcium have been considered to be either non-existent or as "atypical". There is now ample evidence that these receptors exist in numerous regions of the nervous system and in many neuronal as well as non-neuronal cell populations. This evidence has been accumulated by several methods, including electrophysiological recording, calcium imaging and cobalt-loading. Functional AMPA receptors permeable to calcium are already expressed at very early stages of embryonic development, well before the onset of synaptogenesis. They are probably involved in the paracrine signaling necessary for construction of the nervous system before becoming involved in synaptic transmission. In immature cells, cyclothiazide strongly increases the steady-state level of responses not only to AMPA, but also to kainate. Ingestion, during pregnancy, of food or drug substances that can cross the placental barrier and act upon the embryonic receptors may constitute a risk for normal development. In the adult nervous system, synaptic as well as non-synaptic (paracrine) AMPA receptors permeable to calcium are probably widely expressed in both glial and neuronal cells. They may also participate in controlling some aspects related to adult neurogenesis, in particular the migration of newly formed neurons.

AMPA-receptor blockade within the RVLM modulates cardiovascular responses via glutamate during peripheral stimuli

We investigated the effects of AMPA-receptor blockade in the rostral ventrolateral medulla (RVLM) on cardiovascular responses and extracellular concentrations of glutamate during two different types of stimuli that activate peripheral Adelta - and C-fiber polymodal nociceptors using anesthetized rats. First, mechanical stimulation was achieved by applying a bilateral hindpaw pinch for 5 s, and second, thermal stimulation was evoked by immersing bilaterally the hindpaw metatarsi in a 52 degrees C hot water bath for 4 s. Mechanical stimulation increased mean arterial pressure (MAP) by 23 +/- 1 mmHg and heart rate (HR) by 25 +/- 3 bpm (n= 8). Thermal stimuli increased MAP by 32 +/- 3 mmHg and HR by 27 +/- 4 bpm (n= 8). After controlled generation of mechanical or thermal stimulation, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 1.0 microM) was microdialysed bilaterally into the RVLM for 30 min. Administration of CNQX attenuated MAP and HR responses during a subsequent mechanical but not during thermal stimulation. Analyses of extracellular concentrations of glutamate within the RVLM bilaterally revealed an increase of this neurotransmitter within the RVLM during mechanical noxious stimulation. Concomitant with attenuation of the cardiovascular responses, glutamate concentrations were also decreased during the mechanical stimulation after administration of CNQX. These results demonstrate that the AMPA-receptor blockade within the RVLM that attenuates cardiovascular responses during mechanical stimulation is associated with a reduction in extracellular levels of glutamate. In addition, it appears that AMPA receptors in the RVLM do not play a role in mediating cardiovascular responses during thermal stimulation.

Ionotropic glutamate receptor modulation preferentially affects NMDA receptor expression in rat hippocampus

Electrophysiological data suggest that alterations in the function of one glutamate receptor subtype may affect the function of other subtypes. Further, previous studies have demonstrated that NMDA receptor antagonists affect NMDA and kainate receptor expression in rat hippocampus. In order to address the mutual regulation of NMDA, AMPA, and kainate receptor expression in rat hippocampus, we conducted two experiments examining the effects of NMDA and non-NMDA glutamate receptor modulators on NMDA, AMPA, and kainate receptor expression using in situ hybridization and receptor autoradiography. NMDA receptor expression was preferentially affected by systemic treatments, as all drugs significantly altered [(3)H]MK-801 binding, and several drugs increased [(3)H]ifenprodil binding. GYKI52466 and aniracetam treatments resulted in changes in both [(3)H]ifenprodil binding and NR2B mRNA levels, consistent with the association of this subunit and binding site in vitro. There were more modest effects on AMPA and kainate receptor expression, even by direct antagonists. Together, these data suggest that ionotropic glutamate receptors interact at the level of expression. These data also suggest that drug regimens targeting one ionotropic glutamate receptor subtype may indirectly affect other subtypes, potentially producing unwanted side effects.

