Biomedical subjects

L G Magazanik

Publications and source records attributed to L G Magazanik.

At least 19 recordsLinked to original sources

PubMed2007

Effects of ionotropic glutamate receptor channel blockers on the development of pentylenetetrazol kindling in mice.

Experiments on mice were performed to study the ability of monocationic and dicationic adamantane and phenylcyclohexyl derivatives to prevent the development of kindling induced by i.p. administration of pentylenetetrazol (Corasol, 35 mg/kg). The monocationic phenylcyclohexyl derivative IEM-1921 effectively slowed the development of kindling, this being seen over a wide range of doses (0.0001-0.1 micromol/kg). A monocationic adamantane derivative (memantine), also a selective non-competitive blocker of NMDA receptors, produced a similar effect at doses 100 times higher. The anticonvulsive activity of the dicationic phenylcyclohexyl derivative IEM-1925, which could block both types of glutamate receptors, differed from the activity of the monocationic derivative by having a more complex dose-response relationship. Thus, the development of kindling was suppressed by essentially the same doses as needed for the monocation IEM-1921 (0.001 micromol/kg). However, on reducing the dose by a factor of 10 (0.0001 micromol/kg), IEM-1925 facilitated the development of kindling. This difference in the prophylactic activities of selective NMDA receptor blockers and substances able to block both NMDA and AMPA receptors provides evidence that the mechanism of kindling involves both types of ionotropic glutamate receptor and the effects of compounds depend not only on the ratio of the contributions of these receptors, but also on the kinetic characteristics of the blocking action.

Animals↗

PubMed2006

Selective blockade of Ca2+ permeable AMPA receptors in CA1 area of rat hippocampus.

Using whole cell patch-clamp recording from pyramidal cells and interneurons in the CA1 area of hippocampal slices, the effect of IEM-1460, a selective channel blocker of Ca2+ permeable AMPA receptors (AMPARs), on postsynaptic currents (PSCs) was studied. Excitatory postsynaptic currents (EPSCs) were evoked by stimulation of Schaffer collaterals (SCs) in the presence of APV and bicuculline to pharmacologically isolate the EPSCs mediated by AMPAR activation. IEM-1460 (50 microM) did not affect the amplitude of EPSCs in CA1 pyramidal cells but reversibly decreased their amplitude in interneurons of pyramidal layer (15 cells), radiatum (37 cells) and border radiatum-lacunosum-moleculare (R-LM) (55 cells) layers. The ability of IEM-1460 to decrease EPSC amplitude correlated with EPSC rectification properties in CA1 interneurons, providing evidence for synaptic localization of Ca2+ permeable AMPARs at the SC synaptic input. Independent of their localization, the majority of interneurons studied exhibited only modest sensitivity to IEM-1460 (EPSC amplitude decreased by less than 30%), while in 15% of interneurons IEM-1460 induced more than 50% reduction in EPSC amplitude. To reveal possible afferent-specific localization of Ca2+ permeable AMPARs on R-LM interneurons, the effect of IEM-1460 on EPSCs evoked by stimulation of SC was compared with that of perforant path (PP). Although average sensitivities did not differ significantly, in 61% of R-LM layer interneurons, the SC-evoked EPSCs exhibited higher sensitivity to IEM-1460 than the PP-evoked EPSCs. Moreover, in 54% of R-LM layer interneurons the EPSCs evoked by SC stimulation were complex, having an initial peak followed by one or several late components. Kinetics, latency distribution and reversal potential of late components suggest di- and polysynaptic origin of the late components. Late EPSCs were strongly and reversibly inhibited by IEM-1460 indicating that Ca2+ permeable AMPARs are involved in the indirect excitation of R-LM layer interneurons. Despite the ability to decrease the excitatory synaptic input to interneurons, IEM-1460 did not affect interneuron-mediated inhibitory postsynaptic currents (IPSCs) evoked in pyramidal neurons by SC stimulation. These data suggest that interneurons with a synaptic input highly sensitive to IEM-1460 do not contribute specifically to the feed-forward inhibition of hippocampal pyramidal neurons.

Adamantane↗

PubMed2006

[Mechanisms of blockade of glutamate receptors channels: the significance for structural and physiological investigations].

