Biomedical subjects

O A Krishtal

Publications and source records attributed to O A Krishtal.

At least 37 records · Page 2Linked to original sources

PubMed1992

Trans-ACPD selectively inhibits excitability of hippocampal CA1 neurones.

The selective agonist of metabotropic glutamate receptors, t-ACPD (trans-1-aminocyclopentyl-1,3-dicarboxylic acid) (100-250 microM), reversibly inhibited extracellularly recorded EPSP (excitatory postsynaptic potentials) in the CA1 layer of rat hippocampus. This effect was accompanied by depression of electrical excitability of CA1 neurons as revealed by their antidromic stimulation. The excitability of CA3 neurons remained uneffected. Peculiarly, excitatory postsynaptic currents recorded in voltage clamped, internally perfused CA1 cells remained unaltered. Selective depolarization of CA1 neurons may account for the phenomena described.

Animals↗

PubMed1992

A highly potent and selective N-methyl-D-aspartate receptor antagonist from the venom of the Agelenopsis aperta spider.

Agatoxin-489, extracted from the venom of the Agelenopsis aperta spider, was studied on acutely isolated perfused hippocampal neurons of rat using the concentration clamp technique. Agatoxin-489 proved to be a selective N-methyl-D-aspartate antagonist; responses to applications of N-methyl-D-aspartate or L-aspartate were blocked by concentrations of agatoxin-489 ranging between 0.1 nM and 1 microM, while responses to kainate were not affected by agatoxin-489 at concentrations up to 10 microM. The actions of agatoxin-489 against responses to N-methyl-D-aspartate or L-aspartate were use- and voltage-dependent, being less pronounced with an increase in the holding potential from -100 to -30 mV. The action of agatoxin-489 could be completely or partially reversed only after washout in the presence of an N-methyl-D-aspartate agonist. The washout was more effective at positive membrane potentials ranging from 0 to +20 mV. These results imply that the spider toxin agatoxin-489, like dizocilpine, is a potent and selective N-methyl-D-aspartate antagonist which preferentially interacts with activated N-methyl-D-aspartate receptors and/or open N-methyl-D-aspartate-activated ionic channels.

Agatoxins↗

PubMed1992

Two types of steady-state desensitization of N-methyl-D-aspartate receptor in isolated hippocampal neurones of rat.

1. Whole-cell voltage-clamp recordings were made from rat isolated hippocampal neurones. Aspartate (Asp) and/or glycine (Gly) were applied by a method in which the external solution could be changed within 30 ms and thereafter held constant. 2. Asp and Gly applied together at maximal concentrations (5 mM and 10 microM, respectively) evoked an inward current due to activation of N-methyl-D-aspartate (NMDA) receptors. The current peaked and then declined to a steady state during the application. The time constant of desensitization (tau) was about 1 s when the agonists were applied soon after the onset of whole-cell recording. The desensitization became more rapid (tau = 0.3 s) and more complete during the first 15 min of recording, and thereafter remained stable; the amplitude of the peak response did not change throughout. In solutions containing 10 microM-Gly, Asp had an apparent Kd of 51 microM at the peak of response and 20 microM measured at the steady state. The steady-state current was 14% of the peak current. 3. Asp was applied after a conditioning exposure of the cell of Gly (from 1 to 50 microM), together with the same Gly concentration. The maximum current evoked by the application of Asp was increased while increasing Gly in the conditioning solution, with no change in the apparent Kd for Asp at the peak of Asp-activated response. 4. Various concentrations of Asp (plus 10 microM-Gly) were applied after a conditioning exposure to Asp (which alone was without effect). The maximum current induced by Asp applications was only 28% of that observed without conditioning Asp application, but the apparent Kd was unchanged (about 57 microM). 5. Test solution containing maximal concentrations of Asp and Gly was applied after conditioning exposure to both Asp (varying concentrations) and Gly (10 microM). Complete desensitization was caused by 200 microM-Asp. The apparent Kd for Asp to induce desensitization (8.7 microM) was less than the Kd as an agonist (51 microM). 6. Test solution containing maximal concentrations of Asp and Gly was applied after conditioning exposure to both Gly (varying concentrations) and Asp (5 mM). Complete desensitization was caused by 1 microM-Gly. The apparent Kd for Gly to induce desensitization (120 nM) was less than the Kd as a co-agonist (about 1 microM).(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1991

NMDA receptor agonists selectively block N-type calcium channels in hippocampal neurons.

