Voltage-dependent block of native AMPA receptor channels by dicationic compounds

Light-Sensitive Open Channel Block of Ionotropic Glutamate Receptors by Quaternary Ammonium Azobenzene Derivatives

Glutamate ionotropic receptors mediate fast excitation processes in the central nervous system of vertebrates and play an important role in synaptic plasticity, learning, and memory. Here, we describe the action of two azobenene-containing compounds, AAQ (acrylamide-azobenzene-quaternary ammonium) and QAQ (quaternary ammonium-azobenzene-quaternary ammonium), which produced rapid and fully reversible light-dependent inhibition of glutamate ionotropic receptors. The compounds demonstrated voltage-dependent inhibition with only minor voltage-independent allosteric action. Calcium-impermeable AMPA receptors had weaker sensitivity compared to NMDA and calcium-permeable AMPA receptors. We further revealed that the compounds bound to NMDA and calcium-permeable AMPA receptors in different modes. They were able to enter the wide selectivity filter of AMPA receptors, and strong negative voltages caused permeation into the cytoplasm. The narrow selectivity filter of the NMDA receptors did not allow the molecules to bypass them; therefore, QAQ and AAQ bound to the shallow channel site and prevented channel closure by a foot-in-the-door mechanism. Computer simulations employing available AMPA and NMDA receptor structures readily reproduced the experimental findings, allowing for the structure-based design of more potent and selective drugs in the future. Thus, our work creates a framework for the development of light-sensitive blockers of calcium-permeable AMPA receptors, which are desirable tools for neuroscience.

Purification and biochemical analysis of native AMPA receptors from three different mammalian species

The majority of fast, excitatory synaptic transmission in the central nervous system (CNS) is mediated by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs), which are glutamate-activated ion channels integral to synaptic plasticity, motor coordination, learning, and memory. Native AMPARs are multiprotein assemblies comprised of a tetrameric receptor core that co-assembles with a broad range of peripheral auxiliary proteins which shape subcellular localization and signaling properties of the resulting complexes. Structure determination of AMPARs has traditionally relied on recombinant expression systems; however, these methods are not well suited to elucidate the diverse array of AMPAR assemblies that are differentially expressed in mammalian brains. While recent studies of native receptor complexes have advanced our understanding of endogenous assemblies, receptors thus far have only been isolated from rodent brain tissue. Here, we employed an immunoaffinity purification strategy to isolate native AMPARs from the brains of three different mammals-pigs, sheep, and cows. Compared to rodents, pigs, sheep, and cows are ungulate mammals, animals with closer genomic identity with humans. Here we determined the molecular size, overall yield, and purity of native AMPARs isolated from these three mammals, thereby demonstrating that structural determination and biochemical analysis is possible from a clade of mammals evolutionarily distinct from rodents.

Development of a quaternary ammonium photoswitchable antagonist of NMDA receptors

NMDA receptors play critical roles in numerous physiological and pathological processes in CNS that requires development of modulating ligands. In particular, photoswitchable compounds that selectively target NMDA receptors would be particularly useful for analysis of receptor contributions to various processes. Recently, we identified a light-dependent anti-NMDA activity of the azobenzene-containing quaternary ammonium compounds DENAQ (diethylamine-azobenzene-quaternary ammonium) and DMNAQ (dimethylamine-azobenzene-quaternary ammonium). Here, we developed a series of light-sensitive compounds based on the DENAQ structure, and studied their action on glutamate receptors in rat brain neurons using patch-clamp method. We found that the activities of the compounds and the influence of illumination strongly depended on the structural details, as even minor structural modifications greatly altered the activity and sensitivity to illumination. The compound PyrAQ (pyrrolidine-azobenzene-quaternary ammonium) was the most active and produced fast and fully reversible inhibition of NMDA receptors. The IC₅₀ values under ambient and monochromic light conditions were 2 and 14 μM, respectively. The anti-AMPA activity was much weaker. The action of PyrAQ did not depend on NMDA receptor activity, agonist concentration, or membrane voltage, making it a useful tool for photopharmacological studies.

