Ca2+-dependent desensitization of AMPA receptors

Screening for Activity Against AMPA Receptors Among Anticonvulsants-Focus on Phenytoin

The interest in AMPA receptors as a target for epilepsy treatment increased substantially after the approval of perampanel, a negative AMPA receptor allosteric antagonist, for the treatment of partial-onset seizures and generalized tonic-clonic seizures. Here we performed a screening for activity against native calcium-permeable AMPA receptors (CP-AMPARs) and calcium-impermeable AMPA receptors (CI-AMPARs) among different anticonvulsants using the whole-cell patch-clamp method on isolated Wistar rat brain neurons. Lamotrigine, topiramate, levetiracetam, felbamate, carbamazepine, tiagabin, vigabatrin, zonisamide, and gabapentin in 100-µM concentration were practically inactive against both major subtypes of AMPARs, while phenytoin reversibly inhibited them with IC50 of 30 ± 4 μM and 250 ± 60 µM for CI-AMPARs and CP-AMPARs, respectively. The action of phenytoin on CI-AMPARs was attenuated in experiments with high agonist concentrations, in the presence of cyclothiazide and at pH 9.0. Features of phenytoin action matched those of the CI-AMPARs pore blocker pentobarbital, being different from classical competitive inhibitors, negative allosteric inhibitors, and CP-AMPARs selective channel blockers. Close 3D similarity between phenytoin and pentobarbital also suggests a common binding site in the pore and mechanism of inhibition. The main target for phenytoin in the brain, which is believed to underlie its anticonvulsant properties, are voltage-gated sodium channels. Here we have shown for the first time that phenytoin inhibits CI-AMPARs with similar potency. Thus, AMPAR inhibition by phenytoin may contribute to its anticonvulsant properties as well as its side effects.

Metal toxicity at the synapse: presynaptic, postsynaptic, and long-term effects

Metal neurotoxicity is a global health concern. This paper summarizes the evidence for metal interactions with synaptic transmission and synaptic plasticity. Presynaptically metal ions modulate neurotransmitter release through their interaction with synaptic vesicles, ion channels, and the metabolism of neurotransmitters (NT). Many metals (e.g., Pb(2+), Cd(2+), and Hg(+)) also interact with intracellular signaling pathways. Postsynaptically, processes associated with the binding of NT to their receptors, activation of channels, and degradation of NT are altered by metals. Zn(2+), Pb(2+), Cu(2+), Cd(2+), Ni(2+), Co(2+), Li(3+), Hg(+), and methylmercury modulate NMDA, AMPA/kainate, and/or GABA receptors activity. Al(3+), Pb(2+), Cd(2+), and As(2)O(3) also impair synaptic plasticity by targeting molecules such as CaM, PKC, and NOS as well as the transcription machinery involved in the maintenance of synaptic plasticity. The multiple effects of metals might occur simultaneously and are based on the specific metal species, metal concentrations, and the types of neurons involved.

Pharmacological characterization, localization, and regulation of ionotropic glutamate receptors in skate horizontal cells

Glutamate is believed to be the primary excitatory neurotransmitter in the vertebrate retina, and its fast postsynaptic effects are elicited by activating NMDA-, kainate-, or AMPA-type glutamate receptors. We have characterized the ionotropic glutamate receptors present on retinal horizontal cells of the skate, which possess a unique all-rod retina simplifying synaptic circuitry within the outer plexiform layer (OPL). Isolated external horizontal cells were examined using whole-cell voltage-clamp techniques. Glutamate and its analogues kainate and AMPA, but not NMDA, elicited dose-dependent currents. The AMPA receptor antagonist GYKI 52466 at 100 microm abolished glutamate-elicited currents. Desensitization of glutamate currents was removed upon coapplication of cyclothiazide, known to potentiate AMPA receptor responses, but not by concanavalin A, which potentiates kainate receptor responses. The dose-response curve to glutamate was significantly broader in the presence of the desensitization inhibitor cyclothiazide. Polyclonal antibodies directed against AMPA receptor subunits revealed prominent labeling of isolated external horizontal cells with the GluR2/3 and GluR4 antibodies. 1-Naphthylacetyl spermine, known to block calcium-permeable AMPA receptors, significantly reduced glutamate-gated currents of horizontal cells. Downregulation of glutamate responses was induced by increasing extracellular ion concentrations of Zn2+ and H+. The present study suggests that Ca2+-permeable AMPA receptors likely play an important role in shaping the synaptic responses of skate horizontal cells and that alterations in extracellular concentrations of calcium, zinc, and hydrogen ions have the potential to regulate the strength of postsynaptic signals mediated by AMPA receptors within the OPL.