The modulation of voltage-dependent calcium channels by various neurotransmitters has been demonstrated in many neurons. Because of the critical role of Ca2+ in transmitter release and, more generally, in transmembrane signalling, this modulation has important functional implications. Hippocampal neurons possess low-threshold (T-type) Ca2+ channels and both L- and N-type high voltage-activated Ca2+ channels. N-type Ca2+ channels are blocked selectively by omega-conotoxin and adenosine. These substances both block excitatory synaptic transmission in the hippocampus, whereas dihydropyridines, which selectively block L-type channels, are ineffective. Excitatory synaptic transmission in the hippocampus displays a number of plasticity phenomena that are initiated by Ca2+ entry through ionic channels operated by N-methyl-D-aspartate (NMDA) receptors. Here we report that NMDA receptor agonists selectively and effectively depress N-type Ca2+ channels which are involved in neurotransmitter release from presynaptic sites. The inhibitory effect is eliminated by the competitive NMDA antagonist D-2-amino-5-phosphonovalerate, does not require Ca2+ entry into the cell, and is probably receptor-mediated. This phenomenon may provide a negative feedback between the liberation of excitatory transmitter and entry of Ca2+ into the cell, and could be important in presynaptic inhibition and in the regulation of synaptic plasticity.

2-Amino-5-phosphonovalerate↗

PubMed1991

A novel selective NMDA agonist, N-phthalamoyl-L-glutamic acid (PhGA).

Ionic currents elicited by N-phthalamoyl-L-glutamic acid (PhGA) were investigated on freshly isolated hippocampal neurons with the whole-cell voltage clamp and concentration clamp techniques. PhGA elicited desensitizing inward currents in Mg(2+)-free salines only in the presence of glycine. The dose-response relationship for PhGA was close to a Langmuir isotherm with Kd = 3.7 mM and saturating level 0.75 of that for L-aspartate (L-Asp). PhGA-activated currents were blocked by Mg2+, D-2-amino-5-phosphonovalerate and kynurenate, and had the same reversal potential as L-Asp-activated currents. Complete cross-desensitization was obtained between the responses to PhGA and L-Asp. We conclude that PhGA is a new selective 'superacidic' agonist of the N-methyl-D-aspartate type of glutamate receptor.

Animals↗

PubMed1990

The proton-activated inward current of rat sensory neurons includes a calcium component.

Many neurons possess a proton-activated conductance, IH, which supports a large transient inward current at negative potentials and thereby depolarizes cells during rapid drops in external pH. The channels underlying this conductance are permeant to monovalent cations, with a clear preference for sodium. In earlier experiments, it appeared that divalent cations were impermeant: increasing concentrations of extracellular Ca2+ actually decreased the current amplitude. Using whole-cell patch clamp recording techniques, we find that the proton-activated channel is permeant to Ca2+ ions. In the absence of monovalent cations, a substantial current is supported by divalent cations. The previously reported block results from competition between divalents and monovalents. This finding suggests that IH may provide a pathway for Ca2+ entry during the acidification that accompanies normal synaptic transmission, excessive electrical activity, and tissue ischemia.

Animals↗

PubMed1990

Desensitization of NMDA receptors does not proceed in the presence of kynurenate.

The agonist-induced steady-state desensitization of the N-methyl-D-aspartate (NMDA) receptor was investigated by means of whole cell patch and concentration clamp in isolated pyramidal neurons from rat hippocampus. When administered against a background of previously applied agonist, glycine (Gly) produced a response that was smaller than the response elicited by simultaneous application of agonist and Gly. This feature could be explained by an agonist-induced suppression of NMDA receptor sensitivity to the facilitatory action of Gly. This type of 'steady-state desensitization' did not develop when the preincubating solutions contained kynurenate (250 microM). It is proposed that NMDA receptors at all concentrations of agonist may acquire two distinct and interconverting conformations that are sensitive or insensitive to the facilitatory action of Gly. Kynurenate shifts the equilibrium between these states keeping the receptor in a Gly-sensitive conformation.

