Calcium-permeable AMPA and kainate receptors of GABAergic neurons

This Commentary presents a brief discussion of the action of glutamate calcium permeable receptors present with neurons on the release of the neurotransmitter gamma-aminobutyric acid (GABA). In particular, Glutamate sensitive Kainic Acid Receptors (KARs) and α-Amino-3-hydroxy-5-Methyl-4-isoxazole Propionic Acid Receptor (AMPARs) are Na+ channels that typically cause neuronal cells to depolarize and release GABA. Some of these receptors are also permeable to Ca2+ and are hence involved in the calcium-dependent release of GABA neurotransmitters. Calcium-permeable kainate and AMPA receptors (CP-KARs and CP-AMPARs) are predominantly located in GABAergic neurons in the mature brain and their primary role is to regulate GABA release. AMPARs which do not contain the GluA2 subunit are mainly localized in the postsynaptic membrane. CP-KAR receptors are located mainly in the presynapse. GABAergic neurons expressing CP-KARs and CP-AMPARs respond to excitation earlier and faster, suppressing hyperexcitation of other neurons by the advanced GABA release due to an early rapid [Ca2+]i increase. CP-AMPARs have demonstrated a more pronounced impact on plasticity compared to NMDARs because of their capacity to elevate intracellular Ca2+ levels independently of voltage. GABAergic neurons that express CP-AMPARs contribute to the disinhibition of glutamatergic neurons by suppressing GABAergic neurons that express CP-KARs. Hence, the presence of glutamate CP-KARs and CP-AMPARs is crucial in governing hyperexcitation and synaptic plasticity in GABAergic neurons.

[Alpha-adrenergic regulation of two calcium signal pathways in adipocytes]

Using selective receptor's agonist and antagonists we show that mouse white fat cells express alpha1A-, alpha2-adrenergic receptors, which activation with noradrenaline is capable of causing calcium responses different by formation mechanism. Adipocyte's calcium responses to alpha1-adrenoreceptor agonists are caused by alpha1A-type adrenoreceptor and suppressed by inhibitors of PLC-dependent pathway. Calcium responses to alpha2-adrenoreceptors agonists are realized only in the presence of more than 200 microM of L-arginine and suppressed by inhibitors of NOS-PKG-RyR pathway. The incubation of cells with L-arginine creates conditions for switching on the signal pathway with participation of eNOS --> NO --> sGC --> cGMP --> PKG --> CD38 --> RyR --> Ca2+ and for switching of the PLC - IP3R-dependent pathway. Adipocyte's calcium response to L-arginine represents a sharp impulse of the big amplitude and is mediated by alpha2-adrenoreceptors. L-arginine activating alpha2-adrenoreceptors and being the substrate of eNOS, realizes two functions in this pathway.

[Alteration of brown adipocyte Ca2+ responses in culture by adrenergic activation]

Thermogenic capability of brown adipose tissue is controlled by norepinephrine. Interaction of norepinephrine with adipocyte at- and P3-adrenergic receptors results in the increase of Ca2+ and cAMP concentrations. The [Ca2+]i changes initiated by norepinephrine and selective agonists of alpha1- and beta-adrenergic receptors, cirazolin and isoproterenol, were recorded in single cells of primary culture on the 1st, 3rd and 6th days in vitro. On the first day, isoproterenol-induced [Ca2+]i changes as compared to cirazolin-induced ones were characterized by greater amplitude and lesser impulse duration over the entire range of physiological concentrations used. These differences were negligible after 3 days and kinetic differences were practically absent after 6 days of cultivation. The agonist-induced [Ca2+]i changes in proliferating and differentiated cells differed significantly: in the process of cell growth in culture, the amplitude of calcium response increased, the duration of impulse signal decreased and the sensitivity to adrenergic agonists increased. The Ca2+ store in endoplasmic reticulum increased during the cell growth and development in culture, according to thapsigargin-induced Ca2+ response amplitude increase in Ca2+ free medium. The rate of Ca2+ pumping out of cell characterizing PMCA-activity also increased.