Optical Control of N-Methyl-d-aspartate Receptors by Azobenzene Quaternary Ammonium Compounds

Azobenzene-based quaternary ammonium compounds provide optical control of ion channels and are considered promising agents for regulation of neuronal excitability and for restoration of the photosensitivity of retinal cells. However, the selectivity of the action of these compounds remains insufficiently known. We studied the action of DENAQ (diethylamine-azobenzene-quaternary ammonium) and DMNAQ (dimethylamine-azobenzene-quaternary ammonium) on ionotropic glutamate receptors in rat brain neurons. In the dark, both compounds applied extracellularly caused fast and reversible inhibition of NMDA (N-methyl-d-aspartate) receptor-mediated currents with IC₅₀ values of 10 and 5 μM, respectively. Light-induced transformation of DENAQ and DMNAQ to their cis forms caused the IC₅₀ values to increase to 30 and 27 μM, respectively. Detailed analysis of this action revealed a complex nature consisting of fast inhibitory and slower potentiating effects. The AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptors were only weakly affected independently on illumination. We conclude that, in addition to their long-lasting intracellular action, which persists after washout, azobenzene-based quaternary ammonium compounds should affect glutamatergic transmission and synaptic plasticity during treatment. Our findings also extend the list of soluble photoswitchable inhibitors of NMDA receptors. While the site(s) and mechanisms of action are unclear, the effect of DENAQ demonstrates strong pH dependence. At acidic pH values, DENAQ potentiates both NMDA and AMPA receptors.

Reducing Auditory Nerve Excitability by Acute Antagonism of Ca2+-Permeable AMPA Receptors

Hearing depends on glutamatergic synaptic transmission mediated by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs). AMPARs are tetramers, where inclusion of the GluA2 subunit reduces overall channel conductance and Ca²⁺ permeability. Cochlear afferent synapses between inner hair cells (IHCs) and auditory nerve fibers (ANFs) contain the AMPAR subunits GluA2, 3, and 4. However, the tetrameric complement of cochlear AMPAR subunits is not known. It was recently shown in mice that chronic intracochlear delivery of IEM-1460, an antagonist selective for GluA2-lacking AMPARs [also known as Ca²⁺-permeable AMPARs (CP-AMPARs)], before, during, and after acoustic overexposure prevented both the trauma to ANF synapses and the ensuing reduction of cochlear nerve activity in response to sound. Surprisingly, baseline measurements of cochlear function before exposure were unaffected by chronic intracochlear delivery of IEM-1460. This suggested that cochlear afferent synapses contain GluA2-lacking CP-AMPARs alongside GluA2-containing Ca²⁺-impermeable AMPA receptors (CI-AMPARs), and that the former can be antagonized for protection while the latter remain conductive. Here, we investigated hearing function in the guinea pig during acute local or systemic delivery of CP-AMPAR antagonists. Acute intracochlear delivery of IEM-1460 or systemic delivery of IEM-1460 or IEM-1925 reduced the amplitude of the ANF compound action potential (CAP) significantly, for all tone levels and frequencies, by > 50% without affecting CAP thresholds or distortion product otoacoustic emissions (DPOAE). Following systemic dosing, IEM-1460 levels in cochlear perilymph were ~ 30% of blood levels, on average, consistent with pharmacokinetic properties predicting permeation of the compounds into the brain and ear. Both compounds were metabolically stable with half-lives >5 h in vitro, and elimination half-lives in vivo of 118 min (IEM-1460) and 68 min (IEM-1925). Heart rate monitoring and off-target binding assays suggest an enhanced safety profile for IEM-1925 over IEM-1460. Compound potency on CAP reduction (IC₅₀ ~ 73 μM IEM-1460) was consistent with a mixture of GluA2-lacking and GluA2-containing AMPARs. These data strongly imply that cochlear afferent synapses of the guinea pig contain GluA2-lacking CP-AMPARs. We propose these CP-AMPARs may be acutely antagonized with systemic dosing, to protect from glutamate excitotoxicity, while transmission at GluA2-containing AMPARs persists to mediate hearing during the protection.

Protection of cochlear synapses from noise-induced excitotoxic trauma by blockade of Ca2+-permeable AMPA receptors