Animals↗

PubMed1990

Inhibitions of the GABA-induced currents of rat neurons by the alkaloid isocoryne from the plant Corydalis pseudoadunca.

A novel substance, isocoryne, affecting the GABA receptor complex, was isolated from the Corydalis pseudoadunca plant. The inhibitory effect of isocoryne on GABA-activated transmembrane current of single neurons isolated from trigeminal ganglia of rats was tested using concentration-clamp, voltage-clamp and intracellular perfusion techniques. Isocoryne, like other phthalide isoquinoline alkaloids with a 1S,9R isomeric form, produced an inhibitory effect on GABA-activated currents with a Ki = 10(-6) M.

Alkaloids↗

PubMed1990

Proton-induced sodium current in frog isolated dorsal root ganglion cells.

1. The proton-induced current was examined in isolated frog dorsal root ganglion (DRG) cells by the use of the "concentration-clamp" technique, which allows intracellular perfusion and rapid change of external solution with various pH (pHo) within 2 ms under single-electrode voltage-clamp condition. 2. Over one-half of the examined neurons showed no response for a "step" reduction of pHo even in a Ca2(+)-free external solution. In smaller neurons having a diameter less than 20 microns, the persistent and reliable proton-induced responses were obtained, though the current amplitude and the activation and inactivation varied considerably for each cell. 3. The decrease of external Na+ concentration ([Na+]o) reduced the proton response. The proton response reversed the direction and the Na+ equilibrium potential (ENa). 4. With decreasing pHo from 7.4, proton response increased in a sigmoidal fashion. The threshold was around pH 7.0 and the maximum response appeared at pH 5.2, whereas pKa and Hill coefficient were 6.0 and 1.97, respectively. 5. The activation and inactivation phases of the proton-induced current behaved as a single exponential function. The time constants of activation (tau a) and inactivation (tau i) were not affected by changing either the holding membrane potential (VH) or the low external Ca2+ concentration [( Ca2+]o) between 10(-6) and 5 X 10(-3) M. But the decrease of pHo up to 5.2 decreased both tau a and tau i in a saturable manner. 6. In the inactivation curve of proton-induced current obtained by decreasing pHo from various conditioning pHo to 5.5, half inactivation occurred at pHo 7.45.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1989

Hippocampal synaptic plasticity induced by excitatory amino acids includes changes in sensitivity to the calcium channel blocker, omega-conotoxin.

The hippocampus is widely used in investigations of different forms of synaptic plasticity, including long-term potentiation and kindling. Receptors for excitatory amino acids (EAAs) play a prominent role in these phenomena. Recently, is has been demonstrated that exposure of hippocampal slices to EAAs and related agonists produces biphasic effects on excitatory synaptic transmission: initial blockade of synaptic responses is followed by a delayed recovery. The recovered responses demonstrate altered pharmacological properties: they acquire sensitivity to N-methyl-D-aspartate (NMDA) antagonists during L-glutamate (Glu) exposure and lose sensitivity to both NMDA and non-NMDA antagonists under L-aspartate (Asp). These changes persist for many hours. It was suggested that this form of hippocampal plasticity may involve transitions between distinct states of synaptic functioning. To explore this possibility, we investigated several properties of synaptic transmission in the initial and EAA-modified states. Here we report that hippocampal postsynaptic potentials (PSPs) evoked under Glu or Asp exposure completely lose sensitivity to omega-conotoxin GVIA (omega-CgTX), a potent, specific, and irreversible blocker of certain types of neuronal calcium channels. After washout of the EAA, sensitivity to the toxin is regained. These results indicate that prolonged EAA exposure induces profound changes in the machinery of synaptic transmission, which include, but are not limited to, changes in calcium channel functioning.

Amino Acids↗

PubMed1989

Glycine action on N-methyl-D-aspartate receptors in rat hippocampal neurons.