The mechanism of blocking effect of phenylcyclohexyl derivative, IEM-1925, on ionotropic glutamate receptors of the NMDA and AMPA types has been studied on the rat isolated brain neurons. The whole-cell configuration of patck clanp recording technique was used equilibrium conditions and -80 mV holding potential, the IEM-1925 manifests nonselective action on open channels of both receptors. However, the prominent differences in the mechanism of the blocking effect were revealed. Although IEM-1925 can not enter the closed channels of both types, its molecule are able to leave closed channels of the AMPA but not the NMDA receptors. Hyperpolarization reduces removal of blocker from the open channels of the NMDA receptors. Contrary to that, hyperpolarization facilitates going out of the IEM-1925 to cytozol from both open and closed channels. Evidently, the bloker can pass through the AMPA receptor channels into the cell, and the gating mechanism of these channels is located above the binding site for the blocker. The blocking action of the IEM-1925 on the NMDA and AMPA receptors was compared with its potency to weaken the tremor evoked by subcutaneous injection of arecoline to mice. The observed differences in the mechanisms of action help to explain the ambiguous effects of channel blocking drugs on experimental models of pathological processes.

Animals↗

PubMed2005

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.

Algorithms↗

PubMed2005

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.

Animals↗

PubMed2005

Design of antagonists for NMDA and AMPA receptors.

Determinants of antagonism of NMDA and calcium permeable AMPA receptor channels by organic cations were studied using several homologous series of mono- and dicationic derivatives of adamantane, phenylcyclohexyl, triphenylmethane, diphenylmethane. Antagonism by these drugs was studied on native receptors of isolated rat brain neurons and on recombinant GluR1 receptors expressed by Xenopus oocytes. The major action of these compounds was on the open channel, although minor competitive or closed channel antagonism cannot be ruled out. Analysis of structure-activity relationships suggests that all organic monocations are selective antagonists of NMDA receptors. Compounds exhibiting trapping block are more potent than those exhibiting weakly-trapping block. AMPA and NMDA receptor channels are blocked by dicationic organic compounds, the former requiring a certain distance between the hydrophobic moiety and the terminal charged group. Variations of their terminal ammonium group demonstrated that trimethylammonium derivatives are the most potent antagonists of AMPA receptors, whereas the terminal amino group is optimal for block of NMDA receptors. Based on the action of 38 compounds, topographical models of the binding sites of these compounds on NMDA and AMPA receptor channels are presented. These models will help to design channel-blocking drugs with defined potency and selectivity of action.

Adamantane↗

PubMed2005

[Effects of blockade of ionotropic glutamate receptors on the development of pentylenetetrazole kindling in mice].

Effects of mono- and dicationic derivatives of adamantane and phenylcyclohexyl on the petyleneterazole-induced (35 mg/kg i. p.) kindling were studied in the experiments on mice. Monocationic derivative of phenylcyclohexyl IEM-1921, effectively retarded the development of kindling beginning the dose 0.0001 microM/kg. Memantine: derivative of adamantane (derivative of adamatane) produced the same effect with 100-fold increased dose. Dicationic derivative ofphenylcyclohexyl: IEM-1925, is able to block equally the open channels of both NMDA and subtype of Ca-permeable AMPA receptors. Its effect on kindling differed markedly from selective NMDA antagonists (IEM-1921 and memantine) in more complicated dose-dependence. The retardation of kindling IEM-1925 was induced at 0.001 microM/kg. On the contrary, a 10-time lower dose: 0.0001 microM/kg, facilitated the development of kindling. The observed difference in the activity of selective NMDA antagonists and the drugs combining anti-NMDA and anti-AMPA potency indicates that both types of ionotropic glutamate receptors are involved in the mechanism of petyleneterazole-induced kindling. The integral effect of channel blockade evoked by drugs seems to be dependent not only upon the ratio of the receptor types but on the kinetics of drug action, too.

Animals↗

PubMed2004

Comparison of the anticonvulsive activities of organic mono- and dications with their abilities to inhibit NMDA and AMPA glutamate receptors.

The abilities of mono- and dicationic adamantane and phenylcyclohexyl derivatives to (a) block open NMDA and AMPA glutamate receptors in isolated rat brain neurons and (b) prevent convulsions induced in mice by intraventricular NMDA or kainate were studied. Monocations inhibited NMDA receptors in vitro and produced corresponding protection against NMDA-induced convulsions in vivo, but lacked the ability to block AMPA receptors or prevent kainate-induced convulsions. Dications (IEM-1754 and IEM-1925), which inhibited both NMDA and AMPA receptors, were highly effective at protecting against kainate convulsions and were more effective than the corresponding monocations in preventing NMDA convulsions. The origin of convulsions induced by NMDA appears to be based on a component mediated by activation of AMPA receptors. The anticonvulsive activity of IEM-1754 and IEM-1925 were comparable with those of the known NMDA receptor blockers memantine and MK-801. This was combined with an almost complete absence of the side effects characteristic of memantine and MK-801. The complete correspondence between the in vitro data and in vivo results seen with some of the study compounds is evidently associated with their pharmacokinetic properties.