TEA-induced long-term potentiation at hippocampal mossy fiber-CA3 synapses: characteristics of its induction and expression

Potassium ion channel blockade by tetraethylammonium (TEA) reportedly induces long-term potentiation (LTP) at hippocampal mossy fiber (MF)-CA3 synapses, but the characteristics of induction, expression, and modulation of the LTP remain unclear. In the present study, these features of TEA-induced LTP at MF-CA3 synapses were electrophysiologically examined using rat hippocampal slices. Synaptic responses recorded from MF-CA3 synapses were enhanced long-term by TEA application even under the blockade of NMDA receptors with D-AP5, whereas selective pharmacological blockade of T-type voltage-dependent calcium channels (VDCCs) strongly inhibited TEA-induced LTP. Decrease of the paired-pulse facilitation ratio after LTP induction by TEA suggests the involvement of increased neurotransmitter release probability from MF terminals as LTP expression. The facilitative modulation of MF-CA3 LTP by GABA(A) receptor activation reported previously was reversed when bumetanide, a blocker of Na(+)-K(+)-Cl(-) co-transporters (NKCCs), was applied, suggesting that the region-specific modulation of TEA-induced LTP by GABAergic inputs at MF-CA3 synapses is due to the dominance of NKCC action at MF terminals.

Analysis of the excitatory and inhibitory components of postsynaptic currents recorded in pyramidal neurons and interneurons in the rat hippocampus

Postsynaptic currents recorded from interneurons and pyramidal cells in hippocampal slices by local voltage clamping were found to be the sum of excitatory (EPSC) and inhibitory (IPSC) components. An approach allowing quantitative assessment of the amplitude and time course of EPSC and IPSC without pharmacological blockade of the major postsynaptic receptors involved in generating these currents was developed. The approach is based on the existence of a significant difference between reversion potentials of cationic and anionic currents and the presence of a linear zone in the voltage-current characteristics of responses to excitatory and inhibitory transmitters. Comparison of the results of this calculation-based method with those of classical pharmacological analysis of the excitatory and inhibitory components of postsynaptic currents showed them to be virtually identical, which allows synaptic currents in defined neurons to be studied without altering the state of synaptic connections throughout the brain slice. IPSC was found to make a smaller contribution to the total postsynaptic current recorded in interneurons as compared with pyramidal neurons in rat hippocampal field CA1.

Action of extracellular divalent cations on native alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors

The effects of divalent cations on Ca2+-impermeable containing (GluR2 subunit) MPA receptors of hippocampal pyramidal neurones isolated from rat brain was studied using patch-clamping. Ca2+, Mg2+, Mn2+, Co2+, Ni2+ and Zn2+ inhibited currents induced by kainate and glutamate. Inhibition was fast, reversible and voltage independent. The rank order of activities was Ni2+ > Zn2+ > Co2+ > Ca2+ > Mn2+ > Mg2+. Cyclothiazide (0.1 mm) significantly reduced inhibition by divalent cations and 6, 7 dinitroquinoxaline-2.3-dione (DNQX). However, high concentrations of Ni2+ and DNQX inhibited AMPA receptors even in the presence of cyclothiazide. The inhibitory effect of divalent cations as well as DNQX was counteracted by an increase in agonist concentration. In the presence of divalent cations the EC50 values of kainate and glutamate were increased, but the maximal response was not changed. An increase in agonist concentration induced a parallel shift in the concentration-inhibition curve for a divalent cation. These data suggest a competitive-like type of inhibition. However, an increase in agonist concentration reduced the inhibitory action of Ni2+ less than that of DNQX. This gave evidence against direct competition between divalent cations and AMPA receptor agonists. A 'complex-competition' hypothesis was proposed to explain the inhibitory action of divalent cations; it is suggested that divalent cations form ion-agonist complexes, which compete with free agonist for agonist-binding sites on AMPA receptors.