The modulatory effect of glycine and other amino acids on the activity of N-methyl-D-aspartate (NMDA) receptors has been investigated using a concentration clamp method in the acutely isolated pyramidal neurons from rat hippocampus. The dose-response curves for L-aspartate, measured on the background of several glycine concentrations, were close to Langmuir isotherms with Kd values practically independent of the concentration of glycine. The facilitatory action of glycine appeared at concentrations below 0.1 microM and saturated between 10 and 100 microM. This effect demonstrated marked densensitization, at least at concentrations higher than 5 microM. Facilitation of the responses was shared by amino acids with the potencies diminishing as follows: glycine, D-serine, D-alanine greater than L-proline, D-proline, hydroxy-L-proline, taurine greater than L-alanine, L-serine. This sequence did not correlate with the ability of these amino acids to activate strychnine-sensitive glycine receptors.

Amino Acids↗

PubMed1988

Blockade of N-methyl-D-aspartate response in enzyme-treated rat hippocampal neurons.

The responses to excitatory and inhibitory amino acids have been investigated in isolated pyramidal cells from young and adult rat hippocampus using internal perfusion and 'concentration-clamp' techniques. The neurons dissociated in a purely mechanical way were sensitive to all excitatory amino acids (glutamate, kainate, quisqualate and N-methyl-D-aspartate (NMDA] and inhibitory amino acids (glycine, taurine and gamma-aminobutyric acid). The NMDA response was dramatically potentiated by adding glycine at threshold concentration (10(-6) M). The enzyme treatment of hippocampal slices selectively removed the NMDA sensitivity but did not alter all other pharmacological properties of voltage- and agonist-gated ion channels.

Animals↗

PubMed1988

Changes in the state of the excitatory synaptic system in the hippocampus on prolonged exposure to excitatory amino acids and antagonists.

Prolonged exposure of hippocampal slices to L-glutamate was found to produce a biphasic effect on excitatory synaptic transmission from Schaffer collateral-commissural fibers to CA1 pyramidal neurons: an early blockade of postsynaptic responses was followed by a progressive recovery. Antagonists of N-methyl-D-aspartate (NMDA) receptors, which had no effect on the initial responses, blocked the responses which reappeared during continued exposure to L-glutamate. Furthermore, when NMDA antagonists were maintained in the presence of glutamate, excitatory transmission was again restored and the new responses were now insensitive both to 'NMDA' and 'non-NMDA' antagonists. Once elicited, the changes in synaptic transmission appear to be 'memorized' by the slice for at least tens of minutes. These phenomena suggest that the well-known plasticity of the hippocampal synapses may involve a sequence of distinct 'states', and that transitions between these states can be induced by certain pharmacological stimuli.

2-Amino-5-phosphonovalerate↗

PubMed1988

Properties of glycine-activated conductances in rat brain neurones.

The inhibitory responses to glycine, taurine, beta-alanine, 2-amino-5-phosphonovaleric acid (APV) were investigated in freshly isolated neurones from medulla oblongata and hippocampus of rat using concentration clamp, voltage clamp and intracellular perfusion techniques. All substances, when applied in a steplike manner, induced a rapid increase in the chloride permeability of the cellular membrane which slowly desensitized. The responses to all substances were blocked by external strychnine. Cross-desensitization experiments demonstrated that all substances tested activate the same system of membrane receptors.

2-Amino-5-phosphonovalerate↗

PubMed1988

Cationic channels activated by extracellular ATP in rat sensory neurons.

Single channels activated by externally applied ATP were investigated in cultured sensory neurons from nodosal and spinal ganglia of rat using patch clamp and concentration clamp methods. Mean conductance of single ATP-activated channels was 17 pS when measured at a holding potential of -75 mV in saline containing 3 mM Ca2+ and 1 mM Mg2+. Sublevels of conductance were detected in some cases. The current-voltage relationship for a single channel is highly non-linear and demonstrates inwardly directed rectification. The I-V curve obtained for single channels was identical to that for macroscopic current. ATP activated the channels in the absence of divalent cations (in ethylenediaminetetra-acetate-containing medium) as well as in their presence. This indicates that ATP as a free anion can activate the receptor. Ca2+ ions decreased both macro- and microscopic ATP-activated currents. The concentration dependence of this Ca2+ effect does not fit a single site binding isotherm. The single channel current demonstrated prominent fluctuations. When measured in the 0-4 kHz frequency band the amplitude of fluctuations evaluated as a double r.m.s. was about 30% of the mean amplitude of current. The autocorrelation function for the current fluctuations in an open channel could be approximated by a single exponential with the time constant of 0.4 ms. These fluctuations did not depend on the presence of divalent cations in the external medium. The open time distribution for the investigated channels could be described by a sum of two exponentials. Presumably this reflects the existence of two subtypes of ATP-activated channels.