Adamantane↗

PubMed2004

Studies of the roles of NMDA and AMPA glutamate receptors in the mechanism of corasole convulsions in mice.

Experiments on mice were performed to study the ability of monocationic and dicationic adamantane and phenylcyclohexyl derivatives to prevent convulsive syndrome induced by i.p. corasole (pentylenetetrazole; 80 mg/kg). Monocationic phenylcyclohexyl derivatives, which are selective blockers of NMDA glutamate receptor channels, along with memantine and MK-801, effectively prevented the appearance of the clonic and tonic components of convulsions at micromolar concentrations. Their dicationic analogs, which block both NMDA and AMPA receptor channels, were ineffective against clonic convulsions, but prevented corasole-induced tonic convulsions at much lower concentrations. The convulsive action of corasole, whose primary target is weakening of the inhibitory action of GABA, appears to be mediated by glutamatergic synaptic transmission. NMDA receptors have a much greater involvement than AMPA receptors in the genesis of clonic convulsions, while AMPA receptor activation appears to be an important component of tonic convulsions.

Animals↗

PubMed2004

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

Postsynaptic currents recorded in the whole-cell configuration with patch-clamp method are actually the sum ofexcitatory (EPSC) and inhibitory (IPSC) components. An approach has been developed allowing the quantitative evaluation of the amplitude and the time course of EPSC and IPSC without treatment of the brain slice with pharmacological inhibitors. The approach is based on the substantial difference in the equilibrium potential values of incoming cationic and anionic currents as the existence of linear regions of corrent-voltage dependence of these currents. The comparison of the results obtained with the classical pharmacological method and with the suggested one demonstrated their coincidence. It allows analysing the postsynaptic currents in sigle neurons without altering the synaptic transmission in the whole brain slice. The contribution of inhibitory currents in the composite synaptic response of intemeurons turned out to be smaller in comparison with pyramidal neurons of CA1 field of the rat hippocampus.

Animals↗

PubMed2003

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.

Adamantane↗

PubMed2003

Modulatory role of adenosine receptors in insect motor nerve terminals.

The effects of adenosine and ATP were studied on blowfly larvae Calliphora vicina neuromuscular preparation. Adenosine diminished (IC50 = 40 +/- 3 microM) the amplitude of nerve-evoked postsynaptic currents (EPSCs) and slightly decreased the frequency of spontaneous currents without affecting their amplitude. EPSCs were slightly reduced by ATP, and this effect was prevented by concanavalin A. Presynaptic inhibition by adenosine was temperature-dependent and insensitive to pertussis toxin. A1 agonists of vertebrate adenosine receptor CPA and NECA failed to reproduce the effect of adenosine, and 2-CADO enhanced the EPSCs. A1 antagonist DPCPX competitively inhibited adenosine action. A2 agonist DPMA potentiated EPSCs, and its effect was abolished by A2 antagonist DMPX. Adenosine and ATP failed to affect the nonquantal release of glutamate. The results show for the first time the presence of presynaptic adenosine receptors regulating transmitter release at insect motor nerve terminals and point to differences in pharmacological properties of adenosine receptor subtypes in insects and vertebrates.

Adenosine↗

PubMed2003

Determinants of trapping block of N-methyl-d-aspartate receptor channels.

Open channel blockers of NMDA receptors interact with the channel gate in different ways. Compounds like MK-801 and phencyclidine exhibit pronounced trapping block, whereas 9-aminoacridine and tetrapentylammonium cannot be trapped. Some blockers such as memantine and amantadine exhibit intermediate properties, so called 'partial trapping'. To analyze the determinants of trapping we have synthesized a series of mono- and dicationic derivatives of phenylcyclohexyl. The blocking action of these compounds as well as that of amantadine has been studied on native NMDA receptors of hippocampal pyramidal neurons. Use-dependence and kinetics of the blockade have been analyzed to estimate the degree of trapping. Dimensions of the blocking molecules apparently do not correlate with their trapping. However, the degree of trapping is voltage-dependent and correlates with the kinetics of unblock. For instance, amantadine behaved as non-trapping blocker at positive voltages, but demonstrated significant trapping at negative voltages. The data may be explained by the model in which the NMDA receptor channel has two binding sites: the shallow and deep ones. Binding to the deep but not to the shallow site allows trapping of the blockers.

Amantadine↗

PubMed2003

[Role of NMDA and AMPA glutamate receptors in the mechanism of korazol-induced convulsions in mice].