[Comparative analysis of Ca(2+)-signalling in brown preadipocytes of ground squirrel Spermophillus undulatus and mouse]

Analysis of the slow Ca(2+)-responses of brown preadipocytes of ground squirrel Spermophillus undulatus and mouse was carried out. The mouse brown preadipocytes demonstrated low but prominent responses to noradrenalin with the maximum at 3 and 10 microM being the less effective. The ground squirrel brown preadipocytes practically did not practically respond to 10 nM-10 microM, whereas 30-600 microM noradrenalin was able to raise intracellular [Ca2+]i up to 600 nM with 300 microM agonist being the most effective. Stimulation of the plasma membrane Ca(2+)-channels with thimerosal showed considerable reduction of the calcium entry system in the cell precursors of both species comparing with their mature adipocytes. Intracellular calcium stores liberated in preadipocytes of both species by tapsigargin and ionomycin in Ca(2+)-free medium were insignificant, and capacitative Ca(2+)-entry in response to the cellular Ca(2+)-stores depletion was completely absent in Ca(2+)-containing medium. The Ca(2+)-responses of the ground squirrel brown preadipocytes were independent on physiological state of the animals and annual seasons. Preadipocytes of both species showed the same dose-response curves for the Ca(2+)-raise under thimerosal, and the mouse had two-fold higher kinetic constants for the Ca2+ ions entry. The ground squirrel brown adipocytes responded to ionomycin with approximately 25% higher increase in [Ca2+]i and the entry of the ions had 7-10-fold higher kinetic constants for this process. Kinetic constants for the [Ca2+]i raise in mouse preadipocytes were independent of ionomycin concentration, whereas in the ground squirrel brown preadipocytes the constant linearly increased with the ionophore concentration. It is suggested that the found difference in the function of Ca(2+)-signalling in preadipocytes of two species, which becomes apparent in the presence of ionomycin, might be responsible for the observed difference in the noradrenalin induced cellular Ca(2+)-responses as well.

Kyotorphin suppresses proliferation and Ca2+ signaling in brown preadipocytes

A positive correlation was revealed between stimulation of protein and DNA synthesis in preadipocytes by norepinephrine or neokyotorphin and intracellular Ca2+ concentration in these cells. Kyotorphin abolished the stimulatory effect of norepinephrine on proliferation of cultured cells and cold-induced [3H]-thymidine incorporation into DNA of mouse brown adipose tissue in vivo. These changes correlated with peptide-induced suppression of slow calcium signaling in preadipocytes.

Cold adaptation modulates Ca2+ signaling in brown preadipocytes

One-week cold exposure of mice led to a 2-fold increase in the density of alpha1-adrenoceptors in brown adipose tissue. The density of alpha1-adrenoceptors returned to normal after adaptation to cold for 2 weeks. The reduced Ca2+ signaling in stem cells of brown fat activated via beta-adrenoceptors and cAMP was transformed into the Ca2+-system induced by alpha1-adrenoceptors and similar to that in mature brown adipocytes.

[Analysis of the kinetics of Ca2+ signals in Ehrlich ascites tumor cells upon the inhibition of the mitochondrial Na+/Ca2+ exchanger]

Signals of cytosolic Ca2+ (Ca2+c) and mitochondrial Ca2+ (Ca2+m) evoked by the activation of purinoreceptors of Ehrlich ascites tumor cells at different extents of inhibition of the mitochondrial Na+/Ca2+ exchanger by tetraphenylphosphonium (TPP+) were investigated. [Ca2+c] was measured by Fura-2 fluorescence, and [Ca2+m] changes were inferred from NAD(P)H fluorescence. The addition of ATP to the cell suspension induced a NAD(P)H response, which replicated Ca2+c signal with some retardation of the peak and a slower decay. In the presence of increasing TPP+ concentrations, NAD(P)H responses evidenced that the rate of [Ca2+m] decay strongly decreases, while the phase of initial rise does not change. The maximal TPP+ dose did not affect [Ca2+c] and NAD(P)H fluorescence in the resting state, as well as ATP-induced [Ca2+c] responses. These data are described in a mathematical model, which accounts for Ca2+ transport through the membranes of endoplasmic reticulum and mitochondria, as well as through the plasma membrane. The model indicates a low rate of the mitochondrial cycle of Ca2+ uptake/efflux at rest and a strong activation of the uptake with increasing [Ca2+c] to which a Hill coefficient of no less than 4 corresponds. Furthermore, the rise of the uptake rate changes in a short time to a decline, and the peak of the rate is markedly ahead of the peak of [Ca2+c].