Adenosine Triphosphate↗

PubMed1988

Receptors for ATP in rat sensory neurones: the structure-function relationship for ligands.

1. The pharmacological properties of the ATP-activated conductance in isolated sensory neurones of the rat were investigated by use of voltage clamp and concentration clamp techniques. 2. Adenosine 5'-triphosphate (ATP), adenosine 5'-diphosphate (ADP), cytidine 5'-triphosphate (CTP), cytidine 5'-diphosphate (CDP) and some derivatives activate these receptors, whereas adenosine 5'-monophosphate (AMP), cytidine 5'-monophosphate (CMP) and other naturally-occurring nucleotides are competitive blockers. 3. In the sequence of substances, adenosine 5'-(beta,gamma-methylene)-triphosphonate (APPCP), adenosine 5'-(beta,gamma-difluoromethylene)- triphosphonate (APPCF2P), adenosine 5'-(beta,gamma-dichloromethylene)-triphosphonate (APPCC12P) and adenosine 5'-(beta,gamma-dibromomethylene)triphosphonate (APPCBr2P), the properties of ligands depend on the radius of the atom linked to the carbon of the diphosphonate group. Thus, APPCP is an agonist, APPCF2P is a partial agonist, while dichloromethylene and dibromomethylene analogues of adenosine 5'-(beta,gamma-methylene)triphosphonate demonstrate features of competitive blockers. APPCC12P is the most effective blocker of ATP-receptors (inhibition constant Ki = 21 +/- 4 microM). An adenosyl or adenylyl radical, when connected to the terminal phosphate of ATP, converts the agonist into a partial agonist. 4. Two especially important parts of the ATP molecule are crucial for the interactions with receptors. They are: (1) the vicinity of C6 of the purine ring and (2) the polyphosphate chain. Some modifications in these regions of the molecule result in the transformation of an agonist into an antagonist.

Adenosine Monophosphate↗

PubMed1987

Rapid extracellular pH transients related to synaptic transmission in rat hippocampal slices.

Extracellular pH changes were measured in the rat hippocampal slices using the pH-sensitive dye Phenol red. pH changes accompanied artificially evoked synaptic transmission in the dendrite area of dentate gyrus neurones and pyramidal neurones (CA1). Single electrical stimulation of presynaptic pathways produced a rapid acidic pH shift which was followed by a long-lasting alkaline one. The duration (nearly 10 ms) and amplitude of the acidic shift were closely related to the orthodromically evoked population excitatory postsynaptic potential. Population action potential, when elicited antidromically or in conditions of blocked synaptic transmission, did not produce any pH changes which are supposed to be specifically linked to the synaptic transmission.

Animals↗

PubMed1987

Spider toxin blocks excitatory amino acid responses in isolated hippocampal pyramidal neurons.

Using the 'concentration-clamp' technique we have investigated the action of Joro spider toxin (JSTX), as a specific blocker of glutamate receptor, on freshly isolated rat hippocampal pyramidal neurons. The neurons showed prominent responses to L-glutamate (L-Glu), quisqualate (QA) and kainate (KA) and JSTX blocked completely the both responses. The blocking action of toxin was dose-dependent at the concentrations of toxin between 4.8 X 10(-12) and 4.8 X 10(-8) M, and was remarkably similar in cell to cell trials. The kinetics of blockade was revealed by applying the toxin to the non-desensitizing KA response. We suggest that the JSTX spider toxin may be a valuable tool in ligand binding studies of QA/KA receptors in the central nervous system.

Animals↗