The potency of mono- and dikationic derivatives of adamantane and phenylcyclohexyl to prevent seizures induced in mice by intraperitoneal administration of 80 mg/kg pentylenetetrazol (corazol), was studied. Monocationic derivatives of phenylcyclohexyl, being the selective channel blockers of NMDA glutamate receptors, as well memantine and MK-801 in micromolar concentrations, prevented both clonic and tonic components of corazol-induced convulsions. Their dicatonic derivatives which are channel blockers of NMDA and AMPA types of glutamate receptors, failed to prevent clonic seizures but at submicromolar concentrations prevented the tonic extensions provoked by corazol. Evidently, convulsive action of corazol originating from suppression of GABA-ergic inhibition is realized through activation of glutamergic synaptic transmission, and NMDA receptors are mainly involved in genesis of clonic seizures whereas activation of AMPA receptors is important for the tonic component of the corazol-induced syndrome.

Animals↗

PubMed2002

Characterization of acid-sensitive ion channels in freshly isolated rat brain neurons.

Transient proton-activated currents induced by rapid shifts of the extracellular pH from 7.4 to < or =6.8 were recorded in different neurons freshly isolated from rat brain (hypoglossal motoneurons, cerebellar Purkinje cells, striatal giant cholinergic interneurons, hippocampal interneurons, CA1 pyramidal neurons and cortical pyramidal neurons) using whole-cell patch clamp technique. Responses of hippocampal CA1 pyramidal neurons were weak (100-300 pA) in contrast to other types of neurons (1-3 nA). Sensitivity of neurons to rapid acidification varied from pH(50) 6.4 in hypoglossal motoneurons to 4.9 in hippocampal interneurons. Proton-activated currents were blocked by amiloride (IC(50) varied from 3.6 to 9.5 microM). Reversal potential of the currents was close to E(Na), indicating that the currents are carried by sodium ions. The data obtained suggest that the proton-activated currents in the neurons studied are mediated by acid-sensitive ion channels. Strong acidification (pH<4) induced biphasic responses in all neuron types: the transient current was followed by a pronounced sustained one. Sustained current was not blocked by amiloride and exhibited low selectivity for sodium and cesium ions. Slow acidification from pH 7.4 to 6.5 did not induce detectable whole-cell currents. At pH 6.5, most of the channels are desensitized and responses to fast pH shifts from this initial level are decreased at least 10 times. This suggests that slow acidification which is well known to accompany some pathological states should rather desensitize than activate acid-sensitive ion channels and depress their function. Our results provide evidence for a widespread and neuron-specific distribution of acid-sensitive ion channels in the brain. The large amplitudes and transient character of currents mediated by these channels suggest that they could contribute to fast neuronal signaling processes.

Acids↗

PubMed2002

[Comparison of the anticonvulsant activity of organic mono- and di-cations and their potential to inhibit NMDA and AMPA glutamate receptors].

Effects of mono- and dicationic derivatives of adamantine and phenylcyclohexyl were studied on: (i) open channels of NMDA and AMPA glutamate receptors in the experiments on the isolated rat brain neurones, and (ii) convulsions induced by intraventricular injections of NMDA or kainate in mice. Monocations inhibited the NMDA receptors in vitro and prevented convulsions induced by NMDA in vivo, but failed to affect both the AMPA receptors and kainite-induced convulsions. Dications (IEM-1754 and IEM-1925) revealed both anti-NMDA and anti-AMPA potency in vitro, were highly effective against kainite-induced convulsions and excelled monocations in preventing the NMDA-induced ones. Evidently some steps connected with the AMPA receptor activity are involved in the genesis of the NMDA-induced convulsions. Anticonvulsant potency of IEM-1754 and IEM-1925 is comparable with those of known NMDA receptor inhibitors: memantine and MK-801. The IEM-1754 and IEM-1925 show no side effects. An incomplete correspondence between the activity in vitro and in vivo found studying some derivatives, may be due to peculiarities of their pharmacokinetics.

Adamantane↗

PubMed2001

[Structure of glutamate receptor ion channels and mechanisms of their blockade by organic cations].

Structural determinants of blocking the glutamate receptors of AMPA and NMDA subtypes, were studied. Close location of hydrophobic and ammonium groups is necessary for affective blocking of the NMDA receptor channels, whereas blockers of the AMPA receptor channels have a distance of about 10 angstroms between these two groups. Models of the channels meeting these topographic data have been devised using a molecular mechanics approach. The accomplished studies revealed molecular basis of channel blockade of the NMDA and AMPA receptors. This may allow designing predictable new blocking compounds with a desired selectivity.

Adamantane↗

PubMed2000

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.

Animals↗