Exposure to loud sound damages the postsynaptic terminals of spiral ganglion neurons (SGNs) on cochlear inner hair cells (IHCs), resulting in loss of synapses, a process termed synaptopathy. Glutamatergic neurotransmission via α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-type receptors is required for synaptopathy, and here we identify a possible involvement of GluA2-lacking Ca²⁺-permeable AMPA receptors (CP-AMPARs) using IEM-1460, which has been shown to block GluA2-lacking AMPARs. In CBA/CaJ mice, a 2-h exposure to 100-dB sound pressure level octave band (8 to 16 kHz) noise results in no permanent threshold shift but does cause significant synaptopathy and a reduction in auditory brainstem response (ABR) wave-I amplitude. Chronic intracochlear perfusion of IEM-1460 in artificial perilymph (AP) into adult CBA/CaJ mice prevented the decrease in ABR wave-I amplitude and the synaptopathy relative to intracochlear perfusion of AP alone. Interestingly, IEM-1460 itself did not affect the ABR threshold, presumably because GluA2-containing AMPARs can sustain sufficient synaptic transmission to evoke low-threshold responses during blockade of GluA2-lacking AMPARs. On individual postsynaptic densities, we observed GluA2-lacking nanodomains alongside regions with robust GluA2 expression, consistent with the idea that individual synapses have both CP-AMPARs and Ca²⁺-impermeable AMPARs. SGNs innervating the same IHC differ in their relative vulnerability to noise. We found local heterogeneity among synapses in the relative abundance of GluA2 subunits that may underlie such differences in vulnerability. We propose a role for GluA2-lacking CP-AMPARs in noise-induced cochlear synaptopathy whereby differences among synapses account for differences in excitotoxic susceptibility. These data suggest a means of maintaining normal hearing thresholds while protecting against noise-induced synaptopathy, via selective blockade of CP-AMPARs.

CA1 Neurons Acquire Rett Syndrome Phenotype After Brief Activation of Glutamatergic Receptors: Specific Role of mGluR1/5

Rett syndrome (RTT) is a neurological disorder caused by the mutation of the X-linked MECP2 gene. The neurophysiological hallmark of the RTT phenotype is the hyperexcitability of neurons made responsible for frequent epileptic attacks in the patients. Increased excitability in RTT might stem from impaired glutamate handling in RTT and its long-term consequences that has not been examined quantitatively. We recently reported (Balakrishnan and Mironov, 2018a,b) that the RTT hippocampus consistently demonstrates repetitive glutamate transients that parallel the burst firing in the CA1 neurons. We aimed to examine how brief stimulation of specific types of ionotropic and metabotropic glutamate receptors (GluR) can modulate the neuronal phenotype. We imaged glutamate with a fluorescence sensor (iGluSnFr) expressed in CA1 neurons in hippocampal organotypic slices from wild-type (WT) and Mecp2^-/y mice (RTT). The neuronal and synaptic activities were assessed by patch-clamp and calcium imaging. In both WT and RTT slices, activation of AMPA, kainate, and NMDA receptors for 30 s first enhanced neuronal activity that induced a global release of glutamate. After transient augmentation of excitability and ambient glutamate, they subsided. After wash out of the agonists for 10 min, WT slices recovered and demonstrated repetitive glutamate transients, whose pattern resembled those observed in naïve RTT slices. Hyperpolarization-activated (HCN) decreased and voltage-sensitive calcium channel (VSCC) currents increased. The effects were long-lasting and bigger in WT. We examined the role of mGluR1/5 in more detail. The effects of the agonist (S)-3,5-dihydroxyphenylglycine (DHPG) were the same as AMPA and NMDA and occluded by mGluR1/5 antagonists. Further modifications were examined using a non-stationary noise analysis of postsynaptic currents. The mean single channel current and their number at postsynapse increased after DHPG. We identified new channels as calcium-permeable AMPARs (CP-AMPAR). We then examined back-propagating potentials (bAPs) as a measure of postsynaptic integration. After bAPs, spontaneous afterdischarges were observed that lasted for ∼2 min and were potentiated by DHPG. The effects were occluded by intracellular CP-AMPAR blocker and did not change after NMDAR blockade. We propose that brief elevations in ambient glutamate (through brief excitation with GluR agonists) specifically activate mGluR1/5. This modifies CP-AMPAR, HCN, and calcium conductances and makes neurons hyperexcitable. Induced changes can be further supported by repetitive glutamate transients established and serve to persistently maintain the aberrant neuronal RTT phenotype in the hippocampus.

Development of excitatory synaptic transmission to the superior paraolivary and lateral superior olivary nuclei optimizes differential decoding strategies

The superior paraolivary nucleus (SPON) is a prominent structure in the mammalian auditory brainstem with a proposed role in encoding transient broadband sounds such as vocalized utterances. Currently, the source of excitatory pathways that project to the SPON and how these inputs contribute to SPON function are poorly understood. To shed light on the nature of these inputs, we measured evoked excitatory postsynaptic currents (EPSCs) in the SPON originating from the intermediate acoustic stria and compared them with the properties of EPSCs in the lateral superior olive (LSO) originating from the ventral acoustic stria during auditory development from postnatal day 5 to 22 in mice. Before hearing onset, EPSCs in the SPON and LSO are very similar in size and kinetics. After the onset of hearing, SPON excitation is refined to extremely few (2:1) fibers, with each strengthened by an increase in release probability, yielding fast and strong EPSCs. LSO excitation is recruited from more fibers (5:1), resulting in strong EPSCs with a comparatively broader stimulus-response range after hearing onset. Evoked SPON excitation is comparatively weaker than evoked LSO excitation, likely due to a larger fraction of postsynaptic GluR2-containing Ca²⁺-impermeable AMPA receptors after hearing onset. Taken together, SPON excitation develops synaptic properties that are suited for transmitting single events with high temporal reliability and the strong, dynamic LSO excitation is compatible with high rate-level sensitivity. Thus, the excitatory input pathways to the SPON and LSO mature to support different decoding strategies of respective coarse temporal and sound intensity information at the brainstem level.

