Ilona Kovács

Uridine activates fast transmembrane Ca2+ ion fluxes in rat brain homogenates

The excitatory actions of the pyrimidine nucleoside uridine, and the nucleotides UDP and UTP, as well as the purine nucleotide ATP, were studied by fluorescent labeling of Ca2+ and K+ ion fluxes on the time scale of 0.04 ms to 10s in resealed plasmalemma fragments and nerve endings from the rat cerebral cortex. Two phases of Ca2+ ion influx with onsets of a few milliseconds and a few hundred milliseconds, showing different concentration dependencies, agonist sequences and subcellular localizations were distinguishable. [3H]Uridine identified high (K(D) approximately 15 nM) and low affinity (K(D)approximately 1 microM) specific binding sites in purified synaptosomal membranes. Labeled uridine taken up by synaptosomes in a dipyridamole-sensitive process was released by depolarization (1 mM 4-aminopyridine). Taken together, these results may qualify uridine as a neurotransmitter.

Conformational recognition by central benzodiazepine receptors

Abstract In order to distinguish conformational recognition by the receptor from steric effects brought about by substituents attached to C3 of 1,4-benzodiazepines, two series of closely related compounds were tested for binding potency. Increasing size of the 3-substituent up to isopropyl decreases both the binding and its enantioselectivity. Synthesis and X-ray determination of the molecular structure of 3,3-dimethyl derivatives possessing quasi-axial methyl substituents were followed by a mathematical separation of conformational and substituent effects for quartets with successive 3-methylation [(H)2, (S)-Me, (R)-Me, (Me)2 at C3]. Results indicate a very high preference for conformation M of the ligand by the receptor (the primary reason of stereoselectivity) and a large steric hindrance resulting from the axial methyl substituent. A lower but still unexpectedly substantial steric effect is exerted by the equatorial methyl group.

Modulation of GABA flux across rat brain membranes resolved by a rapid quenched incubation technique

The progress and inhibition of [3H]GABA influx in native plasma membrane vesicles from the rat cerebral cortex was studied on a subsecond to minute time scale under different conditions by applying a rapid quenched incubation technique. In the absence of Ca2+ ([Ca2+]free = 10(-8) M), the progress of influx followed by the addition of 10 nM [3H]GABA to the membrane vesicle suspension with time (500 ms to 15 min) can be described by a first-order rate equation giving an overall rate constant, k, of 3.93 +/- 0.48 x 10(-3) s-1 and equilibrium influx value, INFe, of 8.84 +/- 0.41 pmol [3H]GABA/mg protein. In the presence of Ca2+ ([Ca2+]free = 2.4 x 10(-3) M) a significant increase in the INFe value was observed (k = 4.64 +/- 0.41 x 10(-3) s-1 and INFe = 13.9 +/- 0.40 pmol [3H]GABA/mg protein). Multiplicity of GABA transporters was indicated in the time-dependent inhibition of [3H]GABA influx by different uptake blockers. In the absence of Ca2+, depolarization (75 mM KCl) inhibited the influx of [3H]GABA into the vesicles by approximately 70% and initiated the efflux from vesicles loaded with [3H]GABA. Different uptake blockers inhibited the Ca(2+)-independent translocation of [3H]GABA in both directions with similar specificities.

Binding interaction of gamma aminobutyric acid A and B receptors in cell culture

We have examined the effects of 0.1 mM baclofen and pertussis toxin treatment on gamma aminobutyric acid A (GABAA) receptor binding in whole cells and in membrane of cells from cerebellar primary culture. Baclofen, a GABAB receptor ligand, caused a marked decrease in the bicuculline sensitive (GABAA) binding of [3H]muscimol on the whole cells, but failed to inhibit specific [3H]muscimol binding to cells pretreated with pertussis toxin. The effect of baclofen was only seen in whole cells and not in cell membranes. These findings suggest that activation of GABAB receptor reduces GABAA binding sites through a GTP binding protein.

Effect of CGP 36742 on the extracellular level of neurotransmitter amino acids in the thalamus

We have evaluated the effect of the brain penetrating GABAb antagonist, CGP 36742 on GABAb receptors using in vivo microdialysis in the ventrobasal thalamus of freely moving rat. When a solution of 1 mM CGP 36742 in ACSF was dialyzed into the ventrobasal thalamus, 2-3-fold increases of extracellular Glu, Asp and Gly running parallel with significant decreases of contralateral extracellular Asp and Gly were observed. Unilateral applications of Glu receptor antagonists (0.5 mM MK801, 0.1 mM CNQX) evoked 2-3-fold decreases of CGP 36742-specific elevations of extracellular Asp, Glu and Gly. Administration of CNQX and MK801 in the absence of CGP 36742 did not alter the extracellular Glu and Gly concentrations whereas extracellular Asp concentrations diminished by 42-45% at both sides. By contrast, no changes of extracellular Gly accompanied the 5-10-fold enhancements of extracellular Asp and Glu, observed during application of the Glu uptake inhibitor, tPDC (1mM). Suspensions of resealed plasmalemma fragments from the rat thalamus were mixed rapidly with the membrane impermeant form of the fluorescence indicator, bis-fura-2 and the changes in fluorescence intensity in response to CGP 36742 (0.5 mM), and the GABAb agonist, baclofen (0.1 mM), were monitored on the time scale of 0.04 ms(-10)s. Progress of CGP 36742-mediated influx, and baclofen-mediated efflux of Ca++ ion, antagonized by CGP 36742, was observed in the 1 ms(-10s) period of time. These data support the hypothesis that background ventrobasal activities and thalamocortical signaling are under the control of inhibitory GABAb receptors in the ventrobasal thalamus.