(S)-2,3-dihydro-[3,4]cyclopentano-1,2,4-benzothiadiazine-1,1-dioxide: (S18986-1) a positive modulator of AMPA receptors enhances (S)-AMPA-mediated [3H]noradrenaline release from rat hippocampal and frontal cortex slices

The present study describes the effect of (S)-2,3-dihydro-[3, 4]cyclopentano-1,2,4-benzothiadiazine-1,1-dioxide (S18986-1), a positive allosteric modulator of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors with cognitive-enhancing effects, on (S)-AMPA-induced [3H]noradrenaline release in rat hippocampal and frontal cortex slices. (S)-AMPA significantly increased [3H]noradrenaline release in rat hippocampus and frontal cortex slices, whereas S18986-1 (3-1000 microM) alone, was inactive. However, S18986-1 between 30 and 1000 microM potently enhanced (+200%) (S)-AMPA-mediated [3H]noradrenaline release in both hippocampal and frontal cortex slices. The capacity of S18986-1 to potentiate [3H]noradrenaline release was specific for AMPA receptors as S18986-1 failed to potentiate either kainate and N-methyl-D-aspartate (NMDA)-mediated release of [3H]noradrenaline in rat hippocampal slices. Moreover, 1, 2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide (NBQX) and 1-(4-aminophenyl)-3-methylcarbamoyl-4-methyl-3, 4-dihydro-7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI-53655) but not (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5, 10-imine ((+)-MK-801), inhibited (S)-AMPA and S18986-induced stimulation of (S)-AMPA-mediated [3H]noradrenaline release. In addition, S18986-1-induced stimulation of (S)-AMPA-evoked [3H]noradrenaline release was markedly attenuated in the presence of tetrodotoxin (1 microM) and in Ca(2+)-free buffer. S18986-1 enhanced (S)-AMPA-mediated [3H]noradrenaline release to a greater extent than its corresponding (R)-enantiomer S19024-1 and racemic mixture S17951-1. However, positive allosteric modulators of AMPA receptors such as aniracetam failed to potentiate AMPA-mediated noradrenaline release in hippocampal slices, whereas cyclothiazide potently enhanced (S)-AMPA-mediated [3H]noradrenaline release. These results suggest that the capacity of S18986-1 to enhance AMPA receptor-mediated release of noradrenaline in rat hippocampus and frontal cortex, could contribute to the cognition enhancing mechanisms of S18986-1.

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.

Negative allosteric modulators of AMPA-preferring receptors inhibit [(3)H]GABA release in rat striatum

The effect of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), a selective glutamate receptor agonist, on the release of previously incorporated [(3)H]GABA was examined in superfused striatal slices of the rat. The slices were loaded with [(3)H]GABA in the presence of beta-alanine (1 mM) and superfused with Krebs-bicarbonate buffer containing nipecotic acid (0.1 mM) and aminooxyacetic acid (0.1 mM) to inhibit GABA uptake and metabolism. AMPA (0.01 to 3 mM) increased basal [(3)H]GABA outflow and nipecotic acid potentiated this effect. The [(3)H]GABA releasing effect of AMPA was an external Ca(2+)-dependent process in the absence but not in the presence of nipecotic acid. Cyclothiazide (0.03 mM), a positive modulator of AMPA receptors, failed to evoke [(3)H]GABA release by itself, but it dose-dependently potentiated the [(3)H]GABA releasing effect of AMPA. The AMPA (0.3 mM)-induced [(3)H]GABA release was antagonized by NBQX (0.01 mM) in a competitive fashion (pA(2) 5.08). The negative modulator of AMPA receptors, GYKI-53784 (0.01 mM) reversed the AMPA-induced [(3)H]GABA release by a non-competitive manner (pD'(2) 5.44). GYKI-53784 (0. 01-0.1 mM) also decreased striatal [(3)H]GABA outflow on its own right, this effect was stereoselective and was not influenced by concomitant administration of 0.03 mM cyclothiazide. GYKI-52466 (0. 03-0.3 mM), another negative modulator at AMPA receptors, also inhibited basal [(3)H]GABA efflux whereas NBQX (0.1 mM) by itself was ineffective in alteration of [(3)H]GABA outflow. The present data indicate that AMPA evokes GABA release from the vesicular pool in neostriatal GABAergic neurons. They also confirm that multiple interactions may exist between the agonist binding sites and the positive and negative modulatory sites but no such interaction was detected between the positive and negative allosteric modulators. Since GYKI-53784, but not NBQX, inhibited [(3)H]GABA release by itself, AMPA receptors located on striatal GABAergic neurons may be in sensitized state and phasically controlled by endogenous glutamate. It is also postulated that these AMPA receptors are located extrasynaptically on GABAergic striatal neurons.