Inhibition of Spinal Ca(2+)-Permeable AMPA Receptors with Dicationic Compounds Alleviates Persistent Inflammatory Pain without Adverse Effects

Upregulation of Ca²⁺-permeable AMPA receptors (CP-AMPARs) in the dorsal horn (DH) neurons of the spinal cord has been causally linked to the maintenance of persistent inflammatory pain. Therefore, inhibition of CP-AMPARs could potentially alleviate an, otherwise, poorly treatable chronic pain. However, a loss of CP-AMPARs could produce considerable side effects because of the crucial role of CP-AMPARs in synaptic plasticity. Here we have tested whether the inhibition of spinal CP-AMPARs with dicationic compounds, the open-channel antagonists acting in an activity-dependent manner, can relieve inflammatory pain without adverse effects being developed. Dicationic compounds, N1-(1-phenylcyclohexyl)pentane-1,5-diaminium bromide (IEM-1925) and 1-trimethylammonio-5-1-adamantane-methyl-ammoniopentane dibromide (IEM-1460) were applied intrathecally (i.t.) as a post-treatment for inflammatory pain in the model of complete Freund’s adjuvant (CFA)-induced long-lasting peripheral inflammation. The capability of dicationic compounds to ameliorate inflammatory pain was tested in rats in vivo using the Hargreaves, the von Frey and the open-field tests. Treatment with IEM-1460 or IEM-1925 resulted in profound alleviation of inflammatory pain. The pain relief appeared shortly after compound administration. The effects were concentration-dependent, displaying a high potency of dicationic compounds for alleviation of inflammatory hyperalgesia in the micromolar range, for both acute and long-lasting responses. The period of pain maintenance was shortened following treatment. Treatment with IEM-1460 or IEM-1925 changed neither thermal and mechanical basal sensitivities nor animal locomotion, suggesting that inhibition of CP-AMPARs with dicationic compounds does not give rise to detectable side effects. Thus, the ability of dicationic compounds to alleviate persistent inflammatory pain may provide new routes in the treatment of chronic pain.

Specific mechanism of use-dependent channel block of calcium-permeable AMPA receptors provides activity-dependent inhibition of glutamatergic neurotransmission

This study examined the blocking action of the selective channel blocker of calcium-permeable (CP) AMPA receptors, N1-(1-phenylcyclohexyl)pentane-1,5-diaminium bromide (IEM-1925), on excitatory postsynaptic currents in rat neostriatal and cortical neurons and in fly neuromuscular junctions. In both preparations, the blocking of CP-AMPA receptor currents increased along with the stimulation frequency. The continuous presence of kainate, which activates AMPA receptors, in the external solution also caused an enhanced blocking effect. Likewise, decrease of the synaptic release by lowering calcium concentration resulted in significant reduction of the blocking action. The activity dependence of the block is explained using the guarded receptor model. The drug molecule can only bind if the channel is open. After the channel has closed, the drug molecule remains trapped inside. However, the trapped molecule slowly egresses from closed channels to the cytoplasm. The total block effect is determined by the equilibrium between accumulation of the drug in the open channels and relief from the closed channels. Therefore, the conditions that favour the open state result in enhanced inhibition. This significant finding reveals a new way to modulate CP-AMPAR-mediated transmission using a physiologically relevant approach. Moreover, it allows the involvement of CP-AMPARs in the physiological and pathological processes – such as high-frequency synaptic activity or increase of the steady-state glutamate concentration – to be examined.