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

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

Light-induced changes in glutamate release from isolated rat retina is regulated by cyclic guanosine monophosphate.

Isolated rat retina was preloaded with [14C]glutamate and subsequently superfused to follow release of glutamate (Glu). After 20 min of superfusion in the dark, exposure of the [14C]Glu preloaded rat retina to a single train of white light pulses reduced Glu efflux significantly in the absence as well as in the presence of low (4 μM) and high (0.5 mM) concentrations of the Glu uptake inhibitor trans‐L‐pyrrolidine‐2,4‐dicarboxylate (t‐PDC). The dark–light response was the highest in the presence of 4 μM t‐PDC by establishing a plateau at 75% ± 7% of the tonic Glu release in the dark (100%). Displaying transient to saturating responses with increasing relative luminance, time series of four trains of white light pulses arrived at a plateau of 85% ± 10%. The cyclic guanosine monophosphate (cGMP) phosphodiesterase inhibitor Zaprinast (200 μM) antagonized the effect of the light series, leading to a plateau of 115% ± 9%. Exposure of the retina to the guanylyl cyclase inhibitor LY83583 (30 and 100 μM) showed fast, transient responses characterized by peaks at 90% ± 1% and 80% ± 3%, respectively.

Kinetically distinguishable AMPA receptors in rat hippocampus are associated with the loss of glutamate-sensitive conformational transitions.

We describe a stopped-flow method to study alpha-amino-7-hydroxy-5-methyl-4-isoxazole propionate (AMPA)-kainate receptor-mediated Na+ ion flux through native membranes. Resealed plasmalemma vesicles and nerve endings from the rat hippocampus were mixed rapidly with a membrane impermeant form of the fluorescence indicator, sodium binding benzofurane oxazole and the changes in fluorescence intensity in response to various [Glu] on the time scale of 0.04 ms-10 s were monitored at a sampling rate of 6.55 kHz. Inhibitors like ouabain (1 mM) and 5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (dizocilpine, 50 microM) enhanced Na+ ion translocation under low-[Na+] and physiological conditions, respectively. Dependence of AMPA-kainate receptor kinetics on [Glu] was described in a model of channel activation by faster and slower desensitizing receptors. The model accounted for almost all of the Na+ ion flux activity in the 30 microM-10 mM range of [Glu]. We found that the values of the initial rate constant for Na+ ion influx, JA, and rate constant for desensitization, alpha, for the faster desensitizing receptor were dependent on data sampling rate, whereas the initial rate constant for Na+ ion flux through the slower desensitizing receptor, JB, varied much less with the sampling rate. These phenomena can be described by (1) a fractal model of short-lived AMPA-kainate receptor channel with many closely spaced states (fractal dimension approximately 1.8) and (2) a model of long-lived AMPA-kainate receptor channel with two discrete states.

Role of intracellular Ca2+ stores shaping normal activity in brain

The role of intracellular Ca2+ stores in the control of brain activity was investigated in microdialysis experiments by monitoring changes in the extracellular concentration of amino acids (AA) in the hippocampus of the rat after intracerebroventricular (icv) administration of the intracellular Ca2+ release blocker, dantrolene in vivo, as well as in D‐aspartate release and transmembrane Ca2+ flux measurements in dantrolene‐treated (50 μM) hippocampal homogenates containing resealed plasmalemma fragments and nerve endings in vitro. Microdialysis data demonstrate that icv injection of 0.6 mM dantrolene significantly decreases (∼20%) the background (Glu) in the hippocampus. Both the (Glu; ∼300%) and the inhibitory effect of dantrolene thereupon (∼50%) was significantly increased when 0.5 mM of the Glu uptake inhibitor, L‐trans‐pyrrolidine‐2,4‐dicarboxylic acid, was dialysed into the hippocampus. NMDA and (S)‐AMPA induced [3H]‐D‐aspartate release in hippocampal homogenates. Preincubation of these homogenates with 50 μM dantrolene was found to reduce the response to NMDA, but not to (S)‐AMPA, in a NMDA‐dependent manner. Increased rates of transmembrane influx and efflux of Ca2+ in hippocampal homogenates with halftimes of 4 ms and 200 ms, respectively, can be observed by the addition of 100 μM NMDA as recorded using a stopped‐flow UV/fluorescence spectrometer in combination with the Ca2+ indicator dye, bisfura‐2. Both the Ca2+ influx and efflux rates of the NMDA response were reduced (25‐fold and >5‐fold, respectively) in homogenates preloaded with 50 μM dantrolene. These results suggest a role for NMDA‐inducible intracellular Ca2+ stores in the control of normal brain activity in vivo. J. Neurosci. Res. 57:906–915, 1999.

Cu2+ is the active principle of an endogenous substance from porcine cerebral cortex which antagonizes the anticonvulsant effect of diazepam

A boiled extract of porcine cerebral cortex was fractionated on Sephadex G-75 and LH-20 followed by paper chromatography of the active fraction having inhibitory activity towards [3H]GABA binding to rat brain synaptic membranes. A ninhydrin-negative substance migrating more quickly than authentic GABA was identified as a copper-GABA complex. The complex inhibited specific [3H]GABA binding (IC50 approximately equal to 1 microM) and antagonized the anticonvulsant effect exerted by intraamygdaloid injection of diazepam. The effect of a synthetic copper-GABA complex was compared and found to be similar to the endogenous complex. Cu2+ alone has no affinity for the GABA recognition site but antagonizes the anticonvulsant effect of diazepam. Therefore, Cu2+ is suggested to be the pharmacologically active principle of the endogenous substance. Cu2+ does not seem to function via the recognition site of the GABA receptor.