Nucleus-specific expression of ionotropic glutamate receptor subunit mRNAs and binding sites in primate thalamus

Thalamic afferents and efferents utilize glutamate as their primary neurotransmitter. There are four families of glutamate receptors that can transduce this activity, as well as regulate glutamate release from thalamic relay neurons. The three ionotropic subtypes are of particular importance, because subunit composition confers variability in functional properties of each subtype. We have quantified the expression of NMDA, AMPA and kainate receptors in the thalamus of the macaque using receptor autoradiography and in situ hybridization. NMDA receptors are multimeric associations of NR1 and NR2A-NR2D subunits that form ligand-gated ion channels. Particular subunits are associated with modulatory binding sites that affect receptor activity. NR1 was the most abundant subunit mRNA; NR2A, NR2B, and NR2D subunit mRNAs were also present, but were expressed in nucleus-specific patterns. Very high levels of [3H]ifenprodil binding to the polyamine site of the NMDA complex were detected in a fairly homogeneous distribution. Binding of the ion channel ligand [3H]MK-801 was also abundant, and limbic nuclei expressed higher levels than motor nuclei or the reticular nucleus. [3H]CGP39653 binding to the glutamate site of the NMDA receptor was the least abundant of the NMDA receptor binding sites. There was variability in the stoichiometric relationships of binding sites across nuclei, suggesting that there is heterogeneity in the pharmacological properties of NMDA receptors expressed in the thalamus. AMPA and kainate are also multimeric associations of specific subunits that form ligand-gated ion channels. These subunits are encoded by specific genes: gluR1-gluR4 for AMPA receptors, and gluR5-gluR7 and KA1-KA2 for kainate receptors. GluR4 and gluR6 mRNAs were, respectively the most abundant of the AMPA and kainate receptor subunit transcripts. Both AMPA and kainate receptor subunit transcripts were expressed in a nucleus-specific pattern. The binding of [3H]kainate was higher than that of [3H]AMPA throughout the thalamus, but AMPA subunit mRNA levels were three to five orders of magnitude higher than those encoding the kainate receptor subunits. The mismatch between the levels of expression of kainate receptor subunit transcripts and binding sites is suggestive of a presynaptic localization of kainate receptors on thalamic afferents. These results suggest that ionotropic glutamate receptors are heterogeneously expressed in the thalamus of the primate, and that their differential expression is both subunit- and nucleus-specific.

Presynaptic excitability changes following traumatic brain injury in the rat

Pathological processes affecting presynaptic terminals may contribute to morbidity following traumatic brain injury (TBI). Posttraumatic widespread neuronal depolarization and elevated extracellular potassium and glutamate are predicted to alter the transduction of action potentials in terminals into reliable synaptic transmission and postsynaptic excitation. Evoked responses to orthodromic single- and paired-pulse stimulation were examined in the CA1 dendritic region of hippocampal slices removed from adult rats following fluid percussion TBI. The mean duration of the extracellularly recorded presynaptic volley (PV) increased from 1.08 msec in controls to 1.54 msec in slices prepared at 1 hr postinjury. There was a time-dependent recovery of this injury effect, and PV durations at 2 and 7 days postinjury were not different from controls. In slices removed at 1 hr postinjury, the initial slopes of field excitatory postsynaptic potentials (fEPSPs) were reduced to 36% of control values, and input/output plots revealed posttraumatic deficits in the transfer of excitation from pre- to postsynaptic elements. Manipulating potassium currents with 1.0 mM tetraethylammonium or elevating potassium ion concentration to 7.5 mM altered evoked responses but did not replicate the injury effects to PV duration. Paired-pulse facilitation of fEPSP slopes was significantly elevated at all postinjury survivals: 1 hr, 2 days, and 7 days. These results suggest two pathological processes with differing time courses: 1) a transient impairment of presynaptic terminal functioning affecting PV durations and the transduction of afferent activity in the terminals to reliable synaptic excitation and 2) a more protracted deficit to the plasticity mechanisms underlying paired-pulse facilitation.

Depolarization-induced release of [(3)H]D-aspartate from GABAergic neurons caused by reversal of glutamate transporters