[Enzymatic activity of some tissues and blood serum from animals and humans exposed to microwaves and hypothesis on the possible role of free radical processes in the nonlinear effects and modification of emotional behavior of animals]

The dependence of activities of actomyosin ATPase, alkaline phosphatase, aspartataminotranspherase, monoaminoxidase and that of affective rat behavior on frequency of modulation of microwaves (0.8-10 microW/cm2) was explored at short-time actions. Series of nonlinear phenomenons, inexplicable from positions of the energy approaches are revealed, The working hypothesis explaining opportunity of high performance of weak and super-weak microwaves and other revealed phenomena by resonance interaction of such electromagnetic radiofrequency radiation with paramagnetic molecules of biological tissues was proposed. This resonance interaction activate free radicals and initiate auto-supporting and auto-intensifying of chain chemical reactions. The spontaneous autocatalytic oxidation of catecholamines enlarges a common pool of free radicals, capable to participate in such enhanced generating. The protective role of monoaminoxidase is postulated. Monoaminoxidase is basically located on an outer surface of mitochondrias and it is deaminating monoamines. The deaminating prevents penetration of catecholamines inside of mitochondrias and their quinoid oxidation there with formation of free-radical semi-quinons, capable to destroy system of ATP synthesis. These inferences are obliquely confirmed by the experimentally revealed correlation between activity of monoaminoxidase and integrative activity of the rat brain.

[Adenylyl cyclase pathway is involved in the regulation of intracellular calcium level regulation in brown preadipocytes]

Mechanism of adrenergically activated calcium response in freshly isolated brown preadipocytes was studied with fluorescent probe Fura-2. Application of a direct activator of adenylylcyclase forskolin or cell permeable analog BrcAMP caused rise in the intracellular calcium level that was even higher than after the application of norepinephrine. Protein kinase A inhibitor H-89 in a dose-dependent manner reduced, while inhibitor of total phosphodiesterase activity IBMX, or protein phosphatase inhibitor ocadaic acid enhanced norepinephrine or isoproterenol initiated cellular calcium responses. It is concluded that cAMP and protein kinase A mediated phosphorylation play a crucial role in adrenergically initiated calcium signalling in brown preadipocytes.

[Both alpha1- and beta-adrenergic receptors participate in generation of a Ca2+-response to noradrenaline in murine brown preadipocytes]

It has been shown that in Ca(2+)-signal generation, at the addition of norepinephrine (NE) to a suspension of freshly isolated brown preadipocytes, beta-receptors participate alongside with alpha 1-adrenoreceptors. The amplitude of cell response to 10(-5)-10(-8) M NE is approximately equal to the sum of effects of alpha 1-specific agonists (oxymetazoline or cirazoline) and beta-selective agonist isoproterenol. beta-Selective antagonist nadolol practically completely prevented the effect of NE on the intracellular Ca2+ level, while phentolamine, an antagonist of alpha-receptors, caused only a approximately 25% inhibition of the cellular response.

Reduction of the Ca2+ transport systems in mouse young brown fat cells

Principal differences in the kinetics and amplitude of Ca2+ response to norepinephrine were found between freshly isolated young differentiated brown fat cells. An increase in the Ca2+ concentration in the cytoplasm ([Ca2+]i) in the young cells was unusually slow (A[Ca2+]i = 0.03 nM/s) in comparison with that in the differentiated cells, and the Ca2+ influx from the outside was not induced by Ca2+ mobilization agents, such as thapsigargin and ionomycin. Ionomycin increased [Ca2+]i up to 150 nM in a Ca2+-free medium and up to 270 nM in the normal medium. This results in that the intracellular Ca2+ stores in freshly isolated young cells are rather poor, and the mechanism of capacitive calcium entry does not virtually function. The data on chemical modification of Ca2+ channels in the plasma membrane by thimerosal suggest that the conductance of these channels is low and/or their number in young brown fat cells is insignificant.

[Seasonal characteristics of the monoaminergic regulation of the behavior of hibernating animals]

The stimulating effect of L-DOPA on exploratory activity of the ground squirrel in the hole-board and open field experiments was stronger in the spring than in autumn, whereas the regulating effect of the 5-HTP on adaptive behaviour of hibernating animals was stronger in autumn. Results of the biochemical analysis of the MAO A activity with serotonin or noradrenaline as substrates revealed a seasonal dependence of the substrate specific changes of this enzyme activity in hibernating animals. The data obtained suggest that the seasonal distinctions in effects of similar pharmacological agents depend on activity of the brain monoaminergic systems' activity in hibernating animals in different periods of the annual cycle.