Voltage-dependent and -independent block of α-amino-3-hydroxy-5-methylisoxazole-4-propionate receptor channels

Polyamine-containing toxins and synthetic dicationic derivatives of adamantane and phenylcyclohexyl selectively antagonize Ca(2+)-permeable α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor channels. These compounds demonstrate voltage-dependent open-channel block and are trapped by closed channels. In this study, we describe an alternative mechanism of non-competitive AMPA receptor inhibition caused by 9-aminoacridine and some of its derivatives. These compounds exhibit similar potency against Ca(2+)-permeable and Ca(2+)-impermeable AMPA receptors. The inhibition is largely voltage-independent, binding and unbinding do not require presence of agonist. We conclude that 9-aminoacridine binds to a shallow site in the AMPA receptor, which is located above the activation gate. A comparison of three-dimensional structures of the antagonists suggests that the 'V-like' shape of the hydrophobic headgroup favors voltage-dependent binding to the deep site in the channel pore, whereas the compounds possessing flat aromatic headgroups preferably bind to the shallow site. The characterization of the novel mechanism of AMPA receptor channel antagonism opens a way to develop a new family of pharmacological agents, which can be of scientific and practical importance.

Peripheral and central routes of administration of quaternary ammonium compound IEM-1460 are equally potent in reducing the severity of nicotine-induced seizures in mice

Peripheral administration of nicotinic receptor antagonists with a quaternary ammonium group (hexamethonium and chlorisondamine) did not prevent the development of seizures induced by systemic treatment with nicotine in the toxic dose. The Me3N+ group with stable positive charge inhibits transport of these compounds into the brain through the blood-brain barrier. Intracerebral and peripheral (intraperitoneal) administration of compound IEM-1460 with the Me3N+ group was equally potent in reducing the severity of nicotine-induced seizures in mice. This phenomenon is related to the fact that IEM-1460 acts as a nicotinic receptor antagonist and polyamine agonist, which increases blood-brain barrier permeability for polar compounds. These features contribute to IEM-1460 transport into the brain. High anticonvulsant activity of IEM-1460 on the model of nicotine-induced seizures is associated with combined blockade of nicotinic receptors (alpha3beta4 receptors) and glutamate receptors (GluR1 AMPA receptors).

Calcium-permeable presynaptic AMPA receptors in cerebellar molecular layer interneurones

Axons of cerebellar molecular layer interneurones (MLIs) bear ionotropic glutamate receptors. Here, we show that these receptors elicit cytosolic [Ca2+] transients in axonal varicosities following glutamate spillover induced by stimulation of parallel fibres (PFs). A spatial profile analysis indicates that these transients occur at the same locations when induced by PF stimulation or trains of action potentials. They are not affected by the NMDAR antagonist AP-V, but are abolished by the AMPAR inhibitor GYKI-53655. Mimicking glutamate spillover by a puff of AMPA triggers axonal [Ca2+]i transients even in the presence of TTX. Addition of specific voltage-dependent Ca2+ channel (VDCC) blockers such as omega-AGAIVA and omega-conotoxin GVIA or broad range inhibitors such as Cd2+ did not significantly inhibit the signal indicating the involvement of Ca2+-permeable AMPARs. This hypothesis is further supported by the finding that the subunit specific AMPAR antagonist IEM-1460 blocks 75% of the signal. Bath application of AMPA increases the frequency and mean peak amplitude of GABAergic mIPSCs, an effect that is blocked by philanthotoxin-433 (PhTx) and reinforced by facilitating concentrations of ryanodine. By contrast, a high concentration of ryanodine or dantrolene reduced the effects of AMPA on mIPSCs. Single-cell RT-PCR experiments show that all GluR1-4 subunits are potentially expressed in MLI. Taken together, the results suggest that Ca2+-permeable AMPARs are colocalized with VDCCs in axonal varicosities and can be activated by glutamate spillover through PF stimulation. The AMPAR-mediated Ca2+ signal is amplified by Ca2+-induced Ca2+ release from intracellular stores, leading to GABA release by MLIs.

2,3-Benzodiazepine-type AMPA receptor antagonists and their neuroprotective effects