Cultured neocortical neurons, which predominantly consist of GABAergic neurons exhibit a pronounced stimulus-coupled GABA release. Since the cultures may contain a small population of glutamatergic neurons and the GABAergic neurons have a high content of glutamate it was of interest to examine if glutamate in addition to gamma-aminobutyric acid (GABA) could be released from these cultures. The neurons were preloaded with [(3)H]D-aspartate and subsequently its release was followed during depolarization induced by a high potassium concentration or the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor agonists, AMPA and kainate. Depolarization of the neurons with 55 mM potassium increased the release of [(3)H]D-aspartate by more than 10-fold. When the non-specific calcium-channel blockers cobalt or lanthanum were included in the stimulation buffer with potassium, the release of [(3)H]D-aspartate was decreased by about 40%. These results indicated that some of the released [(3)H]D-aspartate might originate from a vesicular pool. When AMPA was applied to the neurons, the release of [(3)H]D-aspartate was increased 2-fold and could not be prevented or decreased by addition of cobalt. Since AMPA has a rapid desensitizing effect on AMPA receptors, it was examined whether AMPA under non-desensitizing conditions was able to induce an increased release of [(3)H]D-aspartate as compared to the conditions of applying AMPA alone. The desensitization of AMPA receptors was blocked by 6-chloro-3,4-dihydro-3-(2-norbornen-5-yl)-2H-1,2, 4-benzothiadiazine-7-sulphonamide-1,1-dioxide (cyclothiazide). Under the non-desensitizing conditions, the AMPA-induced release of [(3)H]D-aspartate was highly enhanced showing about a 10-fold increase over basal release. Addition of cobalt or lanthanum did not decrease the amount of [(3)H]D-aspartate released, indicating that the release originated from a cytoplasmic pool. Kainate, which induces an almost non-desensitizing effect on AMPA receptors, showed similar results as observed for AMPA under non-desensitizing conditions. The NMDA receptor antagonist (5R,10 S)-(+)-5-methyl-10, 11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801) had only minor effects on the [(3)H]D-aspartate release induced by AMPA and kainate. Thus, the depolarization-induced release of [(3)H]D-aspartate from cultured GABAergic neurons appears to be caused mainly by reversal of the glutamate transporters.

Stimulation of nucleoside efflux and inhibition of adenosine kinase by A1 adenosine receptor activation

Adenosine is produced intracellularly during conditions of metabolic stress and is an endogenous agonist for four subtypes of G-protein linked receptors. Nucleoside transporters are membrane-bound carrier proteins that transfer adenosine, and other nucleosides, across biological membranes. We investigated whether adenosine receptor activation could modulate transporter-mediated adenosine efflux from metabolically stressed cells. DDT1 MF-2 smooth muscle cells were incubated with 10 microM [3H]adenine to label adenine nucleotide pools. Metabolic stress with the glycolytic inhibitor iodoacetic acid (1AA, 5 mM) increased tritium release by 63% (P < 0.01), relative to cells treated with buffer alone. The IAA-induced increase was blocked by the nucleoside transport inhibitor nitrobenzylthioinosine (1 microM), indicating that the increased tritium release was primarily a purine nucleoside. HPLC verified this to be [3H]adenosine. The adenosine A1 receptor selective agonist N6-cyclohexyladenosine (CHA, 300 nM) increased the release of [3H]purine nucleoside induced by IAA treatment by 39% (P < 0.05). This increase was blocked by the A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (10 microM). Treatment of cells with UTP (100 microM), histamine (100 microM), or phorbol-12-myristate-13-acetate (PMA, 10 microM) also increased [3H]purine nucleoside release. The protein kinase C inhibitor chelerythrine chloride (500 nM) inhibited the increase in [3H]purine nucleoside efflux induced by CHA or PMA treatment. The adenosine kinase activity of cells treated with CHA or PMA was found to be decreased significantly compared with buffer-treated cells. These data indicated that adenosine A1 receptor activation increased nucleoside efflux from metabolically stressed DDT1 MF-2 cells by a PKC-dependent inhibition of adenosine kinase activity.

Pharmacology of AMPA/kainate receptor ligands and their therapeutic potential in neurological and psychiatric disorders

It has been postulated, consistent with the ubiquitous presence of glutamatergic neurons in the brain, that defects in glutamatergic neurotransmission are associated with many human neurological and psychiatric disorders. This review evaluates the possible application of ligands acting on glutamate alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and kainate (KA) receptors to minimise the pathology and/or symptoms of various diseases. Glutamate activation of AMPA receptors is thought to mediate most fast synaptic neurotransmission in the brain, while transmission via KA receptors contributes only a minor component. Variants of the protein subunits forming these receptors greatly extend the pharmacological and electrophysiological properties of AMPA/KA receptors. Disease and drug use can differentially affect the expression of the subunits and their variants. Ligands bind to AMPA receptors by competing with glutamate at the glutamate binding site, or non-competitively at other sites on the proteins (allosteric modulators). Ligands showing selective competitive antagonist actions at the AMPA/ KA class of glutamate receptors were first reported in 1988, and the systemically active antagonist 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(F)quinoxaline (NBQX) was first shown to have useful therapeutic effects on animal models of neurological diseases in 1990. Since then, newer antagonists with increased potency, higher specificity, increased water solubility, and a longer duration of action in vivo have been developed. Negative allosteric modulators such as the prototype GYKI-52466 also block AMPA receptors but have little action at KA receptors. Positive allosteric modulators enhance glutamatergic neurotransmission at AMPA receptors. Polyamines and adamantane derivatives bind within the ion channel of calcium-permeable AMPA receptors. The latest developments include ligands selective for KA receptors containing Glu-R5 subunits. Evidence for advantages of AMPA receptor antagonists over N-methyl-D-aspartate (NMDA) receptor antagonists for symptomatic treatment of neurological and psychiatric conditions, and for minimising neuronal loss occurring after acute neurological diseases, such as physical trauma, ischaemia or status epilepticus, have been shown in animal models. However, as yet AMPA receptor antagonists have not been shown to be effective in clinical trials. On the other hand, a limited number of clinical trials have been reported for AMPA receptor ligands that enhance glutamatergic neurotransmission by extending the ion channel opening time (positive allosteric modulators). These acute studies demonstrate enhanced memory capability in both young and aged humans, without any apparent serious adverse effects. The use of these allosteric modulators as antipsychotic drugs is also possible. However, the long term use of both direct agonists and positive allosteric modulators must be approached with considerable caution because of potential adverse effects.