[The effect of electromagnetic radiation on the monoamine oxidase A activity in the rat brain]

The effect of the ultralow power pulse-modulated electromagnetic radiation (EMR, power density 10 microW/cm2; carrying frequency 915 MHz; modulating pulses with frequency 2, 4, 6, 8, 12, 16 and 20 Hz) on activity of monoamine oxidase (MAO-A), enzyme involved in the oxidative deamination of monoamines, was investigated. It was established that the increase of activity MAO in hypothalamus reached the maximal meaning at modulation frequency of 6 Hz that corresponded 160% (p < 0.01) of the control level; and at modulation frequency of 20 Hz the decrease of enzyme activity up to 74% (p < 0.01) was found. Mainly the action of ultralow power pulse-modulated EMR on activity of MAO in hippocamp was activating; and the maximal increase of enzyme activity up to 174% (p < 0.01) was registered at modulation frequency of 4 Hz.

Regulation of pyruvate dehydrogenase complex activity in Ehrlich ascites carcinoma cells by Ca2+ and pyruvate

The dependence of pyruvate dehydrogenase complex (PDC) activity on [Ca2+] was determined in Ehrlich ascites carcinoma cells at different pyruvate concentrations. The resulting family of curves had the following characteristics: a) bell-shaped appearance of all curves with maximum activity at 600 nM Ca2+; b) unchanged position of maxima with changes in pyruvate concentration; c) nonmonotonous changes in PDC activity with increasing pyruvate concentration at fixed [Ca2+]. Feasible mechanisms involving Ca2+-dependent phosphatase and kinase which are consistent with the experimental findings are discussed. To determine the steps in the chain of PDC reactions which determine the observed phenomena, a mathematical model is suggested which is based on the known data on the structural--functional relationships between the complex components--pyruvate dehydrogenase (E1), dihydrolipoyl acetyl transferase (E2), dihydrolipoyl dehydrogenase (E3), protein X, kinase, and phosphatase. To adequately describe the non-trivial dependence of PDC activity on [Ca2+] at different pyruvate concentrations, it was also necessary to consider the interdependence of some steps in the general chain of PDC reactions. Phenomenon (a) is shown to be due only to the involvement of protein X in the PDC reactions, phenomenon (b) to be due to changes in the activity of kinase, and phenomenon (c) to be due to dependence of acetylation and transacetylation rates on pyruvate concentration.

[Effect of Ca2+ ions on the activity of pyruvate dehydrogenase complex in ascites Ehrlich carcinoma cells]

The effect of [Ca2+] at saturating concentrations of other substrates on the activity of pyruvate dehydrogenase complex (PDC) from Ehrlich ascite carcinoma cells was studied. The effect is biphasic (bell-shaped) and maximum of PDC activity is at 600 nM [Ca2+]. The experimental curve is in agreement with theoretical plot constructed by analysis of PDC kinetic model. The kinetic model considers the data on the structural and functional relationships between PDC components including pyruvate dehydrogenase (E1), dihydrolipoyl acetyltransferase (E2), dihydrolipoyl dehydrogenase (E3), and protein X. This model can be used for analysis of experimental activation by low Ca2+ concentrations and inhibition by increasing Ca2+ concentrations and can predict changes in the system caused by changes in the substrate concentrations.

[The dynamics of the liver calcium-binding protein in golden hamsters with opisthorchiasis]

By means of immunochemical methods a considerable increase in Ca(2+)-transporting glycoprotein (GP) in liver tissues of golden hamsters at early stages of opisthorchiasis has been established. On the 3rd day after the infection GP amount increases 6 fold, on the 7th day--15 fold. On the 14th day GP amount decreases but does not reach the control level. The control level is reached only on the 22nd day after the infection of animals. Ca(2+)-transporting GP belongs to the system of electrogenic transport of Ca2+ in mitochondria (MCh) and its increase in MCh of the liver is, apparently, the basis of destructive changes of these organelles.

[Effect of antibodies against mitochondrial K+-transporting protein on K+ transport in rat liver mitochondria]

The K+ transport in rat liver mitochondria was studied by the immunochemical method. Antibodies to mitochondrial K+-transporting protein with molecular weight 60 kDa were obtained and used as possible inhibitor or K+ transport. Antibodies depressed the DNP-induced K+ efflux and energy-dependent swelling by 50% without causing changes in respiration and oxidative phosphorylation.