AMPA receptors are fast ligand-gated members of glutamate receptors in neuronal and many types of non-neuronal cells. The heterotetramer complexes are assembled from four subunits (GluR1-4) in region-, development- and function-selective patterns. Each subunit contains three extracellular domains (a large amino terminal domain, an agonist-binding domain and a transducer domain), and three transmembrane segments with a loop (pore forming domain), as well as the intracellular carboxy terminal tail (traffic and conductance regulatory domain). The binding of the agonist (excitatory amino acids and their derivatives) initiates conformational realignments, which transmit to the transducer domain and membrane spanning segments to gate the channel permeable to Na+, K+ and more or less to Ca2+. Several 2,3-benzodiazepines act as non-competitive antagonists of the AMPA receptor (termed also negative allosteric modulators), which are thought to bind to the transducer domains and inhibit channel gating. Analysing their effects in vitro, it has been possible to recognize a structure-activity relationship, and to describe the critical parts of the molecules involved in their action at AMPA receptors. Blockade of AMPA receptors can protect the brain from apoptotic and necrotic cell death by preventing neuronal excitotoxicity during pathophysiological activation of glutamatergic neurons. Animal experiments provided evidence for the potential usefulness of non-competitive AMPA antagonists in the treatment of human ischemic and neurodegenerative disorders including stroke, multiple sclerosis, Parkinson's disease, periventricular leukomalacia and motoneuron disease. 2,3-benzodiazepine AMPA antagonists can protect against seizures, decrease levodopa-induced dyskinesia in animal models of Parkinson's disease demonstrating their utility for the treatment of a variety of CNS disorders.

Mechanisms of the blockade of glutamate channel receptors: Significance for structural and physiological investigations

The mechanism of the blocking action of phenylcyclohexyl derivative IEM-1925 on ionotropic NMDA and AMPA glutamate receptors was studied. Experiments on isolated rat brain neurons (hippocampal pyramidal cells and striatal cholinergic interneurons) were performed using local voltage clamping in the "whole cell" configuration. In equilibrium conditions at a membrane potential of -80 mV, there was no selectivity in the action of IEM-1925 on the open channels of either type of glutamate receptor. However, data were obtained showing significant differences in the mechanisms of the blocking actions. Although IEM-1925 was unable to penetrate into closed channels of either receptor type, molecules were able to leave closed AMPA receptor channels but not closed NMDA receptor channels. In hyperpolarization, the departure of the blocker from open NMDA receptor channels was slowed, while departure from open and closed AMPA receptor channels was accelerated. The blocker thus appeared able to penetrate AMPA receptor channels to enter cells, the gating mechanism of these channels being located above the blocker binding site. The actions of IEM-1925 on NMDA and AMPA receptors were compared with its ability to suppress tremor in mice induced with s.c. doses of arecoline. The results indicated that both types of receptors have a role in producing tremor. The differences in the mechanisms of action on AMPA and NMDA receptors may explain the ambiguous nature of the effects of the glutamate channel blocker in experimental therapy.

Design, Synthesis, and Pharmacological Characterization of Polyamine Toxin Derivatives: Potent Ligands for the Pore-Forming Region of AMPA Receptors

Polyamine toxins, such as philanthotoxins, are low-molecular-weight compounds isolated from spiders and wasps, which modulate ligand-gated ion channels in the nervous system. Philanthotoxins bind to the pore-forming region of AMPA receptors, a subtype of glutamate receptors which are important for memory formation and are involved in neurodegenerative diseases. Previous studies have demonstrated that modification of the polyamine moiety of philanthotoxins can lead to very potent and highly selective ligands for the AMPA receptor, as exemplified with philanthotoxin-56. Much less attention has been paid to the importance of the aromatic head group of philanthotoxins, but herein we demonstrate that modification of this moiety leads to a significant improvement in potency relative to philanthotoxin-56 at cloned AMPA receptors. Interestingly, the incorporation of an adamantane moiety is particularly favorable, and the most potent compound has a Ki value of 2 nM, making it the most potent uncompetitive antagonist of AMPA receptors described to date. Such compounds are potentially useful as neuroprotective agents.

mGluRs induce a long-term depression in the ventral tegmental area that involves a switch of the subunit composition of AMPA receptors

Excitatory glutamatergic synapses on dopamine (DA) neurons of the ventral tegmental area (VTA) undergo long-lasting changes during conditioning of natural rewards and in response to drug exposure. It has been suggested that the ensuing context-dependent behavioural changes are associated with an increased efficacy of synaptic afferents determined by the balance of long-term potentiation (LTP) and long-term depression (LTD). However, the molecular nature of the forms of LTP/LTD involved remains elusive. Here, using acute rat brain slices, we describe a form of long-term depression (LTD) that was engaged by synaptic activity or exogenous agonists activating group I metabotropic glutamate receptors (mGluR) and was sensitive to mGluR1 antagonists. Prior to mGluR-LTD, AMPAR mediated excitatory postsynaptic currents (EPSCs) showed strong rectification at positive potentials and were sensitive to Joro spider toxin (JST), a selective blocker of GluR2-lacking AMPARs. After mGluR-LTD, AMPAR EPSCs had linear current-voltage relations and became insensitive to JST. We conclude that activation of mGluR1s triggers a redistribution exchanging native receptors for GluR2 containing AMPARs, ultimately causing LTD that may oppose pathological neuroadaptation.