Changes in extracellular glutamate and pressor response during muscle contraction following AMPA-receptor blockade in the RVLM and CVLM

We examined whether modulation of cardiovascular responses by administering 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, an AMPA-receptor antagonist) into the rostral (RVLM) or caudal (CVLM) ventrolateral medulla are mediated via changes in extracellular levels of glutamate. Microdialysis probes were inserted bilaterally into the RVLM or the CVLM. For the RVLM experiments (n=8), muscle contraction for 2 min increased mean arterial pressure (MAP) and heart rate (HR) by 18+/-3 mmHg and 24+/-5 bpm, respectively. Extracellular glutamate concentrations increased from 1.5+/-0.3 to 4.3+/-0.9 ng/5 microl during the contraction. Microdialysis of CNQX (1.0 microM) for 30 min into the RVLM attenuated the increases in MAP, HR, and glutamate concentration in response to a muscle contraction (8+/-2 mmHg, 11+/-3 bpm, and 2.2+/-0.7 ng/5 microl, respectively). Developed tensions did not change during contractions before and after CNQX. Microdialysis of CNQX into the CVLM (n=8) potentiated the contraction-evoked responses in MAP (19+/-3 vs. 34+/-3 mmHg) and HR (25+/-4 vs. 49+/-5 bpm) without a change in developed tension. Following CNQX perfusion into the CVLM, the levels of extracellular glutamate in the CVLM were also augmented during the contraction. Results suggests that AMPA-receptors within the RVLM and CVLM differentially modulate cardiovascular responses during static muscle contraction via increasing and decreasing, respectively, extracellular glutamate concentrations.

Inhibition of [3H]D-aspartate release by deramciclane

The effects of the 5-HT2C receptor inverse agonist deramciclane on the gamma-aminobutyric acid (GABA) uptake and excitatory amino acid release processes were compared in rat cerebrocortical homogenates containing resealed plasmalemma fragments and nerve endings. Deramciclane non-competitively inhibited the uptake of [3H]GABA with a Ki value of 13.7 +/- 0.5 microM and partially displaced specifically bound [3H](R,S)-N-[4,4-bis(3-methyl-2-thienyl)-3-butenyl]nipecotic acid ([3H]NNC-328) with high affinity (IC50 = 2.0 +/- 0.7 nM). Depolarization by 4-aminopyridine or by 4-aminopyridine with (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate [(S)-AMPA] induced the release of [3H]D-aspartate. Deramciclane (10 microM) partially (approximately 50%) inhibited the release of [3H]D-aspartate without affecting [3H]D-aspartate uptake. These results suggest a role for presynaptic inhibition of excitatory amino acid release and GABA uptake in the anxiolytic properties of deramciclane.

Presynaptic receptors

Activation of different types of G-protein-linked and ionotropic presynaptic receptors has been shown to regulate neurotransmitter release throughout the central and peripheral nervous systems. In the case of G-protein-linked receptors, three major mechanisms have been suggested: (a) inhibition of Ca channels in the nerve terminal; (b) the activation of presynaptic K channels, resulting in a reduction in the effectiveness of the action potential; and (c) direct modulation of one or more components of the neurotransmitter vesicle release apparatus. In the case of ionotropic presynaptic receptors, inhibition of release may be achieved through depolarization of the terminal and inactivation of Na and Ca channels. Activation of presynaptic ionotropic receptors that are appreciably Ca permeable can also enhance the release of transmitters as a result of their ability to raise [Ca]i in the terminal directly. Many transmitters employ several of these mechanisms, thus allowing considerable flexibility in the presynaptic regulation of transmitter release.