A computer model of field potential responses for the study of short-term plasticity in hippocampus

Activity-dependent synaptic plasticity has important implications for network function. The previously developed model of the hippocampal CA1 area, which contained pyramidal cells (PC) and two types of interneurons involved in feed-forward and recurrent inhibition, respectively, and received synaptic inputs from CA3 neurons via the Schaffer collaterals, was enhanced by incorporating dynamic synaptic connections capable of changing their weights depending on presynaptic activation history. The model output was presented as field potentials, which were compared with those derived experimentally. The parameters of Schaffer collateral-PC excitatory model synapse were determined, with which the model successfully reproduced the complicated dynamics of train-stimulation sequential potentiation/depression observed in experimentally recorded field responses. It was found that the model better reproduces the time course of experimental field potentials if the inhibitory synapses on PC are also made dynamic, with expressed properties of frequency-dependent depression. This finding supports experimental evidence that these synapses are subject to activity-dependent depression. The model field potentials in response to various randomly generated and real (derived from recorded CA3 unit activity) long stimulating trains were calculated, illustrating that short-term plasticity with the observed characteristics could play specific roles in frequency processing in hippocampus and thus providing a new tool for the theoretical study of activity-dependent synaptic plasticity.

A model synapse that incorporates the properties of short- and long-term synaptic plasticity

We propose a general computer model of a synapse, which incorporates mechanisms responsible for the realization of both short- and long-term synaptic plasticity-the two forms of experimentally observed plasticity that seem to be very significant for the performance of neuronal networks. The model consists of a presynaptic part based on the earlier 'double barrier synapse' model, and a postsynaptic compartment which is connected to the presynaptic terminal via a feedback, the sign and magnitude of which depend on postsynaptic Ca(2+) concentration. The feedback increases or decreases the amount of neurotransmitter which is in a ready for release state. The model adequately reproduced the phenomena of short- and long-term plasticity observed experimentally in hippocampal slices for CA3-CA1 synapses. The proposed model may be used in the investigation of certain real synapses to estimate their physiological parameters, and in the construction of realistic neuronal networks.

Aniracetam attenuates apoptosis of astrocytes subjected to simulated ischemia in vitro

The aim of the present study was to establish whether aniracetam is capable of protecting cultured rat astrocytes against ischemic injury. Treatment of the cultures with aniracetam (1, 10 and 100 mM) during 24 h ischemia simulated in vitro significantly decreased the number of apoptotic cells. The antiapoptotic effects of the drug were confirmed by the increase of intracellular ATP and phosphocreatine (PCr) levels and the inhibition of the caspase-3 activity. Aniracetam also attenuated cellular oxidative stress by decreased production of reactive oxygen species (ROS). These effects were associated with the decrease in levels of c-fos and c-jun mRNA in primary astrocyte cultures exposed to 24 h ischemia. When cultured astrocytes were incubated during 24 h simulated ischemia with wortmannin, a phosphatidylinositol 3-kinase (PI 3-kinase) inhibitor or PD98059, a mitogen-activated protein (MAP)/extracellular signal regulated kinase (ERK) (MEK) inhibitor the cell apoptosis was accelerated. This effect was antagonized by adding 100 mM aniracetam to the culture medium. These findings suggest that the protective effect of aniracetam is mediated by PI 3-kinase and MEK pathways in the downstream mechanisms.

The pharmacological manipulation of glutamate receptors and neuroprotection

The overactivation of glutamate receptors is a major cause of Ca(2+) overload in cells, potentially leading to cell damage and death. There is an abundance of agents and mechanisms by which glutamate receptor activation can be prevented or modulated in order to control these effects. They include the well-established, competitive and non-competitive antagonists at the N-methyl-D-aspartate (NMDA) receptors and modulators of desensitisation of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptors. More recently, it has emerged that some compounds can act selectively at different subunits of glutamate receptors, allowing a differential blockade of subtypes. It is also becoming clear that a number of endogenous compounds, including purines, can modify glutamate receptor sensitivity. The kynurenine pathway is an alternative but distinct pathway to the generation of glutamate receptor ligands. The products of tryptophan metabolism via the kynurenine pathway include both quinolinic acid, a selective agonist at NMDA receptors, and kynurenic acid, an antagonist at several glutamate receptor subtypes. The levels of these metabolites change as a result of the activation of inflammatory processes and immune-competent cells, and may have a significant impact on Ca(2+) fluxes and neuronal damage. Drugs which target some of these various sites and processes, or which change the balance between the excitotoxin quinolinic acid and the neuroprotective kynurenic acid, could also have potential as neuroprotective drugs.