AMPA receptor ligands: Synthetic and pharmacological studies of polyamines and polyamine toxins

Alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors (AMPAR), subtype of the ionotropic glutamate receptors (IGRs), mediate fast synaptic transmission in the central nervous system (CNS), and are involved in many neurological disorders, as well as being a key player in the formation of memory. Hence, ligands affecting AMPARs are highly important for the study of the structure and function of this receptor, and in this regard polyamine-based ligands, particularly polyamine toxins, are unique as they selectively block Ca2+ -permeable AMPARs. Indeed, endogenous intracellular polyamines are known to modulate the function of these receptors in vivo. In this study, recent developments in the medicinal chemistry of polyamine-based ligands are given, particularly focusing on the use of solid-phase synthesis (SPS) as a tool for the facile generation of libraries of polyamine toxin analogues. Moreover, the recent development of highly potent and very selective AMPAR ligands is described. Additionally, we provide a detailed account on the mechanism and site of action of AMPAR blockade by polyamine-based ligands, including examples of how these ligands are used as tools to study AMPAR, and a comparison with their action on other ionotropic receptors.

Potassium, sodium, calcium and glutamate-gated channels: pore architecture and ligand action

In the last decade, the idea of common organization of certain ion channel families exhibiting diverse physiological and pharmacological properties has received strong experimental support. Transmembrane topologies and patterns of the pore-facing residues are conserved in P-loop channels that include high-selective cation channels and certain ligand-gated channels. X-ray structures of bacterial K+ channels, KcsA, MthK and KvAP, help to understand structure-function relationships of other P-loop channels. Data on binding sites and mechanisms of action of ligands of K+, Na+, Ca2+ and glutamate gated ion channels are considered in view of their possible structural similarity to the bacterial K+ channels. Emphasized are structural determinants of ligand-receptor interactions within the channels and mechanisms of state-dependent action of the ligands.

The ability of new non-competitive glutamate receptor blockers to weaken motor disorders in animals

The ability of mono- and dicationic phenylcyclohexyl derivatives, which are non-competitive glutamate antagonists, to prevent convulsions induced in mice by intragastric NMDA or kainate, to weaken catalepsy induced in rats by haloperidol and to exert their own influences of movement activity and behavior in animals was studied. The actions of study compounds were compared with those of the known NMDA antagonists memantine and dizocilpine. NMDA-induced convulsions were effectively prevented by both mono- and dications, while only dications were effective against kainate convulsions. Anticataleptic activity was significantly more marked in monocations, which lacked the ability to block non-NMDA receptors. Side effects on motor coordination were less marked with study compounds than with dizocilpine. Thus, the effects of phenylcyclohexyl derivatives in in vivo experimental models correlate with their anti-NMDA and anti-AMPA activity. They can be regarded as potential agents for treating parkinsonism and other motor disorders.

Studies of the structure of glutamate receptor ion channels and the mechanisms of their blockade by organic cations

The structural determinants for blockade of the AMPA and NMDA subtypes of glutamate receptors were studied by analysis of structural-functional relationships in a series of mono- and dicationic compounds. The results showed that the hydrophobic and nucleophilic components of the blocker binding sites are located close to each other in the channel of the NMDA receptor, while they are spatially distant in the channel of the AMPA receptor. Molecular mechanical methods were used to construct models of these channels satisfying these topographic criteria and providing adequate descriptions of the binding of the channel blockers. According to the models, binding of blockers to the NMDA channel occurs in the selective filter of the channel (the N/Q/R site). The nucleophilic region of the AMPA channel is formed by the oxygen atoms of glycine residues in position +2 relative to the selective filter. Identification of the major relationships between the molecular structure of the ion channels of these glutamate receptor subtypes and their blockade by organic cations allows the further synthesis of AMPA and NMDA channel blockers with specified levels of activity and selectivity to be directed.

Evaluation of GluR2 subunit involvement in AMPA receptor function of neonatal rat hypoglossal motoneurons

AMPA receptors (AMPAr) mediate fast synaptic responses to glutamate and, when they lack the GluR2 subunit, are strongly Ca2+ permeable and may increase intracellular Ca2+ levels. Because hypoglossal motoneurons possess restricted ability to buffer internal Ca2+ and are vulnerable to Ca2+ excitotoxicity, we wondered if, in these cells, any significant Ca2+ influx could be generated via AMPAr activity. Using whole cell patch-clamp recording from neonatal rat hypoglossal motoneurons, we tested the AMPAr properties conferred by GluR2 subunits, namely Ca2+ permeability, current rectification and sensitivity to pentobarbital or to the subunit-specific channel blockers, IEM-1460 and IEM-1925. We recorded membrane currents generated by the agonist, kainate, and compared them with those obtained from hippocampal pyramidal neurons (expressing GluR2-containing AMPAr) and from striatal giant aspiny or hippocampal interneurons (with GluR2-lacking AMPAr). Ca2+ vs. Na+ permeability of motoneuron AMPAr was relatively low (0.25 +/- 0.05), although higher than that of pyramidal neurons. With intracellularly applied spermine, significant inward rectification was absent from motoneurons. These data indicated the prevalence of functional GluR2 subunits. However, the sensitivity of motoneuron AMPAr to pentobarbital did not differ from that of GluR2-lacking AMPAr on interneurons. Motoneurons possessed sensitivity to IEM-1460 (IC50 = 90 +/- 10 microm) approximately 10-fold lower than striatal interneurons, although 10-fold higher than hippocampal pyramidal cells. IEM-1925 also reduced the amplitude of excitatory synaptic currents in brainstem slice motoneurons. We hypothesize that hypoglossal motoneuron AMPAr (moderately Ca2+ permeable because they contain few GluR2 subunits) may contribute to intracellular Ca2+ rises especially if persistent AMPAr activation (or the pathological GluR2 down-regulation) occurs.

Modeling of the pore domain of the GLUR1 channel: homology with K+ channel and binding of channel blockers

Molecular models of the M2 segments of the GluR1 channel have been elaborated using a molecular mechanics approach. The models are based on the homology between pore-lining segments of AMPA receptor channels and the KcsA K+ channel and on cyclic H bonds at the Q/R site of the AMPA receptor channel. The N-terminal region of an M2 segment of the channel is assumed, like that of the K+ channel, to adopt a helical conformation. Due to a deletion, the C-terminal end of the M2 segment of the AMPA receptor is more stretched than that of the K+ channel. As a result, only a single oxygen ring may be exposed to the AMPA receptor channel pore. Data on the block of AMPA receptor channels by dicationic adamantane derivatives have been used to select the most relevant model. The model with the oxygen of a Gly residue (position +2 from the Q/R site) exposed to the pore best fits the experimental data. This model also fits experimental data for another class of AMPA receptor antagonists, the polyamine amides. According to the model, the side-chains of the C-terminal residues are involved in intra-receptor interactions that stabilize the structure of the channel rather than in interactions with ions in the pore.

Structural characteristics of ionotropic glutamate receptors as identified by channel blockade

The channels of four types of ionotropic glutamate receptor (NMDA receptors and Ca-permeable AMPA receptors of rat brain neurons, and cation-selective receptors from mollusk neurons and insect postsynaptic muscle membranes) and two subtypes of nicotinic cholinoreceptor (from frog neuromuscular junctions and cat sympathetic ganglia) were studied. The structural characteristics of channels determining their susceptibility to blockade by organic mono- and dications were identified. These studies used homologous series of adamantane and phenylcyclohexyl derivatives. These experiments showed that the receptors studied here could be divided into two groups. The first group included the AMPA receptor and the mollusk and insect receptors. These were characterized by the lack of effect on the part of monocations and a strong relationship between the activity of dications and the distance between nitrogen atoms. The second group included the NMDA receptor and both subtypes of the nicotinic cholinoreceptor (muscular and neuronal). Here, conversely, the activity of monocations and dications, regardless of their lengths, were essentially identical. A model for the binding sites of blockers in channels is proposed, which takes these observations into account.

Different arrangement of hydrophobic and nucleophilic components of channel binding sites in N-methyl-D-aspartate and AMPA receptors of rat brain is revealed by channel blockade

In order to investigate the topography of the channel binding site in (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and N-methyl-D-aspartate (NMDA) types of glutamate receptors, we have compared the blocking actions of mono- and dicationic derivatives of adamantane and phenylcyclohexyl. The potencies of dicationic derivatives to block AMPA receptor channels are about 1000 times higher than those of monocationic ones, whereas NMDA receptors are equally sensitive to both mono- and dicationic derivatives. The dependence of the activity of dicationic compounds on the length of the polymethylene chain between ammonium groups has a pronounced maximum for AMPA receptor channel block. For NMDA receptor channel dicationic compounds with various internitrogen distances produce similar blocking effects. The results show that hydrophobic and nucleophilic components of the binding site are located close to each other in the NMDA receptor channel but are separated by approximately 10 A in the AMPA